North Atlantic tropical storms forecast

Background

'Tropical cyclone' is the generic term for a low pressure system over tropical or sub-tropical waters, with intense convective activity (e.g. thunderstorms) and winds circulating in an anti-clockwise direction in the northern hemisphere (clockwise in the southern hemisphere). A tropical storm is a tropical cyclone with mean wind speeds of at least 39 m.p.h. The terms hurricane and typhoon are region-specific names for strong tropical cyclones with wind speeds more than 73 m.p.h.

The number of tropical storms forming over the North Atlantic is influenced by sea surface temperatures (SST), over both the tropical North Atlantic, and the tropical Pacific. The table below shows numbers of storms in recent years.

Method

At the start of every North Atlantic season (which usually runs from June to November) the Met Office forecasts the number of tropical storms expected throughout the period. This is done using a dynamical seasonal prediction model (called GloSea) which simulates the ocean-atmosphere processes and interactions that determine tropical storm development. Many GloSea forecasts are made (using 'ensemble' forecasting methods) to allow estimation of the range of likely outcomes. In contrast, statistical prediction methods, which have traditionally formed the basis of most published predictions, do not model atmospheric processes, but rely on past relationships between storm numbers and preceding observed conditions (e.g. pre-season SST patterns).

Because the dynamical model grid does not fully resolve relatively small features such as tropical storms, an adjustment is made using storm counts from past forecasts and observations. The GloSea forecast implicitly includes predictions of the sea-surface temperature (SST) anomalies. This season GloSea predicts a warming trend in SST in the tropical North Atlantic, which favours above-average activity. However, continued warming in the tropical east and central Pacific is also predicted. Warm SST in this region tends to suppress tropical storm activity in the North Atlantic. Overall, more tropical storms are predicted this season than seen in the last two seasons (see table above). The competing influences of forecast SST in the tropical Pacific and North Atlantic are consistent with the wide 70% range in this year’s forecast.

Recent studies have shown that GloSea and other European models have considerable skill predicting the number of tropical storms - for example successfully predicting the change from the exceptionally active season of 2005 to the below-normal activity of the 2006 season. Last year the Met Office forecast was for 10 storms with a 70% probability range of 7-13; in the event, 12 storms were observed to occur.

The forecast has been produced following research collaborations with the European Centre for Medium-Range Weather Forecasts (ECMWF).