Papers by Peter Challenor
In the last couple of years the major space agencies have actively supported research and develop... more In the last couple of years the major space agencies have actively supported research and development of altimetry in the coastal zone, with projects like PISTACH funded by the French Space Agency (CNES) and COASTALT funded by the European Space Agency (ESA). In this presentation we will focus on the COASTALT Project, which paves the way to the reprocessing of coastal altimetry data from ESA missions. COASTALT has delivered a number of key contributions to the advancement of the coastal altimetry and has gained visibility within the lively international community of researchers involved in the development of this novel topic. In this talk we will first review what COASTALT has achieved so far, namely:
ABSTRACT Buoy measurements of significant wave height have yielded evidence of increasing wave he... more ABSTRACT Buoy measurements of significant wave height have yielded evidence of increasing wave heights at some sites in both the North Atlantic and the North Pacific. However, it has been difficult to combine the results from these isolated sites into a coherent pic- ture of changes in wave climate in the northern hemisphere, and to establish the causes of these changes. (For example, changes in the atmospheric circulation resulting from either natural variability or anthropogenic forcing). In common with other satellite methods, satellite altimetry provides a detailed picture of changes over the breadth of ocean basins, which would be impossible by traditional methods. There are now some 15 years of altimetric measurements of significant wave height, too little for directly measuring multi-decadal changes, but adequate for analysing inter-annual variability and the relationship of this variability to atmospheric forcing. Waves in the north east Atlantic continued to increase between the late 1980s and the early 1990s, but this increase was reversed during the 1990s. The only clear trend in the North Pacific in the 1990s was an increase in February of successive years. Secular trends appear to be relatively fickle, but there are robust relationships between the atmospheric circulation and wave climate. It is possible to take a hemispheric view of atmospheric circulation, its variability (e.g., "the Arctic Oscillation") and resulting changes in wave climate, but it is still not clear how much of the variability is genuinely coherent between the North Pacific and the North Atlantic. Variability in the wintertime north east Atlantic is exceptionally high, and this variability can be largely explained by two atmospheric modes: the "North Atlantic Oscillation" (NAO) and the "East Atlantic pattern". A large part of the observed increase in wave heights at sites in the north east Atlantic though the latter part of the twentieth century can be explained by the behaviour of the North Atlantic Oscillation in this period. Variability in the north east Pacific is also quite high, but its relationship to atmospheric circulation is relatively obscure.
Environment and Planning A, 2010
Protection of human life and property from flooding is a strategic priority in the UK.
Continental Shelf Research, 2009
Estimates of extreme sea levels and return periods have been based mainly on hourly sampling rate... more Estimates of extreme sea levels and return periods have been based mainly on hourly sampling rates. Technological development has enabled the sampling rates to increase and sampling rates of 5-10 min are becoming increasingly common. In this paper we explore the relationship between extreme sea levels and estimated return periods based on hourly and shorter sampling periods in three tide-gauges one at the Atlantic coasts of Spain (Coruñ a), one in the western Mediterranean (Malaga) and one in the N. Adriatic (Trieste). Significant differences of several centimetres are found in the hourly and 5 min extremes. These reflect in significant underestimation of the 50-year return levels which in Trieste reach 38 cm. A theoretical relationship between the high and the low sampling rate of extremes is also tested. Thus updated 50-year return levels for the Mediterranean and the coasts of the Iberian peninsula are produced assuming that the differences identified in the various stations generalise to other tide-gauge (hourly) records for which hourly values have been analysed earlier.
RAIES is an ESA-funded study for the scientific exploitation of ENVISAT RA2 Individual Echo and S... more RAIES is an ESA-funded study for the scientific exploitation of ENVISAT RA2 Individual Echo and S-band data for ocean, coastal, land and ice remote sensing. This paper will introduce some of the the new scientific applications of RA2 data for the ocean and the coastal zone, including improved rain, wind and wave products using Ku and S-band data. It will also introduce new science applications that will be made possible with RA2 individual echoes or averaged waveforms for ocean, the coastal zone, land and ice remote sensing. The paper will summarise the achievements and scientific findings of the study to date, highlight existing difficulties and provide recommendations to facilitate the exploitation of RA2 data.
