El Nino Holds Down Seasonal North Atlantic TC Activity Below Amount Observed During the 2016-22 Active Period

Explaining The “Atmospheric River” Evolution Into California
01/17/2023, 4:16 am EST
Marine Heat Wave is Catalyst to Historic Flooding and Heat During Summer 2022-23 in New Zealand
02/22/2023, 4:33 pm EST
Show all

Highlight: El Nino holds down seasonal activity below amount observed during the 2016-22 active period.

Executive summary: During the 2023 North Atlantic basin tropical cyclone season El Nino is expected. Consequently, seasonal activity should be lower than the 2016-22 active period. The North Atlantic basin is forecast to be warmer than normal and therefore the seasonal totals (11 storms, 7 hurricanes, and 3 intense hurricanes) are a little higher than the climatology for a typical El Nino year. Presence of increased westerly wind shear caused by El Nino suggests that storms turn away from the U.S. East Coast. However, the warmer than normal Gulf of Mexico may allow storms crossing the Gulf to over-achieve in strength.

Introduction: The North Atlantic basin preliminary seasonal tropical cyclone outlook is based primarily on El Nino southern oscillation (ENSO) using multivariate ENSO index (MEI) and North Atlantic SSTA identified by using the Atlantic multi-decadal oscillation (AMO) and tropical North Atlantic (TNA) index. The ENSO pattern is well-correlated with upper-level windshear across the tropical North Atlantic basin whereas La Nina generally produces a low shear environment supportive of tropical cyclone development while increased upper shear suppressing tropical cyclone development is associated with El Nino. Upper ocean heat as implied by the AMO and TNA patterns contribute to the intensity of seasonal activity with warmer than normal conditions contributing to increased number of stronger storms if ENSO is neutral or La Nina.

Climatology: Since the late 1990’s, long-term cycles of ENSO and AMO have shifted to the cool and warm phase respectively. Generally, the increased number of La Nina years during this time coinciding with a warmer than normal North Atlantic basin has increased the 30-year climatology as compared to long-term averages of seasonal activity (Fig. 1). During the past 15 years, the number of seasonal tropical cyclones has increased while hurricanes and intense hurricanes are unchanged from the 30-year climatology. The higher number of storms is biased by the record 30 events observed in 2020. During the past 7 years, the lack of El Nino has increased seasonal activity above the 30-year normal. The updated North Atlantic basin seasonal tropical cyclone activity climatology based on ENSO phase maintains the higher activity amount for La Nina decelerating for El Nino (Fig. 2).

Fig. 1: Climatology of North Atlantic basin seasonal tropical cyclone activity.

Fig. 2: North Atlantic basin seasonal tropical cyclone activity according to ENSO.

Climate forecast: The ENSO, AMO and TNA forecasts for the 2023 North Atlantic basin seasonal tropical cyclone outlook are based on an analog. ENSO phase is forecast using MEI. MEI is used due to the inherent representation of the global climate pattern rather than just the character of the equatorial East Pacific Ocean as defined by the Nino34 SSTA index. MEI proved especially valuable during the 2020-22 La Nina episode maintain moderate to strong La Nina intensity while Nino34 SSTA occasionally eased into neutral phase.

The ENSO forecast is straight-forward. The outlook is based on the average (ENSO) scenario following 3-year La Nina episodes which have occurred only 3 times since 1950 prior to the 2020-23 event (Fig. 3). Results indicate La Nina ends during mid-to-late meteorological spring and is followed by El Nino emerging during mid-to-late meteorological summer. The ENSO forecast implies the 2023 seasonal activity will be suppressed by El Nino for the first time since 2015.

The AMO index in 2022 produced a very specific regime: Neutral phase to begin the year suddenly warming (dramatically) during the peak of tropical cyclone season and cooling again by late year. During the long-term cool cycle of ENSO/warm cycle of AMO (since the late 1990’s) a total of 5 similar analog years to 2022 are observed. The following year, the analog suggests a similar pattern: Spiking warm AMO during the tropical cyclone season (Fig. 4) which should add upper ocean heat at that time of the year to support stronger hurricanes.

