Present Climate

Beneath the earlier mentioned trends, current climate shows significant variability, on timescales of seasons to decades, which are of importance to agriculture and forestry. Those that are most important interannually are the El Nino/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO), and decadally the recently described Interdecadal Pacific Oscillation (IPO). These quasi-periodic variations are superimposed on the general trend of global warming, but their frequencies may be influenced by global warming.

3.1. INTERANNUAL VARIABILITY

3.1.1. El Niño/Southern Oscillation (ENSO)

ENSO is the primary global mode of natural climate variability in the 2-7 year time band defined by sea surface temperature SST anomalies in the eastern tropical Pacific. The Southern Oscillation is a measure of the atmospheric pressure across the Pacific-Indian Ocean region. Atmospheric and oceanic conditions in the tropical Pacific vary considerably during ENSO, fluctuating somewhat irregularly between the El Nino phase and the opposite La Nina phase. In the former, warm waters from the western tropical Pacific migrate eastwards, and in the latter cooling of the tropical Pacific occurs.

As the El Niño develops, the trade winds weaken and warmer waters in the central and eastern Pacific occur, shifting the pattern of tropical rainstorms eastward. Higher than normal air pressures develop over northern Australia and Indonesia with drier conditions or drought. At the same time lower than normal air pressures develop in the central and eastern Pacific with excessive rains in these areas, and along the west coast of South America. Approximately reverse patterns occur during the La Niña phase of the phenomenon.

The ENSO phenomenon's trigger is in the tropical Pacific Ocean. The observed global influences occur as teleconnections as the atmosphere transmits the anomalous heating in the tropics to large-scale convection and thus to anomalous winds in the atmosphere. The main global impacts are that El Nino events cause above average global temperature anomalies above the trend. Since the mid-1970s El Niño events have been more frequent, and in each subsequent event global temperature anomalies have been higher (Trenberth and Hoar, 1997). Figure 6 shows the Southern Oscillation Index since 1930; the Tahiti minus Darwin normalized pressure

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Figure 6. The Southern Oscillation Index 1930-2000. Negative values represent El Niño and positive values of this index La Niña conditions.

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Figure 6. The Southern Oscillation Index 1930-2000. Negative values represent El Niño and positive values of this index La Niña conditions.

index, which measures whether the climate system is in the El Niño or La Niña state. A negative index indicates the El Niño state, and a positive index the La Niña state.

Reconstructions of ENSO from proxy climate indicators (Stahle et al., 1998; Mann et al., 2000) show that ENSO fluctuations have prevailed since at least 1700, but also suggest that the 1982-83 and 1997-98 very large warm events could be outside the range of variability of the past few centuries. Instrumental records show both the activity and periodicity of ENSO have varied considerably since 1871 with considerable irregularity in time. There was an apparent "shift" in the temperature of the tropical Pacific around 1976 to warmer conditions (Salinger et al., 1996), which appeared to continue until at least 1998 (Figure 6). It is unclear whether this warm state continues now as the current moderate but increasingly long La Nina, that began in late 1998 finally subsided during early 2001. The 1990s have received considerable attention, as the recent behaviour of ENSO has seemed unusual relative to that of previous decades. A protracted period of low SOI occurred from 1990-1995, during which several weak to moderate El Niño events occurred with no intervening La Niña events, which is extreme rare (Trenberth and Hoar, 1997) statistically.

3.1.2. North Atlantic Oscillation (NAO)

This large-scale alternation of atmospheric pressure between the North Atlantic regions of the sub-tropical high (near the Azores) and sub-polar low pressure (extending south and east of Greenland) determines the strength and orientation of the poleward pressure gradient over the North Atlantic, and the mid-latitude westerlies in this area. This is measured by the NAO Figure 7). One extreme of the NAO occurs in winter when the westerlies are stronger than normal, bringing cold winters in western Greenland and warm winters to northern Europe. In the other phase the westerlies are weaker than normal which reverses the temperature anomalies. In addition, European precipitation is related to the NAO (Hurrell, 1995). When this index is positive, as it has been for winters in the last decade, drier than normal conditions occur over southern Europe and the Mediterranean, with above normal precipitation from Iceland to Scandinavia. The NAO also affects conditions in North Africa and possibly the Sahel.

There is a seesaw of atmospheric mass between the polar cap and mid-latitudes in both the Atlantic and Pacific Ocean basins, which has been named the Artic Oscillation (AO). The time series of the AO and NAO (Figure 7) are quite similar (Thompson and Wallace, 2000) and the NAO is regarded by some as the regional expression of the AO.

3.2. INTERDECADAL VARIABILITY

Recently shifts in climate have been detected in the Pacific basin, driven by a newly described climate feature, the IPO, which shifts climate every one to three decades (Power et al., 1999; Salinger et al., 2001). This is an 'ENSO-like' feature of the climate system that operates on time scales of several decades. There is a

Figure 7. December to March North Atlantic Oscillation (NAO) indices, 1864-2000, and Arctic Oscillation (AO) indices, 1900-2000, updated from Hurrell (1995) and updated from Thompson and Wallace (2000) and Thompson et al. (2000) respectively. The indices were normalised using the means and standard deviations from their common period, 1900-2000, smoothed twice using a 21 point binomial filter where indicated and then plotted according to the years of their Januarys.

Figure 7. December to March North Atlantic Oscillation (NAO) indices, 1864-2000, and Arctic Oscillation (AO) indices, 1900-2000, updated from Hurrell (1995) and updated from Thompson and Wallace (2000) and Thompson et al. (2000) respectively. The indices were normalised using the means and standard deviations from their common period, 1900-2000, smoothed twice using a 21 point binomial filter where indicated and then plotted according to the years of their Januarys.

tight coupling between the ocean and atmosphere. The main centre of action in SST is in the north Pacific centred near the Date-Line at 40 °N, with an opposing weaker centre just south of the equator in the eastern Pacific, north of Easter Island at 10 °S. There is also another weaker centre of action, in the southwest Pacific centred near the Cook Islands at 20 °S, which is in the same phase as the north Pacific centre. The matching atmospheric sea level pressure pattern is one of an east/west seesaw at all latitudes, but again centred over the north Pacific, with the centre of action over the Aleutian Islands. The IPO has been shown to be a significant source of decadal climate variation throughout the South Pacific and Australia, and also the North Pacific. Future research may determine whether this feature could contribute to decadal climate variability throughout Pacific-rim countries.

Three phases of the IPO have identified during the 20th century: a positive phase (1922-1946), a negative phase (1947-1976) and the most recent positive phase (1977-1998). There is now evidence that the recent positive phase has ended (Figure 8). Prior to the end of the 19th century there is not enough information to derive an IPO index. Power et al. (1999) show that the two phases of the IPO appear to modulate year-to-year ENSO precipitation variability over Australia. The IPO is

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Figure 8. Smoothed index denoting the phases of the Interdecadal Pacific Oscillation (IPO).

a significant source of decadal climate variation throughout the South Pacific, and modulates ENSO climate variability in this region (Salinger et al., 2001). It may also play a key role in modulating ENSO teleconnections across North America on interdecadal time scales (Livezey and Smith, 1999). The results demonstrate that the IPO is a significant source of climate variation on decadal time scales throughout the South West Pacific region. The IPO also modulates interannual ENSO climate variability over the region.

3.3. GLOBAL WARMING

The three features, ENSO, NAO and the IPO all impinge on aspects of global climate, and two are dominant features of the tropical Pacific and oceanic Southern Hemisphere which effect climate variability of the three southern continents of Southern Africa, Australasia, and South America, as well as the Pacific basin. It is on this background of internal climate variability that external mechanisms such as volcanism and the increase of greenhouse gases from anthropogenic activities have acted (Salinger et al., 2000). Modelling studies are best able to identify the importance of these external factors in the period of current climate.

A climate model can be used to simulate the temperature changes that occur both from natural and anthropogenic causes. Figure 9 shows the results of global mean surface temperature anomalies relative to the 1880-1920 instrumental record compared with ensembles of four simulations with a coupled ocean-atmosphere climate model (Stott et al., 2000; Tett et al., 2000; IPCC, 2001a).

From these simulations IPCC (2001a) concluded that climate forcing from changes in solar radiation and volcanism is likely to have caused fluctuations in global and hemispheric mean temperatures in the first part of the 20th century. However, these have been too small to produce the mean temperature increases in the latter part of the 20th century. Well-mixed greenhouse gases (carbon dioxide, methane, chlorofluorcarbons, etc) must have made the largest contribution in radiative forcing to warm the climate in the late 20th century, as now validated by the above mentioned climate model simulations of global-average surface temperature.

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Figure 8. Smoothed index denoting the phases of the Interdecadal Pacific Oscillation (IPO).

Figure 9. The simulations represented by the band in (a) were done with natural forcings: solar variation and volcanic activity. Those encompassed by the band in (b) were done with anthropogenic forcings: greenhouse gases and an estimate of sulphate aerosols and those encompassed by the band in (c) were done with both natural and anthropogenic forcings included.

Figure 9. The simulations represented by the band in (a) were done with natural forcings: solar variation and volcanic activity. Those encompassed by the band in (b) were done with anthropogenic forcings: greenhouse gases and an estimate of sulphate aerosols and those encompassed by the band in (c) were done with both natural and anthropogenic forcings included.

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