Researchers from the Institute of Atmospheric Physics of the Chinese Academy of Sciences and Xiamen University developed an indirect method for estimating Atlantic meridional freshwater transport (AMFT) at various latitudes. The study was published in Geophysical Research Letters.

The ocean, which contains 97% of the Earth's water, plays a crucial role in the global water cycle. It exchanges freshwater with the atmosphere, land, and cryosphere, and these exchanges are directly reflected in changes in ocean salinity. As a result, variations in ocean salinity are often represented as changes in ocean freshwater in theoretical studies. This approach provides a unified framework for estimating the Earth's water cycle and understanding the dynamics of the global water system.

The redistribution of freshwater in the oceans, mainly driven by freshwater transport, is a crucial part of the Earth's water cycle. The Atlantic Ocean, in particular, plays a significant role because changes in freshwater levels in the subpolar North Atlantic can not only reflect regional freshwater exchanges but also impact global ocean circulation. However, current observations of AMFT are limited to a few specific latitudes (between 26°N and approximately 55°N) due to the high costs associated with establishing observational arrays. This limited coverage significantly hinders the understanding of changes in AMFT and the mechanisms behind them.

The researchers addressed the ocean's freshwater budget by analyzing the ocean's freshwater content, which is derived from ocean salinity, along with surface freshwater flux (precipitation and evaporation), and AMFT at 26.5°N (RAPID array). Changes in the ocean's freshwater content are influenced by surface freshwater flux and AMFT convergence. Therefore, AMFT can be calculated by subtracting surface freshwater flux from the changes in ocean freshwater content. Using this method, the monthly AMFT across the range of 34°S to 66°N was derived for the years 2004 to 2020.

The climatology, inter-annual variability, and trends of AMFT are analyzed based on the new estimates. Climatologically, AMFT extends southwards between 18°S and 34°S, while it extends northwards from 18°S to 66°N. On an inter-annual scale, AMFT exhibits distinct variations in the regions between 34°S to 40°N and from 40°N to 66°N. These variations may be attributed to changes in the drivers of AMFT in these areas.

Moreover, a notable finding from the new estimate is that AMFT showed a trend of increasing intensity as one moves northward across different latitudes from 2004 to 2020, even though the time series is relatively short. However, the strength of this trend varies by latitude, leading to areas of both convergence and divergence in AMFT. These patterns of convergence and divergence have an impact on the acceleration of changes in Atlantic freshwater content.

This method effectively addresses the limited coverage of the observational array and provides valuable data for a deeper understanding of AMFT changes due to global warming.

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