The intra-annual variability as a potential driver for the mean deep circulation in the tropical oceans

<p>The deep tropical ocean circulation is dominated by systems of vertically and meridionally alternating zonal jets, known as the Equatorial Deep Jets (EDJs) and Extra-Equatorial Jets (EEJs) respectively. The energy sources and physical mechanisms responsible for this circulation are still poorly understood. Recent studies have suggested the importance of intra-annual equatorial waves to transfer their energy to the EDJs.</p><p>In this study, we use idealized numerical simulations forced with a wave-like surface momentum flux to investigate how intra-annual variability can be relevant to the formation of the EEJs. It is shown that the amplitude of the jets, their meridional scales and their vertical and latitudinal extensions are sensitive to the period and wavelength of the forced wave. Short intra-annual waves with periods around ~70 days and wavelength ~300 km are found to reproduce the observed circulation most realistically. Focusing on the dominant barotropic mode, the underlying physical processes are detailed. A spectral analysis reveals that the energy transfer between the forced waves and the jet-structured circulation is compatible with a decay instability occurring in waves triadic interactions.</p><p>In parallel, a statistical analysis is performed on observations of the 1000m-velocities inferred from Lagrangian Argo floats drifts to document the amplitude and scales of the deep intra-annual variability in the tropical Pacific and Atlantic oceans. It gives evidence for the presence of short intra-annual waves that share common properties with the most unstable waves found for the EEJ generation.</p>

On Estimating the Surface Wind Stress over the Sea

Our study analyzes measurements primarily from two Floating Instrument Platform (FLIP) field programs and from the Air–Sea Interaction Tower (ASIT) site to examine the relationship between the wind and sea surface stress for contrasting conditions. The direct relationship of the surface momentum flux to U2 is found to be better posed than the relationship between and U, where U is the wind speed and is the friction velocity. Our datasets indicate that the stress magnitude often decreases significantly with height near the surface due to thin marine boundary layers and/or enhanced stress divergence close to the sea surface. Our study attempts to correct the surface stress estimated from traditional observational levels by using multiple observational levels near the surface and extrapolating to the surface. The effect of stability on the surface stress appears to be generally smaller than errors due to the stress divergence. Definite conclusions require more extensive measurements close to the sea surface.

Atmospheric Boundary Layer Turbulence Closure Scheme for Wind-Following Swell Conditions

Over the ocean, atmospheric boundary layer turbulence can be altered by underlying waves. Under swell conditions, the impact of waves on the atmosphere is more complicated compared to that under wind-wave conditions. Based on large-eddy simulation (LES), the wind-following swell impact on the atmospheric boundary layer is investigated through three terms: swell-induced surface momentum flux, the vertical profile of swell-induced momentum flux, and the swell impact on atmospheric mixing. The swell-induced surface momentum flux displays a decreasing trend with increasing atmospheric convection. The swell-induced momentum flux decays approximately exponentially with height. Compared with atmospheric convection, the decay coefficient is more sensitive to wave age. Atmospheric mixing is enhanced under swell conditions relative to a flat stationary surface. The swell impact on the atmospheric boundary layer is incorporated into a turbulence closure parameterization through the three terms. The modified turbulence closure parameterization is introduced into a single-column atmospheric model to simulate LES cases. Adding only the swell impact on the atmospheric mixing has a limited influence on wind profiles. Adding both the impact of swell on the atmospheric mixing and the profile of swell-induced momentum flux significantly improves the agreement between the 1D atmospheric simulation results and the LES results, to some extent simulating the wave-induced low-level wind jet. It is concluded that the swell impact should be included in atmospheric numerical models.

Flow Over Changing Terrain

The micrometeorologist setting out to find a field site that satisfies the requirements of horizontal homogeneity will soon be reminded that most of the earth’s surface is not flat and that most of the flat bits are inconveniently heterogeneous. This is what forced the location of early pioneering experiments to remote sites such as Kansas, Minnesota, or Hay (Chapter 1), where the elusive conditions could be realized. Vital as these experiments were to the development of our understanding, they are merely the point of departure for applications to arbitrary terrain. The components of arbitrariness are two: changes in the land surface and hills. In this chapter we discuss the first of these, flow over changing surface conditions; in Chapter 5 we look at flow over hills. In the real world, the two conditions often occur together — in farmland it is the hills too steep to plow that are left covered with trees — but we separate them here to clarify the explication of phenomena and because treating them in combination would exceed the state of the art. We simplify the problem of horizontal heterogeneity still further and discuss mainly single changes in surface conditions from one extensive uniform surface to another. Furthermore, the change will typically be at right angles to the wind direction so the resulting flow field is two-dimensional. Although multiple changes are now receiving theoretical attention (Belcher et al., 1990; Claussen, 1991), there exist as yet no experimental data for comparison. Two types of surface change may be distinguished at the outset: change in surface roughness, which produces a change in surface momentum flux with a direct effect upon the wind field, and change in the surface availability of some scalar. Those of most interest are the active scalars, heat and moisture. (These are called active because their fluxes and concentrations affect stability and thereby turbulent mixing and momentum transfer, as we saw in Chapters 1 and 3.) We shall discover significant differences in flow behavior according to whether the wind blows from a smooth to a rough surface or a rough to a smooth surface.