Wind-driven currents in the coastal and equatorial upwelling regions

During the last two decades the scientific community has recognised the importance of the tropical Atlantic Ocean and the upwelling regions on the Earth's climate. This recognition has opened new questions such as: ¿What are the mechanisms for the ocean to adjust to variations in atmospheric forcing?, ¿Is there any indirect relation between the atmospheric seasonal cycle and the response of the surface ocean?, ¿How are the meridional boundary flows connected with the zonal jets in the interior ocean?, ¿What is the relevance of these processes in the redistribution of properties such as water mass, heat and fresh water? In this dissertation we explore several elements that determine the effect of the surface wind stress onto the processes within the near-surface ocean. The work focuses on recognizing the (subinertial) response mechanisms of the ocean surface to the spatial and temporal wind variations in two upwelling regions: a coastal region off Northwest Africa, in the area near Cape Blanc, and an oceanic region, in the equatorial Atlantic. With this purpose we use in situ and satellite data as well as numerical data from a high-resolution circulation model. The analysis of these data has been done with several methodologies, in some cases requiring substantial developments and tuning for local applications. The implementation of the Maximum Cross-Correlation Method has allowed determining some of the characteristics of the instantaneous and mean surface fields, during winter and spring, in the upwelling region north and south of Cape Blanc. We have identified three regions which are characterized by different responses to short-time changes of the along-shore wind stress. North of Cape Blanc stands out the intensity of the coastal baroclinic jet, in the Cape Verde basin the mesoscalar structures are relatively weak and large, and off Cape Blanc there is along-shore convergence which traduces in the formation of a normal-to-shore giant surface filament. The analyses of time series corresponding to several upwelling indexes show that the atmospheric forcing and the oceanic response are different north and south of Cape Blanc and during the first and second trimester of the year. The total subinertial flux may be represented as the combination of a surface Ekman flux (calculated as the Ekman transport divided by the thickness of the surface mixed layer) and the surface geostrophic current (deduced from altimetry satellite images). One of the most relevant results is that the temporal and spatial changes in the normal-to-shore Ekman transport influence the intensity of the geostrophic (baroclinic) coastal jet, therefore affecting the corresponding along-shore convergence (e.g. becoming intensified off Cape Blanc) and the offshore transport of upwelled waters. The dissertation has also aimed at understanding the patterns of seasonal variability in the equatorial Atlantic Ocean through the statistical analysis of time series of sea level pressure, sea surface wind stress, sea surface height, and the circulation of the near-surface ocean. The data reveals a predominant annual component in all these variables, closely related to the latitudinal oscillation of the Inter-Tropical Convergence Zone. The equatorial divergence of the Ekman transport is well correlated with the intensity of the zonal system of equatorial currents, which includes the Equatorial Undercurrent and its northern and southern branches. Additionally, the seasonal appearance of the North Equatorial Counter Current during (boreal) summer and fall is related to the meridional convergence of the Ekman transport during those same seasons, which leads to a temporal rise of sea level and the generation of an eastward current in geostrophic balance. In general, the divergence/convergence of meridional Ekman transport is dominant in the northern hemisphere and of lesser relevance in the southern hemisphere.Durante las últimas dos décadas la comunidad científica internacional ha pasado a reconocer la importancia del Océano Atlántico tropical y las regiones de afloramiento en el clima terrestre. Este reconocimiento ha abierto nuevos interrogantes, tales como: ¿Cuáles son los mecanismos de ajuste del océano a las variaciones en el forzamiento atmosférico?, ¿Existe algún tipo de relación indirecta entre el ciclo estacional atmosférico y la respuesta del océano superficial?, ¿Cómo se conectan los flujos oceánicos meridionales en los contornos con los flujos zonales en el océano