Australia: The Land Where Time Began

A biography of the Australian continent 

Climate Change - Patterns of Tropical Warming

The response of the climate to increasing atmospheric levels of greenhouse gases it not uniform around the world. Greater confidence in the pattern of warming in the tropics and its potential impacts has been gained as a result of analysis that has been carried out on the mechanisms that are the basis of  model-projected ocean temperature changes.

According to the authors1 research carried out on climate change over the past decades has shown that human activity is at least partly the cause of the increase of global mean temperature over the past 50 years. As actual global mean temperature is not experienced in any particular region most individuals are interested in the effects of the increase experienced in their local regions. As the response of the climate varies significantly between various locations it is very difficult to determine the changes to be expected in any particular location, with the largest differences being found between high latitudes and low latitudes, and between the warming of the land and the water. It has been argued by Xie et al. (Xie et al., 2009)  in the Journal of Climate that across the tropical and subtropical oceans subtle patterns are also emerging in evolution of temperature and precipitation, and these could possibly have a disproportionate effect on many regional climates around the world.

Regional climate can be influenced by the pattern of ocean warming in the tropics and the subtropics  locally, as well as remotely. An immediate impact can be felt by local ecosystems, such as coral reefs, where coral bleaching is the response of the coral to rising temperatures, the corals expelling their symbiotic algae which often leads to widespread coral mortality. Episodes of bleaching have been predicted to become more frequent and to be more severe as global warming that is human-induced accelerates over the course of the 21st  century. The future of reef ecosystems (Hoegh-Guldberg et al., 2007) are threatened by rising ocean temperature and acidification of the oceans. The survival trajectories of corals are believed to be influenced significantly by regional differences in the warming rate (Donner et al., 2005) and underlying seasonal variability (McClanahan, 2007). It has been revealed by combined model simulations that it could be expected that some coral reef regions may warm almost twice as much as other reef regions by the end of the 21st century. It is critical to the understanding of likely coral reef decline trajectory in different parts of the world (Baker et al., 2008) to know how the variability at the regional scale interacts with seasonal variability and biological responses, such as adaptation.

Regional climate can be influenced by warming of tropical oceans in distant parts of the world. In this article the authors1 consider the case of tropical cyclones. Conditions of the local environment, such as the temperature of the ocean and atmospheric humidity, influence the development of storms, but there are also strong influences from remote  parts of the world. An example of such a long-distance influence is the warm waters in the equatorial parts of the central and eastern Pacific Ocean during El Niņo events increase the wind shear over the Atlantic Ocean which in turn leads to reduced numbers of hurricanes in the Atlantic Ocean during years of El Niņo events. The stability of the atmosphere, that inhibits storm development, is not tied to local temperature, as it varies with the difference between the local temperature and the tropical mean temperature (Vecchi & Sodem, 2007). This is the mechanism by which the number of hurricanes crossing the east coast of the US is affected by the pattern of regional changes of sea surface temperature in the eastern part of the equatorial Pacific Ocean. It is important to have confidence in the projected warming patterns in order to understand how reefs and tropical storms, among other things, will be influenced by future atmospheric levels of greenhouse gases.

The authors1 suggest some confidence can come from most of the models used by the IPCC for their report (Solomon et al., 2007) agreeing on the main features of the pattern of warming in tropical regions. The greatest amount of warming is projected to occur in the central Pacific Ocean, whereas the subtropics, particularly the southern Pacific Ocean, are expected to experience a lesser degree of warming.

Xie and colleagues focused on the physical mechanism that produce these patterns to determine the degree of confidence that can be accorded the predictions of these models occurring in the real world. The proposal of the authors1 is that the warming spatial patterns arise through the relatively simple physics of surface evaporation. The surface of the ocean is warmed as a result of the atmosphere being warmed by the presence of greenhouse gases and increased evaporation is the primary mechanism for balancing this increase in the warming, though the ability of the ocean to lose heat by evaporation varies on a regional basis. In regions such as the subtropics the evaporation rate is high, and strong trade winds and low humidity typically characterise these regions, with evaporative heat loss being increased greatly by a small increase in surface temperature (Seager & Murtgudde, 1997; Leloup & Clement, 2009), and the result is that the ocean responds with relatively modest warming to greenhouse-gas forcing. In regions such as at the equator, winds are weak and humidity is high, substantially higher temperatures are required to produce the same amount of evaporation. Wind speed changes are also part of the picture, as pointed out by Xie and colleagues, such as in the case of the southwest Pacific Ocean where most models predict a strengthening of the trade winds, though the reasons for this have not yet been determined. Heat loss by evaporation increased by high wind speed, which allows the ocean to reduce warming, so gaining less heat than would occur with lower rates of evaporation.

The authors describe these hypotheses as 'elegant in their simplicity'. The authors1 point out that there are artefacts in the datasets used related to measurement technique improvements, result is that the observed trends are not necessarily robust (Vecchi, Clement & Soden, 2008). Then amplitude of natural variability, which is variability that is not related to greenhouse gas forcing, varies greatly between regions, making it extremely difficult to detect trend differences between different regions.

The authors1 suggest that improved use of palaeoclimate records that are specific to the tropics, such as the geochemical records obtained from coral skeletons skeletons, will help understand natural variability differences between regions and extract anthropogenic signals.

Sources & Further reading

  1. Clement, Amy C., Andrew C. Baker, and Julie Leloup. "Climate Change: Patterns of Tropical Warming." Nature Geosci 3, no. 1 (01//print 2010): 8-9.
Author: M. H. Monroe
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Last updated 04/05/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading