Paolo Cipollini

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.

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

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.

Retrieval of sea water optically active parameters from hyperspectral data by means of generalized radial basis function neural networks

The authors present a new methodology for estimating the concentration of sea water optically active constituents from remotely sensed hyperspectral data, based on generalized radial basis function neural networks (GRBF-NNs). This family of NNs is particularly suited to approximate relationships like those between hyperspectral reflectance data and the concentrations of optically active constituents of the water body, which are highly nonlinear, especially in case II waters. Three main water constituents are taken into account: phytoplankton, nonchlorophyllous particles, and yellow substance. Each parameter is estimated by means of a specific multi-input single-output GRBF-NN. The authors adopt a recently proposed network learning strategy based on the combined use of the regression tree procedure and forward selection. The effectiveness of this approach, which is completely general and can be easily applied to any hyperspectral sensor, is proved using data simulated with an ocean color model over the channels of the medium resolution imaging spectrometer (MERIS), the new generation ESA sensor to be launched in 2001. The authors define the estimation algorithms over waters of cases I, II, and I+II and compare their performance with that of classical band-ratio, single-band, and multilinear algorithms. Generally, the GRBF-NN algorithms outperform the classical ones, except for the multilinear over case I waters. A particular improvement Is over case II waters, where the mean square error (MSE) can be reduced by one or two orders of magnitude over the error of multilinear and band-ratio algorithms, respectively.

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.

Exploiting the potential of an improved multimission altimetric data set over the coastal ocean

Until now, most satellite altimetry studies of the coastal ocean have been based on along-track data from a single mission, whereas up to four missions were operative in 2002–2005. Here, to monitor the coastal ocean we have applied specialized corrections and dedicated processing strategies to compute a multimission data set at a mean distance of 32 km of the coast. The resulting altimetric data set is compared with sea level data from three in situ stations over a coastal zone of the northwestern Mediterranean. The mean rms difference between this data set and the sea level stations is 2.9 cm against 3.7 cm when using the AVISO altimetric product. Comparison of altimeter-derived geostrophic velocities with a mooring also shows that the spatial and temporal variability of the surface current field is well reproduced. The agreement with in situ measurements extends to intraseasonal time scales showing a significant improvement compared to previous studies in the 50 km coastal-band.

Propagation characteristics of extratropical planetary waves observed in the ATSR global sea surface temperature record

This paper examines the characteristics of planetary wave signatures that have been found in the Along Track Scanning Radiometer averaged sea surface temperature (ASST) record for 1991–1996. Longitude-time plots for every latitude between 5° and 50°, north and south, reveal westward propagating wave-like patterns at many locations, whose speed decreases with latitude like baroclinic Rossby waves. A two-dimensional Radon transform method is used to measure the wave speed and its variation with location and time, which broadly matches the Rossby wave speeds predicted by the most recent theory and those measured by TOPEX altimetry, although there are some discrepancies. At low latitudes the thermally detected speeds are slower than expected, a possible consequence of sampling limitations. Wave signatures are clearest between 25° and 40°S, where the meridional temperature gradient is strongest. Here observed speeds are 20–30% greater than theoretical predictions. Planetary wave speed varies considerably with longitude. In general, it increases toward the west of ocean basins, and distinct differences between ocean basins are evident. The propagation characteristics of the waves appear to change abruptly at locations consistent with latitudinal variations in seafloor bathymetry, particularly midocean ridges. In addition, eastward propagating signatures are found in the Southern Ocean. The results demonstrate the value of the ASST data set as a tool for studying basin-scale wave processes as a complement to the use of altimetry. By observing the thermal signature of Rossby waves the method has the potential to clarify their influence on air-sea interaction processes and to contribute to climate modeling studies.

