Above: A strong La Niña event spread cooler-than-average waters across the eastern tropical Pacific in 2010–11, as shown by this graphic of departure from seasonal sea surface temperatures on Dec. 15, 2010. The 2010 Atlantic season is tied for the second largest number of hurricanes on record (12) and for the third largest number of named storms (19). (NASA Earth Observatory)
It’s too early to make a confident forecast of how the upcoming hurricane season will evolve, but the tea leaves now on the table suggest that 2020 could be the Atlantic’s fifth season in a row with above-average activity. The latest harbinger is a shift in seasonal computer forecast guidance, heralding the possibility that a La Niña event could emerge later this year.
The mid-month model summary from the International Research Institute for Climate and Society (IRI) and the NOAA/NWS Climate Prediction Center (CPC), released on February 19, shows the odds of a La Niña event pushing above climatology by late summer. By the August-to-October period, the IRI/CPC probabilities include roughly a 35% chance of La Niña, a 20% chance of El Niño, and a 45% chance of neutral conditions. The long-term averages are about 25%, 25%, and 50%, respectively.
Most consistently bullish on a La Niña event is the NASA GMAO model (dark purple boxes below), which has been calling for La Niña conditions to develop for a number of months. A version of the NCAR-based CCSM4 model (light purple boxes) has sounded a similar note since January, and the NOAA CFSv2 model (dark green boxes) has gradually joined the pack (see the figure below).
The state of the El Niño–Southern Oscillation (ENSO) is notoriously difficult to predict. That’s especially the case from February to May—the so-called “spring predictability barrier”, when forecast skill is especially low for a variety of still-unsettled factors.
Keeping this firmly in mind, it’s still noteworthy that multiple models are now trending in the La Niña direction. Dynamic models, like the three highlighted above, tend to perform better during springtime than statistical models, which are leaning toward neutral conditions.
There’s also a tendency for La Niña to emerge in the year after a positive Indian Ocean Dipole—and last fall saw one of the most intensely positive IODs on record. (The +IOD has fed into events ranging from catastrophic drought and fire in Australia to record tropical cyclone activity in the Arabian Sea and flooding in East Africa.)
La Niña events tend to recur about every three to five years. They typically build in the northern fall and fade by spring, then sometimes rebuild for a second or even third year. The last La Niña event was in 2017-18, so it wouldn’t be premature for La Niña to return in 2020.
ENSO has been on the warm side of neutral since last summer, with most of the heat concentrated in the central Pacific (the Niño4 region). In its February discussion, CPC predicts that ENSO indices will move away from warm neutral, but it is not yet forecasting a La Niña event.
“Several models are quite bullish at knocking out the current warm-neutral ENSO conditions and going quickly to La Nina,” noted Phil Klotzbach (Colorado State University, or CSU) in an email. “We've had several healthy westerly wind bursts that have actually increased ocean heat content in the central and eastern Pacific in recent weeks. But, if those don't persist, I could see a quick transition over to La Nina. It's really too early to say. There's a whole lot of noise with ENSO this time of year.”
Why La Niña would spell potential trouble in the Atlantic
When warmer-than-average surface waters spread over the eastern tropical Pacific during El Niño, they foster rising motion that, in turn, favors hurricane-quashing westerly wind shear at upper levels over the Main Development Region (MDR) of the Atlantic. Likewise, the cooler-than-average waters of La Niña are associated with atmospheric subsidence; this supports easterly winds over the MDR and thus makes tropical cyclones more likely to form.
The presence of La Niña isn’t required in order for the Atlantic to have a busy hurricane season, but it certainly helps. The La Niña–flavored years of 2010 and 2011 are among several tied for the third-most-active Atlantic seasons on record (both years saw 19 named storms). The next La Niña year, 2016, was also quite active, with 15 named storms that included Category 5 Matthew and three other major hurricanes. La Niña conditions recurred midway through the hyperactive and catastrophic 2017 season that produced Harvey, Irma, and Maria.
What’s unsettling is that 2018 and 2019 overperformed even though La Niña was absent. The 2018 season brought 15 named storms, including Category 5 Michael’s assault on the Florida Panhandle, and 2019 tied as the fourth-most-active season on record, with 18 named storms, including devastating Category 5 Dorian in the Bahamas.
One factor making these last two seasons so active was consistently and unusually warm sea-surface temperatures in the Atlantic basin. This occurred in tandem with widespread global oceanic warmth in both 2018 and 2019—a flashing-red indicator of human-produced climate change. The Atlantic is starting out 2020 in a similar overheated state. Record-warm conditions were present over parts of the western Main Development Region in January, and sea surface temperatures (SSTs) are running well above average across most of the western Atlantic tropics and subtropics, though cooler than average over the eastern Atlantic tropics.
Klotzbach referred to the current state of tropical Atlantic SSTs as “quite toasty.”
Two other factors Klotzbach watches closely this time of year before preparing his April seasonal outlook for CSU aren’t pointing toward a busy season right now. The Atlantic Multidecadal Oscillation (AMO) is running slightly negative so far this year, he said—mainly because of relatively cold SSTs in the far North Atlantic—and sea level pressures (SLPs) have been running higher than average over the tropical and subtropical Atlantic.
He added a caveat, though: “Normally the tropical Atlantic would have cooler SSTs in association with the higher SLP anomaly. That’s not the case right now.”
Seasonal hurricane outlooks typically show little skill until springtime. For what it’s worth, CSU laid out five potential scenarios involving the AMO and ENSO in its December 12 “qualitative discussion” of potential 2020 hurricane activity. These scenarios led to a spectrum of possibilities from a hyperactive season to a tepid one. Notably, none of the options included a possible La Niña event.
The forecast group at TropicalStormRisk.com issued an extended range forecast in December 2019 calling for a close-to-average season, with the assumption that ENSO will be near neutral. Wisely, they added: "The uncertainties associated with this outlook are large and forecast skill at this extended range is historically low."