scholarly journals North Atlantic Climate Response to Lake Agassiz Drainage at Coarse and Ocean Eddy-Permitting Resolutions

2013 ◽  
Vol 26 (8) ◽  
pp. 2651-2667 ◽  
Author(s):  
Paul Spence ◽  
Oleg A. Saenko ◽  
Willem Sijp ◽  
Matthew H. England
Keyword(s):  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Climate Response ◽  
Model Resolution ◽  
Lake Agassiz ◽  
The North ◽  
Freshwater Forcing ◽  
North Atlantic Climate ◽  
The North Atlantic

Abstract The North Atlantic climate response to the catastrophic drainage of proglacial Lake Agassiz into the Labrador Sea is analyzed with coarse and ocean eddy-permitting versions of a global coupled climate model. The North Atlantic climate response is qualitatively consistent in that a large-scale cooling is simulated regardless of the model resolution or region of freshwater discharge. However, the magnitude and duration of the North Atlantic climate response is found to be sensitive to model resolution and the location of freshwater forcing. In particular, the long-term entrainment of freshwater along the boundary at higher resolution and its gradual, partially eddy-driven escape into the interior leads to low-salinity anomalies persisting in the subpolar Atlantic for decades longer. As a result, the maximum decline of the Atlantic meridional overturning circulation (AMOC) and the ocean meridional heat transport (MHT) is amplified by about a factor of 2 at ocean eddy-permitting resolution, and the recovery is delayed relative to the coarse grid model. This, in turn, increases the long-term cooling in the high-resolution simulations. A decomposition of the MHT response reveals an increased role for transients and the horizontal mean component of MHT at higher resolution. With fixed wind stress curl, it is a stronger response of bottom pressure torque to the freshwater forcing at higher resolution that leads to a larger anomaly of the depth-integrated circulation.

scholarly journals Glacial fluctuations of the Indian monsoon and their relationship with North Atlantic climate: new data and modelling experiments

2013 ◽  
Vol 9 (5) ◽  
pp. 2135-2151 ◽  
Author(s):  
C. Marzin ◽  
N. Kallel ◽  
M. Kageyama ◽  
J.-C. Duplessy ◽  
P. Braconnot
Keyword(s):  
North Atlantic ◽  
Bay Of Bengal ◽  
Indian Monsoon ◽  
Hydrological Cycle ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The North ◽  
North Atlantic Climate ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. Several paleoclimate records such as from Chinese loess, speleothems or upwelling indicators in marine sediments present large variations of the Asian monsoon system during the last glaciation. Here, we present a new record from the northern Andaman Sea (core MD77-176) which shows the variations of the hydrological cycle of the Bay of Bengal. The high-resolution record of surface water δ18O dominantly reflects salinity changes and displays large millennial-scale oscillations over the period 40 000 to 11 000 yr BP. Their timing and sequence suggests that events of high (resp. low) salinity in the Bay of Bengal, i.e. weak (resp. strong) Indian monsoon, correspond to cold (resp. warm) events in the North Atlantic and Arctic, as documented by the Greenland ice core record. We use the IPSL_CM4 Atmosphere-Ocean coupled General Circulation Model to study the processes that could explain the teleconnection between the Indian monsoon and the North Atlantic climate. We first analyse a numerical experiment in which such a rapid event in the North Atlantic is obtained under glacial conditions by increasing the freshwater flux in the North Atlantic, which results in a reduction of the intensity of the Atlantic meridional overturning circulation. This freshwater hosing results in a weakening of the Indian monsoon rainfall and circulation. The changes in the continental runoff and local hydrological cycle are responsible for an increase in salinity in the Bay of Bengal. This therefore compares favourably with the new sea water δ18O record presented here and the hypothesis of synchronous cold North Atlantic and weak Indian monsoon events. Additional sensitivity experiments are produced with the LMDZ atmospheric model to analyse the teleconnection mechanisms between the North Atlantic and the Indian monsoon. The changes over the tropical Atlantic are shown to be essential in triggering perturbations of the subtropical jet over Africa and Eurasia, that in turn affect the intensity of the Indian monsoon. These relationships are also found to be valid in additional coupled model simulations in which the Atlantic meridional overturning circulation (AMOC) is forced to resume.

scholarly journals How Much Has the North Atlantic Ocean Overturning Circulation Changed in the Last 50 Years?

