scholarly journals Mechanisms Affecting the Overturning Response in Global Warming Simulations

2005 ◽  
Vol 18 (23) ◽  
pp. 4925-4936 ◽  
Author(s):  
U. Schweckendiek ◽  
J. Willebrand
Keyword(s):  
Global Warming ◽  
North Atlantic ◽  
Climate Models ◽  
Thermohaline Circulation ◽  
Decadal Variability ◽  
Ocean Model ◽  
Surface Fluxes ◽  
Coupled Models ◽  
The North ◽  
Freshwater Fluxes

Abstract Climate models used to produce global warming scenarios exhibit widely diverging responses of the thermohaline circulation (THC). To investigate the mechanisms responsible for this variability, a regional Atlantic Ocean model driven with forcing diagnosed from two coupled greenhouse gas simulations has been employed. One of the coupled models (MPI) shows an almost constant THC, the other (GFDL) shows a declining THC in the twenty-first century. The THC evolution in the regional model corresponds rather closely to that of the respective coupled simulation, that is, it remains constant when driven with the forcing from the MPI model, and declines when driven with the GFDL forcing. These findings indicate that a detailed representation of ocean processes in the region covered by the Atlantic model may not be critical for the simulation of the overall THC changes in a global warming scenario, and specifically that the coupled model’s rather coarse representation of water mass formation processes in the subpolar North Atlantic is unlikely to be the primary cause for the large differences in the THC evolution. Sensitivity experiments have confirmed that a main parameter governing the THC response to global warming is the density of the intermediate waters in the Greenland–Iceland–Norwegian Seas, which in turn influences the density of the North Atlantic Deep Water, whereas changes in the air–sea heat and freshwater fluxes over the subpolar North Atlantic are only of moderate importance, and mainly influence the interannual–decadal variability of THC. Finally, as a consequence of changing surface fluxes, the Labrador Sea convection ceases by about 2030 under both forcings (i.e., even in a situation where the overall THC is stable) indicating that the eventual breakdown of the convection is likely but need not coincide with substantial THC changes.

scholarly journals Fates and Travel Times of Denmark Strait Overflow Water in the Irminger Basin*

2013 ◽  
Vol 43 (12) ◽  
pp. 2611-2628 ◽  
Author(s):  
Inga M. Koszalka ◽  
Thomas W. N. Haine ◽  
Marcello G. Magaldi
Keyword(s):  
North Atlantic ◽  
Thermohaline Circulation ◽  
Ocean Model ◽  
Potential Density ◽  
Particle Trajectories ◽  
Dense Water ◽  
The North ◽  
Denmark Strait ◽  
Wide Range ◽  
The Mean

Abstract The Denmark Strait Overflow (DSO) supplies about one-third of the North Atlantic Deep Water and is critical to global thermohaline circulation. Knowledge of the pathways of DSO through the Irminger Basin and its transformation there is still incomplete, however. The authors deploy over 10 000 Lagrangian particles at the Denmark Strait in a high-resolution ocean model to study these issues. First, the particle trajectories show that the mean position and potential density of dense waters cascading over the Denmark Strait sill evolve consistently with hydrographic observations. These sill particles transit the Irminger Basin to the Spill Jet section (65.25°N) in 5–7 days and to the Angmagssalik section (63.5°N) in 2–3 weeks. Second, the dense water pathways on the continental shelf are consistent with observations and particles released on the shelf in the strait constitute a significant fraction of the dense water particles recorded at the Angmagssalik section within 60 days (~25%). Some particles circulate on the shelf for several weeks before they spill off the shelf break and join the overflow from the sill. Third, there are two places where the water density following particle trajectories decreases rapidly due to intense mixing: to the southwest of the sill and southwest of the Kangerdlugssuaq Trough on the continental slope. After transformation in these places, the overflow particles exhibit a wide range of densities.

scholarly journals Will Global Warming Suppress North Atlantic Tripole Decadal Variability?

