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James A Yoder - One of the best experts on this subject based on the ideXlab platform.

  • High frequency and Mesoscale variability in SeaWiFS chlorophyll imagery and its relation to other remotely sensed oceanographic variables, Deep Sea Res
    2020
    Co-Authors: James A Yoder
    Abstract:

    Abstract In its typical use for the study of large scale and relatively slow variability of phytoplankton biomass, ocean-color imagery is often binned in space and in time, and variability within the bin is discarded as noise. Since small-to Mesoscale processes at time scales as short as a day may play a significant role in global cycles of carbon and nutrients, characterizing variability at these scales is necessary. With the first four years of nearly continuous daily imagery from the SeaWiFS instrument, we investigated patterns of variability at the Mesoscales, operationally defined as that within a 2 Â 2-degree neighborhood. We show that Mesoscale variability of chlorophyll concentration (Chl) is high near the coasts, in dynamically active areas, and at the oligotrophic centers of subtropical gyres. High apparent variability over the oligotrophic ocean is a surprising contrast to the low variability in composite imagery at the same locations and may be due to increased relative noise at low mean Chl. Low correlation between pairs of images as little as 1 day apart in the oligotrophic ocean is consistent with a noise artifact, or alternatively may indicate that the observed variability is due to high-frequency phenomena. Spatial patterns of variability observed when data are binned into narrow ranges of mean Chl, suggest oceanographic origins. Patterns of variability in Chl and in sea-surface height have little correlation, suggesting that eddy pumping or turbulent diffusion along temporarily slanted isopycnal surfaces are not the major sources of Chl variability. The correlation between Mesoscale anomalies of Chl and sea-surface temperature is not always negative as would have been the case if anomalies were produced mainly by the entrainment of colder, nutrientrich thermocline waters into the euphotic layer. Instead, we find roughly zonal bands of alternating negative and positive correlations determined by the relative directions of the background gradients of Chl and SST. Thus the most obvious influence of Mesoscale Motion on the distribution of Chl is advection of the existing gradients. Both long-term means and local anomalies of scatterometric winds from QuikSCAT are also correlated with mean Chl. Much of this correlation appears to be due to changes in the relationship between surface roughness and wind speed, brought on by factors like surface films, thermal stability of the air column, and surface currents. Our analyses show the feasibility of using ocean-color imagery to study Mesoscale variability but also identify areas where there is room for major improvements. Minimization of speckling due to imperfect atmospheric correction, in particular, would significantly enhance the utility of SeaWiFS data at Mesoscales

Gorriz, Elisa G. - One of the best experts on this subject based on the ideXlab platform.

  • The seasonality of Mesoscale Motion in the northern current of the western Mediterranean Sea: several years of evidence
    2019
    Co-Authors: Font Jordi, García-ladona Emilio, Gorriz, Elisa G.
    Abstract:

    Special issue PRIMO-0.-- 13 pages, 17 figures, 1 tableThe Liguro-Provenco-Catalan or Northern Current, which contours the continental slope of the north-western Mediterranean Sea, has been described as a permanent flow affected by important variability at different scales. Many observations indicate a higher flux in winter than in summer, while the occurrence of Mesoscale events appears to be a common characteristic. The opportunity of examining several years of current meter data at a location close to the inner border of the Northern Current has definitively demonstrated the existence of a seasonality in the Mesoscale variability. Although the records do not concern the core of this current, the consistency of the several characteristics we have determined with results found by other authors in the Ligurian Sea allows us to consider them as representative of its general trends. Three current meters moored at 15, 50 and 100 m depth from an oil drilling ring near the shelf break off the Ebro delta, with a considerable coverage from mid-1987 to mid-1992, provide a clear image of the current behaviour. The general flow is along-slope and presents occasional disruptions for periods of several days, described here as Mesoscale events. Most of the Motion we have recorded displays a marked barotropic character. The intensity of the current is relatively low, increasing suddenly in autumn at the three levels monitored, in a manner that appears not to be connected to the local wind regime. One of the most relevant pieces of information contained in the whole current data set concerns the repeated presence of clockwise rotations in a near-inertial frequency, even at 100 m. The principal objective of the present study was to investigate any seasonal signal in the appearance and characteristics of the Mesoscale Motion. The main observed feature in a low-passed time series below the thermocline, namely a rapid and strong autumn increase in Mesoscale activity, is shown to be consistently present, with very small inter-annual temporal variability. This maximum of Mesoscale activity is followed by a rapid decrease in winter and then by a continuous decline until the end of the summer. A complete characterization of the Mesoscale Motion in the Northern Current, including its origin and interaction with the main flow, requires extensive experimental and numerical work. Our results have demonstrated that this Motion displays a clear seasonal cycle and provided evidence that the recorded Mesoscale activity is linked far more to the shelf/slope front evolution than to local wind variabilityPeer Reviewe