2014 ◽  
Vol 27 (16) ◽  
pp. 6325-6342 ◽  
Author(s):  
Simon F. B. Tett ◽  
Toby J. Sherwin ◽  
Amrita Shravat ◽  
Oliver Browne
Keyword(s):  
North Atlantic ◽  
Mixed Layer Depth ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
The North ◽  
Ocean Reanalyses ◽  
The North Atlantic

Abstract Volume transports from six ocean reanalyses are compared with four sets of in situ observations: across the Greenland–Scotland ridge (GSR), in the Labrador Sea boundary current, in the deep western boundary current at 43°N, and in the Atlantic meridional overturning circulation (AMOC) at 26°N in the North Atlantic. The higher-resolution reanalyses (on the order of ¼° × ¼°) are better at reproducing the circulation pattern in the subpolar gyre than those with lower resolution (on the order of 1°). Simple Ocean Data Assimilation (SODA) and Estimating the Circulation and Climate of the Ocean (ECCO)–Jet Propulsion Laboratory (JPL) produce transports at 26°N that are close to those observed [17 Sv (1 Sv ≡ 106 m3 s−1)]. ECCO, version 2, and SODA produce northward transports across the GSR (observed transport of 8.2 Sv) that are 22% and 29% too big, respectively. By contrast, the low-resolution reanalyses have transports that are either too small [by 31% for ECCO-JPL and 49% for Ocean Reanalysis, system 3 (ORA-S3)] or much too large [Decadal Prediction System (DePreSys)]. SODA had the best simulations of mixed layer depth and with two coarse grid long-term reanalyses (DePreSys and ORA-S3) is used to examine changes in North Atlantic circulation from 1960 to 2008. Its results suggest that the AMOC increased by about 20% at 26°N while transport across the GSR hardly altered. The other (less reliable) long-term reanalyses also had small changes across the GSR but changes of +10% and −20%, respectively, at 26°N. Thus, it appears that changes in the overturning circulation at 26°N are decoupled from the flow across the GSR. It is recommended that transport observations should not be assimilated in ocean reanalyses but used for validation instead.

scholarly journals Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing

2016 ◽  
Vol 12 (8) ◽  
pp. 1663-1679 ◽  
Author(s):  
Nicolas Brown ◽  
Eric D. Galbraith
Keyword(s):  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Abrupt Climate Change ◽  
Freshwater Input ◽  
The North ◽  
Freshwater Forcing ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
Background Climate

Abstract. It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through “hosing” experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we describe spontaneous, unhosed oscillations in AMOC strength that occur in a global coupled ocean–atmosphere model when integrated under a particular background climate state. We compare these unhosed oscillations to hosed oscillations under a range of background climate states in order to examine how the global imprint of AMOC variations depends on whether or not they result from external freshwater input. Our comparison includes surface air temperature, precipitation, dissolved oxygen concentrations in the intermediate-depth ocean, and marine export production. The results show that the background climate state has a significant impact on the character of the freshwater-forced AMOC interruptions in this model, with particularly marked variations in tropical precipitation and in the North Pacific circulation. Despite these differences, the first-order patterns of response to AMOC interruptions are quite consistent among all simulations, implying that the ocean–sea ice–atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. Nonetheless, freshwater addition leads to a more complete shutdown of the AMOC than occurs in the unhosed oscillations, with amplified global impacts, evocative of Heinrich stadials. In addition, freshwater inputs can directly impact the strength of other polar haloclines, particularly that of the Southern Ocean, to which freshwater can be transported relatively quickly after injection in the North Atlantic.

The Adequacy of Observing Systems in Monitoring the Atlantic Meridional Overturning Circulation and North Atlantic Climate

2010 ◽  
Vol 23 (19) ◽  
pp. 5311-5324 ◽  
Author(s):  
S. Zhang ◽  
A. Rosati ◽  
T. Delworth
Keyword(s):  
North Atlantic ◽  
Coupled System ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
The North ◽  
North Atlantic Climate ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
Observing Systems

Abstract The Atlantic meridional overturning circulation (AMOC) has an important influence on climate, and yet adequate observations of this circulation are lacking. Here, the authors assess the adequacy of past and current widely deployed routine observing systems for monitoring the AMOC and associated North Atlantic climate. To do so, this study draws on two independent simulations of the twentieth century using an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled climate model. One simulation is treated as “truth” and is sampled according to the observing system being evaluated. The authors then assimilate these synthetic “observations” into the second simulation within a fully coupled system that instantaneously exchanges information among all coupled components and produces a nearly balanced and coherent estimate for global climate states including the North Atlantic climate system. The degree to which the assimilation recovers the truth is an assessment of the adequacy of the observing system being evaluated. As the coupled system responds to the constraint of the atmosphere or ocean, the assessment of the recovery for climate quantities such as Labrador Sea Water (LSW) and the North Atlantic Oscillation increases the understanding of the factors that determine AMOC variability. For example, the low-frequency sea surface forcings provided by the atmospheric and sea surface temperature observations are found to excite a LSW variation that governs the long-time-scale variability of the AMOC. When the most complete modern observing system, consisting of atmospheric winds and temperature, is used along with Argo ocean temperature and salinity down to 2000 m, a skill estimate of AMOC reconstruction is 90% (out of 100% maximum). Similarly encouraging results hold for other quantities, such as the LSW. The past XBT observing system, in which deep-ocean temperature and salinity were not available, has a lesser ability to recover the truth AMOC (the skill is reduced to 52%). While these results raise concerns about the ability to properly characterize past variations of the AMOC, they also hold promise for future monitoring of the AMOC and for initializing prediction models.

