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1. What is the significance of Level 1B MODIS/TERRA images in remote sensing data?

2. How are ISW signatures visually identified and extracted?

3. How to calculate IT velocities?

4. What is the relationship between the depth of the pycnocline and the generation of higher mode internal tides in area B?

6. What is the seasonal variability of ISW characteristics in terms of velocity, wavelength, and propagation direction on the Amazon shelf?

Accepted Answer

Level 1B MODIS/TERRA images play a crucial role in remote sensing data as they provide high-quality, Level 1B data from the MODIS sensor onboard the TERRA satellite. These images, acquired from January 2005 to December 2021, offer valuable insights into various environmental parameters. Specifically, the Band 6 centered at 1640 nm with a spatial resolution of 500 meters is used to identify ISW signatures in the sun glint region. This data is essential for studying oceanic and atmospheric phenomena, such as sea surface temperature, chlorophyll concentration, and cloud cover. The images are collected from NASA's Earth Science Data System, ESDS, ensuring accessibility and reliability for researchers. However, factors like cloud coverage and the changing position of the sun glint area over the year can limit the availability of samples. Despite these limitations, Level 1B MODIS/TERRA images remain a valuable resource for remote sensing research and applications.

Accepted Answer

ISW signatures are visually identified and manually extracted for each MODIS/TERRA scene in the data set. Non-linear ISWs are visualized as leading bands of increased sea surface roughness followed by bands of decreased roughness. The leading wave of each ISW packet is mapped, and the distance between consecutive leading wave signatures, called wavelength, is calculated. This wavelength is associated with typical IT wavelengths. ISW signatures are often found with mode-1 IT wavelengths, but mode-2 IT wavelengths can also be observed. The average wave propagation velocity is calculated based on the semi-diurnal IT period of 12.42 hours. The ISW propagation direction, pd, is automatically retrieved from the RS data by considering the angle between the North and the direction of the vector connecting the middle points of two consecutive packets. pd = 0 indicates ISWs propagating from the South to the North, while pd = 90 indicates propagation from West to East.

Accepted Answer

The IT velocities can be calculated using the numerical method proposed by Lian et al. (2020) to solve the viscous TGE for both instabilities and waves. The equation (EQUATION EQUATION EQUATION) is used to determine the ITs propagation velocities. The complex vertical structure functions of vertical buoyancy and velocity are considered, and the linearized normal-mode equations are used. The phase speed of internal gravity waves is described by (EQUATION). The nonlinear phase speed can be corrected using the Korteweg-De Vries (KdV) equation, although it applies only to shallow waters with uniform depths. The velocities of all modes are calculated using Equations 8 and 9. Local values of stratification and shear are taken from daily and monthly reanalysis data for each location where ISWs were identified. The current velocities are decomposed in the ISW traveling direction. The separation between different wave modes is based on the probability distribution of the velocities predicted by the viscous TGE for mode-1 and mode-2, considering the monthly reanalysis data.

Accepted Answer

In area B, a shallower pycnocline suggests stronger higher mode internal tide generation, which is in good agreement with the findings. The depth of the pycnocline was defined as the depth corresponding to the maximal value of the Brunt-Vaisala frequency. A shallower pycnocline indicates a higher contrast in velocities and spatial location for mode-2 waves compared to mode-1 waves. This relationship is supported by the observation of an offshore acceleration for the most eastern branch of both mode-1 and mode-2 waves, with mode-2 waves experiencing a slightly more pronounced acceleration in propagation velocities (~ 18%).

Accepted Answer

Spring-neap tidal variability significantly impacts internal tide wave activity. Studies have shown that ISW activity is more pronounced near spring tide conditions for both areas A and B, with 71% and 61% of ISW signatures respectively. The proportion of mode-2/mode-1 signatures increases from spring to neap tides, indicating more mode 2 waves during neap tides. In area A, mode-2 signatures are around 28% near neap tides and 20% near spring tides. Similarly, in area B, mode-2 signatures are about 61% near neap tides and 48% near spring tides. The mean propagation velocity/wavelength values for mode-1 waves follow a normal distribution, with higher values near neap tides. The IT wave propagation velocities calculated using the TGE do not show significant differences between spring and neap tides. For mode-2 waves, no significant differences are found according to the tide conditions. The mean wavelength and propagation velocity for both spring and neap tides are similar for mode-2 waves. Overall, spring-neap tidal variability plays a crucial role in determining the characteristics and distribution of internal tide wave activity.

Accepted Answer

The seasonal variability of ISW characteristics on the Amazon shelf, specifically in terms of velocity, wavelength, and propagation direction, has been characterized for two well-marked seasons: boreal spring (March to July) and boreal summer/fall (August to December). The analysis was focused on A waves, as other sites did not have enough data for both seasons. The mode-1 propagation velocity/wavelength values follow a normal distribution for both seasons, with a statistically significant difference between the two seasons. During boreal summer/fall, there is a higher propagation velocity/wavelength compared to boreal spring. The pycnocline during summer/fall is slightly deeper than in spring. Larger wavelengths (higher velocities) of mode-1 internal tides are expected during summer/fall, as supported by previous studies. The waves in summer/fall exhibit higher diversity in terms of wavelengths, indicating higher variability in local stratification and current shear patterns. The TGE successfully predicts the differences in mode-1 IT velocities/wavelengths between the two seasons, with a similar distribution pattern to the one estimated using RS data. However, the differences are less evident when considering the values predicted by the TGE. The mean values of velocity/wavelength predicted using daily reanalysis data suggest that using monthly data may smooth the variability related to stratification and circulation. The wave propagation velocities/wavelengths calculated using the TGE vary according to the seasons, with higher mean velocities during summer/fall. The mode-2 waves propagate in a wider direction pathway during summer/fall, with an increase in wave propagation velocities with increasing eastern traveling direction. The circulation during summer/fall is characterized by the eastward reinforcement of the NECC, which likely plays a role in refracting the waves to the northeast and increasing their propagation velocities eastward. The ISWs spread over the study area during summer/fall, extending wave penetration further north, especially in the later months of the season. This behavior contrasts with the more straightforward path and lower penetration further north associated with the waves in boreal spring. Most of the ISW signatures in area A above latitude 8* correspond to mode-2 waves from early September 2014 and 2018, but this may be influenced by sampling restrictions due to higher cloudy coverage.

Accepted Answer

The aliasing effect refers to sampling restrictions caused by the sun-synchronous satellite orbit. It can result in images acquired during similar flood-ebb phases of the semi-diurnal tide, affecting the accuracy of data. To mitigate this, a balanced data set can be constructed by considering different seasons and wave baroclinic modes. Using various optical sensors/satellites like AQUA/MODIS, MSI/Sentinel-2, and OLCI/Sentinel-3 can help in obtaining more comprehensive and reliable data.

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Most frequently asked questions

1. What is the significance of Level 1B MODIS/TERRA images in remote sensing data?

2. How are ISW signatures visually identified and extracted?

3. How to calculate IT velocities?

4. What is the relationship between the depth of the pycnocline and the generation of higher mode internal tides in area B?

5. How does spring-neap tidal variability affect internal tide wave activity?

6. What is the seasonal variability of ISW characteristics in terms of velocity, wavelength, and propagation direction on the Amazon shelf?

7. What is the aliasing effect in sun-synchronous satellite orbits?

References

Abyssal recipes II: energetics of tidal and wind mixing

The formation and fate of internal waves in the South China Sea

Theory of radar imaging of internal waves

The Korteweg-de Vries equation and water waves. Part 2. Comparison with experiments

Mapping climatological seasonal variations of surface currents in the tropical Atlantic using ship drifts

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