Summary of the forecasts

On 1 May 2004, because most ENSO forecasts did not predict 2004 to be an El Niño year, we suggested that ENSO might not be a dominant factor in tropical cyclone (TC) activity over the western North Pacific (WNP) in 2004.  Non-ENSO-related predictors did indicate near-normal activity but most of these predictors gave a near to above-normal number of tropical storms and typhoons.  As a result, we called for a normal number of TCs, but a normal to slightly above normal number of TCs reaching at least tropical storm intensity, as well as typhoons (Table 1).  We also pointed out that the spread of the forecasts among the predictors is the smallest for the typhoon category but larger in the other categories, thus giving higher confidence in the prediction for the number of typhoons.  For the South China Sea (SCS), the spread is again quite large but in general the activity should be normal to below normal, including the number of landfalling TCs.

On 18 June 2004, more predictors, especially those related to ENSO, gave above-normal activity for all the categories.  We thus upgraded our forecasts to normal to above normal for all the categories (Table 1).  Similar increases were seen for the predictions of activity over the SCS.  We therefore, called for normal activity for the SCS as well as the number of landfalling TCs.

Summary and discussion of the predictions

Entire WNP

The TC activity in 2004 over the entire WNP was above normal with 21 typhoons according to JTWC [20 according to the Regional Specialized Meteorological Center (RSMC) in Tokyo - see Table 2], which is the highest since 1997 (Fig. 1 and Table 2).  Disagreements occurred among the warning centres on the intensity of some of the systems.  Four systems (01W, 02W, 05W and 21W) were classified by the Joint Typhoon Warning Center (JTWC) as having reached tropical storm intensity but were not warned by the RSMC.  One short-lived system was warned by the Hong Kong Observatory (HKO) and RSMC as a tropical depression but not by JTWC.  RSMC Tokyo issued tropical storm warning for another system (Merbok) but JTWC did not. The system Omais was classified by JTWC as a typhoon but only a severe tropical storm by RSMC.  Details are listed in Table 2.

Regardless of which dataset we use, the verifications show that our forecasts were correct in all categories (Table 1).  In fact, the observed number of typhoons was even more above normal than our predictions.  Was this due to a possible El Niño event that some centres have suggested to have occurred in late 2004?  The mean location of the TCs that formed in 2004 was east of that for normal years and close to that for all  El Niño years (Fig. 2), which might suggest an influence of El Niño.  However, even if such an event did occur, its intensity should be much below that of 1997 when WNP saw 23 typhoons (Fig. 1).  So, it seems that some other yet unknown factors might be operating to drive the number of typhoons up although the formation location might be related to the influence of El Niño.

South China Sea and landfalling TCs

Tropical cyclone activity over the South China Sea (SCS) was much below normal (see Fig. 3) for the second year, which made our predictions substantially wrong again.  None of the predictors gave numbers as low as those observed.  No TC occurred over the SCS during the months of August to October, which never happened since the record began in 1960.

As discussed in the verification of the 2003 forecasts, the number of TCs that formed outside but moved into the SCS has a significant decrease since 1997.  The year 2004 follows the same trend (Figs. 4 and 5).  This year, the anticyclonic anomaly during July to October was just to the east of Japan but extended way down into the tropics (Fig. 6).  This flow pattern apparently tended to steer any TC that formed within the monsoon trough  northward.  This is also probably the reason why a record number of TCs (10) hit Japan in 2004 (see Fig. 3).  The anomalous circulation over the SCS during these few months was also anticyclonic.

We discussed this problem in the verification last year and attempted to include the trend as one of the predictors.  However, the predicted number from the trend predictor was actually much higher.  Apparently, we have not identified the appropriate predictor(s) for such interdecadal variations.  Attempts will be made again this year to see if we can improve the forecasts.

References

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 2004 using the CSL-4 and CSL-6 schemes.

Table 2. 2004 summary of tropical cyclones over the western North Pacific. 

Table 3. 2004 summary of tropical cyclones over the South China Sea and making landfall along the South China coast.

Fig. 1. Annual number of typhoons between 1960 and 2004.

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 2004 is indicated in red (adapted from Wang and Chan 2002).

Fig. 3. Tracks of tropical cyclones in 2004. Typhoon symbols indicates the formation locations.

Fig. 4. (a) Number of TCs formed over the South China Sea (SCS) and (b) number of TCs formed over the western North Pacific (WNP) and entered the SCS from 1960 to 2004. Red and blue dots indicate El Niño and La Niña years respectively.

Fig. 5. (a) Number of TCs formed in the region east of the Philippines and (b) the percentage of these TCs entering the SCS

Fig. 6. Mean 500-hPa wind (vector) and geopotential height (contour) anomalies between July and October in 2004.  Thick green contour indicates the geopotential height (contour interval = 10 m) ³ 5860 m.