Altimeter-Derived Geostrophic Velocities Across the Northeast Channel; Indices for Scotian Shelf Crossovers

This dataset consists of two indices quantifying the geostrophic cross-channel velocity across the Northeast Channel in the Gulf of Maine derived from altimeter data. The purpose of these indices is to quantify the occurrences of Scotian Shelf Crossover events in which water cuts directly across the Northeast Channel from the Scotian Shelf to Georges Bank instead of traveling around the Gulf of Maine. The first index was generated DUACS delayed-time altimeter gridded maps with a spatial resolution of 1/4 degree available from Copernicus Marine Services (https://doi.org/10.48670/moi-00145). We used the absolute geostrophic velocity supplied from the DAUCS product, which is calculated from the absolute dynamic topography. We isolated a region spanning across the NEC, which includes 48 of the SSH grid points (shown as the red box in the included figure). For each grid point within this box, the cross-channel component of the velocity was calculated, with the 500m isobath being the cross-channel direction. The angle () between the 500m isobath and the geostrophic velocity from the gridded SSH product was calculated and used to find the cross-shelf component of the velocity, with the following equation where is the total geostrophic velocity and is the cross-channel component. The average of all values at each grid point was then calculated for each day. The second index included in the dataset was generated from an along-track SSH product, also available from Copernicus Marine Services ( https://doi.org/10.48670/moi-00146). For the cross-channel velocity, we used Track number 24 from the Topex/Jason altimeter product, which passes through roughly the center of the NEC (shown as the green line in the included figure) and has a track repeat time of 10 days. Due to the direction of the track being approximately perpendicular to the cross-channel flow, we used the cross-track geostrophic velocity as an approximation for the cross-channel flow. Along Track 24, the across track geostrophic velocity was calculated from the absolute dynamic topography. The portion of the track within the NEC region was isolated (as shown in the included figure), and the cross-track geostrophic velocity was averaged across the section. Because the along-track altimeter data does not undergo interpolation like the gridded data, gaps can occur along the track due to error flags, extreme weather, or other data-retrieval issues. Due to these gaps, only days in which at least ¾ of the data were usable in the NEC region were included in our study, eliminating 165 of the 1046 available days. Files: sshGridded_CrossChannelVel.csv – contains the daily geostrophic cross-channel velocities derived from the gridded sea surface height product. sshTrack_CrossChannelVel.csv – contains the geostrophic cross-channel velocities derived from Track 24 from the Topex/Jason altimeter product with a track repeat time of 10 days. Days in which there was not enough data to calculate the cross-channel velocities are included as NaN values.