http://oceans.mit.edu America/New_York America/New_York America/New_York 20171105T020000 -0400 -0500 20181104T020000 EST 20180311T020000 -0500 -0400 EDT h4sofbi5ibsgevrin97kepp44k@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Abstract: A major question in how we approach climate change and its impact on the carbon cycle is at what level of granularity must we understand phytoplankton diversity. The phytoplankton that mediate CO2 uptake in the marine biosphere are tremendously diverse, non-homogeneously distributed and oftentimes physiologically ill-characterized. Additionally, they live among non-photosynthetic microbes from the three domains of life – creating a complex network of chemical exchanges and physical interactions. Microbial oceanographers frequently employ genome and environmental sequence analyses to tackle these topics, and yet many genes and features of genomes expressed in nature are of unknown function. By investigating this material in the lab and field we are discovering key, previously unidentified environmental controls and responses of phytoplankton as well as distributional information. Here, we will explore phytoplankton diversity and factors for bloom development at three levels of granularity – basic growth requirements in the context of microbial networks (in this case vitamins), evolutionary divergence (in connection to nutrient limitation), and population connectivity through deep ocean currents (using repetitive, gene-interrupting sequences). Through iterative modeling, lab, and field experiments it should be possible to test and integrate ecologically relevant levels of microbial diversity to understand primary production in transitioning ecosystems. 20150506T121000 20150506T130000 54-915 0

SLS – Alex Worden (Monterey Bay Aquarium Research Institute) Phytoplankton, microbial networks and the global carbon cycle

e0lr5udf6dcsor5o2cbs9fui2o@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The Antarctic Slope Front (ASF) almost completely encircles the Antarctic continent, separating cold shelf waters from relatively warm Circumpolar Deep Water (CDW) at mid-depth offshore. Exchanges across the ASF transport CDW toward marine-terminating glaciers, and export Antarctic Bottom Water (AABW) to the abyssal ocean. Recent studies indicate that this exchange may be modulated by mesoscale eddies, which facilitate cross-slope exchanges via stirring along isopycnals and eddy bolus transports. In this seminar I will discuss physical controls over the rates of cross-slope water mass exchange, and examine the dynamical balances governing cross-slope eddy transfer. I will first present a recently-developed eddy-resolving process model of the Antarctic continental shelf and slope. The model enforces realistic offshore ocean stratification over idealized shelf/slope bathymetry, in order to provide a realistic representation of the water masses in a configuration that can be analyzed cleanly. The model forcing includes a westward wind stress over the continental slope and buoyancy loss on the continental shelf, consistent with prevailing Antarctic easterly winds and brine rejection in coastal polynyas. I will use this model to explore the sources of eddy kinetic energy (EKE) over the continental slope, and its resulting impact on the cross-slope transport of mass and tracers. I will show that the upper-ocean dynamics resemble the Antarctic Circumpolar Current, with wind-driven northward shoaling of the pycnocline resisted by baroclinic conversion of potential energy to EKE. By contrast, close to the ocean bed (at the CDW/AABW interface) potential energy is removed by both the wind-driven mean overturning and the generation of baroclinic eddies, and is instead sourced from the buoyancy loss on the continental shelf. This EKE source turns out to be sensitive to variations in the model surface forcing and bathymetry. Consequently, relatively small changes in the forcing and geometry can produce a substantial rearrangement of the water mass pathways and volume transports across the continental slope. These findings suggest that shoreward eddy transport of CDW should be localized to a few favorable locations around the Antarctic shelf break, and that future changes in the easterly wind strength or coastal polynya productivity could significantly alter the shoreward heat transport and the properties of the outflowing AABW. 20150513T121000 20150513T130000 54-915 0

SLS – Andrew Stewart (UCLA) – Eddy transport and mixing across the Antarctic continental slope

fifj494qb1lu2nt5j49v9jala0@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Abstract: Fixed nitrogen availability can regulate atmospheric carbon dioxide concentrations and climate as a whole. Its loss via two anaerobic microbial processes – anammox and denitrification – only occurs where oxygen is sufficiently depleted. Field experiments in the tropical Pacific oxygen minimum zone resolve two long-standing debates critical to understanding global climate. While very low oxygen concentrations are required for fixed nitrogen loss by either anammox or denitrification, organic matter quantity and quality determine the magnitudes of these rates and the partitioning between the two pathways. Dissolved oxygen concentrations also decouple the denitrification steps, allowing for net production of nitrous oxide and accentuating the importance of the oxygen and nitrogen cycles in regulating climate. 20150514T133000 20150514T143000 54-915 0

SLS – Andrew Babbin (MIT) Anaerobic cycling of marine nitrogen

g3ehmberbdu54vh6631pnu10ug@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Abstract: During the termination of the last ice age, atmospheric CO2 increased by ~30% and global temperature rose by several degrees, yet the mechanism(s) driving these major changes remain elusive. Marine and terrestrial records indicate that the amount of CO2 stored in the deep sea is greater during glacial periods than interglacial periods. The implied net transfer of carbon into and out of the ocean is likely the result of changes in ocean circulation and/or the efficiency of the biologic pump. The resulting atmospheric CO2 fluctuations may play some role in amplifying temperature shifts across ice age cycles. In this study, we reconstruct past variations in seawater carbonate ion concentration in order to gain insight into the relative roles of different oceanic CO2 storage mechanisms and to place constraints on the timing, magnitude, and location of subsequent deep ocean ventilation. Our reconstruction is based on the trace element and stable isotopic composition of calcite shells of the epi-benthic foraminifer Cibicidoides wuellerstorfi from a sediment core in New Zealand's Bay of Plenty. The sediment core site (1,627 m water depth) lies within the upper limit of modern Circumpolar Deep Water (CDW), ~500 m below the local Antarctic Intermediate Water (AAIW) salinity minimum. A record of ΔCO32- derived from the foraminiferal boron to calcium ratio (B/Ca) provides evidence for greater ice-age storage of respired CO2 and also reveals abrupt deglacial shifts in inorganic carbon chemistry up to 30 µmol/kg (5 times larger than the difference between average LGM and Holocene values). The rapidity of these deglacial changes in the ocean interior suggests 1) fluctuations in the intermediate-deep water boundary near the core site, and/or 2) rapid changes in deep water composition. Additional records are currently being reconstructed to probe these possibilities. 20150520T121000 20150520T130000 54 915 0

SLS – Katherine Allen (Rutgers University) The Ocean’s Role in Ending an Ice Age: A View from the South Pacific

vvbkd9tl87kjotiqupdb4ao140@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20150527T121000 20150527T130000 54-915 0

SLS

ff8acuj0ouic1ga8g808v7meo0@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20150603T121000 20150603T130000 54-915 0

SLS

tjt8djodhuj2o0rs2n05q2ncn0@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The abyssal ocean during the last glacial period is widely believed to have been comparatively strongly salt stratified and close to the freezing point of sea water. Much of the support for that view is based on the work of McDuff, Schrag, Adkins, M. Miller and several others who analyzed the chlorinity and oxygen isotope ratios from the pore waters of deep sea cores. The data represent what, in terms of control theory, is termed a "terminal constraint". That theoretical framework is used to revisit the inferences about the LGM abyssal properties, and in particular, to understand the degree to which they are robust to sometimes plausible assumptions about the data. 20150916T121000 20150916T130000 54-915 0

