The recycling of scarce nutrient resources in the sunlit open ocean is crucial to ecosystem funct... more The recycling of scarce nutrient resources in the sunlit open ocean is crucial to ecosystem function. Ammonium oxidation, the first stage of the nitrification process, directs ammonium derived from organic matter decomposition towards the regeneration nitrate, an important resource for photosynthetic primary producers. However, the technical challenge of making nitrification rate measurements in oligotrophic conditions combined with the remote 10 nature of these marine systems means that data availability, and the understanding that provides, is limited. This study reports rate measurements of ammonium oxidation over a 13, 000 km transect within the photic zone of the Atlantic Ocean. These measurements, at relatively fine resolution (order 300 km), permit the examination of interactions with environmental conditions that may warrant explicit development and inclusion in model descriptions. At all locations we report measurable rates with significant variability between and within Atlantic provinces. This adds to evidence 15 that nitrification is an important component of pelagic nitrogen cycling which modifies the inorganic nitrogen inventory of the sunlit ocean. Particular features of interest included a significant hemispheric difference in ammonium oxidation rate and elevated rates associated with mesoscale eddy features. Statistical analysis of potential links between ammonium oxidation rate and routinely measured ecosystem variables indicated significant correlative structure, explaining ~65% of the data variability. Differences between sampling depths were of the same magnitude or greater 20 than horizontally resolved differences along the transect length, identifying distinct biogeochemical niches between depth horizons. Principle component analysis demonstrated that the best overall match between ammonium oxidation rate and environmental variables involved a combination of chlorophyll-a concentration, the duration of the light phase and silicate concentration (which we argue to be a short-term tracer of physical instability). Results allude to an association between ammonium oxidation and potentially short-term product(s) of photosynthetic activity and 25 subsequent degradation. Approximately 35% of data variability was not explained, which may include descriptions of DOM pool dynamics. 35 Moore et al., 2013). Consequently, efficient organic matter decomposition and inorganic nutrient regeneration are crucial to ecosystem function. Decomposition regenerates inorganic nitrogen in the form of ammonium (NH4 +), which rarely accumulates in the open surface-ocean as it is readily used in anabolic and chemotrophic microbial processes. Evidence demonstrates that nitrification, the 2-stage oxidation of NH4 + via nitrite (NO2-) to nitrate (NO3-), takes place in the photic zone of open oceans (Santoro et al., 2013) where it directly competes with photosynthetic cells for the NH4 + 40 resource (Smith et al., 2014). The outcome of this competitive interaction is important; by moderating the composition and concentration of the dissolved inorganic nitrogen pool (NH4 + , NO2-, NO3-), nitrification influences nutrient limitation patterns for primary production at the basin scale (
High-frequency along-track altimetric data only provide direct information on the geostrophic cur... more High-frequency along-track altimetric data only provide direct information on the geostrophic currents orthogonal to the track. A new approach is proposed that combines these across-track current estimates with directional information from remotely-sensed tracer fields, such as surface chlorophyll concentration and sea surface temperature. The analysis focuses on the South Madagascar region characterised by the strong East Madagascar Current and sharp gradients of surface tracers. The results are compared with in-situ observations from three moorings along the Jason-1 track 196. Accurate information on the total velocity direction is the key factor for obtaining accurate estimates of along-track velocities. Surface tracer fronts can be successfully used to retrieve such information, especially when currents intersect the satellite track at low incidence angles (within ±60 • from the perpendicular direction). Errors in the reconstructed total velocities tend to grow rapidly for higher angles. Best performance is obtained by retaining information from the strongest fronts only. However, this significantly limits the resolution at which total currents can be reconstructed along the altimeter
IEEE Geoscience and Remote Sensing Letters, Jul 1, 2009
The normalized backscatter from a radar altimeter, σ 0 , is a measure of the surface roughness at... more The normalized backscatter from a radar altimeter, σ 0 , is a measure of the surface roughness at scales of a few radar wavelengths; over the ocean this is used to infer wind speed. Long-term studies of wind speed rely on consistent measurements within an altimetric mission and good intercalibration between missions. For the Jason-1 and Jason-2 altimeters the derivation of σ 0 from the full waveform data is known to be sensitive to the recovered value for ψ 2 , a term encompassing both mispointing and inhomogeneities within the altimetric footprint. The six months of data from the Jason-1/2 tandem mission reveal that different σ 0 corrections are needed for these two causes of non-zero ψ 2 values. With these corrections implemented, the r.m.s. difference of K u-band σ 0 values for Jason-1 and Jason-2 drops from 0.15 dB to 0.05 dB, with the bias between the two showing a clear trend with wind speed; Jason-1 being 0.04 dB greater in high winds but 0.19 dB greater in low winds. No clear change in offset is noted during the 6 months of overlapping data. Implementation of this correction will improve consistency of Jason-1 σ 0 values and may impact on orbit-fitting procedures.
