Time series representing two of the climate systems' most leading variablity in northern hemisphere, namely El Nino-Southern Oscillation (ENSO) and the Arctic Oscillation (AO), together with 50 years of observed daily mean surface air temperature, precipitation over the conterminous United States, as well as circulation fields from NCEP/NCAR CDAS/Reanalysis are used to investigate the climate impact of ENSO and AO and their combined effects.  Compsites of ENSO only (El Nino, neutral, La Nina) phase, AO only (high, neutral, low) phase, and their 9 combinations are constructed for US temperature, precipitation and the circulation fields for each of the 12 running season.

        The results show that both ENSO and AO have profound impact on US temperature and precipitation in winter.  In general, the low index phase of AO  resemble that of La Nina and high index phase of AO resemble El Nino. Hence during (C,AO-) and (W,AO+) we expect enhanced effects in the temperature and precipitation anomalies field. For example, while it is usually warmer and drier in the northern states, cooler and wetter in the southern states during El Nino years, the high index phase of AO  represent much warmer winter for large area of US from northern border to central and northeast than the low index phase of AO in the same phase of ENSO. The AO enfluence on precipitation is less well organized but the enfluence exist through out the year in the south and east coastal area, the Tennesse and Ohio valley. These composites provide useful tool to support the 6-10 day, monthly and seasonal forecasts in the Climate Prediction Center.