Effects of ENSO diversity and cold tongue bias on seasonal prediction of South China late spring rainfall

Abstract

Seasonal prediction of South China April to May rainfall is examined based on forecasts by the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model. El-Nino Southern Oscillation (ENSO) is a major source of predictability, conveyed by the lower tropospheric anomalous western Pacific anticyclone and cyclone for El-Nino and La-Nina respectively. By separating ENSO into its diversity of eastern Pacific (EP) and central Pacific (CP) ENSO, different effects on South China rainfall are revealed. From observations, while rainfall is enhanced following EP El-Nino and reduced following EP La-Nina, rainfall remains close to climatology following CP ENSO. However, the seasonal forecast model predicts CP ENSO effect on South China rainfall to be similar to EP ENSO. The model develops excessive westward extension of the eastern Pacific cold tongue within seasonal timescale. While model predicts tropical central Pacific anomalous sea surface temperature (SST) following CP ENSO realistically, the colder mean state is proposed to weaken the anomalous convection, which subsequently induces bias in the western Pacific anomalous rotational flow and hinders South China rainfall prediction. Meanwhile following EP ENSO, the colder mean state is proposed to strengthen the thermocline feedback, inducing stronger tropical eastern Pacific anomalous SST. While bias in the western Pacific anomalous rotational flow is also induced, the bias is far away from South China so rainfall prediction is still realistic. This study highlights the importance of model mean state in the fidelity of model ENSO diversity teleconnections on seasonal timescale.