2022 Hurricane Outlook

Tropical cyclone (TC) activity has an adverse and perennial impact on life and property in countries adjacent to ocean basins worldwide (Blake et al., 2011). Therefore, research into tropical meteorology over the last several decades has been vital in increasing understanding of the mechanisms that drive TC formation, steering, and intensification in addition to increasing the capabilities to track these storms. This includes accurate record-keeping of seasonal activity across all TC basins with activity quantified in the North Atlantic and eastern North Pacific (east of 180⁰) through number of named storms (storms that attain winds ≥ 34 knots), number of hurricanes (≥ 64 knots), number of major hurricanes (≥ 96 knots), and Accumulated Cyclone Energy (ACE). ACE is the approximation of wind energy used by a tropical system over its lifetime. This is calculated by summing the squares of the estimated maximum sustained velocity in knots at each observed 6-hour interval for storms that are at least tropical storm intensity (≥ 34 knots). Thus, the seasonal ACE value for an ocean basin is the summation of the ACE of all storms that were observed during a given year.

For this discussion, the primary basin of investigation will be the North Atlantic Ocean which observes high interannual variability in TC activity when compared to other tropical basins (Bell and Muthuvel, 2006). Consequently, there is greater value in assessing what factors can influence TC variability to determine the potential magnitude of TC activity that could occur during hurricane season. The North Atlantic hurricane season spans from June 1 to November 30 and averages fourteen named storms, seven hurricanes, three major hurricanes, and an ACE value of 123 (*10^4 kt2) based on the 1991-2020 mean. TCs typically form in regions of copious moisture, strong atmospheric instability, light vertical wind shear, and sea surface temperatures of 26.5 ⁰C or greater (Gray, 1979). Considering there is an expanding aspect of research focused on understanding how these environments are modulated at temporal scales on the order of weeks, months, and years, these environmental variables are necessary, but not exclusive, for TC development. Specifically, the investigation of how climate teleconnections influence TC activity worldwide when certain modes of variability are present. By leveraging the established literature on the relationships of these climate teleconnections to TC activity in the North Atlantic, scientists through the years have been able to forecast the magnitude of activity expected in the basin in the months preceding the season with reasonable accuracy (Klotzbach et al., 2019). Therefore, these same methods will be used to analyze a multitude of highly correlated variables to North Atlantic TC activity to provide a qualitative discussion and reasoning for the quantitative seasonal hurricane forecast outlined in the conclusion.

1. North Atlantic SSTs

1.1 Background

Sea surface temperatures (SST) in the North Atlantic leading up to hurricane season have been correlated to variability of TC activity in the basin. This is due to the high specific heat of ocean water that allows temperature profiles and atmospheric feedbacks to be maintained into hurricane season. The highest positively correlated region for SSTs (i.e., where warmer waters relate to more hurricane activity) exists along the Canary Current region and Tropical Atlantic. This region coincides with the location of the Atlantic Meridional Mode (AMM) and thus the AMM is commonly invoked in seasonal forecasts of hurricane activity in the basin (Appendix: Figure 1). The AMM describes the meridional difference in SST anomalies between the tropical and equatorial Atlantic (Chiang and Vimont, 2004). When the AMM is positive, the Canary Current region is warmer than average, and the equatorial Atlantic is cooler. The steepening of the temperature gradient between these regions during hurricane season has been attributed to anomalously poleward placement of the Intertropical Convergence Zones (ITCZ) and enhancement of the African Easterly Jetstream which are key components in the production of hurricanes in the Tropical Atlantic (see Section 4).