scholarly journals Hosed vs. unhosed: global response to interruptions of the Atlantic Meridional Overturning, with and without freshwater forcing

2015 ◽  
Vol 11 (5) ◽  
pp. 4669-4700 ◽  
Author(s):  
N. Brown ◽  
E. D. Galbraith
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Abrupt Climate Change ◽  
Freshwater Input ◽  
Global Response ◽  
The North ◽  
Freshwater Forcing ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through "hosing" experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we take advantage of a global coupled ocean–atmosphere model that exhibits spontaneous, unhosed oscillations in AMOC strength, in order to examine how the global imprint of AMOC variations depends on whether or not it is the result of external freshwater input. The results imply that, to first order, the ocean–ice–atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. The exception lies in the impact freshwater inputs can have on the strength of other polar haloclines, particularly the Southern Ocean, to which freshwater can be transported relatively quickly after injection in the North Atlantic.

scholarly journals Freshwater Discharge, Sediment Transport, and Modeled Climate Impacts of the Final Drainage of Glacial Lake Agassiz

2009 ◽  
Vol 22 (8) ◽  
pp. 2161-2180 ◽  
Author(s):  
Garry K. C. Clarke ◽  
Andrew B. G. Bush ◽  
John W. M. Bush
Keyword(s):  
North Atlantic ◽  
Climate Impacts ◽  
Meridional Overturning Circulation ◽  
Glacial Lake ◽  
Lake Agassiz ◽  
Cold Event ◽  
The North ◽  
Glacial Lake Agassiz ◽  
The North Atlantic ◽  
Ice Production

Abstract A cold event at around 8200 calendar years BP and the release, at around that time, of a huge freshwater outburst from ice-dammed glacial Lake Agassiz have lent support to the idea that the flood triggered the cold event. Some suggest that the freshwater addition caused a weakening of the North Atlantic meridional overturning circulation (MOC) thereby reducing the ocean transport of heat to high northern latitudes. Although several modeling efforts lend strength to this claim, the paleoceanographic record is equivocal. The authors’ aim is to use a coupled ocean–atmosphere model to examine the possibility that the two events are causally linked but that MOC reduction was not the main agent of change. It is found that the outburst flood and associated redirection of postflood meltwater drainage to the Labrador Sea, via Hudson Strait, can freshen the North Atlantic, leading to reduced salinity and sea surface temperature, and thus to increased sea ice production at high latitudes. The results point to the possibility that the preflood outflow to the St. Lawrence was extremely turbid and sufficiently dense to become hyperpycnal, whereas the postflood outflow through Hudson Strait had a lower load of suspended sediment and was buoyant.

Aircraft campaigns in support of the North Atlantic Climate System Integrated Studies (ACSIS)

2020 ◽  
Author(s):  
Alexander Archibald ◽  
Keyword(s):  
North Atlantic ◽  
Repeated Measurements ◽  
Climate System ◽  
Atmospheric Composition ◽  
Sea Ice Thickness ◽  
The North ◽  
Climate Interactions ◽  
North Atlantic Climate ◽  
The North Atlantic

<p>The North Atlantic is witnessing major changes during the Anthropocene. These include changes in the physical climate system: in ocean and atmosphere temperatures and circulation; in sea ice thickness and extent; and in atmospheric composition, where ozone, ozone precursors and aerosols have undergone significant changes over the last few decades. Changes in aerosols over the North Atlantic have been linked to changes in sea surface temperatures (SST) and North Atlantic climate variability. A long-term research project, The North Atlantic Climate System Integrated Study (ACSIS), involving data collection and interpretation, has begun to better understand the processes and composition-climate interactions associated with these changes. Here we report on one of the major observational components of the ACSIS programme which involves repeated measurements of the composition of the North Atlantic using the NERC FAAM BAe146. To date six campaigns have taken place including three which coincided with the NASA ATom campaigns (2-4). </p><p><br>In this presentation we will discuss the rationale for the aircraft project and recent results including the observation of transport of biomass burning plumes into the North Atlantic that are estimated to have originated from fires sampled as part of the NASA FIREX campaigns during the summer of 2019. We will highlight results from an intercomparison with the NASA DC-8 during our second campaign and ATom 3, which reveal good agreement in measurements of O3, CO and NOx between the two aircraft but large differences in measurements of non-methane VOCs, and we will summarise our results to-date including the comparison against chemical transport models. </p><p> </p>

