1,720,977 research outputs found
A visible record of eddies in the southern Mozambique Channel
Full text linkThe flows around Madagascar feed into the Agulhas Current, but there have been few hydrographic studies of the flow within the Mozambique Channel. Some cruise and altimetric data point to this being a region of high mesoscale activity, with eddies migrating through the area. Here we show how ocean colour data throw light on the behaviour of eddies in the southern Mozambique Channel
A plankton guide to ocean physics: Colouring in the currents round South Africa and Madagascar
Full text linkThe ocean colour sensor SeaWiFS, launched in August 1997, has been a great boon to those researching large-scale oceanic biological productivity. The sensor can detect variations in the colour of the water due to the presence of chlorophyll in phytoplankton, which essentially changes the water colour from blue to green. SeaWiFS has provided measurements of chlorophyll concentration over nearly all the world’s oceans, and because of their association with fronts, eddies and regions of upwelling, these records of phytoplankton abundance reveal much about physical processes occurring within the ocean
Rain-flagging of the Envisat altimeter
Full text linkAs the goals for altimetric measurements become ever more precise, there is concern about the reliable detection and discarding of rain contaminated data. A dual-frequency rain detection technique developed for the Ku- and C-band TOPEX altimeter, is adapted for the Ku- and S-band RA-2 altimeter on Envisat. Of particular concern is the selection of a suitable threshold to minimise the quantity of good data inadvertently discarded
Spatially implicit plankton population models: transient spatial variability
No full textOcean plankton models are useful tools for understanding and predicting the behaviour of planktonic ecosystems. However, when the regions represented by the model grid cells are not well mixed, the population dynamics of grid cell averages may differ from those of smaller scales (such as the laboratory scale). Here, the ‘mean field approximation’ fails due to ‘biological Reynolds fluxes’ arising from nonlinearity in the fine-scale biological interactions and unresolved spatial variability. We investigate the domain-scale behaviour of two-component, 2D reaction–diffusion plankton models producing transient dynamics, with spatial variability resulting only from the initial conditions. Failure of the mean field approximation can be quite significant for sub grid-scale mixing rates applicable to practical ocean models. To improve the approximation of domain-scale dynamics, we investigate implicit spatial resolution methods such as spatial moment closure. For weak and moderate strengths of biological nonlinearity, spatial moment closure models generally yield significant improvements on the mean field approximation, especially at low mixing rates. However, they are less accurate given weaker transience and stronger nonlinearity. In the latter case, an alternative ‘two-spike’ approximation is accurate at low mixing rates. We argue that, after suitable extension, these methods may be useful for understanding and skillfully predicting the large-scale behaviour of marine ecosystems.<br/
Eddies around Madagascar - the retroflection re-considered
Full text linkThe Agulhas Current with its retroflection and attendant eddy-shedding is the cause of some of the greatest mesoscale variability in the ocean. This paper considers the area to the south and east of Madagascar, which provides some of the source waters of the Agulhas Current, and examines the propagating sea surface height signals in altimetry and output from a numerical model, OCCAM. Both show bands of variability along the axis of the East Madagascar Current (EMC) and along a zonal band near 25°S. Sequences of images plus associated temperature data suggest that a number of westward-propagating eddies are present in this zonal band. The paper then focuses on the region to the south of the island, where ocean colour and infra-red imagery are evocative of an East Madagascar Retroflection. The synthesis of data analysed in this paper, however, show that remotely observed features in this area can be explained by anticyclonic eddies moving westward through the region, and this explanation is consistent with numerical model output and the trajectories of drifting buoys
Skill assessment via cross-validation and Monte Carlo simulation: an application to Georges Bank plankton models
No full textBetter methods are required to assess the skill or uncertainty of plankton model predictions. A method is presented which combines cross-validation with simulated repeat samplings of the data (Monte Carlo simulation), in order to robustly estimate uncertainty in predictions beyond the calibration data (‘extra-sample’). The method is applied to compare two bulk models of chlorophyll on Georges Bank using the GLOBEC data set, accounting for data and forcing errors as well as prior uncertainty in all model parameters and initial conditions. The first model is a simple interpolation of chlorophyll data (‘inductive’ model), and serves as a baseline of predictive skill. The second is a simple process model forced by interannually-variable nutrient and mesozooplankton mean fields. Uncertainty in the process model forcings severely increases the extra-sample prediction variance (over repeat experiments). Although the process model can reproduce some of the interannual chlorophyll variability via top-down control by mesozooplankton, other predictions are strongly biased, possibly due to neglected boundary fluxes of chlorophyll. As a result, the new skill metrics generally favour the inductive model. By contrast, a standard skill metric based on calibration data misfit incorrectly favours the process model, mainly due to the neglect of extra-sample prediction variance.<br/
