ENSO and Pacific Decadal Variability in Community Climate System Model Version 4

The study presents an overview of the El Nino/Southern Oscillation (ENSO) phenomenon and Pacific Decadal Variability (PDV) simulated in a multi-century pre-industrial control integration of the NCAR Community Climate System Model version 4 (CCSM4) at nominal 1 degree latitude/longitude resolution. Several aspects of ENSO are improved in CCSM4 compared to its predecessor CCSM3, including the lengthened period (3-6 years), the larger range of amplitude and frequency of events, and the longer duration of La Nina compared to El Nino. However, the overall magnitude of ENSO in CCSM4 is overestimated by -30%. The simulated ENSO exhibits characteristics consistent with the delayed/recharge oscillator paradigm, including correspondence between the lengthened period and increased latitudinal width of the anomalous equatorial zonal wind stress. Global seasonal atmospheric teleconnections with accompanying impacts on precipitation and temperature are generally well simulated, although the wintertime deepening of the Aleutian Low erroneously persists into spring. The vertical structure of the upper ocean temperature response to ENSO in the north and south Pacific displays a realistic seasonal evolution, with notable asymmetries between warm and cold events. The model shows evidence of atmospheric circulation precursors over the North Pacific associated with the "seasonal footprinting mechanism", similar to observations. Simulated PDV exhibits a significant spectral peak around 15yr, with generally realistic spatial pattern and magnitude. However, PDV linkages between the tropics and extra-tropics are weaker than observed.