Pacific hurricane season more active than Atlantic
Friday, August 31, 2018, 12:30 - This 2018 hurricane season, the east-central Pacific Ocean has been all but "Pacific," and definitely much more active than the Atlantic.
After a frantic 2017 season with 18 storms, 10 hurricanes and 6 major ones, this season, the Atlantic has only seen 6 storms and none have been exceptionally strong.
Only Chris (category 2) and Beryl (category 1) reached hurricane strength, but so far, no major ones are accounted for.
Meanwhile, the east-central Pacific has been experiencing a very different situation. A total of 14 storms have formed west of the Central America and Mexico coast, with 8 of them becoming hurricanes and 5 intensifying to major hurricanes (category 3 or greater).
Just a few weeks ago, while the Atlantic was quiet as a graveyard, Hector, a category 4 storm, moved just south of Hawaii. Two weeks later, the most potent storm this season, hurricane Lane, moved very close to the eastern Hawaii islands, but fortunately it weakened before getting close to land and eventually moving west.
Lane did not make landfall on any of the islands, but extreme rainfall from the storm triggered flooding and landslides. Preliminary rainfall totals of 2-4 feet were reported across the windward side of the Big Island, with two locations recording even higher amounts.
As of August 26th, the National Weather Service in Honolulu reported that former Hurricane Lane produced the third highest storm total rainfall from a tropical cyclone in the United States since 1950.
Most East Pacific storms this year have moved west, away from the continent, after developing. The only major hurricane to touch land along the southern tip of Baja California, Mexico was Bud. At one point it was a category 4 storm but eventually weakened into a tropical storm before landfall.
According to Philip Klotzbach, research scientist in the Department of Atmospheric Sciences at Colorado State University, the Northeast Pacific (to 180o) hurricane season has already generated 17.75 major hurricane days this year, the most through August 30th on record. Prior record through August 30th was 13.5 major hurricane days in 1994.
Also worth noting is that recently formed Norman is the 5th category 4-5 hurricane of the 2018 Northeast Pacific (to 140oW) season to date, the most through August 30th on record. The old record was set in 2014 and equaled in 2015.
On an average year, the East Pacific should see 15.4 named storms, with 7.6 becoming hurricanes and 3.2 reaching major hurricane status. So, why has this average been surpassed with 3 months to go in the season, and the part of the period of peak intensity yet to come?
It is common to see tropical storm activity in opposite modes between the Atlantic and Pacific oceans. While the Atlantic is in a "low" activity mode this season, the Pacific is experiencing the opposite "high" activity described above. This correlation has been observed many years, and was actually forecast to occur this 2018, however, east Pacific storm activity forecasts made early in the season were underestimated.
According to Dr. Klotzbach, the Pacific can get a lot of hurricane activity, more than the Atlantic, but, "most storms just go out in the middle of nowhere and die in open water." This year a few like Lane or other storms that have made landfall in Mexico, have certainly made the headlines.
Warmer than usual Pacific waters key for above average storm formation
Warmer than average water temperatures across much of the tropical north Pacific have been essential for above normal storm development. When the Pacific is warmer than usual, the average wind pattern is altered in such a way that vertical wind shear increases over the Atlantic and decreases over the Pacific. Lower wind shear values in the Pacific have actually helped storms like Lane become strong and move anomalously close to Hawaii, something that has happened only on a few occasions. Observed stronger wind shear in the Atlantic has done the opposite, make it difficult for storms to organize and develop.
Also part of the more active Pacific Ocean scenario is the energy added to the storms by the warmer Pacific waters, a key factor that has to be taken into account as a positive triggering mechanism for storm development and strengthening. The same process, but in opposite direction, has been the norm across the western equatorial Atlantic. Here surface waters during the first part of summer have been colder than average, a negative feedback mechanism for storm development.
Atmospheric circulation changes have favored and suppressed storm development
With the El Niño Southern Oscillation (ENSO) in a near neutral mode most of the summer, the impact of this well know phenomenon should not be a determinant in hurricane activity in either ocean basin. El Niño tends to suppress activity in the Atlantic and enhance it in the Pacific, but it is not expected to evolve into a significant one until late in the season or even winter.
Another form of atmospheric oscillation that factors into the 2018 active Pacific hurricane season is the Julien Madden Oscillation (MJO). The MJO, is a large scale tropical disturbance that circles the globe every 30 to 60 days. During its active mode, more uplift occurs in the region which it affects and a greater amount of moisture can be transported for instance across the Pacific from southeast Asia into the east Pacific.
This summer, during much of July, the Western Pacific that was in a convective mode, but then in August, the convection enhancement shifted to the East-Central Pacific, favoring the intensification of storms like Hector or Lane.
Meanwhile, the tropical Atlantic has seen little chance of activity. Below normal sea surface temperatures, dry air entrainment from the Sahara and strong vertical wind shear have all teamed-up to make storm development virtually impossible.
But it´s not over yet, as we move into September, the peak month in activity, and with much warmer water than a few weeks ago, storm development is likely to increase. Despite this year´s slow Atlantic season, remember it only takes landfall of one hurricane to cause major impact.