Measuring rainfall from above and below the sea surface
No full textSatellites play a major role in the determination of the rainfall at sea. Researchers at Southampton Oceanography Centre (SOC) have been involved in two projects addressing this task. First they have been instrumental in developing techniques to retrieve rain rate information from the 10+ years of dual-frequency altimeter data. The TOPEX radar measures rainfall via the attenuation it causes, producing a climatology that is independent of those derived from passive microwave (PM) and infrared (IR) sensors. Because TOPEX is an active microwave sensor, it can have a much smaller footprint than PM sensors. Therefore it can be used to estimate the size of rain cells, showing that the ITCZ and mid-latitude storm tracks are characterized by larger rain systems than elsewhere. TOPEX’s simultaneous recording of wind and wave data reveal that, for mid-latitude systems, rain is most likely in association with developing seas.All satellite-based datasets require validation, and SOC's work on the development and testing of acoustic rain gauges is the second aspect of this paper. By listening at a range of frequencies, an underwater hydrophone may distinguish the spectra of wind, rain, shipping etc., and estimate the wind speed or rain rate according to the magnitude of the signals. All our campaigns have shown a good acoustic response to changes in wind speed. However the quantitative inversion for recent trials has given values that are too high, possibly because of significant acoustic reflection from the sea bottom. The changes in spectral slope often agree with other observations of rain, although validation experiments in coastal regions are hampered by the extraneous sources present. Acoustic rain gauges would eventually see service not only for routine satellite validation, but also for real-time monitoring of locations of interest
Spatial and temporal scales of variability in Tropical Atlantic sea surface salinity from the SMOS and Aquarius satellite missions
Full text linkTaking advantage of the spatially dense, multi-year time series of global Sea Surface Salinity (SSS) from two concurrent satellite missions, the spatial and temporal decorrelation scales of SSS in the Tropical Atlantic 30°N–30°S are quantified for the first time from SMOS and Aquarius observations. Given the dominance of the seasonal cycle in SSS variability in the region, the length scales are calculated both for the mean and anomaly (i.e. seasonal cycle removed) SSS fields. Different 7–10 days composite SSS products from the two missions are examined to explore the possible effects of varying resolution, bias corrections and averaging characteristics. With the seasonal cycle retained, the SSS field is characterized by strongly anisotropic spatial variability. Homogeneous SSS variations in the Tropics have the longest zonal scales of over ~ 2000 km and long temporal scales of up to ~ 70–80 days, as shown by both SMOS and Aquarius. The longest meridional scales, reaching over ~ 1000 km, are seen in the South Atlantic between ~ 10°–25°S, most discernible in Aquarius data. The longest temporal scales of SSS variability are reported by both satellites to occur in the North-West Atlantic region 15°–30°N, at the Southern end of the Sargasso Sea, with SSS persisting for up to 150–200 days. The removal of the seasonal cycle results in a noticeable decrease in the spatio-temporal decorrelation scales over most of the basin. Overall, with the exception of the differences in the South Atlantic, there is general agreement between the spatial and temporal scales of SSS from the two satellites and different products, despite differences in individual product calibration and resolution characteristics. These new estimates of spatio-temporal decorrelation scales of SSS improve our knowledge of the processes and mechanisms controlling the Tropical Atlantic SSS variability, and provide valuable information for a wide range of oceanographic and modelling applications
Plankton patchiness investigated using simultaneous nitrate and chlorophyll observations
No full textThe complex patterns observed in marine phytoplankton distributions arise from the interplay of biological and physical processes, but the nature of the balance remains uncertain centuries after the first observations. Previous observations have shown a consistent trend of decreasing variability with decreasing length-scale. Influenced by similar scaling found for the properties of the water that the phytoplankton inhabit, ‘universal' theories have been proposed that simultaneously explain the variability seen from meters to hundreds of kilometers. However, data on the distribution of phytoplankton alone has proved insufficient to differentiate between the many causal mechanisms that have been suggested. Here we present novel observations from a cruise in the North Atlantic in which fluorescence (proxy for phytoplankton), nitrate and temperature were measured simultaneously at scales from 10 m to 100 km for the first time in the open ocean. These show a change in spectra between the small scale (10–100 m) and the mesoscale (10–100 km) which is different for the three tracers. We discuss these observations in relation to the current theories for phytoplankton patchiness
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