SLS – Carl Wunsch (MIT)- Salinity and Temperature of the Abyssal Ocean at the Last Glacial Maximum (LGM)

sp1de2gn36rb15tpcgt2vona9k@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20150923T121000 20150923T131000 54-915 0

SLS – Trevor McDougall (UNSW) – The thermodynamics of the turbulent ocean and of ice; what we know and what things are still a puzzle

cro8mu7efln5cf3lqf30ma4u34@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The Brazil Current (BC) is probably the least known and explored of the western boundary currents of the world ocean. In this talk, we offer a brief overview of the research carried out at the Oceanographic Institute of the University of São Paulo (IOUSP) on this current system's general pattern, water masses and meridionally changing vertical structure. We focus here on the latitude range (21S­26S) at which the BC vertical structure very closely approaches the one predicted in the seminal work by Henry Stommel (1965). We explore the spatial­ temporal variability and conduct some first­order dynamical studies of this "Stommelian" BC by blending quasi­synoptic data sets, mooring data and simple process oriented modeling. 20150930T121000 20150930T131000 54-915 0

SLS – Ilson Carlos Almeida da Silveira (LaDO) – A “Stommelian” Brazil Current at 21S-26S: vertical structure and mesoscale variability

hsqb5mfg6baavv7utsoo79kv20@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch With the explosion of exoplanet discoveries and atmospheric characterization over the last decade, there is now the hope that in the near future, it will be possible to study the atmospheres of low mass, possibly Earthlike exoplanets. Interpreting these observations will be a grand challenge, because the diversity of rocky planet climates is likely to be enormous. Here I discuss the role that theory and idealized modeling can play in advancing our understanding of the possibilities. I present results on two key problems in exoplanet climate evolution: the loss of a planet’s water to space and the circulation (and possible nightside collapse) of atmospheres on tidally locked planets. I show that in both cases, scaling analysis allows the fundamentals of the problem to be understood in a robust and general way. I discuss the implications of these results for exoplanet habitability and the future search for biosignatures by groundand spacebased telescopes. 20151007T121000 20151007T131000 54-915 0

SLS – Robin Woodsworth (Harvard) – What can theory teach us about the climates of low-mass exoplanets?

q08g85divlio3s4ek47ihnfoog@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch In situ cosmogenic nuclides are produced in surface materials exposed to cosmic radiation - cover a landscape with ice and production ceases; erode down into a landscape and nuclide concentrations quickly decrease. We used these simple observations to explore a new proxy for reconstructing past ice sheet variability- the 10Be concentration of sand in marine sediments adjacent to glaciated continents, which should reflect the exposure and erosion history on land before the sediment was deposited in the deep sea. This talk will present cosmogenic nuclide records from two Ocean Drilling Program cores off east Greenland and the ANDRILL-1B core next to the East Antarctic Ice Sheet spanning the past 8 Myr, as well as 10Be measurements on contemporary sediments emanating from the southern Greenland Ice Sheet today. Notable features of the Greenland record include a 50-fold long-term decline in 10Be concentration reflecting late Cenozoic ice-sheet growth, pronounced 10Be dips coincident with major ice-rafted debris pulses, considerable variability during the early Pleistocene, and low concentrations indistinguishable from contemporary sediments throughout the past 1 Myr. The Antarctic record, on the other hand, exhibits very low 10Be concentrations over its entire length, suggesting little to no subaerial exposure of land where the sediment was sourced from during the past 8 Myr. 20151014T121000 20151014T131000 54-915 0

SLS – Jeremy Shakun (Boston College)- Using cosmogenic isotopes in marine sediment cores to decipher long-term Greenland and Antarctic Ice Sheet behavior

j02i76ignm8l356jekcpoq13i8@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Many aspects of how natural phytoplankton communities change through time remain poorly understood, in large part because traditional organism-level sampling strategies are not amenable to high frequency, long duration application. To overcome aspects of this limitation, we developed the FlowCytobot series of automated submersible flow cytometers capable of rapid, unattended analysis of individual plankton cells for long periods of time. FlowCytobot and Imaging FlowCytobot use a combination of laser-based scattering and fluorescence measurements and video imaging of individual particles to enumerate and characterize cells ranging from picocyanobacteria to chaining-forming diatoms. When combined with automated processing and image classification, these observations make it possible to characterize taxonomic composition of plankton communities with unprecedented temporal resolution, ranging from hours to years. Multi-year time series from FlowCytobot and Imaging FlowCytobot are now being used to study bloom dynamics and community structure in US coastal waters. The high temporal resolution observations of single cell properties make it possible not only to characterize taxonomic composition and size structure, but also to quantify taxon-specific growth rates. Emerging results provide a wide range of insights including links between climate factors and interannual variability in seasonal blooms, decadal-scale shifts in community composition, ecologically important roles for taxon-specific parasites, and detection and characterization of harmful algal blooms. 20151021T121000 20151021T131000 54-915 0

SLS – Heidi Sosik (WHOI) – Bloom dynamics to climate change: Multi-scale observations of phytoplankton with autonomous flow cytometry

vm82n0tt1i4ag6ja9dn5eut410@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Fjords form a key link in the climate system by connecting glaciers of the Greenland Ice Sheet to the ocean. They are the gateways for importing oceanic heat to melt ice and for exporting meltwater into the ocean. Submarine melting in fjords has been implicated as a driver of dynamic glacier changes in the past several decades. However, there are no direct measurements of this melting, and little is known about the fjord processes that modulate melt rates and export meltwater. Here, we explore the drivers of fjord circulation and heat transport in Sermilik Fjord, near the terminus of Helheim Glacier. We investigate the competing roles of buoyancy forcing from the glacier and remote forcing from the shelf. Building on estuarine studies of salt fluxes, we assess the fluxes of heat and salt through the fjord and develop a new framework for inferring submarine melt rates from glacial fjord budgets. 20151028T121000 20151028T131000 54-915 0

SLS – Rebecca Jackson (MIT/WHOI) – Ocean-glacier interactions in Greenland: fjord dynamics and heat transport

npqli52ji7mdgvlfa620c1bg58@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch During the daytime, under conditions of relatively low winds and high solar insolation, the nearsurface ocean warms. This stratified diurnal warm layer traps momentum from the wind near the surface, generating shear across the layer. Horizontal advection associated with this diurnal warm layer shear is likely responsible for observed nearsurface freshening in the upper meter of the ocean during the SPURSI field campaign. This shear associated with the diurnal warm layer, a fractional depth of the deeper mixed layer, horizontally displaces water near the surface during the daytime converting horizontal gradients into vertical ones. At night, convection vertically mixes the horizontally displaced water throughout the deeper mixed layer. This cycle of advection and then vertical mixing is a mechanism for effective submesoscale lateral diffusion on the scale of 110 km. We will discuss a simplified representation of the mechanism, and estimates of effective submesoscale horizontal diffusivity for the mixed layer. The calculated effective diffusivity associated with this mechanism is 1100 m2/s, depending on the advective timescale, and depth and speed of the diurnal warm layer current. The importance of the daily cycle as a mechanism for smoothing lateral inhomogeneities within the mixed layer will be considered. Additionally, we examine how the advection associated with the diurnal warm layer may locally enhance or suppress the diurnal warming observed at a location. The frequency of occurrence of diurnal warm layers and global perspectives will be discussed. 20151104T121000 20151104T130000 54-915 0