interior?, ¿Cuál es la importancia de estos procesos en la redistribución de propiedades tales como masa, calor y agua dulce? En esta tesis se exploran diversos elementos que determinan el efecto del esfuerzo del viento superficial sobre los procesos que ocurren en el océano superficial. El trabajo se centra en reconocer cuales son los mecanismos (subinerciales) de respuesta de la superficie del océano a las variaciones espaciales y temporales del viento en dos regiones de afloramiento: una costera al Noroeste de África, en el área cercana a Cabo Blanco, y otra oceánica, en el Atlántico ecuatorial. Para ello se emplean observaciones in situ, datos satelitales y datos numéricos provenientes de un modelo de circulación de alta resolución. El análisis de estos datos se ha realizado con diversas metodologías, cuya aplicación en algunos casos ha requerido un esfuerzo substancial de desarrollo y puesta a punto. La implementación del método de Máximas Correlaciones Cruzadas ha permitido determinar algunas de las características de los campos instantáneos y medios de velocidades superficiales, durante invierno y primavera, en la región del afloramiento de Cabo Blanco. Se han identificando tres regiones caracterizadas por tener respuestas distintas a los cambios que el viento paralelo a la costa experimenta en escalas temporales cortas. Al norte de Cabo Blanco destaca la intensidad del chorro baroclino costero, en la cuenca de Cabo Verde se aprecian estructuras mesoscalares relativamente débiles y grandes, y frente a Cabo Blanco existe convergencia paralela a costa que se traduce en flujo normal a costa en forma de un gran filamento superficial. El análisis de las series temporales de diversos índices de afloramiento muestra que los forzamientos atmosféricos y las respuestas oceánicas son distintas al norte y sur de Cabo Blanco y durante el primer y segundo trimestre del año. El flujo subinercial resultante se puede representar como la combinación de un flujo superficial de Ekman (calculado como el transporte de Ekman dividido por la profundidad de la capa de mezcla) y la corriente geostrófica superficial (deducida a partir de imágenes satelitales de altimetría). Uno de los resultados más relevantes es que los cambios espaciales y temporales en el transporte de Ekman perpendicular a costa influyen sobre la intensidad del chorro geostrófico (baroclíno) costero, y por tanto afectan su convergencia a lo largo de la costa intensificándose, por ejemplo, frente a Cabo Blanco) y la transferencia neta de aguas afloradas hacia el océano interior. La tesis también se ha encaminado a investigar los patrones de variabilidad estacional del Océano Atlántico ecuatorial, a través del análisis estadístico de series temporales de presión a nivel de mar, esfuerzo cortante del viento sobre la superficie oceánica, elevación del océano superficial, y la circulación oceánica superficial. Los datos revelan una fuerte componente anual en estas variables, estrechamente vinculada con la oscilación meridional de la Zona de Convergencia Intertropical. La divergencia ecuatorial del transporte de Ekman se correlaciona adecuadamente con la intensidad del sistema de corrientes zonales ecuatoriales, que incluyen la Corriente Ecuatorial Subsuperficial y sus ramales norte y sur. Asimismo, la aparición estacional de la Contra-Corriente Ecuatorial durante verano y otoño (boreal) se relaciona con la convergencia meridional en el transporte de Ekman que tiene lugar durante estas épocas, lo cual conduce a una subida del nivel del mar y la generación de una corriente hacia el este en balance geostrófico. En general se aprecia que los procesos de divergencia/convergencia del transporte meridional de Ekman son dominantes en el hemisferio norte y de menor relevancia en el hemisferio sur. Finalmente, con el fin de comprender mejor la dinámica ecuatorial, se ha desarrollado un modelo sencillo que permite cuantificar el aporte de la divergencia de Ekman al flujo zonal en varias bandas zonales características. Se han identificado dos condiciones típicas extremas, en primavera y otoño, y se han calculado la divergencia/convergencia meridional a través de líneas definidas por un máximo en la elevación de la superficie del mar. Bajo la suposición de que el transporte zonal cerca del contorno oriental (aquí tomada a una longitud de 0º) es nulo, se estima que la franja ecuatorial presenta, en su margen occidental, valores máximos de transporte correspondientes a 58 Sv en primavera y 27 Sv durante otoño, cuyo origen es el sistema de corrientes de frontera oeste

