Abstract's details
Capturing Mesoscale Features In Surface Currents
CoAuthors
Event: 2015 Ocean Surface Topography Science Team Meeting
Session: Science II: Mesoscale and sub-mesoscale ocean processes: current understanding and preparation for SWOT
Presentation type: Type Poster
Contribution: PDF file
Abstract:
Ocean Surface Current Analyses-Realtime currents (OSCAR, podaac.jpl.nasa.gov) are global ocean surface velocities calculated from sea surface height (SSH) gradients, ocean vector winds, and sea surface temperature fields using geostrophy, Ekman, and thermal wind dynamics. OSCAR uses the AVISO gridded MADT SSH fields (http://www.aviso.altimetry.fr/).
OSCAR successfully captures the surface currents in highly geostrophic regions with larger-scale and relatively slowly evolving eddies, such as in the Gulf Stream. When compared with global surface drifters, though, velocity standard deviations are underestimated by 20-50% in OSCAR over much of the oceans. This is in part due to missing dynamics in the simple model for OSCAR -- such as Stokes drift. However, much of the loss of signal is also due to the use of level 4 gridded fields rather than using level 2 fields directly, which is particularly important when taking gradients.
We have been using local polynomial fitting mapping methods to investigate the mesoscale and sub-mesoscale signal in both altimetry and ocean vector wind fields. A main question to address is the degree to which the use of smoothed level 4 fields affects the calculation of surface currents. An important feature of local polynomial fitting over optimal interpolation methods is the order of fit: a first-order (linear) fit calculates gradients as part of the mapping. Geostrophic velocities will therefore be directly calculated from the data, which is expected to result in stronger, more accurate, albeit noisier, velocities.
Here we will contrast AVISO-based geostrophic currents against those obtained directly from along-track SSH anomalies. In addition, wind-driven currents based on ERA Interim winds will be compared with local currents calculated directly from ASCAT winds. These will all be compared with drifting buoy velocities. The main goals are to retain the most information on surface currents possible from high-resolution satellite fields as well as to gain a better understanding of the limits to which simple physical models can accurately reproduce higher-resolution surface currents from satellite sensed fields. Ultimately, how much additional information on the mesoscale surface circulation can SWOT provide?
OSCAR successfully captures the surface currents in highly geostrophic regions with larger-scale and relatively slowly evolving eddies, such as in the Gulf Stream. When compared with global surface drifters, though, velocity standard deviations are underestimated by 20-50% in OSCAR over much of the oceans. This is in part due to missing dynamics in the simple model for OSCAR -- such as Stokes drift. However, much of the loss of signal is also due to the use of level 4 gridded fields rather than using level 2 fields directly, which is particularly important when taking gradients.
We have been using local polynomial fitting mapping methods to investigate the mesoscale and sub-mesoscale signal in both altimetry and ocean vector wind fields. A main question to address is the degree to which the use of smoothed level 4 fields affects the calculation of surface currents. An important feature of local polynomial fitting over optimal interpolation methods is the order of fit: a first-order (linear) fit calculates gradients as part of the mapping. Geostrophic velocities will therefore be directly calculated from the data, which is expected to result in stronger, more accurate, albeit noisier, velocities.
Here we will contrast AVISO-based geostrophic currents against those obtained directly from along-track SSH anomalies. In addition, wind-driven currents based on ERA Interim winds will be compared with local currents calculated directly from ASCAT winds. These will all be compared with drifting buoy velocities. The main goals are to retain the most information on surface currents possible from high-resolution satellite fields as well as to gain a better understanding of the limits to which simple physical models can accurately reproduce higher-resolution surface currents from satellite sensed fields. Ultimately, how much additional information on the mesoscale surface circulation can SWOT provide?