The European Slope Current (SC) is a major section of the warm poleward flow from the Atlantic to... more The European Slope Current (SC) is a major section of the warm poleward flow from the Atlantic to the Arctic, which also moderates the exchange of heat, salt, nutrients and carbon between the deep ocean and the European shelf seas. The mean structure of the geostrophic flow, seasonality, interannual variability and longterm trend of SC are appraised with an unprecedented continuous 20-year satellite altimeter dataset. Comparisons with long term in situ data showed a maximum correlation of r 2 = 0.51 between altimeter and Acoustic Doppler Current Profilers (ADCP), with similar results for drogued buoy data. Mean geostrophic currents were appraised more comprehensively than previous attempts, and the paths of 4 branches of the North Atlantic Current (NAC) and positions of 5 eddies in the region were derived quantitatively. A consistent seasonal cycle in the flow of the SC was found at all 8 sections along the European shelf slope, with maximum poleward flow in the winter and minimum in the summer. The seasonal difference in the altimetry current speed amounted to~8-10 cm s −1 at the northern sections, but only~5 cm s −1 on the Bay of Biscay slopes. This extended altimeter dataset indicates significant regional and seasonal variations, and has revealed new insights into the interannual variability of the SC. It is shown that there is a peak poleward flow at most positions along a~2000 km stretch of the continental slope from Portugal to Scotland during 1995-1997, but this did not clearly relate to the extreme negative North Atlantic Oscillation (NAO) in the winter of 1995-1996. The speed of the SC exhibited a long term decreasing trend of~1% per year. By contrast the NAC showed no significant trend over the 20-year period. Major changes in the NAC occurred three times, and these changes followed decreases in the NAO index.
Hydrographic observations in the 21st century have shown that the flow within the Mozambique Chan... more Hydrographic observations in the 21st century have shown that the flow within the Mozambique Channel is best described by a series of large poleward-propagating anticyclonic eddies, rather than, as previously thought, a continuous intense western boundary current. The portrayal of this region in various runs of the NEMO 75-level model is found to vary between those two descriptions depending upon the resolution used and the implementation of the model's lateral boundary conditions. In a comparison of 1 / 4˚ resolution runs, the change of these conditions from free-slip to no-slip leads to the mean southward flow moving further offshore, with greater variability in the zonal and meridional velocities as the flow organises itself into eddies, and a reduction in total transport. If a realization of a model is unable to get these aspects of the physical flow correct, then this will significantly reduce its ability to show a realistic biological signal or long-term response to climate change. Further south, beyond Durban, the application of no-slip conditions similarly causes the mean Agulhas Current to lie further offshore, making it much more able to simulate Natal Pulses.
IEEE Geoscience and Remote Sensing Letters, Jul 1, 2009
A radar altimeter's normalized backscatter, σ 0 , is used in many oceanographic applications, to ... more A radar altimeter's normalized backscatter, σ 0 , is used in many oceanographic applications, to infer values of wind speed, wind stress, rain rate and the presence of biogenic slicks. The waveform retracker used to estimate the key geophysical variables for the altimeters on the Jason-1 and Jason-2 satellites shows increased small-scale variability since the problem is ill-conditioned. A simple empirical adjustment to σ 0 improves the separability between various parameters and also improves the along-track profiles of σ 0. This leads to i) more realistic wind fields, ii) better discrimination of rain events, and iii) improved comparison between the Jason-1 and Jason-2 altimeters during their tandem mission.
The echo waveforms received by conventional radar altimeters are interpreted by retracking algori... more The echo waveforms received by conventional radar altimeters are interpreted by retracking algorithms to give estimates of range, wave height, and backscatter strength. However, in response to fading noise on the waveform leading edge, common retrackers, such as MLE-3 and MLE-4, show correlated errors in wave height and range. This correlation is used to develop a correction to the wave height data that reduces the high-frequency variability by ∼22%, without affecting the global distribution of values. This correction also results in a closer matchup of Jason-2 and Jason-3 data during their tandem phase. Although the correction is quite straightforward in practice, the appropriate conversion factor has to be determined for each combination of altimeter and retracker. There are also remaining open questions concerning the needed low-pass filtering.
TOPEX and Jason were the first two dual-frequency altimeters in space, with both operating at K u... more TOPEX and Jason were the first two dual-frequency altimeters in space, with both operating at K u-and C-band. Each thus gives two measurements of the normalized backscatter, 0 , (from which wind speed is calculated) and two estimates of wave height. Departures from a well-defined relationship between the K u-and C-band 0 values give an indication of rain. This paper investigates differences between the two instruments using data from Jason's verification phase. Jason's K u-band estimates of wave height are ~1.8% less than TOPEX's, whereas its 0 values are higher. When these effects have been removed the root mean square (r.m.s.) mismatch between TOPEX and Jason's K u-band observations is close to that for TOPEX's observations at its two frequencies, and the changes in 0 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 0 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 0 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.