SLS – Alec Bogdanoff (MIT/WHOI)- Submesoscale lateral mixing by diurnal warm layer shear

9g2sb4helvoome98emc9t6fva8@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch I will present a suite of records from a 950 m-depth sediment core from the western North Atlantic, a site influenced by Antarctic Intermediate Water (AAIW) in the modern ocean. The data suggest that northern sourced waters dominated the water mass mixture during the LGM, and that AAIW was shallower than ~850m. A d18O decrease early in Heinrich Stadial 1 (HS1) is attributed to the incorporation of deglacial meltwater into the northern sourced waters that continued to influence the site. Two interpretations for the mid-to-late HS1 data will be discussed, having opposite implications for the vigor of the Atlantic Meridional Overturning Circulation (AMOC) during this interval. After HS1, the data conform to the consensus view of AMOC variability – increased AAIW presence during the Bolling-Allerod and the Holocene, when the AMOC was strong, and reduced AAIW presence during the Younger Dryas when the AMOC was weak. 20151118T120000 20151118T130000 54-915 0

SLS – Delia Oppo (WHOI) – Deglacial Atlantic circulation:evidence from multiproxy records from shallow western north Atlantic sediment cores

8jlf3c11org5b01aqc8irt4u5g@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Diapycnal turbulent mixing in the Southern Ocean is believed to play a role in setting the rate of the ocean Meridional Overturning Circulation (MOC), an important element of the global climate system. Whether this role is important, however, depends on the strength of this mixing, which remains poorly qualified on global scale. To address this question, a passive tracer was released upstream of the Drake Passage in 2009 as a part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). While the mixing rates inferred from vertical dispersion of the tracer are large and imply a key role played by mixing in setting the MOC, those based on localized microstructure measurements seem to suggest otherwise. In this work we use a high resolution numerical model of the Drake Passage region, sampled in the DIMES experiment and tuned to its observations, to explain that the difference between the two estimates arise from the large values of mixing encountered by the tracer when it flows close to the bottom topography. We conclude that enhanced bottom mixing, in combination with large lateral stirring and mixing by mesoscale eddies, is sufficiently strong to play an important role in setting the Southern Ocean branch of the MOC below 2 km. 20151125T120000 20151125T130000 54-915 0

SLS – Ali Mashayek (MIT) – Topographic Enhancement of Vertical Mixing in the Southern Ocean

e6rv273dmrbnm36gt5jtusdf8o@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Using a dynamically consistent state estimate, the vertical redistribution of oceanic heat is investigated over a 20-year period (1992-2011). The 20-year mean vertical heat flux shows strong variations both horizontally and vertically, consistent with the ocean being a dynamically active and spatially complex heat exchanger. Between mixing and advection, the two processes determining the vertical heat transport in the deep ocean, advection plays a more important role in setting the spatial patterns of vertical heat exchange. The global integral of vertical heat flux shows an upward heat transport in the deep ocean, suggesting an abyssal cooling trend over 1992-2011. The bidecadal change of the ocean vertical heat flux is also examined and provides dynamical insights into the global ocean heat content change. Preliminary results show that above 1500 m more heat is transported downward during 2002 2011 than 1992-2001. The spatial pattern of the vertical heat flux change shows consistent features with previous studies, such as more downward heat transport in the tropical Pacific and the North Atlantic during the last decade. Whereas the spatial pattern of vertical heat flux change is closely related to the advection change, its global integral is largely determined by the change in mixing, indicating a crucial role of ocean mixing in explaining the long-term change of ocean vertical heat exchange. 20151202T120000 20151202T130000 54-915 0

SLS – Xinfeng Liang (MIT) – Global Ocean Vertical Heat Flux and Its Bidecadal

depq9vqumhk21urf23fh7g7qbc@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch There is considerable interest in determining how the full distribution of surface temperature changes with warming. Model ensembles project that extratropical land temperature variability will decrease in the future, consistent with simple physical arguments related to polar amplification. However, observational studies have thus far come to conflicting conclusions. Several analytical pitfalls in interpreting observational records lead to these discrepancies, and highlight the importance of accounting for non-normality and the effects of filtering, time-averaging, gridding, and smoothing. I will present a set of methods designed to overcome these challenges, which I apply to a large set of daily temperature observations to show that a decrease in temperature variability is already robustly detectable in the extratropics. 20151209T121000 20151209T131000 54-915 0

SLS- – Andy Rhines (UofWashington) – Observations and Dynamics of Decreasing Variability of Winter Temperatures

f5lefkd12pr2nsoo70jpmp46ks@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The Southern Ocean is one of the most energetic regions of the world ocean due to intense winds and storm forcing, strong currents in the form of the Antarctic Circumpolar Current (ACC) interacting with steep topography, and enhanced mesoscale activity. Consequently, the Southern Ocean is believed to be a hotspot for enhanced oceanic mixing. Previous work based on finestructure parameterizations has suggested that strong mixing is also ubiquitous below the mixed layer. Results from a US/UK field program, however, showed that enhanced internal wave finestructure and turbulence levels are not widespread, but limited to frontal zones where strong bottom currents collide with steep largeamplitude topography. Direct measurements of turbulence showed that previous estimates of mixing rates in the upper 1km are biased high by up to two orders of magnitude. Despite the prevalence of energetic wind events, turbulence driven by downward propagating nearinertial wave shear is weak below the mixed layer. Inefficient wind forcing at nearinertial frequencies and seasonally varying upper ocean stratification likely contribute to the observed weak mixing rates. Double diffusive processes and turbulence both contribute to buoyancy flux, elevating the effective mixing efficiency above the canonical value of 0.2 in the upper 1km. Ultimately, this work informs largescale modeling efforts through parameterizations of mixing processes in the highly undersampled Southern Ocean. 20151216T121000 20151216T131000 54-915 0

