Fujiwhara effect: a dance of tropical cyclones

Adonis Manzan & Jam Salazar | Dec. 28, 2017

Given the number of constantly active weather systems passing by the Philippines, it is not an unusual event when two or more tropical cyclones enter the Philippine Area of Responsibility (PAR) at the same time.

This phenomenon, a binary vortex interaction, is known as the Fujiwhara effect. Named because of the pioneering work of Dr. Sakuhei Fujiwhara, this effect happens when two or more tropical cyclones pass close enough to each other, while spinning in the same direction. Dr. Fujiwhara was able to explain the notion of the binary vortex by performing a series of laboratory experiments on the interactions between pairs of vortices in a water tank.  Meanwhile, the National Hurricane Center, defines Fujiwhara effect as the tendency of two nearby tropical cyclones to rotate cyclonically about each other.

So what exactly does Fujiwhara effect constitute? It would be safe to say that when two or more tropical cyclones are about 750 to 1,300 km from one another, a Fujiwhara effect is certain. As such, both systems are exhibiting a scenario wherein a fairly stronger system pushing and spinning around a much weaker circulation into orbit before being absorbed into the dominant one. This is where the “merger” phenomenon comes into light, which meteorologists and weather enthusiasts from around the globe are excited to witness.

Annual Phenomenon

Every year, Fujiwhara events may take place indiscriminately, with several possible scenarios taking place: spinning, merger, or dissipation (See Figure 1). All of these have a direct or indirect impact on tropical cyclone behavior. Others might think that when the weaker cyclone becomes ingested by a much stronger system, this increases vigor and intensity. However, studies have proven it inaccurate and a fallacy altogether. Tropical cyclones are a product of warm tropical seas and oceans, where condensation aids to release latent heat and warms the cool air above. This causes it to rise and make way for more warm humid air for the ocean below. As these colossal storms gather pace across vast tracts of warm waters, they tend to convert the warmth of the surface of the oceans into massive wind and rain episodes. Eventually, these are unevenly distributed across the globe.

Figure 1. A conceptual model showing interaction of two (2) tropical cyclones. This diagram was adapted from Lander and Holland (1993). Image courtesy: hko.gov.hk


Adverse impact

Just recently, a slowly intensifying Typhoon LAN (Paolo) emerged into warm East Philippine Sea, pulling in the Southwesterly moisture inflow across Central Philippines. The monsoon rains brought massive flooding and deaths particularly in Zamboanga Peninsula and parts of Western and Central Mindanao, including Negros Oriental with Dumaguete City. To date, these areas are still reeling from the devastating floodwaters from back-to-back torrential rains displacing thousands of residents from their homes.

Fujiwhara effect was also attributed to the sudden high wind and rain event across Northern and Western Visayas at the time where Typhoon LAN was about to absorb a much weather system (see Figure 2) Tropical Depression 26W which developed quite rapidly to the west of mainland Palawan into its massive circulation. The dissipation took place near the vicinity of Masbate Island on October 20, 2017. It only took less than 24 hours for the accelerated dissipation scenario to take hold considering the size and intensity of LAN. Forecast models can only anticipate at certain degree of difficulty, but nevertheless, this year alone, about three (3) Fujiwhara events have persisted well within the PAR.

Figure 2. Surface analysis for 19-20 October 2017 where Typhoon LAN pulls in Tropical Depression 26W into its Southwest circulation. Credits: WeatherPhilippines Foundation, Inc.
Figure 3. Surface analysis for 26-27 July 2017 indicating several circulations interacting with one another. Credits; WeatherPhilippines Foundation, Inc.
Figure 4. Surface analysis for 30-31 July 2017 showing Tropical Storms HAITANG & NESAT orbiting one another. Farther east, Typhoon NORU has already absorbed Tropical Storm KULAP altogether. Credits: WeatherPhilippines Foundation, Inc.