[A-CG29] Extratropical oceans and atmosphere

[ACG29-09] Mechanisms of wintertime recurrence of sea surface temperature anomalies in the southern Indian Ocean

*Kouya Nakamura¹, Kazumichi Murata¹, Shoichiro Kido², Tomoki Tozuka^1,2 (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2.Application Laboratory (APL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

Keywords:Sea surface temperature, Reemergence, Mixed layer depth, Heat budget analysis

Recurrence of wintertime sea surface temperature (SST) anomalies in the extra-tropics is realized by the seasonal variation of the upper-ocean mixed layer depth (MLD). It is considered that the reemergence process, in which the water mass with temperature anomalies generated in the previous winter is reincorporated into the mixed layer, plays an important role. However, a quantitative investigation based on an online mixed layer heat budget analysis with a regional ocean model reveals for the first time that the recurrence in the southern Indian Ocean is not due mainly to this reemergence process. Rather, positive SST anomalies recur through their impact on MLD; anomalously warm and less dense water formed in the previous winter is retained in the subsurface during summer and subsequently favors anomalous deepening of the mixed layer from autumn to winter. This makes the upper ocean less sensitive to the surface cooling. On the other hand, the recurrence of negative SST anomalies is due to anomalously cold and dense water that is subducted below the mixed layer and hampers the mixed layer deepening in the following autumn to winter. The resulting anomalously shallow mixed layer is more effectively cooled by the surface heat flux and negative SST anomalies recur. Therefore, this study suggests that wintertime SST anomalies may contribute to the recurrence of SST anomalies in the following winter not only through the reemergence process, but via their impacts on the MLD that controls the upper ocean heat capacity.