Historically, one of the most well-known modes of variability in North Atlantic SST anomalies that had been linked to TC activity was the Atlantic Multidecadal Oscillation (AMO). The AMO was believed to have a 65 to 80-year period based on the observed data available to researchers (Kerr, 2000). A positive AMO phase was associated with above average tropical cyclone activity due to the above average SSTs observed in the North Atlantic’s Main Development Region (MDR; 10⁰N-20⁰N, 85⁰W-20⁰W) where a majority of TCs form. The negative phase was associated with below average TC activity in the North Atlantic due to the MDR being cooler than average. While the AMO had been valuable in understanding TC variability at extended temporal scales, recent modulation of the North Atlantic SST configuration has raised questions regarding its reliability in seasonal TC forecasting. Specifically, with record Arctic Sea ice melt in recent years and notable freshening (decrease in ocean salinity) of the far North Atlantic, this is slowing the Atlantic Meridional Overturning Circulation (AMOC) and points to the growing evidence that North Atlantic SSTs are being anthropogenically influenced (Mann et al., 2021). The AMOC is regarded as a main component of heat transport via surface and deep ocean currents across the South and North Atlantic basins (Frajka-Williams et al., 2019). These recent findings regarding anthropogenic climate change’s impacts on the basin appear to be validated by the persistent below average SSTs in the Far North Atlantic (45⁰N-60⁰N, 40⁰W-10⁰W) and warmer subtropical North Atlantic and Canadian Maritime in recent years. The modulation of the SST regime in this regard has resulted in a more persistent positive North Atlantic Oscillation (NAO) pattern during the winter months. A positive NAO typically favors stronger high pressure over the northeast Atlantic which enhances easterly trade winds and cooling across the MDR. Consequently, the increased frequency of cooling of the MDR observed in spring SST profile has led to less classically warm MDR years preceding hurricane season despite the recent stretch of more active years (i.e., 2016 through 2021). Further, even years where the spring base state of the tropical Atlantic has been warmer, the SST configuration was not traditional to that of a positive AMM (or AMO) because the subtropics, Canadian Maritime, and equatorial Atlantic remained warm.

1.2 Analysis

The North Atlantic SST profile during Spring 2022 is reminiscent of prior years with near average SSTs in the MDR and anomalously warm waters between 20⁰N to 30⁰N (especially in the western part of the basin) extending into the Canary Current region. Further, a recent reduction in trade winds across the Tropical Atlantic has induced a warming trend in SSTs that have presented a more anomalously warm SST configuration in recent weeks compared to the May average (Figures 2 and 3). These trends do have credence to a more active SST profile for the Atlantic basin but should be approached with caution as SSTs have varied greatly between passages of various equatorial waves (i.e., Madden-Julian Oscillation and Convectively Coupled Kelvin Waves) that enhance and suppress trade winds in the Tropical Atlantic (Figure 4). Regardless, this SST anomaly configuration is comparable to 2020 and 2021 with the primary difference being that SSTs are cooler in the equatorial Atlantic and Gulf of Guinea this year (Figure 5). While it was acknowledged that the existence of Atlantic Niño conditions could suppress ITCZ due to negative AMM conditions last season, the prior literature was conflicting regarding the potential enhancing factors this configuration could have on hurricane activity. However, there was no particular evidence to conclude there were positive enhancements to TC activity in the MDR as a result of the warmer equatorial Atlantic as the number of hurricane and major hurricanes did not exceed abnormally active metrics. More classically, cooler equatorial Atlantic conditions are associated with lifting of the ITCZ poleward and an increase in temperature gradient between the ocean and African continent which enhances the African Easterly Jet. These factors are typically associated with enhancement of TC activity in the Atlantic assuming MDR SSTs are cooperating (i.e., not below average). This difference between equatorial Atlantic warmth can be exemplified in the August-September-October SST profiles for 2020 and 2021 (Figure 6). In addition to the cooler Gulf of Guinea, SSTs in the Canary Current region that highly correlate to hurricane activity are currently above average and generally support a positive AMM configuration that is favorable for hurricane development in the tropical Atlantic. Given the amount of warmth present in the tropical and subtropical North Atlantic at this time that is comparable to recent years, hurricane activity is likely to be enhanced.

2. El Niño Southern Oscillation

2.1 Background

The El Niño Southern Oscillation (ENSO) is defined as the variance of the SSTs in the eastern equatorial Pacific Ocean. The period for ENSO is two to seven years and can vary upwards of 2 to 3⁰C during this period. The fluctuation of the SSTs in this region is considered to be one of the most influential in global climate variability and is associated with significant variations of precipitation in the continents adjacent to the Pacific Ocean. ENSO is classified into three different phases (La Niña, neutral, and El Niño) depending on the mean SST anomaly of the Niño 3.4 ENSO region (5⁰N-5⁰S, 170⁰W-120⁰W). La Niña is the term used for the cool phase of ENSO (≤ -0.5⁰C) and is linked to weaker vertical wind shear across the Atlantic MDR due to the displacement of the Subtropical Jetstream (STJ) further North during the summer months and generally less convective activity is observed in the eastern Pacific. El Niño is known as the warm phase of ENSO (≥ +0.5⁰C) and typically favors upward motion across the eastern Pacific and American continents. This upward motion not only enhances convection across the eastern Pacific that promotes a more active summer STJ, but also generally promotes downward motion and inhibition of convection over the North Atlantic and Maritime Continent (Enfield and Mestas-Nuñez, 1999).