scholarly journals Comparing the spatial patterns of climate change in the 9th and 5th millennia BP from TRACE-21 model simulations

2019 ◽  
Vol 15 (1) ◽  
pp. 41-52 ◽  
Author(s):  
Liang Ning ◽  
Jian Liu ◽  
Raymond S. Bradley ◽  
Mi Yan
Keyword(s):  
Northern Hemisphere ◽  
Spatial Patterns ◽  
North Atlantic ◽  
Volcanic Eruptions ◽  
Meridional Overturning Circulation ◽  
Climate System Model ◽  
Model Simulations ◽  
The North ◽  
The North Atlantic

Abstract. The spatial patterns of global temperature and precipitation changes, as well as corresponding large-scale circulation patterns during the latter part of the 9th and 5th millennia BP (4800–4500 versus 4500–4000 BP and 9200–8800 versus 8800–8000 BP) are compared through a group of transient simulations using the Community Climate System Model version 3 (CCSM3). Both periods are characterized by significant sea surface temperature (SST) decreases over the North Atlantic, south of Iceland. Temperatures were also colder across the Northern Hemisphere but warmer in the Southern Hemisphere. Significant precipitation decreases are seen over most of the Northern Hemisphere, especially over Eurasia and the Asian monsoon regions, indicating a weaker summer monsoon. Large precipitation anomalies over northern South America and adjacent ocean regions are related to a southward displacement of the Intertropical Convergence Zone (ITCZ) in that region. Climate changes in the late 9th millennium BP (the “8.2 ka event”) are widely considered to have been caused by a large freshwater discharge into the northern Atlantic, which is confirmed in a meltwater forcing sensitivity experiment, but this was not the cause of changes occurring between the early and latter halves of the 5th millennium BP. Model simulations suggest that a combination of factors, led by long-term changes in insolation, drove a steady decline in SSTs across the North Atlantic and a reduction in the North Atlantic Meridional Overturning Circulation (AMOC), over the past 4500 years, with associated teleconnections across the globe, leading to drought in some areas. Multi-century-scale fluctuations in SSTs and AMOC strength were superimposed on this decline. This helps explain the onset of neoglaciation around 5000–4500 BP, followed by a series of neoglacial advances and retreats during recent millennia. The “4.2 ka BP Event” appears to have been one of several late Holocene multi-century fluctuations that were embedded in the long-term, low-frequency change in climate that occurred after ∼4.8 ka. Whether these multi-century fluctuations were a response to internal centennial-scale ocean–atmosphere variability or external forcing (such as explosive volcanic eruptions and associated feedbacks) or a combination of such conditions is not known and requires further study.

Preaching and Sermons

2017 ◽  
Author(s):  
Robert H. Ellison
Keyword(s):  
Eighteenth Century ◽  
North Atlantic ◽  
Religious Revival ◽  
Early Nineteenth Century ◽  
Denominational Identity ◽  
The North ◽  
The North Atlantic ◽  
The One ◽  
The Impact

Prompted by the convulsions of the late eighteenth century and inspired by the expansion of evangelicalism across the North Atlantic world, Protestant Dissenters from the 1790s eagerly subscribed to a millennial vision of a world transformed through missionary activism and religious revival. Voluntary societies proliferated in the early nineteenth century to spread the gospel and transform society at home and overseas. In doing so, they engaged many thousands of converts who felt the call to share their experience of personal conversion with others. Though social respectability and business methods became a notable feature of Victorian Nonconformity, the religious populism of the earlier period did not disappear and religious revival remained a key component of Dissenting experience. The impact of this revitalization was mixed. On the one hand, growth was not sustained in the long term and, to some extent, involvement in interdenominational activity undermined denominational identity; on the other hand, Nonconformists gained a social and political prominence they had not enjoyed since the middle of the seventeenth century and their efforts laid the basis for the twentieth-century explosion of evangelicalism in Africa, Asia, and South America.

Sign in / Sign up

Export Citation Format

Share Document