What Affects Tropical Cyclones Environmental Sciences Essay

What are tropical cyclones? A tropical cyclone is the main term used for a low pressure system which gets its energy mainly from evaporation from the sea and condenses in clouds concentrated near its centre (Holland, 1993).

Tropical cyclones occur in tropical latitudes. The central area of the tropical cyclone is called the eye of the storm; this is the area of the lowest pressure. The eye is encircled by dense cloud (cyclone walls) which rotates. The walls are less dense as you move out from the eye as the wind gradually weakens down (Nalivkin, 1983).

Tropical cyclones have three different names that are defined by their wind speed; a tropical depression is a storm with maximum sustained surface winds of less than 17m s ¯¹ but still has a circulation (Wallace and Hobbs 2006: Landsea, 2006). A tropical storm is a storm with winds of at least 17 m s ¯¹ and peak speeds of 32 m s ¯¹ (Wallace and Hobbs 2006: Landsea, 2006). Hurricanes or Typhoons (the regional names for strong tropical cyclones) are when winds reach above 32 m s¯¹ (Wallace and Hobbs 2006: Landsea, 2006).

Tropical cyclones form in ocean basins around the world; these are the North Atlantic Ocean, Northeast Pacific Ocean, North-western Pacific Ocean, North Indian Ocean, South- West Indian Ocean, Australian Ocean and South Pacific Ocean (Aguado and Burt, 2007).This study concentrates on the North Atlantic basin. The tropical cyclone season is from 1st June to 30th November in the Atlantic basin (NOAA, 2007).

1.2.2 What do tropical cyclones need to form?

Tropical cyclones need certain conditions to form. They need a deep surface layer and sea surface temperatures above 26.5°C (81°F) (Goldenberg, et al, 2001: Wallace and Hobbs, 2006: Aguado and Burt, 2007). They need a disturbance near the surface with adequate amounts of spin to start the formation and an atmosphere that is unstable so that it encourages thunderstorm activity. Low vertical wind shear is needed since wind shear can disrupt the organisation of thunderstorms. If the wind shear is too high, it can cause the cyclone to dissipate (Gray 1968: Wallace and Hobbs, 2006). Tropical cyclones also won’t form if the disturbance is too close to the equator. Tropical cyclones need enough coriolis force to maintain the low pressure in their centre, the coriolis is not strong enough to do this near the equator and for this reason tropical cyclones do not form between 0°W and 5°W latitude (Wallace and Hobbs, 2006: Aguado and Burt, 2007). All these conditions are generally needed for a tropical cyclone to form, but even if the conditions are met, it does not mean a tropical cyclone will form every time (Australian Government, Bureau of Meteorology, 2013).

North Atlantic tropical cyclones vary from other basins in the way they form as a high percentage of cyclones form from tropical waves that move across from West Africa (Landsea, 1993). Most major hurricanes form from African waves especially in the area between 10° and 20° North (Goldenberg, 2001). This area 10° and 20° North was termed the main development region by (Goldenberg, 2001). The number of easterly waves remains relatively constant from one year to the next, but the amount that develop into tropical cyclones varies greatly due to other factors (Frank, 1975).

1.2.3 What affects tropical cyclones?

Many different atmospheric factors affect tropical cyclones; the main ones are listed below:

Sea Surface Temperatures (SST’s): SST’s are one of the main factors that affects whether a tropical cyclone forms. Since heat from oceans are the primary energy source for a tropical cyclone, the theory is that if sea surface temperatures were to increase, so too would the number and intensity of tropical cyclones (Goldenberg, et al, 2001). However, this has not been proven by using historical data or models. The research conclusions are mixed with no definitive answer; some studies show that SST’s are increasing intensity of cyclones (Emanuel, 2005), whilst others are coming to the conclusion that the increase in tropical cyclone frequency is due to rising SST’s (Holland and Webster, 2007: Mann and Emanuel, 2006). Some research even points to a decrease in frequency under global warming (Knutson et al 2008). Overall no research has shown conclusively an increase in the frequency or intensity due to rising SST’s and there are many mixed views on the subject.

ENSO: ENSO is very important for tropical storm development and depending on what stage it is in can either cause formation or hinder it. Trenberth (1997) gives a good definition of El Niño and La Niña. El Niño’s effect on tropical cyclones has been well documented and there is a lot of literature on the topic available. It is agreed that El Niño supresses Atlantic hurricane activity (Gray, 1984). During El Niño years there are fewer tropical cyclones and weaker intense storms (Chu, 2004). El Niño years were found to have fewer hurricane days then non El Niño years (Gray 1984: Landsea et al, 1999). La Niña has the opposite effect on tropical cyclones by creating favourable conditions for the development of tropical cyclones (Pielke and Landsea 1999). During El Niño, Pielke and Landsea (1999) found that there are more economical losses during a La Niña year then an El Niño year, this fits in with Atlantic tropical cyclone activity being supressed during El Niño.

Atlantic Multidecadal Oscillation (AMO): This is a pattern of sea surface temperatures which has warm and cold phases, a 25-40 year cycle (Aguado and Burt 2007). The AMO then affects tropical cyclones due to the change in sea surface temperatures. The AMO has been thought to affect the activity of tropical cyclones in the past with the active year of 1995 coinciding with a shift in the AMO (Aguado and Burt 2007). Goldenberg et al (2001) indicates that the Atlantic Multidecadal variation would have a greater impact on tropical cyclones sooner than the warming of SST’s. This is because global warming will only gradually create higher SST’s. If the rise of SST’s due to global warming were to coincide with a change to a warm phase in the AMO there could be a large impact on tropical cyclones.

