A western boundary upwelling system response to recent climate variation (1960–2006)

In this paper, it was hypothesized that coastal upwelling increases in western boundary current influenced regions due to global warming. A state-space decomposition of the alongshore wind stress time-series exhibited an intensification during the “upwelling season” in the last 20 years (1988–2008), which agrees well with the above hypothesis. However, when applying the same decomposition to a temperature time-series at Qinglan station (located at the eastern Hainan coast) over the past 50 years (1960–2006), a negative trend was found. This indicates that the simple relation between alongshore wind stress and upwelling strength is not valid in the study area, but that other non-local factors might influence the inter-annual variability of the upwelling significantly. To identify these possible non-local effects, results of a model simulation were analyzed. From these model data an acceptable relationship between the non-locally driven western boundary current and the strength of the upwelling can be inferred. Furthermore, an “SST (sea surface temperature) upwelling index” was introduced, calculated by the simulated offshore–onshore temperature difference. This index exhibits a good correlation with the Qinglan “observational upwelling time-series”, demonstrating that the latter is a good indicator of the upwelling trend. Finally, it is shown that inter-annual oscillations of employed temperature records correlate well with the El Niño signal.

► We found that the alongshore wind stress revealed an increasing trend.
► We examined a decreasing trend of upwelling off eastern Hainan by means of state-space decomposition.
► The model results showed upwelling time-series based on time-series was a good indicator.
► A strong boundary current could partially explain the missing correlation between the wind and temperature trends.