NOAA releases 2023 Hurricane Season Outlook

NOAA’s North Atlantic Hurricane Season Outlooks are based on predictions of the main climate factors and their associated conditions known to influence seasonal Atlantic hurricane activity. These predictions are based on extensive monitoring, analysis, research activities, a suite of statistical prediction tools, and dynamical models. The dynamical model predictions come from the NOAA Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) HiFLOR-S and SPEAR-MED models, the North American Multi-Model Ensemble (NMME), the United Kingdom Met Office (UKMET) GloSea6 model, and the European Centre for Medium-Range Weather Forecasting (ECMWF) Seas5 model. ENSO forecasts are also provided from the NMME dynamical models contained in the suite of Niño 3.4 SST forecasts, which is compiled by NOAA’s CPC. Newly added this year is a regression model based on the work at AOML. (West et al. 2022).

NOAA’s 2023 North Atlantic hurricane season outlook reflects the expectation of competing climate factors during August-October (ASO), as follows:

1. The underlying climate factor again this season is the continuation of the high-activity era for Atlantic hurricanes, which began in 1995 in association with a transition to the warm phase of the Atlantic Multidecadal Oscillation (AMO) (Goldenberg et al. 2001, Bell and Chelliah 2006, Klotzbach and Gray 2008). The recently observed and predicted atmospheric conditions for ASO 2023 reflect the warm AMV phase, and with several factors conducive for higher levels of activity such as weaker trade winds and warmer SSTs across much of the MDR, a more conducive African easterly jet, weaker vertical wind shear, and an enhanced West African monsoon. When the oceanic and atmospheric conditions are considered as a whole, the variability is being more commonly referred to as Atlantic Multidecadal Variability (AMV) in recent literature. The observations and predictions of these values are not as anomalous as in the prior three years (2020, 2021, and 2022). The trade winds over the Atlantic are weaker over the Gulf of Mexico and the northern portions of the MDR, while stronger near the Lesser Antilles. Flow out of the top of the West African Monsoon is not yet well established, but this is likely an impact of intraseasonal oscillations able to move more freely during ENSO-neutral conditions. Predictions of wind shear during 2020-2022 were for almost record low wind shear, while this year, the predictions are closer to normal. Contrasting that are the SSTs which are warmer than last year in much of the Atlantic including the MDR, so a bit counter to some of the atmospheric signals being observed.
2. The most recent NOAA ENSO probability forecast indicates a 93% chance that El Niño conditions will develop through the hurricane season, and a 7% chance that ENSO-neutral conditions could be in place during the peak months (ASO) of the season. Additionally, the ENSO outlook calls for elevated odds of a moderate to strong El Niño this year, though uncertainty at these leads prevents us from delineating between a moderate or a strong El Niño. Strong El Niño conditions were last present in 2015 during ASO, and that year had below-normal activity.

a.El Niño favored

El Niño represents one phase of the climate phenomenon known as ENSO (El Niño-Southern Oscillation). The three phases of ENSO are El Niño, La Niña, and ENSO-neutral. El Niño tends to suppress Atlantic hurricane activity, while La Niña tends to enhance it (Gray 1984; Goldenberg and Shapiro 1996). These impacts can be strongly modulated by conditions associated with a low- or high-activity era, and also by short-lived conditions during any specific year.

As of May 22, 2023, ENSO-neutral conditions are present. The weekly SSTs are currently slightly above average across much of the central and eastern equatorial Pacific and the SST index for the Niño 3.4 region is +0.5°C. The Niño 3.4 index has shown a significant warming trend since December 2022 and the weekly Niño 3.4 index has increased from -0.5°C in February of 2023 to its current value of +0.5°C. The wind and outgoing longwave radiation patterns over the central Pacific are also reflecting a breakdown of atmospheric response to last year’s La Niña forcings, and are currently showing more influence from intraseasonal modes.

Looking forward, model-predicted SST anomalies in the Niño 3.4 region generally indicate El Niño (Niño 3.4 index greater than +0.5°C) conditions throughout the hurricane season. The dynamical model average (dashed black line) indicates El Niño through the summer and autumn of 2023, with only one modeling system indicating ENSO-neutral conditions, but still showing positive anomalies. When using a larger pool of models that includes multiple dynamical models, multiple statistical models, and unique combinations of those models, 21 of 24 models indicate El Niño conditions, with 3 results not as amplified but still showing positive anomalies.

