Australia: The Land Where Time Began

A biography of the Australian continent 

Climate - The Pacific Ocean

The Pacific Ocean has a large potential for coupled ocean-atmosphere feedbacks as it comprises a large portion of the global ocean surface. An example of efficient coupling is ENSO, which is interannual, has a maximum amplitude in the tropics. A characteristic of decadal and longer timescales is much larger north-south spatial patterns which have extra-tropical amplitudes that are similar to tropical amplitudes. Feedbacks are much weaker and harder to discern outside the tropics because coupling between the atmosphere and the ocean is much weaker.

The authors¹ suggest good resources for many climate modes are found on the various climate web sites of the NOAA, some of these are the Climate Diagnostic Center (Climate Analysis Branch (http://www.cdc.noaa.gov) and the Climate Prediction Center of the National Weather Service (http://www.cpc.ncep.noaa.gov/).

The decadal climate variability of the Pacific Ocean has a timescale of 15-20 years, longer than the dominant decadal timescales of the Atlantic Ocean. The timescale difference may reflect the ocean basin size, and therefore planetary wave propagation timescales. The Pacific decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO; Mantua et al., 1997; Di Lorenzo et al., 2008) are included in the decadal Pacific modes. In the North Pacific these are the first and second EOFs of  the SST (Davis, 1976; Cayan, 1992). The Pacific North American teleconnection pattern (PNA) and the North Pacific Index (NPI; Trenberth & Hurrell, 1994) are 2 related indices that are based on atmospheric pressure. A circumpolar mode that has major impacts in the South Pacific is the Southern Annular Mode (SAM) (Thompson and Wallace, 2000).

The PDO is most robust in the North Pacific, though it spans the entire Pacific, with a pattern that is almost symmetric about the equator, with high amplitude that is centred on the equator and out-of-phase amplitude centred on the subtropical/subpolar North and South Pacific. The PDO is associated with the strength of the Aleutian Low. The westerly winds are strong and displaced somewhat to the south when the Aleutian Low is strong (high PDO); the subpolar gyre of the ocean is strong and less subpolar water enters the California Current system. The entire eastern boundary region, for the subpolar and subtropical gyres, is warmer than normal, while the central Pacific beneath the strengthened westerlies is abnormally cold. Along the coast of Japan the Oyashio is strong, penetrating further to the south.

At sometime around 1976 there was a shift from low to high PDO, that was well documented occurred, and it was at this time that a lengthy period of a particularly strong Aleutian Low began. Almost every environmental variable measured in the North Pacific - fish, crabs, birds, nutrients, etc., following ocean temperature and circulation changes (Mantua et al., 1997). The NPGO is a higher mode EOF, therefore has a tighter spatial pattern than the PDO. The NPGO is much better correlated than the PDO with environmental variables such as upwelling and ecosystem production along extensive lengths of the North Pacific coastline.

Over the region 30^oN-65^oN, 160^oW-130^oW the NPI is the mean sea level pressure and is therefore a direct measure of the strength of the Aleutian Low (Trenberth & Hurrell, 1994). The PNA is an older index of the atmospheric geopotential height, that is summarised from 4 locations, including 2 of which are over North America, and their SST patterns are virtually identical . The time series and the NPI and PDO patterns are very similar. According to the authors¹ the PDO could be considered to be a combination of the NPI and ENSO, which is a combination of tropical forcing and Aleutian Low forcing (Schneider & Cornuelle, 2005).

At high southern latitudes the decadal variability is dominated by the Southern Annular Mode (SAM ), also called the Antarctic Oscillation (AAO). In the western South Pacific one of the centres of maximum amplitude is centred on New Zealand. The South Pacific subtropical gyre circulation variability has been linked to the SAM. (Roemmich et al., 2007).

In the Pacific Ocean, as well as the other major oceans, anthropogenic forcing has been documented. Over the past 50 years the upper 500 m of the Pacific Ocean has warmed as has the remainder of the global ocean (Levitus et al., 2005). There has been a slight, though measureable, decrease in the basin-wide average salinity (Boyer, Antonov, Levitus & Locarnini, 2005). There has been a freshening of intermediate water masses, such as the North Pacific Intermediate Water (NPIW) and Antarctic Intermediate Water (AAIW) (Wong, Bindoff & Church, 2001).

There has been decreasing oxygen content in most parts of the Pacific Oxygen over the past 50 years. There has been an expansion of the tropical oxygen minimum zones (Stramma, Johnson, Sprintall & Mohrholz, 2008) and there has been a decline in the oxygen levels throughout the upper ocean in the North Pacific and Antarctic Circumpolar Current (ACC) regions (Deutsch et al., 2005; Aoki et al., 2005). The water of the Pacific Ocean has become more acidic, and according to the authors¹ there appears to be no possibility of this trend being reversed as the atmospheric CO₂ level continues to rise relentlessly. Increased temperatures and acidity are beginning to be observed as increasing stresses on ecosystems such as coral reefs and continental shelves.

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.