Tidal effects in a global general circulation model: comparison between coarse and high resolution configurations

Low‐mode internal waves propagate over large distances and provide energy for turbulent mixing when they break far from their generation sites. A realistic representation of the oceanic energy cycle in ocean and climate models requires a consistent implementation of their generation, propagation, and dissipation. Here we combine the long‐term mean energy flux from satellite altimetry with results from a 1/10° global ocean general circulation model that resolves the low modes of internal waves and in situ observations of stratification and horizontal currents to study energy flux and dissipation along a 1000 km internal tide beam in the eastern North Atlantic. Internal wave fluxes were estimated from twelve 36‐ to 48‐hr stations in along‐ and across‐beam direction to resolve both the inertial period and tidal cycle. The observed internal tide energy fluxes range from 5.9 kW m−1 near the generation sites to 0.5 kW m−1 at distant stations. Estimates of energy dissipation come from both finestructure and upper ocean microstructure profiles and range, vertically integrated, from 0.5 to 3.3 mW m−2 along the beam. Overall, the in situ observations confirm the internal tide pattern derived from satellite altimetry, but the in situ energy fluxes are more variable and decrease less monotonically along the beam. Internal tides in the model propagate over shorter distances compared to results from altimetry and in situ measurements, but more spatial details close the main generation sites are resolved.

Much of the deep water in the world oceans forms as relatively dense water on shelves in polar regions, followed by downslope flow of the dense water as a boundary layer plume. This process is omitted from current global ocean general circulation models (GCMs) which, instead, form bottom water only as a result of the large‐scale thermohaline overturning and as a result of deep convection. Here the role of polar boundary layer plumes is investigated by coupling a two‐dimensional plume model to a two‐dimensional (latitude‐depth) dynamical ocean model, which, in turn, is coupled to sea ice and surface climate models. The polar plume mass flux at the shelf brink is parameterized in terms of the rate of sea ice formation, net evaporation, and surface cooling over polar shelves. The presence of plumes has only a modest impact on the high‐latitude temperature response to a greenhouse gas increase and the projected sea level rise due to thermal expansion because, in the present model, the overall response is strongly conditioned by changes in the relative strength of northern hemisphere and southern hemisphere bottom waters which occur whether or not plumes are included. These changes, in turn, appear to depend critically on the spin‐up flow field and density fields produced by the coupled model. It is concluded that polar plume processes should be incorporated in coupled atmosphere‐ocean GCMs but that correct simulation of the ocean density field and surface forcing conditions is a more important priority.

Peer Reviewed

A novel vertical mixing scheme to describe the influence of the non-breaking surface waves in ocean general circulation models is proposed based on the second-order turbulence closure model and a mathematical relationship fitted between the in situ observations of the turbulence kinetic energy dissipation rate and the calculated velocity shear module of non-breaking surface waves. The non-breaking-wave-generated turbulence mixing coefficients can be calculated empirically in terms of the wave spectral parameters including ΦSW, K, and ω, where ΦSW is the wave number spectrum of the significant wave height, K is the wave number, ω is the surface wave frequency. The effect of the new mixing scheme on global ocean circulation simulation was tested using the MArine Science and NUmerical Modeling ocean model. The results indicate significant improvement in the upper-ocean temperature structure and the mixed layer depth, i.e., the non-breaking-wave-generated turbulence mixing plays an important role in the upper ocean at high latitudes in summer for both hemispheres compared with traditional Mellor-Yamada 2.5 scheme.

The response of a global ocean general circulation model (OGCM) to different surface heat and freshwater fluxes is examined. The aim is to determine the most appropriate way to spin up an OGCM prior to coupling it with an atmospheric GCM. Attention is focused on surface boundary conditions for salinity since other studies suggest that so-called “mixed” boundary conditions (i.e., a restoring condition on sea surface temperature and flux condition on sea surface salinity) can yield multiple equilibrium states in ocean models. Under the influence of restoring surface boundary conditions, the rate of overturning in the model North Atlantic is in good agreement with observed estimates. Under the influence of mixed surface boundary conditions, however, the rate of deep-water formation in the model North Atlantic is far in excess of the observed estimates. If the equilibrium solution obtained under restoring boundary conditions is subjected to an equivalent set of mixed boundary conditions, the presence...

Based on the National Center for Atmospheric Research (NCAR) Climate System Model version 1 (CSM-1), a Flexible coupled General Circulation Model version 0 (FGCM-0) is developed in this study through replacing CSM-1’s oceanic component model with IAP L30T63 global oceanic general circulation model and some necessary modifications of the other component models. After the coupled model FGCM-0 is spun up for dozens of years, it has been run for 60 years without flux correction. The model does not only show the reasonable long-term mean climatology, but also reproduce a lot of features of the interannual variability of climate, e.g. the ENSO-like events in the tropical Pacific Ocean and the dipole mode pattern in the tropical Indian Ocean. Comparing FGCM-0 with the NCAR CSM-1, some common features are found, e.g. the overestimation of sea ice in the North Pacific and the simulated double ITCZ etc. The further analyses suggest that they may be attributed to errors in the atmospheric model.

The quality of Southern Ocean sea ice simulations in a global ocean general circulation model (GCM) depends decisively on the simulated upper-ocean temperature. This is confirmed by assimilating satellite-derived sea ice concentration to constrain the upper-layer temperature of a sea ice–ocean GCM. The resolution of the model’s sea ice component is about 22 km and thus comparable to the pixel resolution of the satellite data. The ocean component is coarse resolution to afford long-term integrations for investigations of the deep-ocean equilibrium response. Besides improving the sea ice simulation considerably, the simulations with constrained upper-ocean temperature yield much more realistic global deep-ocean properties, in particular when combined with glacial freshwater input. Both outcomes are relatively insensitive to the passive-microwave algorithm used to retrieve the ice concentration being assimilated. The sensitivity of the long-term global deep-ocean properties and circulation to the possible freshwater input from ice shelves and to the parameterization of vertical mixing in the Southern Ocean is reevaluated under the new constraint.

The thermohaline hysteresis response to varying North Atlantic freshwater forcing is studied by means of a three-dimensional global ocean general circulation model (OGCM). The influence of vertical diffusivity is examined using a wide range of mixing coefficients. For sufficiently large vertical diffusivity the model shows a pronounced hysteresis behavior, so that two equilibrium states of the thermohaline circulation are found: one mode with intense deep-water formation in the North Atlantic (“on” mode) and one mode with strongly reduced convective activity (“off” mode). By decreasing the diffusivity, the two branches of the hysteresis merge. In addition, the effect of numerical diffusion is analyzed by applying different advection schemes. A positive feedback between overturning strength and the numerical diffusivity of upstream advection is found, resulting in a reduced stability of the “on” mode. Comparing the results with those from zonally averaged models exhibits substantial differences regarding the stability properties of the thermohaline circulation. Freshwater transports by horizontal gyres have an important effect on the overturning circulation in the OGCM.

The interannual variability of the Kuroshio onshore intrusion (KOI) across the East China Sea (ECS) shelf break and its response to the ECS Kuroshio volume transport (KVT) during 1993 ∼ 2010 are studied based on a high‐resolution, real‐time, and global ocean general circulation model. Since variability of KVT is mainly determined in the interior Pacific, it is rather a remote than a local factor for the local ECS KOI. On interannual time scales, KVT affects KOI not only in the net volume transport across the entire shelf break but also in the spatial pattern along the shelf break. When KVT increases, the intrusion decreases (increases) upstream (downstream) of the major intrusion region northeast of Taiwan, the retreating increases (decreases) upstream (downstream) of the main veering region southwest of Kyushu. These patterns are caused by cyclonic eddies induced by the seaward deflection of the Kuroshio axis from the shelf break, which ultimately results from the KVT increase. A diagnostic mechanism of is proposed, where h is the bottom depth, θ is the angle between isobaths and the vertically averaged current, and M is the absolute volume transport at the shelf break. θ is large in the major intrusion/retreating regions and stable on interannual time scales while M changes with opposite signs between upstream and downstream of the major intrusion/retreating regions. The mechanism explains well the relation of local KVT and the spatial pattern of KOI along the shelf break, in both the mean state and the interannual variations.

