El Niño and Atlantic Tropical Cyclones

El Niño and Atlantic Tropical Cyclones

Mr. Stan Goldenberg

Mr. Anthony G. Barnston (Climate Prediction Center/NCEP/NWS/NOAA)
Dr. Lloyd Shapiro (Colorado State University)

Dr. M. Neil Ward (IMGA-CNR/Italy)


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.


  1. One project has been completed and two others are ongoing. The completed project with regards to the primary temporal and spatial scales governing the relationships (in collaboration with Anthony Barnston) established that the primary region of SSTs in the equatorial Pacific correlated with Atlantic hurricane activity (see Figure) is similar to the Niņo 3.4 region (5°N to 5°S, 120-170°W). This Niño 3.4 region straddles the Niño 3 and Niño 4 regions. The 3.4 index has now been added to the list of various indices published monthly by NOAA's Climate Prediction Center (CPC). Based on the results of this project, Bill Gray and colleagues have recently substituted the 3.4 index for the Niño 3 and Southern Oscillation indices in some of their seasonal forecasting work. Temporally, in the strongest teleconnection, the Niño 3.4 SST anomalies lead the Atlantic hurricane activity by about one to two months.

  2. The first ongoing project (in collaboration with Lloyd Shapiro and Neil Ward) continues to examine the effect of temporal stratification on the various ENSO-Atlantic hurricane relationships. It has already been shown that :

    1. the correlations are much stronger for El Niño events than for La Niņa events
    2. the correlations are only statistically significant for the FIT years.

    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.

  3. The second ongoing project is simply to continue to collect data for the various Niņo indices, hurricane activity and various other related parameters, and examine the interrelationships season by season. A major El Niño event commenced in Spring of 1997 and its effects on the 1997 Atlantic hurricane season were noted. As would be expected, the event acted to greatly inhibit tropical cyclone development particularly in the deep tropics.