Can the mean structure of the tropical pycnocline affect ENSO period in coupled climate models? |
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| Institution: | 1. Atmospheric and Environmental Research (AER), 131 Hartwell Ave, Lexington, MA 02421, USA;2. Massachusetts Institute of Technology (MIT), 77 Massachusetts Ave, Cambridge, MA 02139, USA;1. Institute of Oceanography, CEN, Universität Hamburg, Hamburg, Germany;2. Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, Bremerhaven, Germany;1. Department of Marine Biotechnology and Resources, National Sun Yat-sen University, Kaohsiung 80424, Taiwan;2. Department of Oceanography, National Sun Yat-sen University, Kaohsiung 80424, Taiwan;3. Naval Ocean Analysis and Prediction Laboratory, Naval Postgraduate School, Monterey, CA 93943, USA;4. Institute of Maritime Information and Technology, National Kaohsiung Marine University, Kaohsiung 80543, Taiwan;5. Scripps Institution of Oceanography, La Jolla, CA 92093-0213, USA |
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| Abstract: | The dynamical link between mean state biases and dominant timescales of interannual variability is examined using the output from two state-of-the-art coupled model simulations, results from an ocean-only simulation forced with observed surface fields, and various observational data sets. The focus of this study is the relative role of the mean upper ocean density structure vs. anomalous wind forcing in controlling the spectral characteristics of tropical Pacific interannual variability. It is shown that an extensive South Pacific Convergence Zone (SPCZ) creates a potential vorticity (PV) barrier in the Southern Hemisphere similar to the one associated with the Intertropical Convergence Zone (ITCZ) in the Northern Hemisphere in both climate models. The PV barrier in the Southern Hemisphere strongly constrains the mean equatorward flow in the ocean model pycnocline, creating a “choke point” for the mean flow around 10°S. It is then examined whether the PV barrier can also limit the anomalous flow associated with mass recharge/discharge to/from the equatorial thermocline at interannual timescales. If the anomalous flow were impeded by the mean PV structure the meridional extent of the area involved in the mass recharge/discharge process would be narrower, leading to a shorter adjustment (and ENSO) timescale. Comparison of the two climate models, both of which have similarly erroneous PV structures in the southern tropical Pacific, but different interannual timescales, shows that the meridional extent of the anomalous meridional transport is primarily controlled by the latitudinal location of the wind stress curl anomalies, while the mean state bias in the Southern Hemisphere does not seem to have any significant influence. |
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