2021 Hurricane Outlook

Tropical Storm Ana

On May 19, the National Hurricane Center (NHC) began monitoring an area east of Bermuda for the potential development of a subtropical cyclone from a non-tropical low that was expected to develop. During the following day, a gale-force non-tropical low developed in the left-exit region of a cyclonically curved jet streak approximately 600 miles east-southeast of Bermuda. This low gradually drifted westward into the base of a weakening upper-level low where vertical wind shear was lower and gradually began to acquire subtropical characteristics by May 22. At 0900 UTC that day, the NHC upgraded the system to Subtropical Storm Ana with sustained winds of 40 knots (45 mph) which ultimately was the cyclone’s peak wind intensity through its lifespan. As the nearby upper-level low weakened, Ana’s wind field began to contract and convection, albeit shallow, became more concentrated near Ana’s center. By 0900 UTC on May 23, these characteristics were deemed sufficient in declaring that Ana had fully transitioned into a tropical storm as it began to accelerate northeast. Ana’s status as a tropical storm was short-lived as the cyclone began encountering mid-level dry air and cooling sea surface temperatures which caused it to weaken to a tropical depression. After becoming devoid of convection due to the increasingly hostile environment, Ana became a post-tropical cyclone on May 24 at 0300 UTC just 18 hours after transitioning into a tropical cyclone.

This is the unprecedented seventh consecutive year that a preseason named storm was observed in the North Atlantic since record-keeping in the basin began in 1851. As a result of this more consistent preseason activity in recent, the National Hurricane Center moved their issuance of regular tropical weather outlooks for the North Atlantic to May 15 beginning this year. These operational changes serve to acknowledge the trend in earlier storm formation in the North Atlantic basin over the last 30 years. While these occurrences appear to be becoming more common in recent years, it is noted that the assistance of improved satellite and surface observation capabilities have greatly improved the detection of tropical and subtropical cyclones in the North Atlantic basin over the last several decades. Furthermore, preseason activity does not hold a strong correlation to total seasonal hurricane activity in the North Atlantic.

Tropical cyclone (TC) activity has an adverse and perennial impact on life and property in countries adjacent to these 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 & 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 (*104 kt2) based on the 1991-2020 mean. These 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 these environmental variables are necessary, but not exclusive, for TC development as 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. 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). With this, 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.

I. North Atlantic SSTs

Background

Sea surface temperature (SST) anomalies in the North Atlantic leading up to hurricane season have been correlated to hurricane activity given 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 exists along the Canary Current region and eastern 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 the enhancement of the African Easterly Jetstream which is a key component in the production of hurricanes in the North Atlantic (see Section IV).

When evaluating the entire North Atlantic basin SST profile, one of the most well-known modes of variability in these SST anomalies that have been linked to TC activity is the Atlantic Multidecadal Oscillation (AMO). The AMO refers to the variance of SST anomalies across the North Atlantic Ocean and follows a 65 to 80-year period based on the observed data available to researchers (Kerr, 2000). A positive AMO phase is 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 is associated with below average TC activity in the North Atlantic due to the MDR being cooler than average. While the AMO has 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 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). Considering these recent findings, and their implication on altering the climate of the North Atlantic, the persistent below average SSTs in the Far North Atlantic and warmer subtropical North Atlantic and Canadian Maritime in recent years appear to validate these claims. 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 tropical Atlantic. Consequently, the frequency of negative AMO configurations observed in the spring SST profile appear to have increased recently (2014, 2015, and 2018). Further, even years where the spring base state of the tropical Atlantic has been warmer, such as 2016 and 2017, the SST configuration was not traditional to that of a positive AMO because the subtropics and Canadian Maritime remained warm. Despite these clear differences in SSTs during the months preceding hurricane season, warming of the tropical Atlantic through the peak of hurricane season was observed in each of the last seven hurricane seasons (2014-2020). Most notably, the start of the 2018 hurricane season observed near-record cold Atlantic MDR SSTs and still warmed significantly to near average by peak hurricane season. This was likely in large part due to the anomalous zonal westerlies enhanced by a strong West African Monsoon, or WAM (see Section IV for more). Therefore, the seasonality of these recent trends in North Atlantic SSTs can be explained by tropical Atlantic cooling in the winter months due to enhanced trade winds via positive NAO and warming of this region during summer months when the WAM is active.

Analysis

The May 2021 North Atlantic SST profile once again exhibits the same configuration seen year after year in recent with near or below average SSTs in the MDR, above average SSTs across the region between 20⁰N to 30⁰N with below average SSTs in the higher latitudes. This profile is comparable to 2020 with above average equatorial Atlantic SSTs also present, however, MDR SSTs are starting out marginally cooler this year. The existence of a warm equatorial eastern Atlantic, or Atlantic Niño, has been discussed as a factor that can latitudinally suppress the ITCZ when SSTs are cooler than average in the MDR during peak hurricane season (associated with a negative AMM). However, Atlantic Niño is a seasonally influenced feature that has a low amplitude in comparison to ENSO and is typically influenced by the WAM and subsequent local atmospheric circulations. Since 2017, the equatorial Atlantic has observed warmer than normal SSTs in the spring months preceding monsoon season which is a result of the anomalous 850 hPa zonal westerlies associated with an amplified WAM (O’Reilly et al., 2017; Dippe, Greatbatch, and Ding, 2018). Then, as the WAM strengthened and the ITCZ lifted north during summer, these warmer equatorial SSTs cooled during these years. Further, newer studies have found that the strength of the hemispheric temperature gradient in the Atlantic tends to matter more with regards to African Sahel and WAM intensity (Biasutti, 2019). With an amplified WAM and warmer subtropical SSTs, this can assist in MDR warming during hurricane season by reducing trade winds as evidenced by the warming observed during prior hurricane seasons. This is likely an aspect that will be beneficial to hurricane activity as the season progresses in the North Atlantic, however, it provides a notable level of uncertainty regarding how much SSTs warm in the MDR by peak hurricane season. With the increased reliance on this seasonality, it appears this aspect of the seasonal forecast will be a persistent source of uncertainty in years in which the MDR SSTs are not above average in preseason.

The March-April-May (MAM) mean 850mb zonal wind pattern can be used to explain the appearance of the May SST profile (Figure 2). The presence of anomalous easterly winds in the tropical Atlantic has inhibited warming in the region. Additionally, anomalous westerlies in the lower-latitudes and mid-latitudes of the east Atlantic have enhanced warming in the areas just north and south of the MDR. This zonal wind profile is indicative of a strong WAM and has been observed since 2018. In 2018 and 2019, zonal easterlies were stronger across the MDR that caused more significant cooling whereas 2020 observed weaker zonal easterlies and subsequently observed a warmer MDR at the start of hurricane season. 2021 appears to have had stronger zonal easterlies compared to 2020, however, not to the extent of 2018 which illustrates the difference in MDR SSTs between these years. With the recent uptrend in WAM strength for the first time since the 1960s, it was acknowledged in the 2020 outlook that the 1950s and 1960s featured an 850mb zonal wind pattern similar to 2018-2020 and observed a northward migration of this zonal wind pattern as the WAM amplified into peak hurricane season (Figure 3). This behavior yielded further warming of the tropical Atlantic during hurricane season and enhanced positive (cyclonic) relative vorticity in the region. Considering these enhancing factors in warming the tropical Atlantic and making the MDR more suitable for hurricane activity during peak hurricane season, this leads to the conclusion that this will be sufficient in supporting at least average Atlantic hurricane activity in 2021. However, a higher ceiling for hurricane activity could be possible if warming of the tropical Atlantic exceeds expectations.

