Leading Modes Of The Low-level Circulation Anomalies Over Tropical Pacific And Their Associations With The East Asian Atmospheric Circulation

Using global reanalysis datasets such as the monthly mean NCEP circulation, NOAA sea surface temperature (SST) and so on from 1958 to 2013, the spatio-temporal variation of 10m wind over Tropical Pacific (10°S-10°N,100°E-180°-120°W) and their associations with the atmospheric circulation and climate anomaly on interannual and decadal time scale over East Asia are investigated, with the methods of multivariate empirical orthogonal decomposition (MV-EOF), correlation analysis, regression analysis, composite analysis, harmonic analysis and so on.(1) The time series of 10m zonal wind over tropical western Pacific show obvious inerannual and decadal variability, and the trade winds enhanced after the clear sudden decadal change around 1999.(2) There exsit two leading modes of 10m wind anomalies over Tropical Pacific from 1958-2001. The first mode, which is named "ENSO mode" as it corresponds with the wind anomalies during ENSO warm or cold phase, shows consistent westerly anomalies over western-central Tropical Pacific. The second mode, however, exhibits a characteristic of prominent anticyclonic circulation over western-north Pacific and significant westerly anomalies south of Equator at about 5°S, which corresponds with wind anomalies during ENSO transition phase and thus go by the name of "ENSO transition mode". The two corresponding time series both have a feature of significant interannual variability and seasonal phase locking, but their dominated oscillation periods and relationships with ENSO regime vary. Further analysis shows that persistent remarkable anticyclonic circulation will be set up to the south of Baikal (SBH) when the two-modes-like wind anomalies occur. The connection paths between the two modes and SBH differ from each other. "ENSO mode", which is characterized with abnormal dipole-Walker Circulation forced by warm water in east Tropical Pacific, may be affected by SBH through the anomalous northerly wind and South China Sea trough during its development period. SBH is linked with large-scale vertical motion over mid-east Tropical Pacific and abnormal convergence motion over Southeast China. "ENSO transition mode", which has a feature of abnormal quadrupole-Walker Circulation forced by warm water over west Tropical Pacific-east Tropical Indian Ocean basically and over Tropical Atlantic Ocean secondly, the corresponding SBH may be mainly related to the stronger meridional Hadley Circulation over west Tropical Pacific-east Tropical Indian Ocean. Moreover, the two modes can influence climate anomaly in China remarkably. As far as the rainfall is concerned, there exits less than normal precipitation in eastern China before the "ENSO mode" positive phase. Inflenced by the anticyclone over western-north Pacific, the rainfall in southeast China begins to increase gradually as "ENSO mode" positive phase developing. The positive phase of "ENSO transition mode" corresponds the shift of rainfall anomaly from "dipole" to "tripole" distribution.(3) The spatio-temporal variation of 10m wind is different on different decadal background. From 1999 to 2013, compared with the period discussed before, the consistent westerly anomalies over tropical Pacific of "ENSO mode" turn westard, and the easterly anomalies over the eastern tropical Pacific enhance a lot, and the westerly anomalies south of Equator as well as the prominent anticyclonic circulation over western-north Pacific of "ENSO transition mode" moves westward too. But their relationships with the ENSO phase and the roles they play in ENSO cycle is similar with the former period. Though more Centre-Pacific type of El Nino is observed in the the latter period, there also exsit zonal wind anomalies abruptly shifting southward during the peak of ENSO events, but the position of this meridional wind movement heads west evidently. Further analysis show that the associations between two modes and the atmospheric circulation over East Asia become feeble under this new backgound. In some areas, the obtained relationships between "ENSO transition mode" and atmospheric circulation are almost opposite to the ones got in 1958-2001. The weakened associations are considered to have something to do with the transformation of the El Nino type.(4) Both "ENSO mode" and "ENSO transition mode" have notable interdecadal variability during the period from 1958 to 2013. Power spectrum analysis indicate that the two modes have a cycle of 11a and 14a respectively. They are also tightly linked with the SST anomalies in tropical Pacific. El Nino-like background pattern decadal change is prominent when "ENSO mode" is in its positive phase. However, during "ENSO transition mode" positive phase, SST in central Pacific maybe cooler, but tends to be warmer in west and east Pacific. Composite analysis show that the intensification of El Nino has something to do with the "ENSO mode" decadal phase, but no obvious relationship is detected in terms of the intensification of La Nina and the occurrence frequency of ENSO events. What’s more, remarkable anticyclonic circulation to the south of Baikal is set up, and northerly anomalies are strong in the north of China when "ENSO mode" is in its positive phase. When "ENSO transition mode" is in its positive phase, there is an obvious cyclonic circulation over the east area of Lake Baikal and an prominent anticyclonic circulation near Lake Balkhash, therefore, the northern part of China is controlled by westnortherly anomalies. Further analysis show that maintainess of negative phase of Atlantic Mutidecadal Oscillation (AMO) is propitious to the setup of anticyclonic circulation to the south of Baikal, and then this anticyclonic circulation can lead the transformation of "ENSO mode" via the northerly anomalies over East Asia.