2023 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 periodicity 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 changes in the North Atlantic SST configuration have 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

Since February, the North Atlantic MDR has observed significant warming due to the presence of consistent negative NAO synoptic patterns slowing trade winds across the tropical Atlantic. The most significant warming period occurred in April when a strong negative NAO aligned with passage of the Madden-Julian Oscillation (MJO) causing anomalously weak trade winds that allowed downwelling surface ocean heat across the eastern half of the North Atlantic (Figure 2). In particular, this warming greatly warmed the Canary Current region off the west coast of Africa yielding a more positive AMM and an SST configuration similar to more active hurricane seasons (Figure 3). The extent of ocean heat in this area is particularly important as stronger surface fluxes can increase tropical instability and mixing of the troposphere which helps to mitigate the negative affects of the Saharan Air Layer (SAL) during the peak trade wind season which lasts until August. As of May, the MDR preliminarily ranks as the 3rd warmest since 1950 and warmest among El Niño years (per ERSSTv5). With such warmth present in the North Atlantic, it is apparent that this western hemisphere SST configuration is uncommon and yields higher uncertainty regarding the magnitude of hurricane activity that could occur despite the development of El Niño.

Since El Niño predominantly influences vertical wind shear and suppression of tropical convection across the western half of the Atlantic basin, it’s possible that enhanced favorability of the eastern and subtropical Atlantic allows for enhancement of TC activity in these areas while the rest of the basin remains suppressed. However, it’s worth acknowledging the SST and relative humidity climatology of the eastern and subtropical Atlantic is still less hospitable to stronger, long-lived TCs. This could allow the number of TCs that form to be closer to average in the basin, but the limitations of climatology in these areas yield less hurricane activity as storms are spatially limited and have less ocean heat content to work with.

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 three consecutive years of La Niña influencing the global climate system, observations across the equatorial Pacific indicate that La Niña has dissipated and ENSO neutral conditions are now present. Dissipation of the multi-year La Niña event began back in November with a low-amplitude passage of the MJO which was sufficient enough for relaxing of easterly trades and initiation of a DKW. This began the warming trend of the subsurface equatorial Pacific into early 2023 that was then enhanced further by a record strong Pacific MJO event in March. This event produced anomalous westerlies west of the IDL and across the equatorial eastern Pacific resulting in further downwelling of warmer surface waters (Figure 4). MJO events of this magnitude during transition seasons are harbingers for El Niño development and was last observed during the development of the strong El Niño event of 2015-16. This has caused a more significant warming response in SSTs across the equatorial Pacific in which all ENSO regions are now near or exceeding the El Niño threshold (+0.5⁰C) with Niño 1+2 peaking near record territory around +3.0⁰C in April (Figure 5). This rate of warming coupled with subsequent Pacific MJO activity and reduction of trade winds indicates development of El Niño is imminent during the coming months.

May dynamical climate model forecast consensus (i.e., CPC/IRI) across Niño 3.4 for August-September-October (ASO) is +1.6⁰C and the ENSO analog average is +1.3⁰C, both peaking by October-November-December (OND) between +1.6 to +1.8⁰C (Figures 6 and 7). Based on the earlier onset of this El Niño compared to prior weaker events as well as dynamical climate model forecasts, it is likely that a moderate to strong El Niño will be in place for peak hurricane season. Onset of El Niño conditions will inhibit hurricane development in the Atlantic basin, especially in the Caribbean Sea and Gulf of Mexico, where vertical wind shear will be enhanced by more prevalent deep convection in the eastern Pacific. This will not mitigate hurricane development completely in the basin as corridors of more favorable vertical wind shear can still occur on a more synoptically-driven basis. However, more restricted spatial and temporal domains for hurricane activity in the Atlantic basin will reduce the ceiling on higher-end seasonal activity outcomes. Therefore, onset of moderate to strong El Niño conditions will reduce hurricane activity in the Atlantic basin and greatly decrease the likelihood of above average hurricane activity being observed.

