Linearity in ENSO's Atmospheric Response*

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Abstract
The linearity, or extent of antisymmetry, of El Niño and La Niña heating and circulation anomalies is examined for the period 1950–2000. Characteristic structures are obtained by compositing winter season anomalies for positive and negative values of the Niño-3.4 sea surface temperature (SST) index in excess of one standard deviation. Eight winters meet this condition in each ENSO phase, and the warm and cold years are equitably distributed relative to the 1976/77 climate transition. ENSO SSTs have a direct effect on the large-scale atmospheric circulation through their impact on diabatic heating and subsequent upper-level divergence over the equatorial Pacific. These fields show a significant westward displacement for the La Niña composite compared to the El Niño composite, as expected from the SST threshold condition for convection. But despite the westward shift in convection, the 200-mb height composites are almost antisymmetric over the Pacific, with only a small (∼10°) westward shift for the extratropical La Niña pattern. The upper-level height response in the Tropics, including the position of the El Niño anticyclones, is found to be even more antisymmetric than the extratropical response. The responses are less antisymmetric over eastern North America and the Atlantic. These results are broadly consistent with idealized experiments in which the midlatitude circulation response to equatorial heating is insensitive to shifts in the longitude of the heating. However, the finding of antisymmetry in the upper-level Pacific height responses to warm and cold ENSO events is in disagreement with the observational composites of Hoerling et al., which show a large shift between El Niño and La Niña height patterns over the North Pacific. In their composites, the La Niña response resembles the Pacific–North American (PNA) pattern, a result not in evidence here. This difference can be understood as a consequence of decadal variability, particularly the 1976/77 climate transition.