Sophia Merrifield (MIT/WHOI) – Mechanisms for enhanced turbulence in the Drake Passage region of the Southern Ocean

a2ekjra8t0jbckpkddqqo1uvm4@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Quantitative and mechanistic understanding of the changes in atmospheric CO2 concentrations and ocean carbon content between glacial and interglacial periods remains lacking. An increase in the efficiency of the biological pump has been hypothesized to contribute to higher ocean carbon storage during glacial periods. Here we use a model of the ocean's biogeochemical cycles that includes both carbon (13C) and nitrogen (15N) isotopes but no sediment interactions. We present results from one present day simulation and six simulations of the Last Glacial Maximum (LGM, ~20 ka before the present). The LGM simulations are idealized sensitivity experiments that explore effects of changes in maximum phytoplankton growth rates (mmax). Increasing mmax in the model leads to a more efficient biological pump, more carbon storage, and lower d13CDIC and oxygen concentrations. The resulting increase of denitrification stimulates additional nitrogen fixation and increases the spatial variance of d15NNO3, while decreasing the ocean’s fixed nitrogen inventory. Increased nitrogen fixation lowers surface d15NNO3 in most of the tropics. In the model’s Southern Ocean modest increases in mmax result in higher d15NNO3 due to enhanced local nutrient utilization, consistent with reconstructions, but larger mmax cause declining values there owing to the poleward transport of low tropical d15NNO3. Comparison to reconstructions from LGM sediments indicates that models with moderately increased mmax (by 16 - 33 %) fit both isotope data best, whereas large increases are inconsistent with nitrogen isotopes although they still fit the carbon isotopes reasonably well. The best fitting models reproduce major features of the glacial d13CDIC, d15N, and oxygen reconstructions, while simulating reduced carbon storage, compared with the pre-industrial ocean, due to lower preformed carbon concentrations. We conclude that the biological pump was more efficient during the LGM. However, sediment interactions and whole ocean alkalinity changes may be required to increase ocean carbon storage. Our analysis illustrates interactions between the carbon and nitrogen cycles as well as the complementary constraints provided by their isotopes. 20160203T120000 20160203T130000 54-915 0

SLS – Andreas Schmittner (Oregon State University) – Complementary Constraints from Carbon (13C) and Nitrogen (15N) Isotopes on the Efficiency of the Glacial Ocean’s Biological Pump

uksifljdddbbdn74o6ph44l3pk@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The accuracy of ocean components of climate models is thought to increase with resolution, and we examine this associated change in utility on a range of model fields. A 30-year integration (1978 to 2007) of the NEMO model at 1o, 1/4o and 1/12o is used to investigate the impact of modelling choices associated with horizontal resolution. Changes in degrees of freedom associated with the increasing resolution allow alternative energy dissipation pathways and their impact is assessed. A distinct strengthening of the anti-clockwise component of the overturning is found in the Southern Ocean, primarily owing to the baroclinic component. The mixed layer does not change significantly with resolution, with results comparable to observations. Minor changes with resolution are attributed to increased numbers of fronts with better resolution. In the interior, steric height variability, specifically its covariance between the surface (2000m) does change owing to eddy effects not captured by the Gent-McWilliams parametrization. Topographic interactions are assessed in terms of vortex stretching in the bottom pressure torque term. Major changes are found in the baroclinic component in the Southern Ocean. Low resolution appears appropriate for fields such as the mixed layer depth, but higher resolution is increasingly required for large scale features through allowing eddy activity. 20160210T120000 20160210T130000 54-915 0

SLS- Maike Sonnewald (MIT) – Ocean model utility dependence on horizontal resolution

8fmq0u02dfbgsq7o8fasosd454@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch We investigate how sea surface temperatures (SSTs) around Antarctica respond to the Southern Annular Mode (SAM) on multiple timescales. To that end we examine the relationship between SAM and SST within unforced preindustrial control simulations of coupled general circulation models (GCMs) included in the Climate Modeling Intercomparison Project phase 5 (CMIP5). We develop a technique to extract the response of the Southern Ocean SST to a hypothetical step increase in the SAM index. We demonstrate that in many GCMs, the expected SST step response function is nonmonotonic in time. Following a shift to a positive SAM anomaly, an initial cooling regime can transition into surface warming around Antarctica. However, there are large differences across the CMIP5 ensemble. In some models the step response function never changes sign and cooling persists, while in other GCMs the SST anomaly crosses over from negative to positive values only three years after a step increase in the SAM. This intermodel diversity can be related to differences in the models' climatological thermal ocean stratification in the region of seasonal sea ice around Antarctica. Exploiting this relationship, we use observational data for the time-mean meridional and vertical temperature gradients to constrain the real Southern Ocean response to SAM on fast and slow timescales. 20160217T120000 20160217T130000 54-915 0

SLS- Yavor Kostov (MIT) – Fast and slow responses of Southern Ocean SST to SAM in coupled climate models

rpqjccranqb6v7lfuffo3h6rm4@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20160302T120000 20160302T130000 54-915 0

SLS-Lorenzo Polvani (Columbia)- zone extremes in the Arctic, and their impact on surface climate

u24hh7d4d0vvru16l1rum83b1k@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch We describe and interpret in situ observations of tidally driven turbulence that were obtained in the vicinity of a small channel that transects the crest of the Mendocino Ridge in the north-eastern Pacific. Flows are funneled through the channel and have tidal excursion lengths comparable to the width of the ridge crest. Once per day, energetic turbulence is observed in the channel, with overturns spanning almost half of the full water depth. A high resolution, nonhydrostatic, 2.5-dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave past the site location, and subsequent development of a hydraulic jump. During this phase of the tide the strong transports were associated with full depth flows, however, during the weaker beat of the tide transports were shallow and surface-confined, generating negligible turbulence. A regional numerical model of the area finds that the subinertial K1 (diurnal) tidal constituent generates topographically trapped waves which propagate anticyclonically around the ridge, and are associated with enhanced near-topographic K1 transports. The interaction of the trapped waves with the M2 (semidiurnal) surface tide produces a baroclinic tidal flow that is alternately surface confined and full depth. Consistent with observations, full depth flows are associated with the generation of a large amplitude tidal lee wave on the northward face of the ridge, while surface confined flows are largely nonturbulent. The regional model demonstrates that nearfield dissipation over the entire ridge is diurnally modulated, despite the larger amplitude of the M2 tidal constituent, indicating that the trapped wave modulates near-field dissipation and mixing at this location. 20160309T120000 20160309T130000 54-915 0

SLS- Ruth Musgrave (MIT-MechE) – Tidally driven mixing: breaking lee waves, hydraulic jumps and the influence of subinertial trapped internal tides

0ut5p5eqm7gvf41qhg299rrvf4@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Eddies in the ocean move westwards. Those shed by western boundary currents must then interact with shelf-slope topography at the western boundary. This simple picture is complicated by the presence of other eddies and mean flows, but satellite observations show that many western boundary continental shelves are affected by mesoscale eddies translating near the shelfbreak. In this SST image, a Gulf Stream Warm Core Ring (anticyclone) transports cold fresh shelf water offshore across the Mid-Atlantic Bight shelfbreak. Using idealized numerical simulations, I address three questions: 1. Does the eddy always get to the shelfbreak, or can sloping topography stop an eddy from crossing it? 2. What is the magnitude of offshore transport driven by these eddies? 3. What is the effect of the eddy on the shelf's flow field? 20160316T120000 20160316T130000 54-915 0

