Drivers of heat and freshwater content changes in the North Atlantic

Over the past years the North Atlantic has been the main scene of three interesting phenomena: a long-term warming hole (i.e. Drijfhout et al., 2012), a reoccurring cold blob (i.e. Duchez et al., 2016) and an unusual freshening in recent years (Holliday et al., 2020).

All three have been linked to either changes in ocean circulation causing i.e., anomalous heat transports, atmospheric circulation changes that, i.e., lead to enhanced surface heat loss or changes in precipitation patterns, – or a combination of the two. While it appears that the main drivers of these phenomena have been identified, the relative importance of them as well as the connections between the three are still unclear.

To assess this, we study the correlation of the main atmospheric and oceanic drivers in the North Atlantic region and the upper ocean heat (OHC) and freshwater content (FWC). By looking at OHC and FWC we remove some of the noise visible in the sea surface data, and it further enables us to remove the direct influence of the atmosphere by subtracting the heat and freshwater air-sea fluxes from the data.

The results indicate that long-term changes in the western subpolar North Atlantic are caused by the direct effects of changes in the atmosphere, while the eastern subpolar North Atlantic is more strongly influenced by changes in the ocean circulation causing a simultaneous cooling/freshening or warming/salinification, respectively. This has e.g., implications for the definition of temperature or salinity based AMOC indices (as used in e.g., Boers, 2021; Caesar et al., 2018) that often average quantities over the whole or even just the western subpolar North Atlantic. These should be redefined focusing on the eastern part.  

References

Boers, N. (2021). Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nature Climate Change, 11(8), 680-688. https://doi.org/10.1038/s41558-021-01097-4

Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700), 191-196. https://doi.org/10.1038/s41586-018-0006-5

Drijfhout, S., van Oldenborgh, G. J., & Cimatoribus, A. (2012). Is a Decline of AMOC Causing the Warming Hole above the North Atlantic in Observed and Modeled Warming Patterns? Journal of Climate, 25(24), 8373-8379. https://doi.org/10.1175/jcli-d-12-00490.1

Duchez, A., Frajka-Williams, E., Josey, S. A., Evans, D. G., Grist, J. P., Marsh, R., . . . Hirschi, J. J. M. (2016). Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave. Environmental Research Letters, 11(7), 074004. https://doi.org/10.1088/1748-9326/11/7/074004

Holliday, N. P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-López, C., . . . Yashayaev, I. (2020). Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Communications, 11(1), 585. https://doi.org/10.1038/s41467-020-14474-y