The Seasonal Footprinting Mechanism

in the CSIRO Coupled General Circulation Models

and in Observations

Daniel J. Vimont

2002

A dissertation submitted in partial fulfillment
of the requirements for the degree of

Doctor of Philosophy

University of Washington

Atmospheric Sciences

Abstract:

The Seasonal Footprinting Mechanism (SFM) is defined herein.  The SFM provides a means by which winter mid-latitude atmospheric variability over the North Pacific becomes an important external forcing of ENSO.  qqDuring winter, when the mid-latitude atmosphere is most energetic, mid-latitude atmospheric variability imparts an SST ``footprint'' onto the ocean via changes in the net surface heat flux.  This SST footprint persists into late spring and summer, when its subtropical portion forces an atmospheric circulation that includes zonal wind stress anomalies in the equatorial western Pacific.  The coupled tropical system adjusts to these summer zonal wind stress anomalies, producing an ENSO-like pattern of variability.

This dissertation combines an analysis of the SFM in a model framework with a separate analysis of the SFM in the observed record.  While the latter analysis is more applicable for nature, the former plays a crucial role in providing a laboratory in which to distill the essential physics responsible for the SFM.

The SFM is defined in the CSIRO coupled general circulation models.  It is found that the SFM accounts for 25--50\% of the model's interannual ENSO variability, and up to 75\% of the model's interdecadal ENSO-like variability.  Sensitivity experiments confirm an essential role of the oceanic mixed layer (and hence the SST footprint) in the northern tropics and subtropics, and highlight the importance of the seasonal response in creating zonal wind stress anomalies that influence the tropical ocean waveguide (and hence ENSO).

The observed record suggests the SFM is a leading contributor to the stochastic forcing of ENSO.  Two separate statistical analyses are presented, both of which yield results that closely resemble those from the CSIRO models.  Both analyses indicate that ENSO is preceded by sea level pressure (SLP) anomalies that closely resemble the North Pacific Oscillation (NPO) during the preceding winter.  It is shown that NPO-like SLP anomalies exhibit significant skill in predicting ENSO up to three seasons in advance.   The strong relationships between the NPO and ENSO may enhance predictability of ENSO, and support the notion that ENSO may be in a linearly stable regime in nature.

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