Bifurcation In Climate And Weather

2500

2000

Mean annual precipitation (mm)

2000 1000

SSW

A

NNE

NW B SE

1

' 0 50

100

0 10

:

\ Km

Km

MAY-NOV 1

l DEC-MAY

•=>

/

» MT WILHELM \ * <"\ \ CENTRAL \

ALPS

\ / A ' A

/ \

MADANG

^500

-300

INTERLAKEN

-700

INTERLAKEN

Figure 4.19 The relationship between precipitation (broken line) and relief in the 800 tropics and mid-latitudes. (A) The highly saturated airmasses over the Central Highlands of Papua New Guinea give seasonal maximum precipitations on the windward slopes of the mountains with changes in the monsoonal circulation; (B) Across the Jungfrau massif in the Swiss -400 T Alps the precipitation is much less than in A (A) and is closely correlated with the

0 topography on the windward side of the m mountains. The arrows show the

) prevailing airflow directions.

100 ' Sources: (A) After Barry 1991); (B) After Maurer and Lutschg (from Barry).

^500

-300

wind regime - northwesterly (southeasterly) in the austral summer (winter). By contrast, in more stable mid-latitude airflow, the rainfall maximum is related closely to the topography (see Figure 4.19B for the Swiss Alps).

5 Drought

The term drought implies an absence of significant precipitation for a period long enough to cause moisture deficits in the soil through evapotranspiration and decreases in stream flow, so disrupting normal biological and human activities. Thus a drought condition may obtain after only three weeks without rain in parts of Britain, whereas areas of the tropics regularly experience many successive dry months. There is no universally applicable definition of drought. Specialists in meteorology, agriculture, hydrology and socioeconomic studies, who have differing perspectives, have suggested at least 150 different definitions. All regions suffer the temporary but irregularly recurring condition of drought, but particularly those with marginal climates alternately influenced by differing climatic mechanisms. Causes of drought conditions include:

1 Increases in the size and persistence of subtropical high-pressure cells. The major droughts in the African Sahel (see Figure 13.11) have been attributed to an eastward and southward expansion of the Azores anticyclone.

2 Changes in the summer monsoon circulation. This may cause a postponement or failure of moist tropical incursions in areas such as Nigeria or the Punjab of India.

3 Lower ocean-surface temperatures produced by changes in currents or increased upwelling of cold waters. Rainfall in California and Chile may be affected by such mechanisms (see p. 155), and adequate rainfall in the drought-prone region of northeast Brazil appears to be strongly dependent on high sea-surface temperatures at 0 to 15°S in the South Atlantic.

4 Displacement of mid-latitude storm tracks. This may be associated with an expansion of the circumpolar westerlies into lower latitudes or with the development of persistent blocking circulation patterns in mid-latitudes (see Figure 8.25). It has been suggested that droughts on the Great Plains east of the Rockies in the 1890s and 1930s were due to such changes in

Dinosaurs For Kids Drawing

Figure 4.20 Drought areas of central USA, based on areas receiving less than 80 per cent of the normal July to August precipitation.

Source: After Borchert (1971).

Figure 4.20 Drought areas of central USA, based on areas receiving less than 80 per cent of the normal July to August precipitation.

Source: After Borchert (1971).

the general circulation. However, the droughts of the 1910s and 1950s in this area were caused by persistent high pressure in the southeast and the northward displacement of storm tracks (Figure 4.20).

From May 1975 to August 1976, parts of northwest Europe from Sweden to western France experienced severe drought conditions. Southern England received less than 50 per cent of average rainfall, the most severe and prolonged drought since records began in 1727 (Figure 4.21). The immediate causes of this regime were the establishment of a persistent blocking ridge of high pressure over the area, displacing depression tracks 5 to 10° latitude northward over the eastern North Atlantic. Upstream, the circulation over the North Pacific had changed earlier, with the development of a stronger high-pressure cell and stronger upper-level westerlies, associated perhaps with a cooler than average sea surface. The westerlies were displaced northward over both the Atlantic and the Pacific. Over Europe, the dry conditions at the surface increased the stability of the atmosphere, further lessening the possibility of precipitation. Rainfall for April to August 1995 over England and Wales was only 46 per cent of average (compared with 47 per cent in 1976), again associated with a northward extension of the Azores anticyclone. This deficit has an estimated return period in excess of 200 years! Nevertheless, the fifteen-year period 1983 to 1995 included six summers that were dry and also warm relative to the central England temperature record for 1659 to 1975.

Persistent, severe droughts involve combinations of several mechanisms. The prolonged drought in the Sahel - a 3000 X 700-km zone stretching along the southern edge of the Sahara from Mauritania to Chad - which began in 1969 and has continued with interruptions up to the present (see Figure 13.11), has been attributed to several factors. These include an expansion of the circumpolar westerly vortex, shifting of the subtropical high-pressure belt towards the equator, and lower sea-surface temperatures in the eastern North Atlantic. There is no evidence that the subtropical high pressure was further south, but dry easterly airflow was stronger across Africa during drought years.

Figure 4.21 The drought of northwest Europe between May 1975 and August 1976. (A) Conditions of a blocking high pressure over Britain, jet stream bifurcation and low sea-surface temperatures. (B) Rainfall over western Europe between May 1975 and August 1976 expressed as a percentage of a thirty-year average.

Figure 4.21 The drought of northwest Europe between May 1975 and August 1976. (A) Conditions of a blocking high pressure over Britain, jet stream bifurcation and low sea-surface temperatures. (B) Rainfall over western Europe between May 1975 and August 1976 expressed as a percentage of a thirty-year average.

Source: From Doornkamp and Gregory (1980).

The meteorological definition of drought has become clouded by the subject of desertification, particularly since the UN conference on the subject in 1977 in Nairobi. This concern was sparked by the protracted drought, resulting in desiccation, in much of the Sahel zone. In turn, the removal of vegetation, increasing the surface albedo and lowering evapotranspiration, is believed to result in decreased rainfall. The problem for climatologists is that desertification involves land degradation as a result of human activities, primarily in areas of savanna and steppe surrounding the major desert regions. These areas have always been subject to climatic fluctuations (as distinct from climatic change) and to human impacts (e.g. deforestation, mismanagement of irrigation, overgrazing) initiating changes in surface cover, which modify the moisture budget.

areal extent and frequency (or recurrence interval) of rainstorms. Orography intensifies the precipitation on windward slopes, but there are geographical differences in this altitudinal effect. Global patterns of precipitation amount and annual regime are determined by the regional atmospheric circulation, the proximity to ocean areas, sea-surface temperatures, and the atmospheric moisture budget. Droughts may occur in many different climatic regions due to various causal factors. In mid-latitudes, blocking anticyclones are a major factor. The primary cause of protracted drought in the African Sahel seems to be climatic fluctuations.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment