The Impacts Of Intraseasonal Oscillations Over The Subtropical Pacific On The East Asian Summer Rainfall

The relationships between the precipitation of Eastern Asia (20° -45°N, 110°-135°E) and the 30-60-day intraseasonal oscillations (ISO) over the boreal Pacificduring the summer and its mechanism are studied in the paper. The daily wind andheight fields of NCEP/NCAR reanalysis data and the 24h precipitation data of China687 stations during 1958-2002, and the pentad precipitation of CMAP/NOAA from 1979to 2004, are all analyzed by the Space-time filter analysis. The study resultsare presented as follows:(1) The paper discovers that the zonal propagating directions of the meridionalwind over subtropical North Pacific are reverse with the seasonal shift fromsummer to winter, and then the seasonal variability calculated from monthlyCMAP Precipitation Data for the Northeastern Pacific (160.0W-115.0W,22.5-45.0) is a considerable and significant value, and varies against theseasonal variability of Eastern Asia (110.0-140.0E, 22.5-45.0N). Moreover thezonal wind also presents the reverse changes over the subtropical NorthPacific in turn between summer and winter so the zonally seasonal oscillationsexist between the Eastern Asia and the northeastern Pacific and the climateof the northeastern Pacific may be able to call the unusual monsoon climate.The paper makes clear the dynamic reasons of the unusual monsoon climateemploying the theories of the exchanges between westerly and easterly angularmomentum and the mean vertical circulation of Asian-Pacific region. Inadditional to the similar climate also exists in the region from the easternAtlantic to Mediterranean Sea, the thermal causes of the zonal distributionof the traditional and unusual monsoon in the subtropics of the NorthernHemisphere are probably the "quadruplet heating pattern" in summers, and"reversed quadruplet heating pattern" in winter.(2) The proportion for the meridional wind ISO in the subtropical atmosphere isstrikingly bigger than the one in tropics, which is distinct from the ISO ofthe zonal wind and height. The analysis results, from every drought and floodsummer for 4 different regions of East Asia respectively during 1958-2000,showed that the precipitation amount in the East Asian summer monsoon regionis well correlated to the westward propagation of 30-60-day ISO via the northcentral-eastern Pacific, and depends little on the intensity changes of theEast Asian summer monsoon. And the westward ISO is usually the low-frequencycyclones and anticyclones from the Bay of Alaska in northeastern Pacific andthe Okhotsk in the northwestern Pacific of mid-high latitudes, and the ISOevolution in subtropical easterlies. In mid-high latitude the phenomena arerelated to the westward propagating mid-ocean trough and block high backingoff. Therefore the westward propagating ISO from the north central-easternPacific to East Asia are indispensable for sufficient rainfall occurring inEast Asia in summer, which is resulted from long-wave adjustment process inthe mid-high latitudes and ISO evolving in tropical easterlies.(3) It is found that the mainly relative atmospheric angular momentum (AAM) ofeasterly wind distributes in the low-level troposphere while the main AAM ofwesterly wind distributes in the mid-high levels troposphere from Eastern Asiato the northeastern Pacific. And the easterly AAM convert into the westerlyAAM through the ascending airflow in the subtropical western Pacific whilethe reverse process takes place by the falling airflow in the northeasternPacific. Then it is found that the interannual anomaly mechanism for the zonalpropagations of the meridional wind ISO at 850hPa pressure level over thesubtropical North Pacific(about the south of 30°N) is quite different fromthe one over mid-latitude North Pacific. In the subtropical North Pacific,the strongly (weakly) interannual changes for the westwards propagations ofthe meridional wind ISO are caused by the strongly (weakly) interannualchanges for the relative AAM conversion occurred in the western Pacific (theone through the ascending airflow), and the cross-spectrum analyses showedthat the meridional wind ISO is lagging about 1-18d behind the one of therelative AAM in the subtropics from Eastern Asia to the northwestern Pacific;while the causes of the interannual anomalies for the zonal propagations ISOover the mid-latitude North Pacific are closely interconnected with thelong-wave adjustment process in the mid-high latitudes, e.g. the westwardpropagating mid-ocean trough and block high backing off, etc.(4) The anomalies of the easterly and westerly AAM took on the teleconnectionpattern, i .e. the western Pacific pattern (WP) and the Pacific North-Americapattern (PNA), and the easterly AAM transferring to westerly AAM wouldoccurred easily at negative SLP zones(heating regions) for the region fromthe eastern Asia to the mid-Pacific. And then the convections and westwardpropagations of ISO were enhanced and maintained by the AAM conversions inthe western Pacific, while the strong convections always happened to theoverlapped regions of negative SLP anomalies and the edge of westerly-AAManomalies center. On the contrary, the strong convections and activewestward propagations of ISO would be not occurred without the anomaliesconversions of the easterly AAM to the westerly AAM from the eastern Asiato the mid Pacific. Then E-P fluxes no latitudinal mean is analyzed alongthe path of the teleconnection pattern, which helps to confirm theaforementioned conclusions again.