Science review

Tropical storm forecast

Seasonal forecasting of tropical storms using the Met Office GloSea5 seasonal forecast system.

Whatever offshore industry you work in, timely and accurate weather forecasts are critical in ensuring safety at sea and minimising operational downtime. While daily and weekly planning may depend on short-term forecasts, seasonal forecasts can be useful in longer-term contingency planning. A new publication by Met Office Senior Scientist, Joanne Camp and co-authors at the Met Office Hadley Centre highlights some of the scientific expertise available.

Tropical cyclones are among the most damaging natural hazards on the planet, causing significant socio-economic impacts. Such storms include Typhoon Haiyan, which resulted in over 6,000 casualties when it struck the Philippines in November 2013 (figure 1).

Typhoon Haiyan Figure 1. Typhoon Haiyan at 2230 UTC on 7 November 2013 as it made landfall in the Philippines. Source: NOAA

Seasonal forecasts of tropical storm activity use the latest science to estimate levels of tropical storm activity ahead of a forthcoming season.

The Met Office has been issuing seasonal forecasts for numbers of tropical storms, hurricanes and Accumulated Cyclone Energy (ACE) index - a measure of the strength and duration of storms during the season - over the North Atlantic basin each year since 2007. These forecasts are freely available on the Met Office website[1].

The forecasts are produced using information from the Met Office's seasonal forecast system, GloSea5, which is a fully coupled ocean-atmosphere General Circulation Model (GCM). The system uses the laws of physics to model the complex physical processes in the ocean and the atmosphere that cause tropical storm and hurricane development. This is then used to create a tropical storm forecast for the following six month period (e.g. figure 2).

Tropical storm forecast Figure 2. Met Office seasonal tropical storm forecast for the North Atlantic basin for June-November 2015. Source: Met Office

The new publication by Joanne Camp and co-authors shows that GloSea5 has significant skill for predictions of tropical storms in the North Atlantic as well as other basins, including the west Pacific and Australian region. In the west Pacific the skill for predictions of tropical storms and ACE index is 0.65 and 0.80, respectively, for forecasts starting each May for the period June-November 1992-2013. In the Australian region, the skill for predicting numbers of storms and ACE index over the period November-April 1996-2009 exceeds 0.6. The high skill is likely due to improved predictions of the El Niño-Southern Oscillation (ENSO) - a large-scale, natural fluctuation of the ocean-atmosphere system centred in the tropical Pacific region - which is known to impact tropical storm activity worldwide.

The study also finds skill for seasonal predictions of tropical storm landfall in the Caribbean region. This is the first time that basin-wide as well as more regional skill has been found for tropical storm predictions using GloSea5. However, there is no skill for predictions of tropical storm landfall along the US coast.

The research highlights the benefit of higher resolution modelling for skilful basin-wide and regional tropical storm predictions. The Met Office is now looking to expand its seasonal tropical storm products beyond the North Atlantic basin and into the west Pacific and Australian region.

[1] Met Office 2015 forecast for the North Atlantic: www.metoffice.gov.uk/weather/tropicalcyclone/seasonal/northatlantic2015.

References:

J. Camp, M. Roberts, C. MacLachlan, E. Wallace, L. Hermanson, A. Brookshaw, A. Arribas, A. A. Scaife. (2015) Seasonal forecasting of tropical storms using the Met Office GloSea5 seasonal forecast system. Quarterly Journal of the Royal Meteorological Society. Available freely online: http://onlinelibrary.wiley.com/doi/10.1002/qj.2516/full

Last updated: 24 July 2015

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