Hurricanes and climate modes -- 8-wave simulation with 3 members in 2005.
Interannual variations in seasonal tropical cyclone (TC) activity (e.g., genesis frequency and location, track pattern, and landfall) over the Atlantic are explored by employing observationally-constrained simulations with the NASA Goddard Earth Observing System version (GEOS-5) atmospheric general circulation model. The climate modes investigated are El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Atlantic Meridional Mode (AMM).
The results show that the NAO and AMM can strongly modify and even oppose the well-known ENSO impacts (i.e., La Niña (negative Niño index) acts to drive a strong TC season while El Niño (positive Niño index) acts to drive a weak TC season), like in 2005, when a strong positive phase of the AMM associated with warm sea surface temperature (SST) and a negative sea level pressure (SLP) anomaly over the western tropical Atlantic, led to a very active TC season with enhanced TC genesis over the Caribbean Sea and a number of landfalls over North America, under a neutral ENSO condition. On the other end, the weak TC activity despite the weak negative Niño index during 2013 appears caused by a NAO-induced positive SLP anomaly with enhanced vertical wind shear over the tropical North Atlantic. During 2010 characterized by a strong La Niña phase, the combined impact of the three modes produced positive SST anomalies across the entire low-latitudinal Atlantic and a weaker subtropical high, leading to more early recurvers and thus fewer landfalls despite enhanced TC genesis. The study provides evidence that TC number and track are very sensitive to the relative phases and intensities of these three modes, and not just to ENSO alone. Examination of seasonal predictability reveals that predictive skill of the three modes is limited over tropics to sub-tropics, with the AMM having the highest predictability over the North Atlantic, followed by ENSO and NAO.