Font Jordi - One of the best experts on this subject based on the ideXlab platform.

  • The seasonality of Mesoscale Motion in the northern current of the western Mediterranean Sea: several years of evidence
    2019
    Co-Authors: Font Jordi, García-ladona Emilio, Gorriz, Elisa G.
    Abstract:

    Special issue PRIMO-0.-- 13 pages, 17 figures, 1 tableThe Liguro-Provenco-Catalan or Northern Current, which contours the continental slope of the north-western Mediterranean Sea, has been described as a permanent flow affected by important variability at different scales. Many observations indicate a higher flux in winter than in summer, while the occurrence of Mesoscale events appears to be a common characteristic. The opportunity of examining several years of current meter data at a location close to the inner border of the Northern Current has definitively demonstrated the existence of a seasonality in the Mesoscale variability. Although the records do not concern the core of this current, the consistency of the several characteristics we have determined with results found by other authors in the Ligurian Sea allows us to consider them as representative of its general trends. Three current meters moored at 15, 50 and 100 m depth from an oil drilling ring near the shelf break off the Ebro delta, with a considerable coverage from mid-1987 to mid-1992, provide a clear image of the current behaviour. The general flow is along-slope and presents occasional disruptions for periods of several days, described here as Mesoscale events. Most of the Motion we have recorded displays a marked barotropic character. The intensity of the current is relatively low, increasing suddenly in autumn at the three levels monitored, in a manner that appears not to be connected to the local wind regime. One of the most relevant pieces of information contained in the whole current data set concerns the repeated presence of clockwise rotations in a near-inertial frequency, even at 100 m. The principal objective of the present study was to investigate any seasonal signal in the appearance and characteristics of the Mesoscale Motion. The main observed feature in a low-passed time series below the thermocline, namely a rapid and strong autumn increase in Mesoscale activity, is shown to be consistently present, with very small inter-annual temporal variability. This maximum of Mesoscale activity is followed by a rapid decrease in winter and then by a continuous decline until the end of the summer. A complete characterization of the Mesoscale Motion in the Northern Current, including its origin and interaction with the main flow, requires extensive experimental and numerical work. Our results have demonstrated that this Motion displays a clear seasonal cycle and provided evidence that the recorded Mesoscale activity is linked far more to the shelf/slope front evolution than to local wind variabilityPeer Reviewe

García-ladona Emilio - One of the best experts on this subject based on the ideXlab platform.

  • The seasonality of Mesoscale Motion in the northern current of the western Mediterranean Sea: several years of evidence
    2019
    Co-Authors: Font Jordi, García-ladona Emilio, Gorriz, Elisa G.
    Abstract:

