An Investigation of Snowcover-atmosphere-ocean Interactions in the Northern Hemisphere with a Global Atmospheric Model Coupled to a Slab Ocean Model PDF Download

Author: Gina R. Henderson
Publisher:
ISBN: 9781124240824
Category : Ocean-atmosphere interaction
Languages : en
Pages :

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Book Description
The difference between snow versus snow free conditions is the most significant natural, seasonal change the land surface can experience. Snow affects all aspects of the surface energy balance including albedo, sensible and latent heat fluxes, and soil moisture. In addition, the presence or lack of snow plays an important role in modifying the overlying air temperature, propagating from local climate to neighboring regions and even globally through atmospheric teleconnections. Numerous studies to date have investigated the implications of snow forcing the atmosphere and associated circulation, however the cause and effect relationship or direction of forcing has not been decisively demonstrated from observed data alone. GCM studies investigating snow-atmosphere interaction have focused on interaction of Siberian or Eurasian snow cover anomalies with the atmospheric teleconnection modes such as the Arctic Oscillation. Although the tendency has been to concentrate on Eurasia due to the magnitude of snowmass, North American snow cover also produces a weak relationship with downstream climate and an atmospheric teleconnection via enhanced North Atlantic storm track activity. Recent GCM studies of the effects of snow cover on overlying atmospheric conditions and large-scale circulation have primarily used a data ocean model with a fixed seasonal cycle of sea surface temperature (SST) and sea ice cover, based on historical SST records. We explore the influence of this SST boundary condition by comparing the data-forced model with a mixed-layer slab ocean model underneath the NCAR atmospheric GCM. Experimental runs consist of 40-year simulations where each experiment was run once with the data-forced model and once with the mixed-layer slab ocean model in scenarios of anomalously high and low snow cover patterns. Anomalous snow cover patterns were generated from historical snow cover data by choosing minimum and maximum depths observed on a particular day of the year for each grid point. Surface response results include significant SST cooling under maximum North American and Eurasian snow conditions. Locations of SST cooling include local coastal cooling directly downstream of each individual forcing region in addition to upstream centers of remote cooling; in the Pacific under anomalously high snow conditions in North America and in the Atlantic under anomalously high Eurasian snow conditions. Significant cooling of surface temperature at 2 m under maximum snow conditions local to each forcing region was evident from both experiments, however values were larger in magnitude and greater in spatial extent when using the slab model. Atmospheric responses to anomalous snow conditions are dominated by a barotropic response under maximum snow conditions throughout much of the mid latitudes in both experiments. Consistent upstream anomalously lower geopotential height and sea level pressure over the Pacific during early winter in particular is evident from the North American Slab experiment, implying a reduced north-south gradient indicating a negative AO phase under maximum snow extent and depth. In contrast, the Data experiment is dominated by weaker and less significant downstream response in both atmospheric fields for both experiments. Areas of positive eddy kinetic energy (EKE) correlate well with steep geopotential height gradient differences between maximum and minimum snow experiments. A dipole of EKE in early winter over the Pacific in both experiments with positive values to the south and negative to the north is indicative of reduced poleward heat flux which may be contributing to a decrease in warm SST advection northwards and the ensuing mid Pacific SST cooling. This proposed pathway is supported by increased zonal wind at 250 hPa collocated with identified regions of sharpened geopotential height gradient, strengthened baroclinicity and positive EKE. The Eurasian experiment shows a similar pathway to the North American experiment, however circulation response is focused downstream of the forcing region in early winter for both Slab and Data experiments. A southward shift of the prevailing East Asia storm track is indicated from a dipole pattern of EKE in the Pacific during early winter when using the Slab model in contrast to strengthening alone with no southward shift under Data conditions. Reduced poleward heat transport associated with a southward shift in the prevailing storm tracks of this region may be influencing the cooling SST trend through reduced warm SST advection to the Aleutian area of the North Pacific.