Upwelling Velocity Research Articles

The Role of Wind-Generated Mixing in Coastal Upwelling

Abstract A simple parameterization of mixing processes originally developed by Kraus and Turner (1967) is included in a two-dimensional, two-layer theory of wind-driven coastal upwelling. Mixed-layer deepening is a competition between entrainment due to wind stirring, stabilization due to surface beating, and upwelling vertical velocity. For longshore winds favorable to upwelling, the temperature of the surface water mass being pushed offshore by the wind is determined by that of the incoming subsurface water mass, by the surface heating, and by the mixing dynamics. An upwelling steady state is possible with surface heating. In this state, the vertical upwelling velocity is exactly matched by the entrainment velocity, and the resulting cold water flux into the mixed layer is balanced by surface heating; surface temperature increases linearly with distance offshore. Time-dependent numerical solutions exhibit a, tendency toward this steady-state solution. Winds favorable to downwelling cause detrainment of ...

Turnover of Eastern Caribbean deep water from 14C measurements

A 14C balance for the Eastern Caribbean deep water indicates the average inflow of Atlantic water into the basin to be 2.3 × 10 5 m 3/sec (±30%), or about 2–4 times the values estimated previously. The balance uses a model representation of the deep-water turnover, and is based on 14C concentrations at a station in the Venezuelan Basin which average Δ 14C= 89‰ below 800 m depth with a total range of only 9‰, as well as on a 14C concentration of the Atlantic inflow of Δ 14C= −71% . as obtained from measurements outside the Antilles Arch. The turnover time of the basin water below 2500 m depth is 55 years, which corresponds to an average upwelling velocity at this depth of about 35 m/year. With such upwelling, the temperature profile below 1800 m (the depth of the sill determining the inflow of new water) requires a vertical eddy diffusivity of about 5 cm 2/sec. The oxygen consumption, and silica and CO 2 regeneration, rates below 2500 m depth are obtained as −0.18, + 0.08, and + 0.2 μmole kg −1 yr −1, respectively. The CO 2 regeneration has but a negligible effect on the 14C balance.

The use of oxygen as a test for an abyssal circulation model

Stommel's abyssal circulation velocities are used in the advective-diffusive-decay equation to calculate the dissolved oxygen distribution in an idealized world ocean basin. The use of a weighted difference scheme allows an extension of results obtained earlier to a broader range of values of the parameters for advection, decay and eddy diffusion. Extreme cases of no diffusion and of no advection are used to interpret the results for intermediate values of the parameters. The optimal set of values found for the parameters are: horizontal eddy diffusion = 6 × 10 6 cm 2 sec −1, uniform upwelling velocity = 1.5 × 10 −5 cm sec −1, oxygen consumption rate in abyssal waters = 0.002 ml 1. −1 yr −1, recirculation around the Antarctic Circumpolar Current = 35 × 10 6 m 3 sec −1. The optimal pattern is then compared with (an updated) observed oxygen distribution to determine the merits and deficiencies of the circulation model. The major deficiency appears to be the neglect of topographic effects on the abyssal circulation.

Radium-226 in the eastern equatorial Pacific

226 Ra profiles have been measured at four stations in the vicinity of the East Pacific Rise, one station in the NE Pacific, and one station in the NW Atlantic. We confirm the large increase in 226 Ra in North Pacific Deep Water vs. North Atlantic Deep Water, measured by Broecker, Li, and Cromwell [9], but we find a factor of 3.3 vs. their factor of 2. The vertical diffusion-advection model is applied to the Pacific deep water profiles to evaluate the magnitude of in-situ production of 226 Ra from dissolution of particulate flux — an effect which cannot be determined from ☐-models. The in-situ production rate J and the parametric upwelling velocity w are linearly correlated so that a range of J-w pairs is obtained, all of which yield essentially identical profiles for both positive and negative values of J . However, variance mapping in J-w space gives an upper limit for J , a lower limit for w , and a critical velocity for which J = 0. Thus if the constraint J ≥ 0 is assumed, the Pacific profiles indicate maximum J values of 1.5 × 10 −17 (g/kg)yr, with upwelling velocities limited to values between 1 and 12 m/yr. Three of the profiles can be fit with J = 0, w = −4 m/yr, while two profiles require large upwelling velocities (12–14 m/yr) for J = 0. We conclude that there may indeed be a small in-situ production of 226 Ra in the deep Pacific (replacement residence time > 4000 yr), but if so, it is probably confined to specific areas of the ocean. In contrast, two recently published Pacific barium profiles show no evidence of in-situ production in the water column.

210Pb [sbnd]226Ra: Radioactive disequilibrium in the deep sea

210Pb and 226Ra profiles have been measured in the North Atlantic and North Pacific Deep Water. The 210Pb activity is 25% to 80% of that of 226Ra, averages about 50% in each profile, and is lowest in the bottom water. This deficiency of 210Pb relative to 226Ra shows that 210Pb is rapidly and continually scavenged from deep water, probably by adsorption on particulate material sinking from the surface. A method is developed for simultaneous application of the vertical diffusion-advection model to 226Ra and 210Pb deep-water profiles, and the in-situ source terms for both isotopes are obtained as a function of the parametric upwelling velocity. For a positive source term for 226Ra in deep water, the parametric velocity is limited to a small range (3–12 m/y) and the deep-Pacific residence time of lead is 54 yr, a value two orders of magnitude smaller than residence times estimated from stable lead concentrations. The model calculations show that an in-situ scavenging process, acting throughout the water column, is required to remove the Pb. Box model calculations yield a similar short residence time for lead in North Atlantic Deep Water, indicating that radioactive disequilibrium for 210Pb, due to fast scavenging, is a general phenomenon through the deep sea.

On oceanic boundary mixing

The basic stratification and the sloping boundaries of the ocean induce a mean upwelling velocity at the walls in order to satisfy the no flux condition. In the oceanographically most interesting case, the effect is confined to a boundary of thickness 0((vk) 1 2 /N 1 2 where v, k are the austausch coefficients for momentum and heat respectively, and N is the local Brunt-Väisälä frequency. The effect of strong rotation is to induce an accompanying thermal wind in the interior of the ocean. If boundary mixing processes are sufficiently strong, then this upwelling process might account for most of the vertical velocity needed in the abyssal circulation. This is in accord with earlier suggestions that the boundaries play the essential role in the vertical exchange of oceanic properties.

A contribution to the theory of upwelling and coastal currents

A theory of upwelling and coastal currents induced by longshore and offshore winds is derived, taking the effects of Coriolis forces and horizontal and vertical mixing into account. In the first place, it is shown that an upwelling circulation can be caused in the vertical plane perpendicular to the coast by a wind blowing parallel to the coast in such a manner that, in the northern (southern) hemisphere, the coast is on the left‐hand (right‐hand) side of an observer with his back to the wind. The result agrees with the facts observed in the regions off the Southern Californian and Peruvian coasts very satisfactorily. The velocity of upwelling is computed at about 80 m/month, a value consistent with that estimated from observed hydrographic data. Next, it is shown that an offshore wind blowing perpendicular to the coast can also give rise to a similar circulation in the same vertical plane, but in this case with much weaker intensity. From these results it can be concluded that the most intense upwelling will occur when the wind makes an angle of 21°.5 with coast line in an offshore direction. In addition to these circulations in the vertical plane, there exist horizontal currents parallel to the coast. These will contribute to the formation of the coastal or longshore currents much debated recently. The horizontal components of the motion under consideration show a vertical variation very similar to the Ekman spiral at some distance from the coast.