Cyclones

 

Tropical Cyclones

 

Changes in tropical cyclone frequency and intensity are of considerable interest due to their potential impact on human activities and structures.  However, past trends in tropical cyclone activity – on which future projections are based – are limited by the relatively short observational record of these events and uncertainties in the magnitude of natural variability.  The response of tropical cyclones to numerous natural and anthropogenic factors – such as increasing SST due to increasing greenhouse gases or ozone circulation changes induced by volcanic eruptions – is complex and not yet fully understood.  In addition, shorter-term events like Saharan dust storms have been linked to reduced strengthening of tropical cyclones.  Given the quantity and complexity of factors at play with tropical cyclones, attributing changes in tropical cyclone activity simply to anthropogenic increases in greenhouse gases forcing a long-term trend is an oversimplification.

 

In general, model projections for future changes in tropical cyclone activity are higher confidence at the global scale compared to the regional scale.  This is due to the limited observational record of these events, such that any past trends that the models take into account are based on less data points at the regional scale than at the global scale.  Recent assessments have concluded that it is likely that global tropical cyclone activity will either decrease or remain unchanged, but on the regional scale, there is typically a lack on model consensus on the sign of change for tropical cyclone activity (Fig. 14.17).  Additionally, the frequency of the most intense tropical cyclones is projected to increase substantially, more likely than not to be a >10% increase in some regions.  An increase in tropical cyclone rainfall is also expected due to the enhanced moisture content of the atmosphere in a warmer climate.  In terms of future tropical cyclone tracks, studies suggest that projected changes in tropical cyclone activity are closely associated with projected changes in the spatial distribution of tropical SST.             

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Major uncertainty in predicting future tropical cyclone activity stems from the limited observational record of these events, particularly on the regional scale.  It may take several more decades of observations in order to drive down the signal-to-noise ratio and see the emergence of relevant trends.  This is one reason why current climate models are unable to accurately simulate the observed spatial and temporal variation in tropical cyclone activity.  Since model projections depend critically on these control simulations, they currently do not produce very high confidence projections.  Another limitation in projecting tropical cyclone activity stems from the model’s ability to accurately reproduce major modes of climate variability (e.g., ENSO).  This is because tropical cyclone activity is modulated by these regional climate modes to varying degrees, a relationship that will likely change in some manner in a warming climate.  How these modes of variability change in a warming climate, and how tropical cyclones are physically linked to these modes, is critical to developing higher confidence projections of tropical cyclone activity.

 

Extratropical Cyclones

 

Recent work in the last decade has attempted to predict the response of extratropical storm tracks to anthropogenic forcing.  Several competing factors influence extratropical storm tracks.  One of the most important of these factors is longitudinal temperature gradient, from which extratropical cyclones get most of their energy.  Uncertainties in large-scale warming projections contribute significantly to storm track uncertainty, but local process like sea ice loss could also be important in certain regions.  The increased atmospheric moisture in a warmer climate is expected to have competing effects for extratropical cyclones.  Precipitation in these events is expected to increase, but their wind intensity is projected to increased as poleward heat transport becomes more efficient with a moister atmosphere.

 

One of the most reproducible model projections associated with extratropical cyclones is a poleward shift of the storm track in the Southern Hemisphere.  The weakening of the Mediterranean storm track is another robust model result.  In the North Atlantic, there is a more complex response than just a poleward shift in storm track.  For Europe, there is expected to be an increase in storm activity and a southward shift in the storm track, leading to more frequent intense cyclones affecting the continent.  Overall, there is a general agreement that the total number of extratropical cyclones globally will decrease slightly.  On the regional scale, the majority of models do not reproduce changes exceeding that from natural variability alone.       

 

Summary

 

From the IPCC report: “Although projections under 21st century greenhouse warming indicate that it is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged, concurrent with a likely increase in both global mean tropical cyclone maximum wind speed and rainfall rates, there is low confidence in region-specific projections of frequency and intensity. Still, based on high-resolution modelling studies, the frequency of the most intense storms, which are associated with particularly extensive physical effects, will more likely than not increase substantially in some basins under projected 21st century warming and there is medium confidence that tropical cyclone rainfall rates will increase in every affected region. The global number of extratropical cyclones is unlikely to decrease by more than a few percent due to anthropogenic change. A small poleward shift is likely in the SH storm track, but the magnitude is model dependent. There is only medium confidence in projections of storm track shifts in the Northern Hemisphere. Nevertheless, model results suggest that it is more likely than not that the N. Pacific storm track will shift poleward, and that it is unlikely that the N. Atlantic storm track will respond with a simple poleward shift.”

 

Important limitations remain for our ability to project future changes in cyclone activity, namely the limited observational record (especially on the regional scale), uncertainties surrounding physical links between cyclones and the regional climate modes that modulate them, and interactions with local processes that could have an important region-by-region impact.