Oceanic Thermal Front Research Articles

Southwestern Atlantic Ocean Fronts Detected from Satellite-Derived SST and Chlorophyll

The Southern Ocean front (SOF) is an important factor that affects the heat exchange and material transport of the Southern Ocean. In the past two decades, with the advancements in satellite remote-sensing technology, the study of the spatio-temporal variability of the Southern Ocean front has become a new hot topic. Nevertheless, the southwestern Atlantic, as an important part of the Southern Ocean, is poorly studied in this regard. Based on the 16-year (2004–2019) high-resolution satellite observations of sea surface temperature (SST) and 13-year (2007–2019) observations of chlorophyll (CHL), this study detected and analyzed the position and seasonal variation of the SOF in the southwestern Atlantic using a gradient-based frontal detection method. According to the experimental results, the thermal front (derived from the SST data) disappeared in winter due to the spatially uniform surface cooling, whereas the ocean color front (derived from the CHL data) existed without remarkable spatio-temporal changes. Furthermore, the exact position and seasonal variation of the SOF in the southwestern Atlantic are determined by comparing the paths of the two fronts. Since the formation of the Kuroshio front in the East China Sea (ECS) is similar to the SOF in the southwestern Atlantic, the seasonal distributions of the two fronts were compared. Apart from that, the Kuroshio thermal fronts were mostly distributed in winter and less in summer, while the Southern Ocean thermal fronts showed the opposite. These results indicated that the ocean current properties significantly influence the spatio-temporal variability of the front.

The impact of cloud masking on the climatology of sea surface temperature gradients

ABSTRACT Cloud masking is a critical step in the estimation of Sea Surface Temperature (SST) from satellite observations. It can affect the validation statistics of SST on synoptic scales but also on long-term climatologies. One of the main challenges in cloud masking is the discrimination between clouds and ocean sharp fronts as both of these are associated with high spatial variability. In this study, we investigate the impact of cloud masking on the climatology of SST gradient magnitudes. Using night-time Level 2 SST derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Terra and Aqua for almost the entire mission, i.e. 2003 to 2018 over the Southwestern Atlantic Ocean (SAO), we show that misclassification of sharp ocean thermal fronts as clouds leads to (1) significant underestimation of thermal frontal activity in monthly and longer-term climatologies and (2) different spatial distributions and temporal variability of SST gradient magnitudes.

On the use of satellite-derived frontal metrics in time series analyses of shelf-sea fronts, a study of the Celtic Sea

Satellite-derived frontal metrics describe characteristics of oceanic thermal fronts, such as their strength or persistence. They are used in marine science to investigate spatio-temporal variability of thermal fronts or in ecological studies to assist in explaining animal distributions. Although these metrics represent highly processed data, which is based on sometimes complex algorithms, little guidance is available on their correct application in quantitative analyses, in particular for non-specialist users. This research aims to improve accurate use of frontal data. This case study investigates the inter--annual and seasonal variability of two tidal mixing fronts on the Celtic Sea shelf, based on monthly time series of daily frontal maps at ∼1km2 resolution from 1990 to 2010. Some metrics are almost identical and can be grouped, e.g. frontal probability, persistence and so-called “composites” (Pearson correlation: r = 0.8–1.0; p < 0.001), whereas the metric describing frontal strength is distinct from other ones. Consequently, strength and metrics of the frontal probability group showed pronounced differences in their inter-annual and seasonal variability: Strength displayed an oscillating pattern between 1990 and 2010 while there were no significant changes in probability over time. In addition, seasonal variability was affected by segments from adjacent fronts, not belonging to the fronts of interest, which could result in biased estimates. Most important, there was a doubling of available satellite imagery between 1990 and 2010 due to a greater number of operational satellites, which negatively affected frontal probability, positively frontal strength and consequently, changed the temporal pattern of both. When using frontal maps for temporal analyses, we should choose the metric carefully, be aware of biased estimates caused by variability from unwanted frontal segments in the data and account for the variable data quantity. This guide on the use of frontal metrics will be helpful to improve correct interpretations of statistical analyses.

