Numerical Study on the Variability of Mixed Layer Temperature in the North Pacific

Physical processes important for the interannual variability in mixed layer temperature (MLT) in the North Pacific have been examined by using a three-dimensional bulk mixed layer model. This model was forced by the momentum, heat, and freshwater flux data derived from the NCEP-NCAR reanalysis and geostrophic flow data included in Japanese Ocean Flux datasets with Use of Remote Sensing Observations. The interannual variation in MLT was hindcasted over the course of 11 years from January 1993 to December 2003. The interannual variation in the modeled MLT favorably agreed with that of the available in situ and satellite sea surface temperature (SST) observational data. This agreement depended crucially on whether horizontal heat advection was considered a part of the model dynamics. Although both atmospheric and oceanic processes were required to explain the observed interannual MLT variability, the physical process most important for determining this variability was likely to differ year by year. For example, in the Kuroshio Extension region, it was found that the positive temperature tendency peak in 1997 was attributed to the positive surface thermal forcing, while the temperature tendency in 1998 and 1999 continued to increase in spite of the negative surface thermal forcing. Thus, the abnormal heat loss from the ocean to the atmosphere (hence, the atmospheric circulation change) in 1998-99 was considered to be excited by the ocean dynamics related to the warmer SST. Likewise, it was found that the rapid decrease in SST during 2001-03 was mainly caused by the effect of lateral flux.