Publications

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2016
Xu, Y, Ramanathan V, Washington WM.  2016.  Observed high-altitude warming and snow cover retreat over Tibet and the Himalayas enhanced by black carbon aerosols. Atmospheric Chemistry and Physics. 16:1303-1315.   10.5194/acp-16-1303-2016   AbstractWebsite

Himalayan mountain glaciers and the snowpack over the Tibetan Plateau provide the headwater of several major rivers in Asia. In situ observations of snow cover extent since the 1960s suggest that the snowpack in the region have retreated significantly, accompanied by a surface warming of 2-2.5 degrees C observed over the peak altitudes (5000 m). Using a high-resolution ocean-atmosphere global climate model and an observationally constrained black carbon (BC) aerosol forcing, we attribute the observed altitude dependence of the warming trends as well as the spatial pattern of reductions in snow depths and snow cover extent to various anthropogenic factors. At the Tibetan Plateau altitudes, the increase in atmospheric CO2 concentration exerted a warming of 1.7 degrees C, BC 1.3 degrees C where as cooling aerosols cause about 0.7 degrees C cooling, bringing the net simulated warming consistent with the anomalously large observed warming. We therefore conclude that BC together with CO2 has contributed to the snow retreat trends. In particular, BC increase is the major factor in the strong elevation dependence of the observed surface warming. The atmospheric warming by BC as well as its surface darkening of snow is coupled with the positive snow albedo feedbacks to account for the disproportionately large role of BC in high-elevation regions. These findings reveal that BC impact needs to be properly accounted for in future regional climate projections, in particular on high-altitude cryosphere.

2012
Auffhammer, M, Ramanathan V, Vincent JR.  2012.  Climate change, the monsoon, and rice yield in India. Climatic Change. 111:411-424.   10.1007/s10584-011-0208-4   AbstractWebsite

Recent research indicates that monsoon rainfall became less frequent but more intense in India during the latter half of the Twentieth Century, thus increasing the risk of drought and flood damage to the country's wet-season (kharif) rice crop. Our statistical analysis of state-level Indian data confirms that drought and extreme rainfall negatively affected rice yield (harvest per hectare) in predominantly rainfed areas during 1966-2002, with drought having a much greater impact than extreme rainfall. Using Monte Carlo simulation, we find that yield would have been 1.7% higher on average if monsoon characteristics, especially drought frequency, had not changed since 1960. Yield would have received an additional boost of nearly 4% if two other meteorological changes (warmer nights and lower rainfall at the end of the growing season) had not occurred. In combination, these changes would have increased cumulative harvest during 1966-2002 by an amount equivalent to about a fifth of the increase caused by improvements in farming technology. Climate change has evidently already negatively affected India's hundreds of millions of rice producers and consumers.

2003
Tian, BJ, Ramanathan V.  2003.  A simple moist tropical atmosphere model: The role of cloud radiative forcing. Journal of Climate. 16:2086-2092.   10.1175/1520-0442(2003)016<2086:asmtam>2.0.co;2   AbstractWebsite

A simple moist model for the large-scale tropical atmospheric circulation is constructed by combining the simple models of Gill and Neelin and Held. The model describes the first baroclinic mode of the moist troposphere with variable "gross moist stability'' in response to given thermodynamic forcing from surface evaporation and atmospheric cloud radiative forcing (CRF), which is a measure of the radiative effects of clouds in the atmospheric radiative heating. When the present model is forced solely by the observed atmospheric CRF, quantitatively reasonable Hadley and Walker circulations are obtained, such as the trades, the ascending branches in the intertropical convergence zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), as well as the descending branches in the cold tongue and subtropics. However, when the model is forced only by the observed surface evaporation, the Walker circulation totally disappears, and the Hadley circulation reverses. These results indicate that, in the context of a moist dynamic model, the spatial variations of atmospheric CRF are more important in terms of driving and maintaining the Hadley and Walker circulations than the spatial variation of surface evaporation.

Chung, CE, Ramanathan V.  2003.  South Asian haze forcing: Remote impacts with implications to ENSO and AO. Journal of Climate. 16:1791-1806.   10.1175/1520-0442(2003)016<1791:sahfri>2.0.co;2   AbstractWebsite

