Gavin Gong

Modeled Northern Hemisphere Winter Climate Response to Realistic Siberian Snow Anomalies

Wintertime Northern Hemisphere climate variability is investigated using large-ensemble (20) numerical GCM simulations. Control simulations with climatological surface (land and ocean) conditions indicate that the Arctic Oscillation (AO) is an internal mode of the Northern Hemisphere atmosphere, and that it can be triggered through a myriad of perturbations. In this study the role of autumn land surface snow conditions is investigated. Satellite observations of historical autumn‐winter snow cover are applied over Siberia as model boundary conditions for two snow-forced experiments, one using the highest observed autumn snow cover extent over Siberia (1976) and another using the lowest extent (1988). The ensemble-mean difference between the two snow-forced experiments is computed to evaluate the climatic response to Siberian snow conditions. Experiment results suggest that Siberian snow conditions exert a modulating influence on the predominant wintertime Northern Hemisphere (AO) mode. Furthermore, an atmospheric teleconnection pathway is identified, involving well-known wave‐ mean flow interaction processes throughout the troposphere and stratosphere. Anomalously high Siberian snow increases local upward stationary wave flux activity, weakens the stratospheric polar vortex, and causes uppertroposphere stationary waves to refract poleward. These related stationary wave and mean flow anomalies propagate down through the troposphere via a positive feedback, which results in a downward-propagating negative AO anomaly during the winter season from the stratosphere to the surface. This pathway provides a physical explanation for how regional land surface snow anomalies can influence winter climate on a hemispheric scale. The results of this study may potentially lead to improved predictions of the winter AO mode, based on Siberian snow conditions during the preceding autumn.

A large-ensemble model study of the wintertime AO-NAO and the role of interannual snow perturbations

Abstract Numerous studies have hypothesized that surface boundary conditions or other external mechanisms drive the hemispheric mode of atmospheric variability known as the Arctic Oscillation (AO), or its regional counterpart, the North Atlantic Oscillation (NAO). However, no single external factor has emerged as the dominant forcing mechanism, which has led, in part, to the characterization of the AO–NAO as a fundamental internal mode of the atmospheric system. Nevertheless, surface forcings may play a considerable role in modulating, if not driving, the AO–NAO mode. In this study, a pair of large-ensemble atmospheric GCM experiments (with SST climatology), one with prescribed climatological snow mass and another with freely varying snow mass, is conducted to investigate the degree to which the AO–NAO is modulated by interannual variability of surface snow conditions. Statistical analysis of the results indicates that snow anomalies are not required to produce the AO–NAO mode of variability. Nevertheless...

Sensitivity of atmospheric response to modeled snow anomaly characteristics

[1] The presence of snow over broad land surface regions has been shown to not only suppress local surface temperatures, but also influence various remote climate phenomena. However, the specific mechanisms and snow anomaly characteristics which produce this response are still not well understood. In this study, large-ensemble general circulation model (GCM) experiments are performed to investigate the sensitivity of the atmospheric response to snow cover vs. snow depth anomalies, and the relevant surface thermodynamic processes involved. Realistic, observation-based, autumn-winter snow forcings over Siberia are developed and applied as model boundary conditions, to evaluate the climate response to (1) comprehensive snow forcings including snow cover and snow depth components, (2) snow cover only forcings, and (3) snow forcings in the absence of a surface albedo response. Results indicate that snow cover extent anomalies are not the only significant contributor to the local temperature response; snow depth anomalies are shown to have a comparable effect. Furthermore, the albedo effect is not the predominant thermodynamic mechanism; processes related to the insulative properties of the snowpack (e.g., thermal conductivity and latent heat flux) are also involved. Lastly, we find that realistic snow cover and snow depth anomalies acting in conjunction are required to produce a local temperature response which is strong enough to distinctly modulate the winter Arctic Oscillation (AO) mode as shown in previous studies. Such a detailed understanding of the atmospheric sensitivity to snow anomaly characteristics is beneficial for effectively utilizing any potential climate predictability contained in snow anomaly signals. INDEX TERMS: 1863 Hydrology: Snow and ice (1827); 1833 Hydrology: Hydroclimatology; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; KEYWORDS: snow, climate

North American Snow Depth and Climate Teleconnection Patterns

Abstract Snow–atmosphere relationships have been studied for nearly half a century, but the primary focus has been on snow extent variability, largely because of the relative scarcity of snow depth data. A recently released North American snow depth dataset, with extensive spatial coverage and multidecadal temporal duration, provides a new opportunity to compare snow depth–climate relationships with snow extent–climate relationships over North America. Robust concurrent lead and lag correlations are observed between snow depth and two major climate modes, the Pacific decadal oscillation (PDO) and the Pacific–North America (PNA) pattern, across North America and throughout the snow season. In contrast, snow extent exhibits a less coherent relationship with PDO and PNA except in late spring, which can be interpreted as a residual of the snow depth–climate mode relationship. A regional signature for the snow depth–PDO/PNA relationship is also identified, centered over interior central-western North America. ...

