Distinct sources of interannual subtropical and subpolar Atlantic overturning variability

Kostov, Yavor; Johnson, H. L.; Marshall, David P.; Heimbach, Patrick; Forget, Gaël; Holliday, N. Penny; Lozier, M. Susan; Li, Feili; Pillar, Helen; Smith, Ted

doi:10.1038/s41561-021-00759-4

Cited by 37 publications

(67 citation statements)

References 40 publications

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“…Mechanisms of interannual-to-decadal AMOC variability depend strongly on the region of interest. In the subtropics, high-frequency (sub-annual to interannual) wind forcing dominates AMOC variability, with buoyancy forcing also contributing at low frequencies 28,31 (fig. 1).…”

Section: Amoc Variabilitymentioning

confidence: 99%

The evolution of the North Atlantic Meridional Overturning Circulation since 1980

Jackson

Biastoch

Buckley

et al. 2022

Nat Rev Earth Environ

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The Atlantic Meridional Overturning Circulation (AMOC) (Box 1) is a system of ocean currents in the Atlantic that move warmer, upper waters northwards and cooler, deeper waters southwards. Accordingly, the AMOC is a major source of northward heat transport, accounting for 20-30% of total atmospheric and oceanic heat transport into the mid-latitudes 1 . The AMOC, therefore, has a key role in governing the climate of the North Atlantic region and beyond, influencing European air temperatures and precipitation, the frequency of Atlantic hurricanes and winter storms, spatial patterns of sea level and tropical monsoons 2,3 , and the global carbon budget 4 .The strength of the AMOC is typically 17 Sverdrups (Sv; 1 Sv = 10 6 m 3 s −1 ) 5 . However, both observations and models indicate that the AMOC exhibits substantial variability on daily to multi-decadal timescales. Coupled climate models suggest decadal variability can arise naturally due to internal interactions within the climate system [6][7][8] . The AMOC is also expected to respond to external forcing, including anthropogenic aerosols, volcanic eruptions and solar changes 9,10 , as well as anthropogenic greenhouse gas emissions 11 . Indeed, observations, reanalyses, models and proxies [12][13][14][15][16] indicate substantial contemporary decadal-scale changes in AMOC strength. The RAPID array at 26.5° N (refs 5,17 ), for example, revealed a statistically significant weakening from 2004 (refs 18,19 ), probably representing decadal variability rather than ongoing long-term weakening [20][21][22] . There are indications that the AMOC might be recovering in strength 12 .Despite evidence of decadal variability, many questions remain. For example, high-quality continuous observations, like the RAPID array, are short and sparse, making it difficult to assess longer-term AMOC variability and determine whether decadal changes are representative of those across the wider Atlantic. Moreover, there is uncertainty about the relative roles of internal variability and forced variability, owing to diverse AMOC variability 8,23 and externally forced AMOC trends 10,24 in models. Indeed, the AMOC might have already weakened over the twentieth century 25,26 , potentially implying that it is more sensitive to external forcing than previously thought. Understanding how and why the AMOC has changed on decadal timescales is thus crucial not only to understand the AMOC's role

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Section: Amoc Variabilitymentioning

confidence: 99%

The evolution of the North Atlantic Meridional Overturning Circulation since 1980

Jackson

Biastoch

Buckley

et al. 2022

Nat Rev Earth Environ

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“…Previous studies have highlighted the importance of anomalous surface heat loss in driving anomalies in both surface temperatures and the strength of the convection in the North Atlantic (de Jong and de Steur, 2016;Yeager et al, 2016;Desbruyères et al, 2019;Kostov et al, 2021). From 2014-2015, the subpolar North Atlantic experienced the strongest surface heat loss since the 1980s (Yeager et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

