Deep-water bathymetric features imaged by spaceborne SAR in the Gulf Stream region

Authors


Abstract

[1] Deep-water (>500 m) oceanic bathymetric features are frequently observed in RADARSAT-1 SAR images in the Gulf Stream (GS) region. They are imaged apparently because of the unique environmental conditions in the region, oceanographically characterized by a strong GS current (2 ms−1) and favorable ocean stratification. SAR image analysis shows the basic characteristics of these bathymetric features. A coincident sea surface temperature image shows that the bathymetric feature is only “visible” by SAR within the GS pathway. The dominant wavelength of the wave-like feature is about 2.3 km and their crests are perpendicular to the GS axis. Shipboard sounding measurements confirm the SAR observation. A theoretical consideration of the ocean current and corrugated bathymetry interaction in a 3-layer ocean is presented. Using representative ocean density profile data and the GS current data, we analyze the requirements for the generation and upward propagation of the disturbance induced by the current-bathymetry interaction.

1. Introduction

[2] Ocean surface imprints of shallow-water bathymetry was one of the remarkable oceanic features observed in images from the first spaceborne synthetic aperture radar (SAR) onboard SEASAT, which was launched in 1978 [Fu and Holt, 1982]. With the increasing number of SAR satellites that have become available since early 1990's, these features are frequently observed in SAR images obtained from Europe's ERS-1/2, and ENVISAT satellites; Canada's RADARSAT-1/2 satellites, and Japan's ALOS satellite, among others. Associated theoretical work [Alpers and Hennings, 1984; Shuchman et al., 1985; Vogelzang et al., 1997; Hennings, 1998; Zheng et al., 2006; Li et al., 2009] and field campaigns [McLeish et al., 1981; Valenzuela et al., 1983, 1985; Vogelzang et al., 1992; Cooper et al., 1994; Vogelzang et al., 1997] have been carried out globally. The microwave SAR beam does not penetrate the water column, and only measures the radar backscatter from the ocean's surface. Up to the present, the radar bathymetry imaging mechanism has been thought to have been well understood in that the radar backscatter modulation is caused by the ocean surface Bragg wave modulation induced by current-bathymetry interactions. In the literature, SAR-bathymetry studies have been confined to shallow water coastal areas in zones characterized by strong tidal currents, under low-to-moderate wind conditions. This set of environmental conditions (shallow water regions with depths on the order of tens of meters, strong currents and low winds) in the rough sand bed region is thought to be the necessary conditions for the SAR imaging of bathymetry.

[3] In this study, we present new evidence of the SAR imaging of ocean bathymetry at depths in excess of 500 m, in the Gulf Stream region. While very strong currents (the Gulf Stream current speed is greater than 2 ms−1) and low-to-moderate wind conditions are still true for SAR to image the bathymetry, in this case the SAR demonstrates the ability of imaging a deep-water sand bed. The observations seemingly are made possible only under certain stratification conditions with the water column serving as a waveguide allowing the disturbance induced by current-bathymetry interaction to propagate upward, reaching the ocean surface and modulating the ocean surface Bragg wave spectra. Shipboard sounding measurements confirm the spatial distribution of the sand bed pattern imaged by the SAR within the specific locale in the presence of the Gulf Stream.

2. Observations and Interpretation

[4] The study region covers the southeastern coastal waters of the United States (Figure 1) from Florida to North Carolina. The background is a NOAA/AVHRR sea surface temperature (SST) image acquired from the NOAA-17 satellite on October 15, 2006 and processed using the standard NOAA CoastWatch procedures [Li et al., 2001]. The Gulf Stream, a western boundary current characterized by relatively high temperature and salinity, flows nominally along the shelf-break from south to north transporting significant amounts of heat to higher latitudes. The Gulf Stream can clearly be seen in SST images as a narrow (∼100 km width), high SST meandering oceanic jet.

Figure 1.

An NOAA/AVHRR SST image, acquired coincident with the SAR in Figure 2a on October 15, 2006, shows the study region. The region of interest is off US Southeast Coast. The Gulf Stream, represented by the high SST, flows primarily along the isobaths. Box A represents the SAR image (Figure 2) coverage.

