On the Recent Ambiguity of the North Atlantic Subpolar Gyre Index

Hátún, Hjálmar; Chafik, Léon

doi:10.1029/2018jc014101

Cited by 44 publications

(59 citation statements)

References 28 publications

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“…We note that unlike gyre indexes inferred from sea surface height (SSH; e.g., Häkkinen & Rhines, ; Hátún et al, ), the gyre strength in ECCOv4 (calculated directly from the barotropic stream function) shows no decline in SPG circulation. The trend in the SSH‐based SPG index comes from a basin‐wide sea level rise in the North Atlantic (Foukal & Lozier, ; Hátún & Chafik, ), which does not translate into dynamical SPG changes. Constructing an SPG index based on detrended SSH in ECCOv4 yields similar gyre variations as to that obtained from the barotropic stream function ( r = 0.67 if the SSH‐based index leads by 1 year).…”

Section: Discussionmentioning

confidence: 99%

“…shows no decline in SPG circulation. The trend in the SSH-based SPG index comes from a basin-wide sea level rise in the North Atlantic (Foukal & Lozier, 2017;Hátún & Chafik, 2018), which does not translate into dynamical SPG changes. Constructing an SPG index based on detrended SSH in ECCOv4 yields similar gyre variations as to that obtained from the barotropic stream function (r = 0.67 if the SSH-based index leads by 1 year).…”

Section: Journal Of Geophysical Research: Oceansmentioning

confidence: 98%

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Mechanisms of Ocean Heat Anomalies in the Norwegian Sea

et al. 2019

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Ocean heat content in the Norwegian Sea exhibits pronounced variability on interannual to decadal time scales. These ocean heat anomalies are known to influence Arctic sea ice extent, marine ecosystems, and continental climate. It nevertheless remains unknown to what extent such heat anomalies are produced locally within the Norwegian Sea, and to what extent the region is more of a passive receiver of anomalies formed elsewhere. A main practical challenge has been the lack of closed heat budget diagnostics. In order to address this issue, a regional heat budget is calculated for the Norwegian Sea using the ECCOv4 ocean state estimate—a dynamically and kinematically consistent model framework fitted to ocean observations for the period 1992–2015. The depth‐integrated Norwegian Sea heat budget shows that both ocean advection and air‐sea heat fluxes play an active role in the formation of interannual heat content anomalies. A spatial analysis of the individual heat budget terms shows that ocean advection is the primary contributor to heat content variability in the Atlantic domain of the Norwegian Sea. Anomalous heat advection furthermore depends on the strength of the Atlantic water inflow, which is related to large‐scale circulation changes in the subpolar North Atlantic. This result suggests a potential for predicting Norwegian Sea heat content based on upstream conditions. However, local surface forcing (air‐sea heat fluxes and Ekman forcing) within the Norwegian Sea substantially modifies the phase and amplitude of ocean heat anomalies along their poleward pathway, and, hence, acts to limit predictability.

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Section: Discussionmentioning

confidence: 99%

Section: Journal Of Geophysical Research: Oceansmentioning

confidence: 98%

Mechanisms of Ocean Heat Anomalies in the Norwegian Sea

et al. 2019

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“…The weakening of the SPG from a maximum in the mid-1990s has also been seen in many previous studies (Boning et al, 2006;Danabasoglu et al, 2016;Lohmann et al, 2009). An index of SPG strength based on observed sea surface heights (Häkkinen & Rhines, 2004) also shows a weakening since the mid-1990s; however, modified definitions of the gyre index have shown a partial recovery since 2010 (Foukal and Lozier, 2017;Hatun & Chafik, 2018).…”

Section: Gyre Circulationmentioning

confidence: 99%

The Mean State and Variability of the North Atlantic Circulation: A Perspective From Ocean Reanalyses

et al. 2019

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The observational network around the North Atlantic has improved significantly over the last few decades with subsurface profiling floats and satellite observations and the recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC). These have shown decadal time scale changes across the North Atlantic including in heat content, heat transport, and the circulation. However, there are still significant gaps in the observational coverage. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and can be used to understand the observed changes. However, the ability of the reanalyses to represent the dynamics must also be assessed. We use an ensemble of global ocean reanalyses to examine the time mean state and interannual-decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture processes and whether any understanding can be gained. In particular, we examine aspects of the circulation including convection, AMOC and gyre strengths, and transports. We find that reanalyses show some consistency, in particular showing a weakening of the subpolar gyre and AMOC at 50 • N from the mid-1990s until at least 2009 (related to decadal variability in previous studies), a strengthening and then weakening of the AMOC at 26.5 • N since 2000, and impacts of circulation changes on transports. These results agree with model studies and the AMOC observations at 26.5 • N since 2005. We also see less spread across the ensemble in AMOC strength and mixed layer depth, suggesting improvements as the observational coverage has improved.

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“…Previous studies have defined NASPG climate indices following different methodologies, e.g., Häkkinen and Rhines () linked satellite‐based sea surface height (SSH) to deep convection in the Labrador Sea, and Böning et al () related the NASPG index to volume transport of the deep west boundary current at the Labrador Sea, using ocean general circulation model simulation (OGCM). Recently, Hátún and Chafik (, and reference therein) provided a review on the ambiguity in interpreting the NASPG index in the literature due to different data sources, different data periods, and differences in the application of similar methodology (e.g., region and principal components associated with the NASPG index). They point out that in previous studies the NASPG was studied either relative to its strength or relative to its shape/size separately.…”

Section: Introductionmentioning

confidence: 99%

North Atlantic Sub‐Polar Gyre Climate Index: A New Approach

Biri

Klein

2019

JGR Oceans

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As decadal predictions become operational, the need to use, understand, and extract information from them becomes essential. A climate index is a simple diagnostic quantity that can be used to characterize integral aspects of a geophysical system such as circulation patterns, and thus can be used to evaluate decadal forecasts. One of the most studied and well documented regions of the World Ocean is the North Atlantic. The North Atlantic subpolar gyre is an important region for the modulation of European climate and where skillful predictions of up to a decade can be obtained. Ocean re‐analysis (ORA‐S4) data from 1959 to 2017 are used to introduce a new methodology to compute a climate index of the North Atlantic subpolar gyre that captures both the variability in its strength and shape, the latter was to our knowledge never investigated previously as part of the variability of the gyre on interannual to decadal time scales. The methodology reveals two states of the gyre (before and after 2000), the former is mainly driven by temperature and the latter by a combination of mechanisms that interact to sustain a relatively stable subpolar gyre in terms of strength.

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On the Recent Ambiguity of the North Atlantic Subpolar Gyre Index

Cited by 44 publications

References 28 publications

Mechanisms of Ocean Heat Anomalies in the Norwegian Sea

Mechanisms of Ocean Heat Anomalies in the Norwegian Sea

The Mean State and Variability of the North Atlantic Circulation: A Perspective From Ocean Reanalyses

North Atlantic Sub‐Polar Gyre Climate Index: A New Approach

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