The TNA index evaluates SSTA across the main development region (MDR) for hurricanes in the North Atlantic basin. Given the El Nino forecast for the 2023 tropical cyclone season, a historical review of TNA patterns during onset El Nino years is relevant to the 2023 forecast. During the current long-term cycle of ENSO and AMO, El Nino onset analog years DURING the tropical cyclone season include 2002, 2004, 2006, 2009, and 2018. The 2006 analog is omitted due to the record warm TNA index observed in 2005 not matching the 2022 regime. The analog projection yields likelihood of decelerating warmth in the outer North Atlantic tropics during the first half of 2023 followed by rewarming to the warm phase during the core of the 2023 tropical cyclone season and prevailing through the end of the year (Fig. 5). The TNA index forecast supports development of robust tropical systems in the deep tropics if the upper shear pattern is supportive.

The tropical cyclone season global SSTA forecast by the NCEP CFS V2 model confirms presence of El Nino (Fig. 6). The North Atlantic SSTA forecast is robustly warmer than normal likely more so than the AMO/TNA forecast convey. The SSTA forecasts are made with average to below average forecast confidence implying susceptibility to changes in the seasonal forecast. The next update is in early April.

Fig. 3: The 2023 ENSO phase forecast based on an analog using multivariate ENSO index.

Fig. 4: The 2023 AMO index forecast based on an analog.

Fig. 5: The 2023 TNA index forecast based on an analog.

Fig. 6: The NCEP CFS V2 global SSTA forecast for JUL/AUG/SEP and AUG/SEP/OCT 2023.

Preliminary 2023 outlook: The outlook is based on the ENSO analog years for all tropical cyclones plus the combination of the +AMO/+TNA analog years when El Nino was approaching or already developed. The analog years are equally weighted for 1957, 1976, and 2001 plus a combination of +AMO/+TNA years (2002, 2004, 2009, 2014, and 2018) as the 4th analog. Forecast results yield 11 tropical storms, 7 hurricanes, and 3 intense hurricanes (Table 1). The forecast is below the 30-year normal for number of tropical storms and close to normal for hurricanes and intense hurricanes. The ACE index is markedly below normal and close to last year. The preliminary 2023 forecast is well below normal for the 2016-22 active period for tropical storms and ACE index. The 2023 forecast is more active than weak to moderate El Nino accounted for by the warm North Atlantic.

Tropical CyclonesHurricanesIntense HurricanesACE

Index

195783284
1976106281
20011594106
AMO/TNA11.88.02.8108.8
Forecast11.2 (11)6.5 (7)2.7 (3)94.9 (95)
Last Year148295
2016-202218.68.43.7150
30-Year15.07.43.3126.4
50-Year12.76.52.6103.6
Weak El Nino9.54.31.854.8

Table 1: The Climate Impact Company preliminary analog forecast for the 2023 North Atlantic basin tropical cyclone season.  

Based on the ENSO and AMO/TNA forecast, statements can be made about the expected character of the upcoming tropical cyclone season. The presence of El Nino implies above normal westerly shear which, at times will inhibit development. Additionally, the presence of westerly shear suggests storms moving out of the tropics will turn more quickly north and northeastward. Long distance travelers (“Cape Verde storms”) are less likely during El Nino. The Gulf of Mexico is expected warmer than normal during tropical cyclone season. While long distance travelers entering the warmer than normal Gulf are less likely, systems born in the Gulf can certainly occur and produce quick hazards to coastal communities.

The +AMO/+TNA regime, which based on the analog forecast should feature a warm spike during the core of tropical cyclone season, supports above normal risk of stronger tropical systems. There may be a narrow window of time during late August and into September when the strongest storms of the season form. Of course, these storms will likely need to fight through El Nino westerly shear which inhibits full storm potential. The risk of a major hurricane striking the Gulf Coast is about normal and favors Louisiana while the East Coast landfall threat is below normal.

The next update will be issued in early April.