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Abstract The Atlantic meridional overturning circulation (AMOC) plays an important role in climate, transporting heat and salt to the subpolar North Atlantic. The AMOC’s variability is sensitive to atmospheric forcing, especially the North Atlantic Oscillation (NAO). Because AMOC observations are short, climate models are a valuable tool to study the AMOC’s variability. Yet, there are known issues with climate models, like uncertainties and systematic biases. To investigate this, preindustrial control experiments from models participating in the phase 6 of Coupled Model Intercomparison Project (CMIP6) are evaluated. There is a large, but correlated, spread in the models’ subpolar gyre mean surface temperature and salinity. By splitting models into groups of either a warm–salty or cold–fresh subpolar gyre, it is shown that warm–salty models have a lower sea ice cover in the Labrador Sea and, hence, enable a larger heat loss during a positive NAO. Stratification in the Labrador Sea is also weaker in warm–salty models, such that the larger NAO-related heat loss can also affect greater depths. As a result, subsurface density anomalies are much stronger in the warm–salty models than in those that tend to be cold and fresh. As these anomalies propagate southward along the western boundary, they establish a zonal density gradient anomaly that promotes a stronger delayed AMOC response to the NAO in the warm–salty models. These findings demonstrate how model mean state errors are linked across variables and affect variability, emphasizing the need for improvement of the subpolar North Atlantic mean states in models.
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On the Relation between Thermohaline Anomalies and Water Mass Transformation in the Eastern Subpolar North Atlantic
Abstract Decadal thermohaline anomalies carried northward by the North Atlantic Current are an important source of predictability in the North Atlantic region. Here, we investigate whether these thermohaline anomalies influence surface-forced water mass transformation (SFWMT) in the eastern subpolar gyre using the reanalyses EN4.2.2 for the ocean and the ERA5 for the atmosphere. In addition, we follow the propagation of thermohaline anomalies along two paths: in the subpolar North Atlantic and the Norwegian Sea. We use observation-based datasets (HadISST, EN4.2.2, and Ishii) between 1947 and 2021 and apply complex empirical orthogonal functions. Our results show that when a warm anomaly enters the eastern subpolar gyre, more SFWMT occurs in light-density classes (27.0–27.2 kg m−3). In contrast, when a cold anomaly enters the eastern subpolar gyre, more SFWMT occurs in denser classes (27.4–27.5 kg m−3). Following the thermohaline anomalies in both paths, we find alternating warm–salty and cold–fresh subsurface anomalies, repeating throughout the 74-yr-long record with four warm–salty and cold–fresh periods after the 1950s. The cold–fresh anomaly periods happen simultaneously with the Great Salinity Anomaly events. Moreover, the propagation of thermohaline anomalies is faster in the subpolar North Atlantic (SPNA) than in the Norwegian Sea, especially for temperature anomalies. These findings might have implications for our understanding of the decadal variability of the lower limb of the Atlantic meridional overturning circulation and predictability in the North Atlantic region. Significance StatementAnomalously warm–salty or cold–fresh water, carried by the North Atlantic Current toward the Arctic, is a source of climate predictability. In this study, we investigate 1) how these ocean anomalies influence the transformation of water masses in the eastern subpolar gyre and 2) their subsequent propagation poleward and northwestward. The key findings reveal that anomalously warm waters entering the eastern subpolar gyre lead to increased transformation in lighter water masses, while cold anomalies affect denser water masses. These anomalies propagate more than 2 times faster toward the Greenland coast (northwestward) than toward the Arctic (poleward). Our findings contribute to enhancing the understanding of decadal predictability in the northern North Atlantic, including its influence on the Atlantic meridional overturning circulation.
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- Award ID(s):
- 1948482
- PAR ID:
- 10536339
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 37
- Issue:
- 18
- ISSN:
- 0894-8755
- Format(s):
- Medium: X Size: p. 4821-4834
- Size(s):
- p. 4821-4834
- Sponsoring Org:
- National Science Foundation
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