When El Nino Ends Mid-Year & Influence on North Atlantic Tropical Cyclone Season

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Several forecast models have dissipated El Nino and project either neutral ENSO or weak La Nina emerging during the North Atlantic tropical cyclone season. In previous years when El Nino started the year and dissipated mid-year tropical cyclone activity including numbers of hurricanes in the North Atlantic basin were above normal and sometimes excessive.

Discussion: The Bureau of Meteorology/Australia issued an updated ENSO outlook earlier today indicating El Nino 2018-19 has ended and neutral ENSO is likely the remainder of this year (Fig. 1). Similarly, the NCEP CFS V2 ENSO forecast indicates neutral ENSO or possibly weak La Nina ahead (Fig. 2).

During the modern climate pattern (since the mid-to-late 1990’s when ENSO/PDO and AMO cycles changed) an El Nino ending early-to-mid-year (Fig. 3) was followed by neutral ENSO (2003 and 2005) or a sharp decline into La Nina ((1998, 2010 and 2016). During the 5 analog years tropical cyclone activity (Fig. 4) averaged above average (18.4 storms, 10.2 hurricanes and 4.25 intense hurricanes). One of the analog years (2005) was the most active on record (28 storms, 15 hurricanes and 7 intense hurricanes).

Current seasonal forecasts issued by NOAA, Colorado State University, Tropical Storm Risk U.K. and Climate Impact Company are near normal. The near normal forecast is based on suppressing influence of seasonal activity by an El Nino somewhat negated by the enhancing influence of a warmer than normal North Atlantic. The suppressing influence of El Nino is fading and the season is likely more active than previous forecasts.

Fig. 1: The Bureau of Meteorology/Australia ENSO forecast for 2019.

Fig. 2: The NCEP CFS V2 ENSO forecast for 2019.

Fig. 3: Years when El Nino started the year, ended mid-year and transitioned into neutral ENSO or La Nina in the modern climate cycle.

Fig. 4: North Atlantic tropical cyclone activity when El Nino began the year, ended mid-year and transitioned into either neutral ENSO or La Nina.