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Parameterization of mesoscale eddies in coarse resolution ocean models is necessary to include the effect of eddies on the large‐scale oceanic circulation. We propose to use a multiple‐scale Quasi‐Geostrophic (MSQG) model to capture the eddy dynamics that develop in response to a prescribed large‐scale flow. The MSQG model consists in extending the traditional quasi geostrophic (QG) dynamics to include the effects of a variable Coriolis parameter and variable background stratification. Solutions to this MSQG equation are computed numerically and compared to a full primitive equation model. The large‐scale flow field permits baroclinically unstable QG waves to grow. These instabilities saturate due to non‐linearities and a filtering method is applied to remove large‐scale structures that develop due to the upscale cascade. The resulting eddy field represents a dynamically consistent response to the prescribed background flow, and can be used to rectify the large‐scale dynamics. Comparisons between Gent‐McWilliams eddy parameterization and the present solutions show large regions of agreement, while also indicating areas where the eddies feed back onto the large scale in a manner that the Gent‐McWilliams parameterization cannot capture. Also of interest is the time variability of the eddy feedback which can be used to build stochastic eddy parameterizations.

 

Abstract Symmetric instability is a mechanism that can transfer geostrophic kinetic energy to overturning and dissipation. To date, symmetric instability has only been recognized to occur at the ocean surface or near topographic boundary layers. Analyses of direct microstructure measurements reveal enhanced dissipation caused by symmetric instability in the northwestern equatorial Pacific thermocline, which provides the first observational evidence of subsurface symmetric instability away from boundaries. Enhanced subsurface cross-equatorial exchange provides the negative potential vorticity needed to drive the symmetric instability, which is well reproduced by numerical modeling. These results suggest a new route to energy dissipation for large scale currents, and hence a new ocean turbulent mixing process in the ocean interior. Given the importance of vertical mixing in the evolution of equatorial thermocline, models may need to account for this mechanism to produce more reliable climate projections. 

Abstract. This paper contains a description of recent changes tothe formulation and numerical implementation of the Quasi-GeostrophicCoupled Model (Q-GCM), which constitute a major update of the previousversion of the model (Hogg et al., 2014). The Q-GCM model has been designedto provide an efficient numerical tool to study the dynamics of multi-scalemidlatitude air–sea interactions and their climatic impacts. The presentadditions/alterations were motivated by an inquiry into the dynamics ofmesoscale ocean–atmosphere coupling and, in particular, by an apparent lackof the Q-GCM atmosphere's sensitivity to mesoscale sea-surface temperature (SST)anomalies, even at high (mesoscale) atmospheric resolutions, contrary toample theoretical and observational evidence otherwise. Major modificationsaimed at alleviating this problem include an improved radiative-convectivescheme resulting in a more realistic model mean state and associated modelparameters; a new formulation of entrainment in the atmosphere, whichprompts more efficient communication between the atmospheric mixed layer andfree troposphere; and an addition of a temperature-dependent windcomponent in the atmospheric mixed layer and the resulting mesoscalefeedbacks. The most drastic change is, however, the inclusion of moistdynamics in the model, which may be key to midlatitude ocean–atmospherecoupling. Accordingly, this version of the model is to be referred to as theMQ-GCM model. Overall, the MQ-GCM model is shown to exhibit a rich spectrumof behaviors reminiscent of many of the observed properties of the Earth'sclimate system. It remains to be seen whether the added processes are ableto affect in fundamental ways the simulated dynamics of the midlatitudeocean–atmosphere system's coupled decadal variability. 

We introduce a pseudo‐spectral algorithm that includes full compressible dynamics with the intent of simulating near‐incompressible fluids, CaTSM (Compressible and Thermodynamically consistent Spectral Model). A semi‐implicit scheme is used to model acoustic waves in order to evolve the system efficiently for such fluids. We demonstrate the convergence properties of this numerical code for the case of a shock tube and for Rayleigh‐Taylor instability. A linear equation of state is also presented, which relates the specific volume of the fluid linearly to the potential temperature, salinity, and pressure. This permits the results to be easily compared to a Boussinesq framework in order to assess whether the Boussinesq approximation adequately represents the relevant exchange of energy to the problem of interest. One such application is included, that of the development of a single salt finger, and it is shown that the energetic behavior of the system is comparable to the typical canonical development of the problem for oceanographic parameters. However, for more compressible systems, the results change substantially even for low‐Mach number flows.

 

We describe a form of Atlantic Meridional Overturning Circulation (AMOC) variability that we believe has not previously appeared in observations or models. It is found in an ensemble of eddy‐resolving North Atlantic simulations that the AMOC frequently reverses in sign at ∼35°N with gyre‐wide anomalies in size and that reach throughout the water column. The duration of each reversal is roughly 1 month. The reversals are part of the annual AMOC cycle occurring in boreal winter, although not all years feature an actual reversal in sign. The occurrence of the reversals appears in our ensemble mean, suggesting it is a forced feature of the circulation. A partial explanation is found in an Ekman response to wind stress anomalies. Model ensemble simulations run with different combinations of climatological and realistic forcings argue that it is the atmospheric forcing specifically that results in the reversals, despite the signals extending into the deep ocean.

 

Abstract The structure and variations of the North Equatorial Counter Current (NECC) in the far western Pacific Ocean during 2014-2016 are investigated using repeated in-situ hydrographic data, altimeter data, Argo data, and reanalysis data. The NECC shifted ~1 degree southward and intensified significantly with its transport exceeding 40 Sv (1 Sv = 10 6 m 3 s -1 ), nearly double its climatology value, during the developing phase of the 2015/16 El Niño event. Observations show that the 2015/16 El Niño exerted a comparable impact on the NECC with that of the extreme 1997/98 El Niño in the far western Pacific Ocean. Baroclinic instability provided the primary energy source for the eddy kinetic energy (EKE) in the 2015/16 El Niño, which differs from the traditional understanding of the energy source of EKE as barotropic instability in low latitude ocean. The enhanced vertical shear and the reduced density jump between the NECC layer and the subsurface North Equatorial Subsurface Current (NESC) layer renders the NECC–NESC system baroclinically unstable in the western Pacific Ocean during El Niño developing phase. The eddy-mean flow interactions here are diverse associated with various states of the El Niño Southern Oscillation (ENSO).