The effect of current-induced stress perturbation in a coupled atmosphere-ocean model

In the presence of surface currents, a stress perturbation at the air-sea interface is induced by the surface currents. It is directly correlated to the surface current pattern, and thus causes a feed-back effect in the sea-air system. This effect is studied for an equatorial sea-air system of the geometry of the equatorial Pacific Ocean by using a simple coupled atmosphere-ocean model. It is shown that the effect of the current-induced stress perturbation appears as a damping mechanism for the ocean circulation in general. However, in the presence of a zonally-asymmetric surface wind field, the stress perturbation associated with the zonal-mean current creates an opposing perturbation current. As a result, in the presence of a westward zonal-mean current, the eastward component of the perturbation current is enhanced by the current-induced stress perturbation. In a coupled sea-air system, both the damping and the forcing mechanisms associated with the current-induced stress perturbation can significantly influence the coupling strength, and hence influence the behavior of the coupled modes

Abyssal upwelling in the Indian Ocean: Radiocarbon diagnostics

The GEOSECS Indian Ocean radiocarbon and carbonate chemistry data set are used to estimate the mean upwelling transport of bottom water in the Indian Ocean north of 30S. The study uses an adjusted radiocarbon concentration which is corrected for the effects of addition of particulate radiocarbon to the deep ocean. The cross-basin uniformity in the vertical gradients of adjusted radiocarbon allows quantification of vertical transfer processes using horizontally averaged concentration and fluxes. The estimated total upwelling flux, north of 30S, is 8.2 ± 1.5 × 106 m3 s-1. The mean upwelling velocity and the vertical diffusivity, in the 3000-4500 m depth range, are estimated as 3 × 106 m s-1 and 2.5 × 10-4 m2 s-1, respectively. The results also suggest faster upwelling in the western Indian Ocean

Development and Evaluation of a Global Version of the Miami Isopycnic-Coordinate Ocean Model. Final report

The objective of this project was to test the ability of the Miami Isopycnic-Coordinate Ocean Model (MICOM) to simulate the global ocean circulation, setting the stage for the model's incorporation into coupled global climate models. An existing basin-scale model will be expanded to global domain; suitable atmospheric forcing fields, including precipitation and river runoff, will be selected; the modeling of ayssal flow will be improved by incorporating compressibility and particularly thermobaric effects; a sea-ice model will be added; parameterization options will be explored for subgrid-scale deep convection; parallel coarse- and fine-mesh simulations will be carried out to investigate the impact of grid resolution; the sensitivity of the model's solution to magnitude of vertical (diapycnal) exchange coefficient will be studied; and long-term trends in meridional heat transport and water-mass properties in model solutions will be documented and interpreted

Shear observations in the deep thermocline

Photogrammetric evaluation of the rate of distortion of vertical dye streaks within the main oceanic thermocline indicates that spiraling perturbation structures overlay the large-scale vertical shear. The data show pronounced shear structure with vertical scales of a meter or less at depths of hundreds of meters. The shapes as well as strengths of the observed shear structure have a strong dependence on depth. At 400 m the major variations in shear had vertical length scales of about 10 to 20 m; the velocity amplites were of the order of 1 cm s−1, and the predominant spiral pitch was consistent with downward propagation of energy by internal gravity waves. Shallower observations suggest a lateral intrusion as a predominant shear source

Overflow into the deep Caribbean: Effects of plume variability

The deepest connection between the eastern Caribbean and the Atlantic is over the Jungfern‐Grappler Sill complex at 1815 m depth. Through these gaps flows the sole source of water for the deep Caribbean, presumably balanced by diffusively driven upwelling elsewhere in the basin. Fourteen‐month‐long moored observations at the sills in 1991–1992 reveal a mean transport of 0.11±0.05 Sv (1 Sv ≡ 106 m3 s−1) of Atlantic water colder than θ = 3.965°C flowing into the Caribbean, one quarter of which comes over the previously unmeasured Grappler Sill. This is about twice the transport seen in previous, shorter experiments. The overflow is highly episodic, with ∼10 “events” per year. The range of overflow density is comparable to the total vertical stratification of the deep Caribbean. A numerical stream‐tube model of the overflow plume is run with 13 different initial conditions, representing the observed range of overflow strength and density. Results indicate that much of the overflow only penetrates to 2200–3000 m depth and was dense enough to descend to the bottom of the Caribbean only ∼1% of the time. Combined plume model results are used to drive a model of the basin stratification. A steady balance is found to be possible with small diapycnal diffusivity in the basin, between 0.1 and 0.6 cm2 s−1. The reason for the small diffusivity is that the stratification is essentially built in place by the different detrainment depths, and so less diffusive transformation is needed