Climatology of North Atlantic Tropical Cyclone Activity during Different ENSO Phases

This study produces a North Atlantic tropical cyclone (TC) climatology for different phases of El Niño-Southern Oscillation (ENSO). The hurricane season in North Atlantic basin is June 1-November 30. Neutral years have a mean frequency of 15 TCs per season, with an annual (January-December) mean of 16. La Niña years have more TCs, with 17 per season and 18 annually whereas El Niño years have fewer TCs, with 12 per season and 13 annually. The hurricane season can be divided into early season (June-July), mid-season (August-September), and late season (October-November). The mid-season has the highest mean frequency of TC numbers and landfalls in all phases of ENSO. About 54% of all TCs in Neutral years develop during the mid-season, and the early and late seasons each have 23%. Both El Niño and La Niña phases have their highest percentages of TC occurrence in mid-season and their lowest percentages in the early-season. In all three ENSO phases, the peak month is September, and August is the second most active month, with mean TC numbers of 5 and 4, respectively, for Neutral years, 5 and 3 for El Niño years, and 6 and 5 for La Niña years. Although the mean season and mean annual number of TCs are higher during La Niña years, the La Niña phase has a higher monthly number of TCs than Neutral and El Niño phases only in August to October. The Neutral phase has a higher monthly number of TCs than both La Niña and El Niño phases in June, July, and November.

The North Atlantic hurricane season, during El Niño years, has on average the same start date as Neutral years (June 10), but end slightly earlier (November 6 compared with Neutral, November 9), and has a shorter season length (150 days compared with 154 days for Neutral years) and fewer annual TC days (mean of 77 days while Neutral years have 101 days). In La Niña years, the hurricane season commences a little later (June 15) but also terminate later (November 16), relative to Neutral and El Niño years. The season length is slightly longer (156 days) than that of Neutral years, but has more mean annual TC days (117 days). The season length, start and end dates, in all ENSO phases, exhibit interdecadal variability. There are advancing and retreating start and end dates, and fluctuating season lengths. In Neutral years, the hurricane season start dates are occurring earlier, the earliest is April 6. End dates are a little later, the latest can occur in January 7 of the following year, which is reflected in the increasing trend of the season length. The opposite scenario is seen in El Niño years, with a decreasing trend in the season length, as seen in the season start dates becoming later, the latest is now July 31, while the season end dates are occurring earlier, the earliest is now October 3. In La Niña years, the start dates are occurring slightly earlier; the earliest is April 18, whereas the end dates are occurring a little later, the latest is January 6 of the following year. This is consistent with the increasing trend of the hurricane season length.

The lifetime maximum intensity (LMI) during different ENSO phases also has been examined. La Niña years have a higher mean LMI, 66 knots, than Neutral and El Niño years, which are 64 knots and 62 knots, respectively. The LMI in ENSO phases during the satellite era all have shown an increasing trend. Neutral and El Niño years have the same LMI average latitude which is 26°N, but in La Niña years, it is found in lower latitude, 22°N. The LMI average latitude in Neutral years exhibits no significant trend, but both El Niño and La Niña years reveal a decreasing trend in mean LMI latitude, suggesting an equatorward migration of LMI positions.

The mean monthly number of TCs in each ENSO phase has been standardized to assess the impact of ENSO on the frequency of TCs. The effect of ENSO varies with specific months of the hurricane season. During the early hurricane season (June-July), Neutral years have above normal numbers of TCs, whereas El Niño and La Niña phases have below normal TCs, but are much fewer TCs in El Niño years. In the mid hurricane season (August-September), TC numbers in Neutral and El Niño years are below normal, whereas La Niña years have very much above normal TC numbers. Finally, in the late hurricane season (October-November), El Niño years have below normal TC numbers, but Neutral and La Niña years can experience above normal numbers of TCs.