Claire Levy

Sensitivity of an oceanic general circulation model forced by satellite wind stress fields

Satellite wind and wind stress fields at the sea surface, derived from the scatterometers on European Remote Sensing satellites 1 and 2 (ERS-1 and ERS-2) are used to drive the ocean general circulation model (OGCM) “OPA” in the tropical oceans. The results of the impact of ERS winds are discussed in terms of the resulting thermocline, current structures, and sea level anomalies. Their adequacy is evaluated on the one hand by comparison with simulations forced by the Arpege-Climat model and on the other hand by comparison with measurements of the Tropical Atmosphere-Ocean (TAO) buoy network and of the TOPEX/Poseidon altimeter. Regarding annual mean values, the thermal and current responses of the OGCM forced by ERS winds are in good agreement with the TAO buoy observations, especially in the central and eastern Pacific Ocean. In these regions the South Equatorial Current, the Equatorial Undercurrent, and the thermocline features simulated by the OGCM forced by scatterometer wind fields are described. The impact of the ERS-1 winds is particularly significant to the description of the main oceanic variability. Compared to the TAO buoy observations, the high-frequency (a few weeks) and the low-frequency of the thermocline and zonal current variations are described. The correlation coefficients between the time series of the thermocline simulated by ERS winds and that observed by the TAO buoy network are highly significant; their mean value is 0.73, over the whole basin width, while it is 0.58 between Arpege model simulation and buoy observations. At the equator the time series of the zonal current simulated by the ERS winds, at three locations (110°W, 140°W, and 165°E) and at two depths, are compared to the TAO current meter and acoustic Doppler current profiler (ADCP) measurements. The mean value of the significant correlation coefficients computed with the in situ measurements is 0.72 for ERS, while it is 0.51 for the Arpege-Climat model. Thus ERS wind fields through the OGCM generate more realistic current variations than those obtained with Arpege climate winds, and they are particularly efficient in capturing abrupt changes (“wind bursts”) which may be important regarding ocean dynamics.

Climatology and interannual variability simulated by the ARPEGE-OPA coupled model

A 10-year simulation with a coupled ocean-atmosphere general circulation model (CGCM) is presented. The model consists of the climate version of the Météo-France global forecasting model, ARPEGE, coupled to the LODYC oceanic model, OPA, by the CERFACS coupling package OASIS. The oceanic component is dynamically active over the tropical Pacific, while climatological time-dependent sea surface temperatures (SSTs) are prescribed outside of the Pacific domain. The coupled model shows little drift and exhibits a very regular seasonal cycle. The climatological mean state and seasonal cycle are well simulated by the coupled model. In particular, the oceanic surface current pattern is accurately depicted and the location and intensity of the Equatorial Undercurrent (EUC) are in good agreement with available data. The seasonal cycle of equatorial SSTs captures quite realistically the annual harmonic. Some deficiencies remain including a weak zonal equatorial SST gradient, underestimated wind stress over the Pacific equatorial band and an additional inter-tropical convergence zone (ITCZ) south of the equator in northern winter and spring. Weak interannual variability is present in the equatorial SST signal with a maximum amplitude of 0.5°C.

Long Equatorial Waves in a High-Resolution OGCM Simulation of the Tropical Pacific Ocean during the 1985–94 TOGA Period

Abstract A high-resolution oceanic general circulation model (OGCM) of the three tropical oceans is used to investigate long equatorial wave activity in the Pacific Ocean during the 1985–94 TOGA period. The ARPEGE atmospheric general circulation model simulated zonal wind stress forcing and the OPA OGCM simulated dynamic height are interpreted using techniques previously applied to data. Long equatorial waves of the first baroclinic mode (Kelvin and first-mode Rossby waves) are detected propagating in the model outputs during the entire period. A seasonal cycle and interannual anomalies are computed for each long equatorial wave. In the east Pacific basin, long equatorial wave coefficients are dominated by seasonal variations, while west of the date line they display strong interannual anomalies. Interannual long-wave anomalies are then compared to wave coefficients simulated by a simple wind-forced model. The results presented here indicate the major role played by wind forcing on interannual timescales ...

Simulations couplées globales des changements climatiques associés à une augmentation de la teneur atmosphérique en CO2

Abstract Two transient CO 2 experiments using two coupled general circulation models developed by the French GASTON group have been realized using the same methodology. No flux corrections at the air-sea interface were used in these experiments. The main features of the present climate are reasonably well captured by both coupled models in the control simulations, although the biases are not the same, The transient CO 2 simulations show a global warming, ranging between 1.6 and 2.0 °C at the time of CO 2 doubling (+ 70 years). These values, and the main geographical characteristics of climate change, are in agreement with previous studies published by other research groups, using either flux corrected or non-flux corrected models.

Satellite meteorology and atmospheric spectroscopy: Recent progress in Earth remote sensing from the satellites of the Tiros-N series

Abstract It is now well known that the three-dimensional structure (temperature, moisture, clouds, etc.…) of the atmosphere may be deduced from satellite measurements of the Earths radiance through a procedure called inversion of the radiative transfer equation. The excellent spatial and temporal cover offered by satellite platforms is at the basis of the growing impact of inversion methods, in particular physical, requiring a physical computation of atmospheric transmittances, due to their better accuracies. After having recalled the present status of spectral line data catalogues which are at the basis of transmittance modelling, two such models recently developed at LMD are rapidly described. The way we have approached the inversion problem is then presented and recent results concerning, the meteorological satellites of the TIROS-N series are discussed.

Relaxation processes in liquid and supercooled toluene by light scattering experiments

We report detailed light scattering experiments in toluene comprising measurements of shifts and linewidths of polarized (VV) and depolarized (VH) spectra in the temperature range from 30°C to the glass transition T g = - 156°C. The values of the longitudinal sound velocity and attenuation are analysed in association with the existing ultrasonic data. We find that both types of measurement support the prevalence of a vibrational relaxation process at high temperatures and a viscous one (structural+shear viscosity) at low temperatures. Viscous processes are analysed in the framework of current viscoelastic models. Transverse light scattering data do not agree with the corresponding quantities usually found in ultrasonic shear wave experiments. Although longitudinal properties look similar to those obtained in ultrasonic experiments, we find unusual features in the compressional component.

Seasonal Observations in the Western Indian Ocean

XBT and surface salinity observations which are part of the SINODE (Surface INdian Ocean Dynamics Experiment) program are presented to describe the variations in the surface layers of the western Indian ocean between Cape Guardafui and Reunion island. In the Somali area, the temperature structure presents a high variability related to a strong eddy structure; the surface salinities are a good indicator of the circulation. At the equator, during the two transition periods, after the two monsoons, when the eastward jet is present, the temperature structures are different: the thermocline is broader after the SW monsoon. South of the equator, the South Equatorial Current northern limit wanders between 6° S and 9° S.