Fall persistence barrier of sea surface temperature in the South China Sea associated with ENSO

The authors investigate persistence characteristics of sea surface temperature (SST) in the South China Sea (SCS) in association with El Nino-Southern Oscillation (ENSO). It is found that a persistence barrier exists around October and November. This fall persistence barrier (FPB) is well recognized in the developing phase of strong ENSO cases, but becomes vague in weak ENSO and normal (non-ENSO) cases. During a strong El Nino developing year, salient features of the SCS SST anomaly (SSTA) associated with the FPB include a sign reversal between summer and winter and a rapid warming during fall. One possible cause of these SST changes, as well as the occurrence of the FPB, is the development and evolution of a low-level anomalous anticyclone (LAAC). The analyses show that the LAAC emerges in the northern Indian Ocean in early northern fall, moves eastward into the SCS during fall, and eventually anchors in the Philippine Sea in northern winter. This provides a new scenario for the generation of the anomalous Philippine Sea anticyclone previously studied. Its eastward movement appears to result from an east-west asymmetry, relative to the anticyclonic circulation center, of divergent flow and associated atmospheric vertical motion/moisture fields. The eastward passage of the LAAC across the SCS warms the underlying SST first via increased absorption of solar heating in October as it suppresses convective activities in situ, and next via decreased evaporative cooling in November and December as the total wind speed is weakened by the outer flows of the eastward-displacing LAAC. As such, the SCS SST changes quickly from a cold to a warm anomaly during fall, resulting in an abrupt change in anomaly patterns and the occurrence of the FPB. Analyses also suggest that the LAAC development during fall is relatively independent from the preceding Indian summer monsoon and the longitudinal propagation features of the ENSO-related Pacific SSTA. The aforementioned ocean-atmosphere anomalies contain an opposite polarity in a strong La Nina event. The low-level circulation anomaly weakens in intensity during weak ENSO cases and simply disappears during normal cases. As a result, the SCS FPB becomes indiscernible in these cases.