Autonomous tracking of an oceanic thermal front by a Wave Glider

AbstractTemporal variation in the structure and location of dynamic ocean features make them challenging to observe. Beyond requiring sufficient persistence and speed of the sensor platform, effective observation is augmented by autonomous feature detection coupled with adaptive localization of mapping effort. These capabilities enable observations to remain localized within and around features of interest, thereby enhancing spatial and temporal resolution in their depiction. A Wave Glider is a wave‐propelled unmanned surface vehicle with solar energy collection to support payload and a backup propeller in calm sea state. The efficiency of this platform enables persistence (months), and its propulsion system provides speed (~1 m/s). Building on these capabilities, we aim at enabling the Wave Glider to autonomously recognize and track oceanic features that have strong surface expressions, accessible to near‐surface sensors. We present a method of using a Wave Glider to autonomously track an oceanic thermal front known to have biological importance and dynamic evolution. In October 2017, this method was applied to tracking and mapping an upwelling front in Monterey Bay, CA, over 39 hr. The field result demonstrates adaptive localization of mapping based on autonomous feature detection.

Impact of Ships and Ocean Fronts on Coastal Sea Surface Wind Measurements From the Advanced Scatterometer

In this paper, the capability of the advanced scatterometer (ASCAT) in measuring surface wind speed in coastal waters of China was first evaluated by comparing with in situ wind observations from 13 moored buoys during the period from July 2013 to December 2015. The scatterometer performs well in most coastal regions of China. However, a sharp reduction in wind speed retrieval accuracy is found in areas adjacent to the Yangtze River estuary. The mean bias and root-mean-square error between ASCAT wind speeds and buoy measurements reach up to 3.44 and ${\text{4.06 m}}\cdot {\text{s}}^{- 1}$ , respectively. This large overestimation of surface wind is also observed in Sentinel-1A synthetic aperture radar (SAR) derived wind speeds. The analysis of quasi-synchronous SAR images and sea surface temperature observations shows that the overestimation of the scatterometer or SAR winds is mainly caused by the effects of ships or ocean thermal fronts due to the complicated thermodynamic and hydrodynamic characteristics of the study area on radar backscatter signal. These effects should be taken into account or marked as flags in activities generating or using scatterometer or SAR wind products.

Population genetics of Southern Hemisphere tope shark (Galeorhinus galeus): Intercontinental divergence and constrained gene flow at different geographical scales.

The tope shark (Galeorhinus galeus Linnaeus, 1758) is a temperate, coastal hound shark found in the Atlantic and Indo-Pacific oceans. In this study, the population structure of Galeorhinus galeus was determined across the entire Southern Hemisphere, where the species is heavily targeted by commercial fisheries, as well as locally, along the South African coastline. Analysis was conducted on a total of 185 samples using 19 microsatellite markers and a 671 bp fragment of the NADH dehydrogenase subunit 2 (ND2) gene. Across the Southern Hemisphere, three geographically distinct clades were recovered, including one from South America (Argentina, Chile), one from Africa (all the South African collections) and an Australia-New Zealand clade. Nuclear data revealed significant population subdivisions (FST = 0.192 to 0.376, p<0.05) indicating limited gene flow for tope sharks across ocean basins. Marked population connectivity was however evident across the Indian Ocean based on Bayesian clustering analysis. More locally in South Africa, F-statistics and multivariate analysis supported moderate to high gene flow across the Atlantic/Indian Ocean boundary (FST = 0.035 to 0.044, p<0.05), with exception of samples from Struisbaai and Port Elizabeth which differed significantly from the rest. Discriminant and Bayesian clustering analysis indicated admixture in all sampling populations, decreasing from west to east, corroborating possible restriction to gene flow across regional oceanographic barriers. Mitochondrial sequence data recovered seven haplotypes (h = 0.216, π = 0.001) for South Africa, with one major haplotype shared by 87% of the individuals and at least one private haplotype for each sampling location except Port Elizabeth. As with many other coastal shark species with cosmopolitan distribution, this study confirms the lack of both historical dispersal and inter-oceanic gene flow while also implicating contemporary factors such as oceanic currents and thermal fronts to drive local genetic structure of G. galeus on a smaller spatial scale.

Radar imaging of intense nonlinear Ekman divergence

AbstractIn general, given an oceanic thermal front, there is a strong positive correlation between sea surface temperature (SST) gradients and surface winds, and the marine atmospheric boundary layer is unstable over the warm side of the oceanic thermal front. The Gulf Stream is a notable example of an oceanic thermal front, and its warm side is often detected as enhanced backscatter in synthetic aperture radar (SAR) images. However, in some “anomalous” SAR images, low backscatter is sometimes observed on the warm side of the front, which seems inconsistent. Therefore, we propose a mechanism to interpret the generation of the low backscatter, based on interactions between ocean surface wind waves and intense nonlinear Ekman divergence. This mechanism is verified by showing that patterns in an observed anomalous SAR image are in good agreement with those in the simulated radar signature. In addition, this methodology and analysis demonstrate that SAR is potentially important for detecting and diagnosing small scale air‐sea interactions and upper ocean dynamics with strong vertical transports induced by submesoscale processes.

Modulation mechanisms of marine atmospheric boundary layer at the Brazil‐Malvinas Confluence region

AbstractThe influence of the Brazil‐Malvinas Confluence (BMC) region on the marine atmospheric boundary layer (MABL) is investigated through in situ data analysis of five different cruises (2004 to 2008) and numerical experiments with a regional atmospheric model. Two different groups of numerical experiments were performed in order to evaluate the relevance of static stability and hydrostatic balance physical mechanisms for the MABL instability. The first group used monthly climatological sea surface temperature (SST) as bottom boundary condition while the second used daily updated Advanced Microwave Scanning Radiometer–EOS SST data together with radiosondes and surface data assimilation. A reasonable agreement between numerical results and QuikSCAT wind data was observed through correlation coefficients and mean square error values. In terms of the horizontal structure of the MABL, stronger winds were found over the warm side of the BMC region as well as over the thermal front itself, which supports the coexistence of both modulation mechanisms. The analyzed patterns of surface atmospheric thermal advection showed a clear interaction between the synoptic and regional scales. The signature of the oceanic thermal front (almost meridionally oriented) on the air temperature at 2 m makes the temperature advection strongly determined by the zonal component of the wind. The analysis of momentum budget terms did not show a clear and reasonable explanation of the existence or predominance of the modulation mechanisms, and it also suggested the relevance of other effects, such as the idea based on unbalanced Coriolis force and turbulence/friction effects.

Records of sedimentary dynamics in the continental shelf and upper slope between Aveiro–Espinho (N Portugal)

The sedimentary unconsolidated cover of the Aveiro–Espinho continental shelf and upper slope (NW Portugal) records a complex interplay of processes including wave energy and currents, fluvial input, sediment transport alongshore and cross-shelf, geological and oceanographic processes and sediment sources and sinks. In order to study this record, a set of surface sediment samples was studied. Sediment grain size and composition, as well as the mineralogical composition (by XRD) of the fine (<63μm) and clay (<2μm) fractions and benthic microfaunal (foraminifera) data were analysed. Cluster analysis applied to the sedimentological data (grain size, sediment composition and mineralogy) allowed the establishment of three main zones corresponding to the: inner-, mid- and outer-shelf/upper slope.On the inner-shelf, the sedimentary coverture is composed of siliciclastic fine to very fine sand, essentially comprising modern (immature) terrigenous particles. The sediment grain size, as well as mineralogical and microfaunal composition, denote the high energetic conditions of this sector in which the alongshore transport of sand is predominantly southward and occurs mostly during the spring–summer oceanographic regime, when the main river providing sediments to this area, the River Douro, undergoes periods of drought. This effect may emphasize the erosive character of this coastal sector at present, since the Ria de Aveiro provides the shelf with few sediments.On the mid-shelf, an alongshore siliciclastic band of coarse sand and gravel can be found between the 40m and 60m isobaths. This gravelly deposit includes relic sediments deposited during lower sea-level stands. This structure stays on the surface due to the high bottom energy, which promotes the remobilization of the fine-grained sediments, and/or events of sediments bypassing.Benthic foraminifera density and “Benthic Foraminifera High Productivity” (BFHP) proxy values are in general low, which is consistent with the overall small supply of organic matter to the oceanic bottom in the inner- and mid-shelf. However, the Ria de Aveiro outflow, which delivers organic matter to the shelf, leaves its imprint mainly on the mid-shelf, identifiable by the increase in foraminifera density and BFHP values in front of the lagoon mouth. The higher values of BFHP along the 100m isobath trace the present position of an oceanic thermal front whose situation may have changed in the last 3/5ka BP. This zone marks a clear difference in the density, diversity and composition of benthic foraminifera assemblages. Here, in addition, sediment composition changes significantly, giving rise to carbonate-rich fine to medium sand in the deeper sector. The low bottom energy and the small sedimentation rate of the outer-shelf contributed to the preservation of a discontinuous carbonate-rich gravel band, between the 100m and 140m isobaths, also related to paleo-littorals, following the transgression that has occurred since the Last Glacial Maximum.The winter oceanographic regime favours the transport of fine grained sediments to the outer-shelf and upper slope. The inner- and mid-shelf, however, have low amounts of this kind of sediment and the Cretacic carbonated complexes Pontal da Galega and Pontal da Cartola, rocky outcrops located at the mid- and outer-shelf, act as morphological barriers to the cross-shelf transport of sediments. Thus a reduced sedimentation rate occurs in these deeper sectors, as indicated by the lower abundance of detrital minerals, which is compensated for the high sedimentary content of biogenic carbonates. The relatively high BFHP and Shannon Index values indicate water column stratification, high supply of organic matter and environmental stability, which provide favourable conditions for a diversified benthic fauna to flourish. These conditions also encourage authigenic chemical changes, favourable to glauconite formation, as well as illite and kaolinite degradation. Benthic foraminifera and clay mineral assemblages also reveal the effect of the internal waves pushing upward, and downslope losses of the sediments on the outer-shelf and upper slope.

