Diurnal precipitation and high cloud frequency variability over the Gulf Stream and over the Kuroshio

Recent studies show mid-latitude western boundary currents (WBCs) substantially influence the atmosphere aloft, and an important feature is enhanced rain band over the WBCs in climatological mean field. However, how such long-term, climate phenomena are related to shorter, weather timescale phenomena are generally remained to be explored. In this paper, diurnal precipitation and cloud variations are investigated global mid-latitude oceans with emphasis on air–sea interactions over WBCs using satellite-derived precipitation and outgoing longwave radiation (OLR) datasets. Strong 24-h period components of precipitations are found over the Gulf Stream in summer and over the Kuroshio in the East China Sea in early summer (Baiu–Meiyu season), respectively. Similar diurnal precipitations are not observed in WBCs in the Southern Hemisphere year around. The diurnal precipitation cycles over the Gulf Stream and the Kuroshio exhibit peak phases in the early to late morning for the Gulf Stream and late morning to early afternoon for the Kuroshio, with southeastward phase propagations. High cloud frequency derived from OLR data exhibit consistent diurnal cycles. A substantial difference of diurnal cycles between the Gulf Stream and the Kuroshio regions are associated with the large-scale Baiu–Meiyu rain and cloud bands for the latter region. Diurnal precipitation and high cloud variability is found in the vicinity of the Kuroshio itself, embedded in the Baiu–Meiyu rain and cloud bands distributing in a wider area without a strong diurnal component. The spatial and seasonal distributions of the diurnal variability over these WBCs strongly suggests that the diurnal precipitation and cloud cycles are essential aspects of deep heating mode of atmospheric response recently reported for these WBCs. These results indicate that these WBCs in the Northern Hemisphere play an important role in modulating short-term precipitation variations, and on the other hand diurnal variability can be a substantial agent for the mid-latitude air–sea interaction.

Animations of diurnal precipitation cycles are found at http://www.sci.hokudai.ac.jp/~minobe/animation/diurnal_precip_WBCs/