期刊
JOURNAL OF CLIMATE
卷 33, 期 24, 页码 10419-10436出版社
AMER METEOROLOGICAL SOC
DOI: 10.1175/JCLI-D-20-0119.1
关键词
Synoptic-scale processes; Extreme events; Oceanic mixed layer
资金
- National Natural Science Foundation of China (NSFC) Projects [91858102, 41922039, 41490643/41490640]
- National Key Research and Development Program of China [2016YFA0601804]
- National Postdoctoral Program for Innovative Talents Foundation of China [861805033113]
Change in the extratropical wintertime-mean mixed layer has been widely studied, given its importance to both physical and biogeochemical processes. With a focus on the south of the Kuroshio Extension region where the mixed layer is deepest in March, this study shows that variation of the synoptic-scale extreme mixed layer depth (MLD) is a better precursor than the monthly mean (or nonextreme) MLD for change in the subtropical mode water formation in spring, based on the NCEP Climate Forecast System Reanalysis (1979-2010). It is found that the extreme MLD events are attributable to the accumulation of excessive surface cooling driven by the synoptic storms that characterize cold-air outbreaks. Particularly, the difference between the extreme and nonextreme MLD is primarily related to differences in the cumulative synoptic heat flux anomalies, while a change in the preconditioning upper-ocean stratification contributes almost equally to both cases. Relative contributions of oceanic and atmospheric forcing to the interannual variation of the extreme MLD are quantified using a bulk mixed layer model. Results show comparable contributions: the preconditioning stratification change accounts for;44% of total variance of the extreme MLD, whereas the convective mixing by surface heat flux and the mechanical stirring by wind stress account for;35% and;13%, respectively. In addition, both the reanalysis and observational data reveal that the extreme and nonextreme MLD has been shallowed significantly during 1979-2010, which is accounted for by the strengthened stratification due to the enhanced ocean surface warming by the Kuroshio heat transport.
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