Surface frontogenesis by surface heat fluxes in the upstream Kuroshio Extension region

Tomoki Tozuka,Hiroyuki Tomita,Meghan F Cronin

doi:10.1038/s41598-017-10268-3

Abstract

Western boundary currents bring warm tropical water poleward and eastward and are characterized by a sharp sea surface temperature (SST) front on the poleward edge of the current as it extends into the interior basin. One of the most prominent such front is associated with the Kuroshio Extension (KE) as it extends east of Japan (“upstream KE”). Large latent and sensible heat fluxes that warm the atmosphere and cool the ocean project this front into the atmosphere, thereby affecting weather and climate both locally and remotely. While one might assume that these larger surface heat fluxes on the equatorward side would tend to damp the SST front, here we present observational evidence that the surface heat loss actually strengthens the front during October-April in monthly climatology and about 87% of months from October to January during the 2004/05–2014/15 period, although the percentage lowers to about 38% for February-April of the same period, suggesting some temporal/data dependency in the analysis. The key to understanding this counterintuitive result for frontogenesis is that the effective heat capacity of the surface water depends on mixed layer thickness. SSTs are more (less) sensitive to surface heat fluxes in regions with shallow (deep) mixed layer.

Highlights

A strong sea surface temperature (SST) front exists in the Kuroshio Extension (KE) region off the east coast of Japan[1,2,3] associated with the northwestern branch of the North Pacific’s subtropical gyre
The surface heat flux term contributes to the frontogenesis from October to April, while the oceanic term as a whole contributes to the frontolysis from October through April
The net surface heat flux is strongly negative in winter because cold and dry air from the continent blows across the warm KE6 and there is a strong eddy-driven westerly jet associated with the SST front[1, 6, 10]; the maximum cooling of over 400 W m−2 exists around and south of the SST front (Figs 1c and 2c)

Summary

Introduction

A strong sea surface temperature (SST) front exists in the Kuroshio Extension (KE) region off the east coast of Japan[1,2,3] associated with the northwestern branch of the North Pacific’s subtropical gyre. The strong SST meridional gradient in the 145°E–150°E band (referred to as the “upstream KE”)[4] is associated with the northern branch of the North Pacific’s subtropical recirculation gyre. The destabilization of the marine atmospheric boundary layer over the warm SST results in higher wind speed compared with areas with colder SST16 In addition to these seasonal variations, decadal variations of the SST front are suggested to play an important role in the North Pacific climate variability[17]. The meridional SST gradient has two maxima; one associated with the KE at around 36°N with a maximum of about 2.2 °C/°latitude in February and the other associated with the subarctic front at around 39.5°N with a maximum of 3.0 °C/°latitude in April The latter exists throughout the year, the former, which is the main focus of this study, is conspicuous only from boreal autumn to spring with the maximum in February.

Methods

Results

Conclusion