In winter (Dec-Feb) and spring (Mar-May), El Niņo affects the atmospheric circulation over the northern part of the South China Sea, bringing generally more rainfall to the coastal region compared to normal condition (Figure 1a and 1b).
Figure 1a Winter (Dec-Feb) rainfall of Hong Kong, 1950-2008.
Figure 1b Spring (Mar-May) rainfall of Hong Kong, 1950-2008.
Temperature and rainfall statistics during El Niņo springs, summers, autumns & winters in recent decades
During El Niņo, there are fewer tropical cyclones developed in April and May and the genesis positions are usually located further east over the western North Pacific compared to normal condition. Hence, tropical cyclones are unlikely to affect the territory before June (Figure 2).
Figure 2 Number of tropical cyclones coming within 500 km of Hong Kong, 1961-2008.
During La Niņa, the northeast monsoon over southern China is generally stronger in autumn (Sep-Nov) and winter (Dec-Feb), bringing lower temperature in Hong Kong compared to normal condition (Figure 3a and 3b).
Figure 3a Autumn (Sep-Nov) temperature of Hong Kong, 1950-2008.
Figure 3b Winter (Dec-Feb) temperature of Hong Kong, 1950-2008.
Temperature and rainfall statistics during La Niņa springs, summers, autumns & winters in recent decades
During La Niņa, tropical cyclones in August-October are likely driven by an anomalous steering flow into the South China Sea and hence more tropical cyclones are likely to affect Hong Kong compared to normal condition (Figure 4).
||Number of tropical cyclones coming within 500 km of Hong Kong in August-October, 1961-2008.
1. Normal condition refers to the situation with neither an El Niņo nor a La Niņa in place.