9.5.3.7 Extratropical Cyclones
Simulations of 21st-century climate change in the MMD 20C3M model ensemble generally exhibit a decrease in the total number of extratropical cyclones in both hemispheres, but an increase in the number of the most intense events (Lambert and Fyfe, 2006), although this behaviour is not reproduced by all models (Bengtsson et al., 2006; Section 10.3.6.4). Many 21st-century simulations also show a poleward shift in the storm tracks in both hemispheres (Bengtsson et al., 2006; Section 10.3.6.4). Recent observational studies of winter NH storms have found a poleward shift in storm tracks and increased storm intensity, but a decrease in total storm numbers, in the second half of the 20th century (Section 3.5.3). Analysis of observed wind and significant wave height suggests an increase in storm activity in the NH. In the SH, the storm track has also shifted poleward, with increases in the radius and depth of storms, but decreases in their frequency. These features appear to be associated with the observed trends in the SAM and NAM. Thus, simulated and observed changes in extratropical cyclones are broadly consistent, but an anthropogenic influence has not yet been detected, owing to large internal variability and problems due to changes in observing systems (Section 3.5.3).
9.5.4 Precipitation
9.5.4.1 Changes in Atmospheric Water Vapour
The amount of moisture in the atmosphere is expected to increase in a warming climate (Trenberth et al., 2005) because saturation vapour pressure increases with temperature according to the Clausius-Clapeyron equation. Satellite-borne Special Sensor Microwave/Imager (SSM/I) measurements of water vapour since 1988 are of higher quality than either radiosonde or reanalysis data (Trenberth et al., 2005) and show a statistically significant upward trend in precipitable (column-integrated) water of 1.2 ± 0.3 % per decade averaged over the global oceans (Section 3.4.2.1). Soden et al. (2005) demonstrate that the observed changes, including the upward trend, are well simulated in the GFDL atmospheric model when observed SSTs are prescribed (Figure 9.17). The simulation and observations show common low-frequency variability, which is largely associated with ENSO. Soden et al. (2005) also demonstrate that upper-tropospheric changes in water vapour are realistically simulated by the model. Observed warming over the global oceans is likely largely anthropogenic (Figure 9.12), suggesting that anthropogenic influence has contributed to the observed increase in atmospheric water vapour over the oceans.