In a neutral ENSO state (-0.5⁰C to +0.5⁰C), climatologically average atmospheric conditions favor easterly trade winds across the equatorial Pacific. This is in association with a zonal overturning circulation known as the Walker circulation which is caused by the east-west imbalance in oceanic heat in the deep tropics of the Pacific. This imbalance in heat is accentuated by the eastern equatorial Pacific cool tongue that is caused by the diffluent Ekman transport of ocean water away from the equator that upwells cooler subsurface waters to replace it. The upwelling of these subsurface waters explains the asymmetric appearance of the equatorial Pacific’s subsurface thermocline profile in neutral conditions. Therefore, when the Walker circulation is disrupted, easterly trade winds weaken or reverse (known as a Westerly Wind Burst) and upwelling in the eastern equatorial Pacific ceases. The lack of upwelling, or existence of downwelling, in the eastern equatorial Pacific causes anomalous warming of the region which can lead to genesis of an El Niño event. Contrarily, when the Walker circulation is strengthened, easterly trade winds strengthen (Easterly Wind Burst) causing more significant upwelling in the eastern Pacific and cooling of the region which can be associated with the genesis of La Niña.

Understanding the evolution of ENSO to improve climate prediction is still regarded as a challenging task with many researched methods proposed as factors in ENSO’s variability (Wang, 2018). One method that can be used in real-time application for ENSO prediction is the delayed-oscillator theory. This theory is based on the oceanic response to surface wind stress across the equatorial Pacific. When a Westerly Wind Burst (WWB) occurs, the ocean responds by generating a downwelling Kelvin wave (eastward propagating) and an upwelling Rossby wave (westward propagating) near the International Date Line (180⁰). Commonly the Madden-Julian Oscillation (MJO) is a mechanism that can initiate WWBs as this atmospheric phenomenon propagates across the Pacific. Depending on the persistence of WWB events over the span of a couple of months, the downwelling Kelvin wave (DKW) that is generated will propagate into the eastern equatorial Pacific in the following months causing warming of ENSO and potentially an El Niño event. While this is occurring, the upwelling Rossby wave (URW) propagates westward toward the west Pacific at a slower rate and cools this region. Once these waves reach the periphery of the Pacific basin, they are then refracted by the east-west boundaries of the basin (i.e., continents) or terminated by the atmospheric forcing present at that time. If no new DKW is generated, then the refracted Rossby wave (also known as an upwelling Kelvin wave) will then propagate eastward causing cooling of the eastern equatorial Pacific. Conversely, if an Easterly Wind Burst (EWB) occurs, then an upwelling Kelvin wave (UKW) and a downwelling Rossby wave (DRW) are generated and can lead to the development of La Niña (Roundy and Kiladis, 2007).

2.2 Analysis

After brief termination of La Niña during summer 2021, a series of EWBs and subsequent UKWs were observed through the fall causing significant cooling of the eastern equatorial Pacific and the reemergence of La Niña conditions. The presence of La Niña during peak hurricane season enhanced Atlantic hurricane activity leading to yet another active season in 2021. The initial minima of this second dip in equatorial eastern Pacific water temperatures since 2020 was reached in November-December-January (NDJ) (-1.1⁰C) before beginning to wane at the start of 2022. By December, a strong Pacific MJO event yielded a sufficiently strong WWB to initiate a DKW that warmed the subsurface. Had sufficient reinforcement been achieved in the following months through new WWBs, perhaps there would have been a stronger opportunity for El Niño development by the fall. Instead, the months of February, March, and April observed numerous EWBs and complete suppression of strong MJO propagation along the equator. This not only attenuated the existing DKW, but also initiated a new strong UKW that cooled the subsurface once more and has all but quashed any chance of El Niño conditions developing by the end of 2022 (Figure 7). Further, La Niña conditions have recently strengthened with the latest February-March-April (FMA) ENSO Oceanic Niño (ONI) Index returning to -1.0⁰C, moderate La Niña conditions (-1.0⁰C to -1.4⁰C). Only two other La Niña events since 1950 (1955 and 1975) in the ENSO ONI had observed cooling from Jan-Feb-Mar (JFM) to FMA with both years going on to observe La Niña strengthening into a strong event (< -1.5⁰C) by fall.

