The "Not-so-cold" Surge in March 2016
- Monday, 18th April 2016
We may still recall, after the speculation in the social media on the intense cold surge affecting Hong Kong in late January 2016, rumours of another intense cold surge in March were doing the rounds in mid-February, attracting much public and media interest. Based on the information captured on the social media websites, the intense cold surge "forecast" appeared to hinge on two things: (1) most of the ensemble members of the forecasts by US National Centers for Environmental Prediction (NCEP) predicting the Arctic Oscillation Index (AOI) to become negative again (around -2 to -3), with one of the members forecasting AOI to fall below -4 on 2 March; and (2) AOI also falling below -4 about a week before the intense cold surge reaching southern China in late January 2016. Yet, it turned out that the AOI on 2 March was only about -0.6 and the lowest value in early March was around -2 (Figure 1). As for the cold surge predicted, temperatures did drop to 10oC at the Observatory on 10 and 11 March, but nothing too intense as the figure only ranked about 90th among the daily minimum temperatures for March.
Figure 1 Daily AOI (source: NOAA)
The lesson learnt was that it was never too wise to attach undue importance to the "face value" of long range forecasts produced by computer models and to jump to conclusions about the future weather conditions at a specific location such as Hong Kong. NCEP correctly forecast the trend of AOI changing from positive in mid-February to negative in early March. However, most of the ensemble forecast members provided a quantitative forecast that deviated rather significantly from the actual value of AOI in early March. Just like our regular reminder for users of the Observatory's 9-day forecast, uncertainty and forecast errors normally increased as we made prediction further into the future and as such, the forecasts in terms of such long-term trends should be interpreted objectively and used judiciously for maximum benefits.
Online discussion has also revealed some misunderstanding about the linkage between negative AOI and intense cold surge affecting Hong Kong (or other places in southern China). While negative AOI is a good precursor for the likely southward incursion of cold air from the north, to what extent any particular region would be affected depends on the actual trajectory of the cold air and other location-specific contributing factors involved. Typically, it takes around two to three days for the cold air to reach the south China coast, and the minimum temperature associated with a cold surge is normally recorded around two days after surge arrival. Based on daily AOI data (since 1950) from the US National Oceanic and Atmospheric Administration (NOAA), we have identified days in December - March with AOI at or below -4 and looked at the minimum temperatures at the Observatory in the ensuing seven days (denoted by Tm). As shown in the histogram in Figure 2, Tm was at or below 7oC (very cold weather) for only about 7% of the cases and was actually above 12oC (cold weather) for more than 40% of the cases. It can therefore be readily discerned that there is no general rule linking negative AOI with intense cold surge in Hong Kong.
Figure 2 Minimum temperature at the Observatory in the seven days following a day with AOI ≤ -4 (AOI data source: NOAA)
S M Lee