Changes in storm tracks and energy transports in a warmer climate simulated by the GFDL CM2.1 model

Abstract

Storm tracks play a major role in regulating the precipitation and hydrological cycle in midlatitudes. The changes in the location and amplitude of the storm tracks in response to global warming will have significant impacts on the poleward transport of heat, momentum and moisture and on the hydrological cycle. Recent studies have indicated a poleward shift of the storm tracks and the midlatitude precipitation zone in the warming world that will lead to subtropical drying and higher latitude moistening. This study agrees with this key feature for not only the annual mean but also different seasons and for the zonal mean as well as horizontal structures based on the analysis of Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 model simulations. Further analyses show that the meridional sensible and latent heat fluxes associated with the storm tracks shift poleward and intensify in both boreal summer and winter in the late twenty-first century (years 2081–2100) relative to the latter half of the twentieth century (years 1961–2000). The maximum dry Eady growth rate is examined to determine the effect of global warming on the time mean state and associated available potential energy for transient growth. The trend in maximum Eady growth rate is generally consistent with the poleward shift and intensification of the storm tracks in the middle latitudes of both hemispheres in both seasons. However, in the lower troposphere in northern winter, increased meridional eddy transfer within the storm tracks is more associated with increased eddy velocity, stronger correlation between eddy velocity and eddy moist static energy, and longer eddy length scale. The changing characteristics of baroclinic instability are, therefore, needed to explain the storm track response as climate warms. Diagnosis of the latitude-by-latitude energy budget for the current and future climate demonstrates how the coupling between radiative and surface heat fluxes and eddy heat and moisture transport influences the midlatitude storm track response to global warming. Through radiative forcing by increased atmospheric carbon dioxide and water vapor, more energy is gained within the tropics and subtropics, while in the middle and high latitudes energy is reduced through increased outgoing terrestrial radiation in the Northern Hemisphere and increased ocean heat uptake in the Southern Hemisphere. This enhanced energy imbalance in the future climate requires larger atmospheric energy transports in the midlatitudes which are partially accomplished by intensified storm tracks. Finally a sequence of cause and effect for the storm track response in the warming world is proposed that combines energy budget constraints with baroclinic instability theory.

Appendix

1.1 Change in oceanic heat transport

The meridional oceanic heat transport can be derived from the surface energy balance and is written as

$$ S_o = R_{surface} - SH - LH - \nabla\cdot F_o $$

(8)

where S _o denotes the rate of ocean heat storage, R _surface is the net downward radiative flux at the surface, SH and LH are the surface upward sensible and latent heat fluxes, and F _o denotes the oceanic heat transport. The difference from the derivation in atmospheric energy transport is that the ocean heat storage can’t be neglected in the long term as climate warms. The heat storage rate is defined as the oceanic potential temperature difference between years 1961–2000 and years 2081–2100 integrated over the ocean depth, multiplying by the sea water density and heat capacity, divided by the time period of 110 years. It is quite uniformly distributed over latitude and is calculated to be about 1.5 W/m² on global average (consistent with Russell et al. (2006a)). Increased heat storage is relatively larger over the Southern Ocean and smaller over the North Atlantic region (not shown). Figure 13a shows the climatological meridional total, atmospheric and oceanic energy transport derived from the energy balance requirement. Taking the differential ocean heat storage rate into account, Fig. 13b shows the change in total, atmospheric and oceanic heat transport in the future climate. It is noted that the oceanic heat transport decreases almost everywhere whereas the atmospheric heat transport increases. Held and Soden (2006) pointed out that the reduction in meridional ocean heat transport serves as a compensation for the increased poleward latent heat transport in the atmosphere.