1,721,249 research outputs found
Combined effects of mesoscale processes and atmospheric high-frequency variability on the spring bloom in the MEDOC area
No full textA number of processes are proposed to explain the time and space variability of the onset and decay of the spring phytoplankton bloom. This is done in the modeling framework of a case study most representative of the northwestern Mediterranean Sea (MEDOC area). The strategy followed is to isolate the different possible sources of variability (oceanic mesoscale dynamics, spring warming, wind bursts) in a series of process experiments (no flux, warming and wind experiments). The analysis of these experiments provides information for the analysis of a more realistic experiment, forced with daily atmospheric data (high-frequency experiment). On the basis of this study, we propose a categorization of the processes that control the spring bloom, in terms of their impact on the onset and decay of the bloom, and of the time and space scales on which they apply
Warm and cold water routes of an O.G.C.M. thermohaline conveyor belt
No full textA global general circulation model analyzed with a Lagrangian methodology is used to describe and quantify the paths, transports, and characteristics of the “warm” waters forming the upper branch of the conveyor belt in the North Atlantic Ocean. The total transport for this branch turns out to be 17.8 Sv in the North Atlantic at 20°N: 11.8 Sv are composed of waters coming from the two classical origins, the Drake Passage and the Indonesian Throughflow, which contribute with 6.5 and 5.3 Sv respectively. The remaining 6 Sv find their origins partly in the passage between Antarctica and the Australian Continent (with 3.1 Sv) and partly in the Indo-Atlantic sector itself (i.e., with 2.9 Sv). The geographical structure of the different routes emphasizes the role of the Southern Ocean and large-scale current systems in water mass transformation and distribution
Salt conservation, free surface, and varying levels: a new formulation for ocean general circulation models
Full text linkIn order to clarify the link between ocean salt content (OSC) conservation and the freshwater flux formulation in ocean general circulation models (OGCMs), a varying level thickness, nonlinear free surface version of the OPA is presented. Linear/nonlinear free surface equations are solved using an original approach based on an explicit damping of fast external gravity waves. The method leaves both the potential vorticity equation and the equilibrium state unchanged. Its numerics and cost are quite similar to those of implicit schemes. When nonlinearities are kept, a variable first level thickness is required. Its discretization is determined by volume and energy constraints. The OSC conservation depends on the surface kinematic equation used. Four formulations are presented: (1) virtual salt flux (fixed ocean volume and no volume flux), (2) natural (fixed ocean volume and volume flux), (3) linear free surface (fixed volume and volume flux computed from a linear free surface equation), and (4) assumption free (variable volume computed from a nonlinear free surface equation). Their impact is illustrated in 25 year low-resolution global OGCM simulations. In all cases the first-order ocean response is quite similar, as the concentration-dilution effect always exists. Formulations 4 and 2 ensure a strict conservation of the OSC. Nevertheless, the difference in formulation 3 is not strong enough to play a significant role: the conservation is almost perfect. Only formulation 1 neglects fresh water-driven surface pumping. This mainly modifies the sea surface salinity of the ocean basin where river runoffs are strong. No significant difference is found between the other formulations as a large time step dampens high-frequency free surface motion. The best compromise for climate is the linear free surface formulation. It allows a nearly exact OSC conservation, introduces the fresh water-driven pumping, and runs faster than all the other formulations
Southern Ocean transformation in a coupled model with and without eddy mass fluxes
No full textA coupled air–sea general circulation model is used to simulate the global circulation. Different parameterizations of lateral mixing in the ocean by eddies, horizontal, isopycnal, and isopycnal plus eddy advective flux, are compared from the perspective of water mass transformation in the Southern Ocean. The different mixing physics imply different buoyancy equilibria in the surface mixed layer, different transformations, and therefore a variety of meridional overturning streamfunctions. The coupled-model approach avoids strong artificial water mass transformation associated with relaxation to prescribed mixed layer conditions. Instead, transformation results from the more physical non-local, nonlinear interdependence of sea-surface temperature, air–sea fluxes, and circulation in the model’s atmosphere and ocean. The development of a stronger mid-depth circulation cell and associated upwelling when eddy fluxes are present, is examined. The strength of overturning is diagnosed in density coordinates using the transformation framework
Choice of an advection scheme for biogeochemical models