Given the persistence of EWBs and suppression of MJO-induced WWB events throughout the spring, confidence has increased that La Niña conditions will be observed for the third straight fall and winter seasons. Latest dynamical climate model guidance has continued to adjust in response to the persistence of La Niña conditions this spring and now agree with the continuation of La Niña through the remainder of the year. While most guidance shows some weakening of La Niña during summer, most agree that ENSO cooling will occur in the fall for a third dip in this multi-year La Niña event.

This ENSO forecast is supported by the North American Multi-Model Ensemble (NMME) and the International Multi-Model Ensemble (IMME) which are suites of dynamical climate prediction models (e.g., CFSv2, ECMWF, UKMET). The consensus of these models predicts gradual weakening of La Niña conditions during the summer months to near -0.5C before ENSO cooling resumes and La Niña restrengthens in the fall and winter (Figure 8). The range of uncertainty with the ENSO forecast encompasses solutions from cool neutral to strong La Niña during peak hurricane season. Therefore, it is likely ENSO will continue to suppress convection in the eastern equatorial Pacific leading to a decrease in vertical wind shear in the Atlantic MDR. As a result, North Atlantic hurricane activity is expected to be enhanced by ENSO.

3. PDO and PMM

3.1 Background

Another variable that can be used to anticipate the Pacific’s impact on Atlantic TC activity is the Pacific Decadal Oscillation (PDO). The PDO refers to the variation in SSTs in the mid-latitude Pacific. In the positive phase of the PDO, waters are warmer than average across the eastern part of the mid-latitude Pacific near the west coast of North America with cooler waters in the central mid-latitude Pacific. In a negative PDO, the SST profile is the opposite with cooler than average SSTs off the west coast of the mid-latitude Pacific (Mantua, 1999). The Pacific Meridional Mode (PMM) refers to the difference in SST anomalies between the subtropical Eastern Pacific (i.e., Baja California to Hawaii) and equatorial Pacific and is typically in phase with the PDO. Additionally, the region the PMM examines partially coincides with the Western Development Region (WDR; 10°–20°N, 116°W–180°) of the eastern North Pacific and has a strong correlation to hurricane activity in the basin with the most active hurricane seasons occurring when the PMM was positive (Collins et al., 2016). Given the known inverse relationship between eastern North Pacific and North Atlantic hurricane activity, accounting for the PDO and PMM are important in anticipating how the eastern North Pacific could impact TC activity in the North Atlantic.

3.2 Analysis

The Eastern Pacific basin recorded its second-straight below average hurricane season in 2021 due to the presence of La Niña and the coolest WDR since 2012. This has been a notable change compared to the 2014 through 2019 period which observed more consistent above average TC activity due to the presence of a positive PDO and PMM. With suppressed TC activity in the Eastern Pacific, the Atlantic MDR observed below average vertical wind shear that enhanced TC activity in the basin for the last two years.

In October 2021, the NOAA ERSST PDO index observed its most negative value (-3.11) since 1955 and has remained strongly negative through May 2022 (Figure 9). The PMM has not been as negative as 2021 given that this year’s La Niña in the equatorial Pacific is stronger compared to this time last year. However, the WDR SSTs remain below average and will continue to hinder Eastern Pacific TC activity since the spatial domain of high ocean heat content is confined to the eastern part of the basin. This, in combination with continuation of La Niña conditions, greatly increases the likelihood of below average TC activity in the Eastern Pacific. Despite the less favorable background environmental conditions, it's acknowledged that short periods of enhanced TC activity could still occur in the basin after MJO and Convectively Coupled Kelvin Wave (CCKW) passages. Further, it is acknowledged that La Niña conditions typically confine the region of TC formation closer to the western Mexican coastline which still could provide notable impacts to these areas despite less activity being anticipated (Collins et al., 2016). In conclusion, the negative PDO and PMM will serve to increase the favorability of the MDR for TC activity in 2022 by suppressing eastern Pacific hurricane activity.