    Special issue PRIMO-0.-- 13 pages, 17 figures, 1 tableThe Liguro-Provenco-Catalan or Northern Current, which contours the continental slope of the north-western Mediterranean Sea, has been described as a permanent flow affected by important variability at different scales. Many observations indicate a higher flux in winter than in summer, while the occurrence of Mesoscale events appears to be a common characteristic. The opportunity of examining several years of current meter data at a location close to the inner border of the Northern Current has definitively demonstrated the existence of a seasonality in the Mesoscale variability. Although the records do not concern the core of this current, the consistency of the several characteristics we have determined with results found by other authors in the Ligurian Sea allows us to consider them as representative of its general trends. Three current meters moored at 15, 50 and 100 m depth from an oil drilling ring near the shelf break off the Ebro delta, with a considerable coverage from mid-1987 to mid-1992, provide a clear image of the current behaviour. The general flow is along-slope and presents occasional disruptions for periods of several days, described here as Mesoscale events. Most of the Motion we have recorded displays a marked barotropic character. The intensity of the current is relatively low, increasing suddenly in autumn at the three levels monitored, in a manner that appears not to be connected to the local wind regime. One of the most relevant pieces of information contained in the whole current data set concerns the repeated presence of clockwise rotations in a near-inertial frequency, even at 100 m. The principal objective of the present study was to investigate any seasonal signal in the appearance and characteristics of the Mesoscale Motion. The main observed feature in a low-passed time series below the thermocline, namely a rapid and strong autumn increase in Mesoscale activity, is shown to be consistently present, with very small inter-annual temporal variability. This maximum of Mesoscale activity is followed by a rapid decrease in winter and then by a continuous decline until the end of the summer. A complete characterization of the Mesoscale Motion in the Northern Current, including its origin and interaction with the main flow, requires extensive experimental and numerical work. Our results have demonstrated that this Motion displays a clear seasonal cycle and provided evidence that the recorded Mesoscale activity is linked far more to the shelf/slope front evolution than to local wind variabilityPeer Reviewe

Hogg, Nelson G. - One of the best experts on this subject based on the ideXlab platform.

  • Space and time scales of Mesoscale Motion in the western North Atlantic
    'MBLWHOI Library', 2018
    Co-Authors: Richman, James G., Wunsch Carl, Hogg, Nelson G.
    Abstract:

    Also published as Reviews of Geophysics and Space Physics, Vol. 15, No. 4, November 1977, pp. 385-420From moored data, primarily temperature, of the Mid-Ocean Dynamics Experiment (ModeI) and its successor experiments we find a statistical description of the Mesoscale variability. In the ModeI area itself the spectral characteristics of the thermocline and the deep water are different. The thermocline is conveniently described as being made up of three spectral bands: a ' low-frequency' band dominated by zonal velocity fluctuations, an 'eddy-containing' band in which the velocity field is nearly isotropic, and a 'high-frequency' band consistent with models of geostrophic turbulence. In the deep water the zonal dominance at low frequencies is not apparent, and there is enhanced energy at periods of 20-50 days. Vertical structure scales with WK BJ approximation in the high-frequency band but not in the lower frequencies, where low vertical modes dominate the Motion. Linear models do not adequately describe the data in the ModeI region. Differences between rough and smooth topography regions are clearly seen only at 1500 m, where there is a loss of energy consistent with a reduced barotropic Motion. Other differences, while apparently real, are small. It is found, consistent with the results of Schmitz (1976a), that the ModeI region is atypical of the midocean in that large changes of energy level are found elsewhere. A region due east of ModeI has slightly reduced kinetic energy levels in the main thermocline, but deep energy levels are much lower. Potential energy is less variable than kinetic; in the eastern region the frequency spectra change structure slightly. Linear models may be more adequate there. With more than 2 years of data, no statistically significant heat flux was found in the ModeI area, except for a weak zonal flux in the deep water. There is no direct evidence for baroclinic instability as a significant mechanism of eddy generation; the Gulf Stream is a possible, if unconfirmed, source.Prepared for the Office of Naval Research under Contracts N00014-66-C-0241; NR 083-004~ N00014-74-C-0262; NR 083-004 and N00014-76-C-0197; NR 083-400; and for the National Science Foundation under Grants GX-29054, GX-29034, OCE 75-03962 and ID0-82534