Changes in Subseasonal variability associated with El Nino

Because El Nino/Southern Oscillation (ENSO) produces relatively small shifts in the daily mean of atmospheric variables, previous studies of seasonal-to-interannual variations have focused on seasonal-mean anomalies. We show that ENSO effects on daily variance are not small, have implications for the daily risk of extreme events, and may be predictable.

The present study details a global comparison of changes of variance on synoptic (2-7 days), intraseasonal (8-45 days), monthly, and seasonal timescales associated with ENSO. NCEP/NCAR Reanalysed data (1948-2000) and general circulation model (GCM) simulations of northern hemisphere winter (January through March) are used to investigate these changes. The NCEP atmospheric GCM is integrated with prescribed seasonally evolving sea surface temperatures for warm, cold, and neutral ENSO conditions. A large number of seasonal integrations, differing only in initial condition, are made for observed climatological mean SSTs, SSTs for an observed warm event (1987), and SSTs for an observed cold event (1989). A total ensemble of 540 members is generated, 180 members each for warm, cold, and neutral conditions. Of these, a subset is used at twice-daily resolution. With this large ensemble, changes of variance can be discriminated in regions not usually associated with an ENSO effect.

The results suggest that the patterns of variance changes associated with ENSO, and the altered risk of extreme events, differ in the extratropics depending on timescale, while they are nearly the same in the Tropics across timescales. The results show that the expected variance changes, and the risk of extreme events, for individual ENSO events may differ from a historical composite, particularly over the Atlantic sector and during La Nina. The potential predictability of these changes is discussed.