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Bridging climate science, citizens, and policy

Record High Atlantic SSTs: Active Hurricane Season To Follow?

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Sea surface temperatures across the tropical Atlantic Ocean were higher in March 2010 than any other time in the past 160 years – the UK’s Hadley Center has data going back to 1850.  When temperatures in this region are warmer than average, very active hurricane seasons usually occur.  There are some complex interactions between Atlantic SSTs and other phenomena across the globe, most notably El Niño, so the relationship isn’t exact or direct.  2005 was the last year Atlantic SSTs north of the equator were also significantly warmer than usual.

How much above average were SSTs?  1.26°C above average during March.  That might not sound like it’s too much warmer than usual until you realize the previous record, set back in 1969, was 1.06°C.  This year’s record handily beat that value.  For additional context, the temperatures observed in March are closer to what are normally observed around June.  Finally, the record tied with June 2005 as the biggest positive departure from average in the dataset.

Read on for a more detailed explanation of why this has happened.

So the data geeks have been satisfied.  What about those of us who want to know why these temperatures were recorded?  Is climate change to blame?  It’s actually not the biggest contributor.  Instead, just like northern hemispheric temperatures in the past few months, the extreme Arctic Oscillation conditions shoulder most of the blame.  The Arctic Oscillation was more strongly negative this winter than any time since records on the phenomenon started in 1950.  With strongly negative conditions, the cold air over the Arctic is allowed to pour south, which is why places like the eastern U.S., Europe and northern Russia were colder than normal.  At the same time, the difference in surface pressure between the Icelandic Low and the Azores High decreased.  That caused the trade winds circulating around the High to weaken.  One effect wind has on the oceans is to mix the vertical layers.  Cooler waters exist beneath the warmer surface waters, which are being heated by the sun.  With weaker winds, the mixing decreased, keeping the cooler waters trapped beneath the warming surface.

Now, climate change is warming the oceans.  The vast majority of absorbed heat in the past 150 years has gone to the oceans.  Given their enormous volume, they’ve only warmed a little bit compared to their long-term climatology.  But they have warmed, and one of this year’s weather patterns added even more warming to the surface.  The result is the record high surface temperatures that currently exist.

So U.S. east coast residents might not have experienced the record warmth that most of the rest of the globe did over the winter.  In fact, it was a record-breaking winter in another way for them: record precipitation events.  With a much warmer Atlantic than normal, the east coast could be facing another above-average hurricane season.  The chances of experiencing land-falling hurricanes obviously goes up as the total number per season increases.

Cross-posted at SquareState.


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