Gulf Stream thermal fronts detected by synthetic aperture radar

Our purpose is to detect ocean surface features, specifically oceanic thermal fronts, through analysis of SAR (synthetic aperture radar)‐derived wind stress fields. Fine‐resolution measurements of near‐surface wind speeds over the Gulf Stream region of the Northwest Atlantic were made using SAR images collected by RADARSAT‐2. Linear statistical relationships between the wind stress curl and divergence to the crosswind and downwind components of the SST gradient field were used to derive a new method for detecting Gulf Stream thermal fronts from Synthetic Aperture Radar (SAR) imagery. In particular, sea surface temperature front features, as suggested by corresponding AVHRR and MODIS images, are evident in both of the wind stress curl and divergence fields. The importance of this methodology is that clouds are often present in the Northwest Atlantic in autumn and winter months and they obscure AVHRR and MODIS images, whereas SAR can penetrate clouds.

Wet to dry climatic trend in north-western Iberia within Heinrich events

The direct sea–land correlation applied to core MD99-2331 retrieved from the north-western Iberian margin shows a two-phase pattern within Heinrich events 4, 2 and 1 in the ocean and in the adjacent landmasses. Changes between wet/cold and dry/cool conditions in the Iberian Peninsula detected during these extreme events cannot be explained by a simple oceanographic mechanism related to changes in the strength of the Atlantic Meridional Overturning Circulation. Here we propose an additional atmospheric mechanism able to produce this scenario based on the comparison between the MD99-2331 record and other available palaeoclimate sequences from the North Atlantic region (18–75°N and 0–75°W). The climatic asymmetry observed between mid- and subtropical eastern North Atlantic latitudes (wet/dry) and the Blake Outer Ridge (dry/wet) during H4, H2 and H1 can be explained by changes in the position of the Atlantic jet-stream. During the first phase of H4, H2 and H1 the Atlantic jet-stream was located further south following the southward displacement of the oceanic thermal front as far south as 35°–37°N. On the contrary, during the second phase of H4, H2 and H1 the jet-stream was located further north following the northward displacement of this thermal front as far north as 42°N. From the atmospheric point of view, these two phases are reminiscent of the present-day negative and positive prevailing modes of the North Atlantic Oscillation (NAO), respectively, but high-resolution studies of additional North Atlantic key sites and climate simulations are needed to confirm the hypothesis of a NAO-like mechanism operating on millennial timescales.

