Extratropical Summertime Response to Tropical Interannual Variability in an Idealized GCM

A primitive equation model is used to investigate the role of the tropics in generating seasonal-mean anomalies in the extratropics. A nudging technique is applied to guide selected tropical regions toward 40-yr EuropeanCentre forMedium-RangeWeather Forecasts (ECMWF)Re-Analysis (ERA-40) and theNational Centers for Environmental Prediction (NCEP)/Department of Energy Reanalysis (NCEP-2). The timeindependent linear response to these tropical anomalies is calculated for extratropical basic states taken from reanalysis climatologies and also from the climatological states ofAction deRecherche Petite EchelleGrande Echelle (ARPEGE) and Laboratoire de M et eorologie Dynamique (LMDZ) general circulation model simulations. For summer case studies, time-independent linear solutions show that some seasonal anomalies can be attributed to linear wave propagation from the tropics, especially for lower extratropical latitudes. If nudging is applied to the anomaly part of the tropical flow, the linear response shows little dependence on the basic state. Regional tropical nudging experiments display a global extratropical response. The persistent European summer anomaly in 2003 is partly attributable to a linear response to both Central American and West African monsoon circulations. The African region also triggers a wave train along the Asian subtropical jet. The model is then used in ‘‘simple GCM’’ mode to obtain extratropical responses that include a contribution from transient eddies. Tropical nudging improves the simple GCM’s stationary wave climatology, and transient eddy forcing can produce substantial seasonal anomalies at high latitudes with better correspondence to some observed cases, especially in theWestern Hemisphere, with stronger communication between the Asian monsoon and North America.