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1997 .

(4 publications)

S. Bony, K.-M. Lau, and Y. C. Sud. Sea Surface Temperature and Large-Scale Circulation Influences on Tropical Greenhouse Effect and Cloud Radiative Forcing. Journal of Climate, 10:2055-2077, August 1997. [ bib | DOI | ADS link ]

Two independent sets of meteorological reanalyses are used to investigate relationships between the tropical sea surface temperature (SST) and the large-scale vertical motion of the atmosphere for spatial and seasonal variations, as well as for El Niño/La Niña episodes of 1987-88. Supergreenhouse effect (SGE) situations are found to be linked to the occurrence of enhanced large-scale rising motion associated with increasing SST. In regions where the large-scale atmospheric motion is largely decoupled from the local SST due to internal or remote forcings, the SGE occurrence is weak. On seasonal and interannual timescales, such regions are found mainly over equatorial regions of the Indian Ocean and western Pacific, especially for SSTs exceeding 29.5degC. In these regions, the activation of feedback processes that regulate the ocean temperature is thus likely to be more related to the large-scale remote processes, such as those that govern the monsoon circulations and the low-frequency variability of the atmosphere, than to the local SST change.The relationships among SST, clouds, and cloud radiative forcing inferred from satellite observations are also investigated. In large-scale subsidence regimes, regardless of the SST range, the cloudiness, the cloud optical thickness, and the shortwave cloud forcing decrease with increasing SST. In convective regions maintained by the large-scale circulation, the strong dependence of both the longwave (LW) and shortwave (SW) cloud forcing on SST mainly results from changes in the large-scale vertical motion accompanying the SST changes. Indeed, for a given large-scale rising motion, the cloud optical thickness decreases with SST, and the SW cloud forcing remains essentially unaffected by SST changes. However, the LW cloud forcing still increases with SST because the detrainment height of deep convection, and thus the cloud-top altitude, tend to increase with SST. The dependence of the net cloud radiative forcing on SST may thus provide a larger positive climate feedback when the ocean warming is associated with weak large-scale circulation changes than during seasonal or El Niño variations.

R. D. Cess, M. H. Zhang, G. L. Potter, V. Alekseev, H. W. Barker, S. Bony, R. A. Colman, D. A. Dazlich, A. D. Del Genio, M. DéQué, M. R. Dix, V. Dymnikov, M. Esch, L. D. Fowler, J. R. Fraser, V. Galin, W. L. Gates, J. J. Hack, W. J. Ingram, J. T. Kiehl, Y. Kim, H. Le Treut, X.-Z. Liang, B. J. McAvaney, V. P. Meleshko, J. J. Morcrette, D. A. Randall, E. Roeckner, M. E. Schlesinger, P. V. Sporyshev, K. E. Taylor, B. Timbal, E. M. Volodin, W. Wang, W. C. Wang, and R. T. Wetherald. Comparison of the seasonal change in cloud-radiative forcing from atmospheric general circulation models and satellite observations. Journal of Geophysical Research, 102:16593, July 1997. [ bib | DOI | ADS link ]

We compare seasonal changes in cloud-radiative forcing (CRF) at the top of the atmosphere from 18 atmospheric general circulation models, and observations from the Earth Radiation Budget Experiment (ERBE). To enhance the CRF signal and suppress interannual variability, we consider only zonal mean quantities for which the extreme months (January and July), as well as the northern and southern hemispheres, have been differenced. Since seasonal variations of the shortwave component of CRF are caused by seasonal changes in both cloudiness and solar irradiance, the latter was removed. In the ERBE data, seasonal changes in CRF are driven primarily by changes in cloud amount. The same conclusion applies to the models. The shortwave component of seasonal CRF is a measure of changes in cloud amount at all altitudes, while the longwave component is more a measure of upper level clouds. Thus important insights into seasonal cloud amount variations of the models have been obtained by comparing both components, as generated by the models, with the satellite data. For example, in 10 of the 18 models the seasonal oscillations of zonal cloud patterns extend too far poleward by one latitudinal grid.

