Sea Surface Temperature Satellite Imagery Research Articles

Lateral diffusivity coefficients from the dynamics of a SF6 patch in a coastal environment

The dispersion of a patch of the tracer sulfur hexafluoride (SF6) is used to assess the lateral diffusivity in the coastal waters of the western part of the Gulf of Lion (GoL), northwestern Mediterranean Sea, during the Latex10 experiment (September 2010). Immediately after the release, the spreading of the patch is associated with a strong decrease of the SF6 concentrations due to the gas exchange from the ocean to the atmosphere. This has been accurately quantified, evidencing the impact of the strong wind conditions during the first days of this campaign. Few days after the release, as the atmospheric loss of SF6 decreased, lateral diffusivity coefficient at spatial scales of 10km has been computed using two approaches. First, the evolution of the patch with time was combined with a diffusion–strain model to obtain estimates of the strain rate (γ=2.510−6s−1) and of the lateral diffusivity coefficient (Kh=23.2m2s−1). Second, a steady state model was applied, showing Kh values similar to the previous method after a period of adjustment between 2 and 4.5days. This implies that after such period, our computation of Kh becomes insensitive to the inclusion of further straining of the patch. Analysis of sea surface temperature satellite imagery shows the presence of a strong front in the study area. The front clearly affected the dynamics within the region and thus the temporal evolution of the patch. Our results are consistent with previous studies in open ocean and demonstrate the success and feasibility of those methods also under small-scale, rapidly-evolving dynamics typical of coastal environments.

Probabilistic relationships between wind and surface water circulation patterns in the SE Bay of Biscay

Non-linear K-means classification algorithm was used to obtain a comprehensive description of the winds and high-frequency radar-derived currents in the SE Bay of Biscay (study area), taking into account a wide range of scales, from several days to interannual variability. The results in the study area show that in summer, a stronger variability in winds and surface currents can be expected, while in winter, intense southwesterly winds and a cyclonic circulation prevail. In addition to the seasonal component of the currents, a significant spatial variability in terms of current patterns and a temporal variability at shorter and interannual scales were also identified, highlighting the complexity of the surface current dynamics. Moreover, the probabilistic relationships between wind and current patterns were explored, obtaining conditional probabilities. Most of the surface current patterns are clearly related to specific wind patterns that are recurrent in the study area. However, other common current patterns are not so clearly related to specific wind conditions. The presence of a seasonal slope current (Iberian Poleward Current, IPC) is one of the most relevant features of the local circulation. An IPC occurrence time series based on Sea Surface Temperature satellite imagery was used to obtain conditional probabilities with the high-frequency radar surface current patterns, showing a relation between the strongest IPC events and closed cyclonic currents, which are not linked to specific winds.

Physical characteristics and dynamics of the coastal Latex09 Eddy derived from in situ data and numerical modeling

AbstractWe investigate the dynamics of a coastal anticyclonic eddy in the western part of the Gulf of Lion (GoL) in the northwestern Mediterranean Sea during the Latex campaign in the summer 2009 (Latex09). The sampling strategy combines sea surface temperature satellite imagery, hull‐mounted acoustic Doppler current profiler data, conductivity‐temperature‐depth casts, and drifter trajectories. Our measurements reveal an anticyclonic eddy (Latex09 eddy) with a diameter of ~23 km and maximum depth of 31 m, centered at 3°34′E, 42°33′N. We use a high resolution, three‐dimensional, primitive equation numerical model to investigate its generation process and evolution. The model is able to reproduce the observed eddy, in particular its size and position. The model results suggest that the Latex09 eddy is induced by a large anticyclonic circulation in the northwestern part of the GoL, pushed and squeezed toward the coast by a meander of the Northern Current. This represents a new generation mechanism that has not been reported before. The post generation dynamics of the eddy is also captured by the model. The collision of the Latex09 eddy with Cape Creus results in a transient structure, which is depicted by the trajectories of two Lagrangian drifters during Latex09. The transient structure and its advection lead to a transfer of mass and vorticity from the GoL to the Catalan shelf, indicating the importance of mesoscale structures in modulating such exchanges in the region.

Scales and structure of frontal adjustment and freshwater export in a region of freshwater influence

Sea surface temperature satellite imagery and a regional hydrodynamic model are used to investigate the variability and structure of the Liverpool Bay thermohaline front. A statistically based water mass classification technique is used to locate the front in both data sets. The front moves between 5 and 35 km in response to spring–neap changes in tidal mixing, an adjustment that is much greater than at other shelf-sea fronts. Superimposed on top of this fortnightly cycle are semi-diurnal movements of 5–10 km driven by flood and ebb tidal currents. Seasonal variability in the freshwater discharge and the density difference between buoyant inflow and more saline Irish Sea water give rise to two different dynamical regimes. During winter, when cold inflow reduces the buoyancy of the plume, a bottom-advected front develops. Over the summer, when warm river water provides additional buoyancy, a surface-advected plume detaches from the bottom and propagates much larger distances across the bay. Decoupled from near-bed processes, the position of the surface front is more variable. Fortnightly stratification and re-mixing over large areas of Liverpool Bay is a potentially important mechanism by which freshwater, and its nutrient and pollutant loads, are exported from the coastal plume system. Based on length scales estimated from model and satellite data, the erosion of post-neap stratification is estimated to be responsible for exporting approximately 19% of the fresh estuarine discharge annually entering the system. Although the baroclinic residual circulation makes a more significant contribution to freshwater fluxes, the episodic nature of the spring–neap cycle may have important implications for biogeochemical cycles within the bay.

