2010.09.24
Increased Tropical Activity Across the Northern Hemisphere, Summer 2010
During the last week of August 2010, we saw a sudden increase in tropical activity across the Northern Hemisphere, resulting in seven active tropical systems simultaneously across the Northern Hemisphere. The remnants/extratropical remnants of two more tropical systems, threatened the major shipping lanes of the North Pacific and North Atlantic.
Introduction
On September 1, Hurricane Earl, Tropical Storm Fiona and Tropical Storm Gaston developed in the Atlantic, with Earl threatening the US Eastern Seaboard. TD Ten-E developed off the coast of Mexico near Manzanillo, affecting routes to and from the Panama Canal. Tropical Storm Lionrock was in the northern South China Sea, Typhoon Kompasu was moving towards Seoul, South Korea, and TD Ten-W had developed in the Philippine Sea (which later developed into Tropical Storm Malou on September 3). Additionally, the extratropical remnants of Hurricane Danielle tracked northeastward through the north central Atlantic, and the remnants of Tropical Storm Namtheun dissipated in the Taiwan Strait. These systems are shown below in Figure 1.
Figure 1.
Worldwide IR Satellite composite image for September 1, 2010. Active tropical systems are circled in Red, with dissipating/extratropical systems circled in Orange.
While it is not unusual to have two or three tropical systems at one time in one basin (particularly in the West Pacific), it is unusual to see several tropical systems at one time in each of the Northern Hemisphere basins. Tropical development tends to be cyclical, with flare-ups in the West Pacific, then dying down, the flare-ups then moving into the East Pacific, and finally into the North Atlantic.
The recent marked increase in Northern Hemisphere tropical activity at one time is most likely due to the development of a La Niña event. La Niña is the ‘cold’ counterpart of El Niño; also know as the negative phase of El Niño/Southern Oscillation (ENSO). La Niña is characterized by anomalously cool Sea Surface Temperatures across the central and eastern Equatorial Pacific. El Niño conditions (anomalously warm SSTs) have been observed across the region from late fall 2009.
La Niña events have been related to increased tropical activity in the North Atlantic and Northwest Pacific (with decreased activity in the Central and Eastern Pacific). In the Atlantic, La Niña leads to favorable atmospheric conditions for the development and strengthening of tropical systems. When Sea Surface Temperatures are favorable, and a mechanism, such as a tropical wave or a monsoonal trough, is present to generate development, La Niña conditions help to ensure the system is able to develop further into a Tropical Storm, or Hurricane/Typhoon, by reducing variables that can weaken tropical systems, like mid to upper-level wind shear.
At the same time, La Niña (in addition to very warm sea surface temperatures) has caused conditions across the Atlantic to be very conducive to tropical development. Nearly every tropical wave that has moved off the African coast since August 15 has developed into an organized tropical depression, tropical storm or Hurricane.
Increased Concurrent Tropical Activity and Implications for Tropical Forecasting
Tropical systems in proximity to one another can exert steering forces on one another, along with large scale steering forces caused by the synoptic environment. These effects can change the environment for another tropical system in proximity to or behind the first system, in ways that are difficult to asses and resolve for forecast models.
When two tropical systems are located close enough to exert forces on one another, they are governed by the Fujiwhara Effect, named for Japanese meteorologist Dr. Sakuhei Fujiwhara. The Fujiwhara Effect dictates that when two cyclonic vortices are in close proximity to one another, they will begin to rotate around a common midpoint. Often, if one system is stronger, it will exert the dominant force, which can result in one tropical system weakening or the two systems merging.
The Fujiwhara Effect was observed recently with Tropical Storm Lionrock and Tropical Storm Namtheun in late August. Tropical Storm Lionrock formed as a tropical depression on 27 August west of Luzon in the northern South China Sea. Over the next two days, the system tracked slowly northward while gaining strength, and was forecast to continue tracking northward, making landfall over mainland China around 31 August/1 September.
Tropical Storm Namtheun developed east of Taiwan on 30 August 0000 UTC as Tropical Depression 09W. This system was initially forecast to take a U-shaped track, making landfall over the northern portion of Taiwan. As Tropical Depression 09W strengthened to tropical storm force and was named Namtheun, the two tropical storms north and south of Taiwan began to interact with one another. TS Lionrock was drawn northeastward, towards southern Taiwan, and TS Namtheun took a more west-southwestward track into the Tawain Strait.
As Lionrock was the stronger of the two systems, Namtheun was not able to develop further, and weakened in the Taiwan Strait. Namtheun was absorbed by Lionrock on 1 September (Figure 2.)
Figure 2.
IR Satellite composite image for 31 August (upper lefthand corner). WNI final tracks for Tropical Storm Lionrock and Tropical Storm Namtheun plotted on map. Arrows indicate cyclonic rotation of the two tropical storms, illustrating the Fujiwhara Effect.
The introduction of the Fujiwhara Effect between the two systems caused a significant change in the motion of both systems. Lionrock was much slower than initially forecast, and took a significantly different track. The motion of Namtheun into the Taiwan Strait effectively blocked that major shipping lane.
As model guidance does not effectively resolve these kinds of inter-system interactions, when two tropical systems develop in close proximity to one another, forecasting the motion and intensity of these systems is more difficult, introducing more risk to already dangerous tropical cyclones.
Outlook
Anomalously warm Sea Surface Temperatures remain in place across the main development regions of the North Atlantic, North Pacific, and Eastern Pacific (Figure 3). As such, the fuel for tropical development remains in place.
Figure 3.
Global Sea Surface Temperature Anomaly (degree C) for September 9 2010. Anomalies are departures from 1985-1998 monthly means. Courtesy National Oceanic and Atmospheric Administration.
With the ongoing moderate La Niña event, atmospheric conditions are favorable for tropical development, particularly in the North Atlantic, the potential for unusually active tropical regions remains of the tropical season.
If systems develop in close succession, as we have already seen this season, forecast uncertainty, and its associated risk, will increase. This will require additional monitoring to assess and anticipate changes, and close communication to warn for enhanced risks.
