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  • Author or Editor: A. Capotondi x
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A. Capotondi
and
R. Saravanan

Abstract

The performance of thermal surface boundary conditions based on energy balance models for the atmosphere is tested using a two-dimensional (meridional plane) ocean model. The results are compared to those from an idealized ocean – atmosphere coupled system. The latter consists of a two-dimensional Boussinesq ocean model coupled to a two-layer global atmospheric model. The various thermal boundary conditions are applied to the same ocean model used in the coupled system, and their ability to capture the essential atmospheric feedbacks is investigated. Some of the effects associated with the atmospheric eddy moisture transport are also incorporated by empirically relating variations in the surface freshwater flux to variations in the surface heat flux based on the coupled model results. Comparisons with the coupled results show a considerable improvement in the characteristics of the equilibria of the ocean thermohaline circulation when the alternative thermohaline boundary conditions are used instead of the so-called “mixed boundary conditions” commonly used in ocean-only integrations. Furthermore, the response of the pole-to-pole equilibrium to a freshening of the high northern latitudes is in remarkably good agreement with the one observed in the coupled model. However, a tendency for the “energy balance” boundary conditions to overstabilize the circulation is detected, and limitations in the present treatment of the eddy moisture transport effects are found, especially in the presence of convective adjustment.

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A. Capotondi
and
M. A. Alexander

Abstract

A band of enhanced thermocline variability at 10°–15°N in the Pacific found in nature also occurs in an ocean general circulation model forced with observed fluxes of momentum, heat, and freshwater over the period 1958–97. The variability in the model is primarily associated with long baroclinic Rossby waves characterized by periods in the decadal range (7–10 yr). The waves are forced by westward propagating Ekman pumping anomalies east of the date line and propagate at a speed of ∼13 cm s−1, which is slower than the phase speed of the first mode unforced baroclinic waves (15–16 cm s−1). West of the date line, the correlations between thermocline displacements and local Ekman pumping are relatively small, and the ocean signals have a phase speed of ∼20 cm s−1, very similar to the phase speed of the first baroclinic mode in the western half of the basin (18–20 cm s−1). The phase speeds of the ocean model signals have been estimated using cospectral analysis, while the WKB approximation has been used to evaluate the phase speed of the baroclinic Rossby wave modes for the given model stratification. The thermocline displacements are coherent all the way across the basin in the 10°–15°N latitude band. After reaching the western boundary the signal appears to propagate along the boundary, both to the north and the south. Along the southern branch, the signal reaches the equator and propagates along the equator, contributing to low-frequency equatorial thermocline variability.

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A. Capotondi
and
W. R. Holland

Abstract

Variability in a three-dimensional ocean model of idealized geometry is analyzed. The variability is induced in the model by adding a stochastic component to the surface buoyancy forcing. The influence of the surface thermal forcing on the model variability is investigated under conditions in which the surface freshwater flux is specified. The thermal boundary conditions that have been considered include restoring boundary conditions with different restoring times, fixed surface heat flux, and boundary conditions derived by assuming an energy balance model for the atmosphere. It is found that the ocean model response varies considerably with the thermal boundary conditions used, given the specific ratio of thermal to haline forcing chosen for these calculations. A behavior characterized by sudden transitions between states of strong overturning and states of much weaker overturning dominates the model’s response when a strong restoring is used, while quasi-regular oscillations at a period of approximately 24 years are found with boundary conditions that allow the sea surface temperature to respond to changes in the oceanic heat transport. The spatial pattern of the stochastic forcing is considered here as a variable of the problem, and the model’s response to different spatial patterns is analyzed. The same decadal signal is found for all spatial patterns, suggesting that the variability at this timescale can be considered as an internal mode of the system and not associated with some characteristics of the forcing. However, different special patterns can be more or less effective in exciting the oceanic mode. Large-scale forcing directly contributing to the east–west pressure gradient appears to produce the largest response.

