Mechanisms of tropical Pacific interannual-to-decadal variability in the ARPEGE/ORCA global coupled model |
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| Authors: | Carole Cibot Eric Maisonnave Laurent Terray Boris Dewitte |
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| Institution: | (1) Sciences de l’Univers au CERFACS, CERFACS/CNRS URA 1875, 31057 Toulouse Cedex 1, France;(2) Laboratoire d’Etude en Géophysique et Océanographie Spatiale, IRD - Centre de Nouméa, 101 promenade Roger Laroque- Anse Vata, BP A5, 98848 Nouméa Cedex, New Caledonia, France |
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| Abstract: | Spatial and temporal structures of interannual-to-decadal variability in the tropical Pacific Ocean are investigated using results from a global atmosphere–ocean coupled general circulation model. The model produces quite realistic mean state characteristics, despite a sea surface temperature cold bias and a thermocline that is shallower than observations in the western Pacific. The periodicity and spatial patterns of the modelled El Niño Southern Oscillations (ENSO) compare well with those observed over the last 100 years, although the quasi-biennial timescale is dominant. Lag-regression analysis between the mean zonal wind stress and the 20°C isotherm depth suggests that the recently proposed recharge-oscillator paradigm is operating in the model. Decadal thermocline variability is characterized by enhanced variance over the western tropical South Pacific (~7°S). The associated subsurface temperature variability is primarily due to adiabatic displacements of the thermocline as a whole, arising from Ekman pumping anomalies located in the central Pacific, south of the equator. Related wind anomalies appear to be caused by SST anomalies in the eastern equatorial Pacific. This quasi-decadal variability has a timescale between 8 years and 20 years. The relationship between this decadal tropical mode and the low-frequency modulation of ENSO variance is also discussed. Results question the commonly accepted hypothesis that the low-frequency modulation of ENSO is due to decadal changes of the mean state characteristics. |
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