Wind shear: Wind shear is horizontal wind between the upper and lower troposphere. It inhibits tropical cyclones by stopping organisation of deep convection in the atmosphere, which prevents the organisation of cloud walls (Goldenberg et al, 2001). Wind shear has a big impact on hurricane frequency and intensity (Aiyyer and Thorncroft, 2006). Goldenberg and Shapiro (1996) came to the conclusion that wind shear was the most important environmental factor effecting tropical cyclones since it decreases the chances of cyclones forming and can dissipate them when they have already formed.

Overall, though all of the above factors interlink, for example an increase in SST’s could create an increase in wind shear (Vecchi and Soden, 2007). The above factors that affect tropical cyclones are complex and the knowledge is uncertain. (Emanuel, 2005: Landsea, 2010) argue that there can be no strong conclusion made on research using past data due to the uncertainty of accurate records.

Until a good overall understanding is gained no conclusions can be made on what has happened and what will happen to tropical cyclones, one of the ways to gain this is to look at where they have formed and why and what effected past tracks, leaving room for error in the data.

Tropical cyclone tracks

Tropical cyclone tracks in the North Atlantic are variable. There are no defined paths, but there are rough patterns cyclones will follow, depending on meteorological and oceanographic factors. Cyclones will generally move straight West or East and then may move north to hit land. Some recurve (go West then East). These cyclones are generally known to form by the Cape Verde Islands and will recurve around the periphery of the subtropical cell (Reading, 1990). These are known to form the deadliest hurricanes.

The cyclones as well are known to form in different areas at different times of the hurricane season (Reading, 1990) this then effects the tracks too. Early in the season they form more on the Western side of the Atlantic (Reading, 1990: Willet and Sanders, 1959). They then form throughout most of the basin in the middle of the season and by the end of season most of the cyclones form in the West of the Atlantic basin (Willet and Sanders, 1959).

Changes in intensity, frequency and tracks

Webster et al, (2005) found a small percentage increase in the number and proportion of hurricanes reaching categories 4 and 5 in the North Atlantic, but no increase in the intensity of the strongest hurricanes. Overall Webster (2005) found no global trend for increasing SST’s and number of tropical storms/ hurricanes, but found a global decrease in tropical cyclone days since 1995 to 2004 after a peak in 1995. Webster (2005) concludes that their data points to a 30 year increase in more frequent and intense hurricanes. This links in with Bender et al, (2010) model that found the frequency of intense hurricanes should increase, but the number of hurricanes will decrease in a warmer world due to global warming.

Vecchi and Knutson (2008) found that Atlantic tropical cyclone counts may have increased since the 19th century but with an average decrease in tropical cyclone duration over time. On the other hand Emanuel (2005) found that the duration of storms in the North Atlantic had increased since 1949 but he warns this may be due to changes in reporting practices. Overall the research gives very mixed results.

There is little work done on whether with climate change there will be changes in tracks (Walsh, 2004). Work that has been done varies in the results. Most of the results show little change in tracks, under simulations of enhanced greenhouse conditions and warmer SST’s (Knutson and Tuleya 1999, Tsutsui 2002). This research concentrated on the Pacific, and mainly in the North. This then does not give a picture of what may happen in other basins such as the North Atlantic. The research done has mainly used models to simulate tropical cyclone tracks, with more data every year data analysis and statistical methods could be used better to analyse past tracks. (Walsh and Reading, 1991: Vecchi and Knutson, 2008) have suggested that in the North Atlantic basin there has been a shift in the position of tropical cyclones to the east of the basin using data analysis but did point to possible changes due to better data collection methods.

Wang and Chan (2002) concentrate on tropical cyclone tracks in the North Pacific and demonstrated mainly how El Niño affects the tracks due to changes in El Niño. This then shows that El Niño has this affect in the North Pacific but has not been shown to have an impact on tracks in the North Atlantic or other basins.

1.2.4 What the future holds

Due to the theory of climate change there have been many ideas about how tropical cyclones will react, research using statistics has created many climatic models of tracks and what could happen. Quite a few conclusions have been drawn due to the idea of increasing sea surface temperatures, which could cause increases in frequency and intensity of storms but no firm evidence has yet been found due to uncertainties in the data and the number of parameters which could also affect tropical cyclones. Further uncertainties with how the climate will change mean no conclusions have been made yet for future predictions. Overall there is no agreement on what will happen to tropical cyclones in a warmer climate (Villarini et al., 2011).

Looking at past data has issues due to the inaccuracies that might have occurred before the satellite era. Landsea et al (2010) discusses this issue and found that the occurrence of short lived storms has increased over the last century but medium and intense storms haven’t. He concludes that this is most likely due to better equipment picking up storms, as well as storms being identified as cyclones when in the past they wouldn’t have been.

1.2.5 Why is the research being done, how is it different from other research

Research into tropical cyclones is important due to the impacts cyclones have on the human and natural world. Most research has focused on frequency and intensity of tropical cyclones. Not much work has been previously done on tracks, or the starting points of tropical cyclones. This research looks into where storms form throughout the North Atlantic basin and if this has changed in the basin over time, as well as focusing on recurving cyclones. Wang and Chan (2002) did look at tracks and formation in the Pacific, but one of the issues with the North Pacific was that the record is quite short. The North Atlantic has the longest reliable record (Goldenberg, et al, 2001) and will be examined in this study.