scholarly journals Sea Level controls on Agulhas Leakage Salinity and the Atlantic Overturning Circulation

2021 ◽  
Author(s):  
Sophie Nuber ◽  
James Rae ◽  
Morten Andersen ◽  
Xu Zhang ◽  
Bas de Boer ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Sea Level ◽  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Salt Content ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Land Bridge ◽  
The Indian Ocean

Abstract The Indian Ocean has been proposed as an important source of salt for North Atlantic deep-water convection sites, via the Agulhas Leakage, and may thus drive changes in the ocean’s overturning circulation. However, while past changes in Agulhas leakage volume have been explored, little is known about this water’s salt content, representing a major gap in our understanding of Agulhas salinity supply. Here, we present new planktonic foraminiferal Mg/Ca-derived sea surface temperatures (SST) and stable isotope-derived salinity reconstructions for the last 1.2Ma from the western Indian Ocean source waters of the Agulhas Leakage to investigate glacial-interglacial changes in surface water properties. We find that SST and relative salinity both increase during glaciation, leading to high salinity and SST during glacial maxima. We show that the onset of surface salinification and warming in the Indian Ocean occurs during a phase of rapid land-bridge exposure in the Indonesian archipelago induced by sea level lowering. We link these findings to new global climate model results which show that the export of salt from the Indian Ocean via the Agulhas Leakage can directly impact the deglacial Atlantic meridional overturning circulation and therefore global climate.

scholarly journals An improved parameterization of tidal mixing for ocean models

2014 ◽  
Vol 7 (1) ◽  
pp. 211-224 ◽  
Author(s):  
A. Schmittner ◽  
G. D. Egbert
Keyword(s):  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Tidal Mixing ◽  
Overturning Circulation ◽  
Microstructure Measurements ◽  
High Resolution Bathymetry ◽  
Diurnal Tides

Abstract. Two modifications to an existing scheme of tidal mixing are implemented in the coarse resolution ocean component of a global climate model. First, the vertical distribution of energy flux out of the barotropic tide is determined using high resolution bathymetry. This shifts the levels of mixing higher up in the water column and leads to a stronger mid-depth meridional overturning circulation in the model. Second, the local dissipation efficiency for diurnal tides is assumed to be larger than that for the semi-diurnal tides poleward of 30°. Both modifications are shown to improve agreement with observational estimates of diapycnal diffusivities based on microstructure measurements and circulation indices. We also assess impacts of different spatial distributions of the barotropic energy loss. Estimates based on satellite altimetry lead to larger diffusivities in the deep ocean and hence a stronger deep overturning circulation in our climate model that is in better agreement with observation based estimates compared to those based on a tidal model.

scholarly journals A stable Atlantic Meridional Overturning Circulation in a changing North Atlantic Ocean since the 1990s

2020 ◽  
Vol 6 (48) ◽  
pp. eabc7836
Author(s):  
Yao Fu ◽  
Feili Li ◽  
Johannes Karstensen ◽  
Chunzai Wang
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
North Atlantic Ocean ◽  
Global Climate ◽  
Global Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The Past ◽  
Meridional Overturning

The Atlantic Meridional Overturning Circulation (AMOC) is crucially important to global climate. Model simulations suggest that the AMOC may have been weakening over decades. However, existing array-based AMOC observations are not long enough to capture multidecadal changes. Here, we use repeated hydrographic sections in the subtropical and subpolar North Atlantic, combined with an inverse model constrained using satellite altimetry, to jointly analyze AMOC and hydrographic changes over the past three decades. We show that the AMOC state in the past decade is not distinctly different from that in the 1990s in the North Atlantic, with a remarkably stable partition of the subpolar overturning occurring prominently in the eastern basins rather than in the Labrador Sea. In contrast, profound hydrographic and oxygen changes, particularly in the subpolar North Atlantic, are observed over the same period, suggesting a much higher decoupling between the AMOC and ocean interior property fields than previously thought.

scholarly journals An improved parameterization of tidal mixing for ocean models

2013 ◽  
Vol 6 (3) ◽  
pp. 4475-4509 ◽  
Author(s):  
A. Schmittner ◽  
G. D. Egbert
Keyword(s):  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Tidal Mixing ◽  
Overturning Circulation ◽  
Microstructure Measurements ◽  
High Resolution Bathymetry ◽  
Diurnal Tides

