Friday 17 July 2015

The Hot Pacific

This report goes to considerable lengths to avoid saying that this has anything to do with anthropogenic climate change.

Six Tropical Cyclones At Once in the Pacific Ocean

15 July, 2015

The hyperactive Pacific Ocean pulled off an impressive feat with six tropical cyclones spinning in the Pacific Basin at one time Sunday, July 12.

Tropical Depression Six-E developed in the eastern Pacific Sunday morning becoming the sixth tropical cyclone in the Pacific Ocean to be active at one time – joining Tropical Storm Dolores in its own basin; Typhoon Nangka and then-Tropical StormChan-hom, in the Western Pacific; and Halola and Iune, which were then both Central Pacific tropical storms.

Six at Once - July 12, 2015

Pacific Basin satellite image on the afternoon of July 12, 2015, U.S. EDT, showing six tropical cyclones at once in the basin. 

Although it's not unheard of to see multiple storms at the same time, this is impressive on any sale.

On Sunday morning with an upgrade to Tropical Storm Dolores, it marked the first time in almost 10 years that there have been at least five simultaneous Pacific tropical cyclones of at least tropical storm strength.

Last week, typhoons Linfa, Chan-hom and Nangka were simultaneously active in the Western Pacific basin. According to National Hurricane Center specialist Eric Blake, Wednesday, July 8, marked the first time there had been three typhoons simultaneously in the western Pacific Ocean since Oct. 24, 1994.

Colorado State University's Phil Klotzbach adds it was the earliest occurrence on record of three concurrent typhoons in the western Pacific, breaking the old record from July 10, 1972.

Blake also tweeted that, along with last week's Tropical Storm Ela, made three central Pacific tropical cyclones this month alone. In the historical record from 1949-2014, only three tropical cyclones had ever formed in that basin in the month of July.

Halola has since crossed the International Dateline into the Western Pacific basin, but retains its Central Pacific name.

If Tropical Depression Six-E could have strengthened into Tropical Storm Enrique before Chan-hom became post-tropical, there would have been six named tropical cyclones in the Pacific Ocean at one time. That last occurred over 40 years ago. However, Chan-hom has lost tropical characteristics over North Korea.

Furthermore, Iune became a remnant low the following day, further reducing the simultaneous cyclone count in the Pacific.

How Unusual?

Multiple tropical cyclones occurring at once in the Pacific is not totally rare, given the year-round potential of the western Pacific Ocean.

On average, 36 tropical cyclones form each year in the northwestern Pacific and southwestern Pacific basins, combined. Another 16-17 form each year in the central and eastern north Pacific basin. 

This also happened over the past week when Chan-hom and Nangka were joined by the aforementioned Typhoon Linfa as Tropical Storm Ela briefly spun up east of Hawaii.

In a search through the reliable historical record, hurricane expert and Colorado State University seasonal forecaster Phil Klotzbach said the Pacific Ocean once had six named tropical cyclones in progress at the same time. (To clarify, each of the six systems were at least or had been tropical storm strength to have been given a name, instead of, for instance, "Tropical Depression Four-E".)

"That was in 1974 (August 26) when Ione, Joyce, Kirsten, Lorraine, Maggie and Polly were present at the same time," said Klotzbach.

All but Tropical Depression Polly were in the eastern Pacific Ocean that day. Polly was a tropical depression at the time northeast of Guam.

Five active tropical cyclones in the eastern Pacific Ocean at 5 a.m. PDT on August 26, 1974. Tropical Depression Dolly near Guam was the sixth active Pacific tropical cyclone that day.  (Data: Phil Klotzbach, NHC)

Klotzbach said the last time there were as many as five active tropical cyclones simultaneously in the Pacific Ocean Basin (all at least were, or had been, at least tropical storm intensity) was from September 21-22, 2005 when Jova, Kenneth, Max were in the eastern Pacific, while Damrey and Saola spun in the western Pacific.

In the Atlantic Basin, four hurricanes have only happened simultaneously twice.

Infrared satellite image of four hurricanes at once in the Atlantic Basin on Sep. 26, 1998. (Credit: NOAA) 

Most recently, this occurred September 25-27, 1998Hurricane Georges was heading toward a Gulf Coast landfall while Hurricanes Ivan, Jeanne (not the Ivan and Jeanne you remember from 2004) and Karl meandered harmlessly in the central Atlantic Ocean.

On August 22, 1893, four hurricanes were also active in the Atlantic. One of these was the killer Sea Islands hurricane, which claimed between 1,000 and 2,000 lives in Georgia and South Carolina.

(MORE: The Deadliest U.S. Hurricane)

Incredibly, there have been five active Atlantic tropical cyclones at one time, from September 11-12, 1971. According to the National Hurricane Center, no more than two were of hurricane strength at any one time.

Why Did This Happen?

To answer why we had so many at once, we need to delve into several other factors.

Areas near the equator don't get cold fronts. The only changeable weather over a relatively short period of time is a roughly 30-60 day wet/dry cycle triggered known as the Madden-Julian Oscillation. The MJO is essentially a wave of energy in the atmosphere that propagates eastward around the Earth near the equator once every 30-60 days.

By a "wave," we mean the MJO has a phase where upward motion in the atmosphere is strong, helping to boost the formation of clouds and thundershowers, and a suppressive phase, helping to squelch precipitation.
In this case, a strong MJO was supporting strong upward motion, clouds and thundershowers in the western Pacific Ocean, as the "west-Pac trio" was just about to get going.

Madden-Julian Oscillation European model analysis on June 23, 2015 (top panel) and forecast for June 24-30, 2015 (bottom panel) over the Pacific Ocean (center of each panel). The pink shadings in the center correspond to the most supportive phase of the MJO for rain and thunderstorms, occurring in the western and central Pacific Ocean.  (WSI)

This strong upward motion, as with all MJOs, would eventually push east, giving a boost to thunderstorm clusters in the central Pacific pushing into early July.

The second factor was a strong burst of westerly near-surface winds near the equator in the western Pacific Ocean.

WSI operational scientist Dr. Michael Ventrice noted this would be the strongest such burst on record in the summer for the western and central Pacific.

Eric Blake, hurricane specialist at the National Hurricane Center, later noted this westerly wind burst was on track for a July record.

Generally speaking, the trade winds typically blow from a southeasterly direction south of the equator, and a northeasterly direction north of the equator. 

Keeping in mind winds flow clockwise around low-pressure systems in the Southern Hemisphere, this westerly wind burst near the equator can give a boost to any fledgling areas of low pressure trying to form. 

Finally, contributing to the low-level warmth and humid air needed to fuel convection, are impressive sea-surface temperature anomalies in the region where the two central Pacific systems are located.

Blake tweeted Tuesday parts of the eastern Pacific Ocean were seeing record warm sea-surface temperatures.

El Niños tend to lead to more eastern and central Pacific storms due to reduced wind shear, which would otherwise rip active or developing tropical cyclones apart.

More storms move west from the eastern Pacific basin into the central Pacific basin during El Niños, as well.

MORE ON WEATHER.COM: Cyclone Pam - Vanuatu (March 2015)

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