Overview. The portion of the global overturning circulation that lies in the Southern Ocean is
important not only because of its strong water mass transformations, but also because of its essential role
in redistributing heat, freshwater, carbon and nutrients (e.g. Sarmiento et al., 2004; Sabine et al., 2004;
Marshall and Speer, 2012). We propose to examine crucial hypotheses about the upper cell of the
Southern Ocean’s overturning circulation using output from the data assimilating, eddy permitting
Southern Ocean State Estimate (SOSE) (Mazloff et al., 2010), with new online diagnostics developed
through this funding.
Intellectual Merit. The zonally averaged Southern Ocean overturning circulation is commonly
hypothesized to have an upper cell and a lower cell, fed by inflowing Indian, Pacific and Atlantic Deep
Waters, that upwell to the ocean surface where surface buoyancy fluxes convert them to lighter and
denser waters, respectively; the degree of separation (or not) of these cells will be examined as part of this
work. Observations indicate that the upper cell is most likely fed by nutrient-rich deep waters that
originate in the Indian and Pacific rather than from the Atlantic (Speer et al., 2000; Talley, 2013), a
hypothesis that will be examined in detail.
A second hypothesis is that the two-dimensional, zonally averaged meridional overturning
circulation hides significant zonal asymmetries that are essential to the circulation. A detailed calculation
of the 3D residual circulation in a high-resolution ocean model has not been performed heretofore. Using
a residual circulation framework with proposed new online diagnostics of isopycnal and diapycnal
volume transport in neutral density coordinates (e.g. Abernathey et al., 2011), we will quantitatively
examine regional contributions to the residual circulation in SOSE, including the role of the ACC,
topographic features, and subtropical and Antarctic gyre systems. We will also quantify the relative
contributions of eddy-driven and steady flow in the 3D residual circulation pathways.
Water mass transformation and formation processes in the upper cell are also 3-D; an hypothesis
is that air-sea fluxes dominate with nearly equal importance of freshwater and heat (e.g. Cerovecki et al.,
2013), but that diapycnal mixing, particularly in isopycnal outcrop regions, can also be important, and
that there is enormous heterogeneity in where these processes occur. Likewise short time scale events in
the upper ocean can be important. With the proposed new online SOSE diagnostics, we will quantify the
relative, localized contributions of heat and salinity forcing to transformation at every model time step.
Regionally, with SOSE and these new diagnostics, we will examine the balance of processes that
lead to coherent net heating regions, find the most important upwelling/air-sea exchange sites, and
quantify the role of sea ice processes in the essential freshwater inputs to the upper cell.
Broader Impacts. The proposed work will inform understanding of Southern Ocean response to climate change, including changes in surface temperature, upper ocean heat content and sea ice cover.
The results will be published in scientific journals and presented at major meetings. Our large amount of
public outreach (schools, teacher groups, libraries, university clubs) will be well served by the work,
The proposed work will benefit MITgcm users as well as SOSE users, through development of
online diagnosis of the 3D residual (i.e. isopycnally-averaged) circulation, including the contributions to
the diapycnal velocity from all diabatic processes; the code will be contributed to the community.
Implementation of online neutral density calculations will also be made available to all MITgcm users.
State estimates are increasingly a tool of choice for synthesizing data. The proposed work
supports the rapidly growing userbase for SOSE, including many students. SOSE will be a broadly used
tool for understanding Southern Ocean dynamics, thermodynamics, and biogeochemistry for years to
come, as the numbers of in situ observations under sea ice and of biogeochemical parameters soar.
Crucial verification of its water mass structure and air-sea fluxes will be undertaken. A graduate student
will be mentored.