Indian Ocean | EPFL Graph Search

The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by the Southern Ocean or Antarctica depending on the definition in use. Along its core, the Indian Ocean has some large marginal or regional seas such as the Arabian Sea, Laccadive Sea, Bay of Bengal and Andaman Sea. Etymology The Indian Ocean has been known by its present name since at least 1515 when the Latin form Oceanus Orientalis Indicus ("Indian Eastern Ocean") is attested, named after India, which projects into it. It was earlier known as the Eastern Ocean, a term that was still in use during the mid-18th century (see map), as opposed to the Western Ocean (Atlantic) before the Pacific was surmised. The Hindi name for the Ocean is हिंद महासागर (; ). Conversely, Chinese explorers (e.g., Zheng He during the Ming dynasty) who travele

The role of mixing in controlling resource availability and phytoplankton community composition

Bieito Fernandez Castro, Emilio Marañón

We investigate the role of mixing, through its effect on nutrient and light availability, as a driver of phytoplankton community composition in the context of Margalef’s mandala. Data on microstructure turbulence, irradiance, new nitrogen supply and phytoplankton composition were collected at 102 stations in three contrasting marine environments: the Galician coastal upwelling system of the northwest Iberian Peninsula, the northwestern Mediterranean, and the tropical and subtropical Atlantic, Pacific and Indian oceans. Photosynthetic pigments concentration and microscopic analysis allowed us to investigate the contribution of diatoms, dinoflagellates, pico- and nanoeukaryotes, and cyanobacteria to the phytoplankton community. Simple linear regression was used to assess the role of environmental factors on community composition, and environmental overlap among different phytoplankton groups was computed using nonparametric kernel density functions. Mixing and new nitrogen supply played an important role in controlling the phytoplankton community structure. At lower values of mixing and new nitrogen supply cyanobacteria dominated, pico- and nanoeukaryotes were dominant across a wide range of environmental conditions, and finally enhanced new nitrogen supply was favourable for diatoms and dinoflagellates. Dinoflagellates were prevalent at intermediate mixing levels, whereas diatoms spread across a wider range of mixing conditions. Occasional instances of enhanced diatom biomass were found under low mixing, associated with the high abundance of Hemiaulus hauckii co-occurring with high N2 fixation in subtropical regions, and with the formation of thin layers in the Galician coastal upwelling. Our results verify the Margalef’s mandala for the whole phytoplankton community, emphasizing the need to consider nutrient supply, rather than nutrient concentration, as an indicator of nutrient availability.

2019

Diel quenching of Southern Ocean phytoplankton fluorescence is related to iron limitation

Nina Schuback

Evaluation of photosynthetic competency in time and space is critical for better estimates and models of oceanic primary productivity. This is especially true for areas where the lack of iron (Fe) limits phytoplankton productivity, such as the Southern Ocean. Assessment of photosynthetic competency on large scales remains challenging, but phytoplankton chlorophyll a fluorescence (ChlF) is a signal that holds promise in this respect as it is affected by, and consequently provides information about, the photosynthetic efficiency of the organism. A second process affecting the ChlF signal is heat dissipation of absorbed light energy, referred to as non-photochemical quenching (NPQ). NPQ is triggered when excess energy is absorbed, i.e. when more light is absorbed than can be used directly for photosynthetic carbon fixation. The effect of NPQ on the ChlF signal complicates its interpretation in terms of photosynthetic efficiency, and therefore most approaches relating ChlF parameters to photosynthetic efficiency seek to minimize the influence of NPQ by working under conditions of sub-saturating irradiance. Here, we propose that NPQ itself holds potential as an easily acquired optical signal indicative of phytoplankton physiological state with respect to Fe limitation. We present data from a research voyage to the Subantarctic Zone south of Australia. Incubation experiments confirmed that resident phytoplankton were Fe-limited, as the maximum quantum yield of primary photochemistry, F-v/F-m, measured with a fast repetition rate fluorometer (FRRf), increased significantly with Fe addition. The NPQ "capacity" of the phytoplankton also showed sensitivity to Fe addition, decreasing with increased Fe availability, confirming previous work. The fortuitous presence of a remnant warm-core eddy in the vicinity of the study area allowed comparison of fluorescence behaviour between two distinct water masses, with the colder water showing significantly lower F-v/F-m than the warmer eddy waters, suggesting a difference in Fe limitation status between the two water masses. Again, NPQ capacity measured with the FRRf mirrored the behaviour observed in F-v/F-m, decreasing as F-v/F-m increased in the warmer water mass. We also analysed the diel quenching of underway fluorescence measured with a standard fluorometer, such as is frequently used to monitor ambient chlorophyll a concentrations, and found a significant difference in behaviour between the two water masses. This difference was quantified by defining an NPQ parameter akin to the Stern-Volmer parameterization of NPQ, exploiting the fluorescence quenching induced by diel fluctuations in incident irradiance. We propose that monitoring of this novel NPQ parameter may enable assessment of phytoplankton physiological status (related to Fe availability) based on measurements made with standard fluorometers, as ubiquitously used on moorings, ships, floats and gliders.