4. AEJ AND WAM

4.1 Background

The two main components that contribute to North Atlantic hurricane activity over the African continent are the African Easterly Jet (AEJ) and the West African Monsoon (WAM). The AEJ refers to the 700 hPa Jetstream that exists over the African Sahel region (15⁰N-20⁰N, 15⁰W-35⁰E) where 700 hPa winds are enhanced due to the contrast in temperature and moisture between the Sahara Desert and Gulf of Guinea during boreal summer (Sheen et al., 2017). This Jetstream is known for enhancing and transporting African Easterly Waves (AEWs) across the African continent and into the Atlantic Ocean. AEWs that emerge into the Atlantic account for most of the TC activity that occurs in the basin on average (Landsea, 1993). Similarly, the WAM is also modulated by this temperature and moisture gradient over western Africa and the equatorial Atlantic. When the WAM is strong, 850 hPa zonal westerlies are stronger across the lower latitudes of the Atlantic which enhances convergence over West Africa as well as enhances relative vorticity in relation to the easterly trade wind flow to the north. The differences in placement of the AEJ in relation to WAM strength can have significant effects on the favorability of development of tropical waves into TCs. When the placement of the AEJ is further north, AEWs are enhanced due to the increased latitudinal distance from the equator that enhances vorticity via the Coriolis force. However, this can generate stronger Saharan Air Layer outbreaks especially during the early part of hurricane season (June to August) due to the enhanced easterly trade winds which can inhibit immediate TC formation as AEWs emerge. In contrast, when the AEJ is displaced south, TC development from AEWs is suppressed once they have emerged into the Atlantic.

Useful late spring indicators of the favored placement and strength of the AEJ and WAM during hurricane season are African Sahel rainfall and the AMM (Martin and Thorncroft, 2014). The wetness of the African Sahel can be quantified using merged rain gauge and satellite precipitation datasets (e.g., GPCP v2.3 and CMAP) which provides spatially complete estimates of rainfall on daily and monthly temporal scales. Other variables that can be valuable within reanalysis datasets are columnar precipitable water, or PWAT, values (kg/m3) and upper-level (200 hPa) velocity potential fields. The AMM, as mentioned in Section 1, refers to the variance of SST anomalies between the subtropical and equatorial Atlantic. During a positive AMM, the stronger meridional temperature gradient between the equatorial and tropical Atlantic enhances the AEJ and the West African Monsoon (WAM) which favors stronger zonal westerly anomalies across the tropical Atlantic during hurricane season. This can assist in positive feedback of a wetter African Sahel which then enhances the WAM further and reinforces MDR warming during monsoon season.

4.2 Analysis

Since the conclusion of the 2014-16 El Niño event, the African Sahel region has observed wetter than normal conditions. This wet period has been attributed to an enhanced WAM which has enhanced stronger surface convergence and increased convective activity over western Africa. Further, literature regarding these observations and within modeled environments acknowledge that there is also a two-way soil moisture-atmosphere interaction that likely has assisted in maintaining this convective activity (Berg et al., 2017). The WAM was its strongest from 2018 through 2020 with a strong positive Indian Ocean Dipole (IOD) focusing tropical warmth near Africa while ENSO remained weak and struggled to capture the low-frequency climate state (i.e., temporal scale of months to years). With the development of a multi-year La Niña event beginning in 2020, ENSO finally was able to recapture its dominance in climate variability which has reduced the focus of upward motion over the African continent and shifted it toward the Maritime Continent. Further, recent investigation into the reliability of upper-level velocity potential fields in global weather reanalysis has exposed significant disagreements in the NCEP/NCAR Reanalysis 1 (R1) (1948-present) and independent weather observations in key regions of tropical climate variability (e.g., equatorial Africa, equatorial central Pacific, Amazon basin, and equatorial Indonesia). This has decreased confidence in using NCEP/NCAR R1 to analyze climate change and variability, especially during summer months when tropical convection is most active. Subsequently, the apparent record wet regime over the African continent depicted in the NCEP/NCAR R1 recently appears to be caused by a high bias in the climatological means that are skewing anomalies to appear much wetter than normal. These biases are heavily influenced by highly suspect multidecadal behavior across both the African and South American continents, especially in the early period (i.e., 1948-1978) (Figure 10). Alternatively, confidence was increased in use of the newly extended ECMWF ERA5 as well as ensemble-based approaches to analyze tropical climate variability (Preparing for publication: Stanfield and Ramseyer, 2022). By establishing new literature to identify biases in the global weather reanalysis and using an ensemble-based analysis of upper-level velocity potential fields, the intention is to provide a clearer understanding of how tropical circulations (e.g., Global Walker Circulation and Hadley Circulation) are modulated over time and how this can influence tropical convection on varying temporal scales (See Section 5).

