Peter G. Challenor

Variability and predictability of the North Atlantic wave climate

Wave climate across the ocean basins can be described using satellite altimetry; here, we concentrate on the North Atlantic region. Waves in the North Atlantic are strongly seasonal and peak in the winter season. The northeastern sector of the Atlantic and adjoining shelf seas also exhibit exceptionally high interannual variability in the winter, with monthly average significant wave height varying by up to a factor of 2 from one year to the next. The strength and geographical distribution of variability is broadly consistent throughout the winter months (December–March). A large fraction of these wave height anomalies is associated with a single pattern of pressure anomalies that resembles the North Atlantic Oscillation (NAO). A predictor based on NAO dependence is “trained” from relatively recent satellite data and then tested against earlier satellite and in situ data. The predictor is successful in large areas of the North Atlantic, confirming a robust relationship between wave height anomalies and the NAO over the last few decades. A substantial rise (up to 0.6 m) in monthly mean wave heights on the northeastern Atlantic during the latter part of the twentieth century is attributable to changes in the NAO. Substantial residual anomalies in wave heights exist after the influence of the NAO has been subtracted; these are partly explained by a second pair of North Atlantic patterns in wave height anomalies and sea level pressure anomalies. This “East Atlantic” pattern is particularly influential in midwinter and affects the southern part of the northeastern sector (including the region of Seven Stones Light Vessel)

Rossby waves detected in global ocean colour data

We demonstrate for the first time the detectability of mid-latitude Rossby waves in global ocean colour data from the Japanese Ocean Colour and Temperature Scanner (OCTS) and U.S. Sea-viewing Wide Field-of-view Sensor (SeaWiFS) radiometers. By producing longitude-time plots of the merged OCTS and SeaWiFS datasets we observe at some latitudes westward propagating signals. Their signature is much weaker than the annual phytoplankton cycle, but can be highlighted by filtering the plots. The main propagating speed is estimated with the Radon Transform and increases equatorward, as expected for Rossby waves. A comparison with both speeds derived from altimeter data and the zonal mean of the speed predicted by a recent theory of Rossby wave propagation shows a broad agreement. We conclude that Rossby waves are sometimes observable in the ocean colour field and thus have some effects on biology, and we suggest two simple hypotheses for the underlying interaction mechanism

Numerical simulation of a random sea: a common error and its effect upon wave group statistics

Abstract A commonly used method of simulating ocean waves from a specified frequency spectrum is shown to be incorrect. The method consists of adding numerous sine curves with random phases; and the error arises from assuming that the amplitudes of these component sine waves are deterministic, when they are in fact random variables. Methods of using random amplitudes are described and only one is found to be satisfactory. In this method the number of random values simulated — and then transformed with an inverse FFT — equals the required number of simulated data points. So simulation in the frequency domain can only give relatively short runs; it is necessary to work in the time domain if arbitrarily long runs are required. Errors in wave group statistics derived from the incorrect simulation method are discussed and related to discrepancies reported between groupiness in simulated data and ocean measurements.

A Markov chain Monte Carlo method for estimation and assimilation into models

Abstract The arrival of satellite-borne ocean colour sensors means that there will soon be a wealth of observations of the surface concentration of chlorophyll in the worlds oceans. These observations can be used to improve our understanding of the oceanic ecosystem if the appropriate data assimilation techniques are available to combine them with an ecosystem model. In this paper we explore a novel method, based on Bayes Theorem and a Monte Carlo Markov Chain algorithm, of estimating a subset of the parameters in a seven compartment ecosystem model. The model describes the flows of nitrogen amongst phytoplankton, zooplankton, nitrate, bacteria, ammonium, dissolved organic nitrogen and detritus. We first generate synthetic observations from the model and then, in three separate experiments, try to recover subsets of the model parameters from clean and noisy versions of these. Bayes Theorem allows us to combine both prior information on the parameter values and the observations to generate a posterior probability density function of the parameters. The Metropolis-Hastings algorithm then allows us to produce Markov chains that sample this posterior probability density function and recover the parameter means, variances and standard errors. We find that the technique is very successful in recovering information on a small number of parameters but that the time required to solve the model makes it impractical to find second order properties of more than about ten of the model parameters.

A soft solid surface on Titan as revealed by the Huygens Surface Science Package

The surface of Saturns largest satellite—Titan—is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titans atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

A global study of diurnal warming using infrared satellite-derived sea surface temperature

Ten years of global infrared satellite data from NOAAs Advanced Very High Resolution Radiometer (AVHRR) have been analysed to identify the global temporal and spatial distribution of diurnal warming and assess its impacts. Daily night-time SSTs are subtracted from adjacent day-time SST to give an estimate of diurnal warming (/spl Delta/T /sub day-night/). The results reveal large regions in the tropics and mid-latitudes that are frequently susceptible to diurnal warming each year, dictated by seasonal wind and insolation patterns. The analysis also reveals how the spatial distribution and magnitude of /spl Delta/T varies with the drift of the satellite orbit as it shifts from a local afternoon overpass time of 14h00 to 16h00. The results highlight the sensitivity of SST to its sampling time and reveal the importance of the diurnal cycle for the interpretation of SST measurements. The results also suggest important implications for sea-air interaction and upper ocean processes which are dependent on SST.

