return
On the Decadal and Multidecadal Variability of the Agulhas Current
April 03,2023

Ruize Zhang, Shantong Sun, Zhaohui Chen, Haiyuan Yang, and Lixin Wu

Published in Journal of Physical Oceanography, March 2023

The Agulhas Current (AC) is a critical component of global ocean circulation. However, due to a lack of multidecadal observations, it is not clear how the AC has changed in response to anthropogenic forcing. A recent observational study suggests a broadening and slight weakening of the AC in the past few decades, while others suggest a strengthening of the AC during the historical period. In this paper, we find substantial internal variability of the AC on decadal to multidecadal time scales in high-resolution models. We show that the AC consistently exhibits two modes of decadal and multidecadal (i.e., low frequency) variability in a series of high-resolution climate models: a uniform mode that is largely associated with changes in the AC strength and a dipole mode that is mainly related to width changes of the AC. We demonstrate that the uniform mode is mainly forced externally by the decadal variations of the wind field and presents a decline under global warming, suggesting a weakening of the AC in response to anthropogenic forcing. The dipole mode, on the other hand, is mainly due to internal dynamics and does not show a trend during the historical period. Using a quasigeostrophic model that captures the dipole mode, we attribute the dipole mode to low-frequency potential vorticity changes in the western boundary, driven by a divergence of relative potential vorticity due to eddy activity. Thus, our results present further context for the interpretation of the AC responses in a changing climate based on a short observational record.

Fig. EOF analysis on the low-pass-filtered (>7 years) current speed across the AC section in the preindustrial control experiment, performed over model years 210–490. (a) Spatial pattern of the dipole mode (shading), overlaid with the mean AC speed across the section (gray dashed contour). (b) Time series of the principal component associated with the dipole mode (black). The red line denotes the linear trend of the principal component. (c) As in (a), but for the uniform mode. (d) As in (b), but for the uniform mode.

Zhang, R., S. Sun, Z. Chen, H. Yang, and L. Wu, 2023: On the Decadal and Multidecadal Variability of the Agulhas Current. J. Phys. Oceanogr., 53, 1011–1024, https://doi.org/10.1175/JPO-D-22-0123.1.

return
Copyright @ 2017 NPOCE. All Rights Reserved.