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 Niña years and El Niño 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 Niño 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.