Observed and Modeled Northern Hemisphere Autumn and Winter Climate Responses to Realistic Tibetan Plateau and Mongolia Snow Anomalies
The impact of the Eurasian snow cover extent (SCE) on the Northern Hemisphere (NH) circulation is first investigated by applying a lagged maximum covariance analysis (MCA) to monthly satellite-derived SCE and NCEP reanalysis data. Wintertime atmospheric signals significantly correlated with persistently autumn-early winter SCE anomalies are found in the leading two MCA modes. The first MCA mode indicates the effect of Eurasian snow cover anomalies on the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). The second MCA mode links a persistent dipole of autumn and winter SCE anomalies over the Tibetan Plateau (TP) and Mongolia with winter Pacific-North America (PNA)-like atmospheric variations.
A modeling study further investigates atmospheric responses to above TP and Mongolia snow forcings using multiple ensemble transient integrations of the CAM4 and CLM4.0 models. Model boundary conditions are based on climatological sea ice extent (SIE) and sea surface temperature (SST),and satellite observations of SCE and snow water equivalent (SWE) over the TP and Mongolia from October to March in 1997/98 (heavy TP and light Mongolia snow) and 1984/85 (light TP and heavy Mongolia snow), with model derived SCE and SWE elsewhere. In various forcing experiments, the ensemble-mean difference between simulations with these two extreme snow states identifies local, distant, concurrent, and delayed climatic responses.
The main atmospheric responses to a dipole of high TP and low Mongolia SCE persisting from October to March (versus the opposite extreme) are strong TP surface cooling, warming in the surrounding China and Mongolia region, and a delayed (winter-only) positive PNA-like response over the North Pacific and North America. With a less persistent dipole anomaly, or a persistent anomaly in only the TP or only Mongolia, local responses are similar depending on the specific anomalies, but the winter PNA-like response is noticeably reduced or nearly absent. The localized response is maintained by persistent diabatic cooling or heating, and the remote PNA response results mainly from the increased horizontal eastward propagation of stationary Rossby wave energy due to both snow forcing and transient eddy forcing.