The Role of the Central American Monsoon in the Seasonal Variability of Tropical Cyclone Activity in the Western Hemisphere

ABSTRACT The Central American Monsoon (CAM) is closely linked to the seasonal migration of the Intertropical Convergence Zone (ITCZ) during summer, and its onset is correlated with tropical cyclone (TC) activity in the western hemisphere, comprising the northern tropical eastern Pacific and the tropical Atlantic ocean basins. We find that an early (late) CAM onset is generally associated with reduced (enhanced) May–November TC activity in the eastern Pacific, but with increased (reduced) activity in the eastern Caribbean and western tropical North Atlantic. Similarly, variations in CAM onset are connected to changes in tropical easterly wave (TEW) activity, as identified from multiple reanalysis datasets. These relationships appear to be mediated through concurrent anomalies in mid‐level relative humidity, vertical wind shear, sea surface temperatures (in the eastern Pacific) and North Atlantic subtropical high. We also evaluate the teleconnections of the Oceanic Niño Index (ONI), which represents ENSO variability, with seasonal TC activity in both basins. While the ONI exhibits a stronger and more direct influence on TC variability than CAM onset, the onset variations provide an important additional source of predictability. CAM onset leads the seasonal TC activity in the western hemisphere by 15–30 days, unlike ENSO, whose teleconnections are largely contemporaneous. Notably, the association between CAM onset and ONI is weak, suggesting that these drivers act partly independently. Our results indicate that CAM onset timing can modulate ENSO‐related teleconnections, either reinforcing or offsetting them depending on ENSO phase. This highlights the role of CAM onset as a precursor for seasonal TC variability in the western hemisphere.