Zonal Variations in the Southern Ocean Heat Budget

Abstract

The spatial structure of the upper ocean heat budget in the Antarctic Circumpolar Current (ACC) is investigated using the ⅙°, data-assimilating Southern Ocean State Estimate (SOSE) for 2005–10. The ACC circumpolar integrated budget shows that 0.27 PW of ocean heat gain from the atmosphere and 0.38 PW heat gain from divergence of geostrophic heat transport are balanced by −0.58 PW cooling by divergence of Ekman heat transport and −0.09 PW divergence of vertical heat transport. However, this circumpolar integrated balance obscures important zonal variations in the heat budget. The air–sea heat flux shows a zonally asymmetric pattern of ocean heat gain in the Indian and Atlantic sectors and ocean heat loss in the Pacific sector of the ACC. In the Atlantic and Indian sectors of the ACC, the surface ocean heat gain is primarily balanced by divergence of equatorward Ekman heat transport that cools the upper ocean. In the Pacific sector, surface ocean heat loss and cooling due to divergence of Ekman heat transport are balanced by warming due to divergence of geostrophic heat advection, which is similar to the dominant heat balance in the subtropical Agulhas Return Current. The divergence of horizontal and vertical eddy advection of heat is important for warming the upper ocean close to major topographic features, while the divergence of mean vertical heat advection is a weak cooling term. The results herein show that topographic steering and zonal asymmetry in air–sea exchange lead to substantial zonal asymmetries in the heat budget, which is important for understanding the upper cell of the overturning circulation.

Three‐Dimensional Ageostrophic Motion and Water Mass Subduction in the Southern Ocean

“…While most of the studies focus on the eddy vertical velocity variability induced by frontal instabilities and interactions with the bottom topography, these mean adiabatic vertical fluxes may have an impact on the nutrient fluxes into the euphotic layer and on the mean air‐sea heat fluxes. While this will merit dedicated investigations with longer time series, we note here that the patterns of the vertical velocities in Figure closely remind the residual between the ageostrophic transport and the Ekman transport in Tamsitt et al (). This implies that our purely observation‐based reconstructions are in good agreement with Southern Ocean State Estimate (SOSE).…”

A Multibasin Residual-Mean Model for the Global Overturning Circulation

“…Radko and Marshall (2006) appreciated that zonal variations in this surface buoyancy flux could locally influence the strength of overturning, but recent studies have shown that transitions in the surface buoyancy flux may be more abrupt than a simple sinusoid with the gravest wavenumber (Cerove cki et al 2011;Bishop et al 2016). In particular, Tamsitt et al (2016) show that while there is a large discrepancy in the surface heat flux across different basins, the intrabasin heat flux is largely uniform. This result is consistent with the models derived here in that modifications to the surface buoyancy flux are largely related to changes in the outcrop position across different basins to accommodate zonal convergence/divergence in the ACC.…”

“…Available air-sea buoyancy flux products (Large and Yeager 2009;Cerove cki et al 2011) show large-scale, zonally asymmetric patterns with buoyancy gain in the Atlantic and Indian Oceans (outside of the Agulhas Retroflection) and weaker buoyancy fluxes (both positive and negative) in the Pacific. Tamsitt et al (2016) have analyzed the surface heat budget in the Southern Ocean State Estimate (SOSE) model and showed that topographic steering and zonal asymmetry in air-sea exchange leads to even more dramatic zonal variability in the surface heat flux. In one of the only studies to address the dynamics of this zonal structure, Radko and Marshall (2006) introduced a perturbation with a mode-1 zonal wavenumber to the zonally averaged properties of the ACC.…”

“…However, as the basin widths become comparable, the outcropping sites are pushed further to the south, and the buoyancy forcing changes sign across the interface separating upper and lower CDW y 2 . Tamsitt et al (2016) have recently shown that positive heat fluxes are more prominent in the Atlantic sector of the ACC, as compared to the Pacific sector. This box model suggests that at least part of this zonal asymmetry can be attributed to the narrow width of the …”

SST Dynamics at Different Scales: Evaluating the Oceanographic Model Resolution Skill to Represent SST Processes in the Southern Ocean

“…All regions had latitudes between 77°S and 30°S. The subdivision of SO into the three main oceanic basins corresponds to important zonal differences in the ACC flow and SO physics (e.g., Tamsitt et al, ).…”