8.3.1.3 Extratropical Storms

The impact of extratropical cyclones on global climate derives primarily from their role in transporting heat, momentum and humidity. Regionally and individually, these mid-latitude storms often provide beneficial precipitation, but also occasionally produce destructive flooding and high winds. For these reasons, the effect of climate change on extratropical cyclones is of considerable importance and interest.

Among the several approaches used to characterise cyclone activity (e.g., Paciorek et al., 2002), analysis methods that identify and track extratropical cyclones can provide the most direct information concerning their frequency and movement (Hoskins and Hodges, 2002, 2005). Climatologies for the distribution and properties of cyclones found in models can be compared with reanalysis products (Chapter 3), which provide the best observation-constrained data.

Results from a systematic analysis of AMIP-2 simulations (Hodges, 2004; Stratton and Pope, 2004) indicate that models run with observed SSTs are capable of producing storm tracks located in about the right locations, but nearly all show some deficiency in the distribution and level of cyclone activity. In particular, simulated storm tracks are often more zonally oriented than is observed. A study by Lambert and Fyfe (2006), based on the MMD at PCMDI, finds that as a group, the recent models, which include interactive oceans, tend to underestimate slightly the total number of cyclones in both hemispheres. However, the number of intense storms is slightly overestimated in the NH, but underestimated in the Southern Hemisphere (SH), although observations are less certain there.

Increases in model resolution (characteristic of models over the last several years) appear to improve some aspects of extratropical cyclone climatology (Bengtsson et al., 2006), particularly in the NH where observations are most reliable (Hodges et al., 2003; Hanson et al., 2004; Wang et al., 2006). Improvements to the dynamical core and physics of models have also led to better agreement with reanalyses (Ringer et al., 2006; Watterson, 2006).

Our assessment is that although problems remain, climate models are improving in their simulation of extratropical cyclones.