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lmd_LEGACY1999_abstracts.html

1999 .

(4 publications)

C. Martineu, J.-Y. Caneill, and R. Sadourny. Potential Predictability of European Winters from the Analysis of Seasonal Simulations with an AGCM. Journal of Climate, 12:3033-3061, October 1999. [ bib | DOI | ADS link ]

The potential predictability of European winters on the seasonal scale is investigated with the cycle 5.3 version of the Laboratoire de Météorologie Dynamique general circulation model by analyzing the link between atmospheric low-frequency variability and oceanic temperature prescribed as boundary conditions. The word`potential' refers to the assumption that the SST is a priori known in the experiments, and to the use of a model to evaluate the real climate predictability. Eleven simulations of the 1971-92 winters have been performed with the model in SST-forced mode. The methodology used identifies atmospheric clusters by Ward clustering scheme, and atmospheric variability modes over Europe by matrix analysis of relationships between variables. Tropical Pacific surface temperature fluctuations play a prevailing role in the modulation of European variability:the model preferentially simulates negative phases of the North Atlantic Oscillation during El Niño episodes, and a high pressure pattern in western Europe during La Niña ones. These two situations are associated with modulations in the structure of the North Atlantic jet and of the North Atlantic storm track, in agreement with data analyses synthesized in the literature. They confirm the prevailing role of interactions between different scales of the flow in the maintenance of persistent anomalies in the North Atlantic/European area. The strong link simulated by the model between the Pacific-North American oscillation and the North Atlantic Oscillation plays an important role in the propagation of the impact of the forcing from the tropical Pacific to the North Atlantic.For some winters (1971, 1984, 1989, and 1992), the number of simulations has been increased to 30. The normality of the simulated 1984 winter suggests a weak role of the tropical Atlantic in specifying climate anomalies in Europe. The differences in strength of the European response between the 1971 and 1989 La Niña events are linked to differences in the Pacific/North American area. A stronger spread is found in the El Niño case (1992 winter) than in the two La Niña cases. The sensitivity of the response to the number of realizations demonstrates that one has to reach about 15 simulations to obtain a significant response over Europe.

H. Teitelbaum, M. Moustaoui, R. Sadourny, and F. Lott. Critical levels and mixing layers induced by convectively generated gravity waves during CEPEX. Quarterly Journal of the Royal Meteorological Society, 125:1715-1734, July 1999. [ bib | DOI | ADS link ]

A. Vintzileos, P. Delecluse, and R. Sadourny. On the mechanisms in a tropical ocean-global atmosphere coupled general circulation model. Part II: interannual variability and its relation to the seasonal cycle. Climate Dynamics, 15:63-80, 1999. [ bib | DOI | ADS link ]

The thirty year simulation of the coupled global atmosphere-tropical Pacific Ocean general circulation model of the Laboratoire de Métérologie Dynamique and the Laboratoire d'Océanographie Dynamique et de Climatologie presented in Part I is further investigated in order to understand the mechanisms of interannual variability. The model does simulate interannual events with ENSO characteristics; the dominant periodicity is quasi-biennial, though strong events are separated by four year intervals. The mechanism that is responsible for seasonal oscillations, identified in Part I, is also active in interannual variability with the difference that now the Western Pacific is dynamically involved. A warm interannual phase is associated with an equatorward shift of the ITCZ in the Western and Central Pacific. The coupling between the ITCZ and the ocean circulation is then responsible for the cooling of the equatorial subsurface by the draining mechanism. Cold subsurface temperature anomalies then propagate eastward along the mean equatorial thermocline. Upon reaching the Eastern Pacific where the mean thermocline is shallow, cold subsurface anomalies affect surface temperatures and reverse the phase of the oscillation. The preferred season for efficient eastward propagation of thermocline depth temperature anomalies is boreal autumn, when draining of equatorial waters towards higher latitudes is weaker than in spring by a factor of six. In that way, the annual cycle acts as a dam that synchronizes lower frequency oscillations.

A. Vintzileos, P. Delecluse, and R. Sadourny. On the mechanisms in a tropical ocean-global atmosphere coupled general circulation model. Part I: mean state and the seasonal cycle. Climate Dynamics, 15:43-62, 1999. [ bib | DOI | ADS link ]

The mechanisms responsible for the mean state and the seasonal and interannual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global atmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest. Departures from the mean state are characterized by oscillations with dominant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evolved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Convergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the latitude of organized convention. In turn, the stronger oceanic subsidence reinforces equatorward convergence of water masses at the thermocline depth which, being not balanced by equatorial upwelling, deepens the equatorial thermocline. An equivalent view is that the deepening of the thermocline proceeds from the weakening of the meridional draining of near-surface equatorial waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appear at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacific eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. The whole oscillation is synchronized by the apparent meridional motion of the sun, through the seasonal oscillation of the ITCZ. This possible mechanism is partly supported by the observed seasonal reversal of vorticity between the equator and the ITCZ, and by observational evidence of eastward propagating subsurface temperature anomalies at the thermocline level.

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