The 2018 Atlantic Hurricane Season: Where do we Go from Here?
With the formations of Beryl and Chris during the past week, along with Alberto back in late May, it seems as if the 2018 Atlantic Hurricane Season is off to a quick start. So far, we’ve had 3 named storms, 2 hurricanes, and an Accumulated Cyclone Energy value near 14. Climatologically speaking, we’re above normal for this time of year, and we’re even ahead of last year in terms of hurricanes and ACE.
However, the remainder of the season remains a big mystery. There are a lot of conflicting factors at the moment, either suggesting we will end up having a below or even above average season. The first factor most people look at to determine the future of activity in the Atlantic basin (or any basin, for that matter) is the state of the ENSO, or the El Nino Southern Oscillation. This is defined as a variation in trade winds and therefore, sea surface temperatures across the equatorial Pacific west of South America. The strength of this is usually confined to the Pacific Walker Circulation. A stronger PWC would result in stronger trade winds and would cool the sea surface temperatures, resulting in a La Nina pattern. A La Nina is typically favorable for Atlantic tropical cyclogenesis because it weakens convection over the Pacific through tropospheric descent, which results in rising motion and reduced wind shear across the Atlantic. The opposite of this is an El Nino, where the trades are weaker and any subsurface heat in the form of a Kelvin wave would rise up and result in warmer sea surface temperatures. This enhances Pacific convection, and the shear of which and surrounding vertical descent reduces tropical cyclone activity in the Atlantic. Currently, sea surface temperature anomalies are warmer than normal, but not quite enough to qualify as an El Nino. Climate models suggest a weak-to-moderate El Nino is likely to form by the autumn, which could result in a lessening of Atlantic activity. Recently, Southern Oscillation Index values, which measure the strength of the Walker Circulation, have trended negative, meaning that the environment is more conducive for an El Nino to develop.
In addition to the ENSO, the tropical Atlantic has gone through a multitude of changes over the past several months. SSTAs were significantly cooler than average in May and June, almost record cold. But since the beginning of July, the trade winds have subsided and the West African Monsoon has amplified, resulting in more tropical wave activity in the past 2 weeks. One of these waves developed into Hurricane Beryl. However, there is still more Saharan dust in the atmosphere than usual, which is choking off convection to these tropical waves. Beryl was able to develop into the storm that it did because it was located at a very low latitude within the moisture shield of the InterTropical Convergence Zone, had an incredibly small gale radius, and was in an environment boosted in favorability by a convectively coupled Kelvin wave. Had Beryl formed a few days earlier and about 3 degrees of latitude farther north, it would’ve likely remained either a wave or a weak TC. But the warming of the tropical Atlantic is likely to result in more tropical cyclones in the region than initially thought, which could cause more long-tracked hurricanes to develop.
Finally, the subtropical Atlantic is still warmer than the tropical Atlantic, anomalously speaking. When this is the case, more convergence is focused in the region, resulting in more convection. This causes the air to rise over the region and sink around it, in the tropical regions. This serves as a reversal of the regular Atlantic Hadley Cell, which allows air to rise in the tropics and sink farther north. This can cause more activity to be focused farther north or have tropical cyclones peak in the region. Climate models suggest that this pattern will remain, but might not cause too big of an issue as perviously thought, but can still inhibit activity. This could also cause the Madden-Julian Oscillation to have more negative phases than positive ones for the Atlantic, but if the positive phases arise in August and September, then activity will blossom more towards the peak of the season.
Right now, the most likely outcome is a near average season. An average season usually contains 12 named storms, 6 hurricanes, and 3 major hurricanes (category 3 or greater), along with an ACE value near 100. Most analog years, such as 1989, 2000, 2002, and 2014 are conflicting. The first 2 years were above average while the latter 2 were below average. This recent burst of July activity is likely over now, and due to the MJO configuration for the next few weeks, it likely won’t pick up again until early or mid-August. However, remember that it only takes one storm to make it a bad season for you. Hopefully, the storms can stay away from regions severely impacted last year, such as Puerto Rico, Texas, Florida, Costa Rica, and all of the Lesser Antilles, but always have a plan ready just in case one comes your way. Even if the storm is relatively weak, it can still produce flooding rains that can cause more damage than the high winds of a major hurricane. Take every storm seriously, and try not to make the hurricane’s category dictate your actions regarding the storm.