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Uwe Mikolajewicz
,
Ernst Maier-Reimer
, and
Tim P. Barnett

Abstract

Munk and Forbes have proposed to detect greenhouse gas-induced climate changes in the World Ocean with an array of long-range acoustic transmissions from Heard Island in the southern Indian Ocean. We estimated–assuming a continuously monitorable simplified axial ray propagation–the signal-to-noise ratio for such an experiment in an environment of slow fluctuations of the thermohaline circulation on a decadal time scale. The signal and noise are obtained from two coarse-resolution ocean general circulation model simulations. In the first, prescribed greenhouse atmospheric anomalies forced the ocean and yielded rough estimates of ocean response to greenhouse warming. In the second, some aspects of low-frequency internal variability of the ocean were obtained by stochastic forcing of the same ocean model. By this technique, no oscillations of the coupled ocean-atmosphere system Eke, for instance, El Niño-Southern Oscillation (ENSO) could be stimulated. Both signal and internal variability proved to be strongest at high latitudes, where the depth of the sound channel is small. At lower latitudes the signal is relatively weak, except for the western Atlantic. An array with an acoustic source near Heard Island would monitor primarily temperature changes in the near-surface layers of the Southern Ocean rather than in low-latitude intermediate depths.

The trend detection probability for any single path came out to be weak, at least for a one-decade measuring interval. But using information from at least a two-decade interval and an array of receivers improved the detection probabilities substantially. Two different pattern detection strategies were tested: projecting the natural variability on the expected greenhouse signal and projecting the greenhouse signal onto the major components of the natural variability. Both techniques proved to give almost identical results.

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Arthur J. Miller
,
Tim P. Barnett
, and
Nicholas E. Graham

Abstract

Tropical Pacific SST hindcasts are examined in the Zebiak and Cane (Lamont), Latif (MPIZ), Oberhuber (OPYC), and GFDL ocean models, each forced by the same wind-stress fields over the 1970–85 time interval. Skill scores reveal that, although all the models exhibit significant skill, the regions where the skill is maximized differ from model to model. The simplest model (Lamont) has maximum skills in the eastern basin near the boundary while the three GCMs have maxima in central Pacific regions. We also examine, via canonical correlation analysis (CCA), the heat budgets of the surface layers of the Lamont, MPIZ, and OPYC models. We find that although similar spatial relationships exist for the mechanisms that excite SST anomalies (i.e., zonal advection, meridional advection, and vertical advection/mixing), the balance of the strength of them terms is different for each model. Vertical advection tends to control the large-scale structure of SST in the Lamont model, meridional advection provides the dominant large-scale forcing for SST anomalies in the MPIZ model, and all three terms are important in the region of developing SST in the OPYC model. CCA reconstructions of the El Niño events of 1972–73 and 1982–83 reveal that the Lamont model does not exhibit any clear eastward propagation of SST; the MPIZ model propagates SST anomalies eastward for both the 1972–73 and 1982–83 El Niño events while the OPYC model propagates SST eastward for the 1982–83 El Niño and develops SST in place for the 1972–73 El Niño.

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David W. Pierce
,
K-Y. Kim
, and
Tim P. Barnett

Abstract

The variability of the ocean’s thermohaline circulation in an oceanic general circulation model (OGCM) coupled to a two-dimensional atmospheric energy balance model (EBM) is examined. The EBM calculates air temperatures by balancing heat fluxes, including that from the ocean surface; air temperature and ocean circulation evolve together without imposed temperature restrictions except specification of the solar constant. The heat coupling is scale dependent such that small-scale ocean temperature anomalies are damped quickly while large-scale ones lose heat slowly by longwave emission to space. These boundary conditions are more realistic than restoring conditions even when weak coupling is used, since they allow changes in air temperature and wholesale shifts in the planetary heat balance.

It is found that coupling the EBM to the OGCM increases the stability of the ocean’s thermohaline circulation. This increased stability arises from the ability of the coupled model to develop a four times greater sea surface temperature response to a given change in thermohaline overturning than when traditional restoring boundary conditions are used. The sense of this increased response works to stabilize the thermohaline overturning. The specific value of the small-scale thermal coupling coefficient also influences the stability even though the large-scale coefficient is always small (2 W m−2 C−1); this suggests that small-scale processes might determine the large-scale stability.