The composition of ultraplankton (UP) in near-surface samples collected underway every 1 to 6 h f... more The composition of ultraplankton (UP) in near-surface samples collected underway every 1 to 6 h from a ship sailing from Durban to the Seychelles was determined by flow cytometry, using both autofluorescence pigments and fluorescence DNA staining. Prochlorococcus (Pro) (17 to 160 × 10 3 cells ml-1) numerically dominated the ultraphytoplankton (UPP), followed by Synechococcus (Syn) (4.5 to 57 × 10 3 cells ml-1) and eukaryotic algae (EA) (0.6 to 4.2 × 10 3 cells ml-1). The abundance of heterotrophic bacterioplankton (HB) was 0.4 to 1.3 × 10 6 cells ml-1. A strong correlation (r = 0.8 to 0.97) was observed between sea-viewing wide field of view sensor (SeaWiFS) satellite estimates of total chlorophyll a (chl a) concentration and chl a concentration, abundance and biomass of EA as well as abundance and biomass of HB. This shows the potential for deducing spatial distributions of these 2 groups for ecosystem modelling using satellite data. Although the correlation between satellite chl a estimates and Syn chl a concentration was strong (r = 0.83 to 0.88), the correlation with its abundance and biomass was poor (r < 0.6) due to high variability (factor of 12) in cellular chl a content and to a lesser extent to diurnal cycles. The relationships were similar when either only daytime or all UP measurements were compared with the satellite data. No relationship was found between satellite data and Pro chl a concentration, abundance or biomass, even after correction for a pronounced diel cycle, suggesting that the SeaWiFS instrument might not detect Pro chl a.
IEEE Geoscience and Remote Sensing Letters, Oct 1, 2005
The scientific term "amphidrome" is usually associated with tides in oceanography. The dozen tida... more The scientific term "amphidrome" is usually associated with tides in oceanography. The dozen tidal amphidromes observed in the ocean are critical points that determine the fundamental pattern of the global tidal system. Exploration of the recently available satellite data with an unprecedented 1-2 decades duration suggests that an amphidrome is not a tideonly phenomenon in the ocean. Analysis of altimeter-derived sea level anomaly (SLA) data and radiometer-derived sea surface temperature (SST) data allows 10 amphidromic points to be clearly identified in annual SLA and SST variations. These amphidromes are located in the tropical areas of the Pacific, Atlantic and Indian Oceans. Their existence implies that the annual cycle (in time) of the atmosphere-ocean system is translated into a rotary variation (in space) for many of the geophysical parameters. It can be concluded that annual amphidromes are common, the knowledge of which is of particular interest, given their annually "constant" nature, for the monitoring and understanding of oceanic, climatic, as well as biological variabilities at seasonal to decadal scales, which strongly affect many aspects of the natural and societal activities on the globe.
Dual-frequency rain-flagging has long been a standard part of altimetric data analysis, both for ... more Dual-frequency rain-flagging has long been a standard part of altimetric data analysis, both for quality control of the data and for the study of rain itself, because altimeters can provide a finer spatial sampling of rain than can passive microwave instruments. However, there have been many varied implementations, using different records of the surface backscatter and different thresholds. This paper compares four different measures available for the recently-launched Jason-2. The evaluation compares these measures against clearly desired properties, finding that in most cases the adjusted backscatter and that from the ice retracker perform much better than that recommended in the users' handbook. The adjusted backscatter measure also provides a much better link to observations from Jason-1, opening up a much longer period for consistent rain investigations, and enabling greatly improved analysis of the shortscale variability of precipitation. Initial analysis shows that although the spatial and temporal gradients of backscatter increase at very low winds, the spatial gradients in rain attenuation are concentrated where rainfall is greatest, whilst the temporal changes have a simple broad latitudinal pattern.
The AltiKa altimeter records the reflection of K a-band radar pulses from the Earth's surface, wi... more The AltiKa altimeter records the reflection of K a-band radar pulses from the Earth's surface, with the commonly used waveform product involving the summation of 96 returns to provide average echoes at 40 Hz. Occasionally there are 1-second recordings of the complex individual echoes (IEs), which facilitate the evaluation of on-board processing and offers the potential for new processing strategies. Our investigation of these IEs over the ocean confirms the on-board operations, whilst noting that data quantization limits the accuracy in the thermal noise region. By constructing average waveforms from 32 IEs at a time, and applying an innovative subwaveform retracker, we demonstrate that accurate height and wave height information can be retrieved from very short sections of data. Early exploration of the complex echoes reveals structure in the phase information similar to that noted for Envisat's IEs.
