Hurricanes and long-term climate variability – Lloyds Risk Report

Hurricanes and long-term climate variability – Lloyds Risk Report

The severity and frequency of hurricanes are influenced by changes in the climate of the
Atlantic over the long term, and in particular, the changes over many decades of sea
surface temperatures. This is called Atlantic Multi-decadal Variability (AMV).

This variability of sea surface temperatures occurs naturally1, but is also caused by external factors, such as
volcanic eruptions and changes in man-made emissions.

Warm sea surface temperatures are a pre-condition for tropical disturbances to become
hurricanes, but atmospheric conditions are also important.

The atmosphere must have favourable winds and a seed of circular motion, be unstable to convection2 and
humid enough to make it likely that hurricanes will form. Forecasts and analyses based on the atmospheric
state, as well as Atlantic sea surface temperatures, are likely to be more accurate than those that are not.

Atlantic Multi-decadal Variability of sea surface temperatures is likely to influence
hurricane landfall regionally.

An observed increase in total hurricane activity during positive Atlantic Multi-decadal Variability (AMV) years
is likely to increase the incidence of hurricanes making landfall. It may be possible to see regional differences
in landfall risk between different AMV phases. These differences may also depend on the strength of the
storm. To achieve greater certainty, further research is required to improve the understanding of the
relationship between landfall risks and Atlantic, Pacific and global climate variability.

Some of the most damaging hurricanes may be modulated by eastern Atlantic variability –
this is linked to the Atlantic Meridional Mode (AMM).

As the AMM includes both ocean and atmosphere variability, it can be useful as an indicator of hurricane
activity. Hurricanes that form in the eastern tropical Atlantic are probably those that are most influenced by
the AMM. As these hurricanes travel the furthest before making US landfall, they also have more time to
strengthen their winds and gain moisture

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