An algorithm for oceanic front detection in chlorophyll and SST satellite imagery

An algorithm is described for oceanic front detection in chlorophyll (Chl) and sea surface temperature (SST) satellite imagery. The algorithm is based on a gradient approach: the main novelty is a shape-preserving, scale-sensitive, contextual median filter applied selectively and iteratively until convergence. This filter has been developed specifically for Chl since these fields have spatial patterns such as chlorophyll enhancement at thermohaline fronts and small- and meso-scale chlorophyll blooms that are not present in SST fields. Linear Chl enhancements and localized (point-wise) blooms are modeled as ridges and peaks respectively, whereas conventional fronts in Chl and SST fields are modeled as steps or ramps. Examples are presented of the algorithm performance using modeled (synthetic) images as well as synoptic Chl and SST imagery. After testing, the algorithm was used on > 6000 synoptic images, 1999–2007, to produce climatologies of Chl and SST fronts off the U.S. Northeast.

The physical oceanographic conditions along the Mid-Atlantic Ridge north of the Azores in June–July 2004

Abstract During the MAR-ECO expedition in June and July 2004, 39 deep CTD stations were occupied with uneven spacing along the Mid-Atlantic Ridge (MAR) between 41°N and 61°N. The CTD stations coincided with the biological sampling stations and thus gave a detailed vertical description of the water properties at these locations. Many different water masses were identified. However, using the predominant water mass in the upper 500 m, four different hydrographic regions were identified. Stations in the vicinity of the Reykjanes Ridge north of 57°N were dominated by Modified North Atlantic Water (MNAW). Stations south of 56°30′N and north of the Sub-Polar Front (SPF) were dominated by Sub-Arctic Intermediate Water (SAIW). CTD data combined with along-track continuous surface temperature and salinity measurements, satellite sea-level anomaly data and sea-surface temperature (SST) and along-track current data (ADCP data) showed that the position of the SPF was at about 52°N, at the southern edge of the Charlie–Gibbs Fracture Zone (CGFZ). South of the SPF a Frontal Region with both North Atlantic Central Water (NACW) and SAIW present was observed, and south of about 50°N the predominant water mass in the upper 500 m was NACW. A ship-mounted RDI 75 kHz ADCP was operated during the cruise and yielded high quality data during the first leg. The currents showed large variations over short distances and were strongly affected by topography. The ADCP data was be used to describe the mesoscale eddy field along the MAR and to detect three jets in the North Atlantic Current (NAC). Data from a current meter mooring left for 10 months after the cruise at a seamount just to the south of CGFZ showed the dominance of the tidal currents in the area. Typical spring tidal currents were about 0.15 m s−1 and the mean flow an order of magnitude smaller, about 0.03 m s−1. A short-term mooring on a seamount farther south showed tidal currents of 0.3 m s−1. Binned 8-day, 4-km SST satellite imagery were collected and processed for 2004. The SST data were analysed, and showed characteristic frontal structures in the MAR-ECO region.

Sea surface temperature variability off north-west Africa: 1981-1989

A temporal and spatial Empirical Orthogonal Function (EOF) analysis has been performed on monthly maximum sea surface temperature (SST) satellite data over eight years (July 1981-August 1989) to examine the SST patterns of variability off north-west Africa. The first temporal EOF mode (75 per cent) represents the cooling and warming cycle over the region. The temporal amplitudes show an interannual variability with 1982 summer temperatures abnormally much colder, immediately preceding the largest El Nino phenomenon of the century. The second temporal mode (9 per cent) distinguishes a large-scale behaviour of the northern and southern areas of the region, the two areas being out of phase presumably related to the variability of the wind field. The first spatial EOF mode (66 per cent), which resembles the mean of all images, is stronger in fall, with a smaller second maximum at the beginning of spring. The second spatial mode (10 per cent) shows the coastal upwelling region where a seaward extension of upwelled water around 300km offshore is observed off Cape Blanc. All the modes show complex eastern boundary dynamics with different regimes north and south of Cape Blanc. The Cape Blanc vicinity must have some interesting dynamics with a sharp boundary evident in all the modes. Only these two first temporal and spatial EOF modes are significantly above the noise level. The EOF method is efficient in compressing monthly SST satellite imagery off north-west Africa, with a great percentage of the total variance being accounted by only two modes.