Tropical cyclone forecast verification - southern hemisphere 2023-24

1. Introduction

A summary of tropical cyclone activity in the Southern Hemisphere for the 2023-24 season is presented below together with an assessment of the performance of the Met Office global model in predicting the tracks of these tropical cyclones.

Tropical cyclones are experienced in the South Indian Ocean, tropical seas to the west, north and east of Australia and the western and central South Pacific Ocean. There are also occasional tropical or subtropical storms in the South Atlantic Ocean. For the purpose of tropical cyclone verification the Southern Hemisphere is divided in to three basins; the South-West Indian (west of 90° E), Australian (east of 90° E) and South Atlantic. Mean error statistics for each basin are presented together with a table of statistics for the whole Southern Hemisphere. The global model produces a seven-day forecast every 12 hours. Verification is performed at 12-hour intervals up to forecast time T+168, although statistics are only presented at 24-hour intervals in this report. 

The global model resolution in operation during the season was 0.140625° x 0.09375° x 70 levels. This is equivalent to a horizontal resolution of 16 km × 10 km at the equator.

Tropical cyclone forecast verification - measures of error

Advisory positions from RSMCs La Réunion and Nadi, Fiji, Bureau of Meteorology, Australia and JTWC Hawaii are used as verifying observations of storm location. Best track data from these centres will be obtained once they become available. Past experience shows that use of best track rather than real time data usually only makes minor differences to seasonal track error statistics. Some mean error statistics for last season are also included for the purposes of a comparison. Forecast tracks are only verified when a depression reaches tropical storm status.

2. Tropical cyclone activity

Basin name abbreviations:
SWI : South-West Indian (west of 90° E)
AUS : Australian (east of 90° E)
SAT : South Atlantic

3. Summary of all southern hemisphere storms

3.1 South-west Indian basin storms

* DPE and skill for all south-west Indian storms for the previous five seasons.

Plot of the observed tracks of all storms in the south-west Indian basin
Forecast positional errors in the south-west Indian basin
Forecast skill in the south-west Indian basin

There were more tropical cyclones than last season, but they were shorter lived resulting in a low number of longer lead time forecasts. Track forecast errors were mostly higher than the five year mean. Skill scores against CLIPER were a little down on recent seasons. There was a slow bias in forecasts. There was a positive (weak) central pressure bias, but not as large as last season. 

3.2 Australian basin storms

* DPE and skill for all Australian storms in the previous five seasons.

Plot of the observed tracks of all storms in the eastern Australian basin
Plot of the observed tracks of all storms in the western Australian basin
Forecast positional errors in the Australian basin
Forecast skill in the Australian basin

Activity was higher than last season, but still lower than recent seasons prior to that. Track forecast errors were mostly low compared to recent seasons. There was a slow bias in forecsts. The central pressure bias grew quickly with lead time (weak bias). 

3.3 South Atlantic basin storms

* DPE and skill for all South Atlantic storms in the previous five seasons.

Plot of the observed tracks of all storms in the South Atlantic basin

There was one fairly short-lived storm in the South Atlantic this season.

3.4 Combined Statistics for whole Southern Hemisphere

* DPE and skill for all southern hemisphere storms in the previous five seasons.

There were more storms in the southern hemisphere than last season, but the total number of verified forecasts was similar.

Track forecast errors were similar to or a little higher than the five year mean. Skill scores against CLIPER were high. There was a slow bias in forecasts.

Forecast positional errors for the whole southern hemisphere
Forecast skill for the whole southern hemisphere
Along-track errors for the whole southern hemisphere
Cross-track errors for the whole southern hemisphere

The five-year running mean of track forecast errors are mostly flat-lining.

Five-year running mean track forecast errors for the whole southern hemisphere

The central pressure bias grew quickly during early lead times. This was indicative of a weak bias in forecasts which has been apparent since a major model change in May 2022. The 10m wind bias was negative also indicating a weak bias in forecast intensity. Mean absolute errors for central pressure were similar to recent years at shorter lead times. For 10m winds the mean absolute errors were lower than last season at short lead times, but grew to similar values at longer lead times.

Central pressure forecast bias for the whole southern hemisphere
Peak 10m wind forecast bias for the whole southern hemisphere

Central pressure forecast mean abosolute error for the whole southern hemisphere
Peak 10m wind forecast mean absolute error for the whole southern hemisphere

Further tropical cyclone information

The Met Office tropical cyclone web pages contain information on tropical cyclone forecasting at the Met Office. Updates of tropical cyclone activity and forecasts are made monthly, together with observed and forecast track information of recent storms, track and intensity prediction error statistics, lists of names and real-time tropical cyclone forecast guidance.

For further information on tropical cyclone forecasting at the Met Office please email us.