Revisiting the Tropical Cyclone—Easterly Wave Relationship on Interannual Time Scales

Previous studies that have examined the covarability between the frequency of easterly waves and North Atlantic tropical cyclones (NATL TCs) on interannual time scales have revealed no statistically significant relationship (Thorncroft and Hodges 2001; Hopsch et al. 2007). These findings are reconsidered using a new easterly wave tracking algorithm based on 700 hPa curvature vorticity anomalies and has been applied to four global reanalyses including the CFS-R, ERA-Interim, ERA-40, and NCEP/NCAR Reanalysis. From the reanalysis-derived easterly wave climatologies, a moderately positive and statistically significant relationship is seen with tropical Atlantic TCs, suggesting that approximately 20–30% of the total variance in the number of TCs on interannual time scales may be explained by the frequency of easterly waves. In relation to large-scale climate modes, the Atlantic Multidecadal Oscillation (AMO) and Atlantic Meridional Mode (AMM) exhibit the strongest positive covariability with Atlantic easterly wave frequency. Besides changes in the number of easterly waves, the intensification efficiency of easterly waves, which is the percentage of waves that induce North Atlantic TC formation, has also been evaluated. These findings offer a plausible physical explanation for the recent increase in the number of NATL TCs, as it has been concomitant with an increasing trend in both the number of tropical Atlantic easterly waves and intensification efficiency. In addition, the easterly wave–tropical cyclone pathway is likely an important mechanism governing how the AMO and AMM modulate North Atlantic TC frequency—more so than previously thought (e.g., Thorncroft and Hodges 2001, Hopsch et al. 2007, Kossin and Vimont 2007).