Coupling of internal waves on the main thermocline to the diurnal surface layer and sea surface temperature during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment

A new theory of shear instability in a turbulent environment is applied to eight days of velocity and density profiles from the upper-equatorial Pacific. This period featured a regular diurnal cycle of surface forcing, together with a clear response in upper-ocean mixing. During the day, a layer of stable stratification and shear forms at the surface. During late afternoon and evening, this stratified shear layer descends, leaving the nocturnal mixing layer above it. Using high-resolution current measurements, the detailed structure of the descending shear layer is seen for the first time. Linear stability analysis is conducted using a new method that accounts for the effects of preexisting turbulence on instability growth. Shear instability follows a diurnal cycle linked to the afternoon descent of the surface shear layer. This cycle is revealed only when the effect of turbulence is accounted for in the stability analysis. The cycle of instability leads the diurnal mixing cycle, typically by 2-3 h, consistent with the time needed for instabilities to grow and break. Late at night, the resulting turbulence suppresses further instabilities, lending an asymmetry to the mixing cycle that has not been noticed in previous measurements. Deep cycle mixing is triggered by instabilities formed as the descending shear layer merges with the marginally unstable shear of the Equatorial Undercurrent. In the morning, turbulence decays and the upper ocean restratifies. Wind accelerates the near-surface flow to form a new unstable shear layer, and the cycle begins again.

“…Our shallower instabilities (e.g., Fig. 9d) could account for the modes observed by Walsh et al (1998) and Farrar et al (2007) in connection with modulations of sea surface temperature.…”

Section: Discussionmentioning

confidence: 98%

Diurnal Shear Instability, the Descent of the Surface Shear Layer, and the Deep Cycle of Equatorial Turbulence

Smyth

Thorpe

2013

“…In section 4a we present measurements of the thermal boundary layer for free convection, before NOVEMBER 2009moving on to consider the effects of an imposed upwelling flow in section 4b.…”

Section: Resultsmentioning

confidence: 99%

“…The velocity signals could modify the temperature profiles by acting directly within the skin layer, the thin diffusive thermal boundary layer of approximately 1-mm depth immediately below the ocean surface. Alternatively, velocity signals may modify the skin temperature by indirect mechanisms, such as modulating the level of subsurface turbulence (Walsh et al 1998;Farrar et al 2007), modifying local surfactant concentration (Marmorino et al 2008), or modifying surface wave properties to generate an additional flow. We focus on the direct influence of a velocity signal within the skin layer.…”

Section: Introductionmentioning

confidence: 99%

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Variations in Ocean Surface Temperature due to Near-Surface Flow: Straining the Cool Skin Layer

Wells

Cenedese

Farrar

et al. 2009

The aqueous thermal boundary layer near to the ocean surface, or skin layer, has thickness O(1 mm) and plays an important role in controlling the exchange of heat between the atmosphere and the ocean. Theoretical arguments and experimental measurements are used to investigate the dynamics of the skin layer under the influence of an upwelling flow, which is imposed in addition to free convection below a cooled water surface. Previous theories of straining flow in the skin layer are considered and a simple extension of a surface straining model is posed to describe the combination of turbulence and an upwelling flow. An additional theory is also proposed, conceptually based on the buoyancy-driven instability of a laminar straining flow cooled from above. In all three theories considered two distinct regimes are observed for different values of the Pé clet number, which characterizes the ratio of advection to diffusion within the skin layer. For large Pé clet numbers, the upwelling flow dominates and increases the free surface temperature, or skin temperature, to follow the scaling expected for a laminar straining flow. For small Pé clet numbers, it is shown that any flow that is steady or varies over long time scales produces only a small change in skin temperature by direct straining of the skin layer. Experimental measurements demonstrate that a strong upwelling flow increases the skin temperature and suggest that the mean change in skin temperature with Pé clet number is consistent with the theoretical trends for large Pé clet number flow. However, all of the models considered consistently underpredict the measured skin temperature, both with and without an upwelling flow, possibly a result of surfactant effects not included in the models.

“…Coherent, wavelike bands of higher and lower temperature have been observed in infrared observations of the surface of the ocean while a diurnal warm layer was present (e.g., Walsh et al 1998). Farrar et al (2007) linked such SST structures to internal waves on the diurnal pycnocline, but as they point out, the mechanism is more complex than simple advection of vertical temperature structure; according to classical internal wave theory, fluid particles on the surface remain on the surface.…”

Section: Internal Waves On the Diurnal Pycnoclinementioning

confidence: 99%

AUV Observations of the Diurnal Surface Layer in the North Atlantic Salinity Maximum

Hodges

Fratantoni

2014

Autonomous underwater vehicle (AUV) surveys of temperature, salinity, and velocity in the upper 10 m of the ocean were carried out in low-wind conditions near the North Atlantic surface salinity maximum as part of the Salinity Processes in the Upper Ocean Regional Study (SPURS) project. Starting from a well-mixed state, the development, deepening, and decay of a warm salty diurnal surface layer was observed at ,1-h resolution. The evaporation rate deduced from the freshwater anomaly of the layer corroborates measurements at a nearby flux mooring. Profiles within a few hundred meters of the stationary research vessel showed evidence of mixing, highlighting the effectiveness of AUVs for collecting uncontaminated time series of nearsurface thermohaline structure. A two-dimensional horizontal subsurface survey within the diurnal warm layer revealed coherent warm and cool bands, which are interpreted as internal waves on the diurnal thermocline.