The Atlantic Meridional Overturning Circulation (AMOC) is responsible for a variable and climatically important northward transport of heat. Using data from an array of instruments that span the ...Atlantic at 26°N, we show that the AMOC has been in a state of reduced overturning since 2008 as compared to 2004–2008. This change of AMOC state is concurrent with other changes in the North Atlantic such as a northward shift and broadening of the Gulf Stream and altered patterns of heat content and sea surface temperature. These changes resemble the response to a declining AMOC predicted by coupled climate models. Concurrent changes in air‐sea fluxes close to the western boundary reveal that the changes in ocean heat transport and sea surface temperature have altered the pattern of ocean‐atmosphere heat exchange over the North Atlantic. These results provide strong observational evidence that the AMOC is a major factor in decadal‐scale variability of North Atlantic climate.
Key Points
New data from the RAPID 26°N array show that the AMOC has been in a state of reduced overturning since mid‐2008
Observations of heat content and SSH indicate that the impact of the reduction in the AMOC is similar to that predicted by climate models
The results indicate that changes in ocean heat transport have altered ocean‐atmosphere heat exchange over the North Atlantic
The Atlantic meridional overturning circulation (AMOC) has been observed continuously at 26 degree N since April 2004. The AMOC and its component parts are monitored by combining a transatlantic ...array of moored instruments with submarine-cable-based measurements of the Gulf Stream and satellite derived Ekman transport. The time series has recently been extended to October 2012 and the results show a downward trend since 2004. From April 2008 to March 2012, the AMOC was an average of 2.7 Sv (1 Sv = 106 m3 s-1) weaker than in the first four years of observation (95% confidence that the reduction is 0.3 Sv or more). Ekman transport reduced by about 0.2 Sv and the Gulf Stream by 0.5 Sv but most of the change (2.0 Sv) is due to the mid-ocean geostrophic flow. The change of the mid-ocean geostrophic flow represents a strengthening of the southward flow above the thermocline. The increased southward flow of warm waters is balanced by a decrease in the southward flow of lower North Atlantic deep water below 3000 m. The transport of lower North Atlantic deep water slowed by 7% per year (95% confidence that the rate of slowing is greater than 2.5% per year).
The Gulf Stream plays an important role in North Atlantic climate variability on a range of timescales. The North Atlantic is notable for large decadal variability in sea surface temperatures (SST). ...Whether this variability is driven by atmospheric or oceanic influences is a disputed point. Long time series of atmospheric and ocean variables, in particular long time series of Gulf Stream position, reveal differing sources of SST variability on quasi‐decadal and multidecadal timescales. On quasi‐decadal timescales, an oscillatory signal identified in the North Atlantic Oscillation (NAO) controls SST evolution directly via air‐sea heat fluxes. However, on multidecadal timescales, this relationship between the NAO and SST changes, while the relationship between the NAO and Gulf Stream position remains consistent in phase and resonant in amplitude. Recent changes in the Gulf Stream Extension show a weakening and broadening of the current, consistent with increased instability. We consider these changes in the context of a weakening Atlantic overturning circulation.
Plain Language Summary
The North Atlantic Ocean is a region of remarkable variability in surface temperatures on timescales of decades and longer. Much debate surrounds whether this variability is driven by the atmosphere or by ocean currents, such as the Gulf Stream, moving heat around. In this study, we show that on timescales around 10 years, the atmosphere is the likely cause of Atlantic temperature variability but that this changes when multidecadal variability is considered. Changes ongoing in the Gulf Stream coincide with changes in the broader Atlantic—changes that imply a relatively cooler Atlantic in the coming decades.
