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C. J. Thompson
and
D. S. Battisti

Abstract

In this study the behavior of a linear, intermediate model of ENSO is examined under stochastic forcing. The model was developed in a companion paper (Part I) and is derived from the Zebiak–Cane ENSO model. Four variants of the model are used whose stabilities range from slightly damped to moderately damped. Each model is run as a simulation while being perturbed by noise that is uncorrelated (white) in space and time. The statistics of the model output show the moderately damped models to be more realistic than the slightly damped models. The moderately damped models have power spectra that are quantitatively quite similar to observations, and a seasonal pattern of variance that is qualitatively similar to observations. All models produce ENSOs that are phase locked to the annual cycle, and all display the “spring barrier” characteristic in their autocorrelation patterns, though in the models this “barrier” occurs during the summer and is less intense than in the observations (inclusion of nonlinear effects is shown to partially remedy this deficiency). The more realistic models also show a decadal variability in the lagged autocorrelation pattern that is qualitatively similar to observations.

Analysis of the models shows that the greatest part of the variability comes from perturbations that project onto the first singular vector, which then grow rapidly into the ENSO mode. Essentially, the model output represents many instances of the ENSO mode, with random phase and amplitude, stimulated by the noise through the optimal transient growth of the singular vectors.

The limit of predictability for each model is calculated and it is shown that the more realistic (moderately damped) models have worse potential predictability (9–15 months) than the deterministic chaotic models that have been studied widely in the literature. The predictability limits are strongly correlated with the stability of the models’ ENSO mode—the more highly damped models having much shorter limits of predictability. A comparison of the two most realistic models shows that even though these models have similar statistics, they have very different predictability limits. The models have a strong seasonal dependence to their predictability limits.

The results of this study (with the companion paper) suggest that the linear, stable dynamical model of ENSO is indeed a plausible hypothesis for the observed ENSO. With very reasonable levels of stochastic forcing, the model produces realistic levels of variance, has a realistic spectrum, and qualitatively reproduces the observed seasonal pattern of variance, the autocorrelation pattern, and the ENSO-like decadal variability.

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C. J. Thompson
and
D. S. Battisti

Abstract

Singular vector analysis and Floquet analysis are carried out on a linearized variant of the Zebiak–Cane atmosphere–ocean model of El Niño–Southern Oscillation (ENSO), hereinafter called the nominal model. The Floquet analysis shows that the system has a single unstable mode. This mode has a shape and frequency similar to ENSO and is well described by delayed oscillator physics. Singular vector analysis shows two interesting features. (i) For any starting month and time period of optimization the singular vector is shaped like one of two nearly orthogonal patterns. These two patterns correspond approximately to the real and imaginary parts of the adjoint of the ENSO mode for the time-invariant basic-state version of the system that was calculated in previous work. (ii) Contour plots of the singular values as a function of starting month and period of optimization show a ridge along end times around December. This result along with a study of the time evolution of the associated singular vectors shows that the growth of the singular vectors has a strong tendency to peak in the boreal winter. For the case of a stochastically perturbed ENSO model, this result indicates that the annual cycle in the basic state of the ocean is sufficient to produce strong phase locking of ENSO to the annual cycle; it is not necessary to invoke either nonlinearity or an annual cycle in the structure of the noise.

The structures of the ENSO mode, of the optimal vectors, and of the phase locking to the annual cycle are robust to a wide range of values for the following parameters: the coupling strength, the ocean mechanical damping, and the reflection efficiency of Rossby waves that are incident on the western boundary. Four variant models were formed from the nominal coupled model by changing the aforementioned parameters in such a way as to (i) make the model linearly stable and (ii) affect the ratio of optimal transient growth to the amplitude of the first Floquet multiplier (i.e., the decay rate of the ENSO mode). Each of these four models is linearly stable to perturbations but is shown to support realistic ENSO variability via transient growth for plausible values of stochastic forcing. For values of these parameters that are supported by observations and theory, these results show the coupled system to be linearly stable and that ENSO is the result of transient growth. Supporting evidence is found in a companion paper.

