Worldwide evolution of upwelling and its influence on sst trends

Resumo

To know the mechanisms that govern the ocean is basic since variations in it can lead to dramatic consequences in economy and biodiversity, either in terms of production or fishing migrations. One of the most important phenomena that takes place in some coastal oceanic areas is upwelling. Upwelling systems are known to be areas of great productivity with high fish catch rates due to the input of deep cold water with greater nutrients. Some of the most important and studied upwelling systems in the world are Benguela, Canary, Humboldt and California. However there are also other less studied as Somalia, Java, Guajira or Yucatan among others. Thus, changes in coastal upwelling can suppose variations in the behavior of the coastal areas. This fact have been widely studied since 1990 when Bakun proposed that global warming could induce the intensification of upwelling in coastal areas. Bakun hypothesis was studied in terms of wind stress from 1982 to 2010 obtaining controversial results. Some of the major upwelling systems as Benguela, Peru, Canary and northern California showed increasing trends, but other areas as southern California, Java or Somalia showed decreasing trends. Moreover, a comparison with previous studies indicated that results seem to depend on the study area, the length of time series, the season and, even, the database used. Another important subject of study has been the relationship between upwelling and global warming since, upwelling, contributes to the fact that the coasts affected by this phenomenon have lower temperatures. In this sense, SST trends were analyzed both in coastal and in the adjacent oceanic locations worldwide from 1982 to 2015. Warming was observed at most of the ocean (92%) and coastal locations (~87%), being more intense in ocean areas than in coastal areas in around 66% of the locations. Then, a comparison was established between those areas affected by upwelling and those that not to know the effect of upwelling in the SST trends. The percentage of points where warming was more intense in ocean areas than in coastal areas was around 92% for locations affected by upwelling and only 58% for the areas not affected. Thus, was concluded that upwelling was acting as a moderator of climate change in the upwelling systems. The influence of upwelling on SST trends was also analyzed in three localized areas (La Guajira, Java and Yucatan) which presented different characteristics. La Guajira upwelling system shown a moderate cooling for the months with strong upwelling which contrasts with the general warming observed in the adjacent oceanic Caribbean region. Cooling was related to a reinforcement of upwelling, although the position of the peaks in upwelling and SST trends not coincide due to the presence of the Caribbean Coastal Undercurrent. In the case of Java, the southern coast of the island warmed at a much lower rate than the adjacent ocean locations over the last three decades. This behavior was observed during the upwelling season (July-October) and it was especially patent in August and September when upwelling attains the highest values. Contrary to the case of La Guajira, South Java warming was observed at ocean locations and cooling near shore but under a scenario of decreasing upwelling (~30% in some cases). The origin of coastal cooling was due to changes in the vertical structure of the water column. A vein of subsurface water, which has cooled at a rate higher than 0.3°C per decade, was observed to enter from the northwestern part of the study area following the South Java Current. This water only manifests at surface near coast, where it is pumped up by coastal upwelling. Finally, the coastal area north of Yucatan has experienced a cooling SST trend from 1982 to 2015 during the upwelling season (May-September) that contrasts with the warming observed at the adjacent ocean area. Different drivers were analyzed to identify the possible causes of that unusual coastal cooling. Changes in coastal upwelling and in sea-atmosphere heat fluxes are not consistent with the observed coastal cooling. The eastward shift of the Yucatan Current observed over the last decades is hypothesized as the most probable cause of coastal cooling. This shift enhances the vertical transport of cold deeper water to the continental shelf from where it is pumped to the surface by upwelling favorable westerly winds.