scholarly journals Linking ITCZ Migrations to the AMOC and North Atlantic/Pacific SST Decadal Variability

2020 ◽  
Vol 33 (3) ◽  
pp. 893-905 ◽  
Author(s):  
E. Moreno-Chamarro ◽  
J. Marshall ◽  
T. L. Delworth
Keyword(s):  
Heat Transport ◽  
Time Scales ◽  
North Atlantic ◽  
Time Scale ◽  
Climate Model ◽  
Decadal Variability ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Long Time ◽  
Underlying Mechanisms

AbstractWe examine the link between migrations in the intertropical convergence zone (ITCZ) and changes in the Atlantic meridional overturning circulation (AMOC), Atlantic multidecadal variability (AMV), and Pacific decadal oscillation (PDO). We use a coupled climate model that allows us to integrate over climate noise and assess underlying mechanisms. We use an ensemble of ten 300-yr-long simulations forced by a 50-yr oscillatory North Atlantic Oscillation (NAO)-derived surface heat flux anomaly in the North Atlantic, and a 4000-yr-long preindustrial control simulation performed with GFDL CM2.1. In both setups, an AMV phase change induced by a change in the AMOC’s cross-equatorial heat transport forces an atmospheric interhemispheric energy imbalance that is compensated by a change in the cross-equatorial atmospheric heat transport due to a meridional ITCZ shift. Such linkages occur on decadal time scales in the ensemble driven by the imposed forcing, and internally on multicentennial time scales in the control. Regional precipitation anomalies differ between the ensemble and the control for a zonally averaged ITCZ shift of similar magnitude, which suggests a dependence on time scale. Our study supports observational evidence of an AMV–ITCZ link in the twentieth century and further links it to the AMOC, whose long-time-scale variability can influence the phasing of ITCZ migrations. In contrast to the AMV, our calculations suggest that the PDO does not drive ITCZ migrations, because the PDO does not modulate the interhemispheric energy balance.

scholarly journals Links between the Southern Annular Mode and the Atlantic Meridional Overturning Circulation in a Climate Model

2011 ◽  
Vol 24 (3) ◽  
pp. 624-640 ◽  
Author(s):  
Camille Marini ◽  
Claude Frankignoul ◽  
Juliette Mignot
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
Time Scale ◽  
Climate Model ◽  
Drake Passage ◽  
Atlantic Meridional Overturning Circulation ◽  
Southern Annular Mode ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract The links between the atmospheric southern annular mode (SAM), the Southern Ocean, and the Atlantic meridional overturning circulation (AMOC) at interannual to multidecadal time scales are investigated in a 500-yr control integration of the L’Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL CM4) climate model. The Antarctic Circumpolar Current, as described by its transport through the Drake Passage, is well correlated with the SAM at the yearly time scale, reflecting that an intensification of the westerlies south of 45°S leads to its acceleration. Also in phase with a positive SAM, the global meridional overturning circulation is modified in the Southern Hemisphere, primarily reflecting a forced barotropic response. In the model, the AMOC and the SAM are linked at several time scales. An intensification of the AMOC lags a positive SAM by about 8 yr. This is due to a correlation between the SAM and the atmospheric circulation in the northern North Atlantic that reflects a symmetric ENSO influence on the two hemispheres, as well as an independent, delayed interhemispheric link driven by the SAM. Both effects lead to an intensification of the subpolar gyre and, by salinity advection, increased deep convection and a stronger AMOC. A slower oceanic link between the SAM and the AMOC is found at a multidecadal time scale. Salinity anomalies generated by the SAM enter the South Atlantic from the Drake Passage and, more importantly, the Indian Ocean; they propagate northward, eventually reaching the northern North Atlantic where, for a positive SAM, they decrease the vertical stratification and thus increase the AMOC.

North Atlantic Multidecadal Climate Variability: An Investigation of Dominant Time Scales and Processes

2010 ◽  
Vol 23 (13) ◽  
pp. 3626-3638 ◽  
Author(s):  
Leela M. Frankcombe ◽  
Anna von der Heydt ◽  
Henk A. Dijkstra
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
Climate Model ◽  
Internal Variability ◽  
Meridional Overturning Circulation ◽  
Theoretical Point ◽  
Intergovernmental Panel ◽  
Multidecadal Variability ◽  
The North ◽  
The North Atlantic

Abstract The issue of multidecadal variability in the North Atlantic has been an important topic of late. It is clear that there are multidecadal variations in several climate variables in the North Atlantic, such as sea surface temperature and sea level height. The details of this variability, in particular the dominant patterns and time scales, are confusing from both an observational as well as a theoretical point of view. After analyzing results from observational datasets and a 500-yr simulation of an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate model, two dominant time scales (20–30 and 50–70 yr) of multidecadal variability in the North Atlantic are proposed. The 20–30-yr variability is characterized by the westward propagation of subsurface temperature anomalies. The hypothesis is that the 20–30-yr variability is caused by internal variability of the Atlantic Meridional Overturning Circulation (MOC) while the 50–70-yr variability is related to atmospheric forcing over the Atlantic Ocean and exchange processes between the Atlantic and Arctic Oceans.

