Upper-Ocean Mixed Layer and Wintertime Sea Surface Temperature Anomalies in the North Pacific

Abstract

In the North Pacific, the wintertime sea surface temperature anomaly (SSTA), which is represented by March (SSTAMar), when the upper-ocean mixed layer depth (hMar) reaches its maximum, is formed by the anomalous surface forcing from fall to winter (S?). As a parameter of volume, hMar has a potential to modify the impact of S? on SSTAMar. Introducing an upper-ocean heat budget equation, the present study identifies the physical relationship among the spatial distributions of hMar, S?, and SSTAMar. The long-term mean of hMar adjusts the spatial distribution of SSTAMar. Without the adjustment, the impact of S? on SSTAMar is overestimated where the hMar mean is deep. Since hMar is partially due to seawater temperature, it leads to nonlinearity between the S? and the SSTAMar. When the SSTAMar is negative (positive), the sensitivity to S? is impervious (responsive) with the deepening (shoaling) of the hMar compared to the linear sensitivity. The thermal impacts from the ocean to the atmosphere might be underestimated under the assumption of the linear relationship.