SLS- Deepak Cherian (MIT/WHOI) – Eddy vs. shelf-slope topography

ofoqokjn4nk54q8ac5rmgrqbn4@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20160323T120000 20160323T130000 0

no SLS – Spring break

vuc1onajs0fpct9nl84jltm95g@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Despite the major role played by mesoscale eddies in redistributing the energy of the large-scale circulation, our understanding of their dissipation is still incomplete. This study investigates the generation of internal waves by decaying eddies in the North Atlantic western boundary. The eddy presence and decay are measured from the altimetric surface relative vorticity associated with an array of full-depth current meters extending ~100 km offshore at 26.5N. In addition, internal waves are analysed over a topographic rise from 2-year high-frequency measurements of an Acoustic Doppler Current Profiler (ADCP), which is located 13 km offshore in 600 m deep water. Despite an apparent polarity independence of the eddy decay observed from altimetric data, the flow in the deepest 100 m is enhanced for anticyclones (25.2 cm/s) compared with cyclones (-4.7 cm/s). Accordingly, the internal wave field is sensitive to this polarity-dependent deep velocity. This is apparent from the eddy-modulated enhanced dissipation rate, which is obtained from a finescale parameterization and exceeds 10^-9 W/kg for near-bottom flows greater than 8 cm/s. The present study underlines the importance of oceanic western boundaries for removing the energy of low-mode westward-propagating eddies to higher-mode internal waves. 20160330T120000 20160330T130000 54-915 0

SLS-LOUIS CLEMENT (LDEO) – Generation of internal waves by eddies impinging on the western boundary of the North Atlantic

7f8gqhpvh8tmioqalvkb0i486k@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The ocean is populated by an intense geostrophic eddy field that is unresolved in most numerical ocean models used for climate prediction. A geometric framework for parameterising ocean eddy fluxes will be presented that is consistent with conservation of energy and momentum. The framework involves rewriting the residual-mean eddy force as the divergence of an eddy stress tensor. The magnitude of the eddy stress tensor is bounded by the eddy energy, allowing its components to be rewritten in terms of the eddy energy and non-dimensional parameters describing the mean "shape" of the eddies, analogous to “eddy ellipses” used in observational oceanography. These non-dimensional geometric parameters have strong connections with classical stability theory, for example, the new framework preserves the functional form of the linear Eady growth rate and, with one additional ingredient, Arnold’s first stability theorem. This framework also leads to a simple model of "eddy saturation”: the relative insensitivity of the ocean circulation and stratification to the magnitude of the surface wind stress in ocean models with explicit eddies. These results offer the prospect of improved eddy parameterisations that both preserve the underlying symmetries and conservation laws inherent in the unfiltered equations, and reproduce empirical results that have been obtained with eddy-permitting models. 20160406T120000 20160406T130000 54-915 0

SLS-David Marshall (Oxford) – A geometric interpretation of eddy-mean flow interaction in the ocean

n46c0b7qi2hn84luvfnn1m0ho0@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The ocean is a major sink of anthropogenic CO2 emissions. In order to predict future atmospheric CO2 levels and global climate, we must improve quantification of the ocean carbon pumps, which sequester CO2 from the atmosphere on timescales from years to millennia. In this talk, I will present field data from a quasi-Lagrangian cruise in Monterey Bay, CA and use it to demonstrate how in situ measurements of dissolved gases can be used to quantify the biological and solubility pumps. In particular, O2 concentration and isotopic composition are tracers of gross and net productivity. A persistent challenge in quantifying biological productivity from O2 measurements is the need to accurately parameterize the physical processes that also alter O2 concentration and isotopic composition (e.g., bubble-mediated gas exchange, diffusive gas exchange, and mixing). Measurements of multiple inert gases, such as the noble gases, are used to develop parameterizations for these physical processes. These parameterizations are then applied to bioactive gases such as O2 and CO2, thereby improving estimates of the ocean carbon pumps. Additionally, I will compare in situ gas tracer methods for quantifying productivity with simultaneous incubation- and sediment trap-based productivity estimates. 20160413T120000 20160413T130000 54-915 0

SLS – Cara Manning (MIT-WHOI) – What can oxygen and noble gases teach us about the ocean carbon pumps?

cvfe069npr1tlfqd734tmkb0o8@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The talk will address the implications of a warming Arctic Ocean to the structure and ventilation of the Arctic halocline, and to the overlying sea-ice cover. In recent years, atypically warm intrusions have been observed in the halocline. These derive from intense summertime solar warming on outcropping isopycnals in expansive ice-free regions. Heat that is stored in the shallow halocline can be released in the fall and winter by shear driven mixing, and convective mixing by the release of dense plumes during sea-ice growth. On the other hand, deeper warm ocean layers remain unaffected. I'll show that under continued warming, there exists the possibility for a regime shift in halocline ventilation by these warm waters, and a subsequent cap on the storage of deep-ocean heat. 20160420T120000 20160420T130000 54-915 0

SLS – Mary-Louise Timmermans (Yale) – Summer heat overwinters in the Arctic Ocean

3c7gkt0bk76i6jrn78o4tjl8qc@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Recent work has shown that turbulence in the ocean due to breaking internal gravity is one of the largest uncertainties in climate models. Knowledge of the horizontal and vertical distribution of the turbulence is crucial, which is challenging because internal waves can travel far from their sources and can break via a variety of mechanisms. In this talk I will first introduce internal waves for non-specialists, then walk through an example in the South China Sea where waves can be tracked from their source to their breaking locations, and a rough energy budget determined. Then I’ll discuss recent progress in tracking internal wave energy from generation to cross-basin propagation to dissipation on the globe, focusing on recent efforts to constrain 1) q, the fraction of locally dissipated energy and 2) the reflection coefficient which determines the partition of energy breaking over continental margins versus in the deep basins. A key thread of these analyses is the constant interplay between observations and high-resolution models. 20160427T120000 20160427T130000 54-915 0

SLS – Matthew Alford (Scripps Institute of Oceanography) – Observing the generation, propagation and dissipation of internal waves in the ocean

dbe78mrro0f3pr2cs2afptun68@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20160504T120000 20160504T130000 54-915 0

SLS – Stephanie Dutkiewicz – MIT (EAPS)

086ehfu2ko34c7f70a5qn0m71k@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20160511T120000 20160511T130000 54-915 0

SLS – Isabela Le Bras (MIT-WHOI)

lgt54fubogv1d6gjt18ngh2f4o@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Does size matter for life in the ocean? 20160513T120000 20160513T130000 MIT 54-915 0

SLS – Adrian Martin (NOCS) – Does size matter for life in the ocean?

