A visible record of eddies in the southern Mozambique Channel

The 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

The 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

As 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

Envisat - taking the measure of North Atlantic storms

Envisat carries a number of sensors able to provide quantitative information on raining clouds: AATSR delivers information on cloud microphysics (particle size, temperature etc.), MWR-2 gives columnar totals for liquid and vapour forms of water, and RA-2 yields rain rate and wind speed. This paper examines the complementarity of these sensors, with a focussed study on significant rain events in the N. Atlantic, covering both coherent large storms and fronts with smaller scale structure. The difference in liquid water estimates from the infra-red and passive systems appears to be related to the temperature and sizes of drops being detected

Characterizing and following eddies in Drake Passage

Drake Passage, the gap between South America and Antarctica, is a very energetic region, with strong currents and numerous eddies. These eddies are important for mixing waters across the main three fronts, and for affecting the biological productivity in the region. We use weekly maps of colour images, sea level anomaly and geostrophic velocities to characterize physical and biological activity in the region surrounding Drake Passage. In particular, we note that the largest eddy kinetic energy occurs to the east of the passage and is principally in the area bounded by the mean Subantarctic Front (SAF) and the mean Polar Front, whilst the biological productivity is highest in coastal regions, with the SAF acting as a clear southern boundary. In a number of cases, cyclonic eddies detected by altimetry also show a strong signature in ocean color. Regular XBT (eXpendable BathyThermograph) surveys provide information on sub-surface structure, confirming the altimetric identification of features and showing that they often contain water masses originating from the other side of the front

Application of clustering techniques to multispectral optical data over the ocean

MERIS, on Envisat, provides high-resolution radiometric data at nine discrete channels in the visible band. This paper looks at the potential of an unsupervised classification technique for utilizing these multi-spectral data to provide better discrimination between water masses according to their optical properties, and in particular whether phytoplankton groups can be distinguished. Although the majority of data do show a spectral peak associated with chlorophyll's red fluorescence line, clustering using only the red bands was found to separate out coastal waters according to their sediment content. Red-end classification also appeared to identify sub-pixel cloud, and demonstrate that the smile correction had not removed all the striping from the data. Classification using bands from the blue-green end showed a response to changes in chlorophyll concentration, but also indicated other variations. However, without in situ data no firm conclusions can be drawn on which phytoplankton groupings are present

Sea state and rain: a second take on dual-frequency altimetry

TOPEX and Jason were the first two dual-frequency altimeters in space, with both operating at Ku- and C-band. Each thus gives two measurements of the normalized backscatter, sigma0, (from which wind speed is calculated) and two estimates of wave height. Departures from a well-defined relationship between the Ku- and C-band sigma0 values give an indication of rain.This paper investigates differences between the two instruments using data from Jason's verification phase. Jason's Ku-band estimates of wave height are ~1.8% less than TOPEX's, whereas its sigma0 values are higher. When these effects have been removed the root mean square (r.m.s.) mismatch between TOPEX and Jason's Ku-band observations is close to that for TOPEX's observations at its two frequencies, and the changes in sigma0 with varying wave height conditions are the same for the two altimeters. Rain flagging and quantitative estimates of rain rate are both based on the atmospheric attenuation derived from the sigma0 measurements at the two frequencies. The attenuation estimates of TOPEX and Jason agree very well, and a threshold of -0.5 dB is effective at removing the majority of spurious data records from the Jason GDRs. In the high sigma0 regime, anomalous data can be cause by processes other than rain. Consequently, for these low wind conditions, neither can reliable rain detection be based on altimetry alone, nor can a generic rain flag be expected to remove all suspect data

Realizing Envisat's potential for rain cloud studies

Owing to the highly variable nature of rain both in space and time and the difficulties in obtaining accurate in situ measurements, increasing reliance is being placed on the various types of satellite data now available. The multi-sensor payload of Envisat is of particular interest because the data are co-located and simultaneous, thus reducing some of the uncertainty found in multi-platform analyses. This paper shows variations in cloud and precipitation data derived from AATSR, RA-2 and MWR-2 measurements in an overpass of Hurricane Juan, revealing significant asymmetry in the spatial distribution. The results are discussed in the context of similar data from other tropical and sub-tropical features in the western Atlantic. The combination of data from these sensors allows us to see the effects of different drop-size distribution at varying distances from the eye and to conclude that active microwave systems are needed for studying small-scale variations in rainfall