Collaborative Research: The upper branch of the Southern Ocean overturning in the Southern Ocean State Estimate: water mass transformation and the 3-D residual circulation

Principal Investigator: 
Collaborators: 
Ryan Abernathey, Assistant Professor at Lamont-Doherty Earth Observatory, Columbia University
Proposal Abstract: 

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,

including visualizations.

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.

Start and End Date: 
February 2014 to January 2017
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