Issued on 30 Jan 2004
of the forecasts
On 24 April
2003, we predicted that it would be unlikely for tropical cyclone (TC) activity
over the entire WNP in 2003 to be above normal and thus called for a normal to below normal number of tropical
cyclones, tropical cyclones reaching at least tropical storm intensity, as well
as typhoons (Table
1). While this was based on the
average of the statistical forecasts, the predictions were in fact consistent
with the results of previous studies that TC activity is in general below normal
the year after an El Niņo event (Chan 1985,
and 2002 was one such event. On the
other hand, ENSO predictors predicted above-normal number of tropical cyclones
over the South China Sea (SCS).
Past history suggests that during the year after a warm event, TC
activity over the SCS tends to be above normal if the warm event transitions to
a cold event, but varies from below to above normal if the warm event simply
weakens with the transition. Based
assessment of the ENSO condition, we therefore predicted a normal to above-normal number of tropical
cyclones and of tropical cyclones reaching at least tropical cyclone intensity
for the SCS in 2003 (Table
1). As for the number of
landfalling tropical cyclones over South China, the predictors all called for a
normal number (Table
1), which appeared to be reasonable given the predictions for the TC
activity over the SCS.
On 24 June 2003, we updated our forecasts slightly, with the same qualitative predictions. These are based on both the statistical forecasts as well as our assessment and prediction of the ENSO condition at the time.
and discussion of the predictions
Verifications of both sets of predictions showed that indeed over the
entire WNP, the number of TCs and the number of TCs with at least tropical storm
intensity are below normal, as correctly predicted (Table
1). The number of typhoons is
normal, which is still within the error margin of our predictions.
predictions for the SCS are substantially wrong. Activity over this region was actually
below normal while the predictions called for normal to above-normal
of landfalling TCs over South China was correctly predicted within the error
At first, it
appears that the reason for the erroneous forecasts for the TC activity over the
SCS was the following. Most of the
statistical predictors were ENSO-related, and indeed a La Niņa event was
anticipated (see our ENSO discussion on 16 June
2003), but the cold event never developed. The predictions were therefore
incorrect. However, a more detailed
examination of the atmospheric flow conditions indicated that the atmospheric
flow was very much like that of a La Niņa year (see Chan
and Wu 2003). In addition, the
pattern of TC formation locations in 2003 is quite similar to those of previous
La Niņa years, with many more TCs forming west of 140oE (Fig.
1). Indeed, for the late season (Oct
to Dec), the mean position of formation was even further than that of the
average of all La Niņa years (Fig. 2). The problem is that many of
these TCs did not move into the SCS.
Rather, they migrated northward and/or eventually recurved.
To see how frequent this occurs, we examined the numbers of TCs over the SCS formed within the SCS and outside the SCS but moved into the SCS (Fig. 3). It is obvious that a significant decrease in the number of TCs in the latter group occurs from 1997. While part of this decrease was due to a general decrease in the number of TCs formed in the region east of the Philippines (Fig. 4), the percentage of these TCs entering the SCS is also decreasing. The latter apparently results from an anticyclonic anomaly during June to October over the western North Pacific (Fig. 5) that tends to steer TCs northward. The anomalous circulation over the SCS during these few months is also anticyclonic. Thus, the erroneous predictions of the 2003 TC activity over the SCS are apparently due to an interdecadal variations in the strength of the subtropical high, a factor that so far has not been incorporated into the prediction scheme. We will attempt to include this in the predictions for 2004. Interdecadal variations shown in Figs. 3-5 will also be investigated.
Chan, J. C. L., 1985: Tropical cyclone activity in the northwest Pacific in relation to the El Niņo/Southern Oscillation. Mon. Wea. Rev., 133, 599-606. Abstract
Chan, J. C. L., 2000: Tropical cyclone activity over the western North Pacific associated with El Nio and La Nia Events. J. Climate, 13, 2960-2972. Abstract
Chan, J. C. L. and M. C. Wu, 2003: Outlook for the 2003/04 East Asia winter monsoon. Joint Meeting on Seasonal Prediction of the 2003/04 East Asia Winter Monsoon, Japan Meteorological Agency, Nov 11-13, Tokyo. Download
Wang, B. and J. C. L. Chan, 2002: How strong ENSO events affect tropical storm activity over the western North Pacific. J. Climate, 15, 1643-1658. Abstract
Table 1. Forecasts of TC activity in 2003 using the CSL-4 and CSL-6 schemes.
Western North Pacific
|No. of TCs||29 3||30 3||28||31|
|No. of TCs with at least tropical storm intensity||26 3||26 3||23||27|
|No. of typhoons||16 2||15 2||17||17|
South China Sea
|No. of TCs||14 2||15 2||9||13|
|No. of TCs with at least tropical storm intensity||11 2||12 2||7||10|
|No. of TCs making landfall along the South China coast||5 1||4 1||4||5|
Table 2. 2003 summary of tropical cyclones over the western North Pacific.
Western North Pacific (including South China Sea)
|Tropical cyclones||Tropical cyclones with at least tropical storm intensity||Tropical cyclones with typhoon intensity|
03W and 27W were considered as having TS intensity by JTWC but no name was
given by RSMC Tokyo|
# Tropical storm Jimena formed over the eastern North Pacific and entered the western North Pacific as a tropical depression
(issued in April)
|29 3||26 3||16 2|
(issued in June)
|30 3||26 3||15 2|
Table 3. 2003 summary of tropical cyclones over the South China Sea and making landfall along the South China coast.
South China Sea
|Tropical cyclones||Tropical cyclones with at least tropical storm intensity||Tropical cyclones making landfall along the South China coast|
(issued in April)
|14 2||11 2||5 1|
(issued in June)
|15 2||12 2||5 1|
Fig. 1. Tracks of tropical cyclones in 2003. Typhoon symbols indicates the formation locations.
Fig. 2. Averaged locations of tropical storm formation during (a) JAS, (b) OND, and (c) Jul-Dec of each year from 1965 to 1999. Heavy squares and solids denote strong warm and strong cold years, respectively. The mean locations of TS formation for each of the five categories of SST anomalies are marked by large-size symbols as shown in (c). The averaged location of TS formation for 2003 is indicated in green (adapted from Wang and Chan 2002).
Fig. 3. (a) Number of tropical cyclones formed over the South China Sea (SCS) and (b) number of tropical cyclones formed over the western North Pacific (WNP) and entered the SCS from 1960 to 2003. Red and blue dots indicate El Niņo and La Niņa years respectively.
Fig. 4. (a) Number of TCs formed in the region east of the Philippines and (b) the percentage of these TCs entering the SCS
Fig. 5. Mean 500-hPa wind (vector) and geopotential height (contour) anomalies between June and October in the years 1998-2003.