2012 ◽  
Vol 25 (6) ◽  
pp. 2040-2055 ◽  
Author(s):  
Yun Yang ◽  
Lixin Wu ◽  
Changfang Fang
Keyword(s):  
Global Warming ◽  
North Atlantic ◽  
Time Scale ◽  
Decadal Variability ◽  
Buoyancy Frequency ◽  
The North ◽  
Ocean Atmosphere ◽  
The North Atlantic ◽  
Partial Blocking ◽  
Partial Coupling

Abstract In this paper, the modulations of the North Atlantic tripole (NAT) decadal variability from global warming are studied by conducting a series of coupled ocean–atmosphere experiments using the Fast Ocean Atmosphere Model (FOAM). The model reasonably captures the observed NAT decadal variability with a preferred time scale of about 11 years. With the aid of partial-blocking and partial-coupling experiments, it is found that the NAT decadal cycle can be attributed to oceanic planetary wave adjustment in the subtropical basin and ocean–atmosphere coupling over the North Atlantic. In a doubled CO2 experiment, the spatial pattern of the NAT is preserved; however, the decadal cycle is significantly suppressed. This suppression appears to be associated with the acceleration of oceanic planetary waves due to an increase of buoyancy frequency in global warming. This shortens the time from a decadal to an interannual time scale for the first-mode baroclinic Rossby waves to cross the subtropical North Atlantic basin, the primary memory for the NAT decadal variability in the model. The modeling study also found that the global warming does not modulate the North Atlantic air–sea coupling significantly, but it may be model dependent.

The Sensitivity of a Coupled Climate Model to Its Ocean Component

2010 ◽  
Vol 23 (19) ◽  
pp. 5126-5150 ◽  
Author(s):  
A. P. Megann ◽  
A. L. New ◽  
A. T. Blaker ◽  
B. Sinha
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Climate Model ◽  
Ocean Model ◽  
Intermediate Water ◽  
Mode Water ◽  
Vertical Coordinate ◽  
The North ◽  
Almost Everywhere ◽  
Subantarctic Mode Water

Abstract The control climates of two coupled climate models are intercompared. The first is the third climate configuration of the Met Office Unified Model (HadCM3), while the second, the Coupled Hadley–Isopycnic Model Experiment (CHIME), is identical to the first except for the replacement of its ocean component by the Hybrid-Coordinate Ocean Model (HYCOM). Both models possess realistic and similar ocean heat transports and overturning circulation. However, substantial differences in the vertical structure of the two ocean components are observed, some of which are directly attributed to their different vertical coordinate systems. In particular, the sea surface temperature (SST) in CHIME is biased warm almost everywhere, particularly in the North Atlantic subpolar gyre, in contrast to HadCM3, which is biased cold except in the Southern Ocean. Whereas the HadCM3 ocean warms from just below the surface down to 1000-m depth, a similar warming in CHIME is more pronounced but shallower and confined to the upper 400 m, with cooling below this. This is particularly apparent in the subtropical thermoclines, which become more diffuse in HadCM3, but sharper in CHIME. This is interpreted as resulting from a more rigorously controlled diapycnal mixing in the interior isopycnic ocean in CHIME. Lower interior mixing is also apparent in the better representation and maintenance of key water masses in CHIME, such as Subantarctic Mode Water, Antarctic Intermediate Water, and North Atlantic Deep Water. Finally, the North Pacific SST cold error in HadCM3 is absent in CHIME, and may be related to a difference in the separation position of the Kuroshio. Disadvantages of CHIME include a nonconservation of heat equivalent to 0.5 W m−2 globally, and a warming and salinification of the northwestern Atlantic.

scholarly journals Response of the North Atlantic Thermohaline Circulation and Ventilation to Increasing Carbon Dioxide in CCSM3

2006 ◽  
Vol 19 (11) ◽  
pp. 2382-2397 ◽  
Author(s):  
Frank O. Bryan ◽  
Gokhan Danabasoglu ◽  
Norikazu Nakashiki ◽  
Yoshikatsu Yoshida ◽  
Dong-Hoon Kim ◽  
...  
Keyword(s):  
North Atlantic ◽  
Control Experiment ◽  
Thermohaline Circulation ◽  
Water Mass ◽  
Overturning Circulation ◽  
The North ◽  
Freshwater Fluxes ◽  
The North Atlantic ◽  
Atlantic Thermohaline Circulation ◽  
Water Mass Transformation