International Journal of Remote Sensing, Apr 1, 2004
This paper presents a study of the characteristics of extra-tropical oceanic Rossby waves from da... more This paper presents a study of the characteristics of extra-tropical oceanic Rossby waves from datasets of Sea Surface Height (SSH), Sea Surface Temperature (SST) and ocean colour. We deal in particular with the propagation speed of the waves and compare the observational results with the speeds predicted by the classical theory and by the most recent extended theory of Rossby waves. We also discuss, with an example, the additional information that can be derived by a comparison of the wave signatures in the different datasets.
We assessed the influence of inter-annual changes in primary productivity and local, regional and... more We assessed the influence of inter-annual changes in primary productivity and local, regional and large scale ocean indices on the breeding parameters of roseate terns Sterna dougallii on Aride Island, Seychelles, western Indian Ocean. Productivity (chicks per breeding pair), timing of breeding and clutch and egg sizes were monitored annually for 8 yr and correlated with local ocean productivity (denoted by Sea-viewing Wide Field-of-view Sensor, SeaWiFS, estimates of chlorophyll concentration, CC), sea surface temperature and indices recording the status of the Indian Ocean Dipole and of El Niño. The rate of increase in CC between mean laying date and CC peak value was positively related to roseate tern productivity and mean clutch size over the 1998 to 2005 study period. Colony productivity seemed also to be influenced by the Multivariate El Niño Index. In most years, the breeding phenology of roseate terns corresponded to the local increase in CC around Aride, and failure to adjust timing of reproduction to the timing of the phytopankton bloom decreased the probability of breeding success. This is the first study showing that a tropical seabird species is sensitive to inter-annual variations in the intensity and timing of the phytoplankton bloom, which should be connected to annual variations in the availability of its main fish prey (juvenile goatfishes). Overall, these patterns indicate that the reproduction of this top marine predator is dictated by the temporal variability in oceanographic conditions. We suggest that CC data available over the world's oceans may be a useful tool to develop models predicting the fate of colonies of inshore feeding seabirds when other, more conventional, monitoring methods cannot be used.
Philosophical Transactions of the Royal Society A, Nov 19, 2002
Rossby waves are an important phenomenon, linking processes in the west of ocean basins with forc... more Rossby waves are an important phenomenon, linking processes in the west of ocean basins with forcing that occurred earlier in the east. We show evidence for such features in satellite-derived datasets of sea-surface height, temperature and ocean colour, using a section of the south Indian Ocean as an example. We discuss the possible mechanisms for an effect on chlorophyll, and we investigate this by comparing the ocean colour signal with the other datasets. In this region, the primary mechanism for a Rossby-wave signal in ocean colour appears to be meridional advection of water across a strong chlorophyll gradient. However, this cannot fully explain the observations in the westernmost basin.
Satellite altimetry has revolutionised our understanding of ocean dynamics thanks to frequent sam... more Satellite altimetry has revolutionised our understanding of ocean dynamics thanks to frequent sampling and global coverage. Nevertheless, coastal data have been flagged as unreliable due to land and calm water interference in the altimeter and radiometer footprint and uncertainty in the modelling of high-frequency tidal and atmospheric forcing. Our study addresses the first issue, i.e. altimeter footprint contamination, via retracking, presenting ALES, the Adaptive Leading Edge Subwaveform retracker. ALES is potentially applicable to all the pulse-limited altimetry missions and its aim is to retrack both open ocean and coastal data with the same accuracy using just one algorithm. ALES selects part of each returned echo and models it with a classic "open ocean" Brown functional form, by means of least square estimation whose convergence is found through the Nelder-Mead nonlinear optimisation technique. By avoiding echoes from bright targets along the trailing edge, it is capable of retrieving more coastal waveforms than the standard processing. By adapting the width of the estimation window according to the significant wave height, it aims at maintaining the accuracy of the standard processing in both the open ocean and the coastal strip. This innovative retracker is validated against tide gauges in the Adriatic Sea and in the Greater Agulhas System for three different missions: Envisat, Jason-1 and Jason-2. Considerations of noise and biases provide a further verification of the strategy. The results show that ALES is able to provide more reliable 20-Hz data for all three missions in areas where even 1-Hz averages are flagged as unreliable in standard products. Application of the ALES retracker led to roughly a half of the analysed tracks showing a marked improvement in correlation with the tide gauge records, with the rms difference being reduced by a factor of 1.5 for Jason-1 and Jason-2 and over 4 for Envisat in the Adriatic Sea (at the closest point to the tide gauge).