Key Points
On quasi‐decadal timescales, NAO varies in phase with GSNW and in antiphase with Atlantic SSTs
On multidecadal timescales, NAO continues to vary in phase with GSNW, but the relationship to Atlantic SSTs has changed
The weakening and broadening of the Gulf Stream is consistent with increased instability since 2005 and not with a northward shift
Remote sensing observations and climate models indicate that the Greenland Ice Sheet (GrIS) has been losing mass since the late 1990s, mostly due to enhanced surface melting from rising summer ...temperatures. However, in situ observational records of GrIS melt rates over recent decades are rare. Here we develop a record of frozen meltwater in the west GrIS percolation zone preserved in seven firn cores. Quantifying ice layer distribution as a melt feature percentage (MFP), we find significant increases in MFP in the southernmost five cores over the past 50 years to unprecedented modern levels (since 1550 CE). Annual to decadal changes in summer temperatures and MFP are closely tied to changes in Greenland summer blocking activity and North Atlantic sea surface temperatures since 1870. However, summer warming of ~1.2°C since 1870–1900, in addition to warming attributable to recent sea surface temperature and blocking variability, is a critical driver of high modern MFP levels.
Plain Language Summary
Computer models and satellites show that the amount of snow melting each summer on Greenland has increased since the 1990s, but it is difficult to confirm this directly on the ice sheet. When surface snow melts, the water spreads into deeper layers of snow and refreezes as an ice layer. As fresh snow buries each summer's ice layers, the history of snowmelt is preserved in the ice sheet. We describe seven ice cores collected from western Greenland that contain the history of ice layers back to 1966. We find more ice layers, caused by more summer melting, since the 1990s. By comparing our ice cores to a longer ice core from the same area, we show that today's melt rates are the highest in this region since at least 1550 CE. Year‐to‐year changes in the amount of melting are mostly caused by changes in the number of summer high‐pressure systems and fluctuating ocean temperatures near Greenland. Although both of these processes have contributed to recent high melt rates, Greenland is 1.2°C warmer today than during similar conditions in the 1890s. This “extra” warming is most likely caused by human greenhouse gas emissions, leading to the unusual melt rates of recent years.
Key Points
Ice cores from the West Greenland percolation zone confirm a significant increase in surface melt rates since the early 1990s
Modern melt rates in West Greenland are unmatched in a composite ice core melt record back to 1550 CE
Greenland blocking, regional sea surface temperatures, and a long‐term summer warming trend are required to explain modern melt rates
Atlantic multidecadal variability (AMV) is the term used to describe the pattern of variability in North Atlantic sea surface temperatures (SSTs) that is characterized by decades of basinwide warm or ...cool anomalies, relative to the global mean. AMV has been associated with numerous climate impacts in many regions of the world including decadal variations in temperature and activity, and sea level changes. Given its importance, understanding the physical processes that drive AMV and the extent to which its evolution is predictable is a key challenge in climate science. A leading hypothesis is that natural variations in ocean circulation control changes in ocean heat content and consequently AMV phases. However, this view has been challenged recently by claims that changing natural and anthropogenic radiative forcings are critical drivers of AMV. Others have argued that changes in ocean circulation are not required. Here, we review the leading hypotheses and mechanisms for AMV and discuss the key debates. In particular, we highlight the need for a holistic understanding of AMV. This perspective is a key motivation for a major new U.K. research program: the North Atlantic Climate System Integrated Study (ACSIS), which brings together seven of the United Kingdom’s leading environmental research institutes to enable a broad spectrum approach to the challenges of AMV. ACSIS will deliver the first fully integrated assessment of recent decadal changes in the North Atlantic, will investigate the attribution of these changes to their proximal and ultimate causes, and will assess the potential to predict future changes.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Physiological strain during exercise is increased by mild dehydration (∼1%–3% body mass loss). This response may be sex-dependent, but there are no direct comparative data in this regard. This review ...aimed to develop a framework for future research by exploring the potential impact of sex on thermoregulatory and cardiac strain associated with exercise-induced dehydration. Sex-based comparisons were achieved by comparing trends from studies that implemented similar experimental protocols but recruited males and females separately. This revealed a higher core temperature (T
c
) in response to exercise-induced dehydration in both sexes; however, it seemingly occurred at a lower percent body mass loss in females. Although less clear, similar trends existed for cardiac strain. The average female may have a lower body water volume per body mass compared with males, and therefore the same percent body mass loss between the sexes may represent a larger portion of total body water in females potentially posing a greater physiological strain. In addition, the rate at which T
c
increases at exercise onset might be faster in females and induce a greater thermoregulatory challenge earlier into exercise. The T
c
response at exercise onset is associated with lower sweating rates in females, which is commonly attributed to sex differences in metabolic heat production. However, a reduced sweat gland sensitivity to stimuli, lower fluid output per sweat gland, and sex hormones promoting fluid retention in females may also contribute. In conclusion, the limited evidence suggests that sex-based differences exist in thermoregulatory and cardiac strain associated with exercise-induced dehydration, and this warrants future investigations.
