Observations and Modelling of Deep-Water Circulation in the Colombian Caribbean

Abstract

In the deep waters of the Colombian Caribbean in situ measurements of current speed and direction in the water column are scarce. To address this gap, we conducted a 1-year metocean field campaign that included the deployment of three instrumented moorings at 1400 m, 2160 m and 3675 m water depths. We present results from these new observations and from numerical modelling of the long-term circulation in the Colombian basin. Moorings were designed to obtain current speed and direction through the full water column, with higher resolution near the surface and near the seabed. To examine the spatial variability of the observations, hydrographic measurements were conducted during deployment and service visits to the moorings. To better understand the deep-water dynamics of the region, two numerical models (NCOM and ROMS) were configured to simulate 20-years of circulation in the Colombian Caribbean. Measurements at the northernmost and deepest mooring near the Guajira Peninsula record several anti-cyclonic eddies moving generally towards the west with speeds upwards of 1 m/s in the top 1000 m for periods of up to one month. Observations from 1000 m down to 3 m above the seabed at this deepest mooring are remarkably coherent in speed (~0.2 m/s) and direction (NNE). This mooring also recorded five events resembling topographic Rossby waves with periods of ~60 days and clear bottom current intensification. The upper circulation at the two moorings near the central Colombian Caribbean flows mainly towards the NE and presents several energetic episodes (maximum speeds ~1 m/s). Measurements at all sites show many eddies that are not confined to the near-surface layers but are submerged below 200 m with no clear connection to the upper circulation. Statistical comparison with HYCOM hindcast reanalysis data shows good agreement near the surface, but a rapid deterioration of model skill below 500 m depth. The two models configured to simulate the circulation inside the Colombian basin show good general skill, but still misrepresent some of the observed current features. Our observations show that in the Colombian Caribbean, the northern region’s upper 1000 m are dominated by the NW-flowing Caribbean Current and the anti-cyclonic eddies entrained in it, while below 1000 m to near the seabed, circulation is controlled by a nearly vertically-uniform, deep current moving in the NNE direction. To our knowledge, this is the first time that this deep current system and its oscillations are reported. The central and southern regions are dominated by the variations of the Darien Countercurrent in the upper layers, and by flow-topography interactions near the seabed. Submerged eddies are common to all areas observed.