Inter-Annual Variability of African Easterly Waves in the NCAR Nested Regional Climate Model

Nearly 60% of tropical cyclone (TC) developments in the North Atlantic basin occur in conjunction with African easterly waves (AEWs). Tropical cyclone developments may also occur in association with cyclonic gyres in the Caribbean Sea, remnant frontal zones that penetrate into lower latitudes, organized mesoscale convective systems that originate over land, and Rossby wave breaking in subtropical and middle latitudes. Since much of the variability in TC activity in the North Atlantic is driven by AEW variability, and the most intense Category 4 and 5 TCs tend to originate from AEWs, the aim of this presentation is to examine the interannual variability of AEWs in the NCAR Nested Regional Climate Model (NRCM).

The NRCM is based on the Weather Research and Forecasting Model nested into version 3 of the Community Climate System Model (CCSM) run under the A2 scenario. The CCSM global data are used to drive a NRCM 36-km domain, using one-way nesting, that encompasses a region that stretches from near Hawaii to the Middle East and from the central Amazon to the southern tip of Greenland. The model is run for three periods: a base climate period of 1995–2005, and two future periods of 2020–2030 and 2045–2055. The frequency and variability of AEWs was measured by implementing the Thorncroft and Hodges tracking scheme that identifies and tracks 700 hPa cyclonic vorticity centers.

The results showed a weak increase in TC activity in 2020-2030 and 2045-2055 compared to 1995-2005 that was driven entirely by an increase in TCs that originated from AEWs. Examination of the interannual variability of AEWs shows that the relatively fewer AEWs during 1995-2005 were due to three extremely inactive years in which there was a 75% decrease in the number of AEWs. The interannual variability in AEWs was linked to the (i) strength and persistence of the African easterly jet (AEJ) and waveguide, and (ii) organization and intensity of the AEW-triggering convection upstream over Darfur. During the inactive AEW years, the AEJ was weaker and retracted toward the west and convection was more infrequent over Darfur compared to active years. The weaker AEJ and reduced upstream convection over Darfur contributed to increased difficulty in triggering and subsequently maintaining AEWs via barotropic/baroclinic instability. The structure of the AEJ and upstream convection was modulated on intraseasonal and interannual timescales by the structure of the subtropical anticyclone over the Sahara and the midlatitude storm track.