Early September marks the half way point in the northern hemisphere tropical cyclone season and is often the time when we see the highest levels of activity – so how is this season shaping up?
As reported in a news release last week, tropical cyclone activity across the north Pacific has been extremely high this year with numerous intense typhoons in the west Pacific and hurricanes in the east Pacific. These are different names for the same thing – hurricanes occur east of the International Dateline and typhoons to the west.
There has been a fair amount of discussion recently in social and news media as to how ‘record-breaking’ this season has been so far. Reliable records only go back to about the 1960s or 1970s when satellite coverage of the tropical oceans became available. However, bearing this in mind, here are some of the remarkable statistics for the year up to 1st September:
- There have been 15 tropical cyclones in the northern hemisphere reaching category 4 or 5 on the Saffir-Simpson Hurricane Wind Scale – 6 more than the previous record.
- Tropical cyclone activity across the northern hemisphere as measured by Accumulated Cyclone Energy (a combined measure of intensity and longevity) is 200% of normal and over 20% above any other year.
- Six hurricanes have crossed the central Pacific region – more than any other year.
- Three north Pacific hurricanes have crossed the International Dateline – more than any other year.
- Hurricanes Kilo, Ignacio and Jimena were all at category 4 simultaneously in the Pacific east of the International Dateline – the first time three major hurricanes have been recorded at the same time in this region.
(L-R) Hurricanes Kilo, Ignacio and Jimena on 30 August 2015. Image courtesy of NASA.
Why the high levels of tropical cyclone activity?
One of the main contributing factors is the strong El Niño which has developed. This is characterised by a marked warming of the tropical east Pacific Ocean. Temperature anomalies here are currently at their highest since 1997-98, when high levels of Pacific tropical cyclone activity were also experienced.
What about the Atlantic?
The existence of El Niño conditions usually results in a quiet Atlantic hurricane season. This is primarily as a result of strong wind shear (winds varying in strength and direction with height) across large parts of the region. There have been six Atlantic tropical storms so far this season. Recently Danny became a major hurricane just east of the Caribbean, but quickly succumbed to the strong wind shear as it entered the Caribbean Sea. Erika threatened to develop into a hurricane, but again dissipated in the Caribbean due to a combination of high wind shear and interaction with islands such as Hispaniola.
In the far eastern Atlantic, conditions were favourable enough for a hurricane to quickly spin up as a cluster of thunderstorms moved off the west coast of Africa a few days ago. Fred became the most easterly forming hurricane in the tropical Atlantic Ocean and the first recorded hurricane to hit the Cape Verde Islands since 1892. However, as Fred has continued to move north-westwards it has also been subject to strong wind shear and is weakening rapidly.
What about the rest of the year?
Seasonal model predictions suggest that the strong El Niño will persist for several months to come. Hence it is likely that the high tropical cyclone activity in the Pacific will continue for the remainder of the season. The Atlantic is expected to have a quiet season overall, but this does not exclude the possibility of the development of a major hurricane. There are notable instances of damaging hurricanes occurring in otherwise quiet seasons such as Hurricane Andrew in 1992 which caused devastation in Miami, Florida.
Official warnings for the latest tropical cyclones in the western North Pacific are produced by the Japan Meteorological Agency. Central Pacific warnings are issued by the Central Pacific Hurricane Center and east Pacific and Atlantic warnings by the US National Hurricane Center. The Met Office routinely supplies predictions of cyclone tracks from its global forecast model to regional meteorological centres worldwide, which are used along with guidance from other models in the production of forecasts and guidance. We also provide updates on current tropical storms via @metofficestorms on Twitter.
Statistics on recent northern hemisphere tropical cyclone activity are courtesy of Dr. Phil Klotzbach of Colorado State University (@philklotzbach).