Coolest Main Development Region for North Atlantic Hurricanes Since 1994

Discussion: The tropical North Atlantic (TNA) index for May 2018 was -0.63 which is the coolest monthly value since 1994 when the months of JAN, FEB and MAR were -0.59, -0.61 and -0.60. The last month cooler than May 2018 was in May 1989 (-0.66). The TNA index is a sea surface temperature anomaly (SSTA) measurement across the tropical and lower subtropical latitudes of the North Atlantic basin between the Caribbean Sea and northwest Africa Coast. The TNA measurement zone also matches the main development region (MDR) for North Atlantic hurricanes.

Since 1950 ALL cool regimes in this region have occurred within the 1966-1994 long-term cool phase of the Atlantic multi-decadal oscillation (AMO). The AMO is a decadal ocean/climate cycle identifying periods of persistent cool or warm basin-wide regimes (compared to the long-term average) that influence North America and Europe climate plus North Atlantic tropical cyclone activity. Climate Impact Company has observed that if AMO is sufficiently intense the effect on climate stretches beyond North America and Europe and can (at times) outweigh the climate influence of El Nino southern oscillation (ENSO). TNA is the tropical component of this North Atlantic basin-wide mode of climate variability.

The May 2018 TNA index (-0.63) is approximately -2.5 St Dev. Past months of May since 1950 when TNA index was <-2 St Dev have occurred in 1974, 1975, 1976, 1985, 1986 and 1989 all within the 1966-1994 cool AMO cycle. Tropical cyclone activity that followed in each of these cool TNA years averaged of 9.7 tropical storms, 5.8 hurricanes and 2.0 intense hurricanes. The average accumulated cyclone energy (ACE) index was 79 (114 is the 30-year normal). Clearly, the cool TNA index implies diminished seasonal activity across the North Atlantic basin averaging below the 50-year, 30-year and 15-year climatology (Table 1).

Summary: The tropical and lower subtropical North Atlantic basin in-between the Caribbean Sea and northwest coast of Africa has cooled (compared to normal) significantly during spring 2018. A representative index measuring the sea surface temperature anomalies in this region is the tropical North Atlantic (TNA) index. During May, the TNA index was -0.63 which is the coolest value for any month since a similar (cool) regime observed during quarter 1 of 1994. There are a total of 6 years (since 1950) in which May produced similarly cool TNA index. The seasonal tropical cyclone activity that followed each of the 6 years ranged from 6-11 tropical storms, 4-7 hurricanes and 0-3 intense hurricanes clearly below various climatology for the season. The historical precedent for 2018, based on early season ocean temperatures in the main development region (MDR) for North Atlantic tropical cyclones is a below average season.

Presence of the exceptionally cool tropics and vast cool pool of SSTA south of Greenland support a theory that the long-term warm cycle of the AMO may be reversing to the cool phase.

Table 1: Past months of May (since 1950) when TNA index has been < -0.50 and the tropical cyclone activity that followed.