Low-Frequency Variability in the Midlatitude Baroclinic Atmosphere Induced by an Oceanic Thermal Front

AbstractThis study examines the flow induced by an east–west-oriented oceanic thermal front in a highly idealized baroclinic model. Previous work showed that thermal fronts could produce energetic midlatitude jets in an equivalent-barotropic atmosphere and that barotropic instabilities of this jet had dominant periods of 25–30 and 65–75 days.The present study extends this work to a two-mode baroclinic free atmosphere. The baroclinic jet produced in this case is subject to both barotropic and baroclinic instabilities. A barotropic symmetric instability propagates westward with periods of roughly 30 days and is similar to those found in the equivalent-barotropic model. A baroclinic instability results in standing-dipole anomalies and oscillates with a period of 6–8 months. A mixed barotropic–baroclinic instability results in anomalies that propagate northward, perpendicular to the jet, with a period of 2–3 months. The later anomalies are reminiscent of the 70-day oscillation found over the North Atlantic in observed fields.The atmospheric flow has two distinct states: the flow in the high-energy state exhibits two large gyres and a strong eastward jet; its antisymmetric component is dominant. The low-energy flow is characterized by small gyres and a weak jet.The model’s dynamics depends on the layer-depth ratio. When the model is nearly equivalent-barotropic, symmetric oscillatory modes dominate. As the two layers become nearly equal, antisymmetric oscillatory modes become significant and the mean energy of the flow increases.When the oceanic thermal front’s strength T* is weak (T* ≤ 1.5°C), the flow is steady. For intermediate values of the strength (1.5°C &amp;lt; T* &amp;lt; 3°C), several oscillatory instabilities set in. As the frontal strength increases further (T* ≥ 3°C), the flow becomes more turbulent. These results all depend on the atmospheric model’s horizontal resolution being sufficiently high.

Marine GIS: Identification of mesoscale oceanic thermal fronts

A new method (the ‘sink’ method) is proposed for the mapping of productive mesoscale oceanic thermal fronts based on the combined analysis of satellite imagery for sea surface temperature (SST) distribution and chlorophyll (CHL) concentration under a Geographic Information System (GIS). In an SST lattice data array, data value sinks describe heterogeneous drops in SST distribution as this is compared to more homogeneously distributed SST in the surrounding area. Using Arc/Info GIS, these thermal discontinuities are flagged and applied on SST and CHL imagery for the calculation of differences in SST and CHL patterns (DSST and DCHL) inside and outside of these flagged areas. Spatially connected sinks that are characterized by simultaneous negative DSST and positive DCHL pattern are mapped as thermal fronts. Results include a time series of monthly front occurrence GIS maps in Eastern Mediterranean waters along with three calculated front characteristics (DSST, DCHL and bathymetry). Comparisons between the mapped fronts and isobath distributions present clearly visible front‐to‐isobath spatial and shape associations while spatial analysis between front and wind data explains 66% of the number of mapped fronts.

Low-Frequency Variability in the Midlatitude Atmosphere Induced by an Oceanic Thermal Front

Abstract This study examines the flow induced in a highly idealized atmospheric model by an east–west-oriented oceanic thermal front. The model has a linear marine boundary layer coupled to a quasigeostrophic, equivalent- barotropic free atmosphere. The vertical velocity at the top of the boundary layer drives the flow in the free atmosphere and produces an eastward jet, parallel to the oceanic front's isotherms. A large gyre develops on either side of this jet, cyclonic to the north and anticyclonic to the south of it. As the jet intensifies during spinup from rest, it becomes unstable. The most unstable wave has a length of about 500 km, it evolves into a meander, and eddies detach from the eastern edge of each gyre. The dependence of the atmospheric dynamics on the strength T∗ of the oceanic front is studied. The Gulf Stream and Kuroshio fronts correspond roughly, in the scaling used here, to T∗ ≅ 7°C. For weak fronts, T∗ ≤ 4°C, the circulation is steady and exhibits two large, antisymmetric gyres sepa...

Oceanic survival and movements of wild and captive-reared immature green turtles ( Chelonia mydas) in the Indian Ocean

The ability of captive-reared turtles to survive in the wild is not precisely known, nor are movements of immature turtles in the open ocean. To provide information on these issues, a satellite tracking experiment was conducted in the western Indian Ocean to monitor oceanic movements of immature green turtles. Two wild turtles and four captive-reared individuals were tracked. The latter had been displaced after birth from nesting sites to a distant rearing site. Wild turtles survived after release, but did not move far away from release site. We hypothesize that this resident behaviour may be explained by stage-specific habitat requirements. Captive-reared turtles survived after release and migrated over thousands of kilometres. Among these, the oldest immature turtles retrieved the foraging sites of their native population, with movement patterns similar to those displayed by adults. Observed movements may be linked to hydrographic conditions such as general oceanic circulation, sea temperature and thermal fronts.

