Recorded presentation6.5
The late fall extratropical response to El Nino: The role of coupling and convection in the tropical western Pacific
Ileana Blade, Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Barcelona, Spain; and M. Newman, M. Alexander, and J. D. Scott
We present some modelling and observational evidence for the hypothesis that the atmospheric bridge from the tropical east Pacific to the tropical west Pacific can modify the extratropical ENSO response in the North Pacific region in late fall/early winter.
The model simulations are various sets of regionally-coupled ensemble simulations with the GFDL-R30 model. In these so-called “pacemaker” experiments, observed sea surface temperatures (SST) are prescribed in the tropical eastern Pacific, whereas outside this region a bulk-mixed mixed layer model is coupled to the atmosphere. We also performed simulations with coupling confined to the tropical Indian-Western Pacific (IWP) oceans, which indicate that most of the impact of coupling on the extratropical ENSO response is due to tropical coupling in IWP.
First, we show that the observed circulation in the North Pacific in late fall/early winter (November/December) is very sensitive to forcing in the western tropical Pacific, with warm SSTs and positive convective anomalies being associated with a deeper Aleutian low ? a sensitivity that is well-reproduced in our coupled model simulations. In particular, observed El Niño events characterized by warm (cold) SST anomalies and positive (negative) convective anomalies in the western Pacific result in a strong (weak) extratropical response in the North Pacific. The fact that this sensitivity is also present in the coupled simulations, in which the SST anomalies in the western Pacific are remotely forced by El Niño (through the atmospheric bridge), emphasizes the role of coupling in the western Pacific in modulating the extratropical response to El Niño in early fall. We suggest that this effect might be deterministic and associated with the response of the western tropical Pacific to particular "flavors" of El Niño, which in turn might lead to some predictability of the ENSO response during late fall/early winter.
Session 6, Unresolved issues in ENSO dynamics and prediction
Tuesday, 21 August 2007, 10:30 AM-12:00 PM, Broadway-Weidler-Halsey
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