[5] Since 1997, NOAA has sponsored a multi-year project to determine and demonstrate SAR-derived quantitative and qualitative products in a pre-operational environment for US coastal waters [Pichel and Clemente-Colón, 2000]. RADARSAT-1 SAR data was first used and then ERS-2 SAR, ENVISAT Advanced SAR (ASAR), and ALOS Phased-Array L-Band SAR (PALSAR) data were added. In this study, we primarily use RADARSAT-1 images processed at the Alaska Satellite Facility (ASF). There are 83 SAR images available for analysis in the study area. Among them, 58 images contain the surface signature of the bathymetric feature. These features are observed under low-to-moderate wind condition (wind speed 2–8 m/s). They include all seasons. In these SAR images, we have continuously observed the wave-like pattern in the study area. We do not include all these SAR images in this paper for brevity. However, an example is given in Figure 2 (left). This RADARSAT-1 ScanSAR image coverage is shown as Box A in Figure 1. The SAR and AVHRR/SST images in Figures 1 and 2 were acquired on the same day. Figure 2 (right) shows that the wave-like features exist in the upper left corner of the SAR image. In Figure 2 (right), the two red lines are the Gulf Stream northern and southern boundaries extracted from the AVHRR/SST image in Figure 1. The wave-like features are located within the Gulf Stream main pathway and are perpendicular to the Gulf Stream axis. It is worth noting that: 1) the wave-like features exist persistently at this location. 2) the wave-like features show similarity to well-known ocean surface imprints of oceanic internal waves, but they are not internal waves. This is because the features appear in the same location all-year round. Their dominant wavelength is about 2.3 km (see analysis below), too long for internal waves in this region. In addition, the study area (offshore Charleston, South Carolina) is not known for generating this type of large-scale oceanic internal waves. 3) the features can only be observed within the Gulf Stream indicating that the current-bathymetry interaction must be strong enough to have an impact on the surface; and 4) both published nautical chart and shipboard sounding bathymetric data confirm that the water depth in this image is around 500 m. In some cases, we observe similar wave-like features in the area where the water depth is over 700 m.

Figure 2.

A SAR image (100 m resolution) shows the surface imprints of quasi-linear bathymetric features located in the region with water depth over 500 m. The SAR imaging time is at 23:04 GMT on October 15, 2006. Red lines indicate the Gulf Stream boundaries extracted from the SST image. Bathymetry contour lines are from 200 to 1000 m with interval of 100 m.

[6] Bathymetry survey data obtained from the National Geophysical Data Center (http://www.ngdc.noaa.gov/mgg/mggd.html) were used in this study. The shipboard sounding bathymetric data has a higher resolution than the SAR image spatial resolution (100 m resolution and 50 m pixel size for RADARSAT-1 ScanSAR mode images). For easy comparison, a 512 by 512 pixel full resolution sector of the SAR image containing features of ocean bottom topographic waves was extracted and is presented in Figure 3a. Based on the coordinates of the 512 by 512 pixel SAR sector, we extracted the bathymetric data covering the same domain and resampled the data to 512 by 512 as shown in Figure 3b. Two-dimensional Fast Fourier Transform (FFT) was applied to both data sets to generate the image spectra shown in Figures 3c and 3d. The SAR image spectrum peaks at 2316 m, indicating the wavelength of the bottom topographic features. The FFT of the in situ sounding sonar bathymetric data shows broader spectra with a peak at 2375 m. The wavelength peaks of the spatial pattern of the SAR and sounding sonar bathymetric data agree remarkable well, within 2.5%, while the orientations of these spectral peaks differ by an angle of about 17.5° degree, indicating the bathymetric features do not necessarily orientate the same way as their surface signature. These findings support our interpretation that the observed features are indeed ocean surface imprints of the bathymetric features.

Figure 3.

(a) 512 by 512 pixel full resolution of SAR image containing bathymetric features. (b) The in situ ship ocean depth measurement at the exact same location. (c and d) FFT spectra of images Figures 3a and 3b.

[7] The bathymetric contour map of Figure 3b is presented in Figure 4 (top). Water depth data in the dominant wave spectrum direction (along the white line in the bathymetry contour map) shows the variation of the bathymetric feature (Figure 4, bottom) between 490 m and 550 m.

Figure 4.

(top) Bathymetric contour chart in the same area of Figure 3b. (bottom) Plot of water depth in the dominant wave spectrum direction (along the white line in Figure 4, top).

3. Ocean Bottom Disturbance Generation and Upward Propagation

[8] We adopt a physical model in an effort to demonstrate the mechanisms that created the ocean surface stationary wave-like disturbance that we believe was induced by the corrugation bathymetry in a 3-layer ocean [Zheng et al., 2006]. There are dynamical requirements for both disturbance generation and upward propagation.

[9] In order for the wave-like ocean bottom disturbance to be generated, the lower layer resonance generation condition: σ = kCo must be satisfied, where σ the wave frequency, k is the horizontal wave number and Co is the ocean current velocity. There is also a frequency constraint for upward propagation. Only waves satisfying N3 < σ < N2, where N2 and N3 are the Brunt-Väisälä frequencies in the middle and upper layers of the ocean, will not be damped quickly and can thus penetrate upwards to reach the ocean surface. In this case, the water column hydrographic and dynamic properties serve as a filter and effectively band-passes selected upward propagating waves.

