Causes of low frequency North Atlantic SST variability in a coupled GCM

Research to date has shown strong multi-decadal variability of the North Atlantic Oscillation (NAO) in late winter, particularly in March when correlations to North Atlantic (NA) ocean variability (Atlantic multi-decadal variability (AMV)) are particularly strong. This late-winter low-frequency atmospheric variability appears too weak in the majority of climate models across a range of indices of North Atlantic large-scale atmospheric circulation. It appears that models do not successfully reproduce responses to either (or both) proximal sea-surface temperature (SST) variability at mid-latitudes or teleconnections to SST variability in the sub tropics.&#160;Here, an in-depth analysis of the winter evolution of multiple indices of North Atlantic mid-latitude atmospheric circulation will be presented based on both re-analysis data and historical simulations of coupled climate models (CMIP5 and CMIP6). The atmospheric indices assessed will include the NAO, speed and latitude of the NA eddy driven jet and lower-tropospheric westerly wind strength in a region of maximum variability to the west of the UK. Results so far indicate that the CMIP6 models do not exhibit a clear change from CMIP5 in terms of the representation of low-frequency late-winter atmospheric variability. To diagnose in more detail possible origins of differences between observed and simulated variability, a detailed evaluation of early- to late-winter evolution in variability of the above indices will be presented, with an initial focus on observations (re-analysis and SST re-constructions) and incorporating the following questions: &#160; - Are there significant differences in the relative strength of linkages to tropical and extra-tropical SST variability across the different atmospheric indices?&#160; - Is the observed late-winter maximum in correlations between NA atmospheric indices and North Atlantic SSTs still apparent at sub-decadal timescales? Initial results indicate that there are stronger tropical linkages for jet speed and that at sub-decadal timescales late winter is does not dominate in terms of correlations between atmospheric and SST variability. Updates on these early results will be presented along with implications of the results for differences between observed and simulated variability.&#160;

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Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly

10.5194/egusphere-egu21-2503 ◽

2021 ◽

Author(s):

Marius Årthun ◽

Robert C. J. Wills ◽

Helen L. Johnson ◽

Léon Chafik ◽

Helene R. Langehaug

Keyword(s):

Atmospheric Circulation ◽

North Atlantic ◽

Time Scale ◽

Low Frequency ◽

Meridional Overturning Circulation ◽

The North ◽

Sst Variability ◽

Cold Anomaly ◽

The North Atlantic ◽

Sst Anomalies

There has recently been a large focus on identifying the mechanisms responsible for Atlantic multidecadal variability (AMV). However, decadal-scale variability embedded within the AMV has received less attention, despite being a prominent feature of observed North Atlantic sea surface temperature (SST) and important for the climate of adjacent continents. These decadal fluctuations in the North Atlantic Ocean are also a key source of skill in decadal climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations and pre-industrial control simulations, we identify a decadal mode of atmosphere-ocean variability in the North Atlantic with a dominant time scale of 13-18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air-sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This strongly suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability, and that we might expect it to become less pronounced over the next few years.

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Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly

Journal of Climate ◽

10.1175/jcli-d-20-0464.1 ◽

2020 ◽

pp. 1-52

Author(s):

Marius Ǻrthun ◽

Robert C. J. Wills ◽

Helen L. Johnson ◽

Léon Chafik ◽

Helene R. Langehaug

Keyword(s):

Atmospheric Circulation ◽

North Atlantic ◽

Time Scale ◽

Low Frequency ◽

Meridional Overturning Circulation ◽

The North ◽

Sst Variability ◽

Cold Anomaly ◽

The North Atlantic ◽

Sst Anomalies

AbstractDecadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations and pre-industrial control simulations, we identify a decadal mode of atmosphere-ocean variability in the North Atlantic with a dominant time scale of 13-18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air-sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability.

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Locally and Remotely Forced Subtropical AMOC Variability: A Matter of Time Scales

Journal of Climate ◽

10.1175/jcli-d-19-0844.1 ◽

2020 ◽

Vol 33 (12) ◽

pp. 5155-5172

Author(s):

Quentin Jamet ◽

William K. Dewar ◽

Nicolas Wienders ◽

Bruno Deremble ◽

Sally Close ◽

...

Keyword(s):

Time Series ◽

Time Scales ◽

North Atlantic ◽

Time Scale ◽

Low Frequency ◽

Meridional Overturning Circulation ◽

Open Boundary ◽

Overturning Circulation ◽

Decadal Scale ◽

Meridional Overturning

AbstractMechanisms driving the North Atlantic meridional overturning circulation (AMOC) variability at low frequency are of central interest for accurate climate predictions. Although the subpolar gyre region has been identified as a preferred place for generating climate time-scale signals, their southward propagation remains under consideration, complicating the interpretation of the observed time series provided by the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array–Western Boundary Time Series (RAPID–MOCHA–WBTS) program. In this study, we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low-frequency AMOC variability. We analyze for this a set of four ensembles of a regional (20°S–55°N), eddy-resolving (1/12°) North Atlantic oceanic configuration, where surface forcing and open boundary conditions are alternatively permuted from fully varying (realistic) to yearly repeating signals. Their analysis reveals the predominance of local, atmospherically forced signal at interannual time scales (2–10 years), whereas signals imposed by the boundaries are responsible for the decadal (10–30 years) part of the spectrum. Due to this marked time-scale separation, we show that, although the intergyre region exhibits peculiarities, most of the subtropical AMOC variability can be understood as a linear superposition of these two signals. Finally, we find that the decadal-scale, boundary-forced AMOC variability has both northern and southern origins, although the former dominates over the latter, including at the site of the RAPID array (26.5°N).