3dsmv53iq9qrj9rm821u916qvg@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Recent observations show that the Southern Ocean is dominating anthropogenic ocean heat uptake. Southern Ocean heat uptake is large because the strong northward transport of the heat content anomaly limits warming of the sea surface temperature in the uptake region. Using results from eddy-rich global climate simulations, I will discuss the processes controlling the northward heat transport away from the uptake region and the convergence of the heat content anomaly in the midlatitude Southern Ocean. Heat budget analyses reveal that different processes dominate to the north and south of the main convergence region. The heat transport northward from the high-latitude uptake region is driven primarily by passive advection of the heat content anomaly by the existing time mean circulation, with a smaller contribution from enhanced upwelling. The heat anomaly builds up in the midlatitudes due to a convergent Ekman transport anomaly, combined with limited heat transport further northward into the mode waters. To the north of the peak convergence region, eddy processes drive the warming and account for nearly 80% of the northward heat transport anomaly. 20160525T120000 20160525T125000 54-915 0

SLS – Adele Morrison (Princeton) – Mechanisms of Southern Ocean heat uptake and transport

aaig83l8ph0piup82cpfhgudfk@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch We present a new, steady-state macromolecule-based model to study light-nutrient co-limitation of phytoplankton growth. The model is based on simplified metabolic flux network and resolves key pools of macro-molecules, each of which has different roles for cellular growth. The model is used to predict and interpret the variation of cellular stoichiometry of fresh water Synechococcus sp. under different light and nutrient environment over a range of dilution rates (averaged growth rates) in a steady state culture. The model explains the different response of cellular nitrogen and phosphorus quota to the various light-nutrient environments, predicting protein and RNA as most influential molecules on nitrogen and phosphorus quotas respectively. The model indicates that, though total nitrogen storage is larger than phosphorus storage, relative to requirements, many times more phosphorus can be stored. It accurately predicts the maximum possible growth rate based on the limits of resource allocation within the cell. Finally, the model predicts nutrient-light co-limitation of cell population density under different dilution rates. While the nutrient has a direct effect on the population density, light impacts it by modifying the cellular stoichiometry. This steady-state, macromolecule based model provides bases for predicting phytoplankton growth in different dynamic environments 20160526T140000 20160526T150000 0

SLS – Keisuke Inomura (PAOC-MIT) – A macromolecular model of phytoplankton growth under light and nutrient co-limitation

ll80brjfgrjt197v2jusootht4@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20160527T120000 20160527T130000 54-915 0

Special SLS – Joe LaCasce (U of Oslo) – The buoyancy-driven ocean circulation in idealized and realistic basins

37pufqcsb1t2cohqvog41humpc@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Internal hydraulic jumps in flows with upstream shear are investigated, motivated by applications such as the flow over sills in Knight Inlet and Hood Canal. The role of upstream shear has not previously been thoroughly investigated, although it is important in many natural flows, including exchange flows and flows over topography. Several two-layer theories are extended to include upstream shear, showing that solutions only exist for a limited range of upstream shear values. More realistic two-dimensional numerical simulations are guided by the two-layer theory predictions, and the results are used to evaluate the theories. The simulations also show the qualitative types of hydraulic transitions that occur, including undular bores, fully turbulent jumps, and conjugate state-like solutions. Numerical simulations are also used to investigate the mixing, and a few 3D numerical simulations are found to be consistent with the 2D results. When the upstream shear is increased and the basic two-layer theories no longer exhibit solutions, entrainment is required. Furthermore, the downstream structure of the flow has an important effect on the jump properties. These factors are investigated by modifying a two-layer theory to allow entrainment and account for the downstream vertical velocity structure. The resulting theory indicates that entrainment and jump structure become important factors that influence the jump height. However, the results are very sensitive to how the downstream vertical profiles of velocity and density are incorporated into the layered model, highlighting the limitations of the two-layer approximation when the shear is large. While these two layer theories provide insight into the types of jumps that can occur and the mixing that they cause, jumps such as those that occur in Knight Inlet are significantly influenced by factors such as topography, tidal forcing, and three-dimensional effects. 20160601T121000 20160601T131000 54-915 0

Kelly Anne Ogden (MIT-WHOI) – Internal Hydraulic Jumps with Upstream Shear

6107q6g88bcb157akgbn3j2qb8@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The Antarctic shelf seas are at present receiving increasing amounts of freshwater from the melting of the Antarctic Ice Sheet and its fringing ice shelves. In response, the surface ocean salinity in this region has declined. The talk will investigate the effects of the freshwater input on regional sea level, using satellite measurements of sea surface height (for months with no sea-ice cover) and a global ocean circulation model. It is found that from 1992 to 2011, sea-level rise along the Antarctic coast is at least 2 ± 0.8 mm yr−1 greater than the regional mean for the Southern Ocean. Further, on the basis of the model simulations, we conclude that this sea-level rise is almost entirely related to steric adjustment, rather than changes in local ocean mass, with a halosteric rise in the upper ocean and thermosteric contributions at depth. 20160719T121000 20160719T131000 54-915 0

Special SLS – Craig Rye CUSP (UK) – Trends in Antarctic Subpolar Sea Sea Level: Evidence of Increasing Glacial Melt?

8fq7r97phuk895oaspp7ioq50s@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch El Ninõ–Southern Oscillation is here considered as large (time) scale phenomenon emerging from a complex and fast general dynamical system. This seminar has two main goals. The first one is to give a physically reasonable explanation for the use of stochastic models for mimicking the apparent random features of the El Ninõ–Southern Oscillation (ENSO) phenomenon. The second one is to show how it is possible to obtain, with the present approach, some analytical results concerning the stationary density function of the anomaly sea surface temperature, and the occurring timing of strong El Ninõ events. These results fit well the data from observations, reproducing the asymmetry and the power law tail of the histograms of the NIÑO3 index and the timing of 2-7 years for intermediate El Ninõ events. The approach is based on some of our recent theoretical results in the field of the dynamical origin of stochastic processes. More precisely, we apply this approach to the celebrated recharge oscillator model (ROM), weakly interacting by a multiplicative term with a general deterministic system (Madden-Julian Oscillations, westerly wind burst, etc.), and we obtain a Fokker-Planck Equation that describes the statistical behavior of the ROM. For more details — M. Bianucci, Geophysical Res. Lett., 43(1), 386-394 http://dx.doi.org/10.1002/2015GL066772 — M. Bianucci, Journal of Statistical Mechanics: Theory and Experiment 2015, P05016 (2015), http://stacks.iop.org/1742-5468/2015/i=5/a=P05016; — M. Bianucci, Int. Journal of Mod. Phys. B 0, 1541004 (2015), http://www.worldscientific.com/doi/abs/10.1142/S0217979215410040 20161005T120000 20161005T130000 54-915 0

SLS – Marco Bianucci (ISMAR – CNR) – Modeling El Nino–Southern Oscillation: analytical results

er737t0tcc2jp3q2kbke0fbopc@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Ocean Property Fluxes: a Pseudo-Lagrangian Approach Lagrangian methods can provide insight into complex stirring and transport processes. These methods can provide a skeleton or template that allows one to identify regions of rapid stirring, transport, and transport barriers. 'Lobe dynamics', one of the most beautiful techniques in the toolbox, provides for the measurement and visualization of transport and exchange across moving boundaries. Other methods allow for the identification of natural barriers such as the material boundary of a coherent eddy. The focus is entirely on fluid 'material' transport (volume transport) and fluxes of oceanographically important properties such as heat, salt, vorticity and chemical and biological tracers are relevant only to the extent that they are conserved following fluid motion. In addition, lobe dynamics can become cumbersome when the flow field is complex. I will talk about a generalize approach that enables consideration of a variety of property fluxes, provides a simplified application to complex flow fields, and attempts to preserves the beauty of the original approach. 20161019T120000 20161019T130000 54-209 0

SLS – Larry Pratt (WHOI)