Abstract. Two modifications to an existing scheme of tidal mixing are implemented in the coarse resolution ocean component of a global climate model. First, the vertical distribution of energy flux out of the barotropic tide is determined using high resolution bathymetry. This shifts the levels of mixing higher up in the water column and leads to a stronger mid-depth meridional overturning circulation in the model. Second, the local dissipation efficiency for diurnal tides is assumed to be larger than that for the semi-diurnal tides poleward of 30°. Both modifications are shown to improve agreement with observational estimates of diapycnal diffusivities based on microstructure measurements and circulation indices. We also assess impacts of different spatial distribution of the barotropic energy loss. Estimates based on satellite altimetry lead to larger diffusivities in the deep ocean and hence a stronger deep overturning circulation in our climate model that is in better agreement with observations compared to those based on a tidal model.

scholarly journals Northern Hemisphere storminess in the Norwegian Earth System Model (NorESM1-M)

2014 ◽  
Vol 7 (6) ◽  
pp. 8975-9015
Author(s):  
E. M. Knudsen ◽  
J. E. Walsh
Keyword(s):  
Sea Ice ◽  
Sea Level ◽  
Northern Hemisphere ◽  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
System Model ◽  
High Latitudes ◽  
Earth System ◽  
Projected Changes

Abstract. Metrics of storm activity in Northern Hemisphere high- and midlatitudes are evaluated from historical output and future projections by the Norwegian Earth System Model (NorESM1-M) coupled global climate model. The European Re-Analysis Interim (ERA-Interim) and the Community Climate System Model (CCSM4), a global climate model of the same vintage as NorESM1-M, provide benchmarks for comparison. The focus is on the autumn and early winter (September through December), the period when the ongoing and projected Arctic sea ice retreat is greatest. Storm tracks derived from a vorticity-based algorithm for storm identification are reproduced well by NorESM1-M, although the tracks are somewhat better resolved in the higher-resolution ERA-Interim and CCSM4. The tracks are projected to shift polewards in the future as climate changes under the Representative Concentration Pathway (RCP) forcing scenarios. Cyclones are projected to become generally more intense in the high-latitudes, especially over the Alaskan region, although in some other areas the intensity is projected to decrease. While projected changes in track density are less coherent, there is a general tendency towards less frequent storms in midlatitudes and more frequent storms in high-latitudes, especially the Baffin Bay/Davis Strait region. Autumn precipitation is projected to increase significantly across the entire high-latitudes. Together with the projected increases in storm intensity and sea level and the loss of sea ice, this increase in precipitation implies a greater vulnerability to coastal flooding and erosion, especially in the Alaskan region. The projected changes in storm intensity and precipitation (as well as sea ice and sea level pressure) scale generally linearly with the RCP value of the forcing and with time through the 21st century.

scholarly journals Assessing the Uncertainty in Projecting Local Mean Sea Level from Global Temperature

2014 ◽  
Vol 53 (9) ◽  
pp. 2163-2170 ◽  
Author(s):  
Peter Guttorp ◽  
Alex Januzzi ◽  
Marie Novak ◽  
Harry Podschwit ◽  
Lee Richardson ◽  
...  
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
High Tide ◽  
Deterministic Models ◽  
Total Uncertainty ◽  
Sea Levels ◽  
Global Sea Level

AbstractThe process of moving from an ensemble of global climate model temperature projections to local sea level projections requires several steps. Sea level was estimated in Olympia, Washington (a city that is very concerned with sea level rise because parts of downtown are barely above mean highest high tide), by relating global mean temperature to global sea level; relating global sea level to sea levels at Seattle, Washington; and finally relating Seattle to Olympia. There has long been a realization that accurate assessment of the precision of projections is needed for science-based policy decisions. When a string of statistical and/or deterministic models is connected, the uncertainty of each individual model needs to be accounted for. Here the uncertainty is quantified for each model in the described system and the total uncertainty is assessed in a cascading effect throughout the system. The projected sea level rise over time and its total estimated uncertainty are visualized simultaneously for the years 2000–2100, the increased uncertainty due to each of the component models at a particular projection year is identified, and estimates of the time at which a certain sea level rise will first be reached are made.

