Study On The Regional Mean Anomalous Meiyu Onset And Meridional Seesaw-Like Distribution Of The Meiyu Rainfall

The monthly precipitation data at 160 stations over China and Meiyu datasets in the middleand lower reaches of the Yangtze River valley (MLYRV) issued by National MeteorologicalInformation Center of China were utilized to construct a regional Meiyu onset index (RMOI)near the Yangtze River. Based on the EOF analysis of the June-July precipitation anomalies, thispaper revealed the meridional seesaw-like distribution of the Meiyu rainfall, which previousStudies paid little attention to. Then both the air-sea background singularities were investigatedfor the typical years with anomalous Meiyu onset or the meridional asymmetry of the Meiyurainfall, on the basis of the ERA-40 reanalysis datasets provided by the ECWMF and NCEP1reanalysis datasets as well as the Extended Reconstructed Sea Surface Temperature (ERSST)data supplied by the NOAA. Besides, we also offered some precursory signals.(1) Ocean-atmospheric anomalies for the early/late Meiyu onsetThis paper constructed the RMOI by which the early or late Meiyu onset years were foundout. Then partial correlation between the RMOI, Meiyu duration and the total rainfall during theMeiyu period shows that the Meiyu onset date is well correlated with the duration, while has littledirect relation with the total rainfall that is dominated by the Meiyu duration. Namely, it is theMeiyu length variability that connects the variations of the onset date and total precipitation. InJune, on average, in the lower troposphere the meridional temperature gradient is much moreapparent over the MLYRV in the early than late onset years, with the Meiyu front lies more northin the former case than in the latter. Viewed from the atmospheric circulation pattern, the earlyonset years are often accompanied in the upper troposphere by the extended South Asia High,westerly jets more northward, western Pacific subtropical high more north- and westward, EastAsian monsoon and the Walker circulation stronger in comparison to the late onset years. Andviewed from sea surface temperature (SST) field for the early onset years, SST is lower in thecentral and eastern Pacific near the equator, maybe in association with the cool phase of ENSO,and considerably higher in the subtropical western North Pacific than the late onset years.In addition, there are probably some precursory signals for the early (late) Meiyu onset years,with SST being higher (lower) with respect to the mean around the western Pacific warm pool,the Intertropical Convergence Zone being more (less) active and the local Hadley circulation being stronger (weaker) in previous boreal winter. In northern spring, the convection is probablystronger (weaker) about the Philippines; the North Pacific SST at lower latitudes might bewarmer (cooler) from previous April to May; SST appears higher (lower) over the waters aroundAustralia in proceeding boreal winter and spring.(2) Meridional seesaw-like distribution of the Meiyu rainfallAlthough Meiyu rainfall has its in-phase spatial variability over the MLYRV in most years,it is distributed in some years like a seesaw to the north and south of the Changjiang River,namely, "northern drought and southern flood (NDSF)" and "northern flood and southern drought(NFSD)", when the precipitation tends to be nearly normal throughout the valley, which wouldinevitably increase difficulties of making short-term prediction of the rainfall. For this reason, theauthors investigated ocean-atmospheric characteristics when these two anomalous types occur.The results show that in the NDSF years, the low-level frontal area and moisture convergencecenter lie more southward, accompanied by weaker subtropical summer monsoon over East Asia,with the western Pacific subtropical high and 200 hPa South Asia high being more southward.Both the Northern and Southern Hemisphere Annular Modes are stronger than normal inpreceding February; SST is higher off China during boreal winter and spring and the oppositehappens in the NFSD years. Also, this seesaw-form Meiyu rainfall distribution might be affectedto some degree by the previous ENSO event.