Observed and simulated (CMIP5 and CMIP6) early- to late-winter evolution of North Atlantic atmospheric variability and links to the ocean

2020 ◽  
Author(s):  
Thomas Bracegirdle
Keyword(s):  
Atmospheric Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Decadal Variability ◽  
Low Frequency ◽  
Late Winter ◽  
Atmospheric Variability ◽  
Sst Variability ◽  
Early Results ◽  
Depth Analysis

<p>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. </p><p>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:  <br>- Are there significant differences in the relative strength of linkages to tropical and extra-tropical SST variability across the different atmospheric indices? <br>- Is the observed late-winter maximum in correlations between NA atmospheric indices and North Atlantic SSTs still apparent at sub-decadal timescales?<br>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. </p>

Observed and CMIP-simulated links between North Atlantic climatological winter jet latitude and inter-annual to decadal ocean-atmosphere coupling

2021 ◽  
Author(s):  
Thomas Bracegirdle ◽  
Hua Lu ◽  
Jon Robson
Keyword(s):  
Atmospheric Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Decadal Variability ◽  
Dominant Mode ◽  
Late Winter ◽  
Early Winter ◽  
Atmospheric Variability ◽  
Atmospheric Circulation Indices ◽  
Ocean Atmosphere

<p>Decadal variability in indices of North Atlantic (NA) atmospheric circulation plays a major role in changing climate over western Europe. However, reproducing characteristics of this variability in climate models presents a major challenge. Climate models broadly exhibit weaker-than-observed multi-decadal variability in atmospheric circulation indices. A prominent explanation for this is that model-simulated links between anomalous sea-surface temperatures (SSTs) and atmospheric variability are too weak. The dominant mode of basin-wide NA SST variability is Atlantic multi-decadal variability (AMV), which on multi-decadal timescales is expressed more strongly over the NA sub-polar gyre (SPG). SSTs over the SPG region (SST<sub>SPG</sub>) are therefore the main focus here.</p><p>Studies to date have shown that variability in the North Atlantic Oscillation (NAO) exhibits strongest correlations with AMV indices in late winter, but the reasons for this are not clear. Here we show that this stronger late-winter correlation is particularly clear for SST<sub>SPG</sub> and coincides with a climatological equatorward shift of the eddy-driven NA westerly jet from early-to-late winter. To help gain dynamical insight, indices of eddy-driven jet latitude (JLI) and speed (JSI) were correlated with SST<sub>SPG</sub> and it was found that they exhibit more pronounced early-to-late winter shifts in correlations than for the NAO; In particular,  correlations strengthen from early-to-late winter for JLI while weaken for JSI. Our results suggest that the jet-SST<sub>SPG</sub> linkages progress through winter from JSI dominant in early winter to JLI dominant in late winter.</p><p>CMIP5 and CMIP6 models were then evaluated for representation of these observed characteristics in ocean-atmosphere linkages. Consistent with the observed sub-seasonal links between climatological jet latitude and atmosphere-ocean correlation strength, CMIP models with larger equatorward jet biases exhibit weaker JSI-SST<sub>SPG</sub> correlations and stronger JLI-SST<sub>SPG</sub> correlations. A pronounced early-winter equatorward bias in jet latitude in CMIP models could partially explain the weaker-than observed linkage between SSTs and atmospheric variability.  </p>

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

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

<p>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.</p>

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

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.

scholarly journals Euro-Atlantic Atmospheric Circulation during the Late Maunder Minimum

2018 ◽  
Vol 31 (10) ◽  
pp. 3849-3863 ◽  
Author(s):  
Javier Mellado-Cano ◽  
David Barriopedro ◽  
Ricardo García-Herrera ◽  
Ricardo M. Trigo ◽  
Mari Carmen Álvarez-Castro
Keyword(s):  
Atmospheric Circulation ◽  
North Atlantic ◽  
Decadal Variability ◽  
Complete Classification ◽  
Maunder Minimum ◽  
The North ◽  
Late Maunder Minimum ◽  
The North Atlantic Oscillation ◽  
Nao Pattern

Abstract This paper presents observational evidence of the atmospheric circulation during the Late Maunder Minimum (LMM, 1685–1715) based on daily wind direction observations from ships in the English Channel. Four wind directional indices and 8-point wind roses are derived at monthly scales to characterize the LMM. The results indicate that the LMM was characterized by a pronounced meridional circulation and a marked reduction in the frequency of westerly days all year round, as compared to the present (1981–2010). The winter circulation contributed the most to the cold conditions. Nevertheless, findings indicate that the LMM in Europe was more heterogeneous than previously thought, displaying contrasting spatial patterns in both circulation and temperature, as well as large decadal variability. In particular, there was an increase of northerly winds favoring colder winters in the first half of the LMM, but enhanced southerlies contributing to milder conditions in the second half of the LMM. The analysis of the atmospheric circulation yields a new and complete classification of LMM winters. The temperature inferred from the atmospheric circulation confirms the majority of extremely cold winters well documented in the literature, while uncovering other less documented cold and mild winters. The results also suggest a nonstationarity of the North Atlantic Oscillation (NAO) pattern within the LMM, with extremely cold winters being driven by negative phases of a “high zonal” NAO pattern and “low zonal” NAO patterns dominating during moderately cold winters.

scholarly journals On the Interpretation of the North Atlantic Averaged Sea Surface Temperature

2020 ◽  
Vol 33 (14) ◽  
pp. 6025-6045
Author(s):  
Jing Sun ◽  
Mojib Latif ◽  
Wonsun Park ◽  
Taewook Park
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Climate Models ◽  
Ocean Dynamics ◽  
Sea Surface ◽  
The North ◽  
Sst Variability ◽  
The North Atlantic ◽  
Different Time Scales

AbstractThe North Atlantic (NA) basin-averaged sea surface temperature (NASST) is often used as an index to study climate variability in the NA sector. However, there is still some debate on what drives it. Based on observations and climate models, an analysis of the different influences on the NASST index and its low-pass filtered version, the Atlantic multidecadal oscillation (AMO) index, is provided. In particular, the relationships of the two indices with some of its mechanistic drivers including the Atlantic meridional overturning circulation (AMOC) are investigated. In observations, the NASST index accounts for significant SST variability over the tropical and subpolar NA. The NASST index is shown to lump together SST variability originating from different mechanisms operating on different time scales. The AMO index emphasizes the subpolar SST variability. In the climate models, the SST-anomaly pattern associated with the NASST index is similar. The AMO index, however, only represents pronounced SST variability over the extratropical NA, and this variability is significantly linked to the AMOC. There is a sensitivity of this linkage to the cold NA SST bias observed in many climate models. Models suffering from a large cold bias exhibit a relatively weak linkage between the AMOC and AMO and vice versa. Finally, the basin-averaged SST in its unfiltered form, which has been used to question a strong influence of ocean dynamics on NA SST variability, mixes together multiple types of variability occurring on different time scales and therefore underemphasizes the role of ocean dynamics in the multidecadal variability of NA SSTs.

scholarly journals Variable External Forcing Obscures the Weak Relationship between the NAO and North Atlantic Multidecadal SST Variability

2019 ◽  
Vol 32 (13) ◽  
pp. 3847-3864 ◽  
Author(s):  
Jeremy M. Klavans ◽  
Amy C. Clement ◽  
Mark A. Cane
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
External Forcing ◽  
Atlantic Multidecadal Variability ◽  
The North ◽  
Sst Variability ◽  
Atmospheric Noise ◽  
The North Atlantic Oscillation ◽  
The Relationship ◽  
Spurious Signals