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 PDO has been consistently negative since 2020 and reached its lowest value since 1955 in October 2021 and has stayed between -1.5 and -3.1 since. As such, SSTs across the western coast of North America and WDR have been much lower than average. These cooler SSTs have limited the spatial favorability of TC activity in the eastern and central Pacific over the last few years. However, with much colder equatorial Pacific waters due to La Niña, the PMM became more positive in 2022 which allowed the basin to observe TC activity closer to average. 

With the ongoing development of El Niño and warming of the equatorial Pacific in conjunction with the still negative PDO, the PMM has become negative once more. WDR SSTs are slightly lower than normal and more similar to 2019-20 which does make the eastern Pacific less conducive to long-lived TCs. However, the development of El Niño will overcome any inhibitions that a negative PMM will have by peak season and this is supported by the TC activity observed in the ENSO analogs (Section 6). What this does indicate is that the eastern Pacific probably will not observe record levels of activity like it did in the last two El Niño events (2015-16 and 2018-19), when the spring PDO and PMM were positive. This will leave the door open for the Atlantic basin to be more active than a normal strong El Niño year, especially outside the Caribbean Sea and Gulf of Mexico, in the case that eastern Pacific TC activity is closer to climatological average.

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

After the gradual decline of the wet period over the African continent in recent years, the dissipation of La Niña has allowed the African continent to somewhat regain a foothold in the climate background state (Section 5). This indicated in the increase in magnitude of -VP200 anomalies this spring over the area and slightly wetter anomalies over equatorial Africa. Enhanced upward motion over the African continent has also favored easterlies over the Indian Ocean causing the Indian Ocean Dipole to become more positive (Figure 8). This feedback has allowed warmer waters to focus over the western Indian Ocean and enhance sinking over the Maritime Continent which is favorable for El Niño growth.

With the strengthening of the +AMM regime over the eastern Atlantic, the temperature gradient between the tropical and equatorial Atlantic has been enhanced. This gradient will enhance the AEJ and WAM and result in an anomalous poleward shift in the ITCZ. These factors in conjunction with a warm MDR can increase tropical cyclogenesis (TCG) rates of AEWs as they track further north and are more positively influenced by Coriolis force. However, the aforementioned drier and cooler SST climatology of the east Atlantic still provides spatiotemporal limitations to quantity and intensity of TC activity in this part of the basin. Further, a stronger AEJ (stronger mid-level easterlies) could increase vertical wind shear over these AEWs causing them to struggle before arriving further west where westerly vertical wind shear will be enhanced by El Niño leaving limited areas for development in the tropics. Instead, it’s possible some of these waves continue northwestward, endure vertical wind shear and less favorable conditions, and develop in the subtropics. What this alludes to is spatial restriction of TCG leading to overall less intense, long-lived TCs and a lower quantity of TC activity overall.

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 (VP200) to track these teleconnections (Krishnamurti, 1971, Emanuel, Neelin, and Bretherton, 1994; Trenberth, Stepaniak, and Caron, 2000). VP200 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 VP200 and precipitation and OLR (Section 4). These new applications increased interest in understanding how VP200 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 VP200 can be used to track decadal and multidecadal variability in the global Walker circulation (Figure 9) that influences tropical convection and TC activity (Bell and Muthuvel, 2006). 

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) (Stanfield and Ramseyer, 2022). 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 10). Therefore, the ECMWF ERA5 was preferred for use in this outlook when evaluating variability of VP200 through present day and NCEP/NCAR R1 was employed only for supplemental analysis. 