Assessing temperature fingerprints for the Atlantic overturning in the past two millennia

2020 ◽  
Author(s):  
Reyhan Shirin Ermis ◽  
Paola Moffa-Sánchez ◽  
Alexandra Jahn ◽  
Kira Rehfeld
Keyword(s):  
Time Scales ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Time Scale ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Surface Temperatures ◽  
Temperature Reconstructions ◽  
The North Atlantic

<p>The Atlantic Meridional Overturning Circulation (AMOC) is essential to maintain the temperate climates of Europe and North America. It redistributes heat from the tropics, and stores carbon in the deep ocean. Yet, its variability and evolution are largely unknown due to the lack of long-term direct circulation measurements. Previous studies suggest a connection between the variability of the AMOC strength and a temperature dipole in the North Atlantic. These results suggest a substantial decline in the strength of the overturning at the onset of the industrial era. </p><p>Here we compare temperature reconstructions from four sediment cores in the North Atlantic with model simulations of the Community Earth System Model (CESM1) as well as the Hadley Centre Coupled Model (HadCM3) over the Common Era. By examining the correlation between the surface temperatures in the North Atlantic and the strength of the overturning we test the robustness of previously used temperature fingerprints. Analysing variability in the surface and subsurface temperatures as well as the overturning strength in models we assess possible drivers of variability in ocean circulation. We compare the persistence times and the time scale dependent variability of the AMOC, the surface and ocean temperatures in the model with those in the temperature reconstructions. The sub-surface reconstructions match with the 200m ocean temperatures in persistence times but not with the AMOC in the models. The surface temperatures in the models show persistence times similar to those obtained for the AMOC. However, time scale dependent variabilities in the surface temperatures do not match those found the AMOC. Therefore, temperature fingerprints might not be a reliable basis to reconstruct the ocean overturning strength.</p><p>Due to the systematic comparison of two models on different time scales and an assessment of surface to sub-surface temperatures this study could provide new insights into the variability of Atlantic overturning on decadal time scales and beyond.</p>

scholarly journals Advective Time Scales of Agulhas Leakage to the North Atlantic in Surface Drifter Observations and the 3D OFES Model

2011 ◽  
Vol 41 (5) ◽  
pp. 1026-1034 ◽  
Author(s):  
Erik van Sebille ◽  
Lisa M. Beal ◽  
William E. Johns
Keyword(s):  
High Resolution ◽  
Time Scales ◽  
Transit Time ◽  
North Atlantic ◽  
Time Scale ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Surface Drifter ◽  
The North Atlantic ◽  
Lagrangian Data

Abstract The advective transit time of temperature–salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, because it has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult, because most observational and high-resolution model data are too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of “supertrajectories” of any required length using a Monte Carlo simulation. These supertrajectories allow analysis of the circulation patterns and time scales based on Lagrangian data: in this case, observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OGCM for the Earth Simulator (OFES). The observational supertrajectories can only be used to study the two-dimensional (2D) surface flow, whereas the numerical supertrajectories can be used to study the full three-dimensional circulation. Results for the surface circulation indicate that the supertrajectories starting in the Agulhas Current and ending in the North Atlantic take at least 4 yr and most complete the journey in 30–40 yr. This time scale is, largely because of convergence and subduction in the subtropical gyres, longer than the 10–25 yr it takes the 3D numerical supertrajectories to complete the journey.

scholarly journals A Decadal-Scale Teleconnection between the North Atlantic Oscillation and Subtropical Eastern Australian Rainfall

2015 ◽  
Vol 28 (3) ◽  
pp. 1074-1092 ◽  
Author(s):  
Cheng Sun ◽  
Jianping Li ◽  
Juan Feng ◽  
Fei Xie
Keyword(s):  
North Atlantic Oscillation ◽  
Linear Model ◽  
Time Scales ◽  
North Atlantic ◽  
Decadal Variability ◽  
Meridional Overturning Circulation ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation ◽  
Australian Rainfall

