Uncertainty and resolution dependence of the atmospheric response to decadal variability of the Kuroshio Extension

Bunmei Taguchi, University of Toyama, Faculty of Sustainable Design, Toyama, Japan, Kazuaki Nishii, The University of Tokyo, RCAST, Tokyo, Japan; Mie University, Graduate School of Bioresources, Tsu, Japan and Hisashi Nakamura, University of Tokyo, Research Center for Advanced Science and Technology, Tokyo, Japan
Evidence is increasing for the impact of mid-latitude western boundary currents (WBCs) upon the large-scale atmospheric circulation variability on interannual-to-decadal time scales. Robustness of such large-scale response to WBC-related sea surface temperature (SST) anomalies is, however, still under debate. Some modelling studies suggest the importance of high-resolution, but this could depend on the particular models used for the studies. Here we examine the influence of extra-tropical oceanic frontal zone and its variability in the Kuroshio Extension (KE) regions on winter-season large-scale atmospheric variability in an atmospheric reanalysis data (JRA55) and ensemble historical atmospheric general circulation model (AGCM) simulations configured with two different horizontal resolutions (about 50km and 100km), the both prescribed with satellite-observed, high-resolution SST and sea-ice variability for the period 1982-2013.

The both reanalysis and AGCM simulations show that the response of lower-tropospheric meridional heat flux due to sub-weekly eddies (as a measure of storm track activity) to the decadal variability of KE’s dynamical state is characterized by a northward (southward) shift of the storm track when the KE is stable (unstable). While ensemble-averaged response in the AGCM simulations agrees well with the response detected in the reanalysis, a large spread exists in the response across ensemble members: there is large uncertainty due to atmospheric internal variability in the observational estimate of the atmospheric circulation response to extra-tropical SST anomalies. For the ensemble-averaged response in lower-tropospheric storm track activity, our simulations do not show much difference between 50km and 100km resolutions. We further identify the leading mode of the forced variability of near-surface baroclinicity in the KE region with SVD analysis between the reanalysis and ensemble model simulations. The leading mode is characterized by the strengthening/weakening of the baroclinicity over the KE region, which is nonetheless under the strong influence of remote tropical variability and is not enhanced in the higher-resolution AGCM. Meanwhile, a benefit of the higher resolution is hinted in local vertical motion response to the KE’s dynamical state.