Teleconnections

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Teleconnections are defined as linkages over great distances of atmospheric and oceanic variables; clearly the linkages between climatic conditions in the eastern and western tropical Pacific Ocean represent a 'canonical' teleconnection. Figure 11.52 illustrates the coincidence of ENSO events with regional climates that are wetter or drier than normal.

In Chapter 7C.1, we have referred to Walker's observed teleconnection between ENSO events and the lower than normal monsoon rainfall over South and Southeast Asia (Figure 11.53). This is due to the eastward movement of the zone of maximum convection over the western Pacific. However, it is important to recognize that ENSO mechanisms form only part of the South Asian monsoon phenomenon. For example, parts of India may experience droughts in the absence of El Niño and the onset of the monsoon can also depend on the control exercised by the amount of Eurasian snow cover on the persistence of the continental high-pressure cell.

The eastward movement of the western Pacific zone of maximum convection in the ENSO phase also decreases summer monsoon rainfall over northern Australia, as well as extra-tropical rainfall over eastern Australia in the winter to spring season. During the latter, a high-pressure cell over Australia brings widespread drought, but this is compensated for by enhanced rainfall over western Australia associated with northerly winds there.

Over the Indian Ocean, the dominant seasonal weather control is exercised by the monsoon seasonal reversals, but there is still a minor El Niño-like mechanism over southeast Africa and Madagascar, which results in a decrease of rainfall during ENSO events.

Figure 11.52 The coincidence of ENSO events with regional climates that are wetter or drier than normal. The seasonal occurrences of these anomolies varies geographically.

Sources: After Rasmusson and Ropelowski, also Halpert. From Glantz et al. (1990). Composite reproduced by permission of Cambridge University Press.

Sources: After Rasmusson and Ropelowski, also Halpert. From Glantz et al. (1990). Composite reproduced by permission of Cambridge University Press.

Mapa Por Proyeccion Polar
Figure 11.54 Schematic Pacific-North America (PNA) circulation pattern in the upper troposphere during an ENSO event in December to February. The shading indicates a region of enhanced rainfall associated with anomalous westerly surface wind convergence in the equatorial western Pacific.

Source: After Shukla and Wallace (1983), by permission of the American Meteorological Society.

It is apparent that ENSO teleconnections affect extratropical regions as well as tropical ones. During the most intense phase of El Niño, two high-pressure cells, centred at 20°N and 20°S, develop over the Pacific in the upper troposphere, where anomalous heating of the atmosphere is at a maximum. These cells strengthen the Hadley circulation, cause upper-level tropical easterlies to develop near the equator, as well as subtropical jet streams to be intensified and displaced equatorwards, especially in the winter hemisphere. During the intense ENSO event of the northern winter of 1982 to 1983, such changes caused floods and high winds in parts of California and the US Gulf states, together with heavy snowfalls in the mountains of the western USA. In the northern hemisphere winter, ENSO events with equatorial heating anomalies are associated with a strong trough and ridge teleconnection pattern, known as the Pacific-North American (PNA) pattern (Figure 11.54), which may bring cloud and rain to the southwest United States and northwest Mexico.

The Atlantic Ocean shows some tendency towards a modest effect resembling El Niño, but the western pool of warm water is much smaller, and the east-west tropical differences much less, than in the Pacific. Nevertheless, ENSO events in the Pacific have some bearing on the behaviour of the Atlantic atmosphere-

ocean system (e.g. the establishment of the convective low-pressure centre over the central and eastern Atlantic subtropical high-pressure cell and of the general trade wind flow in the Atlantic). This results in the development of a stronger subsidence inversion layer, as well as subjecting the western tropical Atlantic to greater ocean mixing, giving lower sea-surface temperatures, less evaporation and less convection. This tends to:

1 Increase drought over northeast Brazil. However, ENSO events account for only some 10 per cent of precipitation variations in northeast Brazil.

2 Increase wind shear over the North Atlantic/ Caribbean region such that moderate to strong ENSO events are correlated with the occurrence of some 44 per cent fewer Atlantic hurricanes than occur with non-ENSO events.

A further Pacific influence involves the manner in which the ENSO strengthening of the southern subtropical jet stream may partly explain the heavy rainfall experienced over southern Brazil, Paraguay and northern Argentina during an intense El Niño. Another Atlantic teleconnection may reside in the North Atlantic Oscillation (NAO), a large-scale alternation of atmospheric mass between the Azores high-pressure and the

Icelandic low-pressure cells (see Chapter 7C.2B). The relative strength of these two pressure systems appears to affect the rainfall of both northwest Africa and the sub-Saharan zone.

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