Our goal was to study “tropical instability waves”, 1000km-long waves that propagate along the equator, most commonly in La Nina conditions.  During an equatorial cruise in the Indian Ocean, we were fortunate to observe the passage of a Yanai wave packet, a close cousin of the tropical instability wave, in great detail. We were able to quantify fluxes of heat, mass and energy due to this wave.

A particular focus was the turbulence that the waves cause, which appears to play an important role in El Nino, the Madden-Julian oscillation, and other tropical phenomena that affect the global climate. Besides turbulence in tropical instability waves, the results taught us an important lesson about a much more common phenomenon: stratified shear flows.

Stratified shear flows are ubiquitous in the Earth’s atmosphere and oceans.  These are winds and currents in which both velocity and temperature change with height. Because of the velocity change (shear), the flow is likely to be turbulent. Because of the temperature change (stratification), that turbulence is likely to transport heat vertically across the flow. For example, sheared currents near the ocean surface drive turbulence that transports heat from the atmosphere into the deep ocean, potentially mitigating global warming.  

Based on the results of this project, we believe that turbulence in stratified shear flows can remain near a state we call “marginal instability”, in which energy input from the mean shear balances energy expended in transporting heat. Besides offering insight in the to the physics of turbulence, this finding gives us a new way to predict the rate of heat transport, a critical factor in ecosystem and climate forecasting.

In a marginally unstable flow, turbulence flickers on and off. The turbulent and non-turbulent states have totally different dynamics, and the interaction between those states is a mystery of long standing.  Graduate student Lin Li has completed the bulk of her Ph.D. in physics studying that mystery as part of this project.

Another long-standing mystery is (or was) the cause of the so-called “deep cycle” of equatorial turbulence, a pulsation in turbulence strength that mimics the 24-hour solar cycle even though it occurs too far below the ocean surface to be affected directly by the sun. It turns out that marginal instability is the key to understanding the deep cycle. The ocean at the equator is marginally unstable, most of the time, down to nearly 100 meters depth. As a result, any small perturbation can kick it into a turbulent state.

During the day, trade winds build up a current at the ocean surface. That current is trapped at the surface by heat from the sun, which warms the water so that it cannot sink. At sunset, the sun’s hold is released, and the current spreads down into the deeper ocean. Although this current is not especially strong, it is enough to kick the marginally-unstable flow into the turbulent state. As a result, the solar heat stored near the surface during the day is mixed down into the deep ocean.

The success of the marginal instability idea in explaining the deep cycle has encouraged us to explore the underlying physics more thoroughly, and to apply it to other examples of stratified shear flows in the oceans and atmosphere.

Last Modified: 12/08/2016
Modified by: William D Smyth