Ensemble simulations of the influence of regionally warm sea surface on moisture and rainfall in Tsushima Strait during August 2013

This study investigates the short-term (four days) atmospheric response to regionally high sea-surface temperature (SST) in the Tsushima Strait. During 18–22 August 2013, SST in the strait increased and cloud cover was scarce (sunny period). During 23–26 August, SST decreased and frequent rainfall was associated with a stationary front (rainy period). The moisture response to SST differed between the sunny and rainy periods. Ensemble-mean moisture variation induced by a regionally warm sea surface is well correlated with SST increases during the sunny period. However, they are not clearly correlated with SST increases during the rainy period when water vapor fluctuated because of frequent rainfall. The high SST resulted in locally enhanced precipitation in the central area of the warm core. Unlike the climatological response of precipitation to SST, the ensemble experiment shows that warm SSTs do not always enhance hourly rainfall because of SST-related changes in moisture from prior rainfall events. In a simulation that performs well in reproducing precipitation at Izuhara observatory (located in Tsushima Strait), high SSTs resulted in enhanced precipitation in the morning. Subsequently, water vapor decreased, leading to lower precipitation in the afternoon. In contrast, a low-SST experiment with the warm-SST core removed produced moisture concentrations that were higher than those in the high-SST experiment after weak rainfall during the morning. As a result, low SST led to greater precipitation in the afternoon. Thus, responses of hourly precipitation to SST should be carefully investigated by considering transient moisture variations during each rainfall event and related uncertainties in ensemble simulations.