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Stephen E. Zebiak

Abstract

A parameterization is developed for the feedback between dynamics and heating associated with moisture convergence in the tropical atmospheric boundary layer. The feedback improves the ability of a simple model to simulate observed anomalies of the tropical atmosphere during El Niño events. In particular, two features of the observations are reproduced by including the feedback process: the smaller scale of atmospheric anomalies as compared to SST anomalies, and the focusing of the anomalies in the vicinity of the mean convergence zones. The principal remaining shortcomings of the model are discussed.

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Stephen E. Zebiak

Abstract

Analyses are performed using surface winds derived from the monthly mean FSU Pacific pseudo-stress fields. Vorticity budget calculations reveal the relative contributions of various terms, and allow simplification of the full momentum equations. Based on the simplified equations, a procedure is developed for estimating surface pressure, and then adjusting the winds. The wind field adjustments fall well within the uncertainty of the data, while allowing generally large imbalances in the vorticity budget to be removed completely. Further validation of the procedure is provided by comparisons between estimated and observed pressure anomalies. The adjusted winds and pressure are then used to infer boundary layer and upper level forcing within the context of particular model formulations. These forcing fields an compared with sea surface temperature, outgoing longwave radiation, and highly reflective cloud data in order to infer the dominant mechanisms contributing to observed surface circulation anomalies. It is found that both boundary layer and convective forcing are important. The results show deficiencies in previous model parameterizations based primarily on sea surface temperature and suggest that the horizontal structure of convective heating differs from what is often assumed.

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Stephen E. Zebiak

Abstract

A linear, equatorial, beta-plane atmosphere model with heating parameterized in terms of SST anomalies is developed and used to simulate surface wind anomalies in the equatorial Pacific during El Niño. The model results show some similarity to observations with respect to movement of the major convergence zones, and equatorial wind anomaly patterns in the central and western Pacific. There is considerable discrepancy between the model results and observations in much of the eastern Pacific, especially in the South Pacific high and southeast trades regions. The results suggest that some additional mechanisms may be responsible for these apparently sizeable and spatially coherent fluctuations, but that a direct link between wind anomalies and SST anomalies may indeed exist in much of the equatorial Pacific.

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Stephen E. Zebiak

Abstract

Using a dynamically motivated analysis of observations, and an intermediate-level coupled model, the interannual variability within the equatorial Atlantic is studied. It is found that a significant part of the observed variability can be described by an equatorial coupled mode akin to ENSO (El Niño–Southern Oscillation). The Atlantic mode signature is even more tightly focused on the equator and is situated proportionally farther to the west within the basin than its Pacific counterpart.

Model simulations capture the equatorial coupled mode in relatively pure form and, for what are thought to be the most realistic parameter choices, show interannual oscillations favoring a 4-year period, which are not self-sustaining. The simulated spatial patterns agree well with those extracted from observations, including those features that distinguish the Atlantic from the Pacific.

Sensitivity experiments show that the Atlantic coupled-mode signal is less robust than the corresponding Pacific ENSO signal but is still well-defined qualitatively, within reasonable parameter ranges. The results demonstrate that the primary mechanisms of oscillation for the Atlantic and Pacific are the same but that differences in the zonal structure and strength of air–sea coupling and mean ocean stratification offset the large differences in basin size, allowing similar oscillation periods for the two basin modes. An explanation for the distinct spatial patterns of simulated Atlantic and Pacific anomalies is found in the differences in climatological mean fields and ocean basin configurations.

Together, the observational and model results present a picture of equatorial Atlantic variability in which coupled equatorial dynamics play an important but not exclusive role. It appears that the coupling is sufficiently strong to leave its imprint on the total variability but too weak to dictate it entirely, even at the equator.

