Temporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980–2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) displays a basin-wide shoaling trend that is absent in the other basins within 40°S–40°N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward-shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons.

中文翻译：

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1980-2019年南印度洋海面混合层深度季节性减弱

海洋表面混合层的时空变化对气候和生态系统很重要。 1980-2019年间，南印度洋（SIO）混合层深度（MLD）呈现出全盆地浅滩趋势，而40°S-40°N范围内的其他盆地则没有这种趋势。 SIO MLD浅滩主要在南方冬季较为突出，具有深层气候MLD，大大削弱了MLD的季节性。此外，SIO MLD 的变化主要是由于近几十年来冬季和夏季副热带反气旋风南移以及海洋环流引起的，与南环模的加强有关。然而，在冬季，当经向MLD梯度陡峭时，向极地移动的副热带海洋环流优先浅滩SIO MLD，而在夏季，当梯度平坦时，则不会。这凸显了由于跨季节背景海洋条件不同，亚热带 MLD 对经向风减弱的独特响应。