Regional Impacts of the Westerly Winds on Southern Ocean Mode and Intermediate Water Subduction

Abstract

Subduction processes in the Southern Ocean transfer oxygen, heat, and anthropogenic carbon into the ocean interior. The future response of upper-ocean subduction, in the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) classes, is dependent on the evolution of the combined surface buoyancy forcing and overlying westerly wind stress. Here, the recently observed pattern of a poleward intensification of the westerly winds is divided into its shift and increase components. SAMW and AAIW formation occurs in regional “hot spots” in deep mixed layer zones, primarily in the southeast Indian and Pacific. It is found that the mixed layer depth responds differently to wind stress perturbations across these regional formation zones. An increase only in the westerly winds in the Indian sector steepens isopycnals and increases the local circulation, driving deeper mixed layers and increased subduction. Conversely, in the same region, a poleward shift and poleward intensification of the westerly winds reduces heat loss and increases freshwater input, thus decreasing the mixed layer depth and consequently the associated SAMW and AAIW subduction. In the Pacific sector, all wind stress perturbations lead to increases in heat loss and decreases in freshwater input, resulting in a net increase in SAMW and AAIW subduction. Overall, the poleward shift in the westerly wind stress dominates the SAMW subduction changes, rather than the increase in wind stress. The net decrease in SAMW subduction across all basins would likely decrease anthropogenic carbon sequestration; however, the net AAIW subduction changes across the Southern Ocean are overall minor.

Variability of the Subantarctic Mode Water Volume in the South Indian Ocean During 2004–2018

“…The formation of the SAMW is driven by the combined effect of air-sea fluxes and Ekman pumping (Holte et al, 2012;McCartney, 1977;Rintoul & England, 2002;Sloyan et al, 2010;Sloyan & Rintoul, 2001), which in turn are closely related to overlying westerly winds (Downes et al, 2011(Downes et al, , 2017.…”

“…Therefore, defining the SAMW using a single homogenous temperature might neglect the dynamic differences between ocean basins. Moreover, Downes et al (2017) found that the mixed‐layer depth (MLD) in the Indian and Pacific Oceans responds differently to wind stress perturbations across SAMW formation zones. They further suggested that the poleward shift of westerly winds rather than an increase in the wind stress curl dominates the SAMW subduction variability in the South Indian Ocean (SIO).…”

Historical and Projected Changes in the Southern Hemisphere Surface Westerlies

“…To examine the effect of future changes in surface westerlies, 30–40 previous studies that we are aware of (and possibly many others that we are unaware of) have applied a broad range of poleward shifts and intensifications, with the poleward shift ranging between 0.5° and 10° latitude and wind intensification factors ranging from 10 to over 300% (e.g., Delworth & Zeng, 2008; Downes et al., 2017; Frankcombe et al., 2013; Hogg et al., 2017; Spence et al., 2014; Waugh et al., 2019). Given the wide range of perturbations that have been applied in past studies, some guidance regarding a reasonable estimate of the past and projected changes in the location and strength of the westerly winds in the SH is needed to better facilitate model intercomparison.…”

How Rapidly Do the Southern Subtropical Oceans Respond to Wind Stress Changes?

“…Farneti and Gent (2011) and Waugh (2014) also examined the response of the ideal age tracer in these simulations. More recently, several studies have examined the impact of changes in winds on different ocean properties in the same suite of global eddy‐permitting ocean‐sea ice simulations that include separate abrupt intensification or poleward shift wind stress perturbations, as well as a simulation with a combined increase and shift: Hogg et al (2017) examined the changes in the meridional overturning circulation, Downes et al (2017) changes in subduction rates, and Waugh et al (2019) examined changes in the barotropic stream function (BSF) and ideal age. While the above studies have in combination documented how different ocean properties respond to an increase or shift in the surface winds, they have focused almost exclusively on the steady‐state response, and little attention has been paid to time scales for these responses.…”