No full textFive advection schemes are compared and evaluated in the context of biogeochemical modeling. Using three schemes of comparable quality that have been used in recent biogeochemical models, we found that new production estimates vary by as much as 30%. Test experiments are presented that explain the large discrepancies in terms of the different types of numerical errors inherent to each scheme. One scheme is suggested for eddy-resolving models and another one for coarse resolution models
Response to comment on “Oceanic Rossby Waves Acting as a ‘Hay Rake’ for Ecosystem Floating By-Products"
No full textGlobal estimates of internal tide generation rates at 1/30º resolution
No full textThe dataset contains global estimates of barotropic-to-baroclinic tidal energy conversion at 1/30-degree resolution. Three types of estimates are available: 1. Non-modal conversion rates calculated by Falahat et al. (2014) following the method of Nycander (2005). A map for each of the eight most energetic tidal constituents (M2, S2, K1, O1, N2, K2, P1, Q1) is provided. 2. Mode-by-mode conversion rates calculated by Falahat et al. (2014). A map for each of the three most energetic tidal constituents (M2, S2, K1) and each of vertical normal modes 1, 2, 3, 4, 5 and 6-10 is provided. 3. Mode-by-mode conversion rates calculated by Falahat et al. (2014), to which an ad hoc correction to eliminate negative conversion rates has been applied (following de Lavergne et al., 2019). The correction preserves basin-integrated, depth-dependent conversion rates. A map for each of the three most energetic tidal constituents (M2, S2, K1) and each of vertical normal modes 1, 2, 3, 4, 5 and 6-10 is provided. All maps were computed using the WOCE global climatology of stratification, the ETOPO2v2 bathymetry product and the TXO6.2 atlas of barotropic tidal velocities. Detailed methods and documentation can be found in the following publications: Nycander, J. Generation of internal waves in the deep ocean by tides. Journal of Geophysical Research 110, C10028 (2005). doi:10.1029/2004JC002487 Falahat, S., Nycander, J., Roquet, F., Moundheur, Z. Global calculation of tidal energy conversion into vertical normal modes. Journal of Physical Oceanography 44, 3225-3244 (2014). doi:10.1175/JPO-D-14-0002.1 de Lavergne, C., Falahat, S., Madec, G., Roquet, F., Nycander, J., Vic, C. Toward global maps of internal tide energy sinks. Ocean Modelling,137, 52-75 (2019). doi:10.1016/j.ocemod.2019.03.010.</span
Global maps of internal tide generation and dissipation
No full textThe dataset consists of global two-dimensional maps of internal tide energy sources and sinks, at half-degree horizontal resolution. Estimated energy sources are provided for the three most energetic tidal constituents: M2, S2 and K1. They are decomposed into vertical normal modes. Units are Watts per square meter. Estimated energy sinks are provided for each of M2, S2 and K1 and for 'All constituents'. 'All constituents' is an extrapolation to the eight most energetic tidal constituents, obtained as a weighted sum of M2, S2 and K1 fields. Energy sinks are depth-integrated and decomposed into five process contributions: (i) local dissipation of high modes; (ii) dissipation of low modes via wave-wave interactions; (iii) dissipation of low modes via scattering by abyssal hills; (iv) dissipation of low modes via critical reflection; (v) dissipation of low modes via shoaling. Units are Watts per square meter. Methods and documentation can be found in the following publication: de Lavergne, C., Falahat, S., Madec, G., Roquet, F., Nycander, J., Vic, C. Toward global maps of internal tide energy sinks. Ocean Modelling, 137, 52-75 (2019). doi:10.1016/j.ocemod.2019.03.010. Provided maps of energy sinks correspond to the reference (REF) experiment described in the article.</span
Ocean circulation and sea-ice thinning induced by melting ice shelves in the Amundsen Sea
No full textA 1/12° ocean model configuration of the Amundsen Sea sector is developed to better understand the circulation induced by ice shelf melt and the impacts on the surrounding ocean and sea ice. Eighteen sensitivity experiments to drag and heat exchange coefficients at the ice shelf/ocean interface are performed. The total melt rate simulated in each cavity is function of the thermal Stanton number, and for a given thermal Stanton number, melt is slightly higher for lower values of the drag coefficient. Sub ice shelf melt induces a thermohaline circulation that pumps warm circumpolar deep water into the cavity. The related volume flux into a cavity is 100 to 500 times stronger than the melt volume flux itself. Ice-shelf melt also induces a coastal barotropic current that contributes 45±12% of the total simulated coastal transport. Due to the presence of warm circumpolar deep waters, the melt-induced inflow typically brings 4 to 20 times more heat into the cavities than the latent heat required for melt. For currently observed melt rates, approximately 6% to 31% of the heat that enters a cavity with melting potential is actually used to melt ice shelves. For increasing sub ice shelf melt rates, the transport in the cavity becomes stronger, and more heat is pumped from the deep layers to the upper part of the cavity then advected towards the ocean surface in front of the ice shelf. Therefore, more ice shelf melt induces less sea ice volume near the ice sheet margins. This article is protected by copyright. All rights reserved
- …
