An extratropical cyclone developed along a cold front east of the Canadian maritime on April 15. The system moved slowly southeastward across the North Atlantic with little to no signs of organization of the low. The low began to produce limited convection near an increasingly well-defined low level circulation which eventually led to the development of a curved band feature. The increase in organization of the low prompted the designation of Subtropical Depression One at 1500 UTC April 19. The depression changed little in organization through the following day due to proximity with an encroaching, large non-tropical low to its southwest. On April 20, convection became more concentrated near the center along with the wind field contracting which led to the transition of the cyclone into a fully tropical cyclone and being upgraded to Tropical Storm Arlene. Arlene is only the second named storm recorded in the Atlantic basin during the month of April. As Arlene began to interact with the large non-tropical low it unexpectedly strengthened into a 45 knot cyclone before becoming embedded into the larger aforementioned low. With the formation of Arlene, this marks the third consecutive Atlantic hurricane season with one or more named storms developing prior to the official start of the hurricane season on June 1.
The North Atlantic hurricane season occurs from June 1 to November 30 and averages around twelve named storms, six hurricanes, and three major hurricanes based on the 1981-2010 mean. Despite most tropical cyclones occurring during the six month season, there are occasions in which tropical cyclones do form prior to or after the season. This hurricane outlook will discuss the multitude of variables that can influence tropical cyclone activity in the Atlantic basin as well as what can be expected from this hurricane season given these variables.
The Atlantic Multidecadal Oscillation (AMO) refers to the variance of sea surface temperatures (SST) compared to average across the North Atlantic Ocean. The AMO follows a general 65-80 year period that has an amplitude close to 0.4 ⁰C that has been observed for about 160 years. A warm AMO phase has been associated with above average tropical cyclone activity due to the above normal warmth observed in the Atlantic’s Main Development Region (MDR) where a majority of Atlantic tropical cyclones form. The cool phase is associated with below average tropical cyclone activity in the North Atlantic due to the MDR being cooler than normal. In 1995, the Atlantic Multidecadal Oscillation made a notable transition from a cool phase to a warm phase which is believed to have been the spark that started what is known as an active period in Atlantic tropical cyclones. In 2014, with the onslaught of a warm ENSO event and a persistent positive North Atlantic Oscillation (NAO) winter pattern, the AMO cooled significantly creating speculation of a phase change being possible. 2015 continued such speculation when it became the second consecutive year of cooler than normal SST’s across the North Atlantic. During this timeframe of 2013 to 2015 all three hurricane seasons saw below average tropical cyclone activity. 2016 struck a different chord as the AMO recovered to a more positive state during the fall, and with the assistance of a cool ENSO event, the hurricane season became the first above average year since 2012. Despite this, the questionable state of the AMO still lingers due to differences in the indices that measure the AMO that began in 2013. During the below average seasons of 2013 to 2015, the Atlantic recorded mean Sea Level Pressure (SLP) was well above average during hurricane season. This was mainly due in part to the correlation seen between ocean temperatures and sea level pressure. When ocean temperatures are above normal in the Atlantic the mean sea level pressure typically drops below normal because of the increased warmth which promotes upward motion in the region. The AMO index constructed by Philip Klotzbach and William Gray incorporates mean SLP along with the mean SST anomalies in the North Atlantic to better illustrate how this would impact tropical cyclone development given both factors play into general favorability in the North Atlantic. The NOAA ESRL index is much simpler in its construction using a general mean of SST anomalies from 0-70N in the North Atlantic and then detrending that plotted data. Currently the Klotzbach and Gray AMO index estimate that the AMO has returned to the cool phase as of April 2017 while the ESRL index still displays that the AMO has remained in the warm phase.
Quasi-warm AMO phase characteristics are currently present in the North Atlantic. This warm signal in the AMO has been a recent development as we have approached the summer months and this additional warmth in the tropical Atlantic would tend to promote similar benefits that a warm AMO would supplement to tropical cyclones. Despite this, a classic Atlantic tripole is still not apparent due to an anomalous area of warmth located in the subtropical region of the Atlantic as well as below normal SST’s in the far North Atlantic (Figure 4). This gives us a very mixed signal leaving plenty of room for the SST configuration to sway one way or the other in terms of favorability. Overall, the current tropical Atlantic appears more favorable than seen in the last few years and should be capable of sustaining numerous tropical cyclones should the atmospheric environment be favorable enough.
The El Niño Southern Oscillation (ENSO) is defined as the variance of the SSTs in the eastern Equatorial Pacific Ocean. The period for the ENSO is two to seven years and can vary upwards of 2 to 3 ⁰C in amplitude. This climate oscillation is one of the most influential in overall global climate and is associated with significant variations in precipitation in the continents neighboring the Pacific Ocean. ENSO is categorized by three different phases: La Niña, neutral, and El Niño. A La Niña event is considered the cool phase of the ENSO and is associated with weaker upper level winds across the tropical Atlantic due to the lack of convective enhancement of the Subtropical Jetstream (STJ) in the lower latitudes during the summer months. El Niño is known as the warm phase of the ENSO 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 across surrounding basins like the North Atlantic and western Pacific. In the fall of 2016, a weak La Niña event was observed in the eastern equatorial Pacific. Since then the cool ENSO event has dissipated to a neutral phase, but based on the general consensus from climate modeling there is a possibility 2017 could feature an El Niño toward the end of the year.
