The Southern Ocean meridional overturning circulation as diagnosed from an eddy permitting state estimate

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2008A modern general circulation model of the Southern Ocean with one-sixth of a degree resolution is optimized to the observed ocean in a weighted least squares sense. Convergence to the state estimate solution is carried out by systematically adjusting the control variables (atmospheric state and initial conditions) using the adjoint model. A cost function compares the model state to in situ observations (Argo float profiles, CTD synoptic sections, SEaOS instrument mounted seal profiles, and XBTs), altimetric observations (ENVISAT, GEOSAT, Jason, TOPEX/Poseidon), and other data sets (e.g. infrared and microwave radiometer observed sea surface temperature and NSIDC sea-ice concentration). Costs attributed to control variable perturbations ensure a physically realistic solution. The state estimate is found to be largely consistent with the individual observations, as well as with integrated fluxes inferred from previous static inverse models. The transformed Eulerian mean formulation is an elegant way to theorize about the Southern Ocean. Current researchers utilizing this framework, however, have been making assumptions that render their theories largely irrelevant to the actual ocean. It is shown that theories of the overturning circulation must include the effect of pressure forcing. This is true in the most buoyant waters, where pressure forcing overcomes eddy and wind forcing to balance a poleward geostrophic transport and allows the buoyancy budget to be closed. Pressure forcing is also lowest order at depth. Indeed, the Southern Ocean’s characteristic multiple cell overturning is primarily in geostrophic balance. Several other aspects of the Southern Ocean circulation are also investigated in the thesis, including an analysis of the magnitude and variability of heat, salt, and volume inter-basin transports.This work was supported by CalTech - Jet Propulsion Lab contract #1205624 (Global Oceans Dynamics and Transports). Support for my first 2 years in the MITWHOI Joint Program came from NSF awards #OCE-9901654 (Research in Linear and Nonlinear Waves and Ocean Circulation Theory). I was also supported for two months by NSF awards #OCE-0223434

Production and analysis of a Southern Ocean state estimate

Submitted in partial fulfillment of the requirements for the degree of Master of Science in Physical Oceanography, Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2006A modern general circulation model of the Southern Ocean with one-sixth of a degree resolution is optimized to the observed ocean in a weighted least squares sense. Convergence toward the state estimate solution is carried out by systematically adjusting the control variables (prescribed atmospheric state, initial conditions, and open northern boundary at 24.7°S) using the adjoint method. A cost function compares the model state to data from CTD synoptic sections, hydrographic climatology, satellite altimetry, and XBTs. Costs attributed to control variable perturbations ensure a physically realistic solution. An optimized solution is determined by the weights placed on the cost function terms. The state estimation procedure, along with the weights used, is described. A significant result is that the adjoint method is shown to work at eddy-permitting resolution in the highly-energetic Southern Ocean. At the time of the writing of this thesis the state estimate was not fully consistent with the observations. An analysis of the remaining misfit, as well as the mass transport in the preliminary state, is presented

An advective mechanism for Deep Chlorophyll Maxima formation in southern Drake Passage

We observe surface and subsurface fluorescence-derived chlorophyll maxima in southern Drake Passage during austral summer. Backscatter measurements indicate that the deep chlorophyll maxima (DCMs) are also deep biomass maxima, and euphotic depth estimates show that they lie below the euphotic layer. Subsurface, offshore and near-surface, onshore features lie along the same isopycnal, suggesting advective generation of DCMs. Temperature measurements indicate a warming of surface waters throughout austral summer, capping the winter water (WW) layer and increasing off-shelf stratification in this isopycnal layer. The outcrop position of the WW isopycnal layer shifts onshore, into a surface phytoplankton bloom. A lateral potential vorticity (PV) gradient develops, such that a down-gradient PV flux is consistent with offshore, along-isopycnal tracer transport. Model results are consistent with this mechanism. Subduction of chlorophyll and biomass along isopycnals represents a biological term not observed by surface satellite measurements which may contribute significantly to the strength of the biological pump in this region

