Monday, April 14, 2008

Cycles of climate: The Earth

One way of understanding "climate" is to think of change on time scales longer than the strictly periodic cycles of the day and year set by the rotation and orbit of the Earth. Familiar to everyone, yet usually unrecognized in their true nature, the multiannual oscillations of the coupled oceans and atmosphere possess periods up to 10 years or so, and longer in some cases. Much of what is wrongly attributed to "global warming" is in fact due to these cycles and their longer-period subharmonics.

Oceans as reservoirs and convectors of heat. These oscillations take longer than seasonal and daily changes in the atmosphere, in part, because the oceans possess a much larger heat capacity than does the air. The cycles rely on a "heat reservoir" effect. Oceans have high "thermal inertia" and take a long time to absorb and release their heat. Each cycle, moreover, while grounded in a particular geographic region, also affects heat, wind, and humidity circulation worldwide. While they can be thought of in terms of local thermodynamics (pressure, temperature, humidity, precipitation), it is more illuminating to think of them in terms of flows. Pressure differences give rise to winds; temperature differences, to both winds and heat flow. These in turn determine evaporation rates, cloud cover, and precipitation. The geographic shape of these oscillations is determined by how oceans are contained by the continents. The bigger the ocean, the bigger the impact of climate oscillations.

For unto us a child is born. The best-known is undoubtedly the El Niño-Southern Oscillation (ENSO), which stretches over the South Pacific from Indonesia and Australia in the west to the west coast of South America in the east. The basic cycle takes about three years, although some cycles are irregular and can last up to seven years. The changes in the atmosphere are most commonly tracked by watching oscillations in air pressure, rather than temperature. (Recall these are related by the ideal gas law: pressure proportional to the product of air density and temperature.)

The cycle starts with La Niña conditions: easterly trade winds near the equator* blowing westward and piling up warm water in the southwest Pacific. Off the west coast of South America is, the water is colder, fed by an upwelling of cold water from the depths. These upwelling waters are loaded with nutrients and, in such years, support some of the richest fishing grounds on the planet. The air temperatures are also cooler at the eastern end and support less humidity and rainfall on the western slopes of the Andes. Conversely, Indonesia and Australia experience heavier rains in those years.

As the El Niño pattern develops, the easterly trade winds slacken off, allowing warm water to move back to the southeastern Pacific, weakening the cold-water upwelling off the South American coast. These are bad years for fishermen there; the food flow for fish near the surface slows. They are also years of heavy rains in western South America. Conversely, they are years of frequent drought in Indonesia and Australia.

Like all such internal climate cycles, ENSO is fundamentally a coupling of a broad expanse of ocean of the atmosphere above it. The basic time scale is set by the transfer of excess ocean heat from the southwestern Pacific to the southeastern and back, with related changes in ocean currents and more or less slaved changes in air temperature and winds.

ENSO has global impact and, because the Pacific is the largest ocean, is the most consequential oscillation in the Earth climate system. Some of the excess heat in the southeastern Pacific "leaks" north and eastward, changing seasonal temperature, cloudiness, and rainfall patterns in North America and over the Atlantic. The absence of such excess heat also impacts climate elsewhere.

Other climate oscillations. Tropical storm tracks and changes in the Asian/Pacific Arctic. Other, smaller oceans have their own local versions of the same phenomenon, cycles of excess heat traveling back and forth in ocean basins. Affecting North America and (especially) Europe is the North Atlantic Oscillation (NAO), which is tightly linked to the Arctic Oscillation. Years of strong westerly winds blowing eastward across the Atlantic lead to milder summers and winters and more rain in Europe. These strong westerlies also deflect Atlantic tropical storms eastward and keep them from making landfall. Weakened westerlies lead to more extreme temperatures (colder winters and hotter summers) and European droughts. Tropical storms face less of an obstacle to keeping their tropical westward tracks as they move away from the equator; they are much more likely to make landfall or end up in the Gulf of Mexico, where they are renewed in strength from the warm, shallow waters.

The recent period from 1999 to 2005 was a period of increased incidence and landfall of hurricanes. The North Atlantic westerlies were weak. Europe experienced hotter summers, colder winters, and less rain. The last two years have seen the cycle shift back the other way.

A more recent discovery is the long (10- to 30-year) Pacific (Inter)Decadal Oscillation (PDO), which actually seems to be a collection of several interlinked oscillations cycling at multiple periods. The last decade has seen a warmer west Pacific, which is probably tied to the retreat of Arctic ice cover at those longitudes.** Such retreat has not been seen on the Atlantic/European side. It's local warming, not global.

The oscillations are linked and sync'ed. Considered as a whole, the Earth's climate features all these regional cycles oscillating simultaneously. Their periods are not commensurate, so their collective pattern, while containing repeating elements, does not repeat as a whole. Each moment in the collective system is unique.

Furthermore, moving to the forefront of climate research, we should take note of the apparent fact that the different oscillations, being coupled to one another, get into sync, out of sync, then back into sync in a different way, never exactly repeating. The oscillations are subject to longer-period modulation, with some cycles, for example, of ENSO being stronger and others weaker. When all the oscillations are simultaneously strong, hemispheric or super-hemispheric inter-oscillation "locking" is especially enhanced.

Such a "locking" evidently happened in 1977 and again in 1998, changing the global evolution of air temperature, pressure, and humidity, as well as the exchange of heat between air and oceans. These years were apparently the start and end of the most recent warming period globally. More research is needed to expand the knowledge base of these oscillations worldwide and get a more complete picture. But the "global warming" hysteria slows down such fundamental research and generates "manufactured ignorance" about climate, in place of discovery of new knowledge.

POSTSCRIPT: Over to the right, you'll see some new climate blog links: Steve McIntyre's ClimateAudit, the wonderful plants growing over at CO2 Science, and the venerable World Climate Report.
* Remember the prevailing latitudinal wind patterns? See this old posting.

** It's ocean warmth that is the primary melter of ice, not warm air. The North Pacific ocean currents run west to east and "leak" some warmer water into the Bering Straight area. The main branch of the same current runs down the west coast of North America. It's much colder than the summertime air temperatures over temperate zone land, giving rise to strongly enhanced condensation and some famous fogs - a giant air conditioner, essentially.

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