The Interannual Seesaw Between The Somali And The Australian Cross-equatorial Flow And Its Connection To East Asian Climate

The correlations among the summer low-level cross-equatorial flows (CEFs) over the Indian-west Pacific Ocean region on the interannual timescale are investigated by using both the NCEP/NCAR and ERA40 reanalysis datasets, China 160-station precipitation and Kaplan SST. A significant negative correlation (seesaw) has been illustrated between the Somali CEF and the three CEFs north to Australia (the South China Sea, the Celebes Sea and the New Guinea They are referred to as the Australian CEF in combination) in both reanalyses. A seesaw index is thus defined with a higher (lower) value representing the intensified (weakened) Somali CEF but the weakened (intensified) Australian CEF. The connections of the CEF-seesaw with East Asian summer monsoon (EASM) and with the sea surface temperature anomalies (SSTAs) are investigated. Then the interdecadal shift of the CEF-seesaw relation has been discussed. At last, the CMIP5 model dataset is used to investigate the suggestions derive from the reanalysis datasets, results are shown as following:(1) An enhanced seesaw corresponds to the intensified EASM with more rainfall in North China, the Yellow River valley and the upper reach of the Yangtze River. The higher-(lower-) CEF seesaw index year is characterized by the north-westward (south-eastward) movement of Western Pacific Subtropical High (WPSH) and the strengthened (weakened) South Asia High (SAH). Particularly, the seesaw essentially reflects the opposite co-variability between the two atmospheric action centers in the southern hemisphere, the Mascarene and the Australian subtropical high. During the higher- (lower-) seesaw index years, the Mascarene high moves westward (eastward), along with increased (decreased) southwest monsoon. Meanwhile, the Australian high weakens (strengthens) along with the easterly (westerly) anomalies prevailing over the tropical western Pacific to the South China Sea. All these favor an enhancing EASM.(2) The higher (lower) CEF seesaw tends to happen in the developing phase of La Nina (El Nino) and the negative Indian Ocean Dipole (IOD) episode. Then whether the CEF-seesaw is influenced by El Nino-Southern Oscillation (ENSO) or IOD is analyzed. The results keep unchanged when the ENSO-related or IOD-related signals are excluded, although ENSO exerts a significant influence.(3) The CEF-seesaw relation has undergone a significant interdecadal shift around the late 1960s. Moreover, the interdecadal shift of seesaw-connection may be modulated by Atlantic Multidecadal Oscillation (AMO) and the variation of ENSO variability.(4) Most CIMP5 models can simulate the seesaw connection between the Somali and the Australian CEF, but show a poor performance in describing the time series of seesaw index. However, the simulated seesaw variability in the multi-model ensemble (MME) is consistence with the previous study using the reanalysis datasets.