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Tianming Li

a simple model of ENSO must incorporate both components of the thermocline depth variation. The physical mechanism behind this stationary SST mode involves two essential types of feedback processes, as illustrated in Fig. 13 . In the left side of this schematic diagram, it depicts a positive feedback cycle. Suppose we start from an initial phase of the El Niño, say, a weak warming in the eastern equatorial Pacific. In response to the SST forcing, the atmospheric east–west circulation, the

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Douglas G. MacMynowski and Eli Tziperman

importance of eastern Pacific thermocline depth has been noted in numerous contexts (e.g., Timmermann et al. 1999 , 2005 ; Fedorov and Philander 2001 ; Philander and Fedorov 2003 ; Münnich et al. 1991 ; Galanti et al. 2002 ). Philander and Fedorov also noted the importance of the temperature gradient across the thermocline. The importance of wave reflection coefficients ( Kang and An 1998 ), vertical mixing ( Syu and Neelin 2000 ), and the background mean state ( Codron et al. 2001 ) have been

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C-P. Chang and Tim Li

important if seasonal-mean wind (westerly during northern summer and easterly during northern winter) is considered. Thermocline variation is included because it is very important in the Pacific on the interannual timescale ( Philander 1990 ; Meehl 1993 ). Even though in the equatorial western Pacific vertical motion associated with the mean ocean circulation is downward, the effect of wind-induced vertical mixing, which is proportional to the third power of surface wind speed ( Kraus and Turner 1967

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Heng Xiao and Carlos R. Mechoso

for effects by seasonal variations in other features of the upper-ocean circulation (e.g., the thermocline) and GCMs capable of producing a realistic seasonal cycle in the upper ocean have been used to explore the interactions between ENSO and the seasonal cycle in the upper ocean (e.g., Chang et al. 1995 ; Guilyardi 2006 ). An and Wang (2001) , for example, working with a modified Cane–Zebiak model, found that allowing the basic state thermocline depth (upper-layer depth) to vary seasonally

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Tim Li, Timothy F. Hogan, and C-P. Chang

drives the atmospheric Walker circulation so that winds at the surface are easterlies. The easterly trades converge onto the warmest water in the western Pacific over which convection occurs. The clouds associated with the convection affect the SST by altering net shortwave and longwave radiation at the surface. In addition, the trades also influence the SSTs through surface evaporation and ocean dynamics (by inducing three-dimensional ocean currents and thermocline variations). A strong east

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Jean Philippe Duvel, Rémy Roca, and Jérôme Vialard

equatorial Indian Ocean while the control by the buoyancy plays a role south of our region of interest. This suggests that the interannual variability of the thermocline depth in this region has a systematic influence on the January–March average mixed layer depth and, thus, potentially on the intraseasonal variation of the SST. This thermocline depth may be associated with ENSO, as suggested by the reinforcement of the cyclonic circulation in 1999 that was linked to La Niña conditions. However, the

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Fei-Fei Jin

recharging of the equatorial “reservoir of warm water” as a necessary precondition for the initiation of a warm event. On the basis of his analysis of sea level data, Wyrtki (1986) developed a similar hypothesis. The aftermath of a warm event leaves the thermocline along the equator shallower than normal (i.e., equatorial heat content is low and SST is cold; this is the La Niña phase). Over the next few years the equatorial Kelvin waves allowed by linear equatorial ocean dynamics can move enough of the

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F-F. Jin

/ a W as a nondimensional relative feedback coefficient for the Hadley circulation, and τ EX = − μ Δ T with a specified Δ T in the dimensional unit of degrees Celsius. Equation (4) can be rewritten as τ = − μ / α {( T w − T e ) + μ H ( T w + T e − 2 T n )/2 + Δ T }. (7) The subsurface temperature depends strongly on the thermocline depth. By considering a typical vertical temperature profile of the tropical Pacific, it can be parameterized as in Jin et al. (1996) , T se = T r

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Bin Wang and Zheng Fang

, a theoretical model for E1 Nifio-Southem Oscillation (ENSO) is derived that consistsof prognostic equations for sea surface temperature (SST) and for thermocline variation. Considering only thelargest-scale, equatorially symmetric, standing basin mode yields a minimum dynamic system that highlightsthe cyclic, chaotic, and season-dependent evolution of ENSO. Ftr a steady annual mean basic state, the dynamic system exhibits a unique limit cycle solution for a fairlyrestricted range of air

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Xavier J. Levine and Tapio Schneider

in the subtropics, and an equatorward return flow in the thermocline ( McCreary and Lu 1994 ; Lu et al. 1998 ). This meridional overturning cell dominates the low-latitude ocean heat transport and helps control the surface temperature near the equator ( Klinger and Marotzke 2000 ; Held 2000 ) and the meridional surface temperature gradients ( Trenberth and Solomon 1994 ). It also dominates the total heat transport in the deep tropics because the Hadley circulation there is inefficient at

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