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Spatial analysis of early-warning signals for a North Atlantic climate transition in a coupled GCM

Climate Dynamics ◽

10.1007/s00382-018-4567-7 ◽

2018 ◽

Vol 53 (1-2) ◽

pp. 97-113 ◽

Cited By ~ 2

Author(s):

Andrea Klus ◽

Matthias Prange ◽

Vidya Varma ◽

Michael Schulz

Keyword(s):

Spatial Analysis ◽

North Atlantic ◽

Early Warning ◽

Warning Signals ◽

Early Warning Signals ◽

Coupled Gcm ◽

North Atlantic Climate ◽

Climate Transition

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Multidecadal to millennial marine climate oscillations across the Denmark Strait (~ 66° N) over the last 2000 cal yr BP

Climate of the Past ◽

10.5194/cp-10-325-2014 ◽

2014 ◽

Vol 10 (1) ◽

pp. 325-343 ◽

Cited By ~ 13

Author(s):

J. T. Andrews ◽

A. E. Jennings

Keyword(s):

North Atlantic ◽

Low Frequency ◽

Atlantic Water ◽

Ice Age ◽

The North ◽

Denmark Strait ◽

Water Stratification ◽

The North Atlantic Oscillation ◽

Frequency Variations ◽

Marine Climate

Abstract. In the area of Denmark Strait (~66° N), the two modes of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) are expressed in changes of the northward flux of Atlantic water and the southward advection of polar water in the East Iceland current. Proxies from marine cores along an environmental gradient from extensive to little or no drift ice, capture low frequency variations over the last 2000 cal yr BP. Key proxies are the weight% of calcite, a measure of surface water stratification and nutrient supply, the weight% of quartz, a measure of drift ice transport, and grain size. Records from Nansen and Kangerlussuaq fjords show variable ice-rafted debris (IRD) records but have distinct mineralogy associated with differences in the fjord catchment bedrock. A comparison between cores on either side of the Denmark Strait (MD99-2322 and MD99-2269) show a remarkable millennial-scale similarity in the trends of the weight% of calcite with a trough reached during the Little Ice Age. However, the quartz records from these two sites are quite different. The calcite records from the Denmark Strait parallel the 2000 yr Arctic summer-temperature reconstructions; analysis of the detrended calcite and quartz data reveal significant multi-decadal–century periodicities superimposed on a major environmental shift occurring ca. 1450 AD.

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Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part I: A Simple Model of North Atlantic Oscillations

Journal of the Atmospheric Sciences ◽

10.1175/jas3818.1 ◽

2007 ◽

Vol 64 (1) ◽

pp. 3-28 ◽

Cited By ~ 86

Author(s):

Dehai Luo ◽

Anthony R. Lupo ◽

Han Wan

Keyword(s):

Life Cycle ◽

Theoretical Model ◽

North Atlantic ◽

Low Frequency ◽

Positive Phase ◽

Negative Phase ◽

Synoptic Scale ◽

Planetary Scale ◽

The North ◽

The North Atlantic Oscillation

Abstract A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of the preexisting dipole eddy forcing depending upon the background westerly wind seem to dominate the life cycle of the NAO and its life period. An important finding in the theoretical model is that negative-phase NAO events could be excited repeatedly after the first event has decayed, but for the positive phase downstream isolated dipole blocks could be produced after the first event has decayed. This is supported by observed cases of the NAO events presented in this paper. In addition, a statistical study of the relationship between the phase of the NAO and blocking activity over Europe in terms of the seasonal mean NAO index shows that blocking events over Europe are more frequent and long-lived for strong positive-phase NAO years, indicating that the positive-phase NAO favors the occurrence of European blocking events.

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Surface predictor of overturning circulation and heat content change in the subpolar North Atlantic

Ocean Science ◽

10.5194/os-15-809-2019 ◽

2019 ◽

Vol 15 (3) ◽

pp. 809-817 ◽

Cited By ~ 9

Author(s):

Damien G. Desbruyères ◽

Herlé Mercier ◽

Guillaume Maze ◽

Nathalie Daniault

Keyword(s):

North Atlantic ◽

Spatial Scales ◽

Low Frequency ◽

Heat Content ◽

Climatic Conditions ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

The North ◽

Wide Range ◽

Key Aspects

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) impacts ocean and atmosphere temperatures on a wide range of temporal and spatial scales. Here we use observational datasets to validate model-based inferences on the usefulness of thermodynamics theory in reconstructing AMOC variability at low frequency, and further build on this reconstruction to provide prediction of the near-future (2019–2022) North Atlantic state. An easily observed surface quantity – the rate of warm to cold transformation of water masses at high latitudes – is found to lead the observed AMOC at 45∘ N by 5–6 years and to drive its 1993–2010 decline and its ongoing recovery, with suggestive prediction of extreme intensities for the early 2020s. We further demonstrate that AMOC variability drove a bi-decadal warming-to-cooling reversal in the subpolar North Atlantic before triggering a recent return to warming conditions that should prevail at least until 2021. Overall, this mechanistic approach of AMOC variability and its impact on ocean temperature brings new key aspects for understanding and predicting climatic conditions in the North Atlantic and beyond.

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