Changes to Tropical Eastern North Pacific Intraseasonal Variability
under Global Warming -- Implications for Tropical Cyclogenesis
Abstract
Changes to the eastern North Pacific tropical intraseasonal oscillation
(ISO) at the end of the 21st Century and implications
for tropical cyclone (TC) genesis are examined in the Shared
Socioeconomic Pathways (SSP585) scenario of the Coupled Model
Intercomparison Project phase 6 (CMIP6) data set. Multimodel mean
composite low-level wind and precipitation anomalies associated with the
leading intraseasonal mode indicate that precipitation amplitude
increases while wind amplitude weakens under global warming, consistent
with previous studies for the Indo-Pacific warm pool. The eastern North
Pacific intraseasonal precipitation/wind pattern also tends to shift
southwestward in a warmer climate, associated with weaker positive
precipitation anomalies near the coast of Mexico and Central America
during the enhanced convection/westerly wind phase. Implications for the
modulation of TC genesis by the leading intraseasonal mode are then
explored using an empirical genesis potential index (GPI). In the
historical simulation, GPI shows positive anomalies in the eastern North
Pacific in the convectively enhanced phase of the ISO. The ISO’s
modulation of GPI weakens near the coast of Mexico and Central America
with warming, associated with a southward shift of GPI anomalies.
Further examination of the contribution from individual environmental
variables that enter the GPI shows that relative humidity and vorticity
changes during ISO events weaken positive GPI anomalies near the Mexican
coast with warming and make genesis more favorable to the southwest. The
impact of vertical shear anomaly changes is also to favor genesis away
from the coast. These results suggest a weaker modulation of TCs near
the Mexican Coast by the ISO in a warmer climate.