Intensified upwelling: normalized sea surface temperature trends expose climate change in coastal areas

Abstract

Eastern boundary upwelling systems (EBUSs) provide valuable natural resources due to their high primary production. However, there is significant uncertainty in how climate change may affect the mechanisms that sustain these ecosystems in the future. Therefore, assessing the effects of climate change on EBUSs under the current global warming scenario is crucial for efficient ecosystem management. In 1990, Andrew Bakun suggested an increase in the upwelling intensity due to the rise of the ocean–land pressure gradient. Since there is a significant link between thermal gradients and offshore Ekman transport, we use sea level pressure (SLP) and deseasonalized sea surface temperature (SST) data from remote sensing to elucidate this hypothesis and validate it using in situ observations. SST is an indicator of coastal upwelling, and our long-term analysis of monthly and deseasonalized SST records shows that the seasonal and synoptic processes have minimal influence on the SST–upwelling intensity relationship. Upwelling within the same EBUS is not usually evenly distributed along coastlines, leading to upwelling in specific areas, referred to as upwelling centers. We compare the SST trends in the main upwelling centers of the four EBUSs with those in open ocean waters through a new index, αUI, designed to characterize upwelling changes in the EBUS. An a-dimensional number allows us to normalize the trends independently of the upwelling system and compare all of them. Furthermore, we have complemented the SST index with sea level pressure gradient data. This new index (supported by SLP gradient trends) indicates intensification in all the EBUSs, revealing a coherent pattern within EBUSs in the same ocean (i.e., Canarian and Benguela or Californian and Humboldt upwelling systems).