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Antonietta Capotondi
,
Michael A. Alexander
, and
Clara Deser

Abstract

Observations indicate the existence of two bands of maximum thermocline depth variability centered at ∼10°S and 13°N in the tropical Pacific Ocean. The analysis of a numerical integration performed with the National Center for Atmospheric Research ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat, and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 10°S and 13°N are associated with first-mode baroclinic Rossby waves forced by anomalous Ekman pumping. In this study the factors that may be responsible for the Rossby wave maxima at 10°S and 13°N, including the amplitude and spatial coherency of the forcing at those latitudes, are systematically investigated. A simple Rossby wave model is used to interpret the OGCM variability and to help to discriminate between the different factors that may produce the tropical maxima. These results indicate that the dominant factor in producing the maximum variability at 10°S and 13°N is the zonal coherency of the Ekman pumping, a characteristic of the forcing that becomes increasingly more pronounced at low frequencies, maximizing at timescales in the decadal range. Local maxima in the amplitude of the forcing, while not explaining the origin of the centers of variability at 10°S and 13°N, appear to affect the sharpness of the variability maxima at low frequencies. Although the Rossby wave model gives an excellent fit to the OGCM, some discrepancies exist: the amplitude of the thermocline variance is generally underestimated by the simple model, and the variability along 13°N is westward intensified in the wave model but reaches a maximum in the central part of the basin in the OGCM. Short Rossby waves excited by small-scale Ekman pumping features, or the presence of higher-order Rossby wave modes may be responsible for the differences in the zonal variance distribution along 13°N.

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Antonietta Capotondi
,
Michael A. Alexander
, and
Clara Deser

Abstract

Observations indicate the existence of two bands of maximum thermocline depth variability centered at ∼10°S and 13°N in the tropical Pacific Ocean. The analysis of a numerical integration performed with the National Center for Atmospheric Research ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat, and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 10°S and 13°N are associated with first-mode baroclinic Rossby waves forced by anomalous Ekman pumping. In this study the factors that may be responsible for the Rossby wave maxima at 10°S and 13°N, including the amplitude and spatial coherency of the forcing at those latitudes, are systematically investigated. A simple Rossby wave model is used to interpret the OGCM variability and to help to discriminate between the different factors that may produce the tropical maxima. These results indicate that the dominant factor in producing the maximum variability at 10°S and 13°N is the zonal coherency of the Ekman pumping, a characteristic of the forcing that becomes increasingly more pronounced at low frequencies, maximizing at timescales in the decadal range. Local maxima in the amplitude of the forcing, while not explaining the origin of the centers of variability at 10°S and 13°N, appear to affect the sharpness of the variability maxima at low frequencies. Although the Rossby wave model gives an excellent fit to the OGCM, some discrepancies exist: the amplitude of the thermocline variance is generally underestimated by the simple model, and the variability along 13°N is westward intensified in the wave model but reaches a maximum in the central part of the basin in the OGCM. Short Rossby waves excited by small-scale Ekman pumping features, or the presence of higher-order Rossby wave modes may be responsible for the differences in the zonal variance distribution along 13°N.

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Antonietta Capotondi
,
Michael A. Alexander
,
Clara Deser
, and
Arthur J. Miller

Abstract

The output from an ocean general circulation model (OGCM) driven by observed surface forcing is used in conjunction with simpler dynamical models to examine the physical mechanisms responsible for interannual to interdecadal pycnocline variability in the northeast Pacific Ocean during 1958–97, a period that includes the 1976–77 climate shift. After 1977 the pycnocline deepened in a broad band along the coast and shoaled in the central part of the Gulf of Alaska. The changes in pycnocline depth diagnosed from the model are in agreement with the pycnocline depth changes observed at two ocean stations in different areas of the Gulf of Alaska. A simple Ekman pumping model with linear damping explains a large fraction of pycnocline variability in the OGCM. The fit of the simple model to the OGCM is maximized in the central part of the Gulf of Alaska, where the pycnocline variability produced by the simple model can account for ∼70%–90% of the pycnocline depth variance in the OGCM. Evidence of westward-propagating Rossby waves is found in the OGCM, but they are not the dominant signal. On the contrary, large-scale pycnocline depth anomalies have primarily a standing character, thus explaining the success of the local Ekman pumping model. The agreement between the Ekman pumping model and OGCM deteriorates in a large band along the coast, where propagating disturbances within the pycnocline, due to either mean flow advection or boundary waves, appear to play an important role in pycnocline variability. Coastal propagation of pycnocline depth anomalies is especially relevant in the western part of the Gulf of Alaska, where local Ekman pumping-induced changes are anticorrelated with the OGCM pycnocline depth variations. The pycnocline depth changes associated with the 1976–77 climate regime shift do not seem to be consistent with Sverdrup dynamics, raising questions about the nature of the adjustment of the Alaska Gyre to low-frequency wind stress variability.

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