Characterizing Variability in the Southern Ocean
- (University of California, San Diego)
Altimeter data show that the Southern Ocean is a region of elevated eddy kinetic energy. However, because of the low stratification of the Southern Ocean, eddy length scales in the region can be as short as O(10-20 km), and thus eddy variability may be undersampled by satellite altimeter data, which is often been smoothed to 100 km scales. This has broad implications for our understanding of a range of oceanic processes that are coupled to dynamic topography at the ocean surface, including for example air-sea fluxes of heat and momentum and the interactions between jets and transient eddies. In the future, the Surface Water and Ocean Topography (SWOT) altimeter offers the promise of measurements with higher spatial resolution. However, at high spatial resolution the altimeter response to the strong winds and high significant wave heights that occur in the Southern Ocean has not been fully explored. The new AltiKa altimeter planned for launch in 2012 will provide a first opportunity to examine along-track high-resolution data. In addition targeted processing should allow some higher wavenumber information to be extracted from the TOPEX/Poseidon, Jason-1, and Jason-2 satellite series. The research here will make use of these altimeter data to evaluate the role of small-scale eddy variability in the Southern Ocean.
The work will begin by evaluating the scale dependence of eddy variability (initially focusing on Jason-2 and AltiKa data), with an aim to assessing the relative importance of jet meandering versus other types of eddy variability in governing Southern Ocean climate processes. Hydrographic data are consistent with a hypothesis that long-term warming observed in the Southern Ocean is associated with a poleward displacement of the Antarctic Circumpolar Current (ACC), possibly linked to long-term trends in the Southern Annular Mode. Coarse-resolution climate models also support such an interpretation. However, neither the hydrographic data nor the coarse-resolution models are able to resolve eddies, and thus a lingering question is to uncover the role of eddies in governing the Southern Ocean heat budget and to determine the extent to what extent seasonal to decadal-scale change in the Southern Ocean is linked to migration of the fronts that define the ACC. Small-scale eddies and fronts influence air-sea heat and momentum fluxes, and these processes will also be examined. The ACC frontal features can be detected from satellite altimetry as well as from microwave sea surface temperature data, but there is not currently a clear consensus on how best to identify frontal jets. The proposed work will focus on improving methods for identifying ACC jet positions, by taking advantage of high-resolution altimeter data in conjunction with refined estimates of dynamic ocean topography. The Polar Front, the central jet within the ACC, has a clear temperature signature, so the proposed work will evaluate the feasibility of merging microwave sea surface temperature data (from AMSR-E and WindSat) and altimetry to help to provide clear identification of frontal positions. Results based on analysis of data alone will be compared with findings from the assimilating model, the Southern Ocean State Estimate.
The work responds to NASA solicitation objectives in three ways. The research to assess high resolution length scales in the ocean and their resolution in altimeter observations will contribute to preparation for new altimetry missions, including AltiKa and Jason-3. Applications of these high resolution data to understand the roles of eddy variability in explaining climate processes in the Southern Ocean will contribute to basic physical oceanography research using the extended multi-satellite altimeter record and will employ gravity mission data as a key tool for evaluating how eddies interact with the mean flow.