Orographic Constraints on a Modeled Siberian Snow–Tropospheric–Stratospheric Teleconnection Pathway

Previous modeling studies have identified a teleconnection pathway linking observation-based early season Siberian snow perturbations to a modulation of the winter Arctic Oscillation (AO) mode. In this study, the key role of orography in producing this modeled teleconnection is explicitly investigated using numerical experiments analogous to the previous studies. The climatic response to the same snow perturbation is investigated under modified orographic barriers in southern and eastern Siberia. Reducing these barriers results in a weakening of the prevailing orographically forced region of stationary wave activity centered over Siberia, as well as the snow-forced upward wave flux anomaly that initiates the teleconnection. This diminished anomaly propagates upward, but does not extend into the stratosphere to weaken the polar vortex. Consequently, poleward refraction of upper-tropospheric waves and downward propagation of coupled wave‐mean flow anomalies, which ultimately produce the negative winter AO response, fail to develop. Thus, the mountains represent an orographic constraint on the snow‐AO teleconnection pathway. By reducing the orographic barrier, the snow-forced influx of wave energy remains in the troposphere and, instead, produces a hemispheric-scale equatorward wave refraction.

Modeled Climate State and Dynamic Responses to Anomalous North American Snow Cover

Abstract The radiative and thermal properties of widespread snow cover anomalies have the potential to modulate local and remote climate over monthly to seasonal time scales. In this study, physical and dynamical links between anomalous North American snow conditions and Northern Hemisphere climate are examined. A pair of 40-member ensemble AGCM experiments is run, with prescribed high- and low-snow forcings over North America during the course of an entire year (EY). The difference between the two ensemble averages reflects the climatic response to sustained EY snow forcing. Local surface responses over the snow forcing occur in all seasons, and a significant remote surface temperature response occurs over Eurasia during spring. A hemispheric-scale transient eddy response to EY forcing also occurs, which propagates downstream from the forcing region to Eurasia, and then reaches a maximum in extent and amplitude in spring. The evolution of this transient eddy response is indicative of considerable downstr...

Observed Inconsistencies between Snow Extent and Snow Depth Variability at Regional/Continental Scales

Abstract Snow extent and snow depth are two related characteristics of a snowpack, but they do not need to be mutually consistent. Differences between these two variables at local scales are readily apparent. However, at larger scales, which interact with atmospheric circulation and climate, snow extent is the primary variable considered, owing largely to the scarcity of snow depth data. In this study, three regional-/continental-scale gridded snow depth/snow water equivalent (SWE) datasets, derived from station observations or passive microwave satellite sensors, are utilized to quantitatively evaluate the relationship between snow extent and snow depth/SWE over North America. Various statistical methods are used to ensure the robustness of the results, including correlations, composites, and singular value decomposition analyses. Results indicate that continental-scale snow depth variations are substantial in their own right and that depth and extent anomalies are largely unrelated over broad high-latit...

Regional and Seasonal Estimates of Fractional Storm Coverage Based on Station Precipitation Observations

Abstract Simulated climates using numerical atmospheric general circulation models (GCMS) have been shown to he highly sensitive to the fraction of GCM grid area assumed to be wetted during rain events. The model hydrologic cycle and land-surface water and energy balance are influenced by the parameter k, which is the dimensionless fractional wetted area for GCM grids. Hourly precipitation records for over 1700 precipitation stations within the contiguous United States are used to obtain observation-based estimates of fractional wetting that exhibit regional and seasonal variations. The spatial parameter k is estimated from the temporal raingauge data using conditional probability relations. Monthly k values are estimated for rectangular grid areas over the contiguous United States as defined by the Goddard Institute for Space Studies 4° × 5° GCM. A bias in the estimates is evident due to the unavoidably sparse raingauge network density, which causes some storms to go undetected by the network. This bi...

Physical mechanisms linking the winter Pacific-North American teleconnection pattern to spring North American snow depth.

Abstract The wintertime Pacific–North American (PNA) teleconnection pattern has previously been shown to influence springtime snow conditions over portions of North America. This paper develops a more complete physical understanding of this linkage across the continent, using a recently released long-term, continental-scale gridded North American snow depth dataset and the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis data. An empirical orthogonal function–based filtering process is used to identify and isolate the interannual snow depth variations associated with PNA. Then linear and partial correlations are employed to investigate the physical mechanisms that link winter PNA with spring snow depth. In the positive phase of PNA, the enhanced PNA pressure centers lead to warmer temperatures over northwestern North America and less precipitation at midlatitudes. The temperature and precipitation pathways act independently and in distinct geographical regions, and together they serve ...

North American Temperature, Snowfall, and Snow-Depth Response to Winter Climate Modes

The snowpack is an important seasonal surface water storage reservoir that affects the availability of water resources during the spring and summer seasons in mid‐high latitudes. Not surprisingly, interannual variations in snow cover extent and snow water equivalent have been extensively studied in arid regions such as western North America. This study broadens the focus by examining snow depth as an alternative snowpack metric, and considers its variability over different parts of North America. The authors use singular value decomposition (SVD) in conjunction with linear and partial correlation to show that regional snow-depth variations can be largely explained by the winter North Atlantic Oscillation (NAO) and the Pacific‐North American (PNA) modes of atmospheric variability through distinct mechanistic pathways involving regional winter circulation patterns and hydrologic fluxes. The high index phase of the NAO generates positive winter air temperature anomalies over eastern parts of North America, causing thinning of the winter snowpack via snowmelt. Meanwhile, the high index phase of the PNA generates negative winter snowfall anomalies across midlatitudinal areas of North America, which also serve to thin the snowpack. Positive PNA anomalies have also been shown to increase temperatures and decrease snow depths over western North America. The PNA influence extends across the continent, whereas the NAO influence is limited to eastern North America. The winter snow-depth variations associated with all of these pathways exhibit seasonal persistence, which ultimately yield regional-scale spring snow-depth anomalies throughout much of North America.