The recycling of scarce nutrient resources in the sunlit open ocean is crucial to ecosystem funct... more The recycling of scarce nutrient resources in the sunlit open ocean is crucial to ecosystem function. Ammonium oxidation, the first stage of the nitrification process, directs ammonium derived from organic matter decomposition towards the regeneration nitrate, an important resource for photosynthetic primary producers. However, the technical challenge of making nitrification rate measurements in oligotrophic conditions combined with the remote 10 nature of these marine systems means that data availability, and the understanding that provides, is limited. This study reports rate measurements of ammonium oxidation over a 13, 000 km transect within the photic zone of the Atlantic Ocean. These measurements, at relatively fine resolution (order 300 km), permit the examination of interactions with environmental conditions that may warrant explicit development and inclusion in model descriptions. At all locations we report measurable rates with significant variability between and within Atlantic provinces. This adds to evidence 15 that nitrification is an important component of pelagic nitrogen cycling which modifies the inorganic nitrogen inventory of the sunlit ocean. Particular features of interest included a significant hemispheric difference in ammonium oxidation rate and elevated rates associated with mesoscale eddy features. Statistical analysis of potential links between ammonium oxidation rate and routinely measured ecosystem variables indicated significant correlative structure, explaining ~65% of the data variability. Differences between sampling depths were of the same magnitude or greater 20 than horizontally resolved differences along the transect length, identifying distinct biogeochemical niches between depth horizons. Principle component analysis demonstrated that the best overall match between ammonium oxidation rate and environmental variables involved a combination of chlorophyll-a concentration, the duration of the light phase and silicate concentration (which we argue to be a short-term tracer of physical instability). Results allude to an association between ammonium oxidation and potentially short-term product(s) of photosynthetic activity and 25 subsequent degradation. Approximately 35% of data variability was not explained, which may include descriptions of DOM pool dynamics. 35 Moore et al., 2013). Consequently, efficient organic matter decomposition and inorganic nutrient regeneration are crucial to ecosystem function. Decomposition regenerates inorganic nitrogen in the form of ammonium (NH4 +), which rarely accumulates in the open surface-ocean as it is readily used in anabolic and chemotrophic microbial processes. Evidence demonstrates that nitrification, the 2-stage oxidation of NH4 + via nitrite (NO2-) to nitrate (NO3-), takes place in the photic zone of open oceans (Santoro et al., 2013) where it directly competes with photosynthetic cells for the NH4 + 40 resource (Smith et al., 2014). The outcome of this competitive interaction is important; by moderating the composition and concentration of the dissolved inorganic nitrogen pool (NH4 + , NO2-, NO3-), nitrification influences nutrient limitation patterns for primary production at the basin scale (
High-frequency along-track altimetric data only provide direct information on the geostrophic cur... more High-frequency along-track altimetric data only provide direct information on the geostrophic currents orthogonal to the track. A new approach is proposed that combines these across-track current estimates with directional information from remotely-sensed tracer fields, such as surface chlorophyll concentration and sea surface temperature. The analysis focuses on the South Madagascar region characterised by the strong East Madagascar Current and sharp gradients of surface tracers. The results are compared with in-situ observations from three moorings along the Jason-1 track 196. Accurate information on the total velocity direction is the key factor for obtaining accurate estimates of along-track velocities. Surface tracer fronts can be successfully used to retrieve such information, especially when currents intersect the satellite track at low incidence angles (within ±60 • from the perpendicular direction). Errors in the reconstructed total velocities tend to grow rapidly for higher angles. Best performance is obtained by retaining information from the strongest fronts only. However, this significantly limits the resolution at which total currents can be reconstructed along the altimeter
IEEE Geoscience and Remote Sensing Letters, Jul 1, 2009
The normalized backscatter from a radar altimeter, σ 0 , is a measure of the surface roughness at... more The normalized backscatter from a radar altimeter, σ 0 , is a measure of the surface roughness at scales of a few radar wavelengths; over the ocean this is used to infer wind speed. Long-term studies of wind speed rely on consistent measurements within an altimetric mission and good intercalibration between missions. For the Jason-1 and Jason-2 altimeters the derivation of σ 0 from the full waveform data is known to be sensitive to the recovered value for ψ 2 , a term encompassing both mispointing and inhomogeneities within the altimetric footprint. The six months of data from the Jason-1/2 tandem mission reveal that different σ 0 corrections are needed for these two causes of non-zero ψ 2 values. With these corrections implemented, the r.m.s. difference of K u-band σ 0 values for Jason-1 and Jason-2 drops from 0.15 dB to 0.05 dB, with the bias between the two showing a clear trend with wind speed; Jason-1 being 0.04 dB greater in high winds but 0.19 dB greater in low winds. No clear change in offset is noted during the 6 months of overlapping data. Implementation of this correction will improve consistency of Jason-1 σ 0 values and may impact on orbit-fitting procedures.
