With tropical storm activity in the Atlantic Ocean in a lull, storm watchers are focused on the recent burst of activity in the Pacific, where a train of three storms is swirling off the west coast of Mexico and the first typhoon of the season in the northwest Pacific formed unusually late.

Add to that the fact that one pair of storms did a rare meteorological do-si-do called the Fujiwhara effect and that a second could soon follow, and there’s plenty to keep forecasters busy.

There isn’t one overarching reason for the sudden jump in Pacific storm activity, as there’s no strong El Niño affecting the environment as in the past couple of years, rather a confluence of factors has brought both regions alive.

How the tropical seasons in both the northwestern and eastern Pacific may change as the world steadily warms with the accumulation of heat-trapping greenhouse gases is still something climate scientists are trying to pin down. The evidence to date suggests that there could be fewer storms overall but that the strongest storms will become more common.

An uptick in the number of the most intense typhoons making landfall in the northwest Pacific has already been observed, though the underlying reason isn’t yet clear.

Typhoon Noru is situated to the east of Japan, while in the eastern pacific, Tropical Depression Greg, Hurricane Hilary and Tropical Storm Irwin are strung out like pearls on a necklace, with Greg furthest from land and Hilary closest.

Activity in the eastern Pacific ramped up fairly quickly in recent weeks, though that is not unusual, as late July tends to see an uptick in storms. By a measure of tropical cyclone activity called Accumulated Cyclone Energy the eastern Pacific season went from 61 percent below normal to 51 percent above in just three weeks, hurricane researcher Brian McNoldy, of the University of Miami, said. The index takes into account the number of storms, their strength and how long they last.
 


Abundant moisture in the lower levels of the atmosphere and favorable winds seem to have helped foster storm development, hurricane expert Phil Klotzbach, of Colorado State University, said.

Large-scale waves in the atmosphere could also have given activity there a kick-start, and may actually provide a boost to Hilary and Irwin, Ventrice said. They could also help ramp up Atlantic activity next month.

The northwest Pacific is typically the global hotbed of tropical cyclone activity (tropical cyclone being the blanket term for hurricanes and typhoons), and sees the largest share of intense storms. That area of the ocean has a deep store of warm waters, which fuel the convection that drives tropical systems, and generally more favorable wind conditions.

But so far this year, activity there has been relatively tamped down. ACE in the northwest Pacific is at only 25 percent of normal in the northwest Pacific, Klotzbach said.

The reason for the quiet season is the stable air in the deep tropical parts of the basin that are “the best ‘bang for your buck’ regions, so to speak, for getting strong typhoons,” hurricane expert Phil Klotzbach, of Colorado State University, said in an email. Tropical cyclones need unstable air to form and strengthen.

Typhoon Noru

Typhoon Noru swirls out in the Pacific Ocean on July 24, 2017, as seen by the Suomi NPP satellite.

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Wind shear, or winds that blow in different directions and at different speeds in different layers of the atmosphere, have also stymied storm development, he said.

Michael Ventrice, an operational meteorologist with The Weather Company, said the unfavorable conditions are due to a lingering La Niña signature in the atmosphere (though the National Oceanic and Atmospheric Administration declared the brief, weak La Niña over in February). La Ninas, which feature cooler than normal waters in the eastern tropical Pacific, impact the circulation of the atmosphere overhead in a way that tends to tamp down on typhoon activity.

Typhoon Noru, currently churning southeast of Japan, was the first storm of the season to reach typhoon status, on July 23, the second latest that has ever happened (the latest was Typhoon Otto on August 3, 1998) and another sign of the unusually low-key season there.

Such relatively quiet seasons will likely continue to occur in the future thanks to the natural variations in Earth’s climate, but the most intense typhoons are likely to make up a higher percentage of the storms that occur in the future. Research suggests that the heat building up in the upper ocean layers in this region -- layers already so primed to support storms -- will lead to even stronger typhoons in the future.
 


Research from the same group also suggests that landfalling typhoons are intensifying more than those that stay out at sea, likely because of warmer coastal waters that help them ramp up in strength. Whether those warmer waters have been driven more by warming or natural processes to date isn’t clear, but future warming is likely to keep them that way.

Hilary and Irwin are also poised to come close enough together that they start to orbit each other, a phenomenon called the Fujiwhara effect, after Sakuhei Fujiwhara, the Japanese meteorologist who described it. It is rare to see two hurricanes undergo this effect because they have to come close enough together. It can sometimes cause one storm to absorb another, or weaken one at the expense of the other.

Even rarer is that this would be the second pair of storms to do this in a week, as Typhoon Noru and Tropical Storm Kulap danced around each other earlier, significantly weakening Kulap and sending Noru looping around over its earlier path.

Hilary and Irwin’s dance won’t send either in the direction of land, but could kick up some unusually high surf across the coast of Southern California.