Changes In Mixed Layer Depth And Spring Bloom In The Kuroshio Extension Under Increased Radiation Forcing

The mixed layer depth (MLD) is deep in January-April in the Kuroshio extension region at present-climatology. The response of MLD and the spring bloom initiation there to increased radiation forcing is investigated by using 15 coupled models from the Coupled Model Intercomparison Project Phase 5(CMIP5). The innovative conclusions can be summarized as follows:1. According to the ensemble mean results of CMIP5 data, in the late 21st century, the mixed layer shoals in the Kuroshio extension, especially in March and April. The maximum of shoaling can reach 30m. In March, the maximum MLD reduction takes place in the Kuroshio Extension region of the mixed layer’s front, showing the Southwest-Northeast banded distribution. In April, the maximum MLD reduction takes place in the Kuroshio Extension region of the deep mixed layer and is more obvious than March.2, In March and April, the Kuroshio extension sea water in the southern get more heat (about 15 w/m2), resulting in a shallower mixed layer. However, it cannot explain the different distribution of mixed layer change between March and April. Temperature advection change is the main reason to the shoaling under increased radiation forcing, and the distribution of temperature advection change in both March and April are consistent with the distribution of mixed layer change. The experiments’results show that, as the background east flow, higher in west, lower in east-this distribution of potential temperature produces the warm advection. This distribution of potential temperature is caused by the background flow.3. We verify the theory of Sverdrup in the Kuroshio extension. Through the comparison of the present-climatology and the late 21st century, we found that the spring bloom occurred significantly ahead, probably in about 2-20 days. The number of days change with latitudes and longitudes.4.9 of 15 models have the same results with ensemble mean. Using leading mode of inter-model EOF to get regressions on other physical quantity:the inter-model differences temperature advection leads to the differences of mixed layer depth; the differences of the sea surface temperature leads to differences of the temperature advection, which strengthen the conclusions.The analysis shows that the mixed layer depth become shallower under increased radiation forcing, which mainly occurs in March and April. The MLD change is with seasonality which is due to temperature advection change caused by the different spatial distribution of potential temperature change in different months. While the mixed layer shoaling will lead to the earlier start of spring bloom, affecting biological processes.