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Stephen E. Zebiak

Abstract

The impact of intraseasonal variability on ENSO is studied in the context of the Zebiak and Cane coupled atmosphere–ocean model, and an idealized representation intraseasonal forcing. The effects of the parameterized forcing are examined in both simulation and forecast experiments, with similar results; that is, the intraseasonal variability generally plays a minor role in altering the model behavior. Though the uncertainties inherent in both the coupled model and the specified forcing must be kept in mind, the results support the hypothesis that intraseasonal variability is not an essential component of ENSO. At the same time, they present evidence of occasional sensitive periods or states of the coupled system in which intraseasonal forcing (and possibly other forcings) can indeed disrupt the future course of events.

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Stephen E. Zebiak

Abstract

Tropical Pacific heat content variability is studied in the context of a linear dynamical model forced by observed winds in the period 1970–87. The results confirm the relationships between equatorial heat content, surface winds, and SST predicted by coupled models of ENSO, and are consistent with sea level observations during this period. It is found that the meridional transports at the western boundary generally act to oppose the tendency in equatorial heat content, but are more than compensated for by the interior transports. Such a scenario is consistent with the reflection of equatorial (long) Rossby waves, but not off-equatorial ones, suggesting a dominant role for the former. In agreement with previous studies, an interhemispheric exchange of heat content is found in association with the ENSO cycle. The structure of heat content and transport anomaly fields throughout a composite ENSO cycle is presented and discussed in the context of existing theories, which highlight the role of Rossby waves and western boundary reflections.

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Richard Seager and Stephen E. Zebiak

Abstract

The tropical climate simulated with a new global atmosphere model is presented. The model is purposely designed for climate studies and is still under development. It is designed to bridge the gap between very efficient but simple models of the tropical atmosphere and sophisticated but inefficient general circulation models (GCMs). In this paper the authors examine the sensitivity of the model's climate to specific formulations of convection, boundary-layer physics, and radiation.

The model uses the Betts–Miller convection scheme and a parameterization of the planetary boundary layer (PBL) that combines similarity theory for computation of surface fluxes with a simple scheme for diagnosing PBL depth. Radiative cooling is specified and land surface processes are bypassed by relaxing modeled low-level values to observed quantities. Orography is ignored. The model contains six vertical layers and has a horizontal resolution of about 3° × 5.625°.

The authors compare the climate simulated with two different versions of the Betts–Miller convection scheme. More realistic simulations of rainfall are obtained with the later version, which includes the effects of convective downdrafts. These, by cooling and drying the PBL, act to restrict the areas of convection while strengthening the intertropical convergence zone. The sensitivity to choice of PBL physics is less, and quite similar results were obtained when the PBL scheme was replaced with constant exchange coefficients and PBL depth. In contrast, the amount of precipitation varied strongly with the prescribed radiative cooling. The important role that shallow convection and cloud-radiation interactions play in the spatial organization of deep convection is demonstrated, by default, in an experiment using clear-sky radiative transfer.

The modeled climate, as judged qualitatively by its simulation of quantities of importance to air–sea interaction and climate, such as the low-level wind field and precipitation, is in many ways comparable to that achieved by much more complex GCMs. Indeed the rainfall simulation appears better than obtained by many models that use other convection parameterizations. This adds to the accumulating evidence that the Betts–Miller scheme is a quite reliable scheme, at least for simulation of convection in the current climate. A major model flaw is a very poor Asian summer monsoon, which is attributed to lack of orography in the model. It is demonstrated, by inclusion of a specified monsoonal forcing, that this also has an effect, though modest, on the simulation of the trade winds over the Pacific.

The results suggest there is hope for development of models of intermediate complexity that achieve a degree of realism exceeding the simple models that have often been used in El Niño studies while retaining much of their efficiency.

