Midlatitude storms in a moister world: lessons from idealized baroclinic life cycle experiments

Abstract

The response of midlatitude storms to global warming remains uncertain. This is due, in part, to the competing effects of a weaker meridional surface temperature gradient and a higher low-level moisture content, both of which are projected to occur as a consequence of increasing greenhouse gases. Here we address the latter of these two effects, and try to elucidate the effect of increased moisture on the development and evolution of midlatitude storms. We do this with a set of highly controlled, baroclinic lifecycle experiments, in which atmospheric moisture is progressively increased. To assess the robustness of the results, the moisture content is changed in two different ways: first by using different initial relative humidity, and second by varying a parameter that we insert into the Clausius-Clapeyron equation. The latter method allows us to artificially increase the moisture content above current levels while keeping the relative humidity constant. Irrespective of how moisture is altered, we find that nearly all important measures of storm strength increase as the moisture content rises. Specifically, we examine the storm’s central pressure minimum, the strongest surface winds, and both extreme and accumulated precipitation rates. For all these metrics, increased moisture yields a stronger storm. Interestingly, we also find that when moisture is increased beyond current levels, the resulting storm has a reduced horizontal scale while its vertical extent increases. Finally, we note that for moisture increases comparable to those projected to occur by the end of the twentyfirst century, the actual amplitude of the increases in storm strength is relatively modest, irrespective of the specific measure one uses.

Appendix: Details for Fig. 8

For Fig. 8, we download data from the Program for Climate Model Diagnosis and Intercomparison (PCMDI), CMIP3 archive for the IPCC AR4. We use the historical model integrations to represent the twentieth century and the runs from the Global Warming A2 scenario for the twentyfirst century. Scenario A2 from CMIP3 corresponds to an increase in global temperatures in the range from 2–5 °C. The increase in CO₂ associated with this scenario seemed pessimistic when it was created in 2000, but not anymore. We chose Scenario A2 because it corresponds to a projection in which economies maintain the status quo, which doesn’t seem unreasonable.

1.1 Models used in Fig. 8

BCCR: Bergen Climate Model (BCM) project at the Bjerknes Centre for Climate Research. CCCMA: Canadian Centre for Climate Modeling and Analysis, Victoria, BC, Canada. CCSM: Community Climate System Model project, supported by the, Directorate for Geosciences of the National Science Foundation, and the Office of Biological and Environmental Research of the U.S. Department of Energy. CNRM: Centre National de Recherches Meteorologiques, Meteo-France, Toulouse, France. CSIRO: Atmospheric Research, Melbourne, Australia. ECHAM: Max Planck Institute for Meteorology, Hamburg, Germany. GFDL: US Dept of Commerce/NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA. GISS ModelE: NASA Goddard Institute for Space Studies New York, NY, USA. INMCM: Institute for Numerical Mathematics, Moscow, Russia. IPSL: Institut Pierre Simon Laplace, Paris, France. CCSR/NIES/FRCGC: Center for Climate System Research, Tokyo, Japan/National Institute for Environmental Studies, Ibaraki, Japan/Frontier Research Center for Global Change, Kanagawa, Japan. MRI: Meteorological Research Institute, Tsukuba, Ibaraki, Japan. PCM: National Center for Atmospheric Research, Boulder, CO, USA.