NASA Sea Ice Validation Program for the Defense Meteorological Satellite Program Special Sensor Microwave Imager

The history of the program is described along with the SSM/I sensor, including its calibration and geolocation correction procedures used by NASA, SSM/I data flow, and the NASA program to distribute polar gridded SSM/I radiances and sea ice concentrations (SIC) on CD-ROMs. Following a discussion of the NASA algorithm used to convert SSM/I radiances to SICs, results of 95 SSM/I-MSS Landsat IC comparisons for regions in both the Arctic and the Antarctic are presented. The Landsat comparisons show that the overall algorithm accuracy under winter conditions is 7 pct. on average with 4 pct. negative bias. Next, high resolution active and passive microwave image mosaics from coordinated NASA and Navy aircraft underflights over regions of the Beaufort and Chukchi seas in March 1988 were used to show that the algorithm multiyear IC accuracy is 11 pct. on average with a positive bias of 12 pct. Ice edge crossings of the Bering Sea by the NASA DC-8 aircraft were used to show that the SSM/I 15 pct. ice concentration contour corresponds best to the location of the initial bands at the ice edge. Finally, a summary of results and recommendations for improving the SIC retrievals from spaceborne radiometers are provided

Satellite techniques for studying ocean circulation.

Satellites provide a unique semi-synoptic view of the world's oceans. In recent years, two forms of remotely sensed data have been particularly useful in providing information about ocean circulation, namely altimetric measurements of sea surface height (SSH) and infrared radiometric measurements of sea surface temperature (SST). However, in order to interpret new types of data correctly and obtain meaningful results, new techniques must be developed. In this thesis, techniques to process TOPEX/POSEIDON radar altimeter SSH data and Along-Track Scanning Radiometer (ATSR) SST data are developed. These techniques are tested in the South Atlantic Ocean. The effectiveness of an existing technique to correct for across-track variations in altimeter sampling and the associated SSH errors due to across-track mean sea surface variation is studied. The effects of orbit error removal and interpolation on altimeter data are investigated using ocean model data from the Parallel Ocean Climate Model (POCM). A technique to obtain absolute velocities from altimetry alone is implemented and its accuracy assessed through use of the POCM data. Remnant cloud contamination in the ATSR 0.5° night SST data is discovered and a new technique to remove the cloud contamination is proposed and tested. The seasonality of this cloud contamination is investigated and is found to coincide with the occurrence of marine stratiform clouds. Finally, the relationship between SST and SSH data is examined. It is found that spatial cross-correlations between SST and SSH are surprisingly high (~0.7) in regions associated with fronts and mesoscale variability such as the Agulhas, the Antarctic Circumpolar Current and the Brazil/Falkland regions. In these areas, coherency analysis reveals that the cross-correlations peak at wavelengths of 400-600 km. The strength of the cross-correlations is found to be seasonal, peaking in the winter and minimising in summer

Estimation of Ocean Surface Currents from Maximum Cross Correlation applied to GOCI geostationary satellite remote sensing data over the Tsushima (Korea) Straits

Attempts to automatically estimate surface current velocities from satellite-derived thermal or visible imagery face the limitations of data occlusion due to cloud cover, the complex evolution of features and the degradation of their surface signature. The Geostationary Ocean Color Imager (GOCI) provides a chance to reappraise such techniques due to its multi-year record of hourly high-resolution visible spectrum data. Here we present the results of applying a Maximum Cross Correlation (MCC) technique to GOCI data. Using a combination of simulated and real data we derive suitable processing parameters and examine the robustness of different satellite products, those being water-leaving radiance and chlorophyll concentration. These estimates of surface currents are evaluated using High Frequency (HF) radar systems located in the Tsushima (Korea) Strait. We show the performance of the MCC approach varies depending on the amount of missing data and the presence of strong optical contrasts. Using simulated data it was found that patchy cloud cover occupying 25% of the image pair reduces the number of vectors by 20% compared to using perfect images. Root mean square errors between the MCC and HF radar velocities are of the order of 20 cm s−1. Performance varies depending on the wavelength of the data with the blue-green products out-performing the red and near infra-red products. Application of MCC to GOCI chlorophyll data results in similar performance to radiances in the blue-green bands. The technique has been demonstrated using specific examples of an eddy feature and tidal induced features in the region. This article is protected by copyright. All rights reserved

