The impact of the Galapagos Islands on the dynamics of the equatorial East Pacific

Abstract

Small changes in the sea-surface temperature (SST) distribution in the equatorial Pacific are known to have large consequences for the weather and climate on regional and even global scales. A detailed knowledge of the SST patterns is therefore necessary if we are to understand and predict these effects thoroughly. Effort has hitherto been concentrated on understanding the dynamics of the West Pacific Warm Pool, but attention is now shifting to the eastern Cold Tongue region. This thesis contributes to this programme by assessing the impact of one unique feature of the equatorial East Pacific; the presence on the equator of a medium-sized island group - the Galapagos Archipelago. The project is carried through using a combination of: in situ oceanographic data from ships and moorings; satellite SST and colour images; and diagnostics from both a sophisticated ocean general circulation model and a simplified (reduced gravity) model. Each model was run both with and without a representation of the Galapagos. It is found that under typical conditions the islands form the eastern boundary of the cold tongue; the lowest equatorial SSTs in the Pacific occur on the west coast of the main islands of the group. As well as preventing the cold tongue from extending any further east, the islands also cool the sea surface still further for a distance of some 1000 km to the west. For the first 100 km or so the cooling is of the order of 1C; on the larger scale it is more like 0.1C. This cooling is a result of hydraulically enhanced upwelling of waters from the Equatorial Undercurrent (EUC), which is best explained by a Bernoulli-type (non-rotating, non-linear) theory. This theory also predicts that water parcels in the EUC will undergo vertical excursions on their passage around the north and south coasts of the islands; these excursions are observed in at least one numerical model. Downstream of the archipelago the EUC usually reunites and executes inertial oscillations, meandering about the equator. Transport in the EUC is also reduced because of the presence of the islands, by 30-50% to the east of the islands and by up to 20% to the west. Enhanced SST variability north and south of the equator immediately to the west of the Galapagos indicates that tropical instability waves (TIWs) are more active here than they would be were the islands absent. This is due to a pair of barotropically unstable zonal jets originating at the northern and southern tips of the islands. These jets are consistent with the theory of low-frequency flow past equatorial islands on the beta plane (which predicts that the South Equatorial Current will split into two such jets on encountering such an island), although weaker jets are found in models with no representation of the Galapagos. As with the cold tongue, the islands form the eastern limit of the region where these jets are found.

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