The Atlantic Meridional Overturning Circulation (AMOC) plays a vital role in global climate, redistributing heat, and freshwater. It is predicted to decline due to anthropogenic climate change, with ...major implications for global climate. Accurately assessing AMOC strength with in situ observations has inspired a number of dedicated observing systems in the Atlantic since the 2000s. However, no consensus has been reached on whether the slowdown of the AMOC and its associated heat and freshwater transports is occurring. These dedicated systems are too recent to detect long‐term trends. We have analyzed hydrographic data from zonal sections across the Atlantic for 30 years that predate and overlap the era of AMOC observations. Our results show no changes in the AMOC for all sections analyzed over the whole Atlantic for the last 30 years. We also find an increased export of freshwater from the South Atlantic associated with an increase in upper salinity.
Plain Language Summary
The Atlantic Meridional Overturning Circulation (AMOC) is the oceanic process by which upper warm waters flow northward and cold deep waters flow southward. The AMOC has a large effect on European and global climate. Models have predicted a decline of its strength due to anthropogenic climate change. Across‐ocean systems monitoring the currents on the water column have yet to find this slowdown. We have analyzed hydrographic data collected for the last 30 years and have built a model for each decade of the circulation of the Atlantic, and found no changes in time in the Atlantic Ocean for each hydrographic section. Also, our results present an increase in the amount of freshwater leaving the South Atlantic.
Key Points
No changes at any latitude between hydrographic sections carried out in the last 30 years
The major contributor to the Atlantic Meridional Overturning Circulation at the subpolar North Atlantic is the Eastern subbasin for the three decades
The increase of southward freshwater overturning transport at 30°S indicates a bistable state of the AMOC
Executive Summary Surgical ablation for atrial fibrillation (AF) can be performed without additional risk of operative mortality or major morbidity, and is recommended at the time of concomitant ...mitral operations to restore sinus rhythm. (Class I, Level A) Surgical ablation for AF can be performed without additional operative risk of mortality or major morbidity, and is recommended at the time of concomitant isolated aortic valve replacement, isolated coronary artery bypass graft surgery, and aortic valve replacement plus coronary artery bypass graft operations to restore sinus rhythm. (Class I, Level B nonrandomized) Surgical ablation for symptomatic AF in the absence of structural heart disease that is refractory to class I/III antiarrhythmic drugs or catheter-based therapy or both is reasonable as a primary stand-alone procedure, to restore sinus rhythm. (Class IIA, Level B randomized) Surgical ablation for symptomatic persistent or longstanding persistent AF in the absence of structural heart disease is reasonable, as a stand-alone procedure using the Cox-Maze III/IV lesion set compared with pulmonary vein isolation alone. (Class IIA, Level B nonrandomized) Surgical ablation for symptomatic AF in the setting of left atrial enlargement (≥4.5 cm) or more than moderate mitral regurgitation by pulmonary vein isolation alone is not recommended. (Class III no benefit, Level C expert opinion) It is reasonable to perform left atrial appendage excision or exclusion in conjunction with surgical ablation for AF for longitudinal thromboembolic morbidity prevention. (Class IIA, Level C limited data) At the time of concomitant cardiac operations in patients with AF, it is reasonable to surgically manage the left atrial appendage for longitudinal thromboembolic morbidity prevention. (Class IIA, Level C expert opinion) In the treatment of AF, multidisciplinary heart team assessment, treatment planning, and long-term follow-up can be useful and beneficial to optimize patient outcomes. (Class I, Level C expert opinion)
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