Abstract The response of the North Atlantic thermohaline circulation to idealized climate forcing of 1% per year compound increase in CO2 is examined in three configurations of the Community Climate System Model version 3 that differ in their component model resolutions. The strength of the Atlantic overturning circulation declines at a rate of 22%–26% of the corresponding control experiment maximum overturning per century in response to the increase in CO2. The mean meridional overturning and its variability on decadal time scales in the control experiments, the rate of decrease in the transient forcing experiments, and the rate of recovery in periods of CO2 stabilization all increase with increasing component model resolution. By examining the changes in ocean surface forcing with increasing CO2 in the framework of the water-mass transformation function, we show that the decline in the overturning is driven by decreasing density of the subpolar North Atlantic due to increasing surface heat fluxes. While there is an intensification of the hydrologic cycle in response to increasing CO2, the net effect of changes in surface freshwater fluxes on those density classes that are involved in deep-water formation is to increase their density; that is, changes in surface freshwater fluxes act to maintain a stronger overturning circulation. The differences in the control experiment overturning strength and the response to increasing CO2 are well predicted by the corresponding differences in the water-mass transformation rate. Reduction of meridional heat transport and enhancement of meridional salt transport from mid- to high latitudes with increasing CO2 also act to strengthen the overturning circulation. Analysis of the trends in an ideal age tracer provides a direct measure of changes in ocean ventilation time scale in response to increasing CO2. In the subpolar North Atlantic south of the Greenland–Scotland ridge system, there is a significant increase in subsurface ages as open-ocean deep convection is diminished and ventilation switches to a predominance of overflow waters. In middle and low latitudes there is a decrease in age within and just below the thermocline in response to a decrease in the upwelling of old deep waters. However, when considering ventilation within isopycnal layers, age increases for layers in and below the thermocline due to the deepening of isopycnals in response to global warming.

scholarly journals Decadal Variability in the North Pacific and North Atlantic under Global Warming: The Weakening Response and Its Mechanism

2020 ◽  
Vol 33 (21) ◽  
pp. 9181-9193
Author(s):  
Sheng Wu ◽  
Zheng-Yu Liu
Keyword(s):  
Global Warming ◽  
Interannual Variability ◽  
North Atlantic ◽  
North Pacific ◽  
Decadal Variability ◽  
Spectral Power ◽  
Coupled Model ◽  
The North ◽  
Damping Rate ◽  
The North Pacific

AbstractWe investigate the response of decadal variability in the North Pacific and North Atlantic under global warming and its mechanism in this study. To do so, we use four models (BCC-CSM1–1, CCSM4, IPSL-CM5A-LR, and MPI-ESM-LR) that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5), focusing on three global warming scenarios (RCP2.6, RCP4.5, and RCP8.5). Our analysis shows that the intensified global warming leads to a decrease in amplitude of both the Pacific decadal oscillation (PDO) and Atlantic multidecadal variability (AMV), resulting in reduced decadal variability of sea surface temperature (SST) in both the North Pacific and North Atlantic. In comparison, interannual variability is less impacted by global warming and has a tendency to increase, which leads to a shift of spectral power from decadal toward interannual variability. We then show the weakening decadal variability is caused partly by the weakened forcing of atmospheric heat flux variability, and partly by the increased SST damping rate. In addition, an enhanced upper-ocean stratification under global warming also contributes to the acceleration of Rossby waves, and a shift of decadal variability spectral power toward a shorter period.

Atlantic Salinity Pileup as a Remote Fingerprint of Weakening Atlantic Overturning Circulation under Anthropogenic Warming

2020 ◽  
Author(s):  
Chenyu Zhu ◽  
Zhengyu Liu
Keyword(s):  
Global Warming ◽  
North Atlantic ◽  
Climate Models ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
The South ◽  
Overturning Circulation ◽  
The North ◽  
Direct Measurements ◽  
Meridional Overturning

<p>Climate models show a weakening Atlantic meridional overturning circulation (AMOC) under global warming. Limited by short direct measurements, this AMOC slowdown has been inferred, with some uncertainties, indirectly from some AMOC fingerprints locally over the subpolar North Atlantic region. Here we present observational and modeling evidences of the first remote fingerprint of AMOC slowdown outside the North Atlantic. Under global warming, the weakening AMOC reduces the salinity divergence and then leads to a remote fingerprint of “salinity pileup” in the South Atlantic. Our study supports the AMOC slowdown under anthropogenic warming and, furthermore, shows that this weakening has occurred all the way into the South Atlantic.</p>

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