Abstract The time series of twentieth-century subtropical eastern Australian rainfall (SEAR) shows evident fluctuations over decadal to multidecadal time scales. Using observations from the period 1900–2013, it was found that SEAR is connected to the North Atlantic Oscillation (NAO) over decadal time scales, with the NAO leading by around 15 yr. The physical mechanism underlying this relationship was investigated. The NAO can have a delayed impact on sea surface temperature (SST) fluctuations in the subpolar Southern Ocean (SO), and these SST changes could in turn contribute to the decadal variability in SEAR through their impacts on the Southern Hemisphere atmospheric circulation. This observed lead of the NAO relative to SO SST and the interhemispheric SST seesaw mechanism are reasonably reproduced in a long-term control simulation of an ocean–atmosphere coupled model. The NAO exerts a delayed effect on the variation of Atlantic meridional overturning circulation that further induces seesaw SST anomalies in the subpolar North Atlantic and SO. With evidence that the NAO precedes SEAR decadal variability via a delayed SO bridge, a linear model for SEAR decadal variability was developed by combination of the NAO and Pacific decadal oscillation (PDO). The observed SEAR decadal variability is considerably well simulated by the linear model, and the relationship between the simulation and observation is stable. SEAR over the coming decade may increase slightly, because of the recent NAO weakening and the return of negative PDO phase.

scholarly journals Impact of a realistic Greenland ice sheet melting on the North Atlantic over the period 1920-2014

2020 ◽  
Author(s):  
Didier Swingedouw ◽  
Marion Devilliers ◽  
Juliette Mignot ◽  
Julie Deshayes ◽  
Gilles Garric ◽  
...  
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Decadal Variability ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Subpolar Gyre ◽  
Nordic Seas ◽  
The North ◽  
The North Atlantic

<p>Greenland experienced intensive melting over the last century, especially in the 1920s and over the last decades. The supplementary input into the ocean is influencing the freshwater budget of the North Atlantic. Simultaneously, some signs of a recent weakening of the Atlantic meridional overturning circulation (AMOC) have been reported. In order to better understand the possible impact of the melting on the North Atlantic circulation, salinity and temperature trends, we construct an observation-based estimate of the freshwater fluxes from 1840 to 2014 associated to the runoff fluxes from Greenland ice sheet and surrounding glaciers and ice caps. Input from iceberg melting is also included and spatially distributed over the North Atlantic following an observed climatology. We force historical simulations of the IPSL-CM6A-LR coupled climate model with this reconstruction from 1920 to 2014. The 10-member ensemble mean displays freshened and cooled waters around Greenland, which spread in the subpolar gyre, and then towards the subtropical gyre and the Nordic Seas. Over the whole period, the convection is reduced in the Labrador and Nordic Seas, while it is slightly enhanced in the Irminger Sea, and the AMOC is reduced by 0.32±0.35 Sv at 26°N. This highlights that the AMOC decrease due to Greenland melting remains modest in these simulations and can only explain a very moderate amount of the 3±1 Sv weakening suggested in a recent study. The multi-decadal trend of the North Atlantic surface temperature obtained with the additional freshwater forcing is more in line with observations than in standard historical simulations. We also show a clear improvement of the representation of the 1995 abrupt warming in the subpolar gyre in the melting ensemble, which may thus be partly forced by Greenland ice sheet melting. Mechanisms at play imply changes in the variability of the AMOC in the melting ensemble as compared to the historical one. Such an impact on forced decadal variability has crucial consequences for decadal prediction systems that may gain skill by including observed Greenland ice sheet melting.</p>

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

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).

scholarly journals The Impact of the North Atlantic Oscillation on Climate through Its Influence on the Atlantic Meridional Overturning Circulation

2016 ◽  
Vol 29 (3) ◽  
pp. 941-962 ◽  
Author(s):  
Thomas L. Delworth ◽  
Fanrong Zeng
Keyword(s):  
North Atlantic ◽  
Time Scale ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Hemispheric Temperature ◽  
Meridional Overturning ◽  
The North Atlantic Oscillation ◽  
The Impact ◽  
Model Ocean

