Typhoon

Four Tropical Cyclones At Once : How Rare Is It?

It may be March, with thoughts of spring in the Northern Hemisphere, but half a world away, we're tracking not one, not two, but four tropical cyclones.

Pacific Ocean satellite image showing three western Pacific tropical cyclones and two disturbances in the central Pacific on July 7, 2015 at 1415 UTC. (NASA/MSFC/JMA) — Five Systems - July 7, 2015
Infrared satellite image of Linfa, Chan-hom and Nangka in the western Pacific Ocean, along with the pair of tropical disturbances (circled) in the central Pacific Ocean as of 7:15 a.m. PDT, July 7, 2015.
(NASA/MSFC/JMA)

The strongest of these is Cyclone Pam, whose maximum sustained estimated winds had topped 165 mph as it slammed the Republic of Vanuatu Friday night and Saturday morning, local time.

Meanwhile, Cyclone Olwyn made landfall in far western Australia, Cyclone Nathan made a U-turn in the Coral Sea north of Cairns, Australia and, finally, Tropical Storm Bavi is spinning well east of Guam in the western Pacific Ocean.

Wind field analyses also picked out the four circulations well. You could trace wind streamlines coming from the western side of Tropical Storm Bavi eventually feeding into the eastern side of much stronger Cyclone Pam. The centers of Bavi and Pam were about 1,600 miles apart, at that time. (Remember winds flow clockwise around low-pressure systems, such as tropical cyclones, in the Southern Hemisphere).

Wind streamlines in associated with the four tropical cyclones on Mar, 12, 2015. Purple and red streamlines indicate stronger winds.

How Unusual?

Two tropical cyclones at the same time in the same basin is quite common, particularly in the peak of the season.

In the Atlantic Basin, four simultaneous hurricanes has only happened 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, 1998. Hurricane 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. Hurricanes)

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?

November through April is the season for tropical cyclones in the southwest Pacific Ocean, and they can occur year-round in western North Pacific basin.

To answer why we had four at once, however, we need to delve into two other factors.

Areas near the equator don't get cold fronts. The only changeable weather, there, 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 of the four simultaneous tropical cyclones, a strong MJO was supporting strong upward motion, clouds and thundershowers in the western Pacific Ocean, extending to northern Australia.

Madden-Julian Oscillation forecast for March 6-12, 2015. The brightest pink shading in the center corresponds to most supportive phase of the MJO for rain and thunderstorms, occurring in the western Pacific Ocean near the location of Cyclone Pam.
(WSI, Michael Lowry)

An atmosphere not generally hostile to thundershowers is a prerequisite for the formation of tropical cyclones.

WSI operational scientist Dr. Michael Ventrice blogged in depth on this MJO event, if you're interested in digging deeper.

Interestingly, this strong MJO will contribute to putting a temporary end to the recent spring thaw in parts of eastern and central United States in the week ahead.

The second factor was a strong burst of westerly near-surface winds just south of the equator in the western Pacific Ocean this week. Colorado State University's Dr. Philip Klotzbach noted this burst was the strongest seen in almost two decades.

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.

So there you have it. An atmosphere very supportive of thundershowers (MJO) with areas of low pressure getting a boost from the westerly wind burst during the heart of the southwest Pacific/northern Australia tropical cyclone season has contributed to this conflagration of cyclones.