Upper Ocean Fronts (TS Relationship in the Upper Ocean)

The ocean surface is filled with a convoluted web of fronts that separate waters of different temperatures and salinities. Just as alveoli facilitate rapid exchange of gases in the lungs, fronts are the ducts through which heat, carbon, oxygen and other climatically important tracers enter into the deep ocean. Fronts can be as narrow as hundreds of meters and as wide as tens of kilometers. In our group we develop theories to understand the role of fronts on Earth’s climate and we participate in field campaigns. We are also collaborating with NASA to develop a new altimeter to observe fronts from space, the Surface Water Ocean Topography mission.

The density of seawater is determined by two properties: temperature and salinity. Thus temperature and salinity may vary along a surface of constant density. This variability, sometimes called spiciness, is a result of air-sea fluxes, turbulent mixing, and advection. The goal of this exploratory experiment is to observe spiciness in the upper ocean, including the mixed layer, at horizontal scales of 10 m to 1000 km.

Specific objectives of the Spice Experiment are: (1) to quantify the density ratio in the mixed layer and seasonal thermocline, (2) to find if salinity varies on shorter horizontal scales than does temperature, (3) to confirm that there is more spiciness in the mixed layer than in the seasonal thermocline, and (4) to find where and over what length scales the reduction in spiciness occurs. These objectives are being addressed using data from a cruise in the eastern North Pacific between 25°N and 35°N conducted 24 January - 20 February 1997. Measurements were made using a SeaSoar equipped with a CTD and fluorometer. The SeaSoar was towed in three patterns: (1) sawtooth patterns between 5 and 320 dbar to resolve horizontal length scales down to 3 km, (2) level tows at 50 dbar in the mixed layer and 200 dbar in the seasonal thermocline, and (3) tows following an isopycnal that went from beneath the mixed layer to outcropping (24.8 kg/m³), and following an isopycnal in the seasonal thermocline (25.5 kg/m³). Our newly designed control system worked better than anticipated and the SeaSoar followed an isobar to within a root-mean-square deviation of 0.3 dbar, and an isopycnal to within 0.02 kg/m³. The level and isopycnal tows resolved horizontal scales down to 4 m.

The first objective of the Spice Experiment is to quantify the horizontal density ratio in the mixed layer. The density ratio is defined as the ratio of the effect of a temperature change on density divided by the effect of a salinity change. The isobaric tow at 50 dbar in the mixed layer is ideal to address this first objective. Somewhat surprisingly, temperature and salinity nearly compensate in their effect on density at all scales observed from 1000 km to 10 m. In other words, the density ratio is rougly one on all horizontal scales. The implication is that horizontal mixing in the ocean works to eliminate horizontal density, but not spiciness, gradients. A parameterization of diffusivity in terms of the density gradient is suggested.