II. El Niño Southern Oscillation

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 as 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 overall less convective activity 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 which 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 which is caused by the diffluent Ekman transport of ocean water away from the equator causing 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). Depending on the persistence of this WWB over the span of a couple of months the downwelling Kelvin wave (DKW) that is generated will propagate into the eastern equatorial Pacific over a few 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 (continents) or terminated by the atmospheric forcing present at that time. If no new DKW is generated, then the refracted Rossby wave (now 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).

Analysis

The eastern equatorial Pacific observed significant cooling through the latter half of 2020 due to a series of EWBs which generated consistent UKW activity and subsequent subsurface cooling of the ENSO regions. These events provided a strong background for La Niña development into peak hurricane season and assisted in record-breaking hurricane activity in the North Atlantic in October and November when the La Niña reached its peak at moderate strength (-1.0 to -1.4⁰C). After its peak in boreal fall and winter 2020-21, La Niña significantly weakened as spring MJO activity aided in generating WWBs in the central and eastern equatorial Pacific that were absent in spring 2020. As a result, the subsurface of the equatorial Pacific has warmed through spring with two DKWs observed thus far aiding in this warming (Figure 4). While termination of La Niña is expected to continue into boreal summer, it is worth acknowledging that the subsurface profile and DKW activity does not currently support continued warming into El Niño conditions by boreal fall and winter. For there to be greater confidence of El Niño conditions, a much stronger DKW would need to already be evident in the central equatorial Pacific given the climatology of known preceding El Niño subsurface profiles. Further, despite the MJO having been more active this spring compared to 2020, it has not generated a strong, persistent WWB capable of generating a stronger DKW and initiating El Niño growth. Given these trends, ENSO neutral conditions are expected through the boreal summer months with climatology favoring ENSO cooling into fall with the strengthening WAM and Walker circulation. This solution is indicated in numerous dynamical climate models as the strengthening WAM enhances 850 hPa easterlies across the central equatorial Pacific and subsequent UKW generation that would cool the subsurface profile. The presence of ENSO neutral conditions provides some uncertainty regarding the extent of its impact on hurricane activity in the North Atlantic since this would allow more intraseasonal forcing, such as MJO and Convectively Coupled Kelvin Wave (CCKW) activity, that can aid TC activity in the eastern Pacific. However, without El Niño conditions present, hurricane activity in the North Atlantic will be generally enhanced.

This ENSO forecast is supported by the North American Multi-Model Ensemble (NMME) which is a suite of dynamical climate prediction models. It is also backed by the European Centre for Medium Range Forecasts (ECMWF) and Meteo-France climate models. The consensus of these models predicts cool neutral conditions will be present during peak hurricane season; however, the level of uncertainty encompasses solutions for both warm neutral from models like the ECMWF and weak La Niña in solutions such as the CFSv2 (Figure 5). With this, it is likely that ENSO will be favorable for North Atlantic hurricane activity given the expected absence of El Niño conditions which will generally suppress convective development over the eastern Pacific and provide a below-average wind shear environment across the MDR.

III. PDO and PMM

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. The Pacific Meridional Mode (PMM) refers to the variance in SSTs between the subtropical Eastern Pacific and equatorial Pacific. More specifically, the PMM refers to how the region from Baja California to Hawaii compares with the equatorial Pacific and is typically in phase with the PDO. Additionally, this 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 PMM is important in anticipating how the eastern North Pacific could impact potential activity in the North Atlantic.

Analysis

The eastern Pacific observed one of its quietest hurricane seasons in nearly a decade in 2020 due to a strengthening La Niña and the coolest WDR since 2013. This was a notable change from recent record-breaking years which were fueled by a consistently positive PDO and PMM that dampened Caribbean hurricane activity. With a quieter eastern Pacific, the Caribbean Sea observed one of its most active seasons in recent decades with vertical wind shear during peak hurricane season at record lows.

