the current, and by damping the capillary waves they can help mark circulation
features, showing up as dark lines. Turbulent mixing related to currents, current
shear, and eddies can modify the thermal environment of the ocean near-surface,
which has relevance for coral bleaching. Dramatic increases in coastal river
outflow, especially as a result of flooding, may negatively affect reefs through
salinity reduction and/or increased nutrient and sediment loads (e.g., Devlin and
Brodie 2005; McCulloch et al. 2003).
• Upwelling. Upwelling of cold water decreases the wind stress, resulting in a
dark region in SAR imagery (often accompanied by slick lines after the nutrientrich cold water leads to blooms of algae) (Clemente-Colón 2004; Li et al.
2009b).
• Ocean fronts. Frontal regions may show up as dark or bright lines, depending on
the orientation of the front. When the front is perpendicular to the radar look
direction, bright radar returns are obtained in convergent fronts and dark returns
in divergent fronts (Johannessen et al. 1996; Li et al. 2005).
• Bathymetric features. Shallow-water bathymetric features may be imaged by
SAR instruments in situations where the current interacts with the bottom
topography (Li et al. 2009a).
• Internal waves. Internal wave packets are ubiquitous in SAR imagery of regions
where there is a generation mechanism for these waves (e.g., tidal current
interaction with irregular bathymetry such as a shallow sub-surface sill or
islands; Li et al. 2008b; Jackson 2004; Wolanski and Deleersnijder 1998). These
internal waves, which propagate along the ocean pycnocline, may have amplitudes of 30–40 m and in some cases as large as 100 m (Apel 2004). In coral reef
regions, internal waves can cause both positive and negative impacts on coral
health and growth. On the one hand they can bring cool, nutrient-rich subsurface
water to the coral, mitigating surface warming that can lead to bleaching. On the
other hand, internal waves can produce significant thermal stress, with shortterm fluctuations in temperature of 10 °C, to as much as 20 °C (Wolanski et al.
2004), leading to mortality for species that cannot tolerate these fluctuations.
For the most part, ocean features are identified in SAR data using visual
examination of imagery; however, considerable experience in SAR image interpretation and knowledge of the local oceanographic environment and processes
operating therein are essential for correct interpretation. Often a complex mix of
processes is at work in the ocean modifying the surface roughness. In addition,
atmospheric boundary layer phenomena can also mask or confuse the correct
interpretation of ocean features. For example, atmospheric gravity waves can be
similar in appearance to internal ocean wave signatures and atmospheric fronts can
mimic ocean fronts (Shuming et al. 2010). These same considerations provide
serious challenges for automated ocean feature analysis algorithms, such as those
that utilize wavelet transforms (Wu and Liu 2003).
Wave measurements SAR instruments are able to image the two-dimensional
ocean surface wave field via three imaging processes: (1) wind waves and swell
waves modulate the smaller Bragg waves, leading to changes in backscatter;
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M. L. Heron et al.
features, showing up as dark lines. Turbulent mixing related to currents, current
shear, and eddies can modify the thermal environment of the ocean near-surface,
which has relevance for coral bleaching. Dramatic increases in coastal river
outflow, especially as a result of flooding, may negatively affect reefs through
salinity reduction and/or increased nutrient and sediment loads (e.g., Devlin and
Brodie 2005; McCulloch et al. 2003).
• Upwelling. Upwelling of cold water decreases the wind stress, resulting in a
dark region in SAR imagery (often accompanied by slick lines after the nutrientrich cold water leads to blooms of algae) (Clemente-Colón 2004; Li et al.
2009b).
• Ocean fronts. Frontal regions may show up as dark or bright lines, depending on
the orientation of the front. When the front is perpendicular to the radar look
direction, bright radar returns are obtained in convergent fronts and dark returns
in divergent fronts (Johannessen et al. 1996; Li et al. 2005).
• Bathymetric features. Shallow-water bathymetric features may be imaged by
SAR instruments in situations where the current interacts with the bottom
topography (Li et al. 2009a).
• Internal waves. Internal wave packets are ubiquitous in SAR imagery of regions
where there is a generation mechanism for these waves (e.g., tidal current
interaction with irregular bathymetry such as a shallow sub-surface sill or
islands; Li et al. 2008b; Jackson 2004; Wolanski and Deleersnijder 1998). These
internal waves, which propagate along the ocean pycnocline, may have amplitudes of 30–40 m and in some cases as large as 100 m (Apel 2004). In coral reef
regions, internal waves can cause both positive and negative impacts on coral
health and growth. On the one hand they can bring cool, nutrient-rich subsurface
water to the coral, mitigating surface warming that can lead to bleaching. On the
other hand, internal waves can produce significant thermal stress, with shortterm fluctuations in temperature of 10 °C, to as much as 20 °C (Wolanski et al.
2004), leading to mortality for species that cannot tolerate these fluctuations.
For the most part, ocean features are identified in SAR data using visual
examination of imagery; however, considerable experience in SAR image interpretation and knowledge of the local oceanographic environment and processes
operating therein are essential for correct interpretation. Often a complex mix of
processes is at work in the ocean modifying the surface roughness. In addition,
atmospheric boundary layer phenomena can also mask or confuse the correct
interpretation of ocean features. For example, atmospheric gravity waves can be
similar in appearance to internal ocean wave signatures and atmospheric fronts can
mimic ocean fronts (Shuming et al. 2010). These same considerations provide
serious challenges for automated ocean feature analysis algorithms, such as those
that utilize wavelet transforms (Wu and Liu 2003).
Wave measurements SAR instruments are able to image the two-dimensional
ocean surface wave field via three imaging processes: (1) wind waves and swell
waves modulate the smaller Bragg waves, leading to changes in backscatter;
354
M. L. Heron et al.