In a groundbreaking study, researchers from the Institute of Atmospheric Physics of the Chinese Academy of Sciences and Xiamen University developed an innovative indirect method for estimating Atlantic meridional freshwater transport (AMFT) across various latitudes. The research, published in Geophysical Research Letters, provides critical insights into the dynamics of the Earth’s water cycle, especially concerning how freshwater is redistributed across the Atlantic Ocean.
The Earth’s oceans are essential in regulating the global water cycle. Covering over 70% of the planet’s surface, the oceans hold about 97% of Earth’s water, acting as the main conduit for freshwater exchanges between the atmosphere, land, and cryosphere. These exchanges, especially the input and output of freshwater into the oceans, directly influence ocean salinity. Changes in salinity, in turn, reflect shifts in ocean freshwater content, offering a clear indication of how water is distributed across the globe.
Salinity variations are often used as an indirect measure of freshwater in theoretical oceanic studies. This approach provides a unified framework to estimate freshwater fluxes in the ocean and helps researchers better understand the functioning of the global water system. Freshwater transport, particularly in the Atlantic Ocean, plays a key role in redistributing water around the planet. Specifically, changes in the amount of freshwater in the subpolar North Atlantic are not only indicative of regional freshwater exchanges but also influence larger-scale ocean circulation patterns, which in turn impact global climate.
Despite its significance, research on Atlantic meridional freshwater transport has been constrained by limited data. Direct observations of AMFT are restricted to a few latitudes, predominantly between 26°N and 55°N. Establishing comprehensive observational arrays over the entire Atlantic is challenging and costly. This limitation has hindered efforts to fully understand AMFT’s variations and the mechanisms that drive it. Until now, researchers have had to rely on indirect methods or partial datasets, which left significant gaps in our understanding of how freshwater is transported across the Atlantic Ocean.
To overcome these limitations, the researchers developed a novel method for estimating AMFT across a broader range of latitudes, spanning from 34°S to 66°N. Their approach combines measurements of ocean salinity with surface freshwater flux data, which includes precipitation and evaporation, to estimate AMFT at different latitudes. This method also incorporates the ocean’s freshwater content, which is derived from salinity measurements, to better assess how freshwater is distributed in the ocean.
By focusing on the period from 2004 to 2020, the researchers were able to derive monthly estimates of AMFT across the Atlantic. This data provided new insights into the climatology, inter-annual variability, and long-term trends of AMFT across different latitudes. One of the study’s key findings is that the intensity of AMFT increases as one moves northward across latitudes, with the most pronounced changes occurring in the northern part of the Atlantic.
Climatologically, the study found that AMFT extends southwards from 18°S to 34°S and northwards from 18°S to 66°N. These patterns reflect the natural redistribution of freshwater across the Atlantic, with notable convergence and divergence zones that influence the overall transport of freshwater. The convergence and divergence of AMFT play a significant role in altering the Atlantic’s freshwater content, and these shifts can accelerate or decelerate changes in ocean salinity. Understanding these variations is vital for anticipating how ocean circulation might change in response to global warming.
On an inter-annual scale, the study revealed distinct variations in AMFT between different regions. In the southern portion of the Atlantic (from 34°S to 40°N), AMFT showed fluctuations over time, while in the northern portion (from 40°N to 66°N), the variability was more pronounced. The researchers speculated that these fluctuations were driven by changes in the underlying mechanisms that control AMFT in each region, such as wind patterns, ocean currents, and atmospheric conditions. These findings underscore the complexity of the Atlantic’s freshwater transport and suggest that various factors contribute to its variability.
Another crucial insight from the study is the observed trend of increasing intensity in AMFT, particularly in the northern latitudes, between 2004 and 2020. Although the time series is relatively short, the researchers noted a clear increase in the transport of freshwater in the northern Atlantic. This trend is indicative of larger changes occurring in the global water system, potentially linked to global warming. However, the strength of this trend varied across latitudes, with some areas exhibiting stronger convergence (areas where freshwater is accumulating) and others showing divergence (where freshwater is being depleted).
The study’s findings have significant implications for understanding the impacts of climate change on ocean circulation and the global water cycle. As global temperatures rise, freshwater inputs into the oceans, whether through increased precipitation or melting ice, are expected to alter ocean salinity patterns. These changes could influence the Atlantic Meridional Overturning Circulation (AMOC), which plays a crucial role in regulating global climate. A weakening or disruption of the AMOC could have far-reaching consequences, such as shifts in weather patterns, sea level rise, and changes in marine ecosystems.
The researchers’ new method addresses the issue of limited observational data and provides a more comprehensive view of AMFT across different latitudes. This data could help improve climate models and enhance our understanding of how the Atlantic’s freshwater transport will evolve in the future. Additionally, it offers valuable insights into the dynamics of the global water cycle, allowing for more informed predictions about how changing ocean conditions may affect regional and global climates.
Reference: Huayi Zheng et al, An Observation‐Based Estimate of Atlantic Meridional Freshwater Transport, Geophysical Research Letters (2024). DOI: 10.1029/2024GL110021