5.2 Analysis

Since the development of La Niña back in 2020, the primary areas of anomalous upward motion (-VP200) have been over the Indian Ocean and Maritime Continent. This caused development of a strong negative IOD in 2021-22 reducing the focus of upward motion over the African continent that was present 2018-2021. As a result, the African Sahel became progressively more convectively suppressed and more reliant on subseasonal forcing via CCKW and MJO. This, coupled with Hadley cell stretching due to mid-latitude heatwaves and suppression of the ITCZ during Summer 2022, led to the MDR being more convectively suppressed. Since then, more robust MJO activity in combination with decreasing Southern Oscillation Index (SOI) has indicated that the atmospheric signal associated with La Niña has rapidly weakened (Figure 11). With successive Pacific MJO activity this spring, it is increasingly clear the atmosphere is transitioning into El Niño which will only be further reinforced by warming of the equatorial Pacific.

Examining the low frequency regime and dynamical climate model forecasts for VP200, it is expected that anomalous sinking (+VP200) will continue to be observed over the Indian Ocean and Maritime Continent with anomalous upward motion over the eastern-central Pacific and tropical Africa in a wavenumber two configuration comparable to 2018-19. It’s yet to be seen if this transitions to a full-fledged El Niño standing wave which would suppress tropical Africa more later in the year as El Niño strengthens. Timing of such a change to the background state will be critical to anticipate during peak hurricane season as it could drastically alter the favorability of the Atlantic basin and suppress hurricane activity. This could create a more front-loaded distribution to TC activity in the Atlantic in which October and November are much more hostile than the earlier part of the season. Regardless, the strong agreement of anomalous sinking across the Indian Ocean (MJO Phase 1-2), a highly-correlated region to Atlantic hurricane favorability, is indicative of a background state that could be more limiting as the positive IOD and El Niño strengthens later this 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 key features used for identifying seasonal analogs for this hurricane season were years with moderate to strong El Niño events that began around or before June, negative PMM, positive IOD, and average or above average North Atlantic SSTs. While this configuration is uncommon, the years that best fit this description were 1951, 1957, 1965, 1997, 2002, 2006 and 2009 (Figures 12 and 13). The seasonal analog average activity of these years in the North Atlantic was 9.8 named storms, 4.3 hurricanes, 1.8 major hurricanes, and a seasonal ACE of 75.6 (62% of normal). Every year in this analog set except 1951 observed below average hurricane activity in the Atlantic basin (Table 1). Despite the inactivity and majority of storms developing and remaining out at sea during these years, there were still notable impacts from the few that did hit land (i.e. Charlie 1951, Audrey 1957, Betsy 1965, Isidore and Lili 2002). 

The primary caveat with the selection of these years were that few exhibited the magnitude of above average SSTs observed in the Atlantic basin currently which likely played a bigger role in their inactivity. However, it’s worth acknowledging that ENSO phase, especially strong El Niño, has a stronger relationship to hurricane activity in the North Atlantic than the magnitude of preseason MDR warmth, and the strength of this relationship only increases into peak hurricane season. Other years that exhibited similar characteristics to this year but had key flaws leading to their omission were 1969, 1972, 1976, 1982, and 2012 (Table 2). The majority of these cases either had discrepancies with ENSO progression or were too cold in the North Atlantic that discounted them, but there is still some value in gleaning through their outcomes. While most of these years also observed below average hurricane activity in the North Atlantic, years like 1969 and 2012 represent higher end scenarios. These two years observed above average SSTs in the North Atlantic similar to what is being observed now, but El Niño either failed to develop (i.e., 2012) or was much weaker than what is currently forecast (i.e., 1969). This allowed for hurricane activity to be less suppressed and thus storms that developed were more capable of taking advantage of the warmer than normal SSTs.

This set of analog years concurs that hurricane activity in the North Atlantic will be suppressed given the expected onset of moderate to strong El Niño conditions by peak hurricane season. It is acknowledged that there are outcomes in which more activity could occur, as portrayed by 1969 and 2012, if El Niño is slower to develop and above average North Atlantic SSTs remain. Even though North Atlantic activity was predominantly below average in these years, the analog averages indicate that the Pacific basins were only slightly above climatological average and not record-breaking (Table 3).