西太平洋暖池对流三类显著月际变化及其成因

基于1979～2018年观测的向外长波辐射（outgoing longwave radiation, OLR）资料和其他多种再分析资料，发现西太平洋暖池对流存在3类显著的月际变化。第一类为OLR在6月和8月为负异常而7月为正异常；第二类与第一类完全相反；第三类为OLR在6～7月为正异常，8月为负异常。3类月际变化与ENSO循环的背景有关，前两类发生在较弱的La Ni?a年和El Ni?o发展年，与春季暖池海温异常有关。当前一个月海温偏高时，后一个月对流偏强，造成局地海温降低，偏低的海温又反过来抑制了后一个月的对流发展，因此暖池地区局地海气相互作用在这两类月际变化中起到关键作用。与前两类不同的是，第三类月际变化发生在El Ni?o衰减年，与春季热带印度洋海温偏高有关。热带印度洋海温偏高造成印度附近对流在6～7月间增强，通过东传Kelvin波抑制了暖池对流发展。同时，印度附近对流偏强造成8月印度洋海温降低和对流减弱，对暖池对流的影响因而减弱。另一方面，6～7月暖池对流偏弱造成8月暖池海温升高，结果造成暖池对流增强。因此，第三类月际变化受到热带印度洋强迫以及暖池地区局地海气相互作用的共同影响。

Three Categories of Significant Inter-monthly Variations of the Warm Pool Convection in the Western Pacific and the Related Physical Mechanism

On the basis of the observed outgoing longwave radiation (OLR) data and other reanalysis datasets during 1979–2018, three categories of significant inter-monthly variations of the warm pool convection in the western Pacific are identified. The first category shows a negative OLR anomaly in June and August and a positive OLR anomaly in July. By contrast, the second category shows an opposite OLR anomaly to the first category. Meanwhile, the third category shows a positive OLR anomaly in June and July and a negative OLR anomaly in August. All categories of inter-monthly variations are related to the ENSO background. The first and second categories occur in relatively weak La Ni?a years and El Ni?o developing years, which are closely associated with sea surface temperature (SST) anomaly in spring over the warm pool. When the SST is high in the preceding month, convection in the succeeding month is enhanced along with a reduced SST. Consequently, when the SST is low in the preceding month, convection in the succeeding month is suppressed along with an enhanced SST. The local air–sea interaction in the warm pool plays a key role in the first and second categories. Different from the two other categories, the third category occurs in El Ni?o decaying years, which is related to a high SST in spring over the tropical Indian Ocean. During June and July, convection near India is enhanced because of the high SST in the tropical Indian Ocean. Through the excitation of a Kelvin wave propagating eastward, convection in the warm pool is suppressed. In the meantime, the enhanced convection near India reduces the local SST and suppresses convection in August when the influence from the Indian Ocean on the warm pool convection is considerably weakened. By contrast, the warm pool SST in August tends to increase because of suppressed convection in June and July. As a result, the warm pool convection is enhanced in August. Therefore, the third category results from the combined effects of tropical Indian Ocean forcing and local air–sea interaction in the warm pool.