The Roles of Land and Orography on Precipitation and Ocean Circulation in Global Climate Models
Maroon, Elizabeth Allison
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In this thesis, coupled and atmosphere-only global climate models are used to examine two large-scale climate asymmetries: the zonal asymmetry of tropical precipitation about the equator and the preference for sinking in the North Atlantic Ocean, but not in the North Pacific Ocean. The examination of these two climate asymmetries is performed using models of differing complexity. The first half of this dissertation discusses the influence of land on the distribution of tropical precipitation in idealized geometry models. A continent is added to the Northern Hemisphere subtropics of two aquaplanet models; annual mean insolation is prescribed and the albedo and longitudinal extent of the continent are varied. One of the models, GRaM, has gray-radiation physics with moist dynamics, while the other model, GFDL's AM2.1, has comprehensive physics. In the GRaM model, the pattern of the precipitation response is mostly related to decreased evaporation due to the now unsaturated surface. As the albedo of land is increased, precipitation shifts southward away from the hemisphere with less absorbed energy. In the AM2.1 model, there is a zonally-varying response in tropical precipitation due to the addition of land, but this response is not rubust in simulations that include a seasonal cycle of insolation. As albedo over land is increased, precipitation shifts southward zonally, just as in GRaM. When the width of the continent is increased, tropical precipitation shifts toward the continent, which indicates that continental width plays an important role in setting the distribution of tropical atmospheric overturning circulations. The second half of this dissertation examines the influence of Rocky Mountain orography on the location of Northern Hemisphere sinking of the oceanic meridional overturning circulation (MOC). Warren (1983) found that there is greater transport of salt into the high latitude North Atlantic than into the North Pacific, allowing water to sink in the winter as the downward branch of the Atlantic MOC (AMOC). The southwest-to-northeast tilt of the boundary between the North Atlantic subtropical and subpolar gyres, which is forced by the winds, accomplishes this northward salt transport. Because the Rocky Mountains influence the distribution of wind stress curl in the midlatitude North Atlantic, the presence of the Rockies may be the reason why water sinks in the North Atlantic (Warren, 1983). To test Warren's hypothesis, we remove the Rocky Mountains in a coarse resolution version of GFDL's CM2.1 model that has realistic boundary conditions. In this simulation, the removal of the Rockies causes the strength of the AMOC to decrease by 7 Sv and a Pacific MOC (PMOC) starts. Additional coupled simulations are performed that re-route western North American rivers from the North Pacific to the North Atlantic while retaining the Rocky Mountains. In all of the simulations, decreased runoff to the North Pacific is responsible for the increase in salinity that initiates the sinking in the PMOC. A salinity-advection feedback amplifies the initial density anomaly in the region of Pacific sinking. Similarly, increased runoff to the North Atlantic, not altered wind stress, is responsible for the decrease in AMOC strength, but the AMOC still exists after the Rocky Mountains are removed. Atmosphere-only simulations are used to isolate the changes in surface wind due to the mechanical (orographic) and thermodynamical (buoyancy) impacts of the Rockies on the MOC. We find that the changes in wind stress curl that are responsible for the North Atlantic gyre circulations are strongly affected by the Rocky Mountain orography through its impact on the atmospheric stationary wave (as is well known). Nonetheless, the preference for sinking in the North Atlantic is not affected by changes in the pattern of wind stress that result from the presence of the Rockies.
- Atmospheric sciences