Aerosols are regionally concentrated and are subject to large temporal variations, even on interannual time-scales. In this study, the focus is on the observed large interannual variability of the South Asian (SA) haze, estimating the corresponding variations in its radiative forcing, and using a general circulation model to study their impacts on global climate variability. The SA haze is a widespread haze, covering most of South Asia and the northern Indian Ocean during December-April. The southernmost extent of the haze varies year to year from about 10degreesS to about 5degreesN. In order to understand the impact of this interannual variation in the haze forcing, two numerical studies were conducted with two extreme locations of the forcing: 1) extended haze forcing (EHF) and 2) shrunk haze forcing (SHF). The former has the forcing extending to 10degreesS, while the latter is confined to regions north of the equator. Each of the two haze forcing simulations was implemented into a 3D global climate model (NCAR CCM3) with a prescribed SST seasonal cycle to estimate the sensitivity of the model climate to the aerosol forcing area. In both simulations, the haze forcing was prescribed only during the dry season between November and April. Over India where the forcing is centered, the simulated climate changes are very similar between EHF and SHF. In remote regions, however, the responses differ remarkably. Focusing on the remote effects of the haze, it is shown that some of the recent observed boreal-wintertime changes of the southwest Asian monsoon, El Nino-Southern Oscillation (ENSO), and the Arctic Oscillation (AO) could be explained by the SA haze forcing and its fluctuation. First, both simulations reveal the wintertime drought over southwest Asia, with the EHF generating far more severe drought. Second, the EHF experiment simulates a poleward shift of the Northern Hemisphere (NH) zonal-mean zonal momentum during the winter season, while the SHF effect rather moves the NH extratropical zonal momentum only slightly equatorward. Thus, the interannual fluctuations in the extension of the haze forcing area can explain the recently documented increased variability of the AO. Third, the EHF significantly suppresses the convection in the western equatorial Pacific during the boreal wintertime, and the SHF leads to much less suppression. Since the western Pacific convection suppression would weaken the trade winds over the Pacific and induce warm anomalies in the eastern basin, it is proposed that the SA haze may be partially responsible for the observed El Nino-like warming during the recent decades. When the convection suppression in the EHF experiment is imposed in the Cane-Zebiak Pacific ocean-atmosphere model, the coupled model actually simulates a warm bias similar to the observed El Nino trends of the recent decades. These findings have to be verified with a fully coupled ocean-atmosphere climate model.

Crutzen, PJ, Ramanathan V.  2003.  The parasol effect on climate. Science. 302:1679-1681.   10.1126/science.302.5651.1679   AbstractWebsite
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2002
Krishnan, R, Ramanathan V.  2002.  Evidence of surface cooling from absorbing aerosols. Geophysical Research Letters. 29   10.1029/2002gl014687   AbstractWebsite

[1] Anthropogenic emissions over the Asian region have grown rapidly with increase in population and industrialization. Air-pollutants from this region lead to a brownish haze over most of the North Indian Ocean and South Asia during winter and spring. The haze, with as much as 10-15% of black carbon (by mass), is known to reduce the surface solar insolation by about 10% (-15 Wm(-2)) and nearly double the lower atmospheric solar heating. Here we present an analysis of observed surface-temperature variations over the Indian subcontinent, which filters out effects of greenhouse gases and natural variability. The analysis reveals that the absorbing aerosols have led to a statistically significant cooling of about 0.3degreesC since the 1970s. The seasonally asymmetric cooling, which is consistent with the seasonality of the South Asian aerosol forcing, raises the new possibility that the surface cooling underneath the polluted regions, is balanced by warming elsewhere.

1997
Inamdar, AK, Ramanathan V.  1997.  On monitoring the atmospheric greenhouse effect from space. Tellus Series B-Chemical and Physical Meteorology. 49:216-230.   10.1034/j.1600-0889.49.issue2.8.x   AbstractWebsite

We propose an analysis technique for monitoring the planetary greenhouse effect from space. 2 quantities are used as a measure of the atmospheric greenhouse effect: (1) G(a), which is the reduction in the clear sky outgoing longwave radiation (OLR) due to the atmosphere; it is the radiative heating of the surface-atmosphere column; (2) G(a)(*) which is the back radiation from the atmosphere to the surface; it is the radiative heating of the surface by the atmosphere. G(a) is obtained from satellite observations of OLR and surface temperature. Here, we develop a technique to obtain G(a)(*) from a combination of OLR (broadband and window channel), surface temperature, column water vapor amount and near-surface air temperature. The difference, G(a)-G(a)(*), yields the net radiative cooling of the atmospheric column. While the technique is applicable for any radiometric observations, it is demonstrated here for the proposed cloud and the earth's radiant energy systems (CERES). CERES will have in addition to the broadband channel (4 to 200 mu m), a channel in the atmospheric window (8-12 mu m) to provide continuous radiation flux data. The potential benefits of the window channel in estimating the surface downward longwave flux, as also in addressing the water vapor feedback and continuum-related studies are explored in this paper. The success of the method in unravelling the water vapor - radiative interactions is illustrated by 2 case studies using ship and satellite data for the 1985 to 1990 period: (a) the seasonal and latitudinal variation in the northern tropical oceans; (b) seasonal to inter-annual variations averaged over the entire tropical(30 degrees N to 30 degrees S) pacific ocean. In both instances, the column greenhouse effect (G(a)) varies significantly on seasonal and inter annual time scales and is larger in the warmer months. Furthermore, variations in G(a)(*) exceed those in G(a), such that the atmospheric cooling increases in the warmer, moist atmosphere. The water vapor continuum plays a significant role in the tropical variations of G(a)(*) and tropospheric radiative cooling.