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 experiment was to observe spiciness in the upper ocean, including the mixed layer, at horizontal scales of 10 m to 1000 km.

An objective was 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. Measurements were made by towing a SeaSoar equipped with a CTD and fluorometer. An isobaric tow at 50 dbar in the mixed layer was ideal to address this objective (Figure 1). Somewhat surprisingly, temperature and salinity nearly compensate in their effect on density at all scales observed from 1000 km to 10 m (Figure 2). In other words, the density ratio is roughly 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.

Figure 2. Potential temperature and salinity at 50 dbar. The top panel shows temperature and salinity over the entire tow from 25°N to 35°N. Each succeeding panel is a magnification by a factor of ten of the shaded region in the panel above. Note the change in scale on the horizontal axis of each panel; the bottom panel spans 0.01° in latitude (roughly 1 km). The vertical axes are scaled by the thermal and haline expansion coefficients so that equal excursions of temperature and salinity imply identical effects on density. Note that nearly every feature in temperature is mirrored by one in salinity at all scales. Temperature and salinity structure compensate to produce very small density gradients.

Figure 1. A section of potential density along 140°W in the North Pacific. Note the mixed layer of roughly 100-150 m depth. The white line is the path of a SeaSoar towed at 50 dbar. Data along this line is shown in Figure 2.

Acknowledgements. This work was supported by the National Science Foundation.