Impact of the North Pacific subtropical sea surface temperature front on El Niño‐Southern Oscillation

Abstract

ABSTRACTThe North Pacific subtropical front (NPSTF) of sea surface temperature (SST) is an important subtropical feature in the North Pacific. In this study, we reveal that the inter‐annual variability of the spring NPSTF has a robust correlation with the El Niño‐Southern Oscillation (ENSO) in the following winter over the period of 1961–2016. When there is a strong spring NPSTF, anomalous cyclonic and anticyclonic circulations in the low troposphere occur to its north and south, respectively. The northeasterly wind anomaly associated with the anticyclonic circulation increases the climatological northeasterly wind over the subtropical Northeast Pacific, resulting in SST decrease via the wind–evaporation–SST feedback. The SST and wind perturbations propagate southwards from the subtropical to the eastern central equatorial Pacific in summer and grow via the Bjerknes feedback from summer to winter, eventually leading to a La Niña event in winter.A strong spring NPSTF induces an anomalous anticyclonic circulation to its south through both transient eddy activity and atmospheric heat source anomaly. The enhanced air temperature gradient due to a strong NPSTF increases the atmospheric baroclinicity, favouring an enhanced transient eddy activity over the NPSTF and to the north. The enhanced transient eddy activity triggers a significant negative height anomaly to the north via high‐frequency transient eddy feedback forcing, which is conductive to an anticyclonic circulation over the subtropical North Pacific through the downstream effect of Rossby waves. On the other hand, the convergence and increased atmospheric baroclinicity related to the strong NPSTF act to enhance precipitation over the NPSTF and to the north, corresponding to the increased atmospheric heat source, which can also excite an anomalous anticyclonic circulation over the subtropical North Pacific.