Southern Ocean Mooring: understanding VELPTMN

Dear OOI community,

I was having some trouble understanding the VELPTMN (mean point water velocity) data product from the Southern Ocean Apex surface mooring for deployment 2 and 3.

First, I want to confirm with you that in both deployments the single point velocity meter was implemented at depth of approximately 13m as indicated by the pressure measurement. If I didn’t interpret the depth wrong, then I noticed that the velocity measurements for deployment 2 (year 2016) are almost an order of magnitude larger than that for deployment 3 (year 2017 and 2018).


I doubled checked SST and air temperature data from the Bulk Meteorology Instrument package but they seemed much more continuous than the velocities. I wonder are there some instrument changes/calibrations or data scalings that I wasn’t aware of or should I interpret the results physically?

I further noticed that the VELPTMN data product I acquired from the single velocity meter has frequency of 15 minutes whereas the same VELPTMN data from the Bulk Meteorology Instrument package has frequency of 1 minute. I wonder whether the 1-minute data is from linear interpolation?

Any help or comments are greatly appreciated!

Best,
Luna

Luna,

Thank you for bringing this to our attention!

To begin with, there is only one VELPT (Nortek Aquadopp) instrument deployed on the Surface Mooring, and it is on the Near Surface Instrument Frame (NSIF) at ~12 m water depth. That is the same for all deployments.

Also, the Bulk Meteorology (METBK) data products incorporate the VELPT data in the calculations. The METBK samples at a frequency of 1 min, but the VELPT samples once every 15 minutes, so the VELPT data is interpolated to match the METBK data.

As for the change in magnitude of the velocity data from Deployment 2 to Deployment 3, we’ll have to dig into that a bit further and get back to you.

Thanks and stay safe,
Sheri
OOI/CGSN Data Team

Dear Sheri,

Thank you very much for the clarification! I would be very interested to know the change in the velocity magnitude.

All the best,
Luna