Abstract North Atlantic sea surface temperatures (SST) exhibit a lagged response to the North Atlantic Oscillation (NAO) in both models and observations, which has previously been attributed to changes in ocean heat transport. Here we examine the lagged relationship between the NAO and Atlantic multidecadal variability (AMV) in the context of the two other major components of the AMV: atmospheric noise and external forcing. In preindustrial control runs, we generally find that after accounting for spurious signals introduced by filtering, the SST response to the NAO is only statistically significant in the subpolar gyre. Further, the lagged SST response to the NAO is small in magnitude and offers a limited contribution to the AMV pattern, statistics, or predictability. When climate models include variable external forcing, the relationship between the NAO and AMV is obscured and becomes inconsistent. In these historically forced runs, knowledge of the prior NAO offers reduced predictability. The differences between the preindustrial and the historically forced ensembles suggest that we do not yet have enough observational data to surmise the true NAO–AMV relationship and add evidence that external forcing plays a substantial role in producing the AMV.

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

2004 ◽  
Vol 31 (9) ◽  
pp. n/a-n/a ◽  
Author(s):  
Zhaohua Wu ◽  
E. K. Schneider ◽  
B. P. Kirtman
Keyword(s):  
North Atlantic ◽  
Low Frequency ◽  
Coupled Gcm ◽  
Sst Variability

scholarly journals Arctic/Atlantic Exchanges via the Subpolar Gyre*

2012 ◽  
Vol 25 (7) ◽  
pp. 2421-2439 ◽  
Author(s):  
Helene R. Langehaug ◽  
Iselin Medhaug ◽  
Tor Eldevik ◽  
Odd Helge Otterå
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Decadal Variability ◽  
Sea Water ◽  
Similar Response ◽  
Subpolar Gyre ◽  
North Atlantic Current ◽  
Atmospheric Variability ◽  
The North ◽  
The North Atlantic

Abstract In the present study the decadal variability in the strength and shape of the subpolar gyre (SPG) in a 600-yr preindustrial simulation using the Bergen Climate Model is investigated. The atmospheric influence on the SPG strength is reflected in the variability of Labrador Sea Water (LSW), which is largely controlled by the North Atlantic Oscillation, the first mode of the North Atlantic atmospheric variability. A combination of the amount of LSW, the overflows from the Nordic seas, and the second mode of atmospheric variability, the East Atlantic Pattern, explains 44% of the modeled decadal variability in the SPG strength. A prior increase in these components leads to an intensified SPG in the western subpolar region. Typically, an increase of one standard deviation (std dev) of the total overflow (1 std dev = 0.2 Sv; 1 Sv ≡ 106 m3 s−1) corresponds to an intensification of about one-half std dev of the SPG strength (1 std dev = 2 Sv). A similar response is found for an increase of one std dev in the amount of LSW, and simultaneously the strength of the North Atlantic Current increases by one-half std dev (1 std dev = 0.9 Sv).

scholarly journals Atmospheric Circulation Regimes in a Nonlinear Quasi-Geostrophic Model

2015 ◽  
Vol 2015 ◽  
pp. 1-19 ◽  
Author(s):  
Henriette Labsch ◽  
Dörthe Handorf ◽  
Klaus Dethloff ◽  
Michael V. Kurgansky
Keyword(s):  
Atmospheric Circulation ◽  
Arctic Oscillation ◽  
Significant Contribution ◽  
Decadal Variability ◽  
Low Frequency ◽  
The Arctic ◽  
Satisfactory Model ◽  
Low Frequency Variability ◽  
Interannual And Decadal Variability ◽  
The Arctic Oscillation

Atmospheric low-frequency variability and circulation regime behavior are investigated in the context of a quasi-geostrophic (QG) three-level T63 model of the wintertime atmospheric circulation over the Northern Hemisphere (NH). The model generates strong interannual and decadal variability, with the domination of the annular mode of variability. It successfully reproduces a satisfactory model climatology and the most important atmospheric circulation regimes. The positive phase of the Arctic Oscillation is a robust feature of the quasi-geostrophic T63 model. The model results based on QG dynamics underlie atmospheric regime behavior in the extratropical NH and suggest that nonlinear internal processes deliver significant contribution to the atmospheric climate variability on interannual and decadal timescales.

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