Global dust optical depth climatology derived from CALIOP and MODIS aerosol retrievals on decadal timescales: regional and interannual variability

We derived two observation-based global monthly mean dust aerosol optical depth (DAOD) climatological datasets from 2007 to 2019 with a 2∘ (latitude) × 5∘ (longitude) spatial resolution, one based on Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and the other on Moderate Resolution Imaging Spectroradiometer (MODIS) observations. In addition, the CALIOP climatological dataset also includes dust vertical extinction profiles. Dust is distinguished from non-dust aerosols based on particle shape information (e.g., lidar depolarization ratio) for CALIOP and on dust size and absorption information (e.g., fine-mode fraction, Ångström exponent, and single-scattering albedo) for MODIS, respectively. The two datasets compare reasonably well with the results reported in previous studies and the collocated Aerosol Robotic Network (AERONET) coarse-mode AOD. Based on these two datasets, we carried out a comprehensive comparative study of the spatial and temporal climatology of dust. On a multi-year average basis, the global (60∘ S–60∘ N) annual mean DAOD is 0.032 and 0.067 according to CALIOP and MODIS retrievals, respectively. In most dust-active regions, CALIOP DAOD generally correlates well (correlation coefficient R>0.6) with the MODIS DAOD, although the CALIOP value is significantly smaller. The CALIOP DAOD is 18 %, 34 %, 54 %, and 31 % smaller than MODIS DAOD over the Sahara, the tropical Atlantic Ocean, the Caribbean Sea, and the Arabian Sea, respectively. Applying a regional specific lidar ratio (LR) of 58 sr instead of the 44 sr used in the CALIOP operational retrieval reduces the difference from 18 % to 8 % over the Sahara and from 34 % to 12 % over the tropical Atlantic Ocean. However, over eastern Asia and the northwestern Pacific Ocean (NWP), the two datasets show weak correlation. Despite these discrepancies, CALIOP and MODIS show similar seasonal and interannual variations in regional DAOD. For dust aerosol over the NWP, both CALIOP and MODIS show a declining trend of DAOD at a rate of about 2 % yr−1. This decreasing trend is consistent with the observed declining trend of DAOD in the southern Gobi Desert at a rate of 3 % yr−1 and 5 % yr−1 according to CALIOP and MODIS, respectively. The decreasing trend of DAOD in the southern Gobi Desert is in turn found to be significantly correlated with increasing vegetation and decreasing surface wind speed in the area.

Explicit versus Parameterized Convection in Response to the Atlantic Meridional Mode

This study investigates whether the representation of explicit and parameterized convection influences the response to the Atlantic meridional mode (AMM). The main focus is on the precipitation response to the AMM-SST pattern, but possible implications for the atmospheric feedback on SST are also examined by considering differences in the circulation response between explicit and parameterized convection. On the basis of analysis from observations, SST composites are built to represent the positive and negative AMM. These SST patterns, in addition to the March–May climatology, are prescribed to the atmospheric ICON model. High-resolution simulations with explicit convection (E-CON) and coarse-resolution simulations with parameterized convection (P-CON) are used over a nested tropical Atlantic Ocean domain and a global domain, respectively. Our results show that a meridional shift of about 1° in the precipitation climatology explains most of the response to the AMM-SST pattern in simulations both with explicit convection and with parameterized convection. Our results also indicate a linearity in the precipitation response to the positive and negative AMM in E-CON, in contrast to P-CON. Further analysis of the atmospheric response to the AMM reveals that anomalies in the wind-driven enthalpy fluxes are generally stronger in E-CON than in P-CON. This result suggests that SST anomalies would be amplified more strongly in coupled simulations using an explicit representation of convection.

Test-Size Evolution of the planktonic Foraminifera <i>Globorotalia menardii</i> in the Eastern Tropical Atlantic since the Late Miocene

