Tropical East Pacific Storm Track Statistics in Select IPCC AR5 Models: Historical and RCP 4.5 Projections

This study examines how well select CMIP5 models represent the tropical storm track across the Intra-Americas Sea. We then examine future changes in the storm track position and strength for the RCP 4.5 warming scenario. A shift in the geographic location of the storm track is potentially important to cyclogenesis in this region through interactions with the background vertical wind shear and orography. The tracking method relies on 6-hourly 850hPa relative vorticity and primarily identifies westward moving disturbances such as easterly waves, however tropical cyclones are not specifically removed from the analysis. In all seven CMIP5 models are examined. The multi-model mean track density is in good agreement with ERA-Interim reanalyses, however significant differences are seen with the individual models both in terms of the geographic location and magnitude. The ERA-Interim maximum in track strength in the East Pacific lies along the west coast of Mexico north of the ITCZ and is well captured by the multi-model mean, although it is somewhat stronger than the reanalyses. On the other hand, the multi-model mean strength in the Gulf of Mexico and West Atlantic along the east coast of the US is strongly underestimated compared to ERA-Interim. And unlike for track density, these biases are fairly consistent among the models. The multi-model mean track density for the RCP 4.5 simulations suggest a southward shift in the tropical storm track in the East Pacific moving the tracks away from the west coast of Mexico, with no overall significant change to the mean strength. Such a shift was also seen in the previous generation of global models for the A1B warming scenario. The next step in this study is to examine the impact of this shift in the tropical storm track on East Pacific easterly wave activity and cyclogenesis in dynamically downscaled CMIP5 fields that better capture both orographic influences and convective intensity.