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David S. Battisti

Abstract

A simple coupled ocean–atmosphere model, similar to that of Zebiak and Cane, is used to examine the dynamic and thermodynamic processes associated with El Niño/Southern Oscillation (ENSO). The model is run for 300 years. The interannual variability which results is regular, with a period of either 3 or 4 years, quantized by the annual cycle. The amplitude (∼1.5 m s−1 wind and 2°C SST anomalies), period and structure of the interannual variability compare well with observations. The model warm event is initiated in the spring prior to the event peak, and is well described as an instability of the coupled system. During instability growth, the sea surface temperature (SST) anomaly is primarily generated by vertical upwelling processes. The SST anomaly can be approximately described by the expression ∂T/∂t = KTh − α*T, where T is the SST anomaly, t time, h the upper layer thickness (pycnocline) perturbation and α* an effective damping time which includes heat loss to the atmosphere. KT parameterizes vertical upwelling and mixed layer processes.

Oceanic wave dynamics determines the fate of the growing instability. The warming of the SST produces westerly wind anomalies in the equator central Pacific, forcing equatorially trapped Rossby waves that propagate freely to the western boundary. These waves reflect at the western boundary, sending upwelling equatorial Kelvin waves back to the central basin. These cooling Kelvin waves act to terminate instability growth and rapidly plunge the coupled system into a cold regime. The western boundary reflection is necessary for event termination. The system returns from a cold regime via reduced heat flux to the atmosphere and, to a lesser extent, by wave induced processes like that which lead to the warm event termination. The interannual variability is not produced by vacillation between two equilibrium states: a cold and a warm state. The growth rate to either the cold or warm state is too slow for the system to achieve equilibrium, even for a basin the size of the Pacific. The model results indicate that shortly after the initial set of gravest mode Rossby reflections on the western boundary, the instability growth is already being substantially moderated by the equatorial wave processes in the ocean. Thus the system is oscillatory around a single basic state.

Of the Rossby waves produced in the central Pacific by the warm event, only the two gravest mode symmetric modes are important in the reflection process, which produce the Kelvin waves that terminate the warm event. In nature, the actual western boundary for the equatorial Pacific wave guide is very ambiguous. Calculations indicate, however, that efficient reflection of the gravest symmetric Rossby waves from a more realistic boundary than the meridional wall in the model is possible. Finally, if the model is indeed simulating the correct processes controlling ENSO events, the nature of the instability mechanism that leads to growth and the wave-induced termination of the model warm event suggests that, for realistic instability growth rates for the coupled equatorial ocean-atmosphere system, interannual variability analogous to ENSO should not be possible in equatorial basins significantly smaller than the Pacific.

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David S. Battisti

Abstract

Recent theoretical and numerical modeling studies of the coupled tropical atmosphere-ocean system suggest that equatorial ocean wave dynamics may play an important role in the evolution of ENSO (El Niño/Southern Oscillation). These studies emphasize that the oceanic wave signal is confined to within a narrow equatorial band (within 6° of the equator).

In this study we use a coupled atmosphere–ocean model to investigate the role of off-equatorial Rossby waves observed in the western North Pacific Ocean during the ENSO cycle. We find that these off-equatorial Rossby waves (found poleward of 6° from the equator) are formed through both eastern boundary reflection of the equatorial Kelvin wave signal generated in a warm event (El Niño), and changes in the off-equatorial wind stress curl. Our results indicate that, independent of the generation mechanism, off-equatorial Rossby waves should be thought of as the product and not the triggering mechanism for an ENSO event.

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David S. Battisti

Abstract

No abstract available.

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David S. Battisti
and
David D. Ovens

Abstract

How the time-mean Hadley and Walker circulations affect the formation of a low-level equatorial easterly jet is investigated. Experiments are conducted for equinoctial conditions using a general circulation model, the Community Climate Model (CCM1), that includes a Kuo convective scheme and a lower boundary that is specified to be water at a fixed sea surface temperature (SST). Several zonally symmetric SST forcings are used to determine how various Hadley circulations affect the tropical zonal wind field. A zonal wavenumber one equatorial SST anomaly superimposed on a zonally symmetric SST distribution forces a wind field that includes both Hadley and Walker circulations.

The Hadley circulation experiments produce equatorial easterlies and low-level jets on the poleward sides of the intertropical convergence zone (ITCZ) 10° to 15° from the equator. In an experiment with a single, dominant off-equatorial ITCZ in the Northern Hemisphere, the Southern Hemisphere jet moves to within 7.5° of the equator; yet none of the Hadley circulation cases produce a low-level easterly jet on the equator because they lack a mechanism to vertically confine the flow.

