In the past few months, 2020 has been busy, but not ferocious. There have been 8 tropical storms as of 24th July. As you can see below, no prior observed year has reached 8 named storms sooner.
As if that wasn’t enough, a strong tropical wave is currently traversing the tropical Atlantic that has a good shot at developing into a tropical storm within the next few days. It could even become a hurricane by month’s end (2nd of 2020, after Hanna on 26th July). Imagine that – 9 named storms and 2 hurricanes, by July 31st!
This year’s astonishing pace is clear to see: The 8th storm (Hanna) of 2020 was designated 24/07, which is 9 days earlier than was the case in the previously fastest season, 2005.
Judging by a variety of key indicators, some of which I explore later in this piece, this may just be a taster of things to come during ‘peak season’ 2020 (Aug-Oct; most active trio of months on average).
The July update to the MetSwift analogues predicts high activity for Aug-Oct 2020. Compared to the 1950-2019 average, it predicts an additional 4-5 named storms, 2 hurricanes and 1 major hurricane.
The clear columns show the 1950-2019 average numbers of storms in each strength category, while the striped columns show the mean of the MetSwift analogue years (a selection which are expected to be most similar in the tropical Atlantic & Caribbean).
Combined with near-average activity post-peak, this would make for a very active season overall.
Bringing this together with the pre-peak (Apr-Jul) activity, here’s how the MetSwift analogues prediction compares with the busiest historical years and the long-term average:
This would make 2020 only the 2nd year on record to see 20+ named storms. The hurricane count could be impressive, too. Of the other top years in the list, 1969 saw no pre-peak hurricanes, 2010 one and 1995 two. Statistical prediction models also tend to underestimate the busiest seasons, due to their rarity (inherently lowers probability). So, I believe there’s a good chance 2020 will see more than 10 hurricanes.
In case you’re wondering, 2005 was on another level pre-season. By 2020’s current point in July (24th), it had already unleashed a staggering 3 hurricanes and two major hurricanes! Not only that, but the majors hit category 4 (Dennis) and category 5 (Emily) strength, the latter the strongest pre-August hurricane on record.
That’s the what, now let’s delve into the why – the main indicators for a very active peak to hurricane season 2020.
Atlantic: High-Octane Fuel?
One of the strongest influences on Atlantic hurricane season activity is the warmth of the tropical ocean. Tropical cyclones are fuelled by warm, moist air drawn from just above the ocean surface.
As a rough guide, tropical storms and low-end (category 1-2) hurricanes usually require sea surface temperatures (SSTs) of about 26°C or more, while major hurricanes (category 3-5) tend to require about 28°C or more.
In the past week, SSTs have been above average across nearly all the tropical Atlantic, Caribbean and southern Gulf of Mexico (see plots below). The northern Gulf of Mexico has just been cooled a degree or two by the wind and rain of Hurricane Hanna.
Maps showing sea surface temperature anomalies (differences to the long term average) for the past week (left) and day (right).
This is an impressive feat, rivalling the warmest years since 1950. The warmth has recently stepped up a notch from the Caribbean eastward. There’s now a vast expanse of 26°C+ SSTs and an exceptionally large swathe of 28°C+ SSTs (see left-hand plot below). This tweet by Eric Webb proves this by highlighting regions where SSTs are within the top 5% of historical years.
Maps showing sea surface temperatures (left) and ocean heat content (OHC; right). Notice how OHC is measured in units of energy per square cm i.e. energy density.
It’s not just the surface that matters though. Tropical cyclones bring rainfall that cools the sea surface. At this point, warmer water at depth will mix its way up to the surface. Only if there’s enough of that will SSTs stay at (e.g.) 26°C+ for long.
The temperature the ocean needs to be considered throughout the upper few hundred metres must be considered. This is routinely measured as ‘ocean heat content’ (OHC). As you can see in the right-hand plot above, the highest Atlantic basin values are found along a wide band that extends south-eastward from the Gulf of Mexico.
Sadly, no anomaly maps exist for this measure – but I know from monitoring conditions so far this century that this year’s OHC is among the highest on record in the Caribbean and Gulf of Mexico. You’ll just have to trust me on that one!
Pacific: La Niña to Empower Tropical Cyclones?
The official CPC/IRI probabilistic forecasts for the El Niño Southern Oscillation (ENSO) are slightly favouring a La Niña state to the tropical Pacific Ocean for the peak hurricane season activity months of Aug-Oct.
La Niña has some correlation with atmospheric conditions over the tropical Atlantic that support tropical cyclone development (see graph below).
ENSO = El Niño Southern Oscillation. Note the taller columns i.e. larger average tropical cyclone counts for the La Niña (cold ENSO) category.
It’s no guarantee, though, because the atmospheric response to the sea surface temperature changes is not reliable. It responds more readily on some occasions than others – it’s not always a big factor in season activity. For example, Aug-Oct has seen just 3 hurricanes in four La Niña years since 1950 (1956; 1970; 1973; 1988). That’s well below the 1950-2019 average of 8.5 (i.e. 8 or 9).
In July 2020, the atmosphere has been showing some response, but it’s not been very coherent. Some regions, most notably the Indian Ocean, are seeing weather patterns that don’t ‘mesh’ well with those that a La Niña event encourages.
Africa: Wave Fuel
It’s been a soaking summer so far for much of sub-tropical Africa (see below). Many areas west of Omdurman have seen over 600% of the usual rainfall!
Resulting from this, soil moisture is abundant. Plant growth is enhanced, leading to increased coverage. From these and the sodden soils, moisture readily evaporates in the very warm subtropical climate. That provides more ‘fuel’ for powerful thunderstorms, which often organise into ‘tropical waves’.
In this way, above-average rainfall positively feeds back on itself. It will take a major change in weather patterns to override that. Barring that, tropical waves are likely to be unusually strong (on average) during the next 1-3 months.
This is a big deal for the hurricane season. As they traverse the warm tropical Atlantic waters, these waves can develop into tropical cyclones, if other conditions allow. This typically occurs at least a few times in a season. Some of the largest, most damaging cyclones have originated in this way. This is the reason for the name ‘Main Development Region’ for the area boxed in the image above.
A Recipe for Danger
To summarise, now through Sep or Oct 2020 is likely to feature:
Unusually strong tropical waves, traversing widely warmer than usual waters, while the atmosphere is supportive for development more often than in an average year.
This a top-tier recipe for a highly active Atlantic hurricane season. It’s still not a guarantee of that (the weather is too complex for that!), but it would be surprising to see anything other than an above normal number of tropical storms and hurricanes.
With those comes risks to lives and livelihoods on both land and sea. Considering the complications imposed by COVID-19, this is an especially troubling time to be looking at an active hurricane season.
James Peacock MSc
Head Meteorologist at MetSwift
The featured image for this piece is a satellite view of simultaneous hurricanes Katia, Irma and Jose, captured 8th Sep 2017 by the National Oceanic and Atmospheric Administration‘s Suomi NPP satellite – NOAA View Global Data Explorer.