ARE WE GET­TING MORE HUR­RI­CANES?

Dr Dann Mitchell, cli­mate sci­en­tist at the Uni­ver­sity of Bris­tol

Focus-Science and Technology - - Weather -

In the At­lantic basin, the hur­ri­cane sea­son nor­mally lasts from June to Novem­ber, peak­ing in Septem­ber. The fact that we’ve seen eight hur­ri­canes so far this year [as of 25 Septem­ber] is markedly out of the or­di­nary – the av­er­age per sea­son is six. How­ever, per­haps more pro­found is the sheer num­ber of strong hur­ri­canes: this year was the first time in recorded US his­tory that three Cat­e­gory 4+ hur­ri­canes had made land­fall in a sin­gle sea­son. But what­ever the re­main­der of the sea­son brings, it’s un­likely to be as ac­tive as the fa­mous 2005 sea­son. This saw Ka­t­rina, one of 15 hur­ri­canes that year, dev­as­tate the US coast­line and left much of New Or­leans un­der­wa­ter.

Is there a link with cli­mate change?

We know that a warmer at­mos­phere can hold more rain than a cooler at­mos­phere, so the down­pour over Texas dur­ing Hur­ri­cane Har­vey may have been wors­ened by cli­mate change. We also know that hur­ri­canes get their en­ergy from the heat in oceans, so warmer oceans are the­o­rised to lead to stronger hur­ri­canes, although not nec­es­sar­ily to a change in the num­ber of storms. But it’s too early to say for sure whether the strong hur­ri­canes we’ve been see­ing re­cently are a re­sult of cli­mate change. To know this, we need to see a trend over a longer pe­riod, so time will tell. We’re more sure, how­ever, that cli­mate change will worsen the im­pact of any storm surges caused by a hur­ri­cane’s strong winds. With sea lev­els ris­ing, storm surges can make it fur­ther in­land, po­ten­tially lead­ing to more wide­spread flood­ing.

Are we get­ting bet­ter at pre­dict­ing hur­ri­canes?

The hur­ri­cane path is per­haps the most im­por­tant part of a forecast be­cause you need to pre­pare when a hur­ri­cane’s on its way. This is es­pe­cially true if the sys­tem is trav­el­ling over low- ly­ing is­lands, where storm surges can be dev­as­tat­ing. Due to these risks, the US has in­vested heav­ily in pre­dic­tive re­search, and now a rea­son­able forecast of a hur­ri­cane’s path can be made three to five days ahead of time. A day be­fore the hur­ri­cane hits, we can forecast its cen­tre to within about 64km (40 miles). It was re­mark­able how close to the forecast Hur­ri­cane Irma came, and this un­doubt­edly saved lives.

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