The ocean, which stores 97% of water in the earth, is the core of the global water cycle. Ocean exchange freshwater with atmosphere, land and cryosphere and these exchanges are reflected in ocean salinity changes directly. Ocean salinity changes are therefore often represented by ocean freshwater changes in the theoretical studies to provide a unified framework for estimating earth’s water cycle and understanding global water cycle.

Freshwater transport is relevant to the stability of the Atlantic Meridional Overturning Circulation. (Image by ZHENG Huayi)

The redistribution of ocean freshwater, primarily driven by freshwater transport, is an important component of earth’s water cycle. The Atlantic Ocean, in particular, is a key area, because freshwater changes in subpolar North Atlantic can not only reflect regional freshwater exchanges, but also alter the global ocean circulation. However, the current observation of Atlantic meridional freshwater transport (AMFT) is restricted within a few specific latitudes (26°N, ~55°N) due to the high cost of the observational arrays. The sparse AMFT observation significantly limits the understanding of AMFT changes and mechanisms.

Researchers from the Institute of Atmospheric Physics at the Chinese Academy of Sciences, and Xiamen University developed an indirect method to estimate AMFT at each latitude: solving ocean freshwater budget equation based on the observation of ocean freshwater content (derived from ocean salinity), surface freshwater flux (precipitation and evaporation) and AMFT at 26.5°N (RAPID array). The ocean freshwater changes are driven by surface freshwater flux and AMFT convergence, so it's possible to derive AMFT by subtracting surface freshwater flux from ocean freshwater changes. The monthly AMFT across 34°S-66°N over 2004-2020 is derived with this approach.

The climatology, inter-annual and trend of AMFT are quantified based on the new estimate. Climatologically, AMFT is southward between 18°S and 34°S but northward from 18°S to 66°N. At the inter-annual scale, AMFT shows distinct changes in 34°S-40°N and 40°-66°N, which is potentially explained by the changes in AMFT drivers with the region.

Another finding from the new estimate is that AMFT across different latitudes exhibited a northward intensifying trend over 2004-2020 (Fig. 3) despite the time series being short. However, the magnitude of this trend varies across latitudes, resulting in localized AMFT convergence and divergence. The convergence and divergence in AMFT influence the acceleration of Atlantic freshwater content changes furthermore.

This approach effectively addresses the limited coverage of observational array and provides data for further understanding AMFT changes under global warming. The study is published in Geophysical Research Letters.