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David W. Pierce
,
Tim P. Barnett
, and
Uwe Mikolajewicz

Abstract

The physical mechanisms causing century-scale Southern Ocean thermohaline oscillations in a primitive equation oceanic general circulation model are described. The oscillations have been shown to occur on a 320-year timescale when random fluctuations am added to the freshwater flux field that forces the model; this result is extended to show that they occur in a variety of situations, including ones without added noise. The oscillations involve movement between two model states: one characterized by strong convection and an active thermohaline circulation. and the other with a holocline around Antarctica capping off the water column, thus preventing convection. The physical mechanism that forces the model from the quiescent state to an actively convecting one is subsurface (300 m) heating around Antarctica, which destabilizes the water column; the ultimate source of this heat is advected North Atlantic Deep Water. This leads to a teleconnection between forcing conditions in the North Atlantic and the thermohaline structure of the Southern Ocean. The mechanism that shuts off convection is surface freshening, primarily by precipitation, in the region poleward of the Antarctic Circumpolar Current. The oscillations are analyzed in terms of a simple “flip-flop” model, which indicates that nonlinearities in the seawater equation of state are necessary for the oscillations to occur. The spatial pattern of convection around Antarctica affects the time evolution of the Southern Ocean's thermohaline overturning and the way in which different surface forcings cause the model to oscillate.

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Nicholas E. Graham
,
Tim P. Barnett
,
Vijay G. Panchang
,
Ole M. Smedstad
,
James J. O'Brien
, and
Robert M. Chervin

Abstract

An experiment in which surface wind stress data from the National Center for Atmospheric Research global circulation model (GCM) was used to drive a simple model of the tropical Pacific is described. First, a 15-year integration of the GCM was conducted, forcing the model with observed sea surface temperatures (SSTs) for the period 1958–72 (the FORCED case). A parallel integration was conducted using long-term monthly mean SSTs (the CONTROL case). All other boundary forcing in both GCM simulations was identical. Surface wind stress date from each GCM integration were then used to drive the Florida State University 1½ layer, reduced gravity model of the tropical Pacific. A separate integration of the ocean model was conducted using observed wind stress for the period 1961–72 (the OBSTRESS case).

The goal of this research was to evaluate the response of a sophisticated GCM to tropical Pacific SST variability in terms of the surface wind stress field, and to investigate the sensitivity of a simple wind-driven ocean model to differences between the simulated and observed wind stress data. These are important issues bearing on the potential for accurate modeling of the coupled ocean-atmosphere system over the tropical Pacific. In this paper the results from the ocean model simulations and observations are compared in terms of interannual variability. An earlier paper describes the response of the GCM tropical Pacific surface wind stress field to prescribed SSTs.

The results show that the GCM response to prescribed SSTs produced wind stress anomaly patterns over the tropical Pacific that qualitatively resemble those observed in association with extremes of the El Niño activity, particularly in the central equatorial ocean. These wind stress anomalies produced upper-layer thickness anomalies in the eastern ocean that bore some resembalance to those found in observations and the results of the OBSTRESS integration; i.e., simulated El Niños did occur. In general, however, the El Niño signal in the FORCED case was considerably lower in magnitude and was less organized than in the OBSTRESS simulation. Further, the episode-to-episode changes in magnitude did not agree well with those in the OBSTRESS integration. These results reflect not only important differences between the spatial character of the response of the observed and GCM surface wind stress fields to El Niño SST anomalies, but also the fact that the overall coupling between the GCM atmosphere and the tropical Pacific SST field is not as strong as observed in the real ocean–atmosphere system.

A second interesting result was that quasi-periodic oceanic variability in some ways resembling that associated with El Niñc variability in the OBSTRESS and FORCED experiments was clearly evident in the CONTROL case. Considerations of the response of the model ocean to temporally random atmospheric forcing with large spatial scales shows that such organized low frequency variability may arise from the excitation of preferred resonant frequencies defined by the Rossby wave dispersion relation. This finding may have implications concerning the maintenance and character of the El Niño activity.

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