Freshwater displacement effect on the Weddell Gyre carbon budget

This work was funded by NSF's Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) Project under NSF awards PLR-1425989 and OPP-1936222. G.A.M was additionally supported under UKRI Grant MR/W013835/1. M.R.M. also acknowledges support from NASA grant 80NSSC20K1076 and NSF grants OCE-1924388 and OPP-2149501.The Weddell Gyre mediates carbon exchange between the abyssal ocean and atmosphere, which is critical to global climate. This region also features large and highly variable freshwater fluxes due to seasonal sea ice, net precipitation, and glacial melt; however, the impact of these freshwater fluxes on the regional carbon cycle has not been fully appreciated. Using a novel budget analysis of dissolved inorganic carbon (DIC) mass in the Biogeochemical Southern Ocean State Estimate, we highlight two freshwater-driven transports. Where freshwater with minimal DIC enters the ocean, it displaces DIC-rich seawater outwards, driving a lateral transport of 75 ± 5 Tg DIC/year. Additionally, sea ice export requires a compensating import of seawater, which carries 48 ± 11 Tg DIC/year into the gyre. Though often overlooked, these freshwater displacement effects are of leading order in the Weddell Gyre carbon budget in the state estimate and in regrouped box-inversion estimates, with implications for evaluating basin-scale carbon transport.Publisher PDFPeer reviewe

The effects of enhanced sea ice export from the Ross Sea on recent cooling and freshening of the Southeast Pacific

The top 2000 m of the Southern Ocean has freshened and warmed over recent decades. However, the high-latitude (south of 50°S) southeast Pacific was observed to be cooler and fresher in the years 2008-2010 compared to 2005-2007 over a wide depth range including surface, mode, and intermediate waters. The causes and impacts of this event are analyzed using the ocean—sea-ice data-assimilating Southern Ocean State Estimate (SOSE) and observationally based products. In 2008-2010, a strong positive Southern Annular Mode coincided with a negative El Niño Southern Oscillation and a deep Amundsen Sea Low. Enhanced meridional winds drove strong sea ice export from the eastern Ross Sea, bringing large amounts of ice to the Amundsen Sea ice edge. In 2008, together with increased precipitation, this introduced a strong freshwater anomaly that was advected eastward by the Antarctic Circumpolar Current (ACC), mixing along the way. This anomaly entered the ocean interior not only as Antarctic Intermediate Water, but also as lighter Southeast Pacific Subantarctic Mode Water (SEPSAMW). A numerical particle release experiment carried out in SOSE , showed that the Ross Sea sector was the dominant source of particles reaching the SEPSAMW formation region. This suggests that large-scale climate fluctuations can induce strong interannual variability of volume and properties of SEPSAMW. These fluctuations act at different time scales: instantaneously via direct forcing, and also lagged over advective time scales of several years from upstream regions

The role of air-sea interactions in atmospheric rivers: Case studies using the SKRIPS regional coupled model

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Sun, R., Subramanian, A. C., Cornuelle, B. D., Mazloff, M. R., Miller, A. J., Ralph, F. M., Seo, H., & Hoteit, I. The role of air-sea interactions in atmospheric rivers: Case studies using the SKRIPS regional coupled model. Journal of Geophysical Research: Atmospheres, 126(6), (2021): e2020JD032885, https://doi.org/10.1029/2020JD032885.Atmospheric rivers (ARs) play a key role in California's water supply and are responsible for most of the extreme precipitation and major flooding along the west coast of North America. Given the high societal impact, it is critical to improve our understanding and prediction of ARs. This study uses a regional coupled ocean–atmosphere modeling system to make hindcasts of ARs up to 14 days. Two groups of coupled runs are highlighted in the comparison: (1) ARs occurring during times with strong sea surface temperature (SST) cooling and (2) ARs occurring during times with weak SST cooling. During the events with strong SST cooling, the coupled model simulates strong upward air–sea heat fluxes associated with ARs; on the other hand, when the SST cooling is weak, the coupled model simulates downward air–sea heat fluxes in the AR region. Validation data shows that the coupled model skillfully reproduces the evolving SST, as well as the surface turbulent heat transfers between the ocean and atmosphere. The roles of air–sea interactions in AR events are investigated by comparing coupled model hindcasts to hindcasts made using persistent SST. To evaluate the influence of the ocean on ARs we analyze two representative variables of AR intensity, the vertically integrated water vapor (IWV) and integrated vapor transport (IVT). During strong SST cooling AR events the simulated IWV is improved by about 12% in the coupled run at lead times greater than one week. For IVT, which is about twice more variable, the improvement in the coupled run is about 5%.The authors gratefully acknowledge the research funding (grant number: OSR-2-16-RPP-3268.02) from KAUST (King Abdullah University of Science and Technology). The authors also appreciate the computational resources on supercomputer Shaheen II and the assistance provided by KAUST Supercomputer Laboratory. Additional funding from the NSF (OCE2022846, and OCE2022868) and the National Oceanic and Atmospheric Administration (MAPP NA17OAR4310106 and NA17OAR4310255) is also greatly appreciated. This study is also supported by the U.S. Army Corps of Engineers (USACE)-Cooperative Ecosystem Studies Unit (CESU) as part of Forecast Informed Reservoir Operations (FIRO) under grant W912HZ-15-2-0019. The authors thank Caroline Papadopoulos for important technical support when installing software and using the Shaheen II cluster