The mean test size of planktonic foraminifera (PF) is known to have increased especially during the last 12 Ma, probably in terms of an adaptive response to an intensification of the surface-water stratification. On geologically short timescales, the test size in PF is related to environmental conditions. In an optimal species-specific environment, individuals exhibit a greater maximum and average test size, while the size decreases the more unfavourable the environment becomes. An interesting case was observed in the late Neogene and Quaternary size evolution of Globorotalia menardii, which seems to be too extreme to be only explained by changes in environmental conditions. In the western tropical Atlantic Ocean (WTAO) and the Caribbean Sea, the test size more than doubles from 2.6 Ma to 1.95 Ma and 1.7 Ma, respectively, following an almost uninterrupted and successive phase of test size decrease from 4 Ma. Two hypotheses have been suggested to explain the sudden occurrence of a giant G. menardii form: it was triggered by either (1) a punctuated, regional evolutionary event or (2) the immigration of specimens from the Indian Ocean via the Agulhas Leakage. Morphometric measurements of tests from sediment samples of the Ocean Drilling Program (ODP) Leg 108 Hole 667A in the eastern tropical Atlantic Ocean (ETAO), show that the giant type already appears 0.1 Ma earlier at this location than in the WTAO, which indicates that the extreme size increase in the early Pleistocene was a tropical-Atlantic-Ocean-wide event. A coinciding change in the predominant coiling direction suggests that probably a new morphotype occurred. If the giant size and the uniform change in the predominant coiling direction are an indicator for this new type, the form already occurred in the eastern tropical Pacific Ocean at the Pliocene/Pleistocene boundary at 2.58 Ma. This finding supports the Agulhas Leakage hypothesis. However, the hypothesis of a regional, punctuated evolutionary event cannot be dismissed due to missing data from the Indian Ocean. This paper presents the AMOC/thermocline hypothesis, which not only suggests an alternative explanation for the sudden test-size increase in the early Pleistocene, but also for the test size evolution within the whole tropical Atlantic Ocean and the Caribbean Sea for the last 8 Ma. The test-size evolution shows a similar trend with indicators for changes in the Atlantic Meridional Overturning Circulation (AMOC) strength. The mechanism behind that might be that changes in the AMOC strength have a major influence on the thermal stratification of the upper water column, which is known to be the habitat of G. menardii.

What can we learn from observed temperature and salinity isopycnal anomalies at eddy generation sites? Application in the Tropical Atlantic Ocean.

&lt;p&gt;Potential vorticity (PV) is a key parameter to analyze the generation and dynamics of mesoscale eddies. Numerical studies have shown how adiabatic (displacement of particles within a background gradient of PV) and diabatic (diapycnal mixing and friction) processes can be involved in the generation of localized PV anomalies and vortices. Such processes are however difficult to evaluate in the ocean because PV is difficult to evaluate at mesoscale. In this study, we argue that qualitative analysis can be done, based on the link between PV anomalies and isopycnal temperature/salinity anomalies (&lt;em&gt;&amp;#415;&amp;#8217;&lt;/em&gt;&lt;em&gt;/S&amp;#8217;&lt;/em&gt;). Indeed, in the ocean, eddies created by diapycnal mixing or isopycnal advection of water-masses, are associated with PV anomalies and significant isopycnal &lt;em&gt;&amp;#415;&amp;#8217;&lt;/em&gt;&lt;em&gt;/S&amp;#8217;&lt;/em&gt;. In contrast, eddies created by friction are associated with PV anomalies but without isopycnal &lt;em&gt;&amp;#415;&amp;#8217;&lt;/em&gt;&lt;em&gt;/S&amp;#8217;&lt;/em&gt;. In this study, based on 18 years of satellite altimetry data and vertical &lt;em&gt;&amp;#415;&lt;/em&gt;&lt;em&gt;/S&lt;/em&gt; profiles acquired by Argo floats, we analyze the isopycnal &lt;em&gt;&amp;#415;&amp;#8217;&lt;/em&gt;&lt;em&gt;/S&amp;#8217;&lt;/em&gt; within new-born eddies in the tropical Atlantic Ocean (TAO) and discuss the possible mechanisms involved in their generation. Our results show that on density-coordinates system, both anticyclonic (AEs) and cyclonic (CEs) eddies can exhibit positive, negative, or non-significant &lt;em&gt;&amp;#415;&amp;#8217;&lt;/em&gt;&lt;em&gt;/S&amp;#8217;&lt;/em&gt;. Almost half of the sampled eddies do not have significant &lt;em&gt;&amp;#415;&amp;#8217;&lt;/em&gt;&lt;em&gt;/S&amp;#8217; &lt;/em&gt;at their generation site, indicating that frictional effects probably play a significant role in the generation of their PV anomalies. The other half of eddies, likely generated by diapycnal mixing or isopycnal advection, exhibits significant positive or negative anomalies with typical &amp;#415;&amp;#8217; of &amp;#177;0.5&amp;#176;C. More than 70% of these significant eddies are subsurface-intensified, having their cores below the seasonal pycnocline. Refined analyses of the vertical structure of new-born eddies in three selected subregions of the TAO where the strongest anomalies were observed, show the dominance of cold (warm, respectively) subsurface AEs (CEs) likely due to isopycnal advection of large scale PV and temperature.&lt;/p&gt;