The observed El Nino events are generally stronger than the La Nina events. This property of El Nino–Southern Oscillation (ENSO) is termed as ENSO asymmetry. Evidence is presented to show that this asymmetry has changed since the famous 1976 climate shift. Along the thinking of how the tropical background field modulates ENSO cycle, we explore the effect of the climatological basic‐state change on the ENSO asymmetry by applying the approach of conditional nonlinear optimal perturbation (CNOP) in a theoretical coupled model. CNOP is the initial anomaly pattern that evolves into ENSO event most probably. Observation shows that from the preshift (1961–1975) to the postshift (1981–1995) period, significant changes have occurred in climatological background state, i.e., the mean temperature difference between the equatorial eastern and western Pacific basins and between the mixed‐layer and subsurface‐layer water, which control the ENSO oscillation in the theoretical coupled model. By computing the CNOPs of the climatological basic state corresponding to the 1961–1975 (1981–1995) epoch, we reproduce the observed decadal change of ENSO asymmetry qualitatively. On the basis of the physics described by the model, the mechanism of ENSO asymmetry change in interdecadal scale is explored in depth. It is shown that the decadal change of ENSO asymmetry is induced by the change of nonlinear temperature advection, which is closely related to the decadal change of the tropical background state. These indicate that the decadal change of ENSO asymmetry results from the collective effect of the changes of the tropical background state and the nonlinearity. These findings in this study also suggest that the nonlinearity can explain not only the asymmetry of interannual ENSO, but also that of interdecadal ENSO, which may present a powerful evidence to the ENSO chaotic theory.
This paper is concerned with nonlinear symmetric stability problems. For the moist, adiabatic (saturated) system, the authors utilize the ECM (energy–Casimir method) to establish nonlinear stability criteria, which extends the previous work from the dry atmosphere to the moist case and demonstrates the complexity related to the moist symmetric instability problem. For the nonhydrostatic, Boussinesq equations on an f plane with the northward component of the earth rotation f = 2Ω cosϕ, which has been utilized to show the importance of f term in the mesoscale linear symmetric instability problem, both ECM and the ELM (energy–Lagrange method) are employed to study the “zonal” and“meridional” nonlinear symmetric stability problems. In both cases, the nonlinear stability of the basic states are obtained if the potential vorticity and the vertical component of absolute vorticity of the basic state are positive (for f > 0). In the zonal case, the potential vorticity depends upon f explicitly, and this shows the influence of the f term to the nonlinear symmetric stability. In the meridional case the potential vorticity is independent of the f term, which implies that the f term plays no role in the nonlinear symmetric stability. The upper bounds on the disturbance field to the nonlinearly stable basic state are established, which consists of its initial value multiplied by an amplification factor independent of time. The ELM proposed by Xu is simplifed to be more concise and understandable. The applicable capacity of ECM and ELM is investigated. Both methods are applicable to the symmetric nonlinear stability problem of dry atmosphere, but only ECM has application to the moist problem.
Abstract There are still considerable uncertainties related to the numerical simulation and prediction of net primary production (NPP) as an important part of terrestrial carbon sources and sinks over the Tibetan Plateau (TP). To reduce the uncertainty of numerical simulations and improve the ability of predictions, the key physical processes related to the uncertainty of simulated NPP are identified at nine observational stations over the TP. A sensitivity analysis of parameter combinations based on the Conditional Nonlinear Optimal Perturbation related to Parameters (CNOP‐P) approach, which can be used to assess the sensitivity of a parameter subset, is conducted for 28 target physical parameters in the Lund‐Potsdam‐Jena (LPJ) Wetland Hydrology and Methane Dynamic Global Vegetation Model (LPJ‐WHyMe v1.3.1). Firstly, the numerical results show that the uncertainties of physical parameters do lead to a large error in the simulated NPP over the TP, and the range of error varies from 72.4 (MS 3478) to 150.5 g C m −2 year −1 (Ngari station). Secondly, in areas of moderate precipitation over the TP, the photosynthesis is the main factor leading to high uncertainty in NPP modeling. In areas of low and high precipitation over the TP, the combined influences of hydrological processes and photosynthesis play a key role. Finally, eliminating the errors associated with the most sensitive and important parameter combinations led to the maximum benefit in terms of reducing the uncertainty of simulated NPP, when compared to that obtained with the traditional method. This study suggests that we should prioritize reducing the uncertainty of relatively sensitive parameter combinations among all physical parameters to improve the prediction or simulation ability of NPP over the TP.
Most state‐of‐the‐art climate models have difficulty in the prediction of El Niño‐Southern Oscillation (ENSO) starting from preboreal spring seasons. The causes of this spring predictability barrier (SPB) remain elusive. With a theoretical ENSO system model, we investigate this controversial issue by tracing the evolution of conditional nonlinear optimal perturbation (CNOP) and by analyzing the behavior of initial error growth. The CNOPs are the errors in the initial states of ENSO events, which have the biggest impact on the uncertainties at the prediction time under proper physical constraints. We show that the evolution of CNOP‐type errors associated with El Niño episodes depends remarkably on season with the fastest growth occurring during boreal spring in the onset phase. There also exist other kinds of initial errors, which have either somewhat smaller growth rates or neutral ones during spring. However, for La Niña events, even if initial errors are of CNOP‐type, the errors grow without significant seasonal dependence. These findings suggest that the SPB in this model results from combined effects of three factors: the annual cycle of the mean state, the structure of El Niño, and the pattern of the initial errors. On the basis of the error tendency equations derived from the model, we addressed how the combination of the three factors causes the SPB and proposed a mechanism responsible for the error growth in the model ENSO events. Our results help in clarifying the role of the initial error pattern in SPB, which may provide a clue for explaining why SPB can be eliminated by improving initial conditions. The results also illustrate a theoretical basis for improving data assimilation in ENSO prediction.
Abstract Utilizing the Open Integrated Forecasting System, the responses of Ural blocking (UB) to different stratospheric warming scenarios are investigated. Numerical results show that stratospheric warming with moderate strength in minor patterns prolongs the UB duration and enhances its intensity, while strong stratospheric warming in minor patterns tends to shorten its duration and weaken its intensity, even leading to the collapse of the UB events. Further diagnosis reveals that the planetary wave activity flux propagates downward from the stratosphere to the troposphere after stratospheric warming. Moreover, the convergence of planetary wave activity flux is a key factor for UB enhancement and maintenance. In addition, the weakened meridional temperature gradients, decelerated zonal westerly winds, and a reduced meridional potential vorticity gradient (PVy) result in UB enhancement in response to stratospheric warming with moderate strength. As stratospheric warming strengthens, planetary wave activity flux diverges, westerly winds in the tropospheric mid‐latitudes accelerate and the PVy in the Ural sector enlarges, which further weakens UB. Regarding the stratospheric perturbations in major patterns, they have similar influences on UB events, that is, UB enhances with moderate stratospheric warming and weakens with strong warming. However, the strengthened warming would trigger UB re‐enhancement, which is closely associated with anomalous activities of tropospheric synoptic‐scale waves induced by stratospheric perturbations. These results reveal UB events respond differently to stratospheric warming with various intensities and patterns in the short term, which makes a contribution to understanding stratosphere‐troposphere coupling.
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