The Energetics of Southern Ocean Upwelling

Abstract

The ocean's meridional overturning circulation is closed by the upwelling of dense, carbon-rich waters to the surface of the Southern Ocean. It has been proposed that upwelling in this region is driven by strong westerly winds, implying that the intensification of Southern Ocean winds in recent decades may have enhanced the rate of upwelling, potentially affecting the global overturning circulation. However, there is no consensus on the sensitivity of upwelling to winds or on the nature of the connection between Southern Ocean processes and the global overturning circulation. In this study, the sensitivity of the overturning circulation to changes in Southern Ocean westerly wind stress is investigated using an eddy-permitting ocean-sea ice model. In addition to a suite of standard circulation metrics, an energy analysis is used to aid dynamical interpretation of the model response. Increased Southern Ocean wind stress enhances the upper cell of the overturning circulation through creation of available potential energy in the Southern Hemisphere, associated with stronger upwelling of deep water. Poleward shifts in the Southern Ocean westerlies lead to a complicated transient response, with the formation of bottom water induced by increased polynya activity in the Weddell Sea and a weakening of the upper overturning cell in the Northern Hemisphere. The energetic consequences of the upper overturning cell response indicate an interhemispheric connection to the input of available potential energy in the Northern Hemisphere.

Energetics of the Southern Ocean Mode

“…Figures a–d show P m (integrated over depths of 100–1,500 m) averaged over D as well as the anomalies of the phases A − C with respect to D . The D ‐mean P m distribution (Figure a) is largely similar to the results shown by Wu et al () and Hogg et al (). Noting the logarithmic color scale, this figure shows that the dominant storage of P m occurs at high latitudes around Antarctica, where dense water can be found close to the surface; this water contains energy that is available to be converted to KE by sinking through the depth of the ocean to its reference level.…”

“…Finally, Figure shows the power input by the winds ( G ( K m ) and G ( K e ), respectively) averaged of D as well as the anomalies of the phases A – C with respect to D . The geographic distribution of the D ‐average of the generation from the time‐mean wind stress resembles the distributions described in previous studies (Hogg et al, ; von Storch et al, ; Wu et al, ). That is, the power input from the time‐mean wind stresses is most intense in the region of the eastward flowing ACC where the westerly winds are strongest with generally larger magnitudes south of Africa and in the Indian Ocean sector of the SO than those farther eastward.…”

“…Figures show the four energy conversion terms C ( P m , K m ), C ( P e , K e ), C ( P e , P m ), C ( K e , K m ) (integrated over depths of 100–1,500 m) averaged of D as well as the anomalies of phases A – C with respect to D . For all conversion terms it holds that they are dominated by smaller scale structures of positive and negative values (similarly to the previous studies: Hogg et al, ; von Storch et al, ; Wu et al, ). Consequently, only in an integrated sense a sign/direction can be associated with each of the conversions.…”

“…Moreover, a significant negative anomaly is situated in the Atlantic part of the SO and also peaks in B . A dipole pattern of anomalies is also found in Hogg et al () for their simulations in which convection is triggered. In contrast, in simulations with only increased winds (and no convection triggered) dipole type anomalies are absent (Hogg et al, ; Wu et al, ).…”

“…A dipole pattern of anomalies is also found in Hogg et al () for their simulations in which convection is triggered. In contrast, in simulations with only increased winds (and no convection triggered) dipole type anomalies are absent (Hogg et al, ; Wu et al, ). The changes in the P m distribution may be related to the onset of convection (described in subsection ), subsequently induced changes in the slopes of the isopycnals (reflected in ACC, upper and lower MOC transport changes), and Antarctic shelf dynamics.…”