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Richard Seager and Stephen E. Zebiak

Abstract

A new global atmosphere model purpose designed for climate studies is introduced. The model is solved in terms of the normal modes of the linearized primitive equations on a sphere, which allows use of long time steps without introducing computational instability or phase errors of the linear wave components. The model is tested by attempting to simulate the tropical intraseasonal oscillation using an idealized sea surface temperature distribution. Simple treatments of radiation and boundary-layer processes are used together with the much more complete Betts–Miller convection scheme. The Betts–Miller scheme maintains the atmosphere in a state of near neutrality to reversible saturated ascent. It is found that for different values of the surface evaporation time scale, either the evaporation-wind feedback mechanism postulated by Neelin et al. and Emmanuel or low-level convergence of moisture can create eastward propagating deep convective modes. In general, both mechanisms seem important, but it is the latter mechanism that provides phase speeds more in line with observations. Moisture convergence in this model works to erode the low-level equivalent potential temperature inversion that is ubiquitous in nonconvecting regions, thus triggering convection. In contrast to CISK models, changes in boudary-layer equivalent potential temperature are essential in this model to create propagating modes.

The primary deficiency of the model is the tendency of the model to favor horizontal scales of convective disturbances that are much smaller than the zonal wavenumber one or two disturbances observed. This is related to the absence in the model of any pulsation of convection on an intraseasonal time scale over the warmest water regions that has been observed in satellite OLR data. Possible reasons for these differences are discussed.

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Rong-Hua Zhang and Stephen E. Zebiak

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A simple scheme is proposed for penetrating atmospheric momentum flux over the ocean surface boundary layer or mixed layer (BL/ML) and is tested in the z-coordinate NOAA/Geophysical Fluid Dynamics Laboratory Modular Ocean Model (MOM 3) for improving its performance. Analogous to the treatment in layered ocean models, wind stress is applied, as a body force, to the entire BL/ML whose depth is calculated from a nonlocal K-profile parameterization scheme. The penetrating scheme presents an explicit and effective way to distribute a priori momentum flux throughout the BL/ML that has varying depth in space and time, instead of just over the uppermost model level with fixed thickness. This additional procedure introduces an explicit mechanism that directly relates wind stress to the BL/ML formulation, which in turn controls current and thermal structure in the upper ocean and the interaction with the underlying thermocline. Two penetrating runs, one over the BL and the other over the ML, have similar results that differ systematically from those with the penetration over fixed depths (control run). It is demonstrated that, with coherent and systematic improvements, this penetrating scheme can have significant effects on simulated equatorial ocean currents and thermal structure not only in the surface layer, but also in the thermocline. Besides more reasonable ML depth simulation in the equatorial central basin, there is substantial reduction in the mean offset of simulated isotherm depths and warm bias in the thermocline, due to downward shift of the maximum upwelling zone in the equatorial central Pacific. Consistent with observations, the penetrating scheme realistically reproduces the springtime reversal of the South Equatorial Current and the corresponding surface warming in the central equatorial Pacific, with accompanying surfacing of the Equatorial Undercurrent Current in March–May.

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Ben P. Kirtman and Stephen E. Zebiak

Abstract

A hybrid coupled model (HCM) consisting of a tropical Pacific Ocean and global atmosphere is presented. The ocean component is a linear reduced gravity model of the upper ocean in the tropical Pacific. The atmospheric component is a triangular 30 horizontal resolution global spectral general circulation model with 18 unevenly spaced levels in the vertical. In coupling these component models, an anomaly coupling strategy is employed. A 40-yr simulation was made with HCM and the variability in the tropical Pacific was compared to the observed variability. The HCM produces irregular ENSO events with a broad spectrum of periods between 12 and 48 months. On longer timescales, approximately 48 months, the simulated variability was weaker than the observed and on shorter timescales (approximately 24 months) the simulated variability was too strong. The simulated variability is asymmetric in the sense that the amplitude of the warm events is realistic, but there are no significant cold events.

An ensemble of 60 hindcast predictions was made with the HCM and the skill was compared to other prediction systems. In forecasting sea surface temperature anomalies in the eastern Pacific, the HCM is comparable to the other prediction systems for lead times up to 10 months. The anomaly correlation coefficient for the eastern Pacific SSTA remains above 0.6 for lead times of up to 11 months. Consistent with the 40-yr simulation, hindcasts of cold events have little skill, particularly when compared to hindcasts of warm events. Specific hindcasts also demonstrate that the HCM also has difficulty predicting the transition from warm conditions to normal or cold conditions.

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