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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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R. E. Ronca
and
D. S. Battisti

Abstract

Here 11 years of surface data (1961–72, excluding 1963) taken at ocean weather ship N (OWS N) are analyzed. OWS N is located in the subtropical eastern Pacific Ocean (140°W, 30°N). Bulk formulas are employed to calculate each component of the surface heat flux (sensible, latent, longwave, and shortwave) from the 3-h measurements of sea surface temperature (SST), air temperature, surface humidity, wind speed, and cloudiness. Analyses are performed on fluxes averaged over daily and monthly intervals. Results indicate a large fraction of the variance in net surface energy flux is associated with anomalies in the latent heat flux; the latter are principally due to variability in the surface wind speed. Cross correlation and regression analyses of monthly anomalies of SST and SST tendency (∂SST/∂t) with the surface heat flux components indicate over 50% of the variance in SST-tendency anomalies is accounted for by local anomalies in the net surface energy flux.

In the summer, the summed variance in the four components of the surface heat flux is explained almost completely by two modes of variability that are nearly orthogonal. The first (second) mode is defined as the combination of surface flux components that optimally covaries with the SST-tendency anomaly (SST anomaly) and it contains 74% (26%) of the summed variance in all of the surface heat flux components. In addition, the net heat flux anomaly associated with the the SST-tendency anomaly, which results from the summing of the individual components that define the first mode, accounts for virtually all (96%) of the variance in the net surface flux; it is dominated by the latent heat flux component. The second mode is dominated by the variability in the shortwave flux (mainly due to changes in the cloudiness), but the opposing anomalies in latent and longwave flux largely cancel the anomalies in the shortwave. Hence, the net heat flux associated with the flux components that covary with the SST anomalies is too small to generate significant variability in SST.

The physical scenario consistent with the analyses presented is as follows. Throughout the year, variability that is inherent to the atmosphere causes net surface flux anomalies (mainly due to anomalies in evaporation driven by wind speed anomalies) that account for over 50% of the variability in SST. During the summer months, the changes in the SST that are driven by the aforementioned atmospheric variability, in turn, force changes in the lower troposphere (e.g., in the low-level cloudiness) that are announced by a redistribution of the surface heat flux components, though these changes in the lower atmosphere do not further affect the ocean because there is an insignificant change in the net surface heat flux.

The results obtained from the observations are confirmed using a one-dimensional ocean mixed layer model. Model results also indicate that heat flux anomalies due to entrainment processes act in the same sense as the net surface heat flux anomalies but are small (about 7% of the variance) compared to the surface heat flux anomalies. Anomalies in ocean advection contribute significantly to SST anomalies only during late wintertime and only for seasonally averaged and longer timescales.

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C. M. Bitz
and
D. S. Battisti

Abstract

The authors examine the net winter, summer, and annual mass balance of six glaciers along the northwest coast of North America, extending from Washington State to Alaska. The net winter (NWB) and net annual (NAB) mass balance anomalies for the maritime glaciers in the southern group, located in Washington and British Columbia, are shown to be positively correlated with local precipitation anomalies and storminess (defined as the rms of high-passed 500-mb geopotential anomalies) and weakly and negatively correlated with local temperature anomalies. The NWB and NAB of the maritime Wolverine glacier in Alaska are also positively correlated with local precipitation, but they are positively correlated with local winter temperature and negatively correlated with local storminess. Hence, anomalies in mass balance at Wolverine result mainly from the change in moisture that is being advected into the region by anomalies in the averaged wintertime circulation rather than from a change in storminess. The patterns of the wintertime 500-mb circulation and storminess anomalies associated with years of high NWB in the southern glacier group are similar to those associated with low NWB years at the Wolverine glacier, and vice versa.

