Collaborative Research: Marginal instability and deep cycle turbulence in the equatorial oceans

Oregon State University, Corvallis OR

Investigators

Abstract

Overview: Thirty years after its discovery, the deep cycle of equatorial turbulence remains unexplained. Climate forecast models are unable to represent it accurately, leading to errors in ocean heat uptake. This project brings together two research groups who have made recent progress on the problem to pursue a unified understanding. Combining existing observations and new large-eddy simulations (LES), they will explore deep cycle physics over the range of seasonal and ENSO variability and in all three equatorial oceans. The previously-unexploited deep cycle property of marginal instability will be used together with numerical simulations to (1) document the history of the deep cycle over the past 25 years, (2) describe the mechanics of the deep cycle, and (3) develop improved parameterizations for use in climate models. Intellectual merit: The broad variability of mixing in the upper equatorial Pacific is exemplified by the turbulent diapycnal heat flux sampled at the same location and in the same season, but coinciding with different phases of the El Niño/ Southern Oscillation (ENSO) cycle. Early in the 1991 El Niño, currents were slow and the heat flux was relatively weak. In the 2008 cruise, which coincided with La Niña, currents were much more energetic. The resulting heat flux was stronger by an order of magnitude, and heat was transported to 100 m depth. Despite the extreme difference in magnitude, these mixing regimes exhibited a striking commonality: strong turbulence coincided with a distinct layer in which the gradient Richardson number (Ri) remained within a factor of two of the value of a quarter. Persistent clustering of Ri near 1/4 over a range of depths signifies the state of marginal instability. This near-critical state is maintained in the mean by a balance between large-scale forcing (which reduces Ri) and turbulence (which increases it). Deep cycle mixing is crucial to the climate via its effects on both the zonal current system and the sea surface temperature. In this observational data analyses and large eddy simulations, the investigators will explore the physics of the deep cycle with particular attention to the property of marginal instability. Using marginal instability as a proxy, they will document the history and longitudinal dependence of the deep cycle from existing data. This will lead to both a deeper understanding of the phenomenon and an improved ability to parameterize its effects in large-scale models. Broader Impacts: The causes of long-term Sea Surface Temperature variations will be examined and the results will contribute to ENSO and interdecadal climate prediction. The project will also foster a new collaboration, support a junior researcher, and complement an existing NSF observational project. Two presentations on equatorial oceanography will be developed and presented at high schools and community colleges.