DIFFERENT EVOLUTIONS OF THE PHILIPPINE SEA ANTICYCLONE FOR THE IMPACT OF EL NIÑO IN PEAK PHASES WITH AND WITHOUT A POSITIVE INDIAN OCEAN DIPOLE

The different impacts of El Niño during peak phases with and without a positive Indian Ocean Dipole (P-IOD) on the Northwest Pacific circulation were studied. The authors focused on the Northwest Pacific circulation features in the mature phase of El Niño from September to February of the next year. Composite maps and simulations demonstrate that the atmospheric circulation under the impact of El Niño with and without P-IOD exhibits large differences in temporal evolution and intensity. In single El Niño (SE) years without a P-IOD, an anomalous low-level anticyclonic circulation around the Philippines (PSAC) is instigated by the single El Niño-induced Indonesian subsidence. However, during the years when El Niño and a P-IOD matured simultaneously, a much greater anomalous subsidence over the western Pacific and the Maritime Continent occurred. The PSAC tends to occur earlier, is much stronger and has a longer lifetime than that during SE. More importantly, the PSAC shows a characteristic of an eastward movement from the southern South China Sea (SCS) to the Philippine Sea. This characteristic does not appear during SE. These patterns imply that a positive IOD event tends to exert a prominent influence on the PSAC during El Niño events and there is a combined impact of El Niño and P-IOD on the development of the PSAC.