Tropical Cyclone Trains

An increase of tropical cyclone (TC) activity has been identified in the North Atlantic basin during the past two decades. During these periods of high TC activity there is a greater probability of having multiple TCs occurring at the same time and even in close proximity of each other. Periods of multiple TC activity occurring close to one another will be referred to as TC trains (TCTs). During preliminary studies, a TCT was subjectively defined as a group of at least three storms in which two of the distances between the three storms were ≤ 2000 km.

The motivation to study the processes that allow TCTs to occur comes from a relative lack of literature about them and obvious evidence of mutually interacting storms within the TCTs based on available satellite imagery. An example of this occurred in 2008 within the TCT of Gustav, Hannah, Ike and Josephine. The outflow of Gustav interacted with Hannah and produced 30 – 40 kt of shear over it.

Case studies were conducted of separate TC trains in the North Atlantic (one in 2004 and the other in 2005). Sea surface temperatures (SSTs), precipitable water (PW), 850-200-hPa wind shear, and 850-hPa zonal winds, among other features, during these periods were investigated and compared in order to determine overall characteristic signatures associated with TCTs. Preliminary results suggest that both TCTs occurred in areas of anomalously high PW and SST environments and the 850-hPa zonal winds showed evidence of anomalous westerlies and shear vorticity in the days leading up to the two sets of TCT development.

To correctly classify a TCT, three different distances between storms (1500, 2000, and 2500 km) were tested in the Atlantic basin in order to see how many TCTs occurred between the years 1979 – 2010. When using a distance of 1500 km between storms, only one TCT was found. Eight TCTs were found when the distance was 2000 km and a total of 20 TCTs were found when using a distance of 2500 km between storms. During 1979 – 2010 there were a total of 382 storms, therefore about 1% of the storms were part of a TCT when using a distance of 1500 km between storms, about 6% when using 2000 km, and about 17% of the total number of storms during these years were part of a TCT when using 2500 km between storms. The distances between storms of the TCTs that were missed when using the 2000 km were closer to the 2500 km limit than the 2000 km limit. Therefore a TCT is now considered to be a group of at least three storms in which two of the distances between the three storms are ≤ 2500 km. The group of storms may spread out further than 2500 km as a whole, but as long as two distances between the three storms are within 2500 km then the group is considered a TC train.

Future work will include investigating where and when the TCTs occur more often in the North Atlantic. After finding the result for the North Atlantic, the same procedures will be followed for the East Pacific, West Pacific, and South Indian Ocean, thus creating a climatology of TCTs. After building a climatology, the TCTs in each ocean basin will be compared. The environmental conditions present during the TCTs will be studied in order to see what conditions allow multiple TCs to develop in each ocean basin and see if there is any difference between basins. The mutual interactions within the TCTs will also be investigated in order to see if these interactions are helping generate new TCs or destroying them.