Concurrent altimeter and infrared observations of Rossby wave propagation near 34°N in the northeast Atlantic

We present observations of long-wavelength baroclinic Rossby waves near 34° N in the Northeast Atlantic ocean (east of the Mid-Atlantic Ridge) using both TOPEX/POSEIDON sea surface height (SSH) altimeter data and ERS-1 Along-Track Scanning Radiometer sea surface temperature (SST) measurements. Using the Fourier transforms of longitude-time diagrams of the zonal gradients of SST and SSH we locate the spectral components corresponding to westward wave propagation and estimate wavelength, period and propagation speed. The energy associated with the propagating waves in that region is distinctly higher than in the surrounding areas. The similarities between the properties of the SST and SSH zonal gradients are remarkable with the same propagating signals clearly observed in both fields. It is believed that the interaction between the Rossby waves and the Azores current could play a major role in the amplification of the waves themselves.

A Two-Parameter Wind Speed Algorithm for Ku-Band Altimeters

Globally distributed crossovers of altimeter and scatterometer observations clearly demonstrate that ocean altimeter backscatter correlates with both the near-surface wind speed and the sea state. Satellite data from TOPEX/Poseidon and NSCAT are used to develop an empirical altimeter wind speed model that attenuates the sea-state signature and improves upon the present operational altimeter wind model. The inversion is defined using a multilayer perceptron neural network with altimeter-derived backscatter and significant wave height as inputs. Comparisons between this new model and past single input routines indicates that the rms wind error is reduced by 10%‐15% in tandem with the lowering of wind error residuals dependent on the sea state. Both model intercomparison and validation of the new routine are detailed, including the use of large independent data compilations that include the SeaWinds and ERS scatterometers, ECMWF wind fields, and buoy measurements. The model provides consistent improvement against these varied sources with a wind-independent bias below 0.3 m s21. The continuous form of the defined function, along with the global data used in its derivation, suggest an algorithm suitable for operational application to Ku-band altimeters. Further model improvement through wave height inclusion is limited due to an inherent multivaluedness between any single realization of the altimeter measurement pair [s o, HS] and observed near-surface winds. This ambiguity indicates that HS is a limited proxy for variable gravity wave properties that impact upon altimeter backscatter.

Use of the 3D Radon Transform to Examine the Properties of Oceanic Rossby Waves

One of the most successful applications of satellite-borne radar altimeter data over the oceans in recent years has been the extraction of information about long-wavelength baroclinic Rossby (or planetary) waves, which play a significant role in ocean circulation and climate dynamics. These waves cross ocean basins from east to west at speeds of few centimetres per second at mid-latitudes. The cross-basin propagation time may therefore be several months or even years and an accurate estimation of the speed of the waves is important. We review the methods for obtaining information on Rossby wave velocity from altimetry data, particularly the two-dimensional Radon transform. Unfortunately the use of longitude-time plots, although it allows the estimation of the zonal phase speeds, does not give any information on the speed vector when the propagation of the waves is not purely zonal (east-west). We show how the two-dimensional Radon Transform can be generalised to three dimensions, enabling not only the true propagation velocity component to be determined, but also the direction of the waves and thus any deviation from the pure-westward case. As examples of the application of this extended technique, we show maps of direction, speed and energy of Rossby waves in the North Atlantic Ocean.

Towards a vulnerability assessment of the UK and northern European coasts: the role of regional climate variability

Within the framework of a Tyndall Centre research project, sea level and wave changes around the UK and in the North Sea have been analysed. This paper integrates the results of this project. Many aspects of the contribution of the North Atlantic Oscillation (NAO) to sea level and wave height have been resolved. The NAO is a major forcing parameter for sea-level variability. Strong positive response to increasing NAO was observed in the shallow parts of the North Sea, while slightly negative response was found in the southwest part of the UK. The cause of the strong positive response is mainly the increased westerly winds. The NAO increase during the last decades has affected both the mean sea level and the extreme sea levels in the North Sea. The derived spatial distribution of the NAO-related variability of sea level allows the development of scenarios for future sea level and wave height in the region. Because the response of sea level to the NAO is found to be variable in time across all frequency bands, there is some inherent uncertainty in the use of the empirical relationships to develop scenarios of future sea level. Nevertheless, as it remains uncertain whether the multi-decadal NAO variability is related to climate change, the use of the empirical relationships in developing scenarios is justified. The resulting scenarios demonstrate: (i) that the use of regional estimates of sea level increase the projected range of sea-level change by 50% and (ii) that the contribution of the NAO to winter sea-level variability increases the range of uncertainty by a further 10–20 cm. On the assumption that the general circulation models have some skill in simulating the future NAO change, then the NAO contribution to sea-level change around the UK is expected to be very small (<4 cm) by 2080. Wave heights are also sensitive to the NAO changes, especially in the western coasts of the UK. Under the same scenarios for future NAO changes, the projected significant wave-height changes in the northeast Atlantic will exceed 0.4 m. In addition, wave-direction changes of around 20° per unit NAO index have been documented for one location. Such changes raise the possibility of consequential alteration of coastal erosion.