Mr. Anthony G. Barnston (Climate Prediction Center/NCEP/NWS/NOAA)
Dr. Lloyd Shapiro (Colorado State University)
Dr. M. Neil Ward (IMGA-CNR/Italy)
OBJECTIVE OF THE WORK
Use of objective techniques to better elucidate the role of tropical Pacific SSTs (e.g., ENSO) in Atlantic tropical cyclone fluctuations.
It has long been accepted that there is a strong relationship between the variations in El Niño/Southern Oscillation (ENSO) and tropical cyclone activity in the Atlantic basin. The sense of the relationship is that during an El Niño (La Niña) event (i.e., anomalously warm (cold) SSTs in the equatorial central and eastern Pacific), Atlantic tropical cyclone activity is reduced (enhanced). This relationship is one of the key elements used in seasonal and multi-year forecasting of Atlantic activity. Previous work by some of the investigators has shown that the primary physical mechanism that governs the relationship are ENSO-driven teleconnections that modify the vertical shear in the deep tropics of the Atlantic basin. An El Niño (La Niña) event is generally associated with increased (decreased) vertical shear which acts to suppress (enhance) tropical cyclone activity. Little work has been done, however, in examining the actual spatial and temporal scales of Pacific tropical SST anomalies that are most closely associated with Atlantic-basin tropical cyclone activity. In addition, other investigators have described certain non-homogeneous features of the teleconnections that govern these relationships that would suggest that a stratification of the years is needed to better resolve the different physical mechanisms.
Current work has focused on several methods of temporal stratification of the sample. The primary divisions being stratification based on a certain threshold ENSO-related SST anomaly value, and based on tropic-wide and non-tropic-wide modes (also called 'FIT' and 'NOT FIT') developed by Ward et al. (1994). Correlations and partial correlations are done between annual tropical cyclone data for the Atlantic basin, various measures of ENSO, and monthly/seasonal wind fields (upper- and lower-level winds and vertical shear). To examine spatial and temporal scales, various Atlantic variables (tropical cyclone activity and winds) are correlated at various lags with GFDL-produced SST fields.
We continue to examine the physical mechanisms that govern these relationships. The results of this work strongly suggest that there are certain years (i.e., NOT FIT years) in which there is little or no relationship between ENSO and Atlantic hurricane activity. If the character of a particular year could be determined before the peak portion of the Atlantic season, the seasonal forecast models could be adjusted to use or not to use the ENSO information for their predictions. Much work remains to more completely understand these relationships.