Inflow of bottom water to the Panama Basin

Detailed bathymetric surveying, including use of a deeply-towed transponder-navigated instrument package, shows that the deepest passages into the Panama Basin are along the Ecuador Trench (sill depth 2920m, at 0°21′S) and across the broad central saddle of the Carnegie Ridge between 85 and 86°W (sill depths 2300 to 2330m). The water structure in the trench passage at and up to 7 km beyond the sill was explored for several days with a thermometer attached to the deep-tow instrument, and near-bottom currents there were measured. A well-defined 230- to 300m-thick bottom layer of near-adiabatic Inflow Water (scarcely-modified Peru Basin Water with a potential temperature at the sill of 1.55°C and a salinity of 34.678%) fills a narrow channel only 4.5 to 2.5 km wide. The average inflow velocity was 33.2 cm s−1. Above the Inflow Water near the sill is a steep benthic thermocline with an average gradient of 0.3°C(100m)−1 and measured current velocities of 21.0 and 16.5 cm s−1. The overlying Deep Water is mostly well-stratified except near its base, where it has slight vertical gradients and where a meter at 2365 m measured a slow current that reversed from outflow to inflow during the period of record. A much shallower array of current meters in the trench, high on the flanks of Carnegie Ridge at 1300 to 1400m, measured a slow (< 6 cm s−1) outflow. The average rate inflow along the trenchm estimated from the current meter records, is 0.35 × 106m3s−1. This is sufficient to fill the Panama Basin, below the level of the broad central Carnegie Ridge sill, in less than 50 years. Both velocity and thickness of the inflow change over periods of several days, at semi-diurnal and higher frequencies. The observed characteristics of inflow along the trench, including its velocity and discharge, can be approximated by a model of channel flow over a broad-crested weir (the sill at 0°21′S) and down a spillway. Most of the modification of water moving along the trench passage, which was examined with scattered hydrocasts as well as deep-tow observations, occurs at and immediately downstresm from the sills that interrupt the passage. At these sites rapid changes in flow regime are predictable from the flow dynamics, and intense boundary layer turbulence is engendered by fast currents over rough sea floor. The benthic thermocline (benthic front) forms locally near sills by vertical mixing within the Inflow and Deep Waters, and is broken down my mixing between these two water masses. The thermocline and fast currents in the Inflow Water may be restricted to fairly short reaches of the passage. Hydrocasts and current meter measurements at the central saddle of the Carnegie Ridge indicate that the inflow which crosses this broad sill is seldom dense enough to cascade down to the basin floor. Instead of filling the basin, the slow, unchannelled drift defines the upper limit of distinctive basin water, at about its sill depth. Although there is strong geomorphologic evidence for occasional spillover of water dense enough to cascade down the north flank of the ridge in fast, channelized currents that incise valleys and move sand dunes, there are no hydrographic data that determine the frequency or cause of these events.