Operational Oceanography Comparison of Sea Surface Temperature Measurements and Relation to Mixed Layer Depth Operational Oceanography OC/MR 3570 Sam Poteete LCDR USN

RV New Horizon Summer Cruise 24-28 July 2009

Summer 2009 Cruise Plan

Objectives Make observations concerning the relationship between Sea Surface Temperature (SST) and Mixed Layer Depth (MLD) Compare multiple methods for obtaining SST The purpose of this study is to compare observed in situ Sea Surface Temperatures (SST) measured by XBT drops and CTD casts with SST measured by the Hull Mounted IR Gun and Boom and how this data is used to obtain Mixed Layer Depth (MLD).

Who cares? Any specialist dealing with the upper ocean will need to consider the surface mixed-layer, physical oceanographers, marine biologists, climatologists and WARFIGHTERS! That’s us. In a tactical way, this oceanic feature is very critical in determining Sonic Layer Depth to use in Tactical Decision making.

Ocean Dynamics are Complicated

Sea Surface Temp Mixed Layer Depth Coupled atmosphere-ocean model integration is used to study sea surface temperature (SST) and mixed layer depth (h), and the processes which influence them. The model consists of an atmospheric general circulation model coupled to an ocean mixed layer model in ice-free regions.

Sonic Layer Depth

Turbulent Heat Exchange at the Surface Surface heat exchange Q is made up of longwave and shortwave radiative fluxes plus sensible and latent turbulent heat fluxes. Net surface heat flux strongly regulates both the mean (-) and the anomalous (.) SSTs throughout the year.

More on SST Measurements routinely made from ships are often from the engine water intakes and may be at various depths in the upper 20 m of the ocean. The earliest technique for measuring SST was dipping a thermometer into a bucket of water that was manually drawn from the sea surface.

Mixed Layer Depth The mixed-layer is the oceanic surface zone that responds the most quickly and directly to atmospheric fluxes. It is through the mixed-layer that such influence is transmitted to the whole ocean in the long term. Conversely, the mixed-layer is the part of the ocean through which the ocean influences directly the atmosphere

Mixed Layer Dynamics Ocean Surface Density Changes Surface Cooling Positive Heat Flux (Atmosphere) More Dense above Less Dense Negative Buoyancy Flux Surface Heating Negative Heat Flux (Atmosphere) Less Dense above More Dense Positive Buoyancy Flux

CTD data

XBT data

CTD & XBT data

CTD & XBT data

Radiometer & Boom data vs Time

Conclusions Both CTD &XBT methods of calculating MLD with SST perform well independently. Qualitatively the relationships are clearly evident in the observations When compared to each other, both statistically and observationally they are equally accurate. The relationship between SST and the MLD is useful for the Warfighter in all levels of TDA.

Future A new method is introduced for determining ocean isothermal layer depth (ILD) from temperature profiles and ocean mixed layer depth (MLD) from density profiles that can be applied in all regions of the world's oceans. This method can accommodate not only in situ data but also climatological data sets that typically have much lower vertical resolution. The sensitivity of the ILD and MLD to the temperature difference criteria used in the surface layer depth definition is discussed by using temperature and density data.

Questions? Have a Great Navy Day!