Satellite radar altimetry of sea ice

The thesis concerns the analysis and interpretation of data from satellite borne radar altimeters over ice covered ocean surfaces. The applications of radar altimetry are described in detail and consider monitoring global climate change, the role that sea ice plays in the climate system, operational applications and the extension of high precision surface elevation measurements into areas of sea ice. The general nature of sea ice cover is discussed and a list of requirements for sea ice monitoring is provided and the capability of different satellite sensors to satisfy needs is examined. The operation of satellite borne altimeter over non-ocean surfaces is discussed in detail. Theories of radar backscatter over sea ice are described and are used to predict the radar altimeter response to different types of sea ice cover. Methods employed for analysis of altimeter data over sea ice are also described. Data from the Seasat altimeter is examined on a regional and global scale and compared with sea ice climatology. Data from the Geosat altimeter is compared with co-incident imagery from the Advanced Very High Resolution Radiometer and also from airborne Synthetic Aperture Radar. Correlations are observed between the altimeter data and imagery for the ice edge position, zones within the ice cover, new ice and leads, vast floes and the fast ice boundary. An analysis of data collected by the Geosat altimeter over a period of more than two years is used to derive seasonal and inter-annual variations in the total Antarctic sea ice extent. In addition the retrieval of high accuracy elevation measurements over sea ice areas is carried out. These data are used to produce improved maps of sea surface topography over ice- covered ocean and provide evidence of the ability of the altimeter to determine sea ice freeboard directly. In addition the changing freeboard of two giant Antarctic tabular icebergs, as measured by the Geosat altimeter, is presented. As a summary the achievements are reviewed and suggestions are made towards directions for further work on present data sets and for future data from the ERS-1 satellite

Statistical Modelling and Variability of the Subtropical Front, New Zealand

Ocean fronts are narrow zones of intense dynamic activity that play an important role in global ocean-atmosphere interactions. Of particular significance is the circumglobal frontal system of the Southern Ocean where intermediate water masses are formed, heat, salt, nutrients and momentum are redistributed and carbon dioxide is absorbed. The northern limit of this frontal band is marked by the Subtropical Front, where subtropical gyre water convergences with colder subantarctic water. Owing to their highly variable nature, both in space and time, ocean fronts are notoriously difficult features to adequately sample using traditional in-situ techniques. We therefore propose a new and innovative statistical modelling approach to detecting and monitoring ocean fronts from AVHRR SST images. Weighted local likelihood is used to provide a nonparametric description of spatial variations in the position and strength of individual fronts within an image. Although we use the new algorithm on AVHRR data it is suitable for other satellite data or model output. The algorithm is used to study the spatial and temporal variability of a localized section of the Subtropical Front past New Zealand, known locally as the Southland Front. Twenty-one years (January 1985 to December 2005) of estimates of the front’s position, temperature and strength are examined using cross correlation and wavelet analysis to investigate the role that remote atmospheric and oceanic forcing relating to the El Nino-Southern Oscillation may play in interannual frontal variability. Cold (warm) anomalies are observed at the Southland Front three to four months after peak El Nino (La Nina) events. The gradient of the front changes one to two seasons in advance of extreme ENSO events suggesting that it may be used as a precursor to changes in the Southern Oscillation. There are strong seasonal dependencies to the correlation between ENSO indices and frontal characteristics. In addition, the frequency and phase relationships are inconsistent indicating that no one physical mechanism or mode of climate variability is responsible for the teleconnection

The retrieval of surface parameters from satellite borne infrared radiometers for the study of climate

This thesis concerns the development and application of new infrared remote sensing techniques for measurement of climate-related variables. The nature of the climate system is discussed, and the need for global monitoring is noted, together with the suitability of satellite-based remote sensing for the task. Current applications of data from satellite-borne infrared radiometers are discussed, together with the attendant problems, particularly that of correction for the effects of the atmosphere on remotely-sensed thermal infrared temperatures. In addition, the monitoring of proxy indicators of climatic change, such as the areas of closed lakes, by remote sensing is seen as having great potential, despite the limited research to date. The problem of accurate measurement of lake areas by the necessarily coarse resolution instruments which are capable of providing the required repeat coverage is addressed. An initial case study shows that lakes of order a few hundred km2 can be measured to an accuracy of 1% with 1 km resolution data from the Advanced Very High Resolution Radiometer (AVHRR). A further study of a climatically-sensitive closed lake in Ethiopia demonstrates a qualitative relationship between the measured area cycle and climate records. It is noted that the accurate remote sensing of lake surface temperatures and tropical ocean surface temperatures, both important parameters for climate research, is difficult due to the problem of atmospheric correction. A new correction algorithm is developed which offers an improvement of a factor ~2 over conventional algorithms when applied to AVHRR data. Useful byproducts of the algorithm are accurate atmospheric transmittance and total water vapour. Further developments of the techniques devised are suggested with a view to maximising the exploitation of both new and existing global datasets in order to provide the necessary long time series of accurate measurements required for climate research

Sea surface temperature distribution in the Azores region. Part I: AVHRR imagery and in situ data processing.

