Relationship between cross-equatorial flows over the Bay of Bengal and Australia in boreal summer: Role of tropical diabatic heating

Abstract

The interannual variability of cross-equatorial flows (CEFs) over the Asian–Australian monsoon (AAM) region during boreal summer was analyzed by applying the empirical orthogonal function (EOF) method to the meridional wind at 925 hPa. The first mode (EOF1) exhibits an in-phase relationship among different CEF channels over the AAM region, which has received much attention owing to its tight linkage with ENSO. By contrast, the second mode (EOF2) possesses an out-of-phase relationship between the Bay of Bengal (BOB) CEF (90°E) and Australian CEF, among which the New Guinea CEF near 150°E shows the most significant opposite correlation with the BOB CEF. Observational and numerical model results suggest that the equatorially asymmetric heat source (sink) over the western (eastern) Maritime Continent, closely associated with the in-situ sea surface temperature anomaly, can induce cross-equatorial northerly (southerly) flow into the heating hemisphere, which dominates the out-of-phase relationship between the BOB and New Guinea CEFs. Furthermore, an equatorially symmetric heating over the central Pacific may indirectly change the CEFs by modulating the zonal atmospheric circulation near the Maritime Continent.摘要本文通过对1979–2017年夏季925 hPa经向风异常进行经验正交函数(EOF)分解, 研究了亚澳季风区内越赤道气流的年际变化特征. 结果表明, 越赤道气流的第一模态表现为亚澳季风区内不同通道间的同相变化, 即一致加强或减弱; 第二模态表现为孟加拉湾和澳大利亚越赤道气流的反相变化, 其中新几内亚和孟加拉湾越赤道气流的反相变化最显著. 该反相模态分别对应着赤道以南地区海洋性大陆东西两侧的海温异常, 其纬向偶极型的空间分布直接导致海洋性大陆西部(东部)出现沿赤道非对称的异常热汇(热源), 它可以激发跨越赤道的南风(北风)异常, 从而有利于该反相模态的出现.