The experiment that includes a zonally overturning cell on the equator produces a low-level equatorial easterly jet in the cold tongue region that is similar to the observed jet over the central to eastern Pacific. That case shows that east of the equatorial warm pool the Walker circulation and its induced Kelvin wave response provide the necessary upper-level westerly flow and subsidence to vertically confine the low-level easterlies into a jet. Spring and fall climatological runs of the CCMI with land surfaces, seasonally varying SSTs and insolation, and a moist convective adjustment scheme support the hypothesis that the Walker circulation provides the vertical confinement necessary to form a low-level equatorial easterly jet in the region east of the equatorial convective center, regardless of the Hadley circulation in that region.

The eddy vertical-flux convergence of moisture in the Kuo convective scheme produces a dry tongue in the Walker circulation simulation below the low-level equatorial easterly jet. The CCM1 climatologies show that the dynamics of the jet do not depend on this feature. Betts, Albrecht, and Kloesel have observed a similar feature just above the boundary layer in the central to eastern Pacific and, without referring to the low-level jet, they have hypothesized a mechanism in which convection forms this dry layer. Analysis of the simulations performed here suggests that the model's parameterized convective physics utilize the same mechanism to form the dry tongue in the vicinity of the low-level equatorial easterly jet; however, since the mechanism of Betts, Albrecht, and Kloesel has not yet been confirmed through observational studies, the relationship between the observed and modeled dry tongue remains speculative.

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Aaron Donohoe
and
David S. Battisti

Abstract

The “background” state is commonly removed from synoptic fields by use of either a spatial or temporal filter prior to the application of feature tracking. Commonly used spatial and temporal filters applied to sea level pressure data admit substantially different information to be included in the synoptic fields. The spatial filter retains a time-mean field that has comparable magnitude to a typical synoptic perturbation. In contrast, the temporal filter removes the entire time-mean field. The inclusion of the time-mean spatially filtered field biases the feature tracking statistics toward large cyclone (anticyclone) magnitudes in the regions of climatological lows (highs). The resulting cyclone/anticyclone magnitude asymmetries in each region are found to be inconsistent with the unfiltered data fields and merely result from the spurious inclusion of the time-mean fields in the spatially filtered data. The temporally filtered fields do not suffer from the same problem and produce modest cyclone/anticyclone magnitude asymmetries that are consistent with the unfiltered data. This analysis suggests that the weather forecaster’s assertion that cyclones have larger amplitudes than anticyclones is due to a composite of a small magnitude asymmetry in the synoptic waves and a large contribution from inhomogeneity in the background (stationary) field.

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Robert E. Nicholas
and
David S. Battisti

Abstract

A statistical approach is used to explore the variability of precipitation and meteorological drought in Mexico’s Río Yaqui basin on seasonal-to-decadal time scales. For this purpose, a number of custom datasets have been developed, including a monthly 1900–2004 precipitation index for the Yaqui basin created by merging two gridded land surface precipitation products, a 349-yr tree-ring-based proxy for Yaqui wintertime rainfall, and a variety of large-scale climate indices derived from gridded SST records. Although significantly more rain falls during the summer (June–September) than during the winter (November–April), wintertime rainfall is over 3 times as variable relative to the climatological mean. Summertime rainfall appears to be unrelated to any large-scale patterns of variability, but a strong relationship between ENSO and Yaqui rainfall during the winter months offers the possibility of meaningful statistical prediction for this season’s precipitation. Analysis of both historical and reconstructed rainfall data suggests that meteorological droughts as severe as the 1994–2002 Yaqui drought occur about 2 times per century, droughts of even greater severity have occurred in the past, and such droughts are generally associated with wintertime anomalies. Whereas summertime reservoir inflow is larger in the Yaqui basin, wintertime inflow is more variable (in both relative and absolute terms) and is much more strongly correlated with same-season rainfall. Using the identified wintertime ENSO–rainfall relationship, two simple empirical forecast models for possible use by irrigation planners are demonstrated.

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Robert E. Nicholas
and
David S. Battisti

Abstract

This study describes an EOF-based technique for statistical downscaling of high-spatial-resolution monthly-mean precipitation from large-scale reanalysis circulation fields. The method is demonstrated and evaluated for four widely separated locations: the southeastern United States, the upper Colorado River basin, China’s Jiangxi Province, and central Europe. For each location, the EOF-based downscaling models successfully reproduce the observed annual cycle while eliminating the biases seen in NCEP–NCAR reanalysis precipitation. They also frequently reproduce the monthly precipitation anomalies with greater fidelity than is seen in the precipitation field derived directly from reanalysis, and they outperform a suite of regional climate models over the two U.S. locations. With the relatively high skill achieved over a range of climate regimes, this technique may be a viable alternative to numerical downscaling of monthly-mean precipitation for many locations.

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David S. Battisti
and
Anthony C. Hirst

Abstract

The behavior of a tropical coupled atmosphere/ocean model is analyzed for a range of different background states and ocean geometries. The model is essentially that of Cane and Zebiak for the tropical Pacific, except only temporally constant background states are considered here. For realistic background states and ocean geometry, the model solutions feature oscillations of period of 3–5 yr. By comparing the full model solution with a linearized version of the model, it is shown that the basic mechanism of the oscillation is contained within linear theory.

A simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere–ocean system. The analog model highlights the properties that produce coupled atmosphere–ocean instability in the eastern ocean basin, and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed, negative feedback into this instability growth. The growth rate of the local instability c together with the magnitude b and lag of the wave-induced processes determine the nature of the interannual variability displayed in the coupled model. Specifically, these processes determine the growth rate of the coupled system and, when the solutions are oscillatory, the period of the oscillation. The terms b, c, and are set by the background state of the atmosphere and ocean, and the geometry of the ocean basin.

The simple analog model is used to design and interpret a set of experiments using the full linear and nonlinear numerical models of the coupled atmosphere ocean system in the Pacific. In these experiments, we examine the effects of the assumed basic state and ocean geometry on the interannual variability of the coupled system. The simple model is shown to be a remarkably good proxy of the full linear and nonlinear numerical models. The limiting nonlinearity in the full numerical model is shown to be the dependence of the temperature of the upward water on the thermocline depth. However, we find the essential processes that describe the local instability growth rate and period of the interannual oscillations in the coupled system are linear. Nonlinearities primarily act as a bound on the amplitude of the final state oscillations, and decrease the period of the firm state oscillations by about 10 percent from that obtained in the small amplitude regime of the full coupled model and the linear analog model. The nonlinear analog model for the full numerical model is derived, and compared with that proposed by Suarez and Schopf. The numerical and analog models help to explain why organized, large amplitude, interannual variability is prominent in the tropical Pacific basin, and not in Atlantic and Indian basins.

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Joseph J. Barsugli
and
David S. Battisti

Abstract

Starting from the assumption that the atmosphere is the primary source of variability internal to the midlatitude atmosphere–ocean system on intraseasonal to interannual timescales, the authors construct a simple stochastically forced, one-dimensional, linear, coupled energy balance model. The coupled system is then dissected into partially coupled and uncoupled systems in order to quantify the effects of coupling. The simplicity of the model allows for analytic evaluation of many quantities of interest, including power spectra, total variance, lag covariance between atmosphere and ocean, and surface flux spectra. The model predicts that coupling between the atmosphere and ocean in the midlatitudes will enhance the variance in both media and will decrease the energy flux between the atmosphere and the ocean. The model also demonstrates that specification of historical midlatitude sea surface temperature anomalies as a boundary condition for an atmospheric model will not generally lead to a correct simulation of low-frequency atmospheric thermal variance.

This model provides a simple conceptual framework for understanding the basic aspects of midlatitude coupled variability. Given the simplicity of the model, it agrees well with numerical simulations using a two-level atmospheric general circulation model coupled to a slab mixed layer ocean. The simple model results are also qualitatively consistent with the results obtained in several other studies in which investigators coupled realistic atmospheric general circulation models to ocean models of varying complexity. This suggests that the experimental design of an atmospheric model coupled to a mixed layer ocean model would provide a reasonable null hypothesis against which to test for the presence of distinctive decadal variability.

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Jeffrey H. Yin
and
David S. Battisti

Abstract

Theoretical and modeling studies of nongeostrophic effects in baroclinic waves predict that baroclinic waves should tilt poleward with height, with a larger tilt in total meridional wind than in geostrophic quantities. Regression analysis of NCEP–NCAR reanalysis 6-hourly data demonstrates that observed baroclinic waves do indeed tilt poleward with height, although the observed tilt is smaller than predicted by previous studies. The meridional ageostrophic wind enhances the poleward tilt of meridional wind perturbations, despite being smaller in amplitude than the meridional geostrophic wind by a factor of 5.

An improved estimate of the structure of the meridional ageostrophic wind in baroclinic waves is calculated assuming force balance. Several important terms in this estimate have been left out of previous estimates of the meridional ageostrophic wind. Three terms in the improved estimate produce nearly all of the poleward tilt of the meridional wind: 1) the advection of geostrophic zonal wind perturbations by the mean zonal wind, 2) the convergence of the eddy momentum flux, and 3) the effect of friction.

The poleward tilt with height of baroclinic waves explains why upper-level storm tracks tend to occur poleward of near-surface baroclinic regions, and may play a role in the midwinter suppression of the Pacific storm track.

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