Sixteen months of 1.1 km resolution NOAA-12, -14, and -16 data for the Azores region are investigated. Advanced Very High Resolution Radiometer (AVHRR) derived sea surface temperature (SST) is compared to an extensive in situ temperature measurement database, mainly constituted during fisheries campaigns. This comparison shows that SST maps include numerous pixels with temperature values below the range observed for the Azores. Low temperatures are attributed in literature to pixel contamination by cloud neighbouring and these are usually removed by eroding pixels around clouds. Results of this study show that running an erosion filter removes only two thirds of the contaminated pixels. Remnant clouds are filtered inputting threshold values to SST 8-day temperature histograms. Based on a comparison of the SST values derived on an image-by-image basis, it is also demonstrated that differences among the sensors are lower than the measurement accuracy, whilst, on the contrary, nighttime and daytime SST distributions are statistically different. Based on monthly and 15-day average computations at nighttime, AVHRR-derived SST distribution in the Azores and associated dominant space and time scales are proposed in the second part of this paper (SST distribution in the Azores region. Part II: Space and time variability and its relation to North Atlantic Oscillation)

Sea surface temperature distribution in the Azores region. Part I: AVHRR imagery and in situ data processing.

Sixteen months of 1.1 km resolution NOAA-12, -14, and -16 data for the Azores region are investigated. Advanced Very High Resolution Radiometer (AVHRR) derived sea surface temperature (SST) is compared to an extensive in situ temperature measurement database, mainly constituted during fisheries campaigns. This comparison shows that SST maps include numerous pixels with temperature values below the range observed for the Azores. Low temperatures are attributed in literature to pixel contamination by cloud neighbouring and these are usually removed by eroding pixels around clouds. Results of this study show that running an erosion filter removes only two thirds of the contaminated pixels. Remnant clouds are filtered inputting threshold values to SST 8-day temperature histograms. Based on a comparison of the SST values derived on an image-by-image basis, it is also demonstrated that differences among the sensors are lower than the measurement accuracy, whilst, on the contrary, nighttime and daytime SST distributions are statistically different. Based on monthly and 15-day average computations at nighttime, AVHRR-derived SST distribution in the Azores and associated dominant space and time scales are proposed in the second part of this paper (SST distribution in the Azores region. Part II: Space and time variability and its relation to North Atlantic Oscillation)

A new high-resolution sea surface temperature blended analysis

The National Oceanic and Atmospheric Administration’s (NOAA) office of National Environmental Satellite, Data, and Information Service (NESDIS) now generates a daily 0.05° (∼5 km) global high-resolution satellite-based sea surface temperature (SST) analyses on an operational basis. The new analysis combines SST data from U.S., Japanese, and European geostationary infrared imagers, and low-Earth-orbiting infrared (United States and Europe) SST data, into a single high-resolution 5-km product. An earlier version produced a 0.1° (∼11 km) resolution, a resolution chosen to approximate the Nyquist sampling criterion for the midlatitude Rossby radius (∼20 km), in order to preserve mesoscale oceanographic features such as eddies and frontal meanders. Comparison between the two analyses illustrates that the higher-resolution grid spacing has more success in this regard. The analysis employs a rigorous multiscale optimum interpolation (OI) methodology that approximates the Kalman filter, together with a data-adaptive correlation length scale, to ensure a good balance between detail preservation and noise reduction. The product accuracy verified against globally distributed buoys is ∼0.02 K, with a robust standard deviation of ∼0.25 K. The new analysis has proven a significant success even when compared to other products that purport to have a similar resolution. This analysis forms the basis for other operational environmental products such as coral reef bleaching risk and ocean heat content for tropical cyclone prediction. Forthcoming enhancements include the incorporation of microwave SST products from low-Earth-orbiting platforms [e.g., Global Change Observation Mission for Water-1 (GCOM-W1)] in order to improve the resolution of SST features in areas of persistent cloud and correct for diurnal effects via a turbulence model of upper-ocean heating