1. Spatial and temporal Variability
Characterizing Spatiotemporal Variability in Altimeter-derived Ocean Currents on the coastal NW Atlantic Shelf H. Feng1, D. Vandemark1, R. Scharroo2, J. Wilkin Ocean Process Analysis Lab ,University of New Hampshire, NH, USA ; 2Altimetric LLC, NH, USA 3Institute of Marine and Coastal Sciences, Rutgers University, NJ, USA
Data and Methods
Abstract N
This study focuses on use of the long-term satellite altimeter data record to assess both spatial and temporal variability of ocean current within and surrounding the Gulf of Maine (GoM) region that resides on the NW Atlantic coastal margin. This marginal sea has been studied for many decades and from many perspectives due in part to large fisheries production as well as the energetic ocean circulation dynamics tied to the tied Bay of Fundy, and a persistent along-shelf current from upstream Scotian Shelf and Labrador Sea sources (Figure 1). Feng and Vandemark (2011) recently developed a new regional observational capability by producing satellite altimeter data product using latest coastal data reprocessing approaches. From these new sea surface height anomaly data, the cross-track surface geostrophic velocity anomalies for the suite of regional altimeter satellite tracks (Figure 2) have been estimated from the along-track slope. Here, we assess the ability of altimeter-derived surface geostrophic currents to characterize the surface circulation patterns and the seasonal variability in the region.
For satellite data validation and interpretation, in situ ADCP current measurements at Buoy N (see Figure 2) taken by the Univ. of Maine in the North East Channel (NEC) from 2004 thru 2011 are used to assess the accuracy of altimeter-derived surface geostrophic velocity anomalies. The NEC region is critical for GoM transport as it is the only location for deepwater exchange and dominates the total water mass exchange for the GoM. Buoy time series shows close agreement with altimeter-derived surface geostrophic currents at seasonal time scales. This suggests that altimetry is reliable for synoptic variation characterization. As expected, significant discrepancies still exist between altimeter-derived geostrophic currents and in-situ measurements, with likely explanation tied to ageostrophic parts (i.e. baroclinic and Ekman currents) and remaining high-frequency aliasing. Analyses of the altimeter track data surrounding the NEC and its nearby region, including deeper shelf/slope waters are presented to characterize both the spatial pattern and the seasonal evolution of the surface geostrophic anomaly flows. Discussions follow with respect to present knowledge of regional circulation dynamics and contributions that altimetry can provide towards improved understanding of interannual and seasonal circulation within the satellite altimeter era.
Coastal Altimeter Data Product (UNH-RADS): reprocessed 1-HZ along-track SLA product ( Feng and Vandemark, 2011) for T/P, J1, & J2 altimeters and with along-track filtering and key correction terms being:
Tide: GOT 4.7
Atmos: MOG2dG
Mean Sea Level: DNSC08
Altimeter-based geostrophic current estimation: based on along altimeter track gradients of the sea surface height for calculating cross track geostrophic current anomaly Vg:
centered difference over a ~35 km (7 ground points) distance for Vg estimation
5-point running mean along track filter applied
Altimeter-estimated Vg Validation: Long-term buoy N ( Fig. 2) ADCP velocity measurements ( u, v) near altimeter (<20km) are projected onto the altimeter cross track (Strub et al, 1997) for Jason 1 and Jason 2.
Spatial and temporal analysis based on measurements from altimeters Jason 1 and 2 ( ) , focused on Tracks 24 and 65 .
Figure 2 . Study region the Gulf of Maine and its nearby northwest Atlantic shelf and slope ocean, showing the nominal ground tracks of the TOPEX and Jason-1,2 (odd and even track #s for the ascending and descending tracks, respectively), the depth contours (in m) and the current buoy N (red symbol) mooring site.
Bay of Fundy
Figure 1: Schematic of ocean circulation for this section of the NW Atlantic shelf and Gulf of Maine
Key features: depth to 370m, CCW gyre, large tidal currents, incoming transport is through NE channel and along Scotian Shelf. offshore warm-salty Gulf Stream induced meso- to submeso-scale eddies near the shelf break; a very dynamic oceanic region
Scotian Shelf
Buoy Validation
Spatial and temporal Variability
Track 24
Track 65
Figure 3
(a) Time series of Jason1 (black) and Jason 2 (red) derived cross –track geostrophic current anomaly Vg (positive values for northeastward anomaly) from Track 24, and the 10-day low-pass filtered in situ cross-track projected current VADCP from the 48m depth measurements at Buoy N (see Fig. 2)
(b) Scatter plot of Jason 1 and Jason 2 derived Vg against the buoy N10-day low pass filtered VADCPfrom depth at 48m. Correlation coefficient (Corr. Coef) , difference RMSE and number of instantaneous observations (N) are also given.
Figure 5. Time–space diagrams of 30-day low-pass filtered Jason1a (left) and Jason 2a (right) derived cross –track geostrophic current anomaly Vg (positive for northeastward anomaly) for track 024. The blue line indicates the position of buoy N. Seasonal and interannual variability is apparent, particularly in the shelf/slope region nearer 41 N.
Figure 6. Time–space diagrams of 30-day low-pass filtered Jason1a (left) and Jason 2a (right) derived cross –track geostrophic current anomaly Vg (positive for southeastward anomaly) for track 065. Note the ~60 day signal, particularly near the coastal boundary ( >43o in latitude) but also offshore near 42o N. Between N the seasonal and interannual variability is seen ( Fig. 7).
(b) WINTER
(a) SUMMER
Track 65
Track 24
Figure 7. (a) Summer (Jul-Aug-Sept) and (b) winter (Jan-Feb-Mar) climatology maps of cross-track geostrophic current anomalies derived from altimeter Jason 1 data only ( ) . Blue and read lines indicate descending and ascending tracks, respectively.
Figure 4
(a) Time series of 30-day low-pass filtered Jason1 (black) and Jason 2 (red) derived cross –track geostrophic current anomaly Vg (positive values for northeastward anomaly) from Track 24, and the 30-day low-pass filtered in situ cross-track projected current VADCP from the 48m depth at Buoy N
(b) Scatter plot of the 30-day low-pass filtered Jason 1 and Jason 2 derived Vg against the buoy N 30-day low pass filtered VADCP ( all others are same as in Fig. 3b)
Summary
Buoy validation results are encouraging, showing that altimetry appears to be reliable for synoptic geostrophic current variation characterization (Fig. 3-4) at the mooring site with correlation coefficient of 0.45 ( significance at 99% level) and difference RMSE of 5.3cm/s. Significant discrepancies still exist between altimeter-based and in-situ Vg, most likely related to ageostrophic parts ( e.g. baroclinic and Ekman currents) and remaining high-frequency aliasing ( shallow water tides, ..? ).
Spatial and temporal variability in the altimeter based geostrophic anomaly current data is generally consistent with previous knowledge, particularly in the shelf/slope. Inside the GoM and its coastal zone, currents are weaker and tidal correction errors may still dominate or compete with signal.
References
Feng, H. and D. Vandemark, Altimeter Data Evaluation in the Coastal Gulf of Maine and Mid-Atlantic Bight Regions, Marine Geodesy, 34:3-4,
Strub et al, Altimeter-derived variability of surface velocities in the California Current System: Part 1: Evaluation of TOPEX altimeter velocity resolution, J. Geophys. Res., 102, 12,727– 12,748, 1997
Acknowledgements
NASA’s Science Directorate Physical Oceanography Program
University of Maine NEC Bouy measurement program
6th Coastal Altimetry Workshop, Riva del Garda, Italy, September, 2012 ct