2007 Forecasts

Updated Prediction of Seasonal Tropical Cyclone Activity over the Western North Pacific for 2007

This is an update of the predictions of the annual number of tropical cyclones (TCs) affecting the western North Pacific (WNP) for 2007 that we issued on 23 April 2007. These updates are made based on new information for the months of April and May 2007.

The ENSO condition is currently in a neutral status. In May, SST anomalies remain near-zero in the central equatorial Pacific but below normal in the eastern equatorial Pacific and becoming increasingly negative. The Niño3.4 index in May becomes negative (-0.19), the first time this year. A summary of the various ENSO model forecasts from different climate centres suggests that all of them predict a cool condition in 2 to 7 months time (Table. 1). All the 6 forecasts suggest a possible cold event throughout the summer and fall. Based on these results, it appears that 2007 will likely be a La Niña year. An analysis of the TC activity in the past years with La Niña events suggests that the TC season in 2007 tends to be normal or below-normal, which is consistent with our forecast.

For the total number of TCs, the index of the western extent of the subtropical high (HWNP) gives a predicted number of 28 compared with 30 in the April forecast (cf. Tables 2a and 2b). No change occurs using the Niño4 predictor. The final predicted number therefore decreases from 28 to 27.

For the number of tropical storms and typhoons, the predictions from the index of the strength of the west Pacific index and the Niño3.4 index give decreases in TC number especially for the west Pacific index. The final predicted number therefore decreases from 25 to 24.

No significant change is found on the predicted number of typhoons. All the predictors give similar numbers as compared with the April forecast and therefore the final predicted number is still 14.

With these changes, the average predictions from all the potential predictors therefore call for a below-normal year for all TC categories for the entire WNP (See Table 2b).

The predictor related to the subtropical high (HWNP) continues to suggest a below-normal TC activity. The subtropical high between April and May is even stronger (Fig. 1), resulting in the decreases of the predicted numbers in all categories.

*Entire western North Pacific*
All TC	Below-normal
Tropical storms and typhoons	Below-normal
Typhoons	Below-normal

Entire western North Pacific

All TC
	HWNP	NINO4	Final forecast	Normal
Prediction	30	27	28	31
Weight	0.70	0.73

Tropical storms and typhoons
	HWNP	WP	NINO3.4	Final forecast	Normal
Prediction	24	26	24	25	27
Weight	0.73	0.62	0.67

Typhoons
	HWNP	NINO3.4	ESOI	Final forecast	Normal
Prediction	14	13	16	14	17
Weight	0.60	0.77	0.68

Entire western North Pacific

All TC
	HWNP	NINO4	Final forecast	Normal
Prediction	28	27	27	31
Weight	0.68	0.73

Tropical storms and typhoons
	HWNP	WP	NINO3.4	Final forecast	Normal
Prediction	24	25	23	24	27
Weight	0.70	0.56	0.62

Typhoons
	HWNP	WP	NINO3.4	ESOI	Final forecast	Normal
Prediction	14	14	13	16	14	17
Weight	0.61	0.67	0.75	0.68

HWNP	Index of the westward extent of the subtropical high over the western North Pacific
HIB	Index of the strength of the India-Burma trough (15^o-20^oN, 80^o-120^oE)
WP	Primary mode of low-frequency variability over the North Pacific
NINO3.4	Sea surface temperature (SST) anomalies in the NINO3.4 region (5^oS-5^oN, 170^o-120^oW)
NINO4	Sea surface temperature (SST) anomalies in the NINO4 region (5^oS-5^oN, 160^oE-150^oW)
ESOI	Equatorial Southern Oscillation Index (Equatorial SOI) Equatorial Eastern Pacific SLP - Indonesia SLP (standardized anomalies)

MODEL / GROUP	Forecast Start Date	2-4 MONTHS (Jul to Sep)	5-7 MONTHS (Oct to Dec)
POAMA (run at Bureau of Met)	29/04-28/05	Cool	Cool
System 3 ECMWF	01 May	Cool	Cool #
GloSea UK Met Office	01 May	Cool	Not Available
CSF NCEP	02/05-21/05	Cool	Cool
CGCMv1 GMAO/NASA	May	Cool	Neutral
JMA-CGCM02 Japan Met. Agency	April	Cool	Cool #
# Model forecast is run only until the end of November