Causes of the 2015 North Atlantic cold anomaly in a global state estimate

et al. 2022

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Abstract. The subpolar North Atlantic is an important part of the global ocean and climate system, with SST variability in the region influencing the climate of Europe and North America. While the majority of the global ocean exhibited higher than average surface temperatures in 2015, the subpolar North Atlantic experienced record low temperatures. This interannual cold anomaly is thought to have been driven by surface forcing, but detailed questions remain about how the anomaly was created and maintained. To better quantify and understand the processes responsible for the cold anomaly, we computed mixed-layer temperature budgets in the Estimating the Circulation and Climate of the Ocean (ECCO) Version 4 global ocean state estimate. State estimates have been brought into consistency with a large suite of observations without using artificial sources or sinks of heat, making them ideal for temperature budget studies. We found that strong surface forcing drove approximately 75 % of the initial anomalies in the cooling of the mixed layer in December 2013, while horizontal advection drove the remaining 25 %. The cold anomaly was then sequestered beneath the mixed layer. Re-emergence of the cold anomaly during the summer and autumn of 2014 was primarily the result of a strong temperature gradient across the base of the mixed layer, with vertical diffusion accounting for approximately 70 % of the re-emergence. Weaker surface warming of the mixed layer during the summer of 2015 enhanced the anomaly, causing a temperature minimum. Spatial patterns in the budgets also show large differences between the north and south of the anomaly region, with particularly strong initial surface cooling in the south related to the positive phase of the East Atlantic Pattern. It is important to note that this interannual cold anomaly, which is thought to be primarily driven by surface forcing, is distinct from the multi-decadal North Atlantic “warming hole”, which has been associated with changes in advection.

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confidence: 99%

“…This extreme heat loss spanned the whole subpolar gyre (Piron et al, 2017) and has been linked to anomalously strong westerly and northerly winds transporting colder air over the region (Grist et al, 2016). While there is a strong relationship between variability in upper ocean temperature and the strength of the Atlantic overturning circulation (Desbruyères et al, 2019;Kostov et al, 2021), surface forcing from 2013-14 was strong enough to erode any correlation between the strength of the overturning and the upper ocean heat content (Desbruyères et al, 2019).…”

mentioning

confidence: 99%

Causes of the 2015 North Atlantic cold anomaly in the ECCOv4 state estimates

Sanders

Harle

Josey

et al. 2022

Preprint

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Abstract. The subpolar North Atlantic is an important part of the global ocean and climate system, with SST variability in the region influencing the climate of Europe and North America. While the majority of the global ocean exhibited higher than average surface temperatures in 2015, the subpolar North Atlantic experienced record low temperatures. This interannual cold anomaly is thought to have been driven by surface forcing, but detailed questions remain about how the anomaly was created and maintained. To better quantify and understand the processes responsible for the cold anomaly, we computed mixed layer temperature budgets in two releases of the ECCO Version 4 global ocean state estimate. These state estimates have been brought into consistency with a large suite of observations without using artificial sources or sinks of heat, making them ideal for temperature budget studies. We found that strong surface forcing drove approximately 75 % of the initial anomalies in the cooling of the mixed layer in December 2013, after which the cold anomaly was sequestered beneath the mixed layer. Re-emergence of the cold anomaly during the summer/autumn of 2014 was primarily the result of a strong temperature gradient across the base of the mixed layer, with vertical diffusion accounting for approximately 70 % of the re-emergence. Weaker surface warming of the mixed layer during the summer of 2015 enhanced the anomaly, causing a temperature minimum. Spatial patterns in the budgets also show large differences between the north and south of the anomaly region, with particularly strong initial surface cooling in the south related to the positive phase of the East Atlantic Pattern. It is important to note that this interannual cold anomaly, which is thought to be primarily driven by surface forcing, is distinct from the multi-decadal North Atlantic "warming hole", which is thought to be primarily driven by changes in advection.

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Distinct sources of interannual subtropical and subpolar Atlantic overturning variability

Cited by 37 publications

References 40 publications

The evolution of the North Atlantic Meridional Overturning Circulation since 1980

The evolution of the North Atlantic Meridional Overturning Circulation since 1980

Causes of the 2015 North Atlantic cold anomaly in a global state estimate

Causes of the 2015 North Atlantic cold anomaly in the ECCOv4 state estimates

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