Unusual behavior in atmospheric angular momentum during the 1965 and 1972 El Ninos

The global atmospheric angular momentum (AAM) is known to increase with tropical eastern Pacific sea surface temperature (SST) anomalies during El Nino events. Using a reanalysis dataset, the ratio of the monthly AAM anomaly to El Nino SST anomaly ( based on the Nino-3.4 index) is found to be approximately 1 angular momentum unit (=10(25) kg m(2) s(-1)) per degree Celsius for most post-1975 El Ninos. This ratio is much smaller, however, during the 1965/66 and 1972/73 El Ninos, raising the possibilities that either the early reanalysis data are in error due to sparse observations, or the atmospheric response to the two early El Ninos was unusual. The possibility of a severe data problem in the reanalysis is ruled out by cross-validating the AAM time series with independent measurements of length of day. The latitudinal structures of the zonal wind anomalies in 1965/66 and 1972/73 are examined for both the reanalysis and a set of general circulation model (GCM) simulations. Multiple GCM runs with specified SST produce a more positive ensemble-mean AAM anomaly in 1965 than its counterpart in the reanalysis. The GCM-simulated ensemble-mean zonal wind anomaly resembles the canonical El Nino response with accelerations of subtropical zonal jets in both hemispheres, a pattern that is almost absent in the reanalysis. On the other hand, a large spread exists among the individual ensemble members in the 1965/ 66 GCM simulations. Although the majority of the individual ensemble members shows the canonical El Nino response, two outliers ( out of 12 runs) exhibit very small zonal wind responses in the Northern Hemisphere similar to the reanalysis. Thus, the observed AAM anomaly during 1965/ 66 is interpreted as an outlier with atmospheric noise being strong enough to overwhelm the canonical El Nino response. The low AAM in the 1972/73 event is related in the reanalysis to a significantly negative zonal wind response on the equator. This signal is robustly reproduced, although with a slightly smaller amplitude, in the ensemble mean and all individual ensemble members in the GCM simulations. The small ensemble standard deviation and large ensemble-mean response on the equator indicate that the negative response is due to the lower-boundary forcing related to the SST anomaly. The fact that the AAM anomaly in 1972/73 is not well correlated with the Nino-3.4 index, then, indicates that SST anomalies outside the conventional El Nino region may be responsible for the low AAM. The uncharacteristically low values of global AAM in 1965/ 66 and 1972/73 contribute to a low mean for the decade before 1975, which, combined with high AAM in the post-1980 era, produces a significant upward trend in AAM in the second half of the twentieth century. If the weak AAM anomalies during the two pre-1975 El Ninos are due to random noise or incidental non-El Nino influences, taking them at face value would result in an overestimate of about 15% - 20% in the multidecadal trend of AAM due to boundary forcing alone. Notably, a multidecadal trend in AAM is also simulated in the ensemble mean of the multiple GCM runs, but its magnitude is smaller than the observed counterpart and more consistent with the multidecadal trend of the Nino-3.4 index. The implications of these findings for climate change detection are discussed.