The PDO and PMM are their most negative since 2012 this year. As a result, the WDR SSTs are cooler than 2020 and are their coolest since 2012. This is likely to continue to hinder eastern Pacific hurricane activity since the spatial domain of TC activity and ocean heat content remain meager by comparison to recent active years. This, in combination with the lack of El Niño conditions expected this fall, greatly increases the likelihood of below average hurricane activity in the eastern Pacific. However, as mentioned prior (Section II), the presence of ENSO neutral conditions does provide the opportunity for intraseasonal forcing to aid in TC activity even when background environmental conditions are less favorable. This was evident in August 2020 when the MJO propagated across the Pacific and aided in an uptick in hurricane activity in the eastern Pacific despite TC activity being generally suppressed throughout that hurricane season. In conclusion, the negative PDO and PMM would serve to increase the favorability of the MDR for hurricane activity in 2021 by suppressing eastern Pacific hurricane activity.

IV. AEJ, WAM, and African Sahel Precipitation

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 and its intensity can be characterized by the low-frequency velocity potential fields in this region. 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 & Thorncroft, 2014). The wetness of the African Sahel can be quantified in the NCEP/NCAR dataset (1948-present) through the columnar precipitable water, or PWAT, values (kg/m3) which estimates the amount of condensed water vapor is present in a vertical column of air. The AMM, as mentioned in Section I, 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 a positive feedback of a wetter African Sahel which then enhances the WAM further and reinforces MDR warming during monsoon season.

Analysis

Since 2016, the African Sahel region has become notably wetter than what had been observed in recent decades. This appears to be attributed to the amplification of the WAM which has enhanced stronger surface convergence and more persistent convective activity over the African continent and subsequently has been maintained in part by deep soil moisture content. 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. With the development of a moderate La Niña over boreal winter 2020-21, ENSO finally was able to recapture the climate signal whilst the WAM was out of season. Given this trend, it was of interest to investigate what degree this would impact the strength of the WAM in 2021. Based on NCEP/NCAR reanalysis PWAT data, 2021 observed a lower value (16.08) than 2019 (19.95) and 2020 (17.289) for March-April (Figure 6). This value is near the 2016-2020 (16.94) and 1948-1969 (15.78) PWAT averages that correspond to other active WAM periods. 2021’s PWAT value still indicates that the African Sahel is wetter than normal but just not at the record levels observed in the prior two years. Further, the warming of the equatorial Atlantic in recent months has been correlated to the enhancement of the AEJ and WAM during hurricane season (Dippe, Greatbatch, Ding, 2018; Biasutti, 2019).

With these elements in place, the WAM and AEJ are expected to be favorable to enhance hurricane activity in the North Atlantic once again this season. The tropical Atlantic has observed numerous intense and damaging hurricanes that have originated from AEWs off Africa since 2017. Contrarily, 2020 saw a significant number of these AEWs wait to develop until further west due to their large size and intrusions of drier mid-level air from the northeast Atlantic. This was evident in the geneses of storms like Isaias and Laura in the first half of hurricane season when both systems struggled to intensify before getting further west. This may be less of an issue in 2021 if the WAM is not as amplified and AEWs are not as sprawling, however, with cooler MDR SSTs these systems still may struggle before getting further west or into the subtropics. This may limit seasonal ACE output if storms are unable to develop or intensify until they get further west as what was observed in 2020. Regardless, the state of the African Sahel suggests that the AEJ and WAM will be favorable for an above average North Atlantic hurricane season.

V. Climate Background State

Background

Since the variability of climate teleconnection patterns are observed on longer temporal scales (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 more relevant on smaller temporal scales have been through the use of upper-level (200 hPa) velocity potential fields in tracking these teleconnections (Krishnamurti, 1971, Emanuel, Neelin, and Bretherton, 1994; Trenberth, Stepaniak, and Caron, 2000). Upper-level velocity potential is a scalar field that describes the divergent, irrotational component of the horizontal wind field aloft which 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 discussed the relationship between velocity potential fields and tropical precipitation. These applications increased interest in better understanding how upper-level velocity potential could be used in seasonal and intraseasonal forecasting of tropical convection and subsequent TC activity which are applied in this outlook (Roundy and Schreck III, 2009; Roundy, Schreck III, and Zaniga, 2009; Ventrice et al., 2011). From this, there has been further investigation into how the long-term trends in upper-level velocity potential can be used to track decadal and multidecadal variability in the global Walker circulation (Figure 7) that influences tropical convection and TC activity (Bell and Muthuvel, 2006).

