Midlatitude Wind Forcing and Subduction of Temperature Anomalies

Abstract

An OGCM (MOM1) is used to examine the oceanic response to localized anomalous surface wind and buoyancy forcings. Wind stress and surface cooling anomalies are imposed at several different locations with respect to the positions of the mixed layer front and the LPVP (low potential vorticity path). Surface cooling locally creates sea surface temperature anomalies, which are subducted to the thermocline in remote places. The way in which wind anomalies affect the thermocline structure can be observed by using the following indicator. The LPVP is defined as a line that consists of water with minimum potential vorticity at each latitude. It is defined at each isopycnal surface and is affected through changes in the mixed layer depth or the position of the outcrop lines. Sea surface height (SSH) anomalies created by localized anomalous wind stress forcing propagate westward at the same speed as the lower-thermocline depth anomalies, corresponding to the first baroclinic mode. When the forcing region is east of the LPVP, the depth of various isopycnal surfaces induces large variability in the region of the LPVP, caused either by propagation of the first baroclinic mode wave or variations in the mixed layer front position. These results imply that the subsurface temperature anomalies, associated with the change of isopycnal depths, are large in the vicinity of the LPVP, even if the wind stress anomaly is remote. Previous studies suggest that propagation of subsurface temperature anomalies is forced primarily by surface cooling. In this work, the authors observe that temperature anomalies created by surface cooling primarily follow the subtropical circulation. However, it is shown that the subducted temperature anomalies may also be generated by remote wind-forcing effects, through their impact on the position of the LPVP.