The study of the impact of ENSO and NAO on the rainfall pattern of Hong Kong

Abstract

A study of the impact of large scale atmospheric circulations on the rainfall pattern of Hong Kong has been carried out because rainfall, as a form of major precipitation in Hong Kong, is largely affected by changes in atmospheric circulations. Finding out the possible associated rainfall pattern is beneficial to the prediction of rainfall temporal distribution and hence extreme rainfall events.

The objectives of this study are to identify and examine the possible impact of ENSO as the trade wind factor and NAO as the westerlies factor, on top of the Asian Monsoon system, on the rainfall pattern of Hong Kong. The possible relationship between these large scale atmospheric circulations and the rainfall amount of Hong Kong is examined on monthly and seasonal basis over a 60-year period. Since the effect of the atmospheric circulations is interconnected, the individual and combined effect of ENSO and NAO is also examined. Possible associated rainfall patterns are examined and identified through statistical analysis.

Data has been obtained from the Hong Kong Observatory and the Climate Prediction Center of the National Oceanic and Atmospheric Administration. The relationship between rainfall and the large-scale circulations has been interpreted in relation to the Pearson’s Correlation methods. Particularly, when the large-scale circulations are in their strong or prolonged phases, the resulting rainfall patterns of Hong Kong are evaluated with the time-series plots. Lastly, the possible coupling effect between the large-scale circulations is examined through ANOVA analysis.

It is concluded that statistically the linkage between changes in monthly rainfall anomalies and ENSO or NAO is very weak. The correlation improves a little for seasonal data. The correlation of Spring average rainfall anomalies and ENSO shows a moderate correlation which is the strongest among the four seasons. Nevertheless, the influence of ENSO and NAO on the seasonal rainfall of Hong Kong is considered to be weak and indirect. As figures are compared with a fix time flame over the past 60 years, there may be a ‘cancel-out’ effect during the 60-year period, a delay response of rainfall amount to changes in ENSO and NAO, or impacts from other meteorological factors and hence the weak correlation is considered reasonable.

Study and analyse are focused on the strong El Niño to La Niña years and prolonged NAO positive and negative phases to examine their impacts on the rainfall pattern, as their impacts are anticipated to be more focused which might be able to out-weight the other meteorological factors. By plotting the data in time-series graphs, some rainfall patterns are observed to be associated with the changing conditions between different conditions of ENSO events and during prolonged NAO phases. However, the rainfall patterns observed are not uniform under the strong noise from other meteorological factors.

When it comes to the possible coupling effect between ENSO and NAO on rainfall, no significant interaction effect between the two variables can be identified. However, this may also be largely due to the strong noise of other meteorological factors. Further study in this area is worthwhile.