A metric for surface heat flux effect on horizontal sea surface temperature gradients
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Understanding what controls horizontal variations in sea surface temperatures (SSTs) is one of the key science questions in climate research. Although various oceanic effects contribute to reinforcement/relaxation of horizontal variations in SSTs, the role of surface heat fluxes is surprisingly complex and can lead to significant biases in coupled models if improperly represented. In particular, the contribution of surface heat fluxes to surface frontogenesis/frontolysis depends not just on their gradients, but also on the distribution of mixed layer depth, which controls the effective heat capacity of the upper ocean. In this study, a new metric, referred to as the surface flux frontogenesis metric, is proposed that quantifies the relative importance of horizontal variations in surface heat fluxes and mixed layer depth. Global maps of this metric reveal that the role of surface heat fluxes in determining the horizontal SST gradient is highly variable geographically and by season. Furthermore, the metric can help explain characteristics of SST fronts in the northwestern Pacific, the Southern Ocean, the eastern equatorial Pacific, and the west coast of North America. Implications of this metric in coupled models will also be discussed.
This study greatly benefited from insightful comments by Frank Bryan and an anonymous reviewer. The MIMOC data is available from http://www.pmel.noaa.gov/mimoc/, and the J-OFURO2 data is available from http://dtsv.scc.u-tokai.ac.jp/j-ofuro/. TT was supported by Grant-in-Aid for Scientific Research on Innovative Areas (MEXT KAKENHI Grant number JP16H01589) and the Japan Society for Promotion of Science through Grant-in-Aid for Scientific Research (B) JP16H04047. PMEL contribution 4649.
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