What insights can satellite data provide about the subsurface?

The advent of satellite oceanography in the last 30 years, which has made global measurements of altimetry, sea-surface temperature (SST), chlorophyll, salinity, and winds available on daily to weekly timescales, has revolutionized our understanding of ocean dynamics. However for the most part satellite observations only provide measurements at the surface of the ocean, whereas most oceanographers want data on the interior, subsurface structure of the ocean. What insights, if any, can satellite data provide about the subsurface? The “surface” measured by satellite data depends upon the frequency of the sensor making the measurement. For example, IR measurements of SST measure a depth of about 20 micrometers, while microwave SST radiometers measure a depth of a few millimeters, and visible ocean color sensors integrate over the entire photic zone, which can vary from less than a meter in turbid water to tens of meters in very clear water. These depth differences might seem negligible, given that they all fall within the average mixed layer depth, however diurnal heating and air-sea fluxes of heat can create a variable “surface” temperature structure which can exist even within a “mixed layer” defined by standard in-situ measurements. From altimetry data we can drive subsurface features such as topography, and in some instances thermocline depths. In this presentation we will review the measurement capabilities of the current oceanographic satellite sensors and discuss efforts to extrapolate satellite measurements down into the oceanic subsurface. For example, satellite polarization lidars, developed to detect cloud aerosols, have been used with some success to probe the sub-surface structure of water and detect bio-optical layers and sub-surface processes down to 50-100 m.