Graham D. Quartly

On the role of the Agulhas system in ocean circulation and climate

The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments—backed by palaeoceanographic and sustained modern observations—are required to establish firmly the role of the Agulhas system in a warming climate.

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.

The Effects of Rain on Topex Radar Altimeter Data

Abstract Rain has long been categorized as a contaminant of altimeter data, but little has been done previously to ascertain the magnitude and frequency of its effect or its geographical distribution. Proceeding from recent analysis of ERS-1 data, and the insight gained, this paper addresses all these issues, highlighting the particular advantages of Topexs dual-frequency capability. The primary effect of rain is on the observed backscatter values σ0 through attenuation of the radar signal. An empirical relationship is derived linking the observed σ0 values at C band and Ku band. Through a series of case studies and global analyses, prominent departures from this relationship are shown to be caused by regions of significant precipitation. As it is predominantly the Ku-band returns that are affected, C-band σ0,s are likely to be of great use for the determination of wind speeds in regions or events of intense rain. The different attenuation rates give dual-frequency altimetry the potential to yield inform...

Retracking Altimeter Waveforms Near the Coasts

There has been considerable interest in the past few years in addressing some of the long-term technical difficulties associated with retrieving valid measurements from satellite altimeters in coastal areas, where high levels of human activities are putting increasing demand for information about sea level, wind and wave conditions. Developments of altimeter waveform retracking techniques, together with the now-established practice of giving users access to altimeter waveform data, has led to rapid progress in our understanding of the challenges posed by waveform shapes in the vicinity of land. In this chapter, we present observational evidence of the huge diversity and complexity of waveforms seen by contemporary altimeters in coastal areas. We proceed with a review of waveform retracking methods, examining first empirical methods, then so-called physically-based methods, including discussion of some of their implementation intricacies. We proceed with providing examples of the application of waveform retracking methods to coastal altimeter waveforms in coastal regions around the world. Finally, we explore some of the new ideas on how it may be possible to exploit prior knowledge, for example about the statistics or the along-track evolution of ocean properties in the coastal domain, to improve the estimation of geophysical parameters. Innovative schemes, such as iterative retracking or simultaneous batch retracking, are discussed as new ways to yield unbiased parameter estimation for land-contaminated waveforms much closer to the land/water interface than is currently possible.

Seasonal Variations in the Region of the Agulhas Retroflection: Studies with Geosat and FRAM

Abstract In this paper seasonal variations in the Agulhas Retroflection region are studied by analyzing data from the radar altimeter, flown on the Geosat satellite, and output from the Fine Resolution Antarctic Model (FRAM). The observations of mesoscale variability from Geosat suggest that any seasonal variation that exists is confined to certain limited areas of the Agulhas Retroflection region. Results from FRAM appear to show no seasonal variability, despite the fact that the model is driven by a seasonally varying wind field. A by-product of this study is a comparison between Geosat observations and FRAM output. This shows that, apart from the issue of seasonal variations, there is reasonable agreement, and that such differences as do exist give insights into how the model might be improved.

Global precipitation statistics from dual‐frequency TOPEX altimetry

TOPEX is a nadir-pointing dual-frequency radar altimeter that has been in orbit for more than 6 years. Empirical methods, based on the close correlation between the C band and K u band backscatter, yield an estimate of the rain rate. The principal rain bands of the Intertropical Convergence Zone (ITCZ) are shown to be bounded by marine deserts receiving only one hundredth of the rainfall of the ITCZ. Analysis of the entire data set reveals variations in the frequency of oceanic precipitation over a range of timescales. For most regions the diurnal cycle of precipitation has a minimum likelihood at evening local time, with maximum being early morning for the tropics and northern oceans, but late afternoon for the Southern Ocean. On the seasonal cycle, the Pacific ITCZ reaches its southernmost limit during March-April, and is then nearly always accompanied by a secondary but weaker band an equal distance to the south of the equator. Finally, on the interannual timescale the 1997-1998 El Nino is seen to cause a broadening of the Pacific ITCZ and its translation to the south and east, with increased global levels of rainfall rather than just a geographical redistribution. High-resolution sampling shows that the length scale of rain events varies not only latitudinally, but also longitudinally according to whether they lie in regions of genesis or decay of storms. Simultaneous wind and wave information from TOPEX indicates that the rainfall in midlatitudes occurs preferentially with high winds, but in the tropics is associated with swell. The long-term near-global single-instrument data set from TOPEX is thus complementary to those from other sensors.