S. Bony, Y. Sud, K. M. Lau, J. Susskind, and S. Saha. Comparison and Satellite Assessment of NASA/DAO and NCEP-NCAR Reanalyses over Tropical Ocean: Atmospheric Hydrology and Radiation. Journal of Climate, 10:1441-1462, June 1997. [ bib | DOI | ADS link ]

This study compares the atmospheric reanalyses that have been produced independently at the Data Assimilation Office (DAO) of Goddard Laboratory for Atmospheres and at the National Centers for Environmental Prediction (NCEP). These reanalyses were produced by using a frozen state-of-the-art version of the global data assimilation system developed at these two centers. For the period 1987-88 and for the tropical oceanic regions of 30degS-30degN, surface and atmospheric fields related to atmospheric hydrology and radiation are compared and assessed, wherever possible, with satellite data. Some common biases as well as discrepancies between the two independent reassimilation products are highlighted.Considering both annual averages and interannual variability (1987-88), discrepancies between DAO and NCEP reanalysis in water vapor, precipitation, and clear-sky longwave radiation at the top of the atmosphere are generally smaller than discrepancies that exist between corresponding satellite estimates. Among common biases identified in the reanalyses, the authors note an underestimation of the total precipitable water and an overestimation of the shortwave cloud radiative forcing in warm convective regions. Both lead to an underestimation of the surface radiation budget. The authors also note an overestimaton of the clear-sky outgoing longwave radiation in most tropical ocean regions, as well as an overestimation of the longwave radiative cooling at the ocean surface.Surface latent and sensible heat fluxes differ by about 20 and 3 W m2, respectively, in the two reanalyses. Differences in the surface radiation budget are larger than the uncertainties of satellite-based estimates. Biases in the surface radiation fluxes derived from the reanalyses are primarily due to incorrect shortwave cloud radiative forcing and, to a lesser degree, due to a deficit in the total precipitable water and a cold bias at lower-tropospheric temperatures.This study suggests that individual features and biases of each set of reanalyses should be carefully studied, especially when using analyzed surface fluxes to force other physical or geophysical models such as ocean circulation models. Over large regions of the tropical oceans, DAO and NCEP reanalyses produce surface net heat fluxes that can differ by up to 50 W m2 in the average and by a factor of 2 when considering interannual anomalies. This may lead to vastly different thermal forcings for driving ocean circulations.

K.-M. Lau, H.-T. Wu, and S. Bony. The Role of Large-Scale Atmospheric Circulation in the Relationship between Tropical Convection and Sea Surface Temperature. Journal of Climate, 10:381-392, March 1997. [ bib | DOI | ADS link ]

In this paper, the authors study the influence of the large-scale atmospheric circulation on the relationship between sea surface temperature (SST) and tropical convection inferred from outgoing longwave radiation (OLR). They find that under subsidence and clear sky conditions there is an increase in OLR with respect to SST at a rate of 1.8-2.5 Wm2 (degC)1. In regions of large-scale ascending motions, which is correlated to, but not always collocated with, regions of warm water, there is a large reduction of OLR with respect to SST associated with increase in deep convection. The rate of OLR reduction is found to be a strong function of the large-scale motion field. The authors find an intrinsic OLR sensitivity to SST of approximately 4 to 5 Wm2 (degC)1 in the SST range of 27deg-28degC, under conditions of weak large-scale circulation. Under the influence of strong ascending motion, the rate can be increased to 15 to 20 Wm2 (degC)1 for the same SST range. The above OLR-SST relationships are strongly dependent on geographic locations. On the other hand, deep convection and large-scale circulation exhibit a nearly linear relationship that is less dependent on SST and geographic locations.The above results are supported by regression analyses. In addition, they find that on interannual timescales, the relationship between OLR and SST is dominated by the large-scale circulation and SST changes associated with the El Niño-Southern Oscillation. The relationship between anomalous convection and local SST is generally weak everywhere except in the equatorial central Pacific, where large-scale circulation and local SST appear to work together to produce the observed OLR-SST sensitivity. Over the equatorial central Pacific, approximately 45%-55% of the OLR variance can be explained by the large-scale circulation and 15%-20% by the local SST.Their results also show that there is no fundamental microphysical or thermodynamical significance to the so-called SST threshold at approximately 27degC, except that it represents a transitional SST between clear-sky/subsiding and convective/ascending atmospheric conditions. Depending on the ambient large-scale motion associated with basin-scale SST distribution, this transitional SST can occur in a range from 25.5deg to 28degC. Similarly, there is no magic to the 29.5degC SST, beyond which convection appears to decrease with SST. The authors find that under the influence of strong large-scale rising motion, convection does not decrease but increases monotonically with SST even at SST higher than 29.5degC. The reduction in convection is likely to be influenced by large-scale subsidence forced by nearby or remotely generated deep convection.

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