Historical and Projected Intra-Americas Sea TD-Wave Activity in Select IPCC AR5 Models

The storm track across in the Intra-Americas Sea (IAS) is an important feature of the region, bringing steady rainfall to Central American countries and seeds the majority of tropical cyclones in the region. The storm track consists primarily of easterly waves or “TD-type” disturbances in the easterlies, which originate over West Africa and propagate westward along the southern edge of the North Atlantic subtropical high and into the Caribbean and Gulf of Mexico. A subset of these disturbances also appears to cross into the East Pacific basin, but local Pacific sources of TD-type disturbances have also been investigated. Previous studies have found a southward shift in the tropical storm track across the IAS region in future climate projection scenarios under the Inter-governmental Panel on Climate Change (IPCC) Assessment Report 4 (AR4). A shift in geographic location of TD-type disturbances is potentially important to cyclogenesis in this region through its impact on the interactions of these disturbances with the background vertical wind shear and orography. Here, we investigate the existence of this shift in the most recent climate model projections from the IPCC (AR5) using model historical and RCP 4.5 and 8.5 experiments from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). We additionally investigate environmental factors contributing to the projected shift in the storm track across the region.

Our results suggest that the latest IPCC models also project a southward shift of the tropical storm track across the IAS but with no indication of a change in the strength of the disturbances across the region consistent with the AR4 studies. The shift in the storm track is consistent with a weakening of the Walker circulation across the Pacific and increased wind shear over the IAS both in the West Atlantic and Gulf of Mexico, as noted by previous studies, as well as over the East Pacific. While historical biases in storm track location are well correlated with sea surface temperature biases in the models, the southward shift of the tropical storm track appears to be in partial response to the enhanced wind shear over the region. The potential implications of this shift for tropical storm formation and the rainfall climatology of IAS land areas are discussed.