Coarse-resolution Ocean General Circulation Model Research Articles

Generation Mechanism of Tropical Instability Waves in the Equatorial Pacific Ocean

AbstractTropical instability waves (TIWs) are prominent features in the equatorial Pacific, propagating westward at a speed of ~0.5 m s−1 with a wavelength of ~1000 km. In this study, we show that a linear stability analysis using a 1.5-layer shallow water model can predict successfully an unstable mode whose wavelength, phase speed, growth rate, and meridional structure are all consistent with those of the TIWs simulated by an eddy-resolving ocean general circulation model (OGCM). This unstable mode can be interpreted as resulting from the coupling of two Rossby waves, namely, one trapped just north of the equator (~1°–3.5°N) and the other trapped farther north (~3.5°–8°N). Although these two Rossby waves have opposite intrinsic phase propagation directions reflecting the negative and positive local meridional potential vorticity (PV) gradients, respectively, their actual propagation direction can be adjusted through the advection by the South Equatorial Current and the North Equatorial Countercurrent such that they might propagate westward at the same speed and with the same zonal wavenumber yielding the largest growth rate of TIWs. The unstable mode does not appear during the period in which the negative PV gradient is absent, which demonstrates its essential role in generating TIWs. Indeed, the seasonal and interannual variability of the TIWs simulated by the OGCM is shown to be significantly controlled by the strength of the negative PV gradient just north of the equator, suggesting that it could be a key parameter toward a dynamically based parameterization of the heat and momentum transfer associated with TIWs in coarse-resolution OGCMs.

Mean Subsurface Upwelling Induced by Intraseasonal Variability over the Equatorial Indian Ocean

AbstractA possible formation mechanism of mean subsurface upwelling along the equator in the Indian Ocean is investigated using a series of hierarchical ocean general circulation model (OGCM) integrations and analytical considerations. In an eddy-resolving OGCM with realistic forcing, mean vertical velocity in the tropical Indian Ocean shows rather strong upwelling, with its maximum on the equator in the subsurface layer below the thermocline. Heat budget analysis exhibits that horizontal and vertical heat advection by deviations (i.e., due to deviations of velocity and temperature from the mean) balances with vertical advection caused by mean equatorial upwelling. Horizontal heat advection is mostly associated with intraseasonal variability with periods of 3–91 days, while contributions from longer periods (>91 days) are small. Sensitivity experiments with a coarse-resolution OGCM further demonstrate that such mean equatorial upwelling cannot be reproduced by seasonal forcing only. Adding the intraseasonal wind forcing, especially meridional wind variability with a period of 15 days, generates significant mean subsurface upwelling on the equator. Further experiments with idealized settings confirm the importance of intraseasonal mixed Rossby–gravity (MRG) waves to generate mean upwelling, which appears along the energy “beam” of the MRG wave. An analytical solution of the MRG waves indicates that wave-induced temperature advection caused by the MRG waves with upward (downward) phase propagation results in warming (cooling) on the equator. This wave-induced warming (cooling) is shown to balance with the mean equatorial upwelling (downwelling), which is consistent with simulated characteristics in the OGCM experiments.

An Ensemble Approach for the Estimation of Observational Error Illustrated for a Nominal 1° Global Ocean Model

Least squares algorithms for data assimilation require estimates of both background error covariances and observational error covariances. The specification of these errors is an essential part of designing an assimilation system; the relative sizes of these uncertainties determine the extent to which the state variables are drawn toward the observational information. Observational error covariances are typically computed as the sum of measurement/instrumental errors and “representativeness error.” In a coarse-resolution ocean general circulation model the errors of representation are the dominant contribution to observational error covariance over large portions of the globe, and the size of these errors will vary by the type of observation and the geographic region. They may also vary from model to model. A straightforward approach for estimating model-dependent, spatially varying observational error variances that are suitable for least squares ocean data assimilating systems is presented here. The author proposes an ensemble-based estimator of the true observational error variance and outlines the assumptions necessary for the estimator to be unbiased. The author also presents the variance (or uncertainty) associated with the estimator under certain conditions. The analytic expressions for the expected value and variance of the estimator are validated with a simple autoregressive model and illustrated for the nominal 1° resolution POP2 global ocean general circulation model.