The ENVISAT Radar Altimeter (RA-2) is the first spaceborne altimeter to provide bursts of up to 2... more The ENVISAT Radar Altimeter (RA-2) is the first spaceborne altimeter to provide bursts of up to 2000 un- averaged individual echoes (IE), and for which both amplitude and phase data are available. This paper presents latest findings in our ongoing investigations into the exploitation of these new data products for new scientific applications over the ocean, land and ice surfaces.
The ENVISAT Radar Altimeter (RA-2) is the first spaceborne altimeter to provide bursts of up to 2... more The ENVISAT Radar Altimeter (RA-2) is the first spaceborne altimeter to provide bursts of up to 2000 un- averaged individual echoes (IE), and for which both amplitude and phase data are available. This paper presents latest findings in our ongoing investigations into the exploitation of these new data products for new scientific applications over the ocean, land and ice surfaces.
Climate Dynamics - CLIM DYNAM, 2002
The demonstration that natural climate variability during the last glacial cycle shifted rapidly ... more The demonstration that natural climate variability during the last glacial cycle shifted rapidly between remarkable extremes has dramatically revised the understanding of climate change. To further advance our understanding, research continues into the timings, geographic distribution, and nature of the millennial-scale climate extremes, and into the mechanisms for intra- and inter-hemispheric transmission of variability through the climate/ocean system. Complementing the traditional definition of the timings of millennial-scale climate variability from ice-core delta18O records, we here further narrow down the temporal constraints by determining statistically significant anomalies in the major ion series of the GISP2 ice core. This exercise offers an objective definition of the timing of climatic anomalies in Northern Hemisphere palaeoclimate proxy records of the last 110,000 years that significantly improves the potential for inter-calibration of 'ice-core tuned' chronos...
ABSTRACT Before the advent of radar altimeters our understanding of the world's climate was b... more ABSTRACT Before the advent of radar altimeters our understanding of the world's climate was based on a few instruments moored off the coats of Europe, Japan and North America and visual observations taken from merchant ships. This information was patchy and in ...
The Southland Front is a localised section of the global southern Subtropical Front that runs alo... more The Southland Front is a localised section of the global southern Subtropical Front that runs along the southeast coast of South Island, New Zealand. It is the boundary between warmer subtropical water in the north and colder subantarctic water from the south. It is associated with strong physical and nutrient gradients which lead to enhanced biological productivity. In this paper we discuss a new method to determine the position and gradient (i.e.strength) of a thermal ocean front. A weighted local likelihood estimation technique in which estimates of frontal parameters (position and strength) are based upon weighted contributions from surrounding points is described. We use this new technique to study the characteristics of the Southland Front and the variation in its properties over the period 1985-2005 from AVHRR imagery. We find the mean path of the Southland Front to be bounded by the 500m isobath and observe a gradual increase in meandering intensity northwards along its path...
Continental Shelf Research, 2010
A detailed analysis of the Southland Front, a shelf-break system off the southeast coast of South... more A detailed analysis of the Southland Front, a shelf-break system off the southeast coast of South Island, New Zealand is presented. The position, temperature, temperature range and width of the front are determined using a new statistical front detection algorithm and 21 years worth of Advanced Very High Resolution Radiometer satellite sea surface temperature (SST) data. Overall, the front is strongest (highest SST gradients) in the summer and winter, and the across front gradient decreases northward in all seasons, consistent with an equatorward decrease in stability and divergence of isobaths. The surface expression of the front moves further offshore during the winter months and is found closest inshore in the summer. Seasonality of the front is strongly controlled by the annual cycle of subtropical and subantarctic water mass temperatures. Both the temperature and strength of the front are interannually variable, and correlated with the El Niñ o-Southern Oscillation (ENSO); they both decrease during El Niñ o, and increase during La Niñ a events. ENSO indices lead changes in the fronts temperature by up to 6 months. Conversely, the gradient may change up to 6 months in advance of peak ENSO indices. The strength and sign of correlations is seasonally dependent.
