Wherever low pressure systems (<100 to >1000 km) occur and these intensify over warm waters of the ocean between latitudes circa 5º and 25º. The winds blow onshore and in coastal areas, especially those with broad, shallow shelves which funnel (e.g. Bohai Bay, China, Gulf of Bangladesh, the German Fresian bight), strong surges can occur. They are amplified when they coincide with above average astronomical tides – new or full moon (spring tides) and at the solstices and equinoxes. (Wood, 1986). These are reinforced when they coincide with perigee (moon-earth at its closest) or perihrlion (earth –sun closest) tides. High latitude tides are further reinforced during the 18.6 year lunar nodal cycle when the angle between the moon’s orbit plane and eliptic is at a minimum.
Outside of the Tropics, mid-latitude storms and their associated cold fronts are the main cause of storm surges.
Coastal regions are particularly sensitive to tropical cyclones since the latter are generated over warm ocean bodies. Much of a cyclone’s energy will be dissipated at the coast resulting in weaker wind storms over inland areas. However, the potential threat of heavy rainstorms remains, posing serious flooding hazards for the hinterland.
Some 19.5% (391,812 km2) of the combined coastal territory of 84 countries is vulnerable to inundation from a one-in-100-year storm surge by current reference standards. A 10% future intensification as a consequence of predicted sea level rise, increases the potential inundation zone to 25.7% (517,255 km2), taking into account sea level rise. This translates to an inundation threat for an additional 52 million people; 29,164 km2 of agricultural area; 14,991 km2 of urban area; 9% of coastal GDP and 29.9% of wetlands, with Latin America/Caribbean having the biggest surge impact, whilst the coastal GDP losses could be very severe in East Asia and Latin America.
On Feb 1, 1953.a storm originating in the North Sea hit the Netherlands inundating 150,000 ha and killing >1800 people. Prior to this in 1281, > 80,000 died and in 1421 > 100,000.
The surge of 2.7m at Southend also caused much damage in eastern UK (Alexander, 1993).
Alexander, D. 1993 Natural disasters, Chapman and Hall, New York.
Bangladesh is particularly vulnerable, having a low topography so that the coast is virtually at sea level, shallow continental shelf, high tidal range and a high population density. In 1970 >220,000 people died with a recorded surge of >9m (Pugh, 1987; Murty and Flather, 1994); 11,000 died in May, 1985 and circa 200,000 died in 1991 (Alexander, 1993). The Saxby gale of 1869 caused huge damage along the Bay of Fundy, Canada with a surge >2m The Galveston, USA, hurricane in 1900, raised water levels by >5m killing around 6,000 people; in 1938 a hurricane with a 2m surge swept across Long Island and southern New England killing >700 people (Ludlam, 1988); the Ash Wednesday storm of 1962 affected the entire eastern seaboard of the USA and lasted over five tidal cycles. In 1992, the New York metro was flooded by a >2m surge, resulting in human loss of life; and property damage; the Halloween storm’ of 1991, basis of the film, ‘The Perfect Storm’, Hugo in 1989 in South Carolina, Andrew in 1992 in the Florida Keys and Katrina for New Orleans all give evidence of surges that cost lives and property.
Ludlam, D.M. 1988. The great hurricane of 1938, Weatherwise, 41, 214-216.
Murty, T.S., and Flather, R.A., 1994.Impact of storm surges in the Bay of Bengal, In Finkl, C.W. Jr. (ed.) Coastal hazards: Perception, susceptibility and Mitigation. J Coastal Research, SI 12, 149-161.
Pugh, D.T. 1987. Tides, surges and mean sea level. J Wiley and Sons, Chichester.
Nichols et al (2008) investigated 136 port cities around the world having >one million inhabitants in 2005, to exposure of coastal flooding due to storm surge and damage due to high winds in the 2070’s. Across all cities, about 40 million people (0.6% of the global population or roughly 1 in 10 of the total port city population in the considered cities) are exposed to a 1 in 100 year coastal flood event. By the 2070s, total population exposed could grow to circa 150 million due to the combined effects of climate change (sea-level rise surges and increased storminess), subsidence, population growth and urbanisation. Asset exposure could reach US $35,000 billion by the 2070s; more than ten times current levels rising to some 9% of projected global GDP in this period. On a global-scale, population growth, socio-economic growth and urbanization are the most important drivers of the overall increase in exposure.
Nicholls, R.J., Hanson, S .,. Herweijer, N., Patmore,, S. Hallegatte, J., Corfee-Morlot, J., Château and R. Muir-Wood (2), 2008. Ranking port cities with high exposure and vulnerabilitiy to climate extremes exposure estimates, Environment Working Papers No.1, OECD, 68pp.
Mediterranean cyclones are infrequent to rare with debate concerning their typology (tropical cyclones, subtropical cyclones or polar lows). They develop over open waters under strong, initially cold-core cyclones, similar to subtropical cyclones in the Atlantic Basin with typically non-tropical origins (source: http://en.wikipedia.org/wiki/Mediterranean_tropical_cyclone).
While Mediterranean sea surface temperatures in late-August and early-September range between +24 to +28°C, research indicates water temperatures of 20 °C are normally required for cyclone development and it appears that elevated cold air may bebe the main trigger for instability in the development of these systems.
It has been suggsted that a “hurricane season in the Mediterranean would be June – November ( similar to the North Atlantic hurricane season).
Recent US National Academy of Sciences study has shown powerful links between rising ocean temperatures in the key hurricane breeding grounds of the Atlantic and Pacific and an increase in the intensity of such storms.(source: http://www.theaustralian.news.com.au/story/0,25197,22080458-30417,00.html). For example:
- 2004, Cyclone Catarina, South Atlantic
- 2005 Hurricane Vince, Madeira in Portugal, an area that had never before produced such storms. It even struck Spain – another first.
- 2005, New Orleans overwhelmed by Hurricane Katrina.
- 2005: US Gulf coast, Hurricane Rita, the 4th most intense Atlantic hurricane ever recorded.
It has been further suggested that climate change induced warming in the Mediterranean may be enabling it to store enough heat to trigger the formation of its own hurricanes.
Mediterranean tropical cyclone – October 6-10, 1996 which resulted in serious flooding in the Balearic Islands, Sardinia, and south Italy. Winds of up to 145 km/h were reported over the Eolian Islands, causing infrastructural damage and four deaths. (source: http://en.wikipedia.org/wiki/Mediterranean_tropical_cyclone)