On the seasonal variability of the Oyashio extension fronts

Abstract

Previous study reported that the annual-mean eastern Oyashio Extension (OE) front shifts northward while the western OE front has no obvious poleward shift during 1982–2017 by Wu et al. (Geophys Res Lett 45:9042–9048, 2018). Here we revisit this topic and focus on the seasonal variability and shifts of the OE fronts from 1982 to 2018, with observational reanalysis data and a 1.5-layer reduced-gravity model simulation. In winter, both the western and eastern OE fronts demonstrate consistent northward movement. While in summer, the eastern OE front still moves northward but the western OE front has no obvious and even southward shift. It is shown that the trade wind’s expansion during 1982–2018 favours the northward shift of the OE fronts for both winter and summer. However, there is a local cold Ekman heat transport anomaly along the western OE front in summer, which surpasses the effect of trade wind expansion and prohibits northward movement of the front. This cold Ekman advection is due to a westerly wind anomaly induced firstly by the Atlantic Multi-decadal Oscillation (AMO) and secondly by the Pacific Decadal Oscillation (PDO). In winter, the local Ekman heat transport is less effective than in summer in changing the OE front position because of the deep mixed layer. Our study demonstrates the seasonality of the OE front shift and highlights the importance of local Ekman heat transport associated with the AMO. Our results also partly explain the rainfall changes in both winter and summer in the western Pacific Ocean in the past 37 years, since the rainband east of Japan is affected by the sea surface temperature and its front.