Joint analysis of coastal altimetry and high-frequency (HF) radar data: observability of seasonal and mesoscale ocean dynamics in the Bay of Biscay

Abstract

Abstract. Land-based coastal high-frequency (HF) radar systems provide operational measurements of coastal surface currents (within 1–3 m depth) with high spatial (300 m–10 km) and temporal (≤1 h) sampling resolutions, while the near-continuous altimetry missions provide information, from 1993 until today, on geostrophic currents in the global ocean with typical along-track and temporal sampling resolutions of >7 km and >9 days, respectively. During the last years, the altimetry community has made a step forward in improving these data in the coastal area, where the data present lower quality than in the open ocean. The combination of HF radar and altimetry measurements arises as a promising strategy to improve the continuous monitoring of the coastal area (e.g. by expanding the measurements made by HF radars to adjacent areas covered by the altimetry or by validating/confirming improvements brought by specific coastal algorithms or new altimeter missions). A first step towards this combination is the comparison of both data sets in overlapping areas. In this study, a HF radar system and two Jason-2 satellite altimetry products with different processing are compared over the period from 1 January 2009 to 24 July 2015. The results provide an evaluation of the performance of different coastal altimetry data sets within the study area and a better understanding of the ocean variability contained in the HF radar and altimetry data sets. Both observing systems detect the main mesoscale processes within the study area (the Iberian Poleward Current and mesoscale eddies), and the highest correlations between radar and altimetry (up to 0.64) occur in the slope where the Iberian Poleward Current represents a significant part of the variability in the circulation. Besides, the use of an Ekman model, to add the wind-induced current component to the altimetry-derived geostrophic currents, increases the agreement between both data sets (increasing the correlation by around 10 %).

Integration of HF Radar Observations for an Enhanced Coastal Mean Dynamic Topography

“…Thus, in this study, we apply a larger mask including the land and the pixels close to the coast. Consequently, the first guess of this coastal MDT better resolves the circulation, being in a better agreement with the literature (Manso-Narvarte et al, 2018).…”

“…In this study, we use a 9-year time series of HF radar currents, which have been shown in previous studies (Solabarrieta et al, 2014;Manso-Narvarte et al, 2018) to provide a consistent description of the ocean surface circulation with the one found in the literature (e.g., Le Cann and Serpette, 2009;Charria et al, 2013). When the mean HF radar currents were compared with the mean geostrophic current from the spatial derivatives of already existing MDTs, we found significant differences.…”

“…The Iberian Poleward Current (hereafter IPC) is a slope current flowing eastward along the Spanish coast and northward along the French coast. The IPC is the most salient feature of the winter surface circulation in the SE-BoB; previous studies have shown that its variability can be observed in altimetric data (e.g., Herbert et al, 2011;Xu et al, 2015;Manso-Narvarte et al, 2018). Despite its strong seasonal and interannual variability, its signature in the mean circulation can be expected.…”

“…Some of these operational systems continue providing operational data and have supplied since their installation a historical dataset of surface ocean current fields valuable for, besides other purposes, the study of coastal dynamics and coastal retention conditions at different spatio-temporal scales, the validation of remote ocean currents measurements or the assimilation in ocean models. In the study area (Figure 1), the south-eastern Bay of Biscay (SE-BoB), one of these systems provides since 2009 almost continuous information of the local surface current dynamics (e.g., Solabarrieta et al, 2014;Manso-Narvarte et al, 2018;Rubio et al, 2018) and can be used to obtain a robust estimation of the mean ocean surface circulation.…”

Long-Term Monitoring of the Atlantic Jet through the Strait of Gibraltar with HF Radar Observations

“…As an essential part of the coastal ocean observing system [29], HFR only provide information of surface currents. To better study three-dimensional coastal ocean circulation variability, it would be useful to adopt a multi-platform approach by combining HFR estimations with other observational data sets, such as in situ observation from moored ADCPs and satellite remote sensing products [30,31].…”