[Eoas-seminar] EOAS Colloquium Fri Nov 2 - Xiaolin Zhang

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Tue Oct 30 12:20:12 EDT 2018


This week's speaker at 3:30 Friday in CAR 101 is Xiaolin Zhang from the University of Colorado Boulder


On the Physics of the Warm Water Volume and El Niño /La Niña Predictability

Previous work has shown that warm water volume (WWV), usually defined as the volume of equatorial Pacific warm water above the 20oC isotherm between 5oS and 5oN, leads El Niño. Analysis here shows that, if sea level is adjusted for the near-surface salinity variability associated with the zonal movement of the western equatorial Pacific warm/fresh pool by a shallow, narrow freshwater jet, then the essential relationship between El Niño and WWV can be described by using a 2-layer ocean model with resting lower layer. Building on recent work, WWV is related to “non-equilibrium” ocean adjustment. Specifically, after westerly equatorial wind anomalies in a coupled ocean/atmosphere instability push the warm pool eastward during El Niño, the westerly anomalies follow the warmest water south of the equator in the southern hemisphere summer in December to February. With the wind forcing that causes El Niño in the eastern Pacific removed, the eastern equatorial Pacific sea level and thermocline anomalies decrease to zero. Through long Rossby wave dynamics this decrease results in a westward equatorial flow that tends to push the warm pool westward and often results in the generation of a La Niña during March to June. The eastern equatorial sea level has lower amplitude during La Niña, the negative feedback is not as strong, and El Niños tend to not follow La Niñas the next year. This El Niño /La Niña asymmetry is seen in the WWV/El Niño phase diagram and decreased predictability during “La Niña-like” decades.
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