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Physics of climate change in the western tropical Pacific and its influence on ENSO amplitude

PI: Axel Timmermann
Institution: University of Hawaii
Additional Investigators: Soon-Il, Oliver Timm
Some years ago, the idea had been proposed (Ramanathan and Collins 1993) that clouds play a very important role in establishing the delicate heat balance in the Pacific warm pool area by providing a negative feedback – the so-called cloud thermostat. Several authors (Sun and Liu 1996 and Li 2000) have tried to incorporate ocean dynamical heating into the heat budget analysis. However, their attempts to formulate a dynamical thermostat hypothesis failed due to the inadequate assumption of zonal heat recycling in the tropical Pacific. The issue of cloud- and dynamical thermostats has not been resolved yet from a theoretical perspective.

Moreover, climate model simulations tend to show a large uncertainty in their greenhouse warming temperature projections in the warm pool area, partly due to differences in cloud-representation and partly due to ocean dynamics. In a 4xCO2 simulations performed recently with the HadCM3 model (T. Toniazzo, personal communication) , warm pool temperatures rose up to 36 degree Celsius. Positive feedbacks seem to accelerate this warming and overcome cloud and dynamical thermostat effects. Such a potential change may have severe consequences for the global atmospheric circulation (Hoerling et al. 2001), encompassing the extratropics and also on ENSO amplitude (Sun 2003). So far the detailed reasons for the simulated model discrepancies have not been analyzed thoroughly.

Using the GFDL and NCAR IPCC scenario runs of the 20th-22nd century, we will study the heat balance of the warm pool, in terms of radiative processes and ocean dynamical heating. A revised thermostat theory, capturing meridional heat advection will be formulated and applied to the CGCM data. The goal of our planned research is to investigate the sensitivity of thermostats or super-greenhouse effects under greenhouse warming conditions. Physical processes that lead to different model responses will be identified. The amplitude of strong El Niño events is tightly linked to mean warm pool temperature. Hence, understanding the physics of thermostats or super-greenhouse effects will help to assess also ENSO's response to greenhouse warming. We are planning to generate time-series for the individual simulations of each term in the western and eastern Pacific heat-budget equation. Their behavior will shed light onto which process is dominating the heat-budget and which positive or negative feedbacks will win in the long run. Furthermore, simplified ENSO models will be used in order to understand how the simulated background state changes influence ENSO variability.

Required model data from the GFDL and NCAR IPCC (AR4) scenario runs (1-6, 8-10):

Indo-Pacific ocean temperature data (all z-levels)
Indo-Pacific ocean current data (all z-levels)
Indo-Pacific atmosphere-ocean heat flux
Indo-Pacific cloud data (level-wise and height-integrated water-vapor)

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    Lawrence Livermore National Laboratory  |  Physical & Life Sciences Directorate