Southward Eddy Heat Transport Occurring along Southern Flanks of the Kuroshio Extension and the Gulf Stream in a 1/10° Global Ocean General Circulation Model

Abstract The present study investigates meridional heat transport induced by oceanic mesoscale variability in the World Ocean using a ° global ocean general circulation model (OGCM) running on the Earth Simulator. The results indicate prominent poleward eddy heat transport around the western boundary currents and the Antarctic Circumpolar Current, and equatorward eddy heat transport in the equatorial region, consistent with the previous studies using coarse-resolution OGCMs. Such poleward eddy heat transport in midlatitude oceans suggests that the eddies act to reduce meridional background temperature gradients across the currents, as would be expected based on baroclinic instability. Interestingly, however, along the southern flanks of the eastward jets of the Kuroshio Extension and the Gulf Stream, southward eddy heat transport occurs in subsurface layers. This is likely due to the southward migration of warm water cores originating from southern areas adjacent to these currents. Southward movement of these cores is caused by interactions with unsteady meanders and cold eddies detaching from the meanders. The potential impact on biological production in the subtropical surface layers of these southward-traveling warm water cores is also discussed.

A Theory of Deep Stratification and Overturning Circulation in the Ocean

AbstractA simple theoretical model of the deep stratification and meridional overturning circulation in an idealized single-basin ocean with a circumpolar channel is presented. The theory includes the effects of wind, eddies, and diapycnal mixing; predicts the deep stratification in terms of the surface forcing and other problem parameters; makes no assumption of zero residual circulation; and consistently accounts for the interaction between the circumpolar channel and the rest of the ocean.The theory shows that dynamics of the overturning circulation can be characterized by two limiting regimes, corresponding to weak and strong diapycnal mixing. The transition between the two regimes is described by a nondimensional number characterizing the strength of the diffusion-driven compared to the wind-driven overturning circulation. In the limit of weak diapycnal mixing, deep stratification throughout the ocean is produced by the effects of wind and eddies in a circumpolar channel and maintained even in the limit of vanishing diapycnal diffusivity and in a flat-bottomed ocean. The overturning circulation across the deep stratification is driven by the diapycnal mixing in the basin away from the channel but is sensitive, through changes in stratification, to the wind and eddies in the channel. In the limit of strong diapycnal mixing, deep stratification is primarily set by eddies in the channel and diapycnal mixing in the basin away from the channel, with the wind over the circumpolar channel playing a secondary role. Analytical solutions for the deep stratification and overturning circulation in the limit of weak diapycnal mixing and numerical solutions that span the regimes of weak to strong diapycnal mixing are presented.The theory is tested with a coarse-resolution ocean general circulation model configured in an idealized geometry. A series of experiments performed to examine the sensitivity of the deep stratification and the overturning circulation to variations in wind stress and diapycnal mixing compare well with predictions from the theory.

Interaction of a Coastal Kelvin Wave with the Mean State in the Gulf Stream Separation Area

Abstract The interaction of a coastal Kelvin wave with the mean state in the Gulf Stream separation area is studied using a hierarchy of numerical models including both low- and high-resolution 2.5-layer models and a coarse-resolution ocean general circulation model (OGCM) in a simple configuration. When the Kelvin front reaches the separation area in the low-resolution 2.5-layer model, an anomaly of opposite sign emerges and remains in the upper layer of the separation area. The mechanism leading to its buildup is the following: the variations of thickness in the active midlayer due to the propagation of the Kelvin wave induce current variations that act as a source of thickness anomalies for the upper layer (negative feedback). This source term is proportional to the mean vorticity gradient. The latter therefore must be large enough to obtain a significant response, which explains why this phenomenon occurs only in the separation area. The anomaly remains in the separation area because the advection by the mean zonal current is balanced by the current anomalies due to the variations of thickness in the surface layer. A very similar response is obtained in the high-resolution case and with the OGCM; thus, the mechanism leading to this evolution seems also largely independent of the model. However, in the OGCM, the surface current is sufficiently strong to advect the anomaly in the surface layers (above 200-m depth). Note finally that these anomalies do not prevent the Kelvin waves from pursuing their travel southward and then eastward toward the eastern boundary where Rossby waves are radiated. Numerous recent results based on this adjustment mechanism therefore would be robust.

