Boyle, James S.
Microwave Sounding Unit (MSU) channel 2 temperatures are computed for three sets of model experiments and their interannual variation is compared to that of the observed. The models used are: (1) an ensemble of ten integrations of the NCAR CCM3 using prescribed SSTs for 1979 to 1995, (2) a 300 year integration of the NCAR/DOE Parallel Climate Model (which has the CCM3 as the atmospheric model) and (3) a 300 year integration of the ECHAM4/OPYC coupled model at the Max Planck Institute for Meteorology. In addition, Nino34 and AO indices were computed from SST and MSLP of each data set. The observed data spanned the period of 1979 to 1998. The CCM3 integrations used the observed SSTs from 1979 to 1995. The 300 year coupled runs were divided into non-overlapping 20 year segments, and each segment was processed independently.
The EOFs of the zonally averaged, monthly mean MSU-2 anomalies were computed. An SVD analysis of the covariance of the tropical (30S-30N) precipitation and MSU-2 was carried out.
The first and second modes of the observations are related to the ENSO variations and the Arctic Oscillation, respectively. The Nino34 index leads the ENSO mode by 5 months in the observations. For the nine realizations of the CCM3, all have the ENSO as the leading mode, but one does not have the AO as the second. The lag between the Nino34 and leading EOF decreases to about 3 months. The fourteen PCM 20 year segments show a similar variation to the CCM3, but the lag is decreased to 2 months. All fourteen of the ECHAM segments have the ENSO and AO as the leading and second modes. The fourteen ECHAM data sets evince smaller variations between segments than the PCM and even the CCM3 realizations. The lag between the ECHAM Nino34 and the leading EOF is about 3 months. Thus, both coupled models have a substantially faster response to variations in tropical SSTs. This can affect the way that these models simulate the relation between the seasonal cycle and ENSO.
UCRL-MI-123395