scholarly journals The impact of different glacial boundary conditions on atmospheric dynamics and precipitation in the North Atlantic region

2012 ◽  
Vol 8 (1) ◽  
pp. 63-101
Author(s):  
D. Hofer ◽  
C. C. Raible ◽  
A. Dehnert ◽  
J. Kuhlemann
Keyword(s):  
Boundary Conditions ◽  
North Atlantic ◽  
General Circulation ◽  
Ice Sheet ◽  
Atmospheