Interannual Variations of Mid-Pacific Trough Associated with Asian-Pacific-American Climate

Occurring in the upper-troposphere during boreal summer, the Pacific tropical upper-tropospheric trough (PTUTT) is thought to be an important regulator of the Asian-Pacific-American climate. The PTUTT develops in tandem with the South Asian high and North American ridge, which peaks in July and August. The dominant modes of PTUTT’s variability are explored using the horizontal-wind EOF analysis. The leading mode of variability reflects the deepening and southwest displacement of PTUTT, which presents zonal and meridional teleconnection patterns. The Indian summer monsoon (ISM) tends to reinforce the correlation relationship between ENSO and the leading mode of PTUTT, and vice versa. In non-ENSO years, the ISM could affect the North American climate through the mid-latitude and subtropical routes. In non-ISM years, the ENSO could directly affect the PTUTT by modulating the poleward propagating waves. Besides, the ENSO could also indirectly influence the PTUTT through the Atlantic SSTAs, which subsequently force circulation anomalies over the African-Asian regions. The interannual variations of PTUTT are significantly correlated with the Asian-Pacific Oscillation and the circumglobal teleconnection in mid-to-high latitudes. For convenience, an Asian-Pacific-American index (APAI) is proposed in this study, which reflects the leading mode of variability in the northern subtropics during the boreal summer. In the positive phases of APA, above (below) normal rainfall appears over South Asia (North America). In normal ISM and ENSO years, the APA pattern still shows the zonal and meridional teleconnection patterns that could be related to the land-ocean thermal contrast and the western Pacific warming pool.