Analysis

Since the conclusion of the Strong El Niño of 2015-16, the mean low-frequency (temporal scale of months to years) velocity potential pattern favored a wavenumber three configuration with the most anomalous rising motion present over the African continent and weaker negative (rising) anomalies over the maritime continent and east Pacific (Figure 8). This configuration was associated with the amplification of the WAM that began in late 2016. After the conclusion of the 2018-2019 El Niño, the rising cell associated with the WAM strengthened significantly toward a wavenumber one pattern with rising over Africa and the Indian Ocean in 2020. This transitioned back to a wavenumber two pattern with rising over Africa and the maritime continent with the onset of La Niña during winter 2020-21 which remains the predominant pattern for March-April-May 2021 (Figure 9). Rising over the African continent and Indian Ocean favors strong 850 hPa zonal easterly anomalies across the tropical and equatorial Pacific and enhanced 850 hPa zonal westerly anomalies across the tropical and equatorial Atlantic. The induced slowing of easterly trade winds in the Atlantic serve to enhance surface convergence. Further, the existence of the easterly trade wind belt directly north of the deep tropics has been observed as an enhancing factor of relative vorticity and tropical cyclogenesis accordingly in the MDR during hurricane season. Thus, it is no surprise that a velocity potential pattern such as what has been observed since 2020 is consistent with the most active (seasonal ACE ≥150% of average) North Atlantic hurricane seasons from 1950 to present (Figure 10).

An aspect that will need to be closely monitored into peak hurricane season will be whether the WAM can recapture the low-frequency climate signal as La Niña continues to weaken. If this occurs, the WAM could significantly influence MJO propagation and maintain a more favorable background state for the North Atlantic. The restrengthening of the WAM would also enhance stronger easterly wind anomalies across the tropical and equatorial Pacific and increase the likelihood of ENSO cooling again in boreal fall and winter. However, if this does not occur, the presence of ENSO neutral would likely allow MJO and CCKW activity to be the predominant mode of variability in TC activity during hurricane season and would limit the windows of favorability for Atlantic hurricane activity in comparison to the most active seasons.

VI. Seasonal Analogs for Hurricane Season

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 seem most likely.

Analysis

The base characteristics used to identify analog years were weakening La Niña or ENSO neutral into peak hurricane season, a negative PMM and PDO, and an Atlantic SST configuration where the warmest SST anomalies were between 20⁰N to 30⁰N and the equatorial Atlantic. Additionally, given the evident seasonality of the AMM recently driven by the strong WAM, there is an element of anticipated warming expected in the MDR during the summer months. Another component that was accounted for in identifying analog years was the presence of a wet African Sahel and strong WAM. The years that best fit these characteristics are 1999, 2001, 2008, 2011, and 2020. The analog average activity yielded from these years was: 18.4 named storms, 9.2 hurricanes, 4.8 major hurricanes, and an ACE of 151.0 (*104 kt2) (Table 1). This average would be considered an above average hurricane season with only 2001 not featuring above average activity out of these five years (Table 2). The similarity of the global SST configuration between these years provided higher confidence in these selections, however, it was worth acknowledging that these years were predominantly before the recent strong WAM regime (2016-present) and emphasized the inclusion of 2020 as an analog with a comparable climate background state. These years also encompass the range of ENSO conditions for peak hurricane season from La Niña to ENSO neutral. Other noteworthy years that differed in certain aspects but were similar in evolution of ENSO and climate background state included 1950, 1996, 2000, 2012, and 2017.