Abstract The impact of the North Atlantic Oscillation (NAO) on the Atlantic meridional overturning circulation (AMOC) and large-scale climate is assessed using simulations with three different climate models. Perturbation experiments are conducted in which a pattern of anomalous heat flux corresponding to the NAO is added to the model ocean. Differences between the perturbation experiments and a control illustrate how the model ocean and climate system respond to the NAO. A positive phase of the NAO strengthens the AMOC by extracting heat from the subpolar gyre, thereby increasing deep-water formation, horizontal density gradients, and the AMOC. The flux forcings have the spatial structure of the observed NAO, but the amplitude of the forcing varies in time with distinct periods varying from 2 to 100 yr. The response of the AMOC to NAO variations is small at short time scales but increases up to the dominant time scale of internal AMOC variability (20–30 yr for the models used). The amplitude of the AMOC response, as well as associated oceanic heat transport, is approximately constant as the time scale of the forcing is increased further. In contrast, the response of other properties, such as hemispheric temperature or Arctic sea ice, continues to increase as the time scale of the forcing becomes progressively longer. The larger response is associated with the time integral of the anomalous oceanic heat transport at longer time scales, combined with an increased impact of radiative feedback processes. It is shown that NAO fluctuations, similar in amplitude to those observed over the last century, can modulate hemispheric temperature by several tenths of a degree.

Subpolar Gyre – AMOC – Atmosphere Interactions on Multidecadal Timescales in a Version of the Kiel Climate Model

2021 ◽  
Author(s):  
Jing Sun ◽  
Mojib Latif ◽  
Wonsun Park
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Subpolar Gyre ◽  
Surface Salinity ◽  
The North ◽  
Ocean Coupling ◽  
Meridional Overturning ◽  
The North Atlantic

<p>There is a controversy about the nature of multidecadal climate variability in the North Atlantic (NA) region, concerning the roles of ocean circulation and atmosphere-ocean coupling. Here we describe NA multidecadal variability from a version of the Kiel Climate Model, in which both subpolar gyre (SPG)-Atlantic Meridional Overturning Circulation (AMOC) and atmosphere-ocean coupling are essential. The oceanic barotropic streamfuntions, meridional overturning streamfunctions, and sea level pressure are jointly analyzed to derive the leading mode of Atlantic variability. This mode accounting for about 23.7 % of the total combined variance is oscillatory with an irregular periodicity of 25-50 years and an e-folding time of about a decade. SPG and AMOC mutually influence each other and together provide the delayed negative feedback necessary for maintaining the oscillation. An anomalously strong SPG, for example, drives higher surface salinity and density in the NA’s sinking region. In response, oceanic deep convection and AMOC intensify, which, with a time delay of about a decade, reduces SPG strength by enhancing upper-ocean heat content. The weaker gyre circulation leads to lower surface salinity and density in the sinking region, which eventually reduces deep convection and AMOC strength. There is a positive ocean-atmosphere feedback between the sea surface temperature and low-level atmospheric circulation over the Southern Greenland area, with related wind stress changes reinforcing SPG changes, thereby maintaining the (damped) multidecadal oscillation against dissipation. Stochastic surface heat-flux forcing associated with the North Atlantic Oscillation drives the eigenmode.</p>

scholarly journals Fingerprints of changes in the terrestrial carbon cycle in response to large reorganizations in ocean circulation

2010 ◽  
Vol 6 (5) ◽  
pp. 1811-1852 ◽  
Author(s):  
A. Bozbiyik ◽  
M. Steinacher ◽  
F. Joos ◽  
T. F. Stocker
Keyword(s):  
Carbon Cycle ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Carbon Stocks ◽  
Meridional Overturning Circulation ◽  
Climate Conditions ◽  
Tropical Regions ◽  
The North ◽  
Northern Latitudes

Abstract. CO2 and carbon cycle changes in the land, ocean and atmosphere are investigated using the comprehensive carbon cycle-climate model NCAR CSM1.4-carbon. Ensemble simulations are forced with freshwater perturbations applied at the North Atlantic and Southern Ocean deep water formation sites under pre-industrial climate conditions. As a result, the Atlantic Meridional Overturning Circulation reduces in each experiment to varying degrees. The physical climate fields show changes that are well documented in the literature but there is a clear distinction between northern and southern perturbations. Changes in the physical variables affect, in return, the land and ocean biogeochemical cycles and cause a reduction, or an increase, in the atmospheric CO2 by up to 20 ppmv, depending on the location of the perturbation. In the case of a North Atlantic perturbation, the land biosphere reacts with a strong reduction in carbon stocks in some tropical locations and in high northern latitudes. In contrast, land carbon stocks tend to increase in response to a southern perturbation. The ocean is generally a sink of carbon although large re-organizations occur throughout various basins. The response of the land biosphere is strongest in the tropical regions due to a shift of the Intertropical Convergence Zone. The carbon fingerprints of this shift, either to the south or to the north depending on where the freshwater is applied, can be found most clearly in South America. For this reason, a compilation of various paleoclimate proxy records of Younger Dryas precipitation changes are compared with our model results.

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