Energy and Moisture Transport in the Earth Climate System: Mean State and the Perturbation Response
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Four studies are presented which investigate how energy and moisture transport define the mean state of the Earth Climate System and its response to perturbations. In the first study, we use a global climate model to study the effect of flattening the orography of the Antarctic Ice Sheet on climate. A general result is that the Antarctic continent and the atmosphere aloft warm, while there is modest cooling globally. The large local warming over Antarctica leads to increased outgoing longwave radiation, which drives anomalous southward energy transport towards the continent and cooling elsewhere. Atmosphere and ocean both anomalously transport energy southward in the Southern Hemisphere. Near Antarctica, poleward energy and momentum transport by baroclinic eddies strengthens. Anomalous southward cross-equatorial energy transport is associated with a northward shift of the inter-tropical convergence zone. In the ocean, anomalous southward energy transport arises from a slowdown of the upper cell of the oceanic meridional overturning circulation and a weakening of the horizontal ocean gyres, causing sea ice in the Northern Hemisphere to expand and the Arctic to cool. Comparison with a slab ocean simulation confirms the importance of ocean dynamics in determining the climate system response to Antarctic orography. We conclude this study by briefly discussing the relevance of these results to climates of the past and to future climate scenarios. The remaining studies consider atmospheric moisture transport. First, we develop a new mathematical framework for analyzing results from climate modeling studies that employ numerical water tracers (WTs). Data made available from WTs, which track the movement of water in the aerial hydrological cycle from evaporation to precipitation, are used to analyze the sources and transport of precipitable water in the climate system. The precipitation over a tagged region is subdivided into contributions from local evaporation and remote evaporation. The contribution from remote evaporation, the moisture convergence, can be further subdivided into zonal, meridional (north-to-south and south-to-north), intrabasin, and interbasin parts to yield additional insight into how the aerial hydrological cycle transports water. This theory is applied to the preindustrial mean state climate as simulated by a global climate model in which evaporated water has been tagged in 10-degree latitude bands in each of the major ocean basins, and in which each major land mass has been tagged separately. Findings from analysis of the mean state concur with findings from earlier studies of the hydrological cycle: water evaporated at the equator and in the high latitudes tends to precipitate locally, whereas water evaporated in the subtropics and midlatitudes tends to precipitate remotely; water evaporated in the subtropics precipitates either equatorward or poleward of its source region, while water evaporated in the midlatitudes mostly precipitates poleward. New insights from the method reveal fundamental differences between the major ocean basins in locally-sourced precipitation, remotely-sourced precipitation and their relative partitioning. Per unit area, the subtropical Atlantic is the largest global moisture source, providing precipitable water to adjacent land areas and to the eastern Pacific ITCZ while retaining the least for in situ precipitation. Subtropical moisture is least divergent over the Pacific basin, which is the smallest moisture source (per unit area) for global land areas. Basins also differ in how subtropical moisture sources are partitioned between tropical, midlatitude, and land regions. Next, we use the same matrix operator framework to study the aerial hydrological cycle response to quasi-equilibrium CO2-doubling. The total change in precipitation is separated into contributions from changes in moisture transport and changes in evaporation, and these, in turn, are further separated into changes due to local moisture divergence and remote moisture convergence. While increased surface evaporation increases precipitation everywhere, changes in moisture transport are necessary to create a spatial pattern where precipitation decreases in the subtropics and increases substantially at the equator. This finding agrees broadly with other findings that have emphasized the role of both surface thermodynamics and transport in determining precipitation changes. Overall, changes in the convergence of remotely-evaporated moisture are more important to the overall precipitation change than changes in the amount of locally-evaporated moisture that precipitates in situ. Further decompositions show that CO2-doubling increases the fraction of locally-evaporated moisture that is exported, enhances moisture exchange between ocean basins, and shifts moisture convergence within a given basin towards greater distances between moisture source (evaporation) and sink (precipitation) regions. These changes can be understood in terms of the increased residence timescale of water in the atmosphere with CO2-doubling, which correspond to an increase in the advective length scale of moisture transport. As a result, the distance between where moisture evaporates and where it precipitates increases. Analyses of several heuristic models further support this finding. We conclude by discussing implications of our findings, including effects of changing atmospheric moisture transport on ocean circulation and interpretation of water isotope proxy records. Finally, we consider the role of atmospheric moisture transport in maintaining the high salinity of surface waters in the tropical Atlantic basin. Two independent observational estimates show a 0.5 Sv freshwater deficit over the Atlantic drainage basin, and moisture flux calculations from the ERA interim observational reanalysis shows that at least half of this deficit is due to moisture export from the subtropical Atlantic basin, over the Panama Isthmus, into the tropical Pacific. GCM experiments with water tracers show that most moisture exported from the Atlantic to the Pacific originates between the equator and 30N, with a significant maximum in the 10N to 20N latitude band. Analysis of the CMIP5 abrupt CO2-quadrupling experiment shows striking intermodel agreement between decreased Atlantic drainage basin freshwater input (approximately 0.1 Sv in the multimodel mean) and increased tropical Atlantic sea surface salinity. GCM water tracer experiments reveal that enhanced Atlantic-to-Pacific moisture transport in a quasi-equilibrium CO2-doubling experiment is responsible for approximately one-quarter of the precipitation increase over the equatorial Pacific, resulting in freshening of the Pacific basin and salinizing of the Atlantic. Most of this increased Atlantic-to-Pacific moisture export originates between the equator and 30N in the Atlantic. This intensification of the interbasin moisture flux is due to altered transport attributable to increased atmospheric specific humidity; this results in increased moisture residence time scales and advective length scales, favoring longer distances between moisture source and sink regions. Implications of these findings are discussed.
- Atmospheric sciences