In this case it seems ocean heat content is a depth integrated measure (see page 11 of this document I found by following links in the above article https://upper-ocean-dynamics.earth.miami.edu/_assets/pdf/atb...). Seems like that OHC measure uses some reanalysis model but surface temperatures can be measured by satellite, and temperatures below can be measured from moorings in the Gulf (https://www.ndbc.noaa.gov/obs.shtml) and argo floats currently in the region (https://argo.ucsd.edu/about/status/). I would expect them to all show similar trends.