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What Caused Storm Ophelia?

25 October 2017

Was the exceptional strength of ex-hurricane Ophelia when it hit Ireland on the 16th of October 2017 due to global warming or was it a chance occurrence resulting from natural variability?

Hurricane Ophelia formed in the region of the Azores some days before it reached Ireland. Its track over the period 7th – 17th October can be seen here. A global map of sea surface temperature for the 16th of October (see here) shows a large pool of warm water with surface temperatures up to about 2°C above normal lying along Ophelia’s trajectory.

One of the principal factors determining the strength of a hurricane is the value of the sea surface temperature along its path. Temperatures warmer than normal allow the storm to gather excess energy from the sea and to convert this into stronger wind speeds. Conditions were ideal for Ophelia to gather strength as it moved northward from the area of the Azores across warm seas towards Ireland in the days before the 16th of October.

So was this pool of exceptionally warm water along Ophelia’s path due to global warming?

Evidence that it was not can be seen by referring to the map of sea surface temperature three months earlier (here). This shows that, at that time, the area in question was covered by water cooler than normal, with anomalies of up to -2°C over much of the area. Local changes in sea surface temperature of this magnitude occur regularly on monthly time scales due to natural variability.

Sea surface temperature increases associated with global warming, on the other hand, is a gradual global-scale process occurring on a time scale of decades to centuries. A graph of the monthly-mean values of global-mean sea surface temperature from 1850 to the present illustrates this. The increase in the global-mean values in the period from the 1940s to the present, which is the period when the effect of greenhouse gases due to human activities has been most significant, amounts to about 0.4°C. This is only a tenth of the naturally occurring variation in sea surface temperature seen in the Azores-to-Ireland region in the 3 months prior to Storm Ophelia

This indicates that, insofar as the influence of the sea surface temperature is concerned, the exceptional strength of Storm Ophelia was due to natural variability.

Authors: Professor Ray Bates (UCD) and Ray McGrath (UCD)

*The Royal Irish Academy thanks the authors of this blog post, both of whom are members of the RIA Climate Change and Environmental Sciences Committee. Blog post content and statements are proprietary to the authors. Each author represents only themself and their own opinion.

 

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