Bottom pressure torque and the vorticity balance from observations in Drake Passage

The vorticity balance of the Antarctic Circumpolar Current in Drake Passage is examined using 4 years of observations from current‐ and pressure‐recording inverted echo sounders. The time‐varying vorticity, planetary and relative vorticity advection, and bottom pressure torque are calculated in a two‐dimensional array in the eddy‐rich Polar Frontal Zone (PFZ). Bottom pressure torque is also estimated at sites across Drake Passage. Mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local (50‐km scale) vorticity budget in the PFZ, and are balanced to first order by the divergence of horizontal velocity. Most of this divergence comes from the ageostrophic gradient flow, which also provides a second‐order adjustment to the geostrophic relative vorticity advection. Bottom pressure torque is approximately one‐third the size of the local depth‐integrated divergence. Although the cDrake velocity fields exhibit significant turning with depth throughout Drake Passage even in the mean, surface vorticity advection provides a reasonable representation of the depth‐integrated vorticity balance. Observed near‐bottom currents are strongly topographically steered, and bottom pressure torques grow large where strong near‐bottom flows cross steep topography at small angles. Upslope flow over the northern continental slope dominates the bottom pressure torque in cDrake, and the mean across this Drake Passage transect, 3 to urn:x-wiley:21699275:media:jgrc21771:jgrc21771-math-0001 m s−2, exceeds the mean wind stress curl by a factor of 15–20

How does Subantarctic Mode Water ventilate the Southern Hemisphere subtropics?

In several regions north of the Antarctic Circumpolar Current (ACC), deep wintertime convection refreshes pools of weakly stratified subsurface water collectively referred to as Subantarctic Mode Water (SAMW). SAMW ventilates the subtropical thermocline on decadal timescales, providing nutrients for low-latitude productivity and potentially trapping anthropogenic carbon in the deep ocean interior for centuries. In this work, we investigate the spatial structure and timescales of mode water export and associated thermocline ventilation. We use passive tracers in an eddy-permitting, observationally-informed Southern Ocean model to identify the pathways followed by mode waters between their formation regions and the areas where they first enter the subtropics. We find that the pathways followed by the mode water tracers are largely set by the mean geostrophic circulation. Export from the Indian and Central Pacific mode water pools is primarily driven by large-scale gyre circulation, whereas export from the Australian and Atlantic pools is heavily influenced by the ACC. Export from the Eastern Pacific mode water pool is driven by a combination of deep boundary currents and subtropical gyre circulation. More than 50% of each mode water tracer reaches the subtropical thermocline within 50 years, with significant variability between pools. The Eastern Pacific pathway is especially efficient, with roughly 80% entering the subtropical thermocline within 50 years. The time required for 50% of the mode water tracers to leave the Southern Ocean domain varies significantly between mode water pools, from 9 years for the Indian mode water pool to roughly 40 years for the Central Pacific mode water poo