Effects of the South Asian Monsoon Intraseasonal Modes on Genesis of Low Pressure Systems over Bangladesh

The quasi-biweekly oscillation (QBW) is a dominant intraseasonal mode in summer rainfall over Bangladesh. Active phases of the QBW are often accompanied by low pressure systems (LPSs) such as vortex-type lows. This study investigated the effects of two intraseasonal modes, the QBW and the boreal summer intraseasonal oscillation (BSISO), on the genesis of LPSs over Bangladesh during 29 summer monsoon seasons. Daily lag composites of convection and low-level atmospheric circulation were constructed for active-phase cases with LPSs (LPS case) and without LPSs (non-LPS case) based on rainfall in the QBW over Bangladesh. In the QBW mode, a westward propagation of an anticyclonic anomaly from the western Pacific to the Bay of Bengal (BoB) is common in both cases. However, the anticyclonic center in the LPS case is located slightly to the east of that in the non-LPS case, which results in stronger cyclonic vorticity over and around Bangladesh. In contrast, the BSISO mode shows an opposite phase between the two cases: a cyclonic (anticyclonic) anomaly propagating northward from the equator to the BoB in the LPS case (non-LPS case). In the LPS case, the cyclonic anomaly in the BSISO mode enhances the westerly (easterly) flow over the BoB (Bangladesh) in the active phase, resulting in the enhancement of cyclonic vorticity over the northern BoB and Bangladesh, in cooperation with the QBW mode. These results suggest that both the QBW and BSISO modes have significant influence on the environmental conditions for LPS genesis over Bangladesh

Future Changes in Monthly Extreme Precipitation in Japan Using Large-Ensemble Regional Climate Simulations

This study investigated future changes in monthly extreme precipitation in Japan during summer (June-August). The uncertainties in estimating extreme monthly precipitation were analyzed using large-ensemble regional climate simulations for both present and 4-K warmer climates. The main diagnostics were based on the 100-yr return values of monthly total precipitation P-T100 estimated from a best-fit probability distribution. Under the warmer climate, P-T100 was projected to increase in approximately 87%, 88%, and 78% of the total number of stations for June, July, and August, respectively, suggesting that once-per-century monthly precipitation will increase as temperature increases over a wide area of Japan, although large regional variations will exist. The western part of Kyushu and the Hokkaido region showed significant and moderately robust increases in P-T100 throughout the summer months. In contrast, a considerable and robust increase was projected only in June in the Nansei Islands. The percentage change in P-T100 was small in western and eastern Japan, and thus the sign of the change was uncertain. Further analysis indicated that uncertainty in internal variability is more important than uncertainty in the SST scenario for future projections of monthly precipitation extremes

Impact of SST on Present and Future Extreme Precipitation in Hokkaido Investigated Considering Weather Patterns

This study investigated the impact of sea surface temperature (SST) on extreme precipitation events in North Japan and its relation to synoptic weather patterns using large-ensemble climate simulations. Eight weather patterns were identified by applying cluster analysis to sea level pressure anomalies for selected days with extreme precipitation derived from a 3000-year historical climate experiment. Interannual variability of extreme precipitation days associated with two specific weather patterns, characterized by a weak low-pressure system and an atmospheric river (AR), significantly correlated with that of SST over the Sea of Japan, with a correlation coefficient of 0.37 and 0.53, respectively. In higher SST years, the increased atmospheric moisture can increase the extreme precipitation in the inland area of Hokkaido for the weather pattern associated with a weak low-pressure system, whereas it appears to enhance orographic precipitation along the western slopes of mountains for the pattern resembling AR. These results indicate that the effect of local SST on extreme precipitation strongly depends on the weather patterns. In the future projection, the two weather patterns that are sensitive to SST over the Sea of Japan show a sharp increase of more than 4 times under the 4 K warming climate. Moreover, the magnitude of extreme precipitation was also found to increase with SST, broadly following the Clausius-Clapeyron relation in both historical and warmed climates. These results suggest increased risk of heavy precipitation associated with such weather patterns over North Japan in the future