92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 11:30 AM
A Surface Bjerknes Mode for Generating Pacific Decadal Variability
Room 355 (New Orleans Convention Center )
P.N. DiNezio, University of Miami, Miami, FL; and A. Clement, B. Kirtman, and M. Cane

Observed Pacific decadal climate variability (PDV) has been described as ENSO-like due to the similarity of the sea surface temperature and atmospheric circulation anomalies with ENSO. From this analogy it is also expected that PDV is predictable. However, climate models have a much lower skill predicting PDV compared with ENSO. Why is this? Here we present a Surface Bjerknes mode (SBM) that generates a decadal ENSO-like sea surface temperature (SST) pattern in climate models via surface processes alone. This mode emerges on decadal and longer timescales when the thermocline is closer to equilibrium with the wind changes. The decadal thermocline response to wind changes is consistent with the linear steady-state forced Sverdrup response, which has no signal in the east, and shoaling increasing towards the west. Unlike the ENSO mode, the lack of delayed thermocline adjustment does not lead to a self-sustained oscillation. However, the Bjerknes feedback is still positive on decadal timescales due to the zonal advection and upwelling feedbacks. Thus, the SBM requires external forcing to grow. These fundamental differences between ENSO and PDV could explain the reduced predictability on decadal timescales. Evidence for this mode in observations for the 1970-2010 period is provided.

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