Meridional distribution of aerosol optical thickness (AOT) over the ocean was analyzed by using the 8‐year MISR and MODIS‐Terra data sets from March 2000 to February 2008, as well as the 5‐year MODIS‐Aqua data set from July 2002 to June 2007. The three satellite sensors show that there was a pronounced meridional aerosol asymmetry. It was found that there were strong seasonal variations in the meridional aerosol asymmetry: it was most pronounced in the April–July months. There was no noticeable asymmetry during the season from September to December. The Northern Hemisphere, where the main sources of natural and anthropogenic aerosols are located, contributed to the formation of noticeable aerosol asymmetry. During the season of pronounced hemispheric aerosol asymmetry, an increase in AOT was observed over the Northern Hemisphere, while a decrease in AOT was observed over the Southern Hemisphere. At midlatitudes in the Northern Hemisphere (30–60°N), the main contribution to seasonal variations of AOT over the ocean was made by Pacific Ocean aerosols. At low latitudes in the Northern Hemisphere (0–30°N), aerosols over the Atlantic Ocean contributed to seasonal variations of AOT more significantly than aerosols over the Pacific Ocean. During the 8‐year period under consideration, the brightening phenomenon, detected over the land, was not observed over the ocean at midlatitudes 30–60°N in cloudless conditions.
Moderate Resolution Imaging Spectroradiometer (MODIS) global monthly data from the Terra satellite (MOD08_M3, Collection 4, from March 2000 to May 2006) indicated, with the exception of the tropics, declining trends in aerosol optical thickness (AOD) over much of the globe, in contrast to slightly increasing trends in cloud optical thickness (COT) at many latitudes. In the tropics, increasing AOD trends coincide with increasing COT trends. In the latitudinal distribution of COT, in the Northern Hemisphere, a transition from increasing to declining tendencies was observed between 40°N and 60°N. There is a pronounced hemispheric asymmetry in latitudinal variations of the averaged total AOD, in contrast to those of the averaged total COT.
<p><strong>Abstract.</strong> Previous studies showed that, over the global ocean, there is hemispheric asymmetry in aerosols and no noticeable asymmetry in cloud fraction (CF). In the current study, we focus on the tropical Atlantic (30° N–30° S) which is characterized by significant amounts of Saharan dust dominating other aerosol species over the North Atlantic. Over a limited area such as the tropical Atlantic, our study showed that strong meridional asymmetry in dust aerosols was accompanied by meridional CF asymmetry, by contrast to the global ocean. During the 10 yr study period (July 2002–June 2012), NASA Aerosol Reanalysis (aka MERRAero) showed that, when the meridional asymmetry in dust aerosol optical thickness (AOT) was the most pronounced (particularly in July), dust AOT averaged separately over the tropical North Atlantic was one order of magnitude higher than dust AOT averaged over the tropical South Atlantic. In the presence of such strong meridional asymmetry in dust AOT in July, CF averaged separately over the tropical North Atlantic exceeded CF averaged over the tropical South Atlantic by 20%. In July, along the Saharan Air Layer, Moderate Resolution Imaging Spectroradiometer (MODIS) CF data showed significant cloud cover (up to 0.8–0.9), which contributed to above-mentioned meridional CF asymmetry. Both Multi-Angle Imaging SpectroRadiometer (MISR) measurements and MERRAero data were in agreement on seasonal variations in meridional aerosol asymmetry. Meridional asymmetry in total AOT over the Atlantic was the most pronounced between March and July, when dust presence over the North Atlantic was maximal. In September and October, there was no noticeable meridional asymmetry in total AOT over the tropical Atlantic.</p>
Increasing warming of steadily shrinking Dead Sea surface water compensates for surface water cooling (due to increasing evaporation) and even causes observed positive Dead Sea sea surface temperature trends. This warming is caused by two factors: increasing daytime heat flow from land to sea (as a result of the steady shrinking) and regional atmospheric warming. Using observations from the Moderate Resolution Imaging Spectroradiometer (MODIS), positive trends were detected in both daytime and nighttime Dead Sea sea surface temperature (SST) over the period of 2000-2016. These positive SST trends were observed in the absence of positive trends in surface solar radiation, measured by the Dead Sea buoy pyranometer. We also show that long-term changes in water mixing in the uppermost layer of the Dead Sea under strong winds could not explain the observed SST trends. There is a positive feedback loop between the positive SST trends and the steady shrinking of the Dead Sea, which contributes to the accelerating decrease in Dead Sea water levels during the period under study. Satellitebased SST measurements showed that maximal SST trends of over 0.8 • C decade −1 were observed over the northwestern and southern sides of the Dead Sea, where shrinking of the Dead Sea water area was pronounced. No noticeable SST trends were observed over the eastern side of the lake, where shrinking of the Dead Sea water area was insignificant. This finding demonstrates correspondence between the positive SST trends and the shrinking of the Dead Sea indicating a causal link between them. There are two opposite processes taking place in the Dead Sea: sea surface warming and cooling. On the one hand, the positive feedback loop leading to sea surface warming every year accompanied by long-term increase in SST; on the other hand, the measured acceleration of the Dead Sea water-level drop suggests a long-term increase in Dead Sea evaporation accompanied by a long-term decrease in SST. During the period under investigation, the total result of these two opposite processes is the statistically significant positive sea surface temperature trends in both daytime (0.6 • C decade −1 ) and nighttime (0.4 • C decade −1 ), observed by the MODIS instrument. Our findings of the existence of a positive feedback loop between the positive SST trends and the shrinking of the Dead Sea imply the following significant point: any meteorological, hydrological or geophysical process causing the steady shrinking of the Dead Sea will contribute to positive trends in SST. Our results shed light on continuing hazards to the Dead Sea.
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