The COADS Sea Level Pressure Signal: A Near-Global El Niño Composite and Time Series View, 1946–1993

D. E. Harrison NOAA/PMEL/OCRD, and School of Oceanography and Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Narasimhan K. Larkin School of Oceanography, University of Washington, Seattle. Washington

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Abstract

Using COADS data for the period 1946–1993, the near-global sea level pressure (SLP) patterns associated with interannual variability and the climatological seasonal march are discussed. A particular focus concerns the patterns associated with the two years before and after the South American sea surface temperatures rise (El Niño). The ten El Niño events in this record are composited, and the robustness of the features of this composite is tested.

Many features of the composite are quite robust; they occur during most El Niño events and are infrequent during non-El Niño periods. The most robust feature is an area of negative SLP anomaly (SLPA) in the eastern equatorial Pacific during Year(0) of the composite. This feature exceeds significance thresholds during every El Niño year and never during non-El Niño years; it correlates better with central Pacific SST variability than does the SOI. A west-central North Pacific positive SLPA, occurring late in Year(0) and lasting into the spring of year (+1) is the second most robust feature. Strong SLPA signals occur in the eastern South Pacific and around Australia in many events, but the behavior varies greatly from event to event. Some events show interesting signals in the Indian and Atlantic Oceans, but the behavior is not sufficiently general to be a statistically meaningful element of the composite.

The largest signals in the composite occur in the eastern equatorial and west-central North Pacific and not in the Southern Hemisphere. Thus, the large-scale SLP variations associated with El Niño periods are not dominated by the classical Southern Oscillation. Little evidence is found for phase propagation of the signal in El Niflo years. Although several features of the composite occur during the same season in each El Niño period, so that the main signals are “phase locked” to the seasonal cycle, the patterns of variability have little in common with the patterns of the seasonal march of SLP.

Abstract

Using COADS data for the period 1946–1993, the near-global sea level pressure (SLP) patterns associated with interannual variability and the climatological seasonal march are discussed. A particular focus concerns the patterns associated with the two years before and after the South American sea surface temperatures rise (El Niño). The ten El Niño events in this record are composited, and the robustness of the features of this composite is tested.

Many features of the composite are quite robust; they occur during most El Niño events and are infrequent during non-El Niño periods. The most robust feature is an area of negative SLP anomaly (SLPA) in the eastern equatorial Pacific during Year(0) of the composite. This feature exceeds significance thresholds during every El Niño year and never during non-El Niño years; it correlates better with central Pacific SST variability than does the SOI. A west-central North Pacific positive SLPA, occurring late in Year(0) and lasting into the spring of year (+1) is the second most robust feature. Strong SLPA signals occur in the eastern South Pacific and around Australia in many events, but the behavior varies greatly from event to event. Some events show interesting signals in the Indian and Atlantic Oceans, but the behavior is not sufficiently general to be a statistically meaningful element of the composite.

The largest signals in the composite occur in the eastern equatorial and west-central North Pacific and not in the Southern Hemisphere. Thus, the large-scale SLP variations associated with El Niño periods are not dominated by the classical Southern Oscillation. Little evidence is found for phase propagation of the signal in El Niflo years. Although several features of the composite occur during the same season in each El Niño period, so that the main signals are “phase locked” to the seasonal cycle, the patterns of variability have little in common with the patterns of the seasonal march of SLP.

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