2020

Aerosol Transport and Distribution

Quentin Bourgeois

The fourth report of the Intergovernmental Panel of Climate Change (IPCC) unambiguously indicates that the climate is changing. Climate change is induced by changes in atmospheric abundances of greenhouse gases and aerosols, solar radiation and land surface properties, which all alter the radiative balance of the Earth. Among them, greenhouse gases and aerosols are likely the compounds affecting the climate the most. While greenhouse gases warm the Earth, aerosols influence its radiative balance by scattering and absorbing solar radiation (the direct effect), and by modifying cloud amount and properties (the indirect effect). The radiative forcing of aerosols on the climate is however the less understood among the various forcings currently considered in the IPCC assessment. This is likely related to the lack of comprehensive and accurate observations of aerosols (and in particular of their vertical distribution) which thus makes global aerosol models difficult to constrain. Recent model intercomparisons have indicated that different assumptions regarding aerosol emissions, formation and growing properties, and removal processes generate large diversities within models. These large diversities are found in particular in the Arctic and the upper troposphere region where extreme surrounding conditions take place, limiting instrument sensitivity and accuracy. The objectives of this thesis are to better quantify the processes driving the aerosol distribution in regions where the uncertainties are the largest, including the Arctic region and the upper troposphere, and to assess the quality of CALIOP observations. For this purpose, we used the fully coupled global climate-aerosol-chemistry ECHAM5-HAMMOZ model conjointly with a suite of remote sensing observations including the CALIOP satellite instrument, which provides the vertical distribution of aerosols. The model predicts the size distribution and composition of aerosols as well as the number concentration of cloud droplets and ice crystals. We first investigated the processes that drive the transport of soluble and insoluble compounds toward the Arctic in the ECHAM5-HAMMOZ model. Recognizing that scavenging processes may be an issue in global models, we "re-visited" their properties in ECHAM5-HAMMOZ. We find that the model better reproduces the aerosol vertical distribution in the northern mid- and high-latitudes, especially in the free troposphere, by decreasing aerosol scavenging coefficients in the model. Using smaller aerosol scavenging coefficients results in an increase of aerosol burden and lifetime, especially in the Arctic. In addition, the Arctic haze phenomenon is better represented in the simulation using decreased aerosol scavenging coefficients. The relative contributions of different processes governing the transport of aerosols from the midlatitudes toward the Arctic together with the relative contributions of different geopolitical source regions were then quantified. We find that Europe and Siberia contribute the most to the Arctic pollution even though Asian anthropogenic emissions are much larger. The model suggests that BC emitted in Siberia and Europe is ten times more efficiently deposited onto the Arctic than BC emitted in Asia. We next investigated an aerosol layer that forms in the vicinity of the tropical tropopause layer (TTL) over the southern Asian and Indian Ocean region. Aerosols in this layer are observed with the CALIOP instrument even though ice clouds partly mask their signal. The aerosol layer follows the Inter Tropical Convergence Zone (ITCZ), which indicates that these aerosols are likely transported during convective processes. The ECHAM5.5-HAM2 model reproduces such an aerosol layer in the TTL over the same region but clearly overestimates CALIOP data. Model results suggest that aerosols in this layer are mostly sulfate particles transported during convective processes and originating from natural sources. The shortwave forcing of this aerosol layer is estimated. Finally, we assessed the accuracy of aerosol extinction retrievals from the CALIOP satellite instrument by comparing them with remote sensing observations such as AERONET, MODIS and MISR over the last four years. Overall, standard CALIOP products underestimate by 50% observed aerosol extinctions. However, this comparison is substantially improved by screening CALIOP data. The comparison of the CALIOP screened product with the ECHAM5.5-HAM2 model shows large disparities in model results. The model reproduces quite well the observed interannual variability and vertical distribution of aerosols in Europe and North Africa while it underestimates them in America and Asia. This is likely due to an underestimate of dust and anthropogenic emissions in North America and Asia.

EPFL2012