Impact Propagation of Radiosonde Data Assimilation over the Kuroshio and Kuroshio Extension: Case Study on the Early Summer (Baiu) in 2012

Abstract

 The present study has investigated how the impacts of the inclusion of radiosonde observations conducted locally in early summer of 2012 over the Kuroshio and Kuroshio Extension (KE) can spread over time across the North Pacific basin to influence the predictability of synoptic and large-scale tropospheric circulation. For that purpose, observing system experiments (OSEs) has been performed where each of two extra sets of radiosonde data, one obtained over the East China Sea in mid-May and the other over the KE in early July, is added to an atmospheric ensemble data assimilation system for comparison with the corresponding analyses without those data. The experiments show that the impact of the extra data assimilated propagates eastward mainly due to the advection by the subtropical jet (STJ) in May and July. The strong STJ in May allows the upper-tropospheric impact to travel across the basin only within two days. Under the weaker STJ, the corresponding impact in July tends to remain within the western Pacific, until it eventually reaches the eastern portion of the basin. The assimilation of the extra radiosonde data over the Kuroshio or KE can lead to decrease of pressure over the Gulf of Alaska both in May and July.

 Additional forecast experiments based on the OSEs for May has revealed that the pressure decrease over the Gulf of Alaska can be traced back to the west of the Alaska Peninsula and to east of Japan over three days. The impacts that originate on different dates via different paths merge over the central North Pacific, acting to reinforcing the cyclone over the Gulf of Alaska. This study presents examples where the impacts of atmospheric observations over the western boundary current can propagate across the ocean basin through the westerlies to influence the forecast skill in distant regions.