Atmosphere–Warm Ocean Interaction and Its Impacts on Asian–Australian Monsoon Variation*

Abstract

Asian-Australian monsoon (A-AM) anomalies depend strongly on phases of El Nin ˜o (La Nin ˜a). Based on this distinctive feature, a method of extended singular value decomposition analysis was developed to analyze the changing characteristics of A-AM anomalies during El Nin ˜o (La Nin ˜a) from its development to decay. Two off-equatorial surface anticyclones dominate the A-AM anomalies during an El Nin ˜oone over the south Indian Ocean (SIO) and the other over the western North Pacific (WNP). The SIO anticyclone, which affects climate conditions over the Indian Ocean, eastern Africa, and India, originates during the summer of a growing El Nin ˜o, rapidly reaches its peak intensity in fall, and decays when El Nin ˜o matures. The WNP anticyclone, on the other hand, forms in fall, attains maximum intensity after El Nin ˜o matures, and persists through the subsequent spring and summer, providing a prolonged impact on the WNP and east Asian climate. The monsoon anomalies associated with a La Nin ˜a resemble those during an El Nin ˜o but with cyclonic anomalies. From the development summer to the decay summer of an El Nin ˜o (La Nin ˜a), the anomalous sea level pressure, low-level winds, and vertical motion tend to reverse their signs in the equatorial Indian and western Pacific Oceans (10ЊS-20ЊN, 40Њ-160ЊE). This suggests that the tropospheric biennial oscillation is intimately linked to the turnabouts of El Nin ˜o and La Nin ˜a.The remote El Nin ˜o forcing alone can explain neither the unusual amplification of the SIO anticyclone during a developing El Nin ˜o nor the maintenance of the WNP anticyclone during a decaying El Nin ˜o. The atmosphere-ocean conditions in the two anticyclone regions are similar, namely, a zonal sea surface temperature (SST) dipole with cold water to the east and warm water to the west of the anticyclone center. These conditions result from positive feedback between the anomalous anticyclone and the SST dipole, which intensifies the coupled mode in the SIO during El Nin ˜o growth and maintains the coupled mode in the WNP during El Nin ˜o decay. The interactions in the two anticyclone regions share common wind evaporation/entrainment and cloud-radiation feedback processes but they differ with regard to the oceanic dynamics (vertical and horizontal advection and thermocline adjustment by oceanic waves). The outcome of the interactions in both regions, however, depends crucially on the climatological surface winds. The SIO-coupled mode is triggered by El Nin ˜o-induced subsidence and alongshore winds off the coast of Sumatra. However, other independent El Nin ˜o local and remote forcing can also trigger this coupled mode.The traditional view has regarded SST anomalies in the Indian and western Pacific Oceans as causing the A-AM variability. The present analysis suggests that the SST anomalies in these warm ocean regions are, to a large extent, a result of anomalous monsoons. Thus, the atmosphere-warm ocean interaction may significantly modify the impacts of remote El Nin ˜o forcing and should be ...

Relationship between the tropical Pacific and Indian Ocean sea‐surface temperature and monthly precipitation over the central highlands, Vietnam

“…More SASM rainfall occurs over the Indochina Peninsula during negative SST anomalies in the eastern tropical Pacific (Chen and Yoon, 2000) and vice versa. Warm SST anomalies in the central equatorial eastern Pacific Ocean can strongly suppress the convective activities over the equatorial western Pacific area and subsequently weaken the SASM variation through large-scale circulation (Wang et al, 2003).…”

“…Positive SST anomalies always appear in the Indian Ocean during warm ENSO events, and vice versa, through 'an atmospheric bridge' mechanism (Lau and Nath, 1996;Klein et al, 1999). The dominant inter-annual variation of SST over the Indian Ocean is known as a basin wide warming (cooling), lagging a few months behind a mature warm (cold) phase of ENSO (Chambers et al, 1999;Yu and Rienecker, 1999;Lau and Nath, 2003;Wang et al, 2003). The delayed basin wide warming is remotely forced by SST anomalies in the equatorial eastern Pacific Ocean (Lau and Nath, 2003) and remotely forced winds over the Indian Ocean exciting the down welling Rossby waves and contributing to an off-equatorial warming in the western Indian Ocean (Chambers et al, 1999;Yu and Rienecker, 1999).…”

The tropospheric biennial oscillation in the East Asian monsoon region and its influence on the precipitation in China and large‐scale atmospheric circulation in East Asia

“…Therefore, the anomalous cross-equatorial flows increases the total wind speed that further enhances the surface evaporation, vertical mixing and coastal upwelling there. This positive thermodynamic air-sea feedback (Wang et al, 2000(Wang et al, , 2003 cools the SST over the MC and eastern IO (Figure 12(a)). And an anomalous low-level anticyclone occurs in the southeast Indian Ocean (SEIO) off Sumatra.…”

“…This anomalous anticyclone, together with the anomalous cyclone over the midlatitudes of East Asia (140°E, 35°N), favours the supply of tropical water vapour and its subsequent convergence to the south of the Yangtze River with the mid-and high-latitude water vapour transport ( Figure 13(b)). Furthermore, through the aforementioned positive air-sea feedback (Wang et al, 2000(Wang et al, , 2003, the cold SSTA over eastern IO and the MC develops significantly (Figure 12(b)). This SST cooling further leads to suppression of convection in the MC and a weak Walker circulation over the equatorial Pacific Ocean, and enhances the SEIO anticyclone in the preceding summer (Figure 14(b)).…”

Impact of the MJO on the interannual variation of the Pacific–Japan mode of the East Asian summer monsoon

“…In particular, enhanced/reduced heating over the tropical eastern Indian Ocean favors the positive/negative phase of the PJ-mode. On the other hand, El Niño-Southern Oscillation (ENSO) exerts an influence on the subtropical WNP anticyclonic (during decaying El Nino) or cyclonic (during decaying La Nina) anomaly through its effect on convection anomalies over the tropical Indian and Pacific Oceans (e.g., Wang et al 2000Wang et al , 2003Yang et al 2007;Xie et al 2009Xie et al , 2016. While the connection between the PJ-mode and ENSO is only significant during the period after 1979 (Sun et al 2010;Xie et al 2010;see;Ding et al 2014Ding et al , 2015, for a detailed discussion), even during this period, ENSO only explains 10-20% of the variance of the interannual variability of the PJ-mode, with the highest correlation coefficient around 0.40 (see Figs.…”