Abstract. We describe the seasonal and interannual variability of volume transports in the Weddell and Ross Seas using the 1/12° 20-yr simulation of the OCCAM global ocean general circulation model. The average simulated full-depth cumulative volume transports were 28.5 ± 2.9 Sv (1 Sv ≡ 106 m3 s−1) and 13.4 ± 5.2 Sv, across the main export regions of the Weddell and Ross Seas, respectively. The values of mean outflow of Antarctic Bottom Water (AABW) (defined by neutral density γn ≥ 28.27 kg m−3) from the Weddell and Ross Seas of 10.6 ± 3.1 Sv and 0.5 ± 0.7 Sv, respectively, agree with the range reported in historical observational studies. The export of Weddell Sea dense water in OCCAM is primarily determined by the strength of the Weddell Gyre. Variability in AABW export is predominantly at periods of ~1 yr and 2–4 yr.

Observations show a multidecadal signal in the North Atlantic ocean, but the underlying mechanism and cause of its timescale remain unknown. Previous studies have suggested that it may be driven by the North Atlantic Oscillation (NAO), which is the dominant pattern of winter atmospheric variability. To further address this issue, the global ocean general circulation model, Nucleus for European Modelling of the Ocean (NEMO), is driven using a 2,000 years long white noise forcing associated with the NAO. Focusing on key ocean circulation patterns, we show that the Atlantic Meridional Overturning Circulation (AMOC) and Sub-polar gyre (SPG) strength both have enhanced power at low frequencies but no dominant timescale, and thus provide no evidence for a oscillatory ocean-only mode of variability. Instead, both indices respond linearly to the NAO forcing, but with different response times. The variability of the AMOC at 30°N is strongly enhanced on timescales longer than 90 years, while that of the SPG strength starts increasing at 15 years. The different response characteristics are confirmed by constructing simple statistical models that show AMOC and SPG variability can be related to the NAO variability of the previous 53 and 10 winters, respectively. Alternatively, the AMOC and the SPG strength can be reconstructed with Auto-regressive (AR) models of order seven and five, respectively. Both statistical models reconstruct interannual and multidecadal AMOC variability well, while on the other hand, the AR(5) reconstruction of the SPG strength only captures multidecadal variability. Using these methods to reconstruct ocean variables can be useful for prediction and model intercomparision.

ORCA025: Performance Analysis on Scalar Architecture

We investigated seasonal variations of the upper ocean temperature and the mixed layer depth (MLD) in an eddy-permitting global ocean general circulation model (OGCM) to assess the OGCM perfermance. The OGCM is based on the GFDL MOM3 which has a horizontal resolution of 0.5 degree and 30 vertical levels. The OGCM was integrated for 68 years using a monthly-mean climatological wind stress forcing. The model sea surface temperature (SST) and sea surface salinity were restored to the Levitus climatology with a time scale of 30 days. Annual-mean model SST shows a cold bias <TEX>$(<\;-2^{\circ}C)$</TEX> in the summer hemisphere and a warm bias <TEX>$(>\;1^{\circ}C)$</TEX> in the winter hemisphere mainly due to the restoring boundary condition of temperature. The model MLD captures well the observed features in most areas, with a slightly deep bias. However, in the Ross Sea and Weddell Sea, the model shows significantly deeper MLD than the climatology-mainly due to weak salinity stratifications in the model. For amplitude of seasonal variation, the model SST is smaller <TEX>$(1{\sim}3^{\circ}C)$</TEX> than the observation largely due to the restoring surface boundary condition while the model MLD has larger seasonal variation <TEX>$({\sim}50m)$</TEX>. It is suggested that for more realistic simulation of the upper ocean structure in the present eddy-permitting ocean model, more refinements in the surface boundary condition for the thermohaline forcing and parameterization for vertical mixing are required, together with the incorporation of a sea-ice model.

10.1016/0967-0653(96)89845-0

The Chandler wobble is an excited resonance of the Earth's rotation having a period of about 14 months. Although it has been under investigation for more than a century, its excitation mechanism has remained elusive. Here, the angular momentum of the atmosphere computed from the products of a numerical weather prediction analysis system and the angular momentum of the oceans computed from a global oceanic general circulation model driven by observed surface winds and fluxes are used to show that during 1985.0–1996.0 the Chandler wobble was excited by a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being ocean‐bottom pressure fluctuations.