Impact of the Equatorial Wind Stress on the Indian Ocean Shallow Meridional Overturning Circulation During the IOD Mature Phase

2020 ◽  
Author(s):  
Linfang Zhang ◽  
Yaokun Li ◽  
Jianping Li
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Overturning Circulation ◽  
Mature Phase ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Upper Level ◽  
The Impact

<p>            This paper investigates the impact of the equatorial wind stress on the Indian Ocean Shallow Meridional Overturning Circulation (SMOC) during the India Ocean Dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper level. In normal years, the wind stress in the Indian Ocean is easterly between 30°S-0°and the westerly wind is between 0°and 30°N, which contributes to a southward Ekman transport at the upper level to form the climatological SMOC. During the years of positive IOD events, abnormal easterly wind near the equator, accompanying with the cold sea surface temperature anomaly (SSTA) along the coast of Sumatra and Java and the warm SSTA along the coast of East Africa, brings southward Ekman transport south of the equator while northward Ekman transport north of the equator. This leads the seawaters moving away from the equator and hence upwelling near the equator as a consequence, to form a pair of small circulation cell symmetric about the equator.</p>

scholarly journals Northern Hemisphere storminess in the Norwegian Earth System Model (NorESM1-M)

2016 ◽  
Vol 9 (7) ◽  
pp. 2335-2355 ◽  
Author(s):  
Erlend M. Knudsen ◽  
John E. Walsh
Keyword(s):  
Sea Ice ◽  
Sea Level ◽  
Northern Hemisphere ◽  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
System Model ◽  
High Latitudes ◽  
Earth System ◽  
Projected Changes

Abstract. Metrics of storm activity in Northern Hemisphere high and midlatitudes are evaluated from historical output and future projections by the Norwegian Earth System Model (NorESM1-M) coupled global climate model. The European Re-Analysis Interim (ERA-Interim) and the Community Climate System Model (CCSM4), a global climate model of the same vintage as NorESM1-M, provide benchmarks for comparison. The focus is on the autumn and early winter (September through December) – the period when the ongoing and projected Arctic sea ice retreat is the greatest. Storm tracks derived from a vorticity-based algorithm for storm identification are reproduced well by NorESM1-M, although the tracks are somewhat better resolved in the higher-resolution ERA-Interim and CCSM4. The tracks show indications of shifting polewards in the future as climate changes under the Representative Concentration Pathway (RCP) forcing scenarios. Cyclones are projected to become generally more intense in the high latitudes, especially over the Alaskan region, although in some other areas the intensity is projected to decrease. While projected changes in track density are less coherent, there is a general tendency towards less frequent storms in midlatitudes and more frequent storms in high latitudes, especially the Baffin Bay/Davis Strait region in September. Autumn precipitation is projected to increase significantly across the entire high latitudes. Together with the projected loss of sea ice and increases in storm intensity and sea level, this increase in precipitation implies a greater vulnerability to coastal flooding and erosion, especially in the Alaskan region. The projected changes in storm intensity and precipitation (as well as sea ice and sea level pressure) scale generally linearly with the RCP value of the forcing and with time through the 21st century.

scholarly journals The Global Climate Response to Lowering Surface Orography of Antarctica and the Importance of Atmosphere–Ocean Coupling

2016 ◽  
Vol 29 (11) ◽  
pp. 4137-4153 ◽  
Author(s):  
Hansi K. A. Singh ◽  
Cecilia M. Bitz ◽  
Dargan M. W. Frierson
Keyword(s):  
Energy Transport ◽  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Longwave Radiation ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
The Arctic ◽  
System Response ◽  
The Antarctic

Abstract A global climate model is used to study the effect of flattening the orography of the Antarctic Ice Sheet on climate. A general result is that the Antarctic continent and the atmosphere aloft warm, while there is modest cooling globally. The large local warming over Antarctica leads to increased outgoing longwave radiation, which drives anomalous southward energy transport toward the continent and cooling elsewhere. Atmosphere and ocean both anomalously transport energy southward in the Southern Hemisphere. Near Antarctica, poleward energy and momentum transport by baroclinic eddies strengthens. Anomalous southward cross-equatorial energy transport is associated with a northward shift in the intertropical convergence zone. In the ocean, anomalous southward energy transport arises from a slowdown of the upper cell of the oceanic meridional overturning circulation and a weakening of the horizontal ocean gyres, causing sea ice in the Northern Hemisphere to expand and the Arctic to cool. Comparison with a slab-ocean simulation confirms the importance of ocean dynamics in determining the climate system response to Antarctic orography. This paper concludes by briefly presenting a discussion of the relevance of these results to climates of the past and to future climate scenarios.

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