Sensitivity of the Global Thermohaline Circulation to Interbasin Freshwater Transport by the Atmosphere and the Bering Strait Throughflow

Abstract Sensitivity of the global thermohaline circulation to interbasin freshwater transport by the atmosphere and the Bering Strait throughflow is investigated by using a free-surface, coarse-resolution ocean general circulation model. The model is run by prescribing freshwater flux at the sea surface without restoring the sea surface salinity to climatology in order that effects of salinity advection are properly represented. Comparison of experiments with the open and closed Bering Strait shows that the throughflow reduces the intensity of the Atlantic deep circulation by ∼17%, while minimally affecting the Pacific deep circulation. Increase in the atmospheric freshwater transport from the Atlantic to the Pacific intensifies both the Atlantic deep circulation and the Bering Strait throughflow. On the other hand, changes in the throughflow transport under a fixed amount of atmospheric interbasin freshwater transport are found to have a minor impact on the global thermohaline circulation. This insensit...

An efficient climate model with a 3D ocean and statistical-dynamical atmosphere*

We describe a coupled climate model of intermediate complexity designed for use in global warming experiments. The atmospheric component is a two-dimensional (zonally averaged) statistical–dynamical model based on the Goddard Institute for Space Study's atmospheric general circulation model (GCM). In contrast with energy-balance models used in some climate models of intermediate complexity, this model includes full representation of the hydrological and momentum cycles. It also has parameterizations of the main physical processes, including a sophisticated radiation code. The ocean component is a coarse resolution ocean GCM with simplified global geometry based on the Geophysical Fluid Dynamics Laboratory modular ocean model. Because of the simplified geometry the resolution in the western boundary layers can be readily increased compared to conventional coarse resolution models, without increasing the model's computational requirements in a significant way. The ocean model's efficiency is also greatly increased by using a moderate degree of asynchronous coupling between the oceanic momentum and tracer fields. We demonstrate that this still allows an accurate simulation of transient behavior, including the seasonal cycle. A 100 years simulation with the model requires less than 8 hours on a state-of the art workstation. The main novelty of the model is therefore a combination of computational efficiency, statistical–dynamical atmosphere and 3D ocean. Long-term present-day climate simulations are carried out using the coupled model with and without flux adjustments, and with either the Gent-McWilliams (GM) parametrization scheme or horizontal diffusion (HD) in the ocean. Deep ocean temperatures systematically decrease in the runs without flux adjustment. We demonstrate that the mismatch between heat transports in the uncoupled states of two models is the main cause for the systematic drift. In addition, changes in the circulation and sea-ice formation also contribute to the drift. Flux adjustments in the freshwater fluxes are shown to have a stabilizing effect on the thermohaline circulation in the model, whereas the adjustments in the heat fluxes tend to weaken the global "conveyor". To evaluate the model's response to transient external forcing global warming simulations are also carried out with the flux-adjusted version of the coupled model. The coupled model reproduces reasonably well the behavior of more sophisticated coupled GCMs for both current climate and for the global warming scenarios.

The Mechanism for Antarctic Intermediate Water Renewal in a World Ocean Model

Abstract Realistic representation of the low-salinity tongue of Antarctic Intermediate Water (AAIW) has been achieved in a coarse-resolution ocean general circulation model. The authors find that this water mass is not generated by direct subduction of surface water near the polar front. Instead, the renewal process is concentrated in the southeast Pacific Ocean off southern Chile. The outflow of the East Australian Current progressively cools (by heat loss to the atmosphere) and freshens (by assimilation of polar water, carried north by the surface Ekman drift) during its slow movement across the South Pacific toward the AAIW formation zone. Further, deep, warm advection near Chile enables more convective overturn, resulting in very deep mixed layers from which AAIW is fed into the South Pacific and also into the Malvinas Current. Along with this isolated region of AAIW renewal, the model relies on alongisopycnal mixing of fresh surface water from the polar front to capture a realistic circumpolar tongue...

Acoustic Detection of Greenhouse-induced Climate Changes in the Presence of Slow Fluctuations of the Thermohaline Circulation

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.