Seasonal variability of thermal fronts in the northern South China Sea from satellite data

The 8‐year (1991–1998) Pathfinder sea surface temperature data have been applied here to produce the objectively derived seasonality of the oceanic thermal fronts in the northern South China Sea from 17°N to 25°N. Several fronts have been clearly distinguished, namely, Fujian and Guangdong Coastal Water, Pear River Estuary Coastal, Taiwan Bank, Kuroshio Intrusion, Hainan Island East Coast and Tonkin Gulf Coastal fronts. The frontal patterns in winter, spring and summer are quite similar, whereas individual fronts display different modes of seasonal variability due to different mechanisms favoring those fronts.

Climatology and seasonal variability of ocean fronts in the East China, Yellow and Bohai seas from satellite SST data

Front detection and declouding algorithms developed at the URI have been applied to the 12‐year (1985–1996) Pathfinder SST data set to produce the first comprehensive, objectively derived, year‐round climatology of ocean thermal fronts in the eastern China Seas, from 24°N to 41°N. Ten fronts have been distinguished, namely Kuroshio, Zhejiang‐Fujian, Jiangsu, Shandong Peninsula, Bohai Sea, Seohan Bay, Kyunggi Bay, Western and Eastern Chejudo (South Korean Coastal Front), and Yangtze Bank Ring Front. The fronts are seasonally persistent: they emerge and disappear in the same seasons of different years. The frontal pattern remains fairly stable from one year to another, whereas individual fronts display different modes of seasonal variability.

A Front-Following Algorithm for AVHRR SST Imagery

A Front-Following Algorithm provides a new approach to determining the position and characteristics of thermal oceanic fronts using Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) imagery. This algorithm differs from standard line enhancement, threshold edge detection, or classical contour techniques. Instead it utilizes a hyperbolic tangent function in a surface-fitting technique to follow an oceanic front. It has the advantage of describing the characteristics of an oceanic thermal front (including mean SST, SST difference, width and gradient across the front) and extracting information on the position and characteristics of the front into parameter form. Thus, the algorithm has the added benefit of recording the changes in the characteristics of the thermal front as it tracks along the front. The algorithm was applied to AVHRR SST imagery on part of the Subtropical Front known as the Southland Front (SF) off the east coast of the South Island of New Zealand, where subantarctic surface waters and subtropical surface waters converge. The algorithm was tested on examples of the SF and also compared with a standard gradient operator. The results showed that the algorithm performed well when following the SF, with low standard errors of parameter estimates, good visual verification of tracking, and consistent standards of accepted data. The algorithm estimated gradients better than a gradient operator.

Thermal Feedback on Wind Stress as a Contributing Cause of Climate Variability

A model that isolates the interaction between midlatitude ocean gyres and the wind stress due to atmospheric baroclinic eddies is formulated. The ocean and atmosphere are coupled through their respective heat balances and global heat and momentum conservations are enforced. The ocean flow creates a steep oceanic thermal front at the midlatitude intergyre boundary. This frontogenesis sharpens the atmospheric temperature gradients and locally increases the atmospheric eddy heat transport. The result is a well-defined storm track that, because of the delayed adjustment of the gyres to the wind stress, oscillates in time with a period of about 18 yr.

Exploring the association between swordfish catch rates and thermal fronts on U.S. longline grounds in the western North Atlantic

Associations between ocean surface thermal fronts and the swordfish catch rates of U.S. longline vessels were explored. The study area was the western North Atlantic off the United States, extending from 32°N to 45°N and from 76°W to 63°W. To locate and describe fronts, we used three variables computed from satellite-derived sea surface temperature (SST): horizontal gradient, distance to nearest thermal surface front, and frontal density. Most of the fishing effort analysed occurred along the edge of the continental shelf, where there was a high frequency of frontal presence. Very high catch per unit effort (CPUE) occurred more frequently in the vicinity of fronts than would be expected by chance. The high variability of CPUE that could not be explained by our frontal parameters suggested other, unmeasured, factors also influenced catch rates.