Upwelling along the western coast of the Iberian Peninsula: dependence of trends on fitting strategy

We investigated upwelling variability along the western coast of the Iberian Peninsula for the period 1948−2009 by means of wind data extracted from the NCEP/NCAR reanalysis project. According to most previous studies, a remarkable weakening in upwelling intensity has been observed in the area. However, the observed linear trends are highly dependent on the length of the series. Here we show how the upwelling index signal can be fitted to different functions with diametrically opposed results. Both a simple linear regression and the fit to an equation including a linear term and a sinusoid with periodicity of 10.9 yr show weakening of upwelling intensity. In contrast, the fit to a more complex function including a linear term and a double sinusoid with short and long periodicities of Ts = 10.9 and Tl = 89 yr show strengthening of upwelling intensity. Both periodicities can be related to the Schwabe and Gleissberg cycles of solar activity. Although this last function showed a greater significance than the others, some uncertainty remains in the actual duration of the long period due to the short length of the upwelling index signal.