The independent precipitation and outgoing longwave radiation (OLR) datasets indicate that precipitation has been above average across central and southern Africa while the African Sahel and eastern Africa have been drier than the prior years (i.e., 2019-2021) during February-March-April (FMA) (Figure 11). This is indicative of the slow decline in focus of upward motion over the region as the multi-year La Niña has progressed and promoted development of negative IOD. This was a natural progression observed in prior multi-year La Niña events (e.g., 1998-2001 and 2010-2012) where tropical Africa was wettest leading up to the development of La Niña and gradually dries in the following years as the focus of upward motion favors the Maritime Continent. However, further investigation is warranted to fully understand how these complex interactions in the global tropical climate system influence ENSO and vice versa.

In conclusion, it is apparent that the climate state is gradually shifting away from the amplified WAM regime of prior years which could dampen the enhancement of AEWs and early development of TCs in the eastern tropical Atlantic compared to prior years. Nevertheless, intraseasonal forcing (i.e., MJOs and CCKWs) will be important in anticipating periods of greater enhancement of AEWs as these events pass over the African continent and emphasizes the range of uncertainty that could play a role in deciding lower or higher end outcomes for TC activity this season.

5. Climate Background State

5.1 Background

Since climate teleconnections can be observed on varying temporal scales from shorter (e.g., daily, weekly, monthly) to longer (e.g., seasonal, interannual, decadal), it can be challenging to fully understand their scope and impact in real-time. Applications that have made climate teleconnections in the tropics more relevant on shorter temporal scales have been by employing upper-level velocity potential fields to track these teleconnections (Krishnamurti, 1971, Emanuel, Neelin, and Bretherton, 1994; Trenberth, Stepaniak, and Caron, 2000). Upper-level velocity potential is a measurement of the horizontal wind field aloft (in relation to divergence) that is useful in identifying regions of upward and downward atmospheric motions given the known principles of mass continuity in the atmosphere. Through this relationship, subsequent literature has identified strong correlations between upper-level velocity potential fields and precipitation and OLR (Section 4). These new applications increased interest in understanding how upper-level velocity potential could be used in monthly and seasonal forecasting of tropical convection and TCs that subsequently form out of this convection which are applied in this outlook (Roundy and Schreck III, 2009; Roundy, Schreck III, and Zaniga, 2009; Ventrice et al., 2011). Through this increased understanding, there has been further investigation into how the long-term trends (i.e., longer temporal scales) in upper-level velocity potential can be used to track decadal and multidecadal variability in the global Walker circulation (Figure 12) that influences tropical convection and TC activity (Bell and Muthuvel, 2006).

As discussed in Section 4, recent investigation into global weather reanalysis agreement with independent weather observations have revealed discrepancies in performance between commonly used datasets (i.e., NCEP/NCAR R1, JMA JRA-55, and ECMWF ERA5). In the early literature regarding this topic, the NCEP/NCAR R1 dataset was predominantly used to evaluate tropical climate variability and change. However, the findings of the study decreased confidence in its use (especially during the summer months when tropical convection is most active) and instead found the newest reanalysis, ECMWF ERA5, was more reliable and in better agreement with independent weather observations (Figure 13). Therefore, the ECMWF ERA5 was preferred for use in this outlook when evaluating variability of the upper-level velocity potential fields through present day and NCEP/NCAR R1 was employed only for supplemental analysis.

