Australia: The Land Where Time Began

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Climate Variability - Natural Modes

There are 3 climate modes, all of which are tropical variability modes, that have dominantly interannual variability, ENSO, Atlantic Meridional Mode (AMM), Atlantic Niņo and the Indian Ocean Dipole (IOD) mode. The relatively high frequency of these modes, all of which include strong feedback between ocean and atmosphere, is set by tropical dynamics. Globally, ENSO is by far the most energetic of these, having a sea level pattern that includes the central-eastern tropical Pacific (one sign), western tropical Pacific and eastern Indian Ocean (opposite sign), and the tropical Atlantic (opposite sign), and it also has a signature over North America and in the western part of the tropical Atlantic. The intrinsic Atlantic Ocean mode and Indian Ocean mode are both mostly confined within their own ocean basins.

Climate modes that have dominantly decadal variability are the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO or Northern Atlantic mode, NAM), that is closely related, the Antarctic Oscillation (AAO or Southern Annular Mode, SAM), and the Pacific Decadal Oscillation (PDO), with its modes that are closely related that are defined within the North Pacific. These modes do not tend to have feedbacks between the ocean and atmosphere that are obvious, though a lot of work on such mechanisms has been done and continues to be done.

The AMO, that also affects the Arctic Ocean, is the only centennial mode described here, is defined in terms of average sea surface temperature (SST), that is presumed to be linked to the meridional overturning circulation (MOC). Both the AAO (SAM) and the PDO have similar spatial patterns and similarities with the ENSO pattern, all 3 having the strongest signatures in the Pacific, Indian and Antarctic regions, as if the Pacific is connected primarily zonally to the Indian and to the Antarctic meridional. There is little correlation with the Arctic, with the NAO (AO) pattern connecting the Arctic meridionally to the Atlantic Ocean, and with little signature in the Southern Hemisphere, even in the tropical regions of the Atlantic Ocean.

The authors1 describe these climatic variability modes as simply as possible as if they were standing patterns, with oceans and land determining, to some extent, the node locations. Analysis of many of the modes is in terms of lagged correlations and large-scale wavelike propagation.

The natural climatic variability modes are of importance with regard to the regional climate variation in the oceans as well as for the natural modes of the entire system that could be forced anthropogenically. Climate prediction models sometimes suggest shifts into a particular phase of modes such as ENSO, the PDO, SAM, and NAO/AO.

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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