The North Atlantic subtropical high and tropical cyclone tracks

We examine the relationship between North Atlantic tropical cyclone (TC) tracks and the large-scale steering flow using best-track data for the 1950 to 2007 hurricane seasons. Limiting the cyclones to the main development region (MDR), we categorize each cyclone into one of three groups according to their track type. Straight moving (SM) cyclones are defined as those which enter the Gulf of Mexico, recurving landfall (RCL) cyclones threaten the east coast of the United States, and recurving ocean (RCO) cyclones stay within the North Atlantic never threatening land. Preliminary results suggest an influence of climatological events, such as El Niño, on the frequency of different track types. Although a common forecasting tool for the TC track is the strength and location of the subtropical high, little research has been done to demonstrate the high's direct influence on the steering flow. Using NCEP-NCAR Reanalysis data, the characteristics of the subtropical high were categorized by track type. A westward extension and strengthening of the subtropical high is observed for SM cyclones, and a receding and weakening of the subtropical high occurs for RCO cyclones. The influence of the subtropical high is further diagnosed using the Beta and Advection Model (BAM) to generate synthetic tracks by uniformly seeding the MDR and steering storms using reanalysis winds. A parallel set of calculations is performed using climate model simulations of the circulation changes in response to increased CO₂ to evaluate the potential impact of anthropogenically-forced changes in the characteristics of the subtropical high on TC tracks. Results from these analyses will be presented at the meeting.