The moment that all field scientists crave has arrived – preliminary data!  Team T-Beam ran two successful profiles and are hot on the trail of identifying the area of highest energy of the internal tide.  Here’s an update of their progress so far.  During a preliminary 30 hour period, the ship steamed back and forth across a 200km transect thought to be the most likely location of the T-Beam (Transect C1 – C2).  The midpoint of this transect marked the location of one of the moorings deployed by the Revelle. During this trek we traversed the transect six times and the shipboard ADCP collected water velocity data down to 800 meters depth.  As we’ve discussed before, the ADCP measures current velocity in two orthogonal directions, North-South (the V-velocities) and East-West (the U-velocities).  Taking these two directional velocities together gives a picture of the overall trajectory of the current.  Remember that the internal tide is driven by the semi-diurnal tidal cycle and the tides move in a back and forth direction (like when the tide comes in and then goes back out on a beach). The semi-diurnal tide in the Tasman Sea should move bi-directionally along a NW – SE line.

 

This contour plot shows the amplitude of the current velocity with depth along a 200km transect (100km on either side of mooring A1 at 0).  The blue astrix indicates the location north (negative numbers) of the mooring buoy that shows unusually high velocities that are likely associated with the internal tide. SOI/Amy Waterhouse, Sam Kelly and Luc Rainville

A sign of the sine wave

In addition, the velocities of the semi-diurnal tide over time will fit on a sine curve.  What this shows is the velocity speeds up and slows down at different times in the cycle (for example, when the tide is all the way in, the current speed approaches zero).  The velocity also changes direction (positive and negative) depending on whether the tide is coming or going.  Here’s where the preliminary data provide some promising results.  When the science team plotted all of the ADCP velocity data onto a graph, the velocities over time fit onto a sine curve with the correct period – a good indication that the data are representing the expected tidal cycle.  Also exciting is that the relationship between the U velocities (N-S) and the V velocities (E-W) indicates that the current is moving in the same directional angle as the T-Beam.  So they are pretty sure that they’ve found the internal tide beam!!  Previously the location of the internal tide beam was only predicted by a (very sophisticated) computer model.  This is what scientists call ground-truthing a model.