The Met Office Seasonal Forecast for Atlantic Tropical Cyclone Activity in 2023

The Met Office has been issuing seasonal forecasts of tropical cyclone activity in the Atlantic since 2007, based on its seasonal forecast model GloSea. Previous publications have evaluated the skill of the model to predict the variability of Atlantic tropical cyclone activity (Camp et al., 2015: Camp et al., 2018).

GloSea forecasts produced during May 2023 showed a high likelihood of above average tropical cyclone activity in the Atlantic for the for the following six months, despite forecasting the development of an El Niño, which historically acts to supress activity by generating anomalously high wind shear over the tropical North Atlantic. Before issuing a public forecast in late May 2023, further model diagnostics were evaluated to provide an understanding of this unusual forecast. These diagnostics included forecast anomalies of sea-surface temperatures, precipitation, sea-level pressure, vertical wind shear and geopotential height. These all gave support to the forecast of most-likely above average tropical cyclone activity for the season ahead. Based on this evidence the GloSea forecast for Atlantic tropical cyclone activity was issued publicly by the Met Office in late May 2023.

An updated forecast issued in early August reduced the forecast values slightly, but still called for most-likely above average activity across the whole season. In the event observed activity in the June-November season was above the 30-year long term average for tropical storm numbers and accumulated cyclone energy index and near to average for hurricanes and major hurricanes. By all these measures observed activity was well above what would normally be seen in an El Niño year. Observed activity was less than the most-likely GloSea forecast values produced in May, but was still within the 70% forecast ranges for most measures of activity.

This presentation will review the GloSea forecasts for Atlantic tropical cyclone activity, including underlying model diagnostics and how these compared to what was observed across the whole 2023 season.

Camp, J., Roberts, M., MacLachlan, C., Wallace, E., Hermanson, L., Brookshaw, A., Arribas, A., Scaife, A. A., (2015). Seasonal Forecasting of Tropical Storms Using the Met Office GloSea5 Seasonal Forecast System. Q.J.R. Meteorol. Soc, 141: 2206-2219.

Camp, J., Scaife, A. A., Heming, J. (2018). Predictability of the 2017 North Atlantic hurricane season. Atmos Sci Lett. 2018; 19: e813.