| A study was performed of the winter climate in the Northeast United States and its relationship to the large-scale circulation. Temperature, radiation, precipitation, and circulation features of the La Nina winter of 1998--1999 were analyzed through observations, NCEP-NCAR Reanalysis, and model simulations by SUNYA regional climate model (RCM). The relationship between the Pacific North American (PNA) pattern and regional winter climate of the Northeast was also investigated. Ten Decembers during the 1980s and 1990s were simulated, five with the most positive and five with the most negative PNA index.; RCM reproduced the key climate features of the Northeast during the winter of 1998--1999. The model's circulation closely agreed with the reanalysis, particularly in the mid- and upper-troposphere, and with surface wind observations. Spatial and temporal patterns of temperature and precipitation agreed well with observations, despite a cold bias in the boundary layer (2--3°C) and dry bias in precipitation. The use of six-hourly, rather than twelve-hourly, reanalysis boundary conditions improved the diurnal cycle and increased the success at capturing fast-moving systems, such as fronts, and reproducing hourly weather variations.; The relationship of the PNA pattern, and other teleconnection patterns, to the Northeast winter climate was investigated. Positive PNA pattern was associated with a stronger, southeastward shifted jet and colder, drier conditions in the Northeast, while mild surface southerlies were more frequent with negative PNA pattern. In the positive PNA simulations, there was a large air-water thermal gradient over the Great Lakes, enhancing evaporation and fluxes of sensible and latent heat. Precipitation and clouds during positive PNA pattern were less abundant across the domain, although lake-effect maxima were well defined. The PDO (Pacific Decadal Oscillation), PNA, and ENSO (El Nino/Southern Oscillation) teleconnections significantly influenced the initial date, final date, and duration of the Great Lakes' ice season. Observed snowfall in the Northeast exhibited a stronger relationship to the North Atlantic Oscillation (NAO) than PNA pattern. Frontal passages were most frequent under a negative PNA and positive NAO pattern, characterized by the jet stream centered over New York. Finally, the tracks of highly positive quasi-geostrophic potential vorticity anomalies were influenced by the modes of PNA and PDO. |