The European Slope Current (SC) is a major section of the warm poleward flow from the Atlantic to... more The European Slope Current (SC) is a major section of the warm poleward flow from the Atlantic to the Arctic, which also moderates the exchange of heat, salt, nutrients and carbon between the deep ocean and the European shelf seas. The mean structure of the geostrophic flow, seasonality, interannual variability and longterm trend of SC are appraised with an unprecedented continuous 20-year satellite altimeter dataset. Comparisons with long term in situ data showed a maximum correlation of r 2 = 0.51 between altimeter and Acoustic Doppler Current Profilers (ADCP), with similar results for drogued buoy data. Mean geostrophic currents were appraised more comprehensively than previous attempts, and the paths of 4 branches of the North Atlantic Current (NAC) and positions of 5 eddies in the region were derived quantitatively. A consistent seasonal cycle in the flow of the SC was found at all 8 sections along the European shelf slope, with maximum poleward flow in the winter and minimum in the summer. The seasonal difference in the altimetry current speed amounted to~8-10 cm s −1 at the northern sections, but only~5 cm s −1 on the Bay of Biscay slopes. This extended altimeter dataset indicates significant regional and seasonal variations, and has revealed new insights into the interannual variability of the SC. It is shown that there is a peak poleward flow at most positions along a~2000 km stretch of the continental slope from Portugal to Scotland during 1995-1997, but this did not clearly relate to the extreme negative North Atlantic Oscillation (NAO) in the winter of 1995-1996. The speed of the SC exhibited a long term decreasing trend of~1% per year. By contrast the NAC showed no significant trend over the 20-year period. Major changes in the NAC occurred three times, and these changes followed decreases in the NAO index.
Hydrographic observations in the 21st century have shown that the flow within the Mozambique Chan... more Hydrographic observations in the 21st century have shown that the flow within the Mozambique Channel is best described by a series of large poleward-propagating anticyclonic eddies, rather than, as previously thought, a continuous intense western boundary current. The portrayal of this region in various runs of the NEMO 75-level model is found to vary between those two descriptions depending upon the resolution used and the implementation of the model's lateral boundary conditions. In a comparison of 1 / 4˚ resolution runs, the change of these conditions from free-slip to no-slip leads to the mean southward flow moving further offshore, with greater variability in the zonal and meridional velocities as the flow organises itself into eddies, and a reduction in total transport. If a realization of a model is unable to get these aspects of the physical flow correct, then this will significantly reduce its ability to show a realistic biological signal or long-term response to climate change. Further south, beyond Durban, the application of no-slip conditions similarly causes the mean Agulhas Current to lie further offshore, making it much more able to simulate Natal Pulses.
IEEE Geoscience and Remote Sensing Letters, Jul 1, 2009
A radar altimeter's normalized backscatter, σ 0 , is used in many oceanographic applications, to ... more A radar altimeter's normalized backscatter, σ 0 , is used in many oceanographic applications, to infer values of wind speed, wind stress, rain rate and the presence of biogenic slicks. The waveform retracker used to estimate the key geophysical variables for the altimeters on the Jason-1 and Jason-2 satellites shows increased small-scale variability since the problem is ill-conditioned. A simple empirical adjustment to σ 0 improves the separability between various parameters and also improves the along-track profiles of σ 0. This leads to i) more realistic wind fields, ii) better discrimination of rain events, and iii) improved comparison between the Jason-1 and Jason-2 altimeters during their tandem mission.
The echo waveforms received by conventional radar altimeters are interpreted by retracking algori... more The echo waveforms received by conventional radar altimeters are interpreted by retracking algorithms to give estimates of range, wave height, and backscatter strength. However, in response to fading noise on the waveform leading edge, common retrackers, such as MLE-3 and MLE-4, show correlated errors in wave height and range. This correlation is used to develop a correction to the wave height data that reduces the high-frequency variability by ∼22%, without affecting the global distribution of values. This correction also results in a closer matchup of Jason-2 and Jason-3 data during their tandem phase. Although the correction is quite straightforward in practice, the appropriate conversion factor has to be determined for each combination of altimeter and retracker. There are also remaining open questions concerning the needed low-pass filtering.
TOPEX and Jason were the first two dual-frequency altimeters in space, with both operating at K u... more TOPEX and Jason were the first two dual-frequency altimeters in space, with both operating at K u-and C-band. Each thus gives two measurements of the normalized backscatter, 0 , (from which wind speed is calculated) and two estimates of wave height. Departures from a well-defined relationship between the K u-and C-band 0 values give an indication of rain. This paper investigates differences between the two instruments using data from Jason's verification phase. Jason's K u-band estimates of wave height are ~1.8% less than TOPEX's, whereas its 0 values are higher. When these effects have been removed the root mean square (r.m.s.) mismatch between TOPEX and Jason's K u-band observations is close to that for TOPEX's observations at its two frequencies, and the changes in 0 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 0 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 0 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.
