2022 August Update

Through the first two months of the North Atlantic hurricane season three named storms and no hurricanes have developed with a seasonal ACE value of 2.7 (10^4*kt^2). By the first week of August the North Atlantic on average (1991-2020) has observed 3.3 named storms, 0.9 hurricanes, and 0.2 major hurricanes with a seasonal ACE of 12 indicating that activity thus far is near the climatological average. While activity was anticipated to be more robust to an extent through the early portion of the season, it must be acknowledged that the North Atlantic’s climatology for hurricane formation is far more peaked around September than any other basin globally. Therefore, the basin is no stranger to quiet periods through the early and late season.

The season kicked off on time with the formation of Tropical Storm Alex in early June which originated from the Central American Gyre and partially from the remnants of Hurricane Agatha. Alex’s interaction with an upper trough kept the system highly sheared and lopsided for its entire lifetime as it traversed Florida and moved into the southwest Atlantic before becoming extratropical. By late June, attention turned to a vigorous tropical wave in the central tropical Atlantic which benefited from the convectively enhanced phase of a strong convectively coupled Kelvin wave (CCKW). Although, due to its low latitude and broad nature, the system struggled to organized as it moved through the southern Caribbean before eventually becoming Tropical Storm Bonnie in the southwest Caribbean on July 1. Bonnie was a healthy early-season tropical storm but was limited by its lack of time over water as it made landfall in southern Nicaragua the next day. However, Bonnie’s healthy structure enabled it to survive the crossing of Nicaragua and continue on to become a major hurricane in the eastern tropical Pacific thereafter. Nearly simultaneously, a well-defined low formed in association with a weak surface trough interacting with an upper-low off the coast of Georgia on July 2 and was promptly named Tropical Storm Colin. Environmental conditions were largely unfavorable, and the combination of dry air and wind shear quickly degraded the tropical cyclone’s structure later that day leading to the dissipation of the system on July 3.

With the lack of long-lived, stronger tropical cyclones there has been concern over the validity of the June seasonal forecasts that called for a well-above average hurricane season. While there are some valid questions to discuss regarding whether the season will live up to some of the lofty forecasts or not, it must be acknowledged that on average 90% of the Atlantic’s hurricane activity is still to come based on seasonal ACE. Other valuable examples in history to reiterate this point are seasons such as 1955, 1988, 1999, 2000, and 2011 which observed most, or all, of their hurricane activity after the first week of August. Even in recent memory, years like 2019 had two storms through August 20 before going on to produce 16 more storms, two of which being Category 5 hurricanes (Dorian and Lorenzo). In conclusion, the observed seasonal activity thus far has been near the climatological average and now is not the time to let your guard down.

1. North Atlantic SSTs

In the June discussion, North Atlantic sea surface temperatures (SSTs) had trended more favorably above average across the Main Development Region (MDR) of the North Atlantic. This yielded the potential for higher outcomes in seasonal activity denoted in the June range forecast with acknowledgement that the magnitude of additional warming would be crucial in determining the outcome of the season. The North Atlantic SST anomaly configuration has changed significantly since the beginning of hurricane season with significant cooling of the equatorial (5°N-5°S) Atlantic and lower subtropical (20-30°N) Atlantic extending along the Canary Current in addition to high-latitude (40-60°N) warming near the Canadian maritime. This was a result of a persistent high-latitude ridging regime over northeastern Atlantic which favored a stronger Azores high and subsequent strong trade winds along the Canary Current throughout the month of July. Consequently, the Atlantic MDR does not reflect the same hyperactive look it once did earlier in the season with SST anomalies generally near average and thus the higher end outcomes have been reduced as a result. What should be acknowledged, however, is that average SSTs will not be an inhibiting factor to hurricane activity in the basin especially during the climatological peak of hurricane season so long as further cooling does not occur. Further, the cooling of the subtropics is not necessarily an inhibitor either as many analog years observed similar SSTA configurations and had little trouble producing long-lived hurricanes during peak season. Rather, it appears that the trajectory of this hurricane season is moving toward a more climatological normal season rather than one that leads with a spurt of early season activity as observed in recent years. While this creates concern regarding the probability of verification in higher end outcomes for hurricane activity, confidence remains adequate for average to above average (100-125% of normal) hurricane activity to occur through the remainder of the season based on the current SST configuration.

2. El Niño Southern Oscillation

The 2021-22 La Niña reached its initial peak strength in November-January (NDJ) with average SST anomalies of -1.0°C (Ensemble ONI) and appeared poised to weaken into spring as a December MJO event induced a downwelling Kelvin wave (DKW) that warmed the subsurface equatorial Pacific into February. However, the tide quickly turned against ENSO warming after a series of strong Easterly Wind Bursts (EWBs) that caused significant upwelling of the subsurface and resulted in an anomalous cooling trend in ENSO through May. This led to the strengthening of the La Niña into a top 2 strongest event on record behind only 1893 as hurricane season began (Webb and Magi, 2021). While SST anomalies of April-June (AMJ) would not be considered to be in the strong La Niña (-1.5°C or colder) category, La Niña events weaken on average during summer months as result of the preceding transition season and monsoon circulation forcing. Therefore, it wasn’t that this La Niña was anomalously cold all-time, but rather it was anomalous that the event strengthened into summer rather than weakening.

