Investigating the Influence of Systematic Biases on the Annual Cycle and ENSO Variability in the Coupled GCMs Using Flux Correction MethodPrimary Author: Manganello, Julia
Additional Authors: Bohua Huang
Investigating the Influence of Systematic Biases on the Annual Cycle and ENSO Variability in the Coupled GCMs Using Flux Correction Method
Julia V. Manganello Center for Ocean-Land-Atmosphere Studies, Calverton, Maryland Bohua Huang Climate Dynamics Department, College of Science, George Mason University, Fairfax, Virginia Center for Ocean-Land-Atmosphere Studies, Calverton, Maryland
A series of coupled GCM experiments are conducted to test the hypothesis that the existence of systematic biases in the Tropics could have serious consequences for the quality of model-simulated variability on various temporal-spatial scales. To reduce annual mean errors in the coupled models, an empirical method is used in this study, such as the flux adjustment strategy, in which a prescribed correction term is added to the surface heat flux into the ocean. This correction term varies spatially but is constant in time, and its magnitude is proportional to the annual mean local SST error in the directly coupled GCM integration. By design, it only affects the mean surface heat balance directly. It must be noted that in this study we intentionally keep the flux correction at a minimum and target those errors with relatively clear physical error sources. In the first part of our study the above heat flux correction method is applied in the Tropics, and is implemented in the coupled GCM, where the atmospheric component is the Center for Ocean-Land-Atmosphere Studies (COLA) AGCM (Version 2) and the oceanic component is a quasi-isopycnal reduced-gravity OGCM. As expected, this constant heat flux correction eliminates most large mean SST errors in the directly coupled run, including the warm bias in the southeast Pacific and Atlantic. As a result, the corrected mean climate exhibits stronger SST asymmetry relative to the equator. It is found that, given better mean SST field, mean distributions of surface wind stress and precipitation are also improved in these regions. Due to the improvement of the model mean state, the annual cycles of the SST and surface wind stress in the eastern equatorial Pacific become more realistic as a result of enhancement of their annual, rather than semi-annual, harmonics. The annual cycle of precipitation in the eastern Pacific is also improved due to more realistic seasonal SST variations in this region. Both directly coupled and the flux corrected models simulate interannual variability in the tropical Pacific with some ENSO characteristics. However, in the first version phase locking of ENSO to the annual cycle is unrealistic. In the second, phase locking is well reproduced, likely due to more realistic seasonal evolution of the mean state. The development of ENSO events at the equator and tropical teleconnections are also more realistic. On the other hand, the simulated interannual variability is weaker and the timescale of ENSO cycle is longer in both runs compared to observations, but more so in the flux corrected simulation. In the second part of our study we apply the heat flux correction method to the NCEP Climate Forecast System (CFS), which is a state-of-the-art fully coupled ocean-land-atmosphere dynamical seasonal prediction system for operational forecasts. In the first experiment, heat flux correction is applied to the whole tropical oceans. In the second, it is further localized to include only the southeast Pacific and Atlantic, where the SST bias is the largest. Preliminary analysis shows that the mean SST errors in the southeast Pacific and Atlantic are significantly reduced in both experiments. As a result of stronger SST gradient, equatorial easterlies are significantly enhanced and are closer to the observed, which is likely responsible for the deepened equatorial thermocline. On the other hand, heat content and SST variability in the tropical Pacific is weaker in both runs compared to the directly coupled model simulation and the observations. However, the ENSO cycle is less regular, and the duration of ENSO episodes is shortened and more in line with the observed variability. Experiments with the uncoupled version of the Modular Ocean Model Version 3 (MOM3), which is the oceanic component of the CFS, are currently underway to test the hypothesis that the deepening of the equatorial thermocline in response to more realistic zonal wind stress has effectively weakened ENSO variability in the model. In addition, we are currently introducing global wind stress correction into the CFS to test whether correcting wind stress, as opposed to SST, is a more efficient way to improve the simulation of the mean climate and ENSO variability.
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