NOAA’s Climate Forecast System (CFS) and the North American Multi-model Ensemble (NMME) are predicting El Niño. The CFS predicts above-normal shear over the MDR while the NMME predicts about near-normal shear for the same region. The shear predicted by the NMME this year is closer to climatology than what was predicted last year for much of the Atlantic basin. This is likely associated with the model predictions of El Niño, which is typically associated with near- or above-normal vertical wind shear.

The official NOAA ENSO outlook from May of 2023 indicates about a 93% chance of El Niño during the peak months (ASO) of the hurricane season, only a 7% chance of ENSO-neutral and negligible chance of La Niña. Furthermore, the ENSO team also has indicated a 72% chance of a moderate to strong El Niño event (i.e. ASO Niño 3.4 SST anomaly of +1.0°C or more), making the likelihood of teleconnections to the Atlantic more likely if we reach those higher Niño 3.4 values (L’Heureux et al. 2019). These teleconnections typically increase shear over the MDR thus reducing Atlantic basin hurricane activity. Therefore, the current hurricane season outlook reflects the expectation that the El Niño conditions will compete with, and would likely interfere with, the ongoing set of conditions associated with the current high-activity era for Atlantic hurricanes (discussed below).

b. Predicted conditions within the MDR

SSTs are currently above-average across nearly all of the MDR, with an area-averaged anomaly during April of +0.46°C, compared to only +0.02°C during April last year. For the MDR as a whole, both the CFS and NMME models predict slightly above-average SSTs during ASO. The differences between MDR SSTs and the global tropics is another predictor favoring an above-normal season, and has been linked to some hyper-active years (Blake et al. 2018). Among the NMME models, the predicted values of MDR SST anomalies range from +0.17°C to +0.62°C, with the multi-model, spatial average being +0.48°C. The outlook for SSTs in the MDR is warmer than last year’s prediction, but this year the difference in the MDR from the global tropics is about the same as 2022. However, most models tend to have only modest skill in predicting the strength of the Atlantic SST anomalies this far in advance, though these model predictions are consistent with the ongoing warm phase of the AMV. SSTs across the North Atlantic basin are presently above-normal and predicted to stay above-normal at least through ASO.

Two inter-related atmospheric features, also related to the warm phase of the AMO/AMV that are typically analyzed, are anomalous winds at 700-hPa across the central and eastern tropical Atlantic and the strength of the West African monsoon system. The 700-hPa winds show a weak anticyclonic circulation, which would be indicative of weaker trade winds. The outflow at the top of the West African Monsoon, as analyzed by 200-hPa Velocity Potential Anomaly and divergent wind is showing a weaker than normal circulation. The Madden-Julian Oscillation (MJO) may have played a role in some early suppression of upper-level divergence and rainfall totals, so if the MJO continues to propagate eastward, some more favorable circulations would likely develop. If the monsoon stays enhanced as indicated in many of the model outlooks, these conditions are expected to contribute to an interrelated set of conditions during ASO within the MDR, all of which favor increased Atlantic hurricane activity. These interrelated features include 1) anomalously warm SSTs and decreased vertical wind shear, 2) an African Easterly Jet structure that allows for increased rotation of low-pressure cloud systems (aka African easterly waves) moving westward from Africa, and 3) the combination of increased moisture and decreased atmospheric stability. Because of these conditions, the enhanced African easterly waves can develop more easily into tropical storms and hurricanes, thus creating a forcing that would be in opposition to those expected and associated with El Niño, increasing the uncertainty in the outlook and supporting a forecast for near-normal activity this season rather than below-normal typically associated with El Niño.

c. Factors increasing the uncertainty

The main uncertainty in this seasonal hurricane outlook is the interplay between the predicted El Niño which favors lower activity and the predicted set of conditions in the MDR that are conducive to more activity. So even though El Niño conditions will probably develop in the Pacific and create the traditional teleconnection patterns associated with lower tropical storm and hurricane activity in the Atlantic Basin, the local (MDR) conditions could be strong enough to dominate and thus allow high-activity era conditions to rule, resulting in a season near the upper end of our forecast range. SSTs in the MDR are much warmer, relative to normal, but exhibit a different spatial structure than at this same point last year, suggesting there are differences in the overlying atmospheric circulation patterns. This suggests that the change from La Niña to El Niño already being felt in the MDR In addition, the NMME model-predicted shear values are in the near normal range, suggesting some local feedback to counteract the traditional El Niño influence, so that interplay creates a heightened amount of uncertainty for this outlook. The impact of combination of these various climate factors should be much more clear by the time of the August update of this outlook.