Modeling Envisat RA-2 Waveforms in the Coastal Zone: Case Study of Calm Water Contamination

Radar altimeters have so far had limited use in the coastal zone, the area with most societal impact. This is due to both lack of, or insufficient accuracy in the necessary corrections, and more complicated altimeter signals. This letter examines waveform data from the Envisat RA-2 as it passes regularly over Pianosa (a 10-km2 island in the northwestern Mediterranean). Forty-six repeat passes were analyzed, with most showing a reduction in signal upon passing over the island, with weak early returns corresponding to the reflections from land. Intriguingly, one third of cases showed an anomalously bright hyperbolic feature. This feature may be due to extremely calm waters in the Golfo della Botte (northern side of the island), but the cause of its intermittency is not clear. The modeling of waveforms in such a complex land/sea environment demonstrates the potential for sea surface height retrievals much closer to the coast than is achieved by routine processing. The long-term development of altimetric records in the coastal zone will not only improve the calibration of altimetric data with coastal tide gauges but also greatly enhance the study of storm surges and other coastal phenomena.

Analyzing Altimeter Artifacts: Statistical Properties of Ocean Waveforms

In this paper waveforms, that is, returns from the ocean surface, from a number of spaceborne radar altimeter instruments [European Remote-sensing Satellites (ERS-1 and -2), TOPEX, and Poseidon] are examined. This is the first paper to analyze waveform data from a number of altimeters in a consistent manner. Mean shapes and various statistical properties (bin-to-bin correlations, number of independent samples) were determined and the authors comment on their anomalies. The analyses were performed for data over the deep ocean, as that is the best understood surface. However, the determined functional characteristics of the individual altimeters are applicable to their operation over all surfaces. The implications of the existence of these anomalies for the retrieval of geophysical parameters from radar altimeter data are discussed. It is argued that the need for physically based theories, in order to understand radar altimeter returns from the ocean (or indeed any other) surface, implies a need for the engineering and software design of the instrument to be such as to avoid spurious anomalies in the waveforms.

Observations of Rossby wave propagation in the Northeast Atlantic with TOPEX/POSEIDON altimetry

Abstract A technique is described to observe Rossby waves in the oceans using altimeter data, which makes use of longitude/time diagrams of sea surface height anomalies at a given latitude, relying on the zonal propagation of those waves. By means of the Fast Fourier and Radon Transforms of the longitude/time diagrams it is possible to locate the spectral components corresponding to wave propagation and to give an approximate estimation of some of their characteristics such as their height and speed. The results obtained by using data from the TOPEX/POSEIDON radar altimeters in the Northeast Atlantic highlight a narrow ‘preferred’ zonal band of propagation for Rossby waves around 33°–34° N. The speeds of the dominant signals at different latitudes have been computed both in Fourier space and in Radon space: they are in good agreement with the prediction of a revised theory of Rossby wave propagation, which has recently been introduced to explain the discrepancy between observed wave speeds and the speeds computed from the classical theory.

An intercomparison of global oceanic precipitation climatologies

Large-scale patterns of precipitation are important for the changes they may effect upon the circulation of the ocean. However, marine precipitation is very hard to quantify accurately. Four independent climatologies are examined to compare their estimates of the annual mean precipitation, and the seasonal and interannual variations. One data set, Global Precipitation Climatology Project (GPCP), is based upon satellite data, the other three on output of weather forecast reanalyses from the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-range Weather Forecasts (ECMWF). Although all datasets have their errors, there is general agreement on the geographical patterns of precipitation. All the models had higher rain rates in the tropics than shown by the satellite data, and also greater seasonal ranges. However, GPCP has 10-25% more precipitation than NCEP and ECMWF in most of the southern regions, because of their weak representation of convergence zones; NCEP2, a more recent version of the NCEP reanalysis, shows a marked improvement in this area. However, in most regions NCEP2 exhibits a larger seasonal range than shown by other datasets, particularly for the tropical Pacific. Both NCEP and NCEP2 often show a seasonal cycle lagging two months or more behind GPCP. Of the three reanalysis climatologies, ECMWF appears best at realising the position and migration of rain features. The interannual variations are correlated between all four datasets, however the correlation coefficient is only large for regions that have a strong response to El Nino and La Nina event, or for comparisons of the two NCEP reanalyses. Of the datasets evaluated, GPCP has the most internal consistency, with no long-term trend in its regional averages, and it alone shows the deficit in Mediterranean precipitation coincident with the Eastern Mediterranean Transient.