A

Iquitos, Peru

Charleston, South Carolina, USA

Iquitos, Peru

Charleston, South Carolina, USA

MJ JASOND Month

Fig. 7.16. Monthly mean precipitation for four stations showing the influence of latitude on rainfall seasonality. (Data courtesy of NOAA and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oa/climate/ghcn-monthly/index.php.)

MJ JASOND Month

Fig. 7.16. Monthly mean precipitation for four stations showing the influence of latitude on rainfall seasonality. (Data courtesy of NOAA and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oa/climate/ghcn-monthly/index.php.)

precipitation regime at Iquitos, Peru (3° S) in Figure 7.16 displays a double maximum that is stronger in March than in November and a dry period in July and August. The ITCZ movers further south of the equator in January over South American than it does north of the equator in July in response to changes in the general circulation driven by temperature gradients. The southward movement of the ITCZ is accompanied by warm, unstable northeasterly winds off the Atlantic Ocean. In July, the position of the ITCZ just north of the equator promotes zonal easterly winds that traverse the Amazon Basin before reaching Iquitos.

Seasonal precipitation contrasts become more pronounced at latitudes approaching 20° as the influence of the annual movement of the ITCZ produces a single rainy season. In these realms, precipitation is most abundant during the high-sun season and least during the low-sun season. This effect is evident in the monthly precipitation regime at Cairns, Australia (17° S) (see Fig. 7.16). The difference in mean monthly precipitation between March and July is 419 mm. The effect of the seasonal shift in the ITCZ is magnified at Cairns by its location relative to the confluent zone between the warm, moist air from the northeast and the southeast.

For latitudes between 20° and 35°, precipitation amount and seasonality are dependent upon continental location. Precipitation is meager and seasonality is of little consequence for the western portion of continents at these latitudes. Subsiding air associated with subtropical highs is dominant and most rain-producing storm systems are prevented from entering the region except in the winter. San Diego, California (33° N), has a precipitation regime that illustrates the effect of persistent subsiding air on the southwestern coast of the United States (see Fig. 7.16). Eight months receive precipitation of 25 mm or less, and 75% of annual precipitation occurs from December through March. In contrast, subtropical highs are weaker over the eastern portion of continents at these latitudes and prevailing winds are dominantly onshore. Precipitation is relatively abundant in all months, but some locations have an identifiable warm season concentration. June to September precipitation accounts for 51% of the annual total at Charleston, South Carolina (33° N), on the southeastern coast of the United States (see Fig. 7.16). All months are wetter than at San Diego which is on the west coast and at the same latitude.

Poleward of 35° latitude, precipitation results from cyclonic activity related to storms imbedded in the westerlies. The core regions of the oceanic subtropical high-pressure cells display seasonal shifting of 5° to 7° of latitude as illustrated by the conditions in the Northern Hemisphere. This characteristic has important significance for precipitation at mid-latitude locations on the west coast of continents because it is related to seasonal variations in frontal tracks associated with the most vigorous and frequent mid-latitude cyclones (Hartmann, 1994). In January, the oceanic subtropical highs in the Northern Hemisphere retreat equatorward as the zone of maximum solar radiation is near the Tropic of Capricorn in the Southern Hemisphere and the global circulation responds by shifting southward. The equator to pole temperature and pressure gradients in the Northern Hemisphere are greatest at this time, the polar front jet stream is stronger and makes more equatorward excursions, and the westerlies are more intense. By July, the oceanic subtropical highs assume a more poleward position as the zone of maximum insolation moves toward the Tropic of Cancer and the Hadley cell circulation strengthens in the Northern Hemisphere. In their higher latitude location, the subtropical highs support weaker westerly winds because the hemispheric temperature and pressure gradients are weakened. Cyclonic storms imbedded in the westerlies are forced poleward by the more northerly location of the subtropical high pressure and a distinct seasonal variation in storm frequencies is observed. The result is predominantly winter rainfall along the west coast of continents poleward to about 50° latitude. The length of the wet season decreases at lower latitudes. The eastern margin of the subtropical highs dominates the east coast of continents poleward of 30° latitude with the result that the eastern portions of continents receive rainfall in all months.

Lisbon, Portugal (39° N), and Washington, D.C. (39° N), in Figure 7.17 show representative precipitation regimes for opposite coast locations. Lisbon receives 30% less annual precipitation than Washington, D.C., and October to March precipitation at Lisbon accounts for 78% of the annual total. April to

Month

Fig. 7.17. Monthly mean precipitation for two stations illustrating the influence of subtropical high pressure on rainfall seasonality. (Data courtesy of NOAA and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oa/ climate/ghcn-monthly/index.php.)

Month

Fig. 7.17. Monthly mean precipitation for two stations illustrating the influence of subtropical high pressure on rainfall seasonality. (Data courtesy of NOAA and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oa/ climate/ghcn-monthly/index.php.)

Month

Fig. 7.18. Monthly mean precipitation for three stations displaying the influence of coastal and inland locations on precipitation seasonality at high latitudes. (Data courtesy of NOAA and the National Climate Data Center from their website at http:// www.ncdc.noaa.gov/oa/climate/ghcn-monthly/index.php.)

Month

Fig. 7.18. Monthly mean precipitation for three stations displaying the influence of coastal and inland locations on precipitation seasonality at high latitudes. (Data courtesy of NOAA and the National Climate Data Center from their website at http:// www.ncdc.noaa.gov/oa/climate/ghcn-monthly/index.php.)

September accounts for 57% of annual precipitation at Washington, D.C. August is the wettest month at Washington, D.C., averaging 124 mm while the August average in Lisbon is 4 mm.

At 50° latitude and higher, precipitation seasonality is largely controlled by location influences. Continental west coasts between 40° and 50° latitude have a slightly dry summer like the regime for Brest, France (48° N), in Figure 7.18. However, the summer dryness is much less severe than at Lisbon (see Fig. 7.17) or San Diego (see Fig. 7.16). Increased cyclonic activity at latitudes above 50° delivers precipitation in all months with a slight tendency for winter to be wetter. Annual precipitation is 1958 mm at Bergen, Norway (60° N), and each month from August to January receives more than 150mm (see Fig. 7.18). Continental interiors and east coast locations at these latitudes receive more precipitation during the summer when the warmer atmosphere has a greater moisture capacity (Shelton, 1988). The precipitation regime for Moscow, Russia (56° N), in Figure 7.18 is a nearly inverted pattern of monthly precipitation at Brest, France. The months of May to October account for 65% of annual precipitation at Moscow and 40% at Brest.

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