Evaluation of the Tropical Storm Track Across the Intra-Americas Sea in IPCC AR5 Models and the Mechanisms of Change in a Warmer Climate

The Intra-Americas Sea (IAS) includes the Gulf of Mexico, Caribbean Sea and tropical northeast Pacific Ocean, the latter of which is the most prolific hurricane formation region in the world per square meter. Heavy rains arrive over the IAS during boreal summer, when the Inter- Tropical Convergence Zone (ITCZ), or axis of the tropical storm track, migrates north off the equator and SSTs warm throughout the region. Localized moisture convergence over land areas within the IAS is important for hydropower, agriculture and fresh water supplies. IAS moisture transport into northern Mexico and Southwest U.S. is also important for agriculture and populations in these regions.

Several studies point out the critical role that orography plays in present day mid-latitude and tropical storm tracks. Recent work also suggests that the Caribbean low-level jet (CLLJ) influences storm track activity within the IAS. Studies of tropical storm tracks within the projected warmer conditions of the 21st century find reduced storm track activity in the tropical Atlantic and a shift of the tropical northeast Pacific storm track southward. The intensity of tropical storms overall appears to remain unchanged in studies that have accounted for a mean shift in the tropical mean sea surface pressure due to warmer temperatures. However, predicting storm intensity changes remains a difficult task, as this parameter is more dependent on model resolution than storm frequency.

The following questions are raised by these studies: i) Will the roles of orography and the CLLJ change if the storm track in the tropical eastern Pacific shifts southward in the 21st century? ii) How would such a change affect intensity of storms in the tropical eastern Pacific? iii) How would changes in the position of the tropical eastern Pacific storm track affect the precipitation over land areas of the IAS and NAM regions?

This study proposes to investigate these questions through the following set of analyses:

-Obtain 20th century tropical storm track statistics using state-of-the-art reanalyses.

-Assess tropical storm track statistics of all AR5 model 20th century scenario data available at greater than daily resolution against the reanalyses’ statistics.

-Use high-resolution regional model simulations to assess physical mechanisms associated with several real cases of developing and non-developing tropical depressions within the IAS using the reanalyses as boundary forcing for these simulations.

-Force the regional model for actual cases of 21st century storms/waves from AR5 models that produce realistic track statistics for the 20th century and compare mechanisms of storm initiation and intensification with the cases from step 3. 

This project will make use of a numerical technique in which actual features of the tropical storm track (easterly waves and mature storms) in the AR5 models will be simulated using a high-resolution regional model, rather than using idealized simulations of a mature tropical storm forced with the general conditions of a warmer climate. This approach permits changes in genesis mechanisms to be evaluated.