An Improved Scheme for Interpolating between an Atmospheric Model and Underlying Surface Grids near Orography and Ocean Boundaries

ABSTRACT To take into account the strong nonlinearities of vertical fluxes due to small-scale heterogeneities of surface properties, more and more coupled general circulation models compute part of their atmospheric physical parameterizations, either the surface fluxes or the whole package, on the finer grid of their ocean or land model. A modification of a traditional interpolation scheme is presented to calculate the values of atmospheric variables over surface model grid points. In addition to the desirable properties of flux conservation and preservation of a constant field, the new scheme allows discontinuities in the interpolated fields at the surface model's boundaries and orographic jumps, while remaining continuous elsewhere. It can also be tuned separately for each variable. The modified scheme is then evaluated using the circulation model of the Laboratoire de Météorologie Dynamique coupled to the Laboratoire d'Océanographie Dynamique et de Climatologie tropical Pacific Ocean model using the delocalized physics method. The results show a large improvement of heat and humidity fluxes near the focus region of the South American coast in the southeastern equatorial Pacific, and a subsequent westward propagation of significant cold SST anomalies

The tropical rain belts with an annual cycle and a continent model intercomparison project: TRACMIP

This paper introduces the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project (TRACMIP). TRACMIP studies the dynamics of tropical rain belts and their response to past and future radiative forcings through simulations with 13 comprehensive and one simplified atmosphere models coupled to a slab ocean and driven by seasonally-varying insolation. Five idealised experiments, two with an aquaplanet setup and three with a setup with an idealized tropical continent, fill the space between prescribed-SST aquaplanet simulations and realistic simulations provided by CMIP5/6. The simulations reproduce key features of present-day climate and expected future climate change, including an annual-mean intertropical convergence zone (ITCZ) that is located north of the equator and Hadley cells and eddy-driven jets that are similar to present-day climate. Quadrupling CO2 leads to a northward ITCZ shift and preferential warming in Northern high-latitudes. The simulations show interesting CO2-induced changes in the seasonal excursion of the ITCZ and indicate a possible state-dependence of climate sensitivity. The inclusion of an idealized continent modulates both the control climate and the response to increased CO2; for example, it reduces the northward ITCZ shift associated with warming and, in some models, climate sensitivity. In response to eccentricity-driven orbital seasonal insolation changes, seasonal changes in oceanic rainfall are best characterized as a meridional dipole, while seasonal continental rainfall changes tend to be symmetric about the equator. This survey illustrates TRACMIP's potential to engender a deeper understanding of global and regional climate and to address questions on past and future climate

The extraordinary March 2022 East Antarctica "heat" wave. Part I: observations and meteorological drivers

peer reviewe

The extraordinary March 2022 East Antarctica "heat" wave. Part II: impacts on the Antarctic ice sheet

peer reviewe

How Future Circulation Changes are modified by the Coupling of Atmosphere and Ocean Energy Transports.

International audienceA number of studies have shown how in isolation the atmospheric circulation would balance changes in the top-of-atmosphere energy budget or ocean energy transport, leading to climate impacts such as shifts of the ITCZ or mid-latitude jets. The atmosphere and ocean circulation, and so their respective meridional energy transports, are however tightly coupled through the surface wind forcing of the ocean. We use here the simplest setup for including the effects of the ocean circulation : an aqua-planet with the atmosphere coupled to a 2-layer slab ocean in which the Ekman currents and eddy diffusion can be parameterized. We first show how the interactive ocean strongly damps in the Tropics the atmospheric response to a prescribed inter-hemispheric heat transport. We then apply the same method for a doubling of CO2 : the response of the total meridional energy transport remains weak in the Tropics, but with strong compensation between the atmosphere and ocean components. As a result, the classic circulation response - weaker and broader Hadley cell, poleward jet shift, precipitation changes - is significantly modified

Ekman heat transport for slab oceans

International audienceA series of schemes designed to include various representations of the Ekman-driven heat fluxes in slab-ocean models is introduced. They work by computing an Ekman mass flux, then deducing heat fluxes by the surface flow and an opposite deep return flow. The schemes differ by the computation of the return flow temperature: either diagnosed from the SST or given by an active second layer. Both schemes conserve energy, and use as few parameters as possible. Simulations in an aquaplanet setting show that the schemes reproduce well the structure of the meridional heat transport by the ocean. Compared to a diffusive slab-ocean, the simulated SST is more flat in the tropics, and presents a relative minimum at the equator, shifting the ITCZ into the summer hemisphere. In a realistic setting with continents, the slab model simulates correctly the mean state in many regions, especially in the tropics. The lack of other dynamical features, such as barotropic gyres, means that an optimal mean-state in regions such as the mid-latitudes will require additional flux corrections. © 2011 Springer-Verlag

13. Reproduction du climat actuel

Aujourd’hui, on demande non seulement aux modèles de reproduire correctement la circulation générale moyenne de l’atmosphère et de l’océan mais aussi de simuler les caractéristiques principales des variations climatiques observées. C’est une condition nécessaire pour espérer effectuer des prévisions de l’évolution du climat à court terme et en réponse à une perturbation, cette réponse ressemblant souvent à la « variabilité naturelle ». De façon plus fondamentale, un modèle simulant une variab..

Differing impacts of resolution changes in latitude and longitude on the midlatitudes in the LMDZ atmospheric GCM

International audienceThis article examines the sensitivity of the Laboratoire de Météorologie Dynamique Model with Zoom Capability (LMDZ), a gridpoint atmospheric GCM, to changes in the resolution in latitude and longitude, focusing on the midlatitudes. In a series of dynamical core experiments, increasing the resolution in latitude leads to a poleward shift of the jet, which also becomes less baroclinic, while the maximum eddy variance decreases. The distribution of the jet positions in time also becomes wider. On the contrary, when the resolution increases in longitude, the position and structure of the jet remain almost identical, except for a small equatorward shift tendency. An increase in eddy heat flux is compensated by a strengthening of the Ferrel cell. The source of these distinct behaviors is then explored in constrained experiments in which the zonal-mean zonal wind is constrained toward the same reference state while the resolution varies. While the low-level wave sources always increase with resolution in that case, there is also enhanced poleward propagation when increasing the resolution in longitude, preventing the jet shift. The diverse impacts on the midlatitude dynamics hold when using the full GCM in a realistic setting, either forced by observed SSTs or coupled to an ocean model. © 2011 American Meteorological Society