Abstract <p>In this paper, trends and characteristics of low-frequency variability in the intensity of the Canary upwelling (CU) have been estimated using satellite data on the wind field and sea surface topography (which allow one to calculate the necessary parameters of Ekman and gradient currents, as well as the water rise rate) starting from 1993 and up to 2023. It has been obtained that the contribution of geostrophic transport to the formation of vertical water movements in the CU region is approximately half as much as compared with the Ekman surge and is comparable to the contribution of the vorticity of the wind field. On average, geostrophic currents forming in the upper ocean layer in the upwelling region lead to a decrease in the intensity of water rise. The trend in the total rate of the rise of the upper water layer averaged over the whole CU region is about 1.3 × 10<sup>–6</sup> m/s over 30 years. The significance of the trend does not exceed 85%, which is a consequence of the presence of intense interannual and interdecadal variability of wind fields and geostrophic currents, as well as noisiness of the gradient current field by subgrid (mesoscale) fluctuations. The obtained estimate of the long-term trend of the total rate of the upper water rise averaged over the entire CU region is close to the estimate of the vertical velocity trend at the lower boundary of the upper mixed layer calculated from ocean reanalysis data with a spatial resolution of 1/12° × 1/12°. The statistical significance of the trend estimated using the reanalysis data is smaller than 80%. At the same time, a significant parabolic trend is distinguished in the time variation of the average annual wind velocity and the Ekman upwelling index. The trend makes a significant contribution to the total variance of the primary series (more than 15%). This can be considered as a manifestation of the Atlantic Multidecadal Oscillation the periodicity of which is approximately twice the length of the analyzed series.</p>

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Long-Term Trends and Low-Frequency Variability of the Canary Upwelling Intensity and Their Causes

  • A. B. Polonskii,
  • A. N. Serebrennikov

摘要

Abstract

In this paper, trends and characteristics of low-frequency variability in the intensity of the Canary upwelling (CU) have been estimated using satellite data on the wind field and sea surface topography (which allow one to calculate the necessary parameters of Ekman and gradient currents, as well as the water rise rate) starting from 1993 and up to 2023. It has been obtained that the contribution of geostrophic transport to the formation of vertical water movements in the CU region is approximately half as much as compared with the Ekman surge and is comparable to the contribution of the vorticity of the wind field. On average, geostrophic currents forming in the upper ocean layer in the upwelling region lead to a decrease in the intensity of water rise. The trend in the total rate of the rise of the upper water layer averaged over the whole CU region is about 1.3 × 10–6 m/s over 30 years. The significance of the trend does not exceed 85%, which is a consequence of the presence of intense interannual and interdecadal variability of wind fields and geostrophic currents, as well as noisiness of the gradient current field by subgrid (mesoscale) fluctuations. The obtained estimate of the long-term trend of the total rate of the upper water rise averaged over the entire CU region is close to the estimate of the vertical velocity trend at the lower boundary of the upper mixed layer calculated from ocean reanalysis data with a spatial resolution of 1/12° × 1/12°. The statistical significance of the trend estimated using the reanalysis data is smaller than 80%. At the same time, a significant parabolic trend is distinguished in the time variation of the average annual wind velocity and the Ekman upwelling index. The trend makes a significant contribution to the total variance of the primary series (more than 15%). This can be considered as a manifestation of the Atlantic Multidecadal Oscillation the periodicity of which is approximately twice the length of the analyzed series.