Abstract: This study investigates the non linear processes by which the ocean diurnal variations can affect the intraseasonal sea surface temperature (SST) variability in the Atlantic Ocean. The CNRMOM1D 1-dimensional ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes over the 1959â2001 period with a 1-hour frequency in solar heat flux in a first simulation and with a daily forcing frequency in a second simulation. This model has a vertical resolution of 1m near the surface. The comparison between both experiments shows that the daily mean surface temperature is modified by about 0.3°C to 0.5°C if the ocean diurnal variations are represented and this correction can persist for 15 to 40 days in the midlatitudes and more than 60 days in the Tropics. The so-called ârectificationâ mechanism, by which the ocean diurnal warming enhances the intraseasonal SST variability by 20% to 40%, is found to be robust in the Tropics. In contrast, in the mid-latitudes, diurnal variations in wind stress and non solar heat flux are shown to affect the daily mean SST. For example, an intense wind stress or non solar heat flux toward the atmosphere during the first half of the day followed by weak fluxes during the second half result in a shallow mixed layer. The following day, this preconditioning results in heat being trapped near the surface and the daily mean surface temperature is higher than if these diurnal variations in surface forcings were not resolved.
Abstract:
We study the sensitivity of the LMDZ grid-point atmospheric GCM to changes in the resolution in latitude or in longitude, focusing on the mid-latitudes. 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 where the zonal-mean zonal wind is constrained towards a 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 mid-latitude dynamics hold when using the full GCM in a realistic setting, either forced by observed SSTs or coupled to an ocean model.
Abstract: This study aims at understanding the winter interactions between aerosols and weather regimes in the North-Atlantic European (NAE) region. As a first step, a six year simulation of sulphate, black carbon (BC) and dust is performed with a Chemical Transport Model (CTM), forced by the meteorological data issued from the European Centre for Medium-range Weather Forecast Integrated Forecast System (ECMWF IFS) model. The CTM uses the emissions inventory of aerosols and precursors gases provided by the âAERosol Comparison between Observations and Modelsâ (AEROCOM) project. In this experiment, atmospheric dynamical processes associated with the different regimes can impact by up to 25% the burden of sulphate and BC and by up to 80% the burden of mineral dust, through the modification of deposition, transport and chemistry processes. As a second step, the patterns of aerosol anomalies induced by each weather regime are used to force experiments performed with an Atmosphere General Circulation Model (AGCM). The mean persistence of the negative phase of the North Atlantic Oscillation (NAO-) and the Zonal regime are reduced by 1.78 and 0.88 days respectively when the atmosphere is forced with the patterns of aerosols induced by the Zonal regime and the Blocking regime respectively. This suggests that the interaction between the atmosphere and its aerosol concentration could destabilize the NAO- regime which occurs after a Zonal episode. Same conclusion can be set out for a Zonal regime which occurs after a Blocking episode.
Abstract: This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes with a 1-hour frequency in solar heat flux (6h hours for the other forcing fields) over the 1959-2001 period. The model has 124 vertical levels with a vertical resolution of 1m near the surface and 500m at the bottom. This ocean forced experiment is used to assess the impact of the North Atlantic weather regimes on the surface ocean. Composites of Sea Surface Temperature (SST) anomalies associated with each weather regime are computed and the mechanisms explaining these anomalies are investigated. Then, the SST anomalies related to each weather regime in the ocean-forced experiment are prescribed to the ARPEGE Atmosphere General Circulation Model. We show that the interaction with the surface ocean induces a positive feedback on the persistence of the Blocking regime, a negative feedback on the persistence of the NAO- regime and favours the transition from the Atlantic Ridge regime to the NAO- regime and from the Atlantic Low regime toward the Blocking regime.
Abstract: This study aims at understanding the winter marine surface/atmosphere interactions in the North Atlantic European (NAE) region on intraseasonal timescales. The CNRMOM1d ocean model coupled with the GELATO3 sea ice model is forced with the ERA40 surface fluxes over the 1959â2001 period. Composites of the simulated Sea Surface Temperature (SST) and sea ice concentration anomalies associated with each weather regime are computed. These are then prescribed to the ARPEGE Atmosphere General Circulation Model. We show that the interaction with the marine surface induces a negative feedback on the persistence of the NAOâ regime, favours the transition from the Zonal regime toward the Atlantic Ridge regime and destabilizes the transition from the Blocking regime toward the Atlantic Ridge regime.
Abstract: A preindustrial climate experiment was conducted with the third version of the CNRM global atmosphereâoceanâsea ice coupled model (CNRM-CM3) for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). This experiment is used to investigate the main physical processes involved in the variability of the North Atlantic ocean convection and the induced variability of the Atlantic meridional overturning circulation (MOC). Three ocean convection sites are simulated, in the Labrador, Irminger and GreenlandâIcelandâNorwegian (GIN) Seas in agreement with observations. A mechanism linking the variability of the Arctic sea ice cover and convection in the GIN Seas is highlighted. Contrary to previous suggested mechanisms, in CNRM-CM3 the latter is not modulated by the variability of freshwater export through Fram Strait. Instead, the variability of convection is mainly driven by the variability of the sea ice edge position in the Greenland Sea. In this area, the surface freshwater balance is dominated by the freshwater input due to the melting of sea ice. The ice edge position is modulated either by northwestward geostrophic current anomalies or by an intensification of northerly winds. In the model, stronger than average northerly winds force simultaneous intense convective events in the Irminger and GIN Seas. Convection interacts with the thermohaline circulation on timescales of 5â10 years, which translates into MOC anomalies propagating southward from the convection sites.