0l8inq5v99j6i45p17dim1skj0@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Redox chemistry and ecological dynamics as underlying mechanisms for chemoautotrophic control of the primary nitrite maximum The primary nitrite maximum (PNM) – the accumulation of nitrite at the base of the euphotic zone – is a ubiquitous but poorly understood oceanographic feature. I will discuss how ecological dynamics and redox chemistry can explain the formation of the PNM by chemoautotrophic nitrifying microorganisms. Theoretical descriptions of nitrifying metabolisms, reflecting their underlying redox chemistry, results in the emergence of a PNM in a marine ecosystem model. I also discuss the implications for understanding rates of new production using this more mechanistic representation of nitrification. 20161026T120000 20161026T130000 54-915 0

SLS – Emily Zakem (MIT)

loqqebtvf9gka4i5h01a0ujl90@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Scaling properties of Arctic sea ice deformation in high-resolution viscous-plastic sea ice models Many climate models use a rheology of the viscous-plastic type to simulate sea ice dynamics. With this rheology, large scale velocity and thickness fields can be realistically simulated, but the representation of small scale deformation rates and Linear Kinematic Features (LKF) is thought to be inadequate. However, at high resolution (< 5 km) the rheology starts to produce lines of localised deformation rates. In this study we use results from a 1-km Pan-Arctic model to investigate the influence of these deformation features on the scaling properties of sea ice deformation. For evaluation the EGPS satellite data set of small-scale sea ice kinematics for the Central Arctic (successor of RGPS) is used. The modelled sea ice deformation shows multi-fractal spatial scaling and, in this sense, agrees with the satellite data. In addition, the temporal coupling of the spatial scaling is reproduced as well. Furthermore, we examine the regional and seasonal variations of spatial scaling properties and its dependence on the ice condition, i.e. sea ice concentration and thickness, which are in agreement with previous RGPS studies. 20161031T150000 20161031T160000 54-209 0

SLS – Nils Hutter (Alfred Wegener Institute)

2mkb1gkqss0ulbueppk98dal4k@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20161109T120000 20161109T130000 54-915 0

Glenn Flierl (MIT)

9qn6emngpg2i831ratfv3u8p14@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Deciphering deep ocean circulation changes between the present and the last glacial The paleoclimate record indicates that the deep ocean circulation and water masses have undergone major rearrangements between glacial and interglacial climates, which have likely played an important role in the observed atmospheric carbon dioxide swings by affecting the partitioning of carbon between the atmosphere and ocean. The mechanisms by which the deep ocean circulation changed, however, are still unclear and represent a major challenge to our understanding of past and future climates. We address this question using a hierarchy of numerical models of varying complexity, ranging from a highly idealized ocean-only model to coupled climate simulations from the Paleoclimate Modelling Intercomparison Project (PMIP). The results suggest that various inferred differences in the deep ocean circulation and stratification between glacial and interglacial climates can be attributed to increased Antarctic sea-ice formation in a colder world. Colder temperatures lead to thicker ice, which is exported by winds. The associated increased freshwater export leads to saltier and denser Antarctic Bottom Water, consistent with high abyssal salinities inferred for the Last Glacial Maximum (LGM). The enhanced deep ocean stratification moreover results in a weakening and shoaling of the inter-hemispheric overturning circulation, again consistent with proxy evidence for the LGM. The results also highlight the importance to distinguish between the equilibrium and transient response of the ocean circulation to climatic changes. The adjustment of the deep ocean circulation is found to be highly non-monotonic, with the response on centennial time-scales differing qualitatively from the equilibrium results. This distinction is rarely observable in complex coupled climate models, which cannot be integrated for sufficiently long times. 20161116T120000 20161116T130000 54-915 0

SLS – Malte Jansen (University of Chicago)

a0cjs9f8f6q551t8rifnp5lb3c@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch The vertical structure of ocean eddies Since we began observing the ocean surface with satellites, it's been of interest to understand how the surface fields reflect motion at depth. A series of recent modeling studies suggest the vertical structure is fairly well-captured by a single mode, intensified near the surface and decaying to zero with depth. A study of 69 globally-distributed current meters supports this, in many locations outside of the tropics. The reason for the dominance of a surface is explored theoretically, using a simple two layer model. The latter predicts a wavenumber frequency spectra which resembles that in the ocean, except at small scales. The latter are shown to be more likely to transfer energy to large scales, leaving the (non-dispersive) large scale waves in tact. A similar conclusion was made previously from idealized numerical experiments. 20161122T120000 20161122T130000 54-915 0

SLS – Joe Lacasce (University of Oslo)

pk0iqfejbpehn1plfruqhpea5o@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Stability and Internal Flow Variability of Ice Sheets Ice streams are regions of fast-flowing ice embedded within ice sheets that account for the majority of mass transport from ice sheet interiors to the ocean. Variability of ice stream flow on centennial to millennial time scales plays an important role in the present mass balance of the West Antarctic Ice Sheet. In this talk, I show how a simple model of subglacial meltwater production coupled to ice flow explains the underlying physical mechanism for millennial-scale, unforced ice stream variability and predicts the transition to steady ice stream flow. The model equally well reproduces modern ice stream variability in the Siple Coast region of West Antarctica and Heinrich events, periods of increased ice discharge from the Laurentide Ice Sheet during the last glacial period. In a more realistic, purpose-built model, the same mechanism produces variability and rapid migrations of the ice stream grounding line. These migrations are always associated with mass imbalance near the grounding line, but not necessarily in the ice stream at large, which is important to consider when interpreting modern observations of grounding line variability. Under certain conditions, this ice stream variability may cause the grounding line to slow down for hundreds to thousands of years even as it retreats onto a reverse bed slope, before readvancing. Such behavior runs counter to the conventional theories predicting the instability of ice sheets on reverse bed slopes. Determining if such behavior occurs in real ice sheets is important when evaluating the likelihood of irreversible ice sheet collapse and rapid sea level rise in the future. 20161130T120000 20161130T130000 54-915 0

SLS – Alexander Robel (California Institute of Technology & University of Chicago)

5g6bl6kgr3mktf5110aik0lplk@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch 20161207T120000 20161207T130000 54-915 0

SLS – Oliver Andrews (Tyndall Centre for Climate Change Research, University of East Anglia)

4976enni5i3j1pu5nflav90cs0@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Chasing Water: Lagrangian tracking of tracers, plastic and plankton through the global ocean The ocean is in constant motion, with water circulating within and flowing between basins. As the water moves around, it caries heat and nutrients, as well as larger objects like planktonic organisms and litter around the globe. The most natural way to study the pathways of water and the connections between ocean basins is using particle trajectories. The trajectories can come from either computing of virtual floats in high-resolution ocean models, or from the paths of free-flowing observational drifters (surface buoys or Argo floats) in the real ocean. In this seminar, I'll give an overview of some recent work with Lagrangian particles. I will show applications to dynamical oceanography, marine ecology, palaeoclimatology and marine plastic pollution. Central to each of these studies is the question on how connected the different ocean basins are, and on what time scales water flows between the different regions of the ocean. 20170207T000000 20170207T010000 54-915 0

