What Controls Early or Late Onset of Tropical North Atlantic Hurricane Season

The occurrence of first hurricane in early summer signifies the onset of an active Atlantic hurricane season.The interannual variation of this hurricane onset date is examined for the period 1979-2013.It is found that the onset date has a marked interannual variation.The standard deviation of the interannual variation of the onset day is 17.5 days,with the climatological mean onset happening on July 23.A diagnosis of tropical cyclone（TC） genesis potential index（GPI） indicates that the major difference between an early and a late onset group lies in the maximum potential intensity（MPI）.A further diagnosis of the MPI shows that it is primarily controlled by the local SST anomaly（SSTA）.Besides the SSTA,vertical shear and mid-tropospheric relative humidity anomalies also contribute significantly to the GPI difference between the early and late onset groups.It is found that the anomalous warm（cold） SST over the tropical Atlantic,while uncorrected with the Nino3 index,persists from the preceding winter to concurrent summer in the early（late） onset group.The net surface heat flux anomaly always tends to damp the SSTA,which suggests that ocean dynamics may play a role in maintaining the SSTA in the tropical Atlantic.The SSTA pattern with a maximum center in northeastern tropical Atlantic appears responsible for generating the observed wind and moisture anomalies over the main TC development region.A further study is needed to understand the initiation mechanism of the SSTA in the Atlantic.