1 NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado
2 Laboratori d'Enginyeria Maritima. Universitat Politècnica de Catalunya,
Barcelona, Spain
3 NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey
Submitted to the Journal of Climate, July 2001
Special issue: Reviews of Climate and its Variability
Observations, model experiments and a review of previous studies are used to investigate the influence of El Niño - Southern Oscillation (ENSO) on the global atmosphere-ocean system. In the "MLM" experiment, observed sea surface temperatures (SSTs) in the tropical Pacific are specified as boundary conditions in an atmospheric general circulation model (AGCM) while the upper ocean over the remainder of the globe is simulated by a predictive mixed layer model. The simulated SST anomalies during El Niño are fairly realistic with warm water in the north tropical Atlantic, Indian Oceans and along the west coast of the Americas and cold water in the central North and South Pacific in boreal winter and spring. While most SST anomalies peak approximately 2-5 months following the maximum El Niño signal in December, the western North Pacific cools rapidly during the previous summer and an east-west dipole structure forms in the Indian Ocean in the previous fall. In addition, thermal anomalies that form in the surface waters of the North Pacific during winter partially recur in the following winter after being sequestered beneath the mixed layer in the intervening summer. The model results indicate that a significant fraction of the dominant pattern of low frequency (> 10 years) SST variability in the North Pacific is associated with tropical forcing. The ENSO-related SST anomalies over most of the globe are primarily generated by changes in the net surface heat flux, although Ekman transport also affects SSTs in the central North Pacific. Changes in the surface fluxes also create sizeable mixed layer depth (MLD) anomalies over the North Pacific.
Two additional model experiments are performed to investigate how the SST anomalies outside of the equatorial Pacific feed back on the original atmospheric response to ENSO. Both have observed SSTs specified in the tropical Pacific, but in the "Control" experiment, climatological SSTs are specified at all other ocean gridpoints, while in the "NP-MLM" experiment, the ocean model is active in the North Pacific and climatological SSTs are specified elsewhere outside the tropical Pacific. The extratropical height anomalies are similar in all three experiments and have the familiar wavetrain structure in the Pacific/North America (PNA) region in winter. During January-February the anomalies are weaker in the MLM experiment than in the Control, but the most striking difference occurs in March when the amplitude of the 500 mb height anomalies are substantially stronger in the NP-MLM than in the Control. A similar but less conspicuous effect is found in the MLM experiment. Sensitivity analysis suggests that air-sea coupling in the North Pacific and in other locations, including the Indian Ocean, Maritime continent and tropical Atlantic influence the atmospheric circulation in the PNA region.
Outstanding issues pertaining to various aspects of the bridge mechanism are discussed.