Since its secondary peak, La Niña has been in gradual decay with coldest anomalies focused in the central Pacific and south of the equator. This decay has largely been induced by intraseasonal forcing favoring the Central American Gyre (CAG) during an MJO passage in May-June that weakened trade winds and enhanced downwelling in the eastern tropical Pacific. This, in combination with ITCZ being latitudinally enhanced poleward due to cooler equatorial waters, subsequently led to enhanced Pacific hurricane activity through June and July.

By the end of June, enhanced trade winds associated with a record strong and prolonged EWB returned to the equatorial Pacific. This has led to significant upwelling and the initiation of a new UKW that will restrengthen La Niña into peak hurricane season. This restrengthening is supported by the climate model ensemble guidance (NMME & IMME) with the mean focused around moderate La Niña before gradual weakening commences through Winter 2022-23. With the persistence of a formidable La Niña event, the Atlantic MDR will be on average more favorable for hurricane activity through the remainder of the season.

3. Pacific tropical cyclone activity

The eastern North Pacific has been off to a busy start with 9 named storms, 7 hurricanes, 2 major hurricanes and a seasonal ACE of 78 (~151% of normal) through the first week of August. This anomalously active beginning to the season was kickstarted by an active CAG regime in May and June that assisted in the formation of Agatha, Blas, and Celia and aided localized downwelling of ocean waters in the eastern Pacific. This warming yielded a localized more positive Pacific Meridional Mode that has amplified the ITCZ poleward. The basin has also benefited from a consistent tropical wave train provided catalysts for new hurricane development on a regular basis. While this two-month period has overperformed seasonal forecasts for the basin, this magnitude of hurricane activity is not completely unheard of even in multi-year La Niña years. As mentioned in section 2, La Niña typically weakens into spring and summer as intraseasonal forcing, like MJO, become a more favored mode of variability for tropical convection. This leads to more of a Modoki, or central-Pacific based, configuration to the La Niña SST pattern. This typically causes the strongest downward motion and suppression of convection to occur in the central Pacific and can open the door for a more active eastern Pacific than first year La Niña events.

Given what has been observed thus far, the likelihood of the eastern North Pacific observing a near average, or even slightly above average, hurricane season have increased significantly. However, barring periods in which intraseasonal forcing becomes more favorable via CCKW and MJO, the basin should remain spatially limited to where hurricanes can form as is common in La Niña. It would also be plausible that the basin quiets down substantially into fall as La Niña restrengthens which would further aid hurricane activity in the Caribbean Sea.

The western North Pacific has been the relative quietest basin under current La Niña conditions with the basin observing a seasonal ACE value of only 22 (26% of normal) thus far, the quietest since 1998. The SST configuration in the basin is indicative of many quiet years with meager anomalous warmth in the tropics and La Niña conditions dominating the equatorial Pacific keeping the areas of tropical cyclone formation in the basin rather limited close to land areas. Further, the lack of MJO activity and strong EWBs have inhibited much assistance from intraseasonal forcing. Based on prior analog years, it’s possible that 2022 will finish in the top 10 quietest years in the western North Pacific if current trends hold through the fall.

The top analog years remain largely unchanged from the June outlook with the primary focus in selection prioritizing a near average tropical Atlantic, cooler subtropical Atlantic, and multi-year La Niña conditions. 2001 was removed from the top analog set given its warmer ENSO conditions and more active western North Pacific tropical cyclone activity and was substituted with 2000 as a more comparable SST anomaly configuration. 1988 was also added to the top analog set as another strong La Niña year with a near average eastern North Pacific hurricane season and comparable SST anomalies to the North Atlantic which could lend as a scenario on the lower end of the range of outcomes. The new analog average is slightly lower from the June outlook which is indicative of the recent trends denoted in this update.

2011 remains the top analog of the group with the reminiscent central US ridging regime and east coast troughing shaping much of the summer weather thus far. Additionally, 2011 observed a more active than expected eastern North Pacific with 6 major hurricanes and a near average seasonal ACE value and a below normal western North Pacific typhoon season. This lends confidence in this solution as a more probable outcome for tropical cyclone activity in the Northern Hemisphere.

Trends in Mid-Latitude Steering

The mean mid-level steering dominating the North American weather pattern throughout the summer months has featured anomalous central U.S. ridging that has led to significant heat and drier conditions across the region. Simultaneously, significant flooding has occurred around the northern periphery of the ridge as persistent mesoscale convective features have tracked over the same areas. Additional synoptic features that have frequented the North Atlantic and North America have been the high-latitude troughing from Ontario to the northeast U.S. as well as the anomalous North Atlantic ridging in the high-latitudes and mid-latitude troughing underneath.

The seasonal analogs can also provide some indication of what track patterns might be more favored into peak season (Figure 1). The currently observed pattern is quite similar to what was observed in 2011 with some minor differences. Assuming this pattern persists like it did in 2011, this would enhance the likelihood of storms traveling out to sea due to the predominance of east coast troughing which aids in recurving systems. Meanwhile, the central U.S. ridging would keep systems more latitudinally suppressed into Central America if they take their time to develop and move into the Caribbean. If this were to occur, it would decrease storm longevity and limit intensification of storms due to their reduced time in favorable environments for strengthening. Alternatively, this would increase the likelihood of seeing land impacts for systems that take their time to develop and subsequently enhance the risk that a storm rapidly intensifies prior to landfall regardless of its shorter lifespan.