After looking into extra tropical cyclones in some detail, I would like to move on to another important climate hazard, tropical cyclones, in terms of the science, response to a changing climate and their impact on people. The Met Office describes Tropical cyclones as as amongst the most powerful and destructive meteorological systems on earth. I have produced two simple graphics using data from Munich Re (2013). The first shows the costliest natural disasters in term of monetary loss whilst the second depicts the same story for human fatalities. Costs consequent of tropical cyclone appear high up on these lists.
The awesome might of these tropical systems can be seen in the video below which shows scenes from the aftermath of hurricane sandy, the second costliest natural disaster of all time behind the recent Japanese Tsunami. Tropical storms have the potential to wreck cities, livelihoods and cause long-term demographic, social and political change.
The mechanics of tropical cyclones are relatively well understood. It is generally accepted that there are 6 critical antecedent conditions which must prevail in order for such systems to develop, intensify and in turn propagate. Of critical importance is the presence of warm ocean waters, at least 27 degrees centigrade. Related to warm waters is the depth of the thermocline, that is, the depth at which there is a sharp transition from warmer waters to deeper cold water. A deep thermocline means that if a tropical cyclone were to develop the associated turbulence only returns more warm water to the surface of the ocean and hence continues to fuel the genesis process In contrast, a shallow thermocline might mean that the strong winds of a hurricane bring cold waters up from below and self prohibit serious intensification. In terms of Atlantic systems, the position of the loop current may play an important role in allowing cyclones to intensify en route to Louisiana or Florida. (Will be considered in more detail in a later post.) The second key precursory condition is an atmosphere which is unstable. That is, an atmosphere which cools significantly wit height. Other requirements are moist layers in the mid-troposphere, a minimum distance of 500km from the equator (so that there is sufficient Coriolis Force to spin the cyclone), pre-existing near-surface disturbances, and low levels of vertical wind shear. The figure below illustrates the tracks of all historical Atlantic tropical storms. The tropical Atlantic source is evident as a favourable location for genesis, as is the northwards trajectory where storms pick up the Gulf Stream's warm ocean waters. Furthermore, It is clear that coastal regions bare the brunt of the tropical storm's force and that inland regions are relatively secure.
The next image documents the anatomy of a hurricane. The eye which is roughly 20-50 km in diameter is found at the centre of the system. The eye wall immediately adjacent presents very strong winds, intense rain and thunderstorms. It is important to note that the direction of winds at the bottom of the system are in the opposite direction of rotation than at the top. Rising air at the centre of the storm is opposed by subsidence on the outer edges. Energy for intensification of the hurricane comes from sensible heat and latent heat as vapour changes state which creates high pressure, divergence aloft and lower pressure at the surface. The increased pressure gradient generates strong surface level winds. Just as there are conditions that favour the development of tropical storms, there are conditions that inhibit their formation too. Strong trade wind inversion, cooler sea surface temperatures and strong upper level winds play a role.
Storms can leave distinctive sedimentary deposits in shallow marine deposits and coastal lagoons. Davis et al (1989) inferred that 'hurricanes produced graded or homogeneous facies of sand, shell gravel, and mud found in predominantly clastic sediments of late Holocene age in coastal lagoonal bays of Florida'. The hurricane signature is largely a consequence of their immense energy. As such, they can act as geomorphic agents wit the potential to cause coastal landform changes, particularly when hurricanes reach categories 4 or 5 when the winds get particularly strong. Liu and Fearn (1993) suggest that sediment cores from Lake Shelby on the Alabama coast show more tranquil climatic regimes prior to 3.2 ka BP as a result of a complete absence of sand layers in the cores. Furthermore, the authors propose that a more mesic climate post 3.2 ka BP was responsible for an unusual incursion of hurricanes into southwest Texas. Donnelly and Woodruff (2007) examined the millenial scale variability of Caribbean hurricane activity by reconstructing hurricane-induced overwash events from Laguna Playa Grande, Puerto Rico. The data reveals large scale fluctuation in the frequency of intense hurricanes. Relatively frequent occurrences of intense hurricanes are shown to have occured between 5.4 ka BP and 3.6 ka BP with the exception of a short quiet period from 4.9 ka BP to 5.1 ka BP. Following this period is a spell of quiet activity from 3.6 ka BP to 2.5 ka BP. Another active spell occurs between 2.5 ka BP and 1 ka BP. This is depicted in the figure below. A key future research goal is to relate these activities more reliably to past SSTs based on coral or sediment records. Such issues are at the centre of current debate on the impact of changing SSTs in the face of anthropogenic climate change will have on future hurricane activity. One this that is more certain, however, is that there is a link between La Nina years and intensified NAHU activity and supression during El Nino. This is chiefly due to increased vertical wind shear (noted above as a controlling factor) in strong El Nino seasons.
That's it for now, later this week I hope to discuss a case study of the recent cyclone to have devastated the Philippines as well as prompting a discussion on the future for hurricanes in the face of anthropogenic climate change. The latest scientific hurricane research will be debated and participation is encouraged!