The high variability nature of ENSO causes a large margin for error when making a seasonal forecast. This is especially true during the spring months when equatorial Pacific trade winds are at their most variable. Due to this high uncertainty during this time, the term “spring barrier” has been coined to describe the forecasting skill barrier climate models face in attempting to best depict the general global SST profile months in advance. The swift shifts in SST anomalies during the spring add extra challenge to the climate model initialization of SSTs. A noteworthy change in recent ENSO forecasts have been the initialization of the ENSO regions closer to what is currently observed as opposed to the warm bias in their ENSO forecasts due to poor initialization in prior forecasts. This more accurate initialization of the SST profile in the eastern Pacific has resulted in most of the climate models, such as the CFS, forecasting a much weaker warm ENSO event for fall 2017. The strength of the potential El Niño event will help determine exactly how significant the effects on Atlantic tropical cyclone development are. If El Niño is weaker and/or later in evolution this could mean less impacts will be seen to Atlantic tropical cyclone activity during peak hurricane season.
The evolution of this El Niño would be non-traditional compared to what we have observed from the progressions of prior El Niño’s dating back to 1980. Most El Niño events, that have been observed, develop through westerly wind bursts coupled with downwelling oceanic Kelvin waves that shift warmer western Pacific waters eastward toward the central and eastern Pacific. Contrary to this, we have seen warming occurring in the eastern Pacific with a notable increase in ocean heat content off of South America due to weak trade winds across the eastern equatorial Pacific the past several months. This promoted a warming of the ENSO regions, but with a recent easterly trade wind burst it is evident that we haven’t seen any indication of the Walker Circulation weakening which is crucial to El Niño development. In addition to this, we have seen the complete dissipation of the downwelling oceanic Kelvin wave that was evident near the dateline which would mean there is no subsurface momentum to see further warming across the eastern equatorial Pacific. This makes it plausible that this stale mate in the equatorial Pacific could lead to no development of a full-fledged El Niño event. With this uncertainty of the exact strength of the warm ENSO event it could leave room for less inhibition of Atlantic tropical cyclones than initially anticipated, but there is still a chance as well that a warm ENSO event eventually hampers Atlantic tropical cyclone development as we approach fall depending on how the event evolves.
The African Easterly Jet (AEJ) refers to the region where easterly trade winds are enhanced due to the contrast in temperatures between the Sahel and the Gulf of Guinea during the summer. This Jetstream is known for transporting tropical waves across the African continent and into the Atlantic Ocean. Tropical waves that emerge into the Atlantic account for the majority of tropical cyclone genesis that occurs in the basin. The differences in placement of the AEJ can have significant effects on the overall capabilities tropical waves will have in the basin. When the placement of the AEJ is further north, tropical waves are enhanced due to the increased latitudinal distance from the equator that allows for greater vorticity. Contrarily, a southerly displaced AEJ is associated with the suppression of tropical waves which inhibits further development once the waves have emerged into the Atlantic. Good indicators on the favored placement of the AEJ as we approach summer are differences in western African rainfall and SSTs in the Gulf of Guinea.
Based on the available observations we have, the Sahel has been wetter for the third straight year. Wetter years for the Sahel typically bring along with them more limited Saharan Air Layer (SAL) outbreaks during the summer. SAL is at its most active during the early part of the hurricane season and can inhibit tropical cyclone formation as well as cool the MDR by the dust particles increasing the reflection of solar rays. With less SAL outbreaks and a more active AEJ it is more likely to have tropical cyclone formation across the Main Development Region.
Analog years are important to identify because they can give insight, although not always, into the type of outcome that can occur in the following months. 2017 has featured an interesting 500mb geopotential height pattern with anomalous ridging bridging across much of the North Atlantic from North America to Europe. This ridging has promoted a relative lowering of heights to its south that have promoted troughing and weaker easterly trade winds across the Atlantic thus far. The ENSO background of a dissipating La Niña transitioning into a warm ENSO event as well as a weaker AMO signal were important in selecting these analog years. Based on those parameters the years of 1932, 1957, 1965, 1976, 2002, and 2012 best fit 2017’s profile. The non-traditional AMO profile warrants the inclusion of both AMO backgrounds in the analogs to account for the uncertainty. All of these years observed a warm ENSO through the summer and fall months which overall did inhibit activity in the Main Development Region in most analog years. 1932 and 2012 are the only two years of the six that featured above average tropical cyclone activity in the Atlantic which was mainly due to the strengthening of the AMO. It is also worth noting that if an El Niño event does not materialize that activity would also follow much closer to that of 1932 and 2012. The average activity from these years is 11.7 named storms, 5.5 hurricanes, and 2.2 major hurricanes.
The 2017 Atlantic hurricane season will feature above normal tropical cyclone activity with 15 named storms, 8 hurricanes, and 4 major hurricanes, but this will be heavily dependent on the materialization of an El Niño event this fall. It is worth noting that if an El Niño event develops much stronger than anticipated that Atlantic tropical cyclone activity would be inhibited to a further degree. This scenario would be reflected by the lower end of the Range Forecast shown above. Despite the uncertainty that remains regarding ENSO, I have reasonable confidence in the likelihood that if an El Niño event were to occur that it would be much weaker than originally anticipated, and may not materialize at all. Due to this increased confidence in the ENSO we can anticipate a higher amount of tropical cyclone activity in the North Atlantic due to less inhibition by El Niño. This development along with the quasi-warm state of the AMO should be sufficient in enhancing Atlantic tropical cyclone activity and my hurricane outlook reflects such. This numerical forecast does not include tropical cyclones that form outside of the traditional hurricane season such as Arlene.
With above normal tropical cyclone activity this will generally increase the risk for landfall of one of these tropical cyclones. Regardless, the 2017 hurricane season is officially underway and it is crucial to be prepared ahead of the storm because no matter how many storms we anticipate in a given season it only takes one of them striking land to make it a bad season.