How Wind and Stratification Control Upper

How Wind and Stratification Control Upper-Ocean Mixing
A. Natarov1, K. Richards1, Y. Kashino2
Top: Southern oscillation index. Negative
(positive) values correspond to El Niño (La Niña)
events. Bottom: Observed stratification in the
western equatorial Pacific during SURTROPAC.
Depth profiles of the horizontally averaged
vertical mixing coefficient from the four
numerical experiments.
Latitudinal structure of the vertical mixing
coefficient averaged between 150 and 300 meter
depths from the four numerical experiments.
ENSO modulates upper ocean stratification in the Western Equatorial Pacific (left panel). Recent microstructure
measurements carried out by IPRC and JAMSTEC scientists indicate that ENSO may also control mixing.
Specifically, they discovered that the 2010 La Niña was accompanied by much higher mixing levels than the
2009 El Niño even though the wind bursts were more intense during the El Niño year. To further investigate the
dependence of mixing on wind intensity and stratification, the scientists used high resolution Regional Ocean
Modeling System (ROMS) to conduct four numerical experiments: {weak (w), strong (s)} x {wind stress (τ),
stratification (N)} = {wτ&sN, …}. They found that the level of mixing is controlled by stratification in the
thermocline and by the wind intensity above the thermocline (middle panel). Moreover, the meridional extent
of the region of enhanced mixing depends on both wind intensity and stratification (right panel). These
findings will help to understand and quantify processes that generate mixing in the upper Western Equatorial
Pacific ocean and so to improve ocean and climate models.