A Southern Ocean mode of multidecadal variability

Abstract

A 250 year simulation of a strongly eddying global version of the Parallel Ocean Program (POP) model reveals a new mode of intrinsic multidecadal variability, the Southern Ocean Mode (SOM), with a period of 40–50 year. The peak‐to‐peak difference in the global ocean heat content within a multidecadal cycle is up to 60 ZJ. This change results from surface heat flux variations in the South Atlantic and propagation of temperature anomalies along the Antarctic Circumpolar Current and into the Weddell gyre around 30°E. The temperature anomalies propagate as deep as 5000 m along the isopycnals between 50°S and 30°S and induce multidecadal changes in the Atlantic Meridional Overturning Circulation. A positive feedback loop between the generation of eddies through baroclinic instability and the dynamics of the mean circulation is essential for the existence of the SOM. The dominant physics appears similar to that responsible for variability found in a three‐layer quasi‐geostrophic eddy‐resolving model. This combined with the fact that the SOM is not found in a noneddying version of the same global POP model further suggests that eddy processes are crucial for its existence and/or excitation.

Eastward propagating decadal temperature variability in the South Atlantic and Indian Oceans

“…A previous study by Le Bars et al . [] discussed a key role of eddy‐mean flow interaction in generating decadal temperature variation in the South Atlantic, but our CGCM did not show this effect due to insufficient horizontal resolution. Since the decadal variability is generated even without oceanic mesoscale eddies, the role of atmospheric forcing may be significant, but the relative role of atmospheric forcing and oceanic eddies in generating the decadal temperature variability should be investigated by using eddy‐resolving CGCM.…”

“…Note that in Le Bars et al . [], the atmospheric forcing used in their OGCM experiment is based on annual cycle of atmospheric variables, so a possible influence of atmospheric interannual‐to‐decadal variability in the South Atlantic remains unclear. Also, the multidecadal variability reproduced in their OGCM has much longer timescale than the observed decadal variability with a period of 20 years, and physical mechanisms behind the eastward propagation of multidecadal temperature variability are not fully examined.…”

Energetics of the Southern Ocean Mode

“…Recently, output from a 326‐year simulation with a strongly eddying (a horizontal grid spacing of 0.1° ) version of the Parallel Ocean Program (POP) revealed multidecadal (40‐ to 50‐year) spatiotemporal variability in the Southern Ocean (Le Bars et al, ), termed the Southern Ocean Mode (SOM). Although the spatial pattern of the SST anomalies spans the full Southern Ocean, large‐scale anomalies occur in the Weddell Sea area.…”

“…A first description of the mechanisms of the SOM was presented in Le Bars et al (). It was shown that the changes in overall potential energy (PE) and kinetic energy (KE) in the region south of 30° S (associated with the SOM) could be linked to a dynamical mechanism described by Hogg and Blundell () who used an idealized quasi‐geostrophic model.…”

Role of subsurface ocean in decadal climate predictability over the South Atlantic

“…On the other hand, one recent study 18 demonstrated in an eddy-resolving ocean general circulation model (OGCM) that the multi-decadal SST variability in the Atlantic Sector of the Southern Ocean can be generated as an intrinsic ocean variability mode through eddy-mean interaction and propagate eastward along the background eastward current. Since the decadal SST variability generated in the South Atlantic tends to migrate eastward and have potential influences on decadal climate variability in the southern Indian Ocean 14,19 , skillful prediction of the decadal SST variability in the South Atlantic is crucial for that of decadal climate variability in the southern Indian Ocean.…”