Simulation of ocean circulation around the Galapagos Archipelago using a hybrid coordinate ocean model (HYCOM)

The ocean circulation around the Galapagos Archipelago is simulated using the University of Miami's Hybrid Coordinate Ocean Model (HYCOM) using a high-resolution (0.04 degree resolution) nested domain. The model generally reproduces the major ocean circulation features around the Galapagos showing that the 0.04 degree resolution is sufficient for studying the circulation and water mass structure local to the Galapagos. A variety of winds forcing were tried with high frequency wind forcing (daily, 6-hrly) proving to be more reliable than the monthly forcing. The Archipelago has a large range in temperature and salinity associated with four different current systems and topographically and wind forced upwelling. West of Isabella and Fernandina Islands are the largest and most consistent upwelling zones, resulting from the collision of the Equatorial Undercurrent with the islands, bring relatively colder and salty waters to the surface in a region of topographically induced upwelling. This upwelling zone marks the location of high biological production at the surface with the EUC shoaling to approximately 20m from its normal depth of 50m just to the west of the islands. This results not only in a strong upwelling zone just to the west of the islands, but also in shallow mixed layers over the entire western portion of the Archipelago. The model results, which agree quite well with observations of the EUC by the TAO array, show a seasonal cycle in the speed and transport of the EUC, reaching a maximum during the late spring/early summer and minimum in the late fall. This seasonal cycle is disrupted during El Nino when the collapse of the westerlies results in a greatly weakened or even a total lack of the undercurrent. The far northern region of the Archipelago is characterized with warmer, fresher water and had the deepest mixed layer depth as a result of Panama Current waters from the northeast. The water masses over the remainder of the region result from a mixing of cool Peru Current waters and upwelled Cold Tongue waters entering from the east to the southeast mixing with the upwelled EUC waters. The regional impact of global climate change and ENSO events on the ocean circulation and water mass characteristics around the Galapagos Archipelago is studied using the Hybrid Coordinate Ocean Model (HYCOM). The result indicates that with prescribed atmospheric forcing, the major characteristics of the ocean circulation and its variability in the Galapagos region, including the ENSO events, annual and seasonal cycles, upwelling system, local and regional current systems, and tropical instability waves, can be reproduced by the nested HYCOM model. Results show that the oceanic variability in the Galapagos region is sensitive to global climate change. The regional impact of global climate change in the Galapagos region varies from region to region. Results show the western Galapagos upwelling region is affected by the EUC and Panama currents; while central/east Galapagos is affected by both Peru and EUC currents. The north region is affected by the Panama Current only. The southeast region is affected by the Peru Current only. These regional differences explain the variability of the responses in different regions of the Galapagos Archipelago to global climate change.