SLS — Erik van Sebille (Imperial College London)

pjlef2grif27kg8g886drvn09k@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Warm-route versus cold-route interbasin exchange in the meridional overturning circulation - Why is the Atlantic saltier than the Pacific? The interbasin exchange of the meridional overturning circulation (MOC) is studied in an idealized domain with two basins connected by a circumpolar channel in the southernmost region. Gnanadesikan’s (1999) conceptual model for the upper branch of the MOC is extended to include two basins of different widths connected by a re-entrant channel at the southern edge and separated by two continents of different meridional extents. Its analysis illustrates the basic processes of interbasin flow exchange either through the connection at the southern latitude of the long continent (cold route) or through the connection at the southern latitude of the short continent (warm route). A cold-route exchange occurs when the short continent is poleward of the latitude separating the sub-polar and sub-tropical gyre in the southern hemisphere, otherwise there is warm-route exchange. The predictions of the conceptual model are compared to primitive equation computations in a domain with the same idealized geometry forced by wind-stress, surface temperature relaxation and surface salinity flux. A visualization of the horizontal structure of the upper branch of the MOC illustrates the cold and warm routes of interbasin exchange flows. Diagnostics of the primitive equation computations show that the warm-route exchange flow is responsible for a substantial salinification of the basin where sinking occurs. This salinification is larger when the interbasin exchange is via the warm route, and it is more pronounced when the warm-route exchange flows from the wide to the narrow basin. 20170301T120000 20170301T130000 54-915 0

SLS — Paola Cessi (Scripps)

lmlhi8pt0d9au0c0m4m7tg5ke0@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Barotropic turbulence above topography: form stress and eddy saturation Wind is an important driver of large-scale ocean currents, imparting momentum into the ocean at the sea surface. This force is almost entirely balanced by topographic form stress (that is the correlation of bottom pressure and topographic slope). The direct effect of bottom or skin friction in turbulent boundary layers is almost negligible for the momentum balance. We use a one-layer barotropic model to study the effect of a random monoscale bottom topography on beta-plane geostrophic turbulence. The model forcing is a uniform steady wind stress that produces both a uniform large-scale flow and smaller-scale macroturbulence. The macroturbulence is characterized by both standing and transient eddies and the large-scale flow is retarded by a combination of bottom drag and domain-averaged topographic form stress produced by the standing eddies. A main control parameter is the ratio of beta to the root mean square gradient of the topographic potential vorticity (PV). We derive asymptotic scaling laws for the strength of the large-scale flow in the limiting cases of weak and strong forcing. If beta is comparable to, or larger than, the topographic PV gradient there is an “eddy saturation” regime in which the large-scale flow is insensitive to large changes in the wind stress. We show that eddy saturation requires strong transient eddies that act effectively as PV diffusion. This diffusion does not decrease the strength of the standing eddies but it does increase the topographic form stress by enhancing the correlation between topographic slope and the standing eddy pressure field. 20170308T120000 20170308T130000 54-915 0

SLS — William Young (Scripps)

282jnfvshnuoiapnbkcm8bf17g@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Southern Ocean eddies: generation, propagation and decay The Southern Ocean has high concentrations of eddy kinetic energy, but analysis of altimeter data and an ocean state estimate show that the generation of relatively large amplitude eddies is not a ubiquitous feature of the Southern Ocean but rather a phenomenon that is constrained to isolated, well-defined regions. Five “hotspots” of high probability of eddy generation are identified using altimeter data. These hotspots are located downstream of major topographic features, with their boundaries closely following f/H contours. Eddies generated in these locations do not propagate far but decay within the boundaries of the generation area. The anisotropy of the dispersion of eddy tracks in geographical coordinates versus a rotated coordinates system aligned with f/H contours, shows that eddies tend to follow f/H contours rather than f. Maps of buoyancy and shear production terms computed from a state estimation model show enhanced values of both conversion terms inside the hotspots, with buoyancy production two orders of magnitude larger than shear production. The mean potential density field estimated from Argo floats shows that inside the hotspots isopycnal slopes are steep, indicating availability of potential energy and providing further evidence of the main generation mechanism. The hotspots identified in this paper overlap with previously identified regions of standing meanders. We hypothesize that hotspot locations can be explained by the combined effect of topographic features, standing meanders which enhanced baroclinic instability and availability of potential energy to generate eddies via baroclinic instabilities. 20170315T120000 20170315T130000 0

SLS — Uriel Zajaczkovski (Scripps)

gva7pvgq40gn82og8h8hrmoqug@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch High-resolution observations of internal wave induced turbulence in the deep ocean An overview is presented of high-resolution temperature observations above underwater topography in the deep, generally stably stratified ocean. The Eulerian mooring technique is used to monitor temperature variations by typically 100 sensors distributed over lines between 40 and 400 m long. The independent sensors sample at a rate of 1 Hz for up to one year with a precision better than 0.1 mK. This precision and sampling rate are sufficient to resolve the large, energy containing turbulent eddies and all of the internal waves and their breaking above underwater topography. Such underwater wave breaking is the key mechanism for the redistribution of nutrients and heat (to maintain the ocean stably stratified), and the resuspension of sediment. Under conditions of tight temperature-density relationship, the temperature data are used to quantify turbulent overturns. These observations show two distinctive turbulence processes that are associated with different phases of a large-scale, mainly tidal, internal gravity wave: i) highly nonlinear turbulent bores during the upslope propagating phase, and ii) Kelvin-Helmholtz billows, at some distance above the slope, during the downslope phase. While the former may be associated in part with convective turbulent overturning following Rayleigh-Taylor instabilities, the latter are mainly related to shear-induced instabilities. Under weaker stratified conditions, away from boundaries, free convective mixing appears more often, but a clear inertial subrange in temperature spectra is indicative of dominant shear-induced turbulence. With stratification, turbulence is seen to increase in dissipation rate and diffusivity all the way to the bottom, which challenges the idea of a homogeneous bottom boundary layer. With a newly developed five-lines mooring, the transition from isotropy (full turbulence) to anisotropy (stratified turbulence/internal waves) is revealed. 20170405T120000 20170405T130000 54-915 0

SLS — Hans van Haren (NIOZ)

asmkr42urnv29bn7kehvf58q8s@google.com 20180503T075357Z MIT Seminar | PAOC Oceanography and Climate Sack Lunch Modeling iceberg drift and decay in modern and glacial climates Under global warming, the calving of icebergs into the polar oceans is expected to increase. As a result, the role that icebergs play in Earth's climate system has received a recent surge of interest, and efforts are underway to explicitly represent icebergs in GCMs. A better understanding of how icebergs drift and decay will help facilitate an accurate representation of icebergs and guide the interpretation of GCM results. In this talk I will present an idealized analytical model that we developed to aid this effort. I will use the model to address (i) which climate model variables are most important to accurately model iceberg evolution and (ii) whether climate models do a good job simulating these variables. I then will turn to episodes of massive iceberg discharge, called Heinrich Events, which occurred during the last glacial period. These events are believed to have had large-scale impacts on the global climate system. However, modeling icebergs that lived and melted more than 10,000 years ago comes with its own challenges, as we will see. 20170419T120000 20170419T130000 54-915 0

SLS — Till Wagner (Scripps)