Abstract
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 oceanatmosphere 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 EI Nino-Southern Oscillation (ENSO) events. Its temporal variability is characterized by alternations between the maximum phases of the EI Nino and La Nina events. Its spatial structure is characterized by an elongated positive (negative) SST anomaly over the tropical eastern Pacific during the EI Nino (La Nina) 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 tropical central and eastern Pacific during the developing stage of the EI Nino (La Nina) 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 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 EI Nino 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 EI Nino 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.
Highlights
In the Asian-Pacific region, the most significant interannual variability in the oceanic system is the EI Nifio-La Nifia oscillation
These spatial relationships indicate that the centers of Outgoing Longwave Radiation (OLR) anomalies and tropical vertical motion are more consistent in geographical distribution in the western Pacific than in the central and eastern Pacific
The first type is characterized by the interannual variabil ity associated with variations of the mature phase of the El Nifio-Southern Oscillation (ENSO) events
Summary
In the Asian-Pacific region, the most significant interannual variability in the oceanic system is the EI Nifio-La Nifia oscillation. The above reviews reveal that past studies dealing with the interannual variability of the northern summer ocean-atmosphere system paid particular attention to the relationships be tween the Pacific SSTs, monsoon precipitation, and atmospheric circulation. We analyze the interannual characteristics of the following oceanic and atmospheric fields: SST, tropical cumulus convection inferred from Outgoing Longwave Radiation (OLR), Walker circulation, atmospheric circulation, transient activity, and hydrological processes. Each of these fields is selected for a particular reason. In order to examine the interannual variability phenomenon the interdecadal trend embedded in the 1979-95 northern summer mean fields is removed from the analyses using a least-square-fit method
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