Changes in hydrography and circulation in the eastern South Pacific Ocean between 1967 and 1995

Changes in water properties in the ocean interior over time provide information on ocean variability and climate change. There are few high quality repeat hydrographic sections in the worlds oceans. This makes the reoccupation in 1995 with RV Sonne of three hydrographic transects off the Chilean coast potentially important, particularly as the sparsely-sampled eastern South Pacific is the site of several processes of importance for global circulation and water mass formation. The three transects, which lie along 43º15'S, 35º15'S, and 28º15'S, were first sampled in 1967 as part of the SCORPIO expedition.

Over the 28 years between occupations of the transects there have been marked changes in the observed oceanographic properties on all of the three transects. Warming is observed along all three sections above 1000 m. On the northernmost transect, the warming penetrates deeper and is accompanied by a distinct decrease in dissolved oxygen concentrations. On the southernmost transect, warming and salinity increase is most significant between 300 db and 800 db. On both transects warming is largest towards the centre and eastern end of the transect.

The observed differences are probably not related to seasonal or interannual variations as both cruises were conducted during the same season and when the El Niño-Southern Oscillation was in a similar phase. Instead the variations probably reflect longer term climate shifts in this region, apparently related to different processes along the different sections. Analysis of the results suggests that along the 43º15'S transect the warming above 800 m is caused by production of warmer Subantarctic Mode Water in recent decades. In contrast, the warming above 1600 m on the 28º15' S section is due to downward isotherm displacement and southward movement of the water masses by between 100 km and 200 km as infered from joint analysis of temperature and oxygen data.. One possible mechanism to explain the latter results would be a weakening of the subtropical gyre circulation of the South Pacific Ocean.