The composition of ultraplankton (UP) in near-surface samples collected underway every 1 to 6 h f... more The composition of ultraplankton (UP) in near-surface samples collected underway every 1 to 6 h from a ship sailing from Durban to the Seychelles was determined by flow cytometry, using both autofluorescence pigments and fluorescence DNA staining. Prochlorococcus (Pro) (17 to 160 × 10 3 cells ml-1) numerically dominated the ultraphytoplankton (UPP), followed by Synechococcus (Syn) (4.5 to 57 × 10 3 cells ml-1) and eukaryotic algae (EA) (0.6 to 4.2 × 10 3 cells ml-1). The abundance of heterotrophic bacterioplankton (HB) was 0.4 to 1.3 × 10 6 cells ml-1. A strong correlation (r = 0.8 to 0.97) was observed between sea-viewing wide field of view sensor (SeaWiFS) satellite estimates of total chlorophyll a (chl a) concentration and chl a concentration, abundance and biomass of EA as well as abundance and biomass of HB. This shows the potential for deducing spatial distributions of these 2 groups for ecosystem modelling using satellite data. Although the correlation between satellite chl a estimates and Syn chl a concentration was strong (r = 0.83 to 0.88), the correlation with its abundance and biomass was poor (r < 0.6) due to high variability (factor of 12) in cellular chl a content and to a lesser extent to diurnal cycles. The relationships were similar when either only daytime or all UP measurements were compared with the satellite data. No relationship was found between satellite data and Pro chl a concentration, abundance or biomass, even after correction for a pronounced diel cycle, suggesting that the SeaWiFS instrument might not detect Pro chl a.
IEEE Geoscience and Remote Sensing Letters, Oct 1, 2005
The scientific term "amphidrome" is usually associated with tides in oceanography. The dozen tida... more The scientific term "amphidrome" is usually associated with tides in oceanography. The dozen tidal amphidromes observed in the ocean are critical points that determine the fundamental pattern of the global tidal system. Exploration of the recently available satellite data with an unprecedented 1-2 decades duration suggests that an amphidrome is not a tideonly phenomenon in the ocean. Analysis of altimeter-derived sea level anomaly (SLA) data and radiometer-derived sea surface temperature (SST) data allows 10 amphidromic points to be clearly identified in annual SLA and SST variations. These amphidromes are located in the tropical areas of the Pacific, Atlantic and Indian Oceans. Their existence implies that the annual cycle (in time) of the atmosphere-ocean system is translated into a rotary variation (in space) for many of the geophysical parameters. It can be concluded that annual amphidromes are common, the knowledge of which is of particular interest, given their annually "constant" nature, for the monitoring and understanding of oceanic, climatic, as well as biological variabilities at seasonal to decadal scales, which strongly affect many aspects of the natural and societal activities on the globe.
Dual-frequency rain-flagging has long been a standard part of altimetric data analysis, both for ... more Dual-frequency rain-flagging has long been a standard part of altimetric data analysis, both for quality control of the data and for the study of rain itself, because altimeters can provide a finer spatial sampling of rain than can passive microwave instruments. However, there have been many varied implementations, using different records of the surface backscatter and different thresholds. This paper compares four different measures available for the recently-launched Jason-2. The evaluation compares these measures against clearly desired properties, finding that in most cases the adjusted backscatter and that from the ice retracker perform much better than that recommended in the users' handbook. The adjusted backscatter measure also provides a much better link to observations from Jason-1, opening up a much longer period for consistent rain investigations, and enabling greatly improved analysis of the shortscale variability of precipitation. Initial analysis shows that although the spatial and temporal gradients of backscatter increase at very low winds, the spatial gradients in rain attenuation are concentrated where rainfall is greatest, whilst the temporal changes have a simple broad latitudinal pattern.
The AltiKa altimeter records the reflection of K a-band radar pulses from the Earth's surface, wi... more The AltiKa altimeter records the reflection of K a-band radar pulses from the Earth's surface, with the commonly used waveform product involving the summation of 96 returns to provide average echoes at 40 Hz. Occasionally there are 1-second recordings of the complex individual echoes (IEs), which facilitate the evaluation of on-board processing and offers the potential for new processing strategies. Our investigation of these IEs over the ocean confirms the on-board operations, whilst noting that data quantization limits the accuracy in the thermal noise region. By constructing average waveforms from 32 IEs at a time, and applying an innovative subwaveform retracker, we demonstrate that accurate height and wave height information can be retrieved from very short sections of data. Early exploration of the complex echoes reveals structure in the phase information similar to that noted for Envisat's IEs.