Global fields of sea surface dimethylsulfide predicted from chlorophyll, nutrients and light

Abstract The major difficulty in estimating global sea–air fluxes of dimethylsulphide (DMS) is in interpolating measured seawater DMS concentrations to create seasonally resolved gridded composites. Attempts to correlate DMS with variables that can be mapped globally, e.g. chlorophyll, have not yielded reliable relationships. A comprehensive database of DMS measurements has recently been assembled by Kettle et al. [Global Biogeochem. Cycles 13 (1999) 399]. This database, which contains chlorophyll as a recorded variable, was extended by merging nutrients and light from globally gridded fields. A new equation was developed whereby DMS is predicted from the product of chlorophyll ( C , mg m −3 ), light ( J , mean daily shortwave, W m −2 ) and a nutrient term ( Q , dimensionless) using a “broken-stick” regression: DMS =a, log 10 (CJQ)≤s DMS =b[ log 10 (CJQ)−s]+a, log 10 (CJQ)>s where Q =N/( K N +N), N is nitrate (mmol m −3 ) and K N is the half saturation constant for nitrate uptake by phytoplankton (0.5 mmol m −3 ). Fitted parameter values are: a =2.29, b =8.24, s =1.72. Monthly maps of global DMS were generated by combining these equations with ocean color data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). The resulting high DMS concentrations in high latitude, upwelling and shelf areas are consistent with observed patterns. Predicted global seasonally averaged mean DMS is 2.66 nM. The further application of gas transfer equations to these fields leads to estimates of globally integrated DMS fluxes from ocean to atmosphere of 0.86 and 1.01 Tmol S year −1 for two formulations of piston velocity. The simplicity of the new relationship makes it suitable for implementation in global ocean general circulation models. The relationship does not however resolve DMS variability in low-DMS areas, which constitute large tracts of the open ocean, and should therefore be used with caution in localized studies.

A study of the Alboran sea mesoscale system by means of empirical orthogonal function decomposition of satellite data

Abstract This paper presents the results of a combined empirical orthogonal function (EOF) analysis of Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data and sea-viewing wide field-of-view sensor (SeaWiFS) chlorophyll concentration data over the Alboran Sea (Western Mediterranean), covering a period of 1 year (November 1997–October 1998). The aim of this study is to go beyond the limited temporal extent of available in situ measurements by inferring the temporal and spatial variability of the Alboran Gyre system from long temporal series of satellite observations, in order to gain insight on the interactions between the circulation and the biological activity in the system. In this context, EOF decomposition permits concise and synoptic representation of the effects of physical and biological phenomena traced by SST and chlorophyll concentration. Thus, it is possible to focus the analysis on the most significant phenomena and to understand better the complex interactions between physics and biology at the mesoscale. The results of the EOF analysis of AVHRR-SST and SeaWiFS-chlorophyll concentration data are presented and discussed in detail. These improve and complement the knowledge acquired during the in situ observational campaigns of the MAST-III Observations and Modelling of Eddy scale Geostrophic and Ageostrophic motion (OMEGA) Project.

Physical and biological mechanisms for planetary waves observed in satellite-derived chlorophyll

We examine the evidence for global propagation of planetary wavelike features in sea-surface chlorophyll. Over much of the midlatitude ocean, westward propagating signals are seen that travel at the same speed as that predicted for long planetary waves. We then test three mechanisms for production of this signal. These are: horizontal (passive) north-south advection by the wave against a mean background gradient; vertical upwelling of nitrate, which is converted into chlorophyll; and vertical upwelling of chlorophyll itself. The tests involve comparisons of the amplitude and phase of the predicted signal with observations. The horizontal advective process predicts an amplitude for chlorophyll fluctuations that is in fair agreement with the data, though both overestimating and underestimating in places. The predictions for the phase difference between the chlorophyll and sea surface height signatures are in good agreement with the data. The upwelling biological mechanism could potentially give a large signal in the chlorophyll field, but the predicted amplitude patterns and the predicted phase difference (which is everywhere negative) are not in accord with the observations. Except in a few regions, the amplitude predicted by upwelling of chlorophyll is small compared with the horizontal advection mechanism. We conclude that over most of the ocean, the chlorophyll signal is well explained by horizontal advective processes, although we cannot rule out that there exist locations where additional biological mechanisms may be responsible for at least part of the signal.