International Journal of Remote Sensing, 2001
ABSTRACT This study presents a basic analysis of ERS-1 altimeter data, from its 3-day repeat trac... more ABSTRACT This study presents a basic analysis of ERS-1 altimeter data, from its 3-day repeat track periods taken over a section of the Gulf Stream. Along track variations of radar backscatter (0) have been compared with positions of the Gulf Stream derived from the composite maps of sea surface temperature from ship and satellite-sensor observations, produced by NOAA. Variations in 0 , indicating potential interaction with the Gulf Stream, are found at both high and low wind speed conditions. At high wind speeds, comparison with model wind data indicates that peaks in altimeter-derived wind speeds along the path of the North Wall of the Gulf Stream may be related to real increases in wind speed. A possible reason may be a tendency for winter storm tracks to follow the same path as sections of the Gulf Stream. At low wind speeds, minima in altimeterderived winds are associated with sections of the path of the Gulf Stream but there were no comparable occurrences of low wind speeds in model wind fields, with implied consequences for winds derived from satellite sensor data. Although, for low wind speeds, the cause of these 'anomalously high' backscatter regions is unknown, such associated relatively 'calmer' waters have been previously noted as the result either of a tendency for surface slicks to be confined to the shears associated with local current systems or by wave-current interaction mechanisms causing a local reduction in the sea surface roughness.
The Gulf of Guinea is an important region in the tropical Atlantic by virtue of its geographical ... more The Gulf of Guinea is an important region in the tropical Atlantic by virtue of its geographical position with a closed boundary to the north and an open South Atlantic Ocean to the south. This leads to a direct impact on local oceanic and atmospheric forcings. Equatorial Kelvin waves bring sub-thermocline waters to the near surface, where the thermocline was already shallow. At the same time, freshening from river runoff and upwelling changes the density structure considerably. We use Argo data, Sea Surface Height (SSH) from altimetry and winds from scatterometers to study changes in water properties in the region. We characterise the core depth of the equatorial undercurrent (EUC) to observe changes at sub-seasonal to annual time scales at four different chosen areas in the region. We further examine the impact of remote forcing with a view to understanding its influence on the core depth of the EUC and the resulting consequence to local oceanic and atmospheric circulation. Results show that the EUC shallows east of the Guinea Gulf, and south of the equator at the beginning of June as the monsoon winds strengthens. The core depth variability show annual and sub-seasonal cycles which are also reproduced by the SSH data.
Environmetrics, 2008
The world's climate is to a large extent driven by the transport of heat and fresh water in the o... more The world's climate is to a large extent driven by the transport of heat and fresh water in the oceans. Regular monitoring, studying, understanding and forecasting of temperature and salinity at different depths of the oceans are a great scientific challenge. Temperature at the ocean surface can be measured from space. However salinity cannot yet be measured by satellites, and space-based measurements can only ever give us values at the surface. Until recently temperature and salinity measurements within the oceans have had to come from expensive research ships. The Argo float program has been funded by various nations to collect actual measurements and rectify this problem. A Bayesian hierarchical model is proposed in this paper describing the spatio-temporal behavior of the joint distribution of temperature and salinity levels. The model is obtained as a kernel-convolution effect of a single latent spatio-temporal process. Additional terms in the mean describe non-stationarity arising in time and space. Predictive Bayesian model selection criteria have been used to validate the models using data for the year 2003. Illustrative annual prediction maps along with their uncertainty maps are also obtained. The Markov chain Monte Carlo methods are used throughout in the implementation.
Journal of Atmospheric and Oceanic Technology, 2005
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Papers by Peter Challenor