The purpose of this study is to analyze the dynamic and hydrological characteristics of the interannual variability of the northern summer (June-August) ocean-atmosphere system in the Asian-Pacific region. In this ocean atmosphere system, there are two types of interannual variability modes. As indicated by the sea surface temperature (SST) variability, the first type is related to the variations of the mature phase of the El Niño-Southern Oscillation (ENSO) events. Its temporal variability is characterized by alternations between the maximum phases of the El Niño and La Niño events. Its spatial structure is characterized by an elongated positive (negative) SST anomaly over the tropical eastern Pacific during the El Niño (La Niño) event. The second type is related to SST variability between the developing and decaying stages of the ENSO events. This mode is characterized by warm (cold) SST anomalies in the El Niño (La Niño) event, and warm (cold) SST anomalies near the Peruvian coast during the decaying stage.
In accordance with these two types of interannual SST variability, tropical convection and the upward branches of Walker circulation are found enhanced (suppressed) in association with the warm (cold) SST anomalies. The centers of tropical convection anomaly coincide well spatially with the centers of major vertical motion branches and SST anomalies in the tropical western Pacific. The centers are to the west of the centers of vertical motion branches and SST anomalies in the central and eastern Pacific.
In the atmospheric system, the lower-tropospheric circulation anomalies corresponding to the first interannual mode contain spatial structures largely opposite to the climatological mean circulation. These anomalies represent the weaker Asian low and Pacific subtropical high during the El Niño event, which lead to weaker tropical monsoon westerlies and Pacific trade winds. The lower-tropospheric circulation anomalies corresponding to the second interannual mode are characterized by an anomalous low centered in the western Pacific during the developing stage of the El Niño event. This anomalous low later develops into an anomalous high during the decaying stage. For both types of interannual mode, water vapor convergence toward the convection-enhanced region is observed. Such convergence results in an increase in atmospheric water vapor and thus maintains the positive precipitation anomalies. Enhanced precipitation and tropical convection are found embedded in the lower-tropospheric anomalous lows and accompanied by intensified transient activity. Water vapor divergence, negative precipitation anomaly, and weaker transient activity are found for the convection-suppressed region.