Most of us are familiar with surface weather maps that would usually depict the surface wind patterns for a particular time across a large area, such as the Northeastern States or the entire continental United States. We have been introduced to the prevailing semi-permanent surface wind patterns across the globe in the Earth Climate Studies text. Knowledge of the gross features of these surface winds has been known for centuries by mariners who sailed the oceans. In fact, some of the names, such as the trade winds, originated from this nautical experience.
By the late 19th century, sea-level pressure data from around the global had been collected, that showed the prevailing surface wind patterns across the globe to be related to the semi-permanent sea level pressure features. Specifically, the circulation around the large relatively permanent subtropical high pressure systems resulted in trade winds on the equatorward flanks of these hemispheres in both hemispheres, while westerly winds are found on the poleward flanks. Farther poleward, circulation out of the high-pressure centers over the polar caps produces the polar easterlies.
Since the atmosphere is a three-dimensional entity, we need to inspect the long-term planetary-scale atmospheric circulation regime at several levels in the troposphere above the earth's surface. We will consider climatological maps depicting the monthly wind fields generated by NOAA's Climate Prediction Center (CPC) from upper air data collected from 1979 through 1995. CPC has generated these climatological data sets (that appear at http://www.cpc.ncep.noaa.gov/products/precip/CWlink/climatology/ ) for analysis of monthly upper air conditions and the preparation of monthly and seasonal outlooks.
The suite of upper air maps that CPC has produced is based data collected, analyzed and displayed at constant pressure levels, rather than at constant altitude levels. Specifically, the monthly mean flow is displayed for the lower troposphere at the 850-hectoPascal (hPa) or (mb) constant pressure level and for the upper troposphere at the 200-hPa constant pressure level. These upper air maps have wind arrows plotted at grid points spaced at 10-degree latitude and longitude intervals following spatial interpolation of the observational data collected by radiosondes. The direction of these arrows shows direction of the wind flow at each grid point, while the lengths of these arrows provide an indication of the wind speed. Color coded bands of isotachs (lines connecting equal values of wind speed) are also indicated with the legend below the maps. The units for wind speed are meters per second (1 m/s=2.27 mph).
LOWER TROPOSPHERIC FLOW
The CPC climatology charts for the lower troposphere have been selected on the 850-mb (or hPa) constant pressure level, which is approximately at an altitude of 5000 feet above sea level. This level is above approximately 15 percent of the atmosphere in terms of mass. For many areas, especially across the tropics, this level is at or slightly above the friction level, meaning that the effects of friction from the surface have been minimized. However, this pressure surface is below the higher terrain of the western Cordillera of both North and South America, along with the Himalayan Massif in Asia.
The mean annual flow at 850 mb shows a pattern that deviates slightly from the surface wind pattern, mainly because the effects of friction are minimal at this level. Easterly winds are found across the tropics in both hemispheres, extending from approximately 30 degrees north to 30 degrees south latitude. These easterly winds, which are strongest over the tropical ocean basins, are part of the trade wind regime found at the Earth's surface. Winds at approximately 30 degrees in each hemisphere are relatively light and variable near the centers of the large subtropical high pressure centers that extend upward through the troposphere. Poleward of these high pressure areas, westerly winds are found across the midlatitudes, with the strongest winds at 850 mb found over the Southern Ocean south of Africa and Asia, due to the absence of land masses that permits an uninterrupted flow. Polar easterly winds are found along the periphery of the Antarctic continent as strong katabatic winds flow off the ice sheet and down toward the coast.
Seasonal variations in the 850-mb flow pattern can be discerned from inspection of the January and July maps. As at the surface, the patterns "follow the sun." On the January map, the winds across the Northern Hemisphere are relatively strong, since this month is in the heart of boreal winter. The strongest winds in this hemisphere are over the North Pacific and North Atlantic at approximately 35 degrees north latitude. In the Southern Hemisphere, the strongest summertime winds are over the Southern Oceans at approximately 50 degrees south. Also, note that the winds across the North Indian Ocean are from the east and northeast as part of the wintertime Indian monsoon circulation regime. The July map at the height of boreal summer (austral winter) shows weaker winds across the North Pacific and North Atlantic, which are displaced farther to the north to approximately 40 degrees north latitude. Winds across the Southern Ocean are much stronger and displaced slightly northward. Strong southwest winds are found across the western North Indian Ocean and Arabian Sea as part of the summer Indian monsoon circulation regime.
Monthly variations in the 850-mb flow can be found at http://www.cpc.ncep.noaa.gov/products/precip/CWlink/climatology/Wind-850.shtml. Run your cursor across the list of month names above the map to see the variation from month to month.
UPPER TROPOSPHERIC FLOW
The CPC maps of the upper tropospheric flow are on the 200-mb (hPa) constant pressure surface, selected because this is near the maximum winds in the upper tropospheric jet stream. This level is at an altitude between approximately 11,000 to 12,000 meters (or nearly 37,000 feet) near the tropopause, and where many long-distance commercial jet aircraft fly. Only 20 percent of the mass of the atmosphere lie above this level.
The mean annual 200-mb flow pattern shows a smoother flow than that found in the lower troposphere. Broad westerly winds are found centered between latitudes of 40 and 50 degrees in each hemisphere. The strongest winds are found over the western North Pacific just off the Japanese archipelago. These strong winds are considered the Polar Front Jet. A weak easterly flow is found within a few degrees of the equator.
A seasonal variation in the upper tropospheric flow is also detected. The monthly maps also show a wavelike feature to the flow, especially in the Northern Hemisphere, with troughs where the winds are deflected equatorward and ridges where the winds migrate poleward. The January monthly mean chart for boreal winter has exceptionally strong winds of over 150 mph off the eastern coast of Asia on the equatorward side of a trough pattern located over the cold Eurasian continent. Strong winds are also detected over the eastern coastal sections of North America, where another trough is found. Ridges are located over the eastern North Pacific and the western coastal sections of North America, as well as over the North Atlantic and western Europe. Upper tropospheric winds across the Southern Hemisphere are from the west, with little north-south variation. The monthly mean 200-mb chart for July shows weaker winds in the Northern Hemisphere, with only a hint of ridges over the continents and troughs over the ocean basins. Stronger jet stream winds are found across the Southern Oceans, displaced equatorward, especially near Australia.
The monthly variations in the 200-mb flow are found at http://www.cpc.ncep.noaa.gov/products/precip/CWlink/climatology/Wind-200.shtml.
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2009, The American Meteorological Society.
