INTRODUCTION


Like black velvet, the dark ocean surface provides an ideal backdrop for the viewing of atmospheric dust, smoke, and haze from space. When illuminated by the sun, aerosols backscatter a fraction of the radiation and this signal is detectable by satellites (1., .2). This simple remote sensing principle allows the routine global-scale monitoring of atmospheric aerosols over the oceans.

Recognizing the importance of atmospheric aerosols for global climate and biogeochemistry, National Oceanic and Atmospheric Administration (NOAA) has implemented an operational program to routinely derive and archive aerosol backscattering estimates from polar-orbiting meteorological satellites (3). As a result, it is now possible to begin constructing a comprehensive global aerosol climatology which could benefit many diverse research areas such as aeolean dust transport (4), biomass burning (5), biogeochemical coupling of the atmosphere and oceans (6), and the anthropogenic influences on these cycles (7). Aerosols also interfere with the remote sensing of land features (8), ocean surface temperature and chlorophyll concentration (9) as well as with the detection of most atmospheric constituents (10). A recurring major concern is the effect of aerosols on the climate through their direct (11) and indirect (12) perturbation of the radiative energy field. In this capacity, the global aerosol mix, particularly the trends of anthropogenic aerosols have been implicated in counteracting the anticipated warming from greenhouse gases (13). Such a hypothesis is plausible since brightly reflecting, 1,000 km-scale aerosol plumes originating from urban-industrial regions have been repeatedly observed through satellites (14) and the anthropogenic industrial aerosol levels have increased markedly during the past 40 years, in accordance with the sulfur emission trends (15). However, the verification of these aerosol-climate linkages and the investigation of the other aerosol influences is hampered by the lack of a consistent global scale aerosol data set.

This work presents a summary of the global oceanic aerosol pattern detected by polar-orbiting satellites between July 1989 and June 1991 (16). The results are presented in four seasonal maps (Figures 1a-d) and regionally aggregated seasonal time charts (Figures 2a-f).



Figures 1a-d: Equivalent aerosol optical depth (x 1000) over the oceans derived routinely from operational meteorological satellites.



Figures 2a-f: Seasonal variation of equivalent optical depth (x 1000) aggregated over characteristic oceanic regions. In 2e, the southern hemispheric signal was shifted by six months to show the north-south difference in seasonality.

The magnitude of the backscattered columnar aerosol signal is expressed as radiatively equivalent vertical optical depth (17). A source of confidence in the presented oceanic aerosol pattern is the favorable spatial and seasonal correspondence with the haze climatology derived from ship observations (18). However, the expectations from the data presented here need to be tempered by the absence of data over land and by the semi-quantitative nature (17) of these measurements. Sensor limitations (19), interferences (20), and instrumental drift (21) will require significant further consideration.


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