Verification of Forecasts of Tropical Cyclone
Issued on 06 Jan 2004
of the forecasts
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
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,
the number of landfalling TCs.
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
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.
and discussion of the predictions
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
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.
China Sea and landfalling TCs
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.
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
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
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.
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.
Western North Pacific
|No. of TCs||31||33||32||31|
|No. of TCs with at least tropical storm intensity||29||29||31||27|
|No. of typhoons||18||19||21||17|
South China Sea
|No. of TCs||13||13||6||13|
|No. of TCs with at least tropical storm intensity||11||11||6||10|
|No. of TCs making landfall along the South China coast||5||5||1||5|
Table 2. 2004 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|
|JTWC||All Centres||JTWC||RSMC Tokyo||JTWC||RSMC Tokyo|
(issued in April)
(issued in June)
Table 3. 2004 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|
|JTWC||RSMC Tokyo||JTWC||RSMC Tokyo||JTWC||RSMC Tokyo|
(issued in April)
(issued in June)
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.