Structure, Propagation and Growth of Caribbean Easterly Waves: What Do They Teach Us about Tropical Atmospheric Dynamics?

Caribbean easterly waves (CEWs) propagate in an environment that is distinct from that of other easterly waves since it exhibits substantial westerly vertical wind shear. In spite of this distinction, their structure, propagation and growth have not received much attention. A linear regression analysis reveals that these systems exhibit features consistent with moisture modes that are destabilized by moisture-vortex instability. They exhibit large moisture fluctuations, are in WTG balance, and MSE growth is partly driven by meridional mean MSE advection by the anomalous winds. However, its circulation tilts vertically against the mean shear, a feature that is often associated with baroclinic instability. To reconcile these differences, a linear stability analysis employing a moist two-layer model is performed using a basic state that resembles the Caribbean Sea during boreal summer. The unstable wave solution from this analysis exhibits a structure that resembles observed CEWs. Excluding the upper troposphere from the stability analysis has little impact on the propagation and growth of the wave, and its circulation still exhibits a westward tilt in height. Thus, baroclinic instability is not the main growth mechanism in CEWs despite their structural similarity to baroclinic waves. Instead, the instability is largely rooted in how the lower tropospheric circulation interacts with water vapor, as expected from moisture mode theory. These results suggest that tilting against the shear should not be used as the sole diagnostic for baroclinic instability, and that baroclinic instability is unlikely to be a primary driver of growth for most tropical weather systems.