Subseasonal to Interannual Variability of Tropical Cyclone Activity in the Eastern Pacific. Towards a Comprehensive Forecasting System with Significant Lead Time

Julien Boucharel, University of Hawaii at Manoa, Meteorology, Honolulu, HI, United States, Fei-Fei Jin, University of Hawaii at Manoa, Honolulu, HI, United States, I-I Lin, National Taiwan University, Department of Atmospheric Sciences, Taipei, Taiwan, Matthew H England, University of New South Wales, Climate Change Research Centre, Sydney, NSW, Australia and Hsiao-Ching Huang, NTU National Taiwan University, Taipei, Taiwan
Abstract:
The Eastern Pacific (EPac) is the second most active region in terms of Tropical Cyclone (TC) activity, and yet environmental factors regulating this activity on subseasonal to interannual timescales are poorly documented. Here, we present a fundamental mechanism related to the El Niño Southern Oscillation (ENSO), the dominant mode of interannual variability in the Tropics, as one of the main driver of TC intensity in the EPac. It operates through meridional redistribution of ocean subsurface heat following an El Niño towards the EPac cyclogenesis region, consistently with the well-known recharge-discharge (RD) mechanism. We show that this process plays a major role during Eastern Pacific (EP) ENSO events, which have a stronger influence in the EPac but remains only marginal during Central Pacific (CP) El Niño. CP events are characterized by a meridional heat discharge in the center of the basin, mostly directed to the south of the Equator, far from the EPac cyclogenesis region. In contrast, the altered atmospheric circulation, in particular the reduction of vertical wind shear is more influential in controlling TC activity during these CP events. Although every El Niño is quite different, the RD mechanism overall accounts for a significant part of the interannual variability of TC intensity in the EPac and sheds light on the previously overlooked subsurface ocean dynamics as a major driver of hurricane activity.
We further explore the relationship between the EPac TC activity and the oceanic variability associated with timescales shorter than ENSO’s. In particular, we highlight the role of intraseasonal equatorial Kelvin waves and subsequent coastal-trapped and reflected Rossby waves as major provider of subsurface heat to the EPac. This oceanic process, well monitored by satellite altimetry, has the ability to enhance the EPac hurricane forecasting skills, almost up to bi-monthly prediction of TC activity and with a lead-time up to 6 months.
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