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Tropical Weather |

What to expect from La Niña and El Niño this year

Dr. Mario Picazo
Meteorologist, PhD

Friday, June 15, 2018, 16:38 - Since the beginning of 2018, computer models had been anticipating that the first half of the year would be one marked by the vanishing of La Niña from the equatorial Pacific, while the beginning of the summer would mark the development of a weak El Niño. 

The first prediction was right on the money, but latest calculations now show a trend, that delays and weakens the predicted El Niño episode until at least, the fourth quarter of the year.  

Several atmospheric conditions, like the intensity of the trade winds and the sea level pressure, together with the Sea Surface Temperature (SST) Anomaly pattern in the east tropical Pacific, suggests that ENSO (El Niño Southern Oscillation) conditions will remain near neutral during most of this summer.

ENSO forecasts tend to be much more uncertain when performed during the March-June period, and achieve a higher level of confidence once the summer has started. Despite the mentioned forecasts for ENSO this summer, we will have to follow-up closely on the outcome of model runs performed during the summer, to have a more reliable prediction of what the tropical Pacific SSTs pattern will look like for the last stretch of 2018.

Deep water temperature measurements from the central Pacific eastward, have been climbing steadily above average since early April 2018. This indicates, that there will be a tendency for that warm water to extend to the surface, and thus bump-up sea surface temperature values from current neutral conditions to a warmer than average pattern.

Most models predict that SSTs in tropical Pacific will remain in a near neutral mode (0.5oC above or below average) between June and August. What occurs beyond the third quarter of the year is still up in the air, although current data, despite low confidence levels, favors a warming trend. There is a 75 per cent probability that by August neutral conditions will persist, and only a 25 per cent probability of seeing a weak El Niño develop by that time.  

What neutral ENSO conditions mean for Pacific and Atlantic hurricane seasons

In 2017, with a lingering El Niño, the east Pacific hurricane season turned out to be very active, with 20 storms developing out of which 18 ended up named. Nine reached hurricane status but only four touched land in Mexico: Beatriz, Calvin, Lidia and Max.   

This year the timing of the transition between La Niña and El Niño will have a lot to say about the number and intensity of storms in the region. With La Niña gone and neutral ENSO conditions in place, we should expect a close to average season. If El Niño does form later in the season, say between September and November, even if it is weak, we could have a more active late season.

La Niña, the cooling of the Pacific equatorial waters, tends to increase wind shear in the tropical east Pacific thus tearing apart many of the storms that form and move northward. With El Niño, the warming phase of the equatorial Pacific, conditions are opposite, as wind shear relaxes and the clusters of thunderstorms that form around tropical cyclones tend to grow with ease and eventually develop into larger more powerful systems. 

On an average year, we should see 15.4 named storms, with 7.6 becoming hurricanes and 3.2 reaching major hurricane status. Predictions indicate that this will be a near normal season, although in just a month, since it began, we have already seen 2 early major hurricanes, Aletta and Bud. 

WATCH BELOW: 2018 East Pacific hurricane season outlook

As for the Atlantic Ocean, this season expect anywhere from 10 to 16 named storms, with 5 to 9 becoming hurricanes and 1 to 4 of those becoming major hurricanes. With these numbers published by NOAA just a few weeks ago, it appears the 2018 season will be nowhere near what we saw in 2017, one of the most active seasons registered. There is a 75 per cent chance it might be slightly more active than a normal season, but NOAA also highlights the other side of the spectrum, where there is a 25 per cent chance it could be less active.  

With a neutral ENSO pattern in the Pacific, we should see near neutral wind shear conditions overall. El Niño tends to increase upper-level westerly winds across the Caribbean into the tropical Atlantic, tearing apart developing tropical storms that can eventually evolve into hurricanes. It is hard to say at this point how strong the El Niño anomaly will be later in the season, but if we do end-up seeing one beyond August, other factors being average, we could have a less active end of the season. Another determining factor to keep in mind, is the large swath of the very cold SST anomalies seen at this point in the season across much of the equatorial east Atlantic (see diagram above). This pattern could very well limit storm formation and development throughout the season. 

Time will tell, and we will update this information as it becomes available. 

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