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Ocean Surface Topography from Space
SCIENCE
Mixed-layer Interannual variability in the Southern Ocean (MISO)


Author:

Frédéric Vivier - (LOCEAN)

Co-Investigator(s):
  Young-Hyang Park
Jacqueline Boutin
Julien Le Sommer
Jean-Baptiste Sallee
(LOCEAN)
(LOCEAN)
(LEGI)
(BAS)

Abstract:


Mixed-layer Interannual variability in the Southern Ocean (MISO)
ENSO-locked heat budget for the period 1992-2002, from a 2D diagnostic model. Regression of dominant heat budget terms onto ENSO index: (a) anomalous air-sea heat flux (b) anomalous Ekman HF (c) HF associated with anomalous geostrophic current and (d) advection of SST anomalies by mean flow. Units are 10
-7 C s
-1 . Gray lines in subplots (a),(b) indicate 10m isolines of the geopotential height anomaly at 500mb regressed onto ENSO. Pink (green) lines in subplot (c) denote contours of positive (negative) SSH anomalies regressed onto the index. From Vivier et al., [2010], Climate Dynamics.
The Southern Ocean is major compartment of the meridional overturning circulation (MOC) where large volumes of mode and intermediate waters are formed and exported from the surface, contributing to make it the largest sink of CO2 of the World Ocean. Substantial changes have occurred in the Southern Hemisphere atmospheric circulation over the past few decades with measurable impact on the properties of these water masses. Climate simulations project a continuation of intensification trends in the winds: assessing how the MOC might ultimately be affected in a changing climate is among the most pressing questions. A full understanding of the physical processes involved in the variability of the formation rate and properties of mode and intermediate water, and thus of the MOC is however still missing. The rate of formation and properties of these waters depend on exchanges with the atmosphere through ocean mixed-layer (ML) processes. The scope of this project, using intermediate complexity modeling tools, is to extract the maximum of information from existing databases in the Southern Ocean to provide insight on the ML variability at interannual time scales, its interplay with subsurface layers underneath, and on the dominant mechanisms driving its response.

The ML heat budget will be analyzed for the period 1992-present based on simulations from a simple, insightful, model with advection prescribed from altimetry which ensures a correct positioning of the currents. This model will explicitly consider ML depth and entrainment anomalies, which can either be modeled or mapped from in situ observations. Although the Southern Ocean is a data depleted region, expectations for mapping ML parameters for recent years are much better with the advent of the Argo program. An important prerequisite, however, is to assess whether this improved data coverage is sufficient to map interannual changes with a reasonable accuracy and acceptable spatial resolution. Here mapping procedure will be tested on synthetic hydrographic profiles with realistic space-time distribution, and mapped fields compared with relevant satellite datasets. Analyses will particularly focus on wintertime budgets, relevant for mode water formation, and investigate the relation of the dominant term of the budget with the main climate modes of the Southern Hemisphere.

In a complementary fashion, we will also examine to what extent nonseasonal SSH anomalies reflect changes in the upper ocean heat content, and investigate factors driving its variability. This first requires to sort out the ocean's barotropic response at interannual time scales, which will be done with a finite element barotropic model, assessed against gravimetric observations from Grace. A joint analysis of the ML temperature and steric SSH has the potential to shed light on the interplay between the ML and the subsurface, and buoyancy anomalies stored below the ML.

Finally, we will attempt a similar diagnostic analysis for Sea Surface Salinity (SSS) anomalies based on the same model architecture. Investigating anomalies of SSS, which is a critical ML parameter in the Southern Ocean, is both important and timely and should be possible for recent years owing both to improved hydrographic data coverage (Argo) and dedicated satellite mission (SMOS).



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