International Journal of Remote Sensing, Apr 1, 2004
This paper presents a study of the characteristics of extra-tropical oceanic Rossby waves from da... more This paper presents a study of the characteristics of extra-tropical oceanic Rossby waves from datasets of Sea Surface Height (SSH), Sea Surface Temperature (SST) and ocean colour. We deal in particular with the propagation speed of the waves and compare the observational results with the speeds predicted by the classical theory and by the most recent extended theory of Rossby waves. We also discuss, with an example, the additional information that can be derived by a comparison of the wave signatures in the different datasets.
We assessed the influence of inter-annual changes in primary productivity and local, regional and... more We assessed the influence of inter-annual changes in primary productivity and local, regional and large scale ocean indices on the breeding parameters of roseate terns Sterna dougallii on Aride Island, Seychelles, western Indian Ocean. Productivity (chicks per breeding pair), timing of breeding and clutch and egg sizes were monitored annually for 8 yr and correlated with local ocean productivity (denoted by Sea-viewing Wide Field-of-view Sensor, SeaWiFS, estimates of chlorophyll concentration, CC), sea surface temperature and indices recording the status of the Indian Ocean Dipole and of El Niño. The rate of increase in CC between mean laying date and CC peak value was positively related to roseate tern productivity and mean clutch size over the 1998 to 2005 study period. Colony productivity seemed also to be influenced by the Multivariate El Niño Index. In most years, the breeding phenology of roseate terns corresponded to the local increase in CC around Aride, and failure to adjust timing of reproduction to the timing of the phytopankton bloom decreased the probability of breeding success. This is the first study showing that a tropical seabird species is sensitive to inter-annual variations in the intensity and timing of the phytoplankton bloom, which should be connected to annual variations in the availability of its main fish prey (juvenile goatfishes). Overall, these patterns indicate that the reproduction of this top marine predator is dictated by the temporal variability in oceanographic conditions. We suggest that CC data available over the world's oceans may be a useful tool to develop models predicting the fate of colonies of inshore feeding seabirds when other, more conventional, monitoring methods cannot be used.
Philosophical Transactions of the Royal Society A, Nov 19, 2002
Rossby waves are an important phenomenon, linking processes in the west of ocean basins with forc... more Rossby waves are an important phenomenon, linking processes in the west of ocean basins with forcing that occurred earlier in the east. We show evidence for such features in satellite-derived datasets of sea-surface height, temperature and ocean colour, using a section of the south Indian Ocean as an example. We discuss the possible mechanisms for an effect on chlorophyll, and we investigate this by comparing the ocean colour signal with the other datasets. In this region, the primary mechanism for a Rossby-wave signal in ocean colour appears to be meridional advection of water across a strong chlorophyll gradient. However, this cannot fully explain the observations in the westernmost basin.
Satellite altimetry has revolutionised our understanding of ocean dynamics thanks to frequent sam... more Satellite altimetry has revolutionised our understanding of ocean dynamics thanks to frequent sampling and global coverage. Nevertheless, coastal data have been flagged as unreliable due to land and calm water interference in the altimeter and radiometer footprint and uncertainty in the modelling of high-frequency tidal and atmospheric forcing. Our study addresses the first issue, i.e. altimeter footprint contamination, via retracking, presenting ALES, the Adaptive Leading Edge Subwaveform retracker. ALES is potentially applicable to all the pulse-limited altimetry missions and its aim is to retrack both open ocean and coastal data with the same accuracy using just one algorithm. ALES selects part of each returned echo and models it with a classic "open ocean" Brown functional form, by means of least square estimation whose convergence is found through the Nelder-Mead nonlinear optimisation technique. By avoiding echoes from bright targets along the trailing edge, it is capable of retrieving more coastal waveforms than the standard processing. By adapting the width of the estimation window according to the significant wave height, it aims at maintaining the accuracy of the standard processing in both the open ocean and the coastal strip. This innovative retracker is validated against tide gauges in the Adriatic Sea and in the Greater Agulhas System for three different missions: Envisat, Jason-1 and Jason-2. Considerations of noise and biases provide a further verification of the strategy. The results show that ALES is able to provide more reliable 20-Hz data for all three missions in areas where even 1-Hz averages are flagged as unreliable in standard products. Application of the ALES retracker led to roughly a half of the analysed tracks showing a marked improvement in correlation with the tide gauge records, with the rms difference being reduced by a factor of 1.5 for Jason-1 and Jason-2 and over 4 for Envisat in the Adriatic Sea (at the closest point to the tide gauge).
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Papers by Graham Quartly