Session 3 Surface Currents III

Transcription

1 Session 3 Surface Currents III

2 Mesoscale dynamics along the Western Australia coastline from a combined analysis of HF radar, altimetry, SST and subsurface observations (Simone Cosoli 1 and Chari Pattiaratchi 1 ) 1: Ocean Graduate School and the UWA Oceans Institute The University of Western Australia 35 Stirling Highway, Crawley, WA 6009, Australia Corresponding author s simone.cosoli@uwa.edu.au 1 Introduction This paper focuses on the analysis of a novel and integrated data set collected from a network of SeaSonde and WERA radars deployed along the coast of Western Australia. The main objectives are the following: 1, determine the accuracy and understand the differences between WERA and SeaSonde data; 2, define a merged data set in the area of common overlap; 3, identify and characterize the main oceanographic structures in the region using the merged data set. 2 Results 2.1 Quality-control methods and radar accuracy estimates Specifically designed quality-control procedures have been developed for the WERA and the SeaSonde systems. They are capable of removing artifacts on the radial current maps both in near real-time and offline mode. A novel approach is used for the QC parameter selection and tuning, in which thresholds are defined through quantitative comparisons with independent measurements (drifting buoys, including gliders; subsurface measurements from moored current meters; or similar). Despite the different sampling strategies, accuracies of the two instruments are consistent with estimates in other deployments. Data agreement improves significantly when daily averages are used in the comparisons..

3 2.2 Major oceanographic structures The sequence of monthly-averaged currents well capture the dominant ocean circulation feature along the western Australia coast, and in particular the warm southwards Leeuwin Current (LC) and the colder northwards Capes Current (CC), as well as seasonal zonal shift of the southwards LC current (Figure 1). Persistent mesoscale eddies generated at the interface of the LC and CC currents occur, prevalently confined in the deep-water region (minimum depth 100m) or constrained by the Perth canyon bathymetric feature. Their signature is particularly strong to reflect also on the monthly current pattern, particularly in October Typical values for eddy Rossby number are clustered between +/- 0.6 f, with f the planetary vorticity. While cyclonic eddies are predominant in the canyon region, cyclonic and anticyclonic mesoscale eddies equally occur north of the Rottnest shelf, bounded by the meandering LC current. The horizontal wavenumber spectral decomposition showing the cascading of energy or the transfer of energy between spatial scales suggests that the transfer follows a k -3 trend across the cpkm horizontal scales ( km), and that this pattern is consistent over time and season despite a lower energy content in January April than July October months. A real-valued empirical orthogonal function (EOF) analysis shows that the major variability in the monthly fields is captured by a limited number of modes. The 10 modes explain 95% total variance, the first 5 contribute to more than 84% variance and the remaining 5 modes cumulatively explain the remaining 10% variance. EOF-1 (36.3% explained variance) shows a large scale cyclonic-anticyclonic pattern with the presence of a seasonal cycle which intensifies during March-July and a reverses during August-December. EOF-2 (23.3% explained variance) shows a single large scale cyclonic pattern to the north of the Rottnest shelf driven by the large-scale meander-type flow that dominates over the Rottnest shelf. Amplitudes are relatively weak, although they show an increasing trend in when a seasonal (yearly) cycle appears, and tend to oppose EOF-1. 3 Main operational challenges During the initial stages of the Australian HF radar network, systems were operated using scientific radiodetermination licences. Following the 2012 International Telecommunication Union (ITU) World Radiocommunication Conference, the radar operating frequencies were changed to match the Resolution 612 outcomes, in particular in relation to the operation within the 3

4 and 50 MHz to support oceanographic radar operations. However, the new ITU frequency bands are currently much noisier than the original operating frequency bands, which are characterised with stronger average noise levels and more pronounced diurnal variability resulting in reduced operational ranges. Further to that, the new frequency bands are also congested with several primary users and the systems are forced to operate at extremely low power (as low as 1W) and reduced bandwidth, however surprisingly with little impact on the overall operational range and also with improved data quality. 4 Acknowledgment ACORN is an IMOS facility supported by the Australian Government through the National Collaborative Research Infrastructure Strategy and the Super Science Initiative. HRPT AVHRR SST retrievals were produced by the Australian Bureau of Meteorology as a contribution to the Integrated Marine Observing System - an initiative of the Australian Government being conducted as part of the National Collaborative Research Infrastructure Strategy and the Super Science Initiative. The imagery data were acquired from NOAA spacecraft by the Bureau, Australian Institute of Marine Science, Australian Commonwealth Scientific and Industrial Research Organization, Geoscience Australia, and Western Australian Satellite Technology and Applications Consortium. Subsurface current data was sourced from the Integrated Marine Observing System (IMOS) - IMOS is supported by the Australian Government through the National Collaborative Research Infrastructure Strategy and the Super Science Initiative. A particular acknowledgment to Mark Snell and Ian Darby (CSIRO) for making this data set available.

