El Niño and La Niña transitions affect tropical cyclone development half a world away

The butterfly effect suggests that small changes in a system can make a large impact on eventual outcomes. One metaphor used to illustrate this concept is a butterfly flapping its wings only to cause a hurricane across the ocean.

While meteorologists’ current cause-and-effect understanding of weather isn’t this granular, researchers are actively investigating how changes in temperature, rainfall, wind patterns, etc. can impact weather phenomena halfway across the world.

Cyclical variation in the temperature of surface waters in the central and eastern Pacific Ocean, called the El Niño-Southern Oscillation (ENSO) cycle, moves between warmer-than-average water temperatures during El Niño years and colder-than-average water temperatures during La Niña years. Remarkably, scientists have observed that the surface temperatures of water in the central and eastern Pacific affect the number and strength of tropical cyclones (TCs) that develop in the North Atlantic Ocean and the probability those TCs will make landfall. El Niño phases typically result in fewer TCs in the North Atlantic Ocean compared to La Niña phases, and TC intensity increases during an El Niño phase while landfall probability increases in a La Niña phase.

Meteorologists do not completely understand how the ENSO cycle, occurring in the central and eastern Pacific Ocean, affects TC development in the North Atlantic. To address this knowledge gap, a group of scientists from Hohai University, the National Marine Data and Information Service and Fudan University analyzed the effects of El Niño and La Niña dissipation (ELD, LAD) events on sea surface temperatures and how those changes could affect the development of TCs in the North Atlantic Ocean.

The team published their research on February 10 in the journal Ocean-Land-Atmosphere Research.

The scientists measured the frequency, duration, intensity, location and density of North Atlantic TCs during ELD and LAD events along with North Atlantic Sea surface temperatures, and other large-scale environmental factors. Subsequent data analysis revealed that the transition out of El Niño or La Niña can have a large impact on TC formation over the North Atlantic.

“During [LAD], hurricanes [in the North Atlantic Ocean] become more frequent, stronger, and last longer. More hurricanes also pass through the Caribbean Sea and the eastern part of the main development zone. Studies show that during [LAD] events, certain types of hurricanes occur twice as often in this eastern area due to the weaker vertical wind shear and the warmer sea surface temperature. These changes are connected to the preceding phases of El Niño and La Niña, showing how ENSO dissipation plays a special role in shaping hurricane behavior,” said Xidong Wang, professor at Hohai University and first author of the research paper.

Vertical wind shear refers to the change in wind direction and speed in the atmosphere with altitude. Stronger vertical wind shear can push the top of a TC hundreds of miles downstream, weakening the storm. On the other hand, during LAD events, weaker vertical wind shear allows TCs to organize more easily, contributing to more TC development in the North Atlantic during LAD.

Higher sea surface temperatures and tropical cyclone heat potentials also contribute to enhanced TC formation during LAD events. Tropical cyclone heat potentials measure the vertical thermal structure of the upper ocean. Importantly, warmer sea temperatures provide the energy necessary to fuel TCs. In contrast, the higher vertical wind shear, lower sea surface temperatures and lower tropical cyclone heat potentials observed in the North Atlantic during ELD events reduce TC formation.

The research team investigated the effects of ENSO on TC development in the North Atlantic, but this same analysis could also be performed for other oscillating ocean phenomena and their effects in other parts of the world. For now, the research team looks forward to validating the results of their study.

“Although we employed linear regression and composite methods to investigate the effects of ENSO evolution on the large-scale environments during both ELD and LAD events, coupled atmosphere–ocean model simulations should be performed to further confirm the results reported herein,” said Wang.

Mengyuan Quan from the National Marine Data and Information Service at the Ministry of Natural Resources in Tianjin, China and Kaigui Fan from the Department of Atmospheric and Ocean Sciences in the Institute of Atmospheric Sciences at Fudan University in Shanghai, China also contributed to this research.

This study was supported by the National Natural Science Foundation (41776004), the Fundamental Research Funds for the Central Universities (B210203041, 2016B12514), the Opening Project of Key Laboratory of Marine Environmental Information Technology, the Fundamental Research Funds for the Central Universities (2019B62914) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJKY19_0416).