5.2 Analysis

The low-frequency (i.e., temporal scale of months to years) variability of the tropical climate depicted by the ECMWF ERA5 over the key regions of variability can be used to identify numerous events in long-term modulation of the tropical circulations that could be attributed to periods of enhanced or suppressed tropical convection. However, for the sake of simplifying this analysis, further investigation and discussion of this variability will be completed at another time (Figure 14).

Since the conclusion of the Strong El Niño event of 2014-16, the mean low-frequency has shifted toward a more La Niña oriented mode with more rising over the African and Maritime continents. This was first initiated with the 2016-18 La Niña event followed by a strong positive IOD that focused warmth closer to the African continent during the weaker warm ENSO event from 2018-20. With the weakening of upward motion over the Maritime Continent and a strong positive IOD, this left the African continent as the sole area of anomalous upward motion when ENSO began to cool again during summer of 2020. This anomalous upward motion (i.e., negative upper-level velocity potential) over tropical Africa was the strongest such values since the multi-year La Niña event of 1998-2001 and assisted in significant cooling of the equatorial Pacific through anomalously strong easterly trade winds that caused significant upwelling through the remainder of 2020. The anomalous upward motion also fueled a significantly strong WAM and subsequent AEWs that enhanced TC activity during the 2020 hurricane season. During 2021, La Niña began to take hold as the predominant mode of the climate state as anomalous rising over Africa waned and gave way to a stronger focus of upward motion closer to the Maritime Continent. With La Niña present, the feedbacks enhanced development of a more negative IOD as anomalous westerly winds transported warmer waters toward the Maritime Continent and cooled the western Indian Ocean thus reducing the amount of warmth focused near Africa. While the anomalous rising has persisted to a lesser degree over western Africa thus far this year, the rising motion over the Maritime Continent remains in firm control of the global climate state with strong downward motion dominating the eastern Pacific Ocean, South American continent, and western Indian Ocean (Figure 15). This global upper-level velocity potential anomaly pattern is in accordance with correlated regions to increased North Atlantic ACE that are statistically significant (Figure 16). This dominance has also been quite apparent in the suppression of MJO activity across the equatorial regions throughout Spring 2022 which enabled strengthening of La Niña conditions. Therefore, this configuration is favorable for TC activity in the North Atlantic as it suppresses convective activity in the Eastern Pacific and reduces vertical wind shear across the MDR. Additionally, based on dynamical climate model guidance, there is no expectation that this will significantly change with La Niña anticipated to persist through the remainder of the year.

6. Seasonal Analogs for Hurricane Season

6.1 Background

With advancement in understanding of TC variability in the North Atlantic, the variables discussed in this outlook that possess strong relationships to this variability can also be used to find commonalities in history to better understand the potential outcomes of the upcoming season. Through this approach, the identification of years which featured similar modes and climate backgrounds can be used to deduce which outcomes appear most likely.

6.2 Analysis

The base characteristics used to identify analog years were within multi-year La Niña events that persisted into peak hurricane season, a negative PMM and PDO, and an Atlantic SST configuration with near average MDR SSTs, above average subtropical Atlantic SSTs, and near average to below average equatorial Atlantic (Figure 17).

Identification of analogs that fit these characteristics was somewhat difficult given the infrequency of strengthening multi-year La Niña events. However, the years that best fit these characteristics are 1999, 2001, 2008, 2011, and 2021 (Figure 18). The analog average activity yielded from these years was: 16.6 named storms, 7.8 hurricanes, 4.4 major hurricanes, and an ACE of 141.0 (*10^4 kt2). This average would be considered an above average hurricane season with 2001 observing the lowest ACE (110) out of these five years (Table 1). Further, the averaged TC activity from these years for the Western and Eastern Pacific was below the 1991-2020 average indicating that TC activity was more favored elsewhere in the tropics (Table 3. The primary caveats to using these analog years were the predominance of warmer equatorial Atlantic SSTs in most cases, the Maritime Continent SSTs being cooler than 2022, and only 1999 having as cool of SST anomalies in the far eastern equatorial Pacific. These years also encompass a range of MDR warmth for peak hurricane season from near to above average that aim to exemplify the range of possible outcomes. Other noteworthy years that differed in certain aspects but were similar in evolution of ENSO and climate background state included 1955, 1985, 1989, 1996, 2000, and 2020 (Table 2).