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lmd_LEGACY2001_abstracts.html

2001 .

(3 publications)

V. Ramanathan, P. J. Crutzen, J. Lelieveld, A. P. Mitra, D. Althausen, J. Anderson, M. O. Andreae, W. Cantrell, G. R. Cass, C. E. Chung, A. D. Clarke, J. A. Coakley, W. D. Collins, W. C. Conant, F. Dulac, J. Heintzenberg, A. J. Heymsfield, B. Holben, S. Howell, J. Hudson, A. Jayaraman, J. T. Kiehl, T. N. Krishnamurti, D. Lubin, G. McFarquhar, T. Novakov, J. A. Ogren, I. A. Podgorny, K. Prather, K. Priestley, J. M. Prospero, P. K. Quinn, K. Rajeev, P. Rasch, S. Rupert, R. Sadourny, S. K. Satheesh, G. E. Shaw, P. Sheridan, and F. P. J. Valero. Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze. Journal of Geophysical Research, 106:28371, November 2001. [ bib | DOI | ADS link ]

Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo-Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one- and four-dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long-range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single-scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo-Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (-204 W m-2) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.

M. Bonazzola, L. Picon, H. Laurent, F. Hourdin, G. SèZe, H. Pawlowska, and R. Sadourny. Retrieval of large-scale wind divergences from infrared Meteosat-5 brightness temperatures over the Indian Ocean. Journal of Geophysical Research, 106:28113, November 2001. [ bib | DOI | ADS link ]

Over the tropics the atmospheric general circulation models usually fail in predicting horizontal wind divergence, which is closely related to atmospheric heating and to the vertical exchanges associated with convection. With the aim of forcing atmospheric models we present here a reconstruction of wind divergences based on the links between infrared brightness temperatures, convective activity, and large-scale divergence. In practice, wind divergences are reconstructed from brightness temperatures using correlations obtained from numerical simulations performed with a general circulation model. When building those correlations, a distinction must be made between the brightness temperatures of opaque clouds and those of semitransparent clouds, only the former being directly associated with convection. In order to filter out semitransparent clouds we use radiative thresholds in the water vapor channel in addition to the window channel. We apply our approach to Meteosat-5 data over the Indian Ocean. Comparison with wind divergences reconstructed independently from Meteosat water vapor winds partially validates our retrieval. Comparison with European Center for Medium-Range Weather Forecasts analyses indicates that much can be gained by adding information on the wind divergence in the tropics to force an atmospheric model.

F. Codron, A. Vintzileos, and R. Sadourny. Influence of Mean State Changes on the Structure of ENSO in a Tropical Coupled GCM. Journal of Climate, 14:730-742, March 2001. [ bib | DOI | ADS link ]

This study examines the response of the climate simulated by the Institut Pierre Simon Laplace tropical Pacific coupled general circulation model to two changes in parameterization: an improved coupling scheme at the coast, and the introduction of a saturation mixing ratio limiter in the water vapor advection scheme, which improves the rainfall distribution over and around orography. The main effect of these modifications is the suppression of spurious upwelling off the South American coast in Northern Hemisphere summer. Coupled feedbacks then extend this warming over the whole basin in an El Niño-like structure, with a maximum at the equator and in the eastern part of the basin. The mean precipitation pattern widens and moves equatorward as the trade winds weaken.This warmer mean state leads to a doubling of the standard deviation of interannual SST anomalies, and to a longer ENSO period. The structure of the ENSO cycle also shifts from westward propagation in the original simulation to a standing oscillation. The simulation of El Niño thus improves when compared to recent observed events. The study of ENSO spatial structure and lagged correlations shows that these changes of El Niño characteristics are caused by both the increase of amplitude and the modification of the spatial structure of the wind stress response to SST anomalies.These results show that both the mean state and variability of the tropical ocean can be very sensitive to biases or forcings, even geographically localized. They may also give some insight into the mechanisms responsible for the changes in ENSO characteristics due to decadal variability or climate change.

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