5 HF radar applications in the Gulf of Naples Enrico Zambianchi 1, Daniela Cianelli 1,2, Domenico D Alelio 3, Luigi De Luca 1, Paola De Ruggiero 1, Roberta Di Lemma 1, Giovanni Esposito 1, Pierpaolo Falco 1, Alberto Giordano 1, Stefano Pierini 1, Maurizio Ribera d Alcalà 3, Diana Sarno 3, Simona Saviano 1, Marco Uttieri 1,3, Giovanni Zambardino 1 and Adriana Zingone 3 1: Department of Science and Technology, Parthenope University of Napoli and CoNISMA, Italy 2: Italian Institute for Environmental Protection and Research, Roma, Italy 3: Department of Integrated Marine Ecology, Stazione Zoologica Anton Dohrn, Napoli, Italy Corresponding author s enrico.zambianchi@uniparthenope.it 1 Introduction An HF radar system has been operating in the Gulf of Naples (GoN - Southeastern Tyrrhenian Sea) since The system is a SeaSonde manufactured by CODAR Ocean Sensors Ltd. Three mono-static radar units working at about 25 Mhz ensure the surface current mapping over nearly the entire Gulf of Naples area. The grid resolution is 1 Km with a range of approximately 40 Km. In this presentation we illustrate a number of very recent applications of HF radar data in our coastal area that extend the range of applications and might be of general interest to coastal radar operators. 2 Wave parameters retrieved from the HF radar network in the GoN A yearly assessment of seasonal wave characteristics has been carried out, comparing measurements from a wave buoy and a network of three HF radar stations operating in the GoN (Saviano et al., 2018). HF radar-derived measurements depict the same wave pattern retrieved by the buoy, both under calm (Hs between 0.5 m and 1.0 m) and stormy conditions (Hs greater than 2.5

6 m). The agreement between the two observation systems points out the ability of both platforms to resolve the main structure of the wave field. The results indicate that the wave field is predominantly locally wind-driven, with specific patterns depending on the sub-basin of the GoN under analysis. The different orientation of the bathymetry and the consequent refraction patterns are very well accounted for by the recordings by the three stations. Fig. 1 Map of the Gulf of Naples, with the location of the three HF radar sites (red stars), of the ISPRA weather station (orange sun), of the wave buoy (green donut), of the LTER-MC (cyan cross). 3 HF radar-derived wind observations in the GoN In the framework of an analysis and comparison between observed and simulated winds in the GoN, HF radar-derived wind direction data have been collected and validated. The wind observations are measured by two almost co-located weather stations: one managed by the Italian Institute for Environmental Protection and Research (ISPRA) and one managed by the Department of Science and Technology of the Parthenope University. The wind model data are drawn from the SKIRON/Eta atmospheric model with a spatial resolution of 10 Km and temporal resolution of 1 hour. The analysis was carried out for two periods: a stormy (2nd February - 6th

7 March) and a relatively calm one (1st - 29th August 2009). The analyses showed a high correlation between the radar-derived wind direction with the in situ measurements during intense events that are characterized by strong winds from a prevailing direction for a long period of time; these conditions are typically verified in winter and generate waves with height > 1.5 m. In summer, the establishment of the Azores anticyclone determined the onset of moderate breeze systems, with lower Hs ~ 0.50 m and a consequent reduction of back-scattering from the sea surface (Falco et al., 2016). 4 An investigation on physical and biological drivers of phytoplankton dynamics in the Gulf of Naples (as an example of a generalised coastal system) The continuous mapping of surface currents in the Gulf of Naples by our HF radar system was combined with ecological observations performed at the reference site LTER-MareChiara (LTER-MC), in order to explore the link between coastal circulation and plankton dynamics (Cianelli et al., 2017). The high resolution of both physical and ecological observations allowed us to detect short-term changes in species abundance and to relate them to either horizontal transport or biological processes intrinsic to the phytoplankton community, in different seasons and phases of coastal circulation. The study was focused on the year 2009, which offered the best coincidence between ecological measurements and continuity of radar coverage. Annual and seasonal regimes of currents at the LTER-MC location were investigated; thereafter, lagrangian backtracking simulations were performed on the basis of the radar-derived surface currents in order to identify the origin of water masses/populations observed at LTER-MC. Backward assessed origin zones, along with a normalized index of source spatial distribution and arrival times were analysed in conjunction with surface salinity, chlorophyll-a and phytoplankton diversity in order to identify the distinct states of the system based on both physical and ecological proxies and allowing to evaluate the relative importance of the two main drivers, i.e. the physical and the biological one. References D. Cianelli, D. Sarno, D. D Alelio, A. Zingone, E. Zambianchi, M. Uttieri and M. Ribera d Alcalà, 2017: Disentangling physical and biological drivers of

8 phytoplankton dynamics in a coastal system. Scientific Reports, 7(1), 15868, DOI: /s x P. Falco, B. Buonocore, D. Cianelli, L. De Luca, A. Giordano, I. Iermano, A. Kalampokis, S. Saviano, M. Uttieri, G. Zambardino and E. Zambianchi, 2016: Dynamics and sea state in the Gulf of Naples: potential use of HF radar data in an operational oceanographic context, Journal of Operational Oceanography, doi: / X S. Saviano, A. Kalampokis, E. Zambianchi and M. Uttieri, 2018: A year-long assessment of wave measurements retrieved from a HF radar network in the Gulf of Naples (Tyrrhenian Sea, Western Mediterranean Sea), Journal of Operational Oceanography, submitted