The decadal ENSO-like climate phenomenon discussed by Zhang et al. has a large impact on the NWB and NAB of these maritime glaciers, accounting for up to 35% of the variance in NWB. The 500-mb circulation and storminess anomalies associated with this decadal ENSO-like mode resemble the Pacific–North American pattern, as do 500-mb composites of years of extreme NWB of South Cascade glacier in Washington and of Wolverine glacier in Alaska. Hence, the decadal ENSO-like mode affects precipitation in a crucial way for the NWB of these glaciers. Specifically, the decadal ENSO-like phenomenon strongly affects the storminess over British Columbia and Washington and the moisture transported by the seasonally averaged circulation into maritime Alaska. In contrast, ENSO is only weakly related to NWB of these glaciers because (i) the large-scale circulation anomalies associated with ENSO do not produce substantial anomalies in moisture advection into Alaska, and (ii) the storminess and precipitation anomalies associated with ENSO are far to the south of the southern glacier group.

Finally, the authors discuss the potential for short-term climate forecasts of the mass balance for the maritime glaciers in the northwest of North America.

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Ying-Quei Chen
,
D. S. Battisti
, and
E. S. Sarachik

Abstract

A 21/2-layer ocean model is developed to investigate the role of the first two baroclinic modes in determining the interannual variations of the sea surface temperature (SST) associated with the El Niño–Southern Oscillation (ENSO) phenomenon. Rather than simply adding an additional mode to the ocean component of the Zebiak–Cane coupled atmosphere–ocean model, it proved necessary to completely rethink all parts of the model. This allowed the external parameters to be specified more realistically. For example, the drag coefficient used in calculating the surface wind stress in the model is now consistent with that empirically derived, and the temperature of the water entrained in the surface layer that affects SST is now more carefully parameterized.

When forced by observed wind stress anomalies for 1961–93, the ocean model reproduces the interannual variations of SST satisfactorily. The quantitative discrepancies between the model hindcast and observed SST anomalies are limited to an excessive cooling of 0.5–1°C in the eastern/central Pacific during the period of 1989 to early 1991, and weaker warm phases in the central/western Pacific than observed. Both of the two gravest baroclinic modes are shown to be important in affecting the interannual variability in SST. A critique of the ocean model is presented at the end of this work.

When the ocean model is coupled with a simple atmosphere model, the resulting model exhibits quasi-periodic ENSO cycles with a period of ∼5 years. The variability in the coupled model is sensitive to the strength of the coupling and to the model parameterization of subsurface temperature. This model provides an opportunity to gain a better insight into the instability and variability of large-scale, low-frequency phenomena in the coupled atmosphere–ocean climate system and to bridge the gap between the simple Zebiak–Cane model and the more complex and computationally intensive coupled general circulation models in which more vertical modes are present.

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S. M. Penny
,
G. H. Roe
, and
D. S. Battisti

Abstract

Penny et al. recently showed that the midwinter suppression in storminess over the western and central Pacific Ocean is due to a reduction in the number and amplitude of “seed” disturbances entering the Pacific storm track from midlatitude Asia. In this reply, the authors strengthen the conclusions that were originally put forth and show that the apparent departure from this behavior presented in a recent comment originates in the commenters having undersampled the full dataset of interannual variability. It is shown that when the Pacific storm track is only weakly “seeded” by an upstream source, as is common during winter and uncommon during fall and spring, it is likely to be weaker than average, and this reduction is highly statistically significant and the amplitude compares well with the midwinter suppression.

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Scot D. Johnson
,
David S. Battisti
, and
E. S. Sarachik

Abstract

An empirically derived linear dynamical model is constructed using the Comprehensive Ocean–Atmosphere Data Set enhanced sea surface temperature data in the tropical Pacific during the period 1956–95. Annual variation in the Markov model is sought using various tests. A comparison of Niño-3.4 forecast skill using a seasonally varying Markov model to forecast skill in which the seasonal transition matrices are applied during opposite times of the year from which they were derived is made. As a result, it is determined that the seasonal transition matrices are probably not interchangeable, indicating that the Markov model is not annually constant. Stochastic forcing, which has been hypothesized to exhibit seasonality, is therefore not the sole source of the annual variation of El Niño–Southern Oscillation (ENSO) dynamics and the phase locking of ENSO events to peak during November.

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M. Biasutti
,
D. S. Battisti
, and
E. S. Sarachik

Abstract

An atmospheric GCM coupled to a slab ocean model is used to investigate how temperature and precipitation over South America and Africa affect the annual cycle of the Atlantic ITCZ. The main conclusion of this study is that variations in precipitation and temperature forced by the annual cycle of insolation over the continents are as important as variations in insolation over the ocean and in ocean heat transport convergence in forcing the annual march of the Atlantic ITCZ observed in the control simulation. The processes involved are as follows.

The intensity of precipitation over land affects the stability of the atmosphere over the tropical Atlantic Ocean, and thus modulates the intensity of deep convection and convergence in the ITCZ. Both the imposed changes in land precipitation and the subsequent changes in the strength of the ITCZ drive surface wind anomalies, thereby changing the meridional gradient of SST in proximity of the basic-state ITCZ. Finally, atmosphere–ocean feedbacks cause the ITCZ to be displaced meridionally.

Seasonal changes in surface temperature in the Sahara also have a strong influence on the position of the Atlantic ITCZ. Cold wintertime temperatures produce high surface pressure anomalies over Africa and into the tropical North Atlantic and drive stronger trade winds, which cool the North Atlantic by evaporation. The coupled interactions between the SST, the wind, and the ITCZ intensify the anomalies in the equatorial region, causing the southward displacement of the ITCZ in boreal spring.

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D. S. Battisti
,
U. S. Bhatt
, and
M. A. Alexander

Abstract

A new model for the upper North Atlantic Ocean is presented and used to hindcast the SST from 1950 to 1988. The model consists of a matrix of one-dimensional (independent) columns in which a variable-depth, bulk mixed layer overlies a diffusive convective thermocline. The climatological annual cycle of heat flux convergence by the oceanic circulation is implicitly included in the formulation of the forcing. The 39-yr control integration of the model includes as surface forcing the shortwave and net longwave radiation from a control integration of the community climate model. Sensible and latent heat fluxes are determined from instantaneous values of surface temperature, humidity, and wind speed from the atmospheric model, and the SST simulated by the ocean model using the bulk formulae. The hindcast is performed by repeating the control integration, adding the observed, monthly mean surface anomalies in surface temperature, humidity, and wind speed for the period 1950–88. Thus, the simulated SST anomalies are generated explicitly by anomalies in the latent and sensible heat fluxes. A separate hindcast integration is presented, using as forcing the “observed” sensible plus latent beat flux anomalies rather than the surface atmospheric field anomalies to demonstrate that the major results are not predetermined by the formulation of the coupling.

The ability of the, model to hindcast the wintertime interannual variations in SST is demonstrated by simple correlations with observed anomalies and by comparing the composite of warm and cold events observed with those simulated by the model. There is a good quantitative agreement between simulated and observed SST anomalies throughout most of the North Atlantic Ocean. Since the model formulation explicitly excludes any effects due to anomalies in the ocean advection, our results confirm the hypothesis that wintertime interannual to subdecadal variability in SST is mainly due to local anomalies in the air-sea flux of sensible and latent heat and not to anomalies in oceanic advection. Significant disagreement between hindcast and simulated SST anomalies is limited to a small region extending from Cape Hatteras to Nova Scotia along the U.S. coast. Here, the observed surface flux anomalies are anticorrelated with the SST anomalies, implicating important changes in oceanic advection in the generation of interannual wintertime SST and surface flux anomalies.

Both the sensible and latent heat flux anomalies are shown to contribute substantially to the wintertime anomalies in SST in the subpolar Atlantic, while the heat flux anomalies are predominantly determined by the latent heat flux in the subtropics. Entrainment anomalies contribute to a lesser extent to the mixed layer temperature anomalies throughout the basin. Sensitivity studies are performed to highlight the atmospheric processes and variability that account for the surface heat flux anomalies.

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