Australia: The Land Where Time Began

A biography of the Australian continent 

Oceans and the Climate

 Variability in any part of the ocean-atmosphere-land-ecology system, on timescales that are longer than seasonal, is what is called "climate". There are a number of factors that contribute to climate variations, such as natural, internal interactions between the various components of the system; natural, external forcing; or anthropogenic external forcing. Climate variability, as it is used at the present, usually refers to natural variability of climate, climate change refers to climate variations that are anthropologenically forced. Some of the internal forcings are include some of the feedbacks such as Bjerknes tropical ocean-atmosphere feedback, and the ice-albedo feedback, and many others that are described elsewhere in journals and books on climate variations. Natural external forcings include examples such as Earth-Moon-Sun orbit variations, volcanism, a forcing that is of random occurrence, though it is ongoing. Anthropogenic forcings include fossil fuel burning, land use patterns that change the land surface, the changes in these forcings then leading to other feedbacks that could shift the climate in a different direction. Timescales are categorised as interannual, approximately longer than 1 year and shorter that 8 years, decadal, approximately 1- several decades, centennial, millennial or longer. Time series observations of the property of interest are used for shorter timescales. When considering the ocean these include temperature, salinity, oxygen, nutrients, carbon parameters, current velocities and surface height, etc. Longer timescales are considered to be palaeoclimate and are based on "proxy" records of some feature depending on the property of interest, such as the abundance or different types of benthic foraminifera or the size of tree rings, which change with temperature, precipitation, etc.

In this book the authors1 are concerned mainly with the circulation and properties of the ocean, and for seasonal variability of some regions. As climate affects the variability of the ocean in properties and circulation on years to millennia timescales. Small changes, on the order of 10%, usually result from climate variability and climate change, in the mean structures. Much larger changes in the structure result from the most energetic variability modes, which are the tropical modes such as ENSO, though even these do not eliminate the mean pycnocline or vertical temperature structure. Another example is the importance of the western boundary currents would never be removed by climate change or climate variability, as they result from the rotation of the Earth and the presence of ocean boundaries, neither of which will vanish, though the boundaries will change over geological timescales. Also, there will be no change from the present state of the tropical low latitudes being heated by the Sun and cooled in the high latitudes because of less radiant energy from the Sun reaching the Earth in the polar regions, and it is unlikely the major wind systems will change. Included in these are the easterly and westerly winds that are associated with the Hadley Cell, Ferrel Cell and Polar Cell, and the tropical circulation such as the Walker circulations and the monsoons. It is therefore not likely the demise or complete reorganisation of the Gulf Stream, or any of the western boundary currents, will result from variations of the climate. The basic structure would remain as long as the general surface wind forcing patterns exist, though there might be a change in the strength, there might be a change in the position of separated currents, and there might be some degree of alteration of the advected properties.

The strength and structure of the overturning circulation could be changed by major changes in the stratification of the ocean resulting from heating, precipitation and evaporation, and sea ice. An example of such changes that are apparent in the palaeoclimate record is production and properties of North Atlantic Deep Water that were altered vastly during the last glaciation. In the tropics stratification changes could alter the ENSO feedbacks, with the ocean being much less capable of producing the cold tongue of the eastern tropical Pacific if the ocean became warmer. Such changes could have widely spread effects as the ENSO has an effect on temperature and precipitation over a large part of the world.

Sources & Further reading

  1. Talley, Lynne D., Pickard, George L., Emery, William J., and Swift, James H., 2011, Descriptive Physical Oceanography: An Introduction 6th ed.., Academic Press.
Author: M. H. Monroe
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