Predictions of changes in midlatitude storminess

Hall et al. (1994) summarised the achievements and limitations of early dynamic models of the climate system in addressing the mid-latitude windstorm hazard. Typical models used in the 1980s had relatively poor spatial resolution, resulting in a poor simulation of the growth and decay of individual extratropical cyclones. Therefore they were unable to make accurate predictions of changes to the mid-latitude windstorm hazard with an enhanced greenhouse effect. However, they did all predict three global-scale changes that can affect storm frequency and intensity (see Section 2.3.1). The first is an increase in atmospheric water vapour, due to increased evaporation as the ocean surface warms. This tends to produce more intense storms through increased latent heat release within the cyclone. The second is an enhanced warming of the surface and lower troposphere at high latitudes in winter (due to a positive feedback, as a reduction in ice and snow cover leads to increased absorption of sunlight), though later coupled models generally show this effect restricted to the Northern Hemisphere (e.g. Carnell and Senior, 1998). This will reduce the surface meridional temperature gradient, leading to a weaker jet stream and less intense storms. The third is an enhanced warming of the upper troposphere at low latitudes, due to increase latent heating there, which provides a greater energy source for storms. Thus the expected changes are likely to have competing effects on the frequency and intensity of extratropical cyclones.

Another possible global-scale effect was pointed out by Trenberth (1999) (see also Section 2.2.2 and Chapter 3). He notes that there will be an increased poleward heat transport associated with the increased latent heat release in extratropical cyclones, and this may be balanced by there being reduced overall numbers of storms. The result of global warming may therefore be fewer but more intense extratropical cyclones.

In addition, local factors, such as the differential heating of land and ocean (affecting the temperature gradient), and the frequency and position of blocking features, are of crucial importance (Carnell and Senior, 1998). Any long-term trends in regional decadal-scale oscillations such as the PNA pattern or the NAO would also have an effect. There are also regional factors such as changes in the intensity and location of the shallow continental winter highs, or possible alterations to the North Atlantic Ocean circulation. In the far North Atlantic the surface water is dense because it has cooled and has increased in salinity following evaporation and sea ice formation. It therefore sinks and flows southwards at depth as it is replaced by warm northward-flowing surface water from the Gulf Stream. Most models predict a weakening of this thermohaline circulation (sometimes known as the Atlantic conveyor belt), mainly due to increased precipitation at high latitudes reducing the surface water salinity. This leads to reduced warming or even cooling at high latitudes.

More sophisticated models have since been used for predicting the effects of global warming on mid-latitude storms. Carnell and Senior (1998) noted that the most recent models are coupled ocean-atmosphere models with improved horizontal resolution. Such models not only can resolve individual extratropical cyclones and blocking anticyclones, but also can more accurately simulate aspects of natural variability such as ENSO. Nevertheless, they may still be inadequate to predict accurately the paths of individual cyclone tracks and the positions of blocking highs. Kattenberg et al. (1996) summarised some of the early results from the 1990s. The models simulated the storm track positions but tended to underestimate their strength. These results show little agreement in predicting changes in storminess due to enhanced C02, partly because of their different resolutions and methods of studying the storm tracks.

Since then, a number of different models have been used to investigate the effect of an enhanced greenhouse effect (in both equilibrium and transient experiments) on extreme wind events, either directly or via numbers of intense cyclones. In each case we note the predicted effects for COz doubling, unless stated. In an equilibrium experiment, Lambert (1995) examined the frequency of winter lows (noted once per day, i.e. allowing many lows per cyclone system) as a function of central pressure. In the Southern Hemisphere he found a 5 per cent reduction in the total frequency of lows, with no significant increase in intense ones. Katzfey and Mclnnes (1996) investigated changes in Australian east-coast low systems in an equilibrium experiment. They found an overall reduction of almost 50 per cent in the frequency of such systems, but with a tendency towards lower minimum pressures. In the

Northern Hemisphere, Lambert (1995) found a 4 per cent decrease in the total frequency of lows, accompanied by a considerable increase in the numbers of more intense lows near the termination of the Pacific and Atlantic storm tracks. In a transient experiment, Carnell et al. (1996) too found fewer but deeper cyclones (each cyclone being noted only once) at the ends of these storm tracks. They also found an increase in wind speeds in the eastern North Atlantic area, with a 30-40 per cent increase in the numbers of gales over the British Isles. A similar wind speed trend was found by Lunkeit et al. (1996) in a transient experiment with a larger COz increase. In an equilibrium experiment, Zwiers and Kharin (1998) examined global changes in wind speeds having a 20-year return period; in mid-latitudes the only (marginally significant) change is an increase in north-west Europe. They found that the number of strong-wind days per year also increases there, but decreases within the main storm tracks. Schubert et al. (1998), however, in a transient experiment, found little change in the frequency of lows or the intensity of cyclones in the North Atlantic, though there is a shift in frequency toward the north-east. Using ensembles of transient experiments, Carnell and Senior (1998) found an overall decrease of a few per cent in the number of Northern Hemisphere winter storms (each being counted once), but an increase in the number with minimum pressure <970 mb (by 11 per cent for runs including aerosol effects). They also found an increase (decrease) in the frequency of blocking highs in the eastern (central) Pacific, and noted a strong tendency for the positive PNA index pattern to become predominant in winter. Ulbrich and Christoph (1999), in a transient experiment, found a slight tendency for the positive NAO index pattern to become more prevalent, with a shift of its low-pressure centre to the north-east.

Thus, despite the problems outlined earlier, we find a consensus is beginning to emerge from recent models that with enhanced greenhouse warming there is likely to be an increase in the frequency of intense extratropical cyclones, particularly at the end of the storm tracks where the storms reach maturity. However, this will probably be accompanied by a reduction in the overall frequency of lows. Such changes accord with Trenberth's suggestion based on theoretical arguments (Trenberth, 1999), and can be explained in terms of the various competing mechanisms, as discussed earlier. In particular, most studies indicate consequent increases in the frequency of extreme wind events in the north-east Atlantic and north-west Europe. Positive phases of the PNA and NAO circulation patterns are also predicted to be more prevalent. Nevertheless, the predicted trends are relatively small compared with the observed decadal-scale natural variability. Because of this, and because of the difficulties in observing trends in the most extreme events, it is still unclear whether these predicted anthropogenic effects are actually beginning to occur in the observed record, despite a number of recent severe storms. Nevertheless, as models improve, particularly in their resolution, their predictions are likely to become more accurate and reliable, even without strong observational support.

Renewable Energy 101

Renewable Energy 101

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. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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