Toward a Nonlinear Identification of the Atmospheric Response to ENSO

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Abstract
Motivated by the need to understand the nature of the remote atmospheric climate signal associated with El Niño–Southern Oscillation (ENSO), the question is addressed of estimating the nonlinear atmospheric response to ENSO using state-of-the-art general circulation models (GCMs). A set of multidecadal integrations of the Hadley Centre GCM model, HadAM1, is considered and the focus is on the variability of the winter 500-mb heights over the North Pacific and North Atlantic basins. The method is based on optimally filtering the signal out given an estimate of the covariance matrices of the ensemble mean and the internal noise, respectively, and requires that the ensemble mean be split into clusters according to the phase of the Southern Oscillation and then the signal in each cluster found. Over the North Pacific, La Niña appears to trigger the negative Pacific–North American (PNA) oscillation while during El Niño the response is degenerate, that is, with more than one response pattern, where the first one has a zonally stretched PNA-like structure with a north–south seesaw signature and the second one is similar to the tropical Northern Hemisphere pattern. None of them is precisely the reverse of the response corresponding to La Niña (−PNA). A similar behavior is observed over the North Atlantic where a tripole pattern emerges during La Niña, whereas the first pattern obtained during El Niño shows a (tilted) dipole structure with a north–south seesaw.