Decadal Modulation of Transbasin Variability on Extended Boreal Summer Tropical Cyclone Activity in the Tropical North Pacific and Atlantic Basins

This study analyzes decadal modulation of transbasin variability (TBV) on extended boreal summer (May-October) tropical cyclone frequency (TCF) over the western North Pacific (WNP), central-eastern North Pacific (CENP), and North Atlantic (NATL) basins. There are distinct decadal regimes (P1: 1979-97, P2: 1998-2008, and P3: 2009-19) with changes in the interannual relationship between TBV and TCF over these three basins. During P1 and P3, there is a significant interannual TBV-TCF relationship over the CENP and NATL, but these relationships become insignificant during P2. Changes in the interannual TBV-TCF relationship over the WNP are opposite to those over the CENP and NATL basins, with significant relationship during P2 but insignificant relationship during P1 and P3. Changes in all three basins coincide with decadal changes in large-scale parameters associated with TBV. Consistent basinwide changes in lower-tropospheric vorticity (vertical wind shear) associated with TBV appear to be largely responsible for changes in total TCF over the NATL (CENP) during P1 and P3. In contrast, a dipole pattern in lower-tropospheric vorticity and vertical wind shear anomalies associated with TBV over the NATL and CENP basins occurs during P2, leading to an insignificant interannual TBV-TCF relationship over the NATL and CENP basins. Over the WNP, a basinwide consistent distribution of lower-tropospheric vorticity associated with TBV is consistent with changes in total TCF during P2, whereas a dipole correlation pattern between TBV-associated factors and TCF during P1 and P3 leads to a weak correlation between TBV and WNP TCF. These three distinct observed decadal regimes may be associated with interactions between ENSO and the Pacific decadal oscillation on decadal time scales.