[10] Field-measured density profiles and calculated vertical distributions of the Brunt-Väisälä frequencies at two stations in the study area during the summer and winter months are shown in Figure 5. One can see that the ocean can be represented as a 3-layer system with an upper mixed layer and a lower layer separated by a sharp pycnocline. From Figure 5, we derive the average Brunt-Väisälä frequencies, N2 = 8.0 cph and N3 = 1.5 cph. This means that the wave-like disturbances with frequencies between 1.5 and 8.0 cph have the potential to propagate to the ocean surface and subsequently be imaged by SAR. The Gulf Stream current speed is on the order of 2 ms−1. Thus, from the lower layer resonance generation condition, we derive a wavelength range of 0.9 km < λ < 4.8 km. This agrees well with the SAR observations shown in Figure 3.

Figure 5.

(a) In Situ ship measurement of oceanic interior density profiles in the study region in summer (solid line) and winter (dash line). (b) The corresponding distributions of the summer and winter Brunt-Väisälä frequencies. The summer measurement was taken by a CTD instrument at Station 1770962 (31.88°N, 79.18°W) at 08:14 GMT on June 25, 2002. The winter measurement was taken by a XBT instrument at Station 5895883 (31.25°N, 79.25°W) at 04:17 GMT on November 11, 2008 (http://www.nodc.noaa.gov/GTSPP/access_data/).

[11] Repeated SAR observations do not show ocean surface imprints of bottom features outside of the Gulf Stream region. Curiously, the feature lines stop abruptly at the Gulf Stream boundary. To understand this phenomenon, we rewrite the lower layer resonance generation condition, σ = kCo in a wavelength form, λ = 2πCo/σ. In the regions outside Gulf Stream, the ocean current velocity is very small, typically an order of magnitude smaller than that of the Gulf Stream (Co/10). Under the same stratification conditions, the resonant bottom ocean sand waves would have a wavelength an order of a magnitude smaller (λ/10) than that within the Gulf Stream region. This much-smaller-wavelength wave would not satisfy the upward propagation condition, and thus, the smaller waves have no signatures on ocean surface detectable by SAR.

4. Summary

[12] We have reported on SAR observations of a deep-water bathymetric feature appearing in the surface waters of the Gulf Stream offshore Charleston, South Carolina. These stationary wave-like features are persistently observed in RADARSAT-1 SAR images in this region. A case study shows that the dominant wavelength of the quasi-linear bathymetric feature is about 2.3 km and that the orientation of the wave-like bottom feature is perpendicular to the Gulf Stream current axis. Shipboard sounding measurements show that the dominant wavelength calculated from the bathymetric data agrees well with that from the SAR image. The orientation of features calculated from this set of image SAR and sonar data have a 17.5° degree difference indicating the bathymetric feature does not necessarily orientate the same way as their surface signature. SST image analysis shows that the SAR imaging of the deep-water bathymetric feature only happens within the Gulf Stream current, per se. No SAR observations of deep-water bathymetric features have been found outside of the Gulf Stream pathway.

[13] Theoretical considerations are presented to explain the physical and imaging mechanisms of the observations of wave-like deep-water bathymetric features manifested in SAR imagery. Two requirements, the lower layer and resonance generation condition and the frequency constraint condition, are needed for ocean bottom disturbance generation and the upward propagation of the disturbances. Using representative ocean column density profiles and the Gulf Stream current velocity, we find that both conditions are satisfied for the generation and upward propagation of wave-like disturbances with wavelengths between 0.9 to 4.8 km to be generated and to appear in SAR imagery.

[14] The SAR capability shown herein is in keeping with the early use of gravity data in characterizing bottom configurations regionally. However, SAR has far greater coverage and repeated observations, and thus allows for dynamic water column conditions to be considered. We have shown that, for the first time, SAR also displays the mechanistic ability to detect the translation of bottom effects through extensive deep water columns, which would seem relevant to the mechanics of very large scale bedforms; some of which could migrate. The significance of these findings is that there are no prior reports of such bottom features in deep waters which appear in the peer reviewed literature. This also suggests that other bottom features of like kind may be identifiable in SAR imagery of the surface waters of Western Boundary Currents globally.

Acknowledgments

[15] The RADARSAT-1 SAR data were obtained under the NASA RADARSAT ADRO-2 program (RADARSAT-0011-0071) and processed by the Alaska Satellite Facility (ASF). The views, opinions, and findings contained in this report are those of the authors and should not be construed as an official NOAA or U.S. government position, policy, or decision.

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