Dr. Kelvin Richards, the Principal Investigator on the Mixing Up The Tropical Pacific Cruise beams with excitement at the prospect of collecting new data. For him, this cruise is a significant step forward, or east, to obtain long awaited information. After working in the western equatorial Pacific for many years, Dr. Richards is curious to see if the same physics he is witnessing there extend into the central Pacific. The namesake of Lord Kelvin, it is no coincidence that small scale ocean mixing is his passion.
Lord Kelvin and Hermann von Helmholtz made significant contributions to fluid dynamics. They found that a velocity difference (or shear) at different heights in a fluid flow can induce the flow to become unstable, while if the density of the fluid decreases with height (i.e. light fluid overlays heavy fluid) there is a tendency to damp any disturbance. The battle between these two forces (shear and buoyancy) decides if the flow will go unstable. The mechanism is known as Kelvin-Helmholtz instability which when active forms beautiful “billows”. Under the right conditions these billows are seen in cloud formations in the sky. The billows undergo further instabilities leading to turbulence and the mixing of properties such as heat, water vapor (in the atmosphere) and salt (in the ocean).
Another significant player was Lewis Fry Richardson. In looking at measurements of the wind at two different heights above the ground he noted the fluctuations in wind increased as the difference in wind speed at the two heights increased. On the other hand, when the temperature of the air closer to the ground became colder than that above, the fluctuations decreased. These observations inspired him to consider the energetics of flow and to encapsulate the physics in a number, now known as the Richardson number. The Richardson number essentially is the ratio of the vertical density gradient, squared, to the shear, squared. Later theory showed that a necessary condition for instability is that the Richardson number is less than a quarter. The science party on Falkor will use this concept to analyze the data being collected on the cruise in order to advance our understanding of turbulent mixing.
Ocean models are not yet powerful enough to resolve small-scale mixing processes on a global scale. By collecting oceanographic data on turbulence activity and relating this to the shear and stratification, scientists can better understand what is producing the turbulence. This in turn will lead to developing models that are better in representing the turbulence and its effect on the large-scale ocean. The mixing of heat and salt associated with the turbulence is known to have a large impact on the density structure of the ocean. This affects the way the sea surface temperature changes and thus the way the ocean interacts with the atmosphere across the tropical Pacific.
Dr. Kelvin Richards is interested in expanding his previous research that focused on the western equatorial Pacific, operating off ships from Japan, Korea, and the US. The present Falkor cruise allows him to take measurements further east towards the central equatorial Pacific. In the west he has found that the shear generating the observed turbulence is associated with relatively small vertical scale flow features that themselves are produced by a combination of fluctuating winds over the ocean and instabilities of the major ocean currents. Now the questions is, are the same flow features found in the central Pacific and do they dominate the mixing?
Falkor is now in the central Pacific, and measurements have started. Initial results show the presence of strong small-scale flow features with large shear. The science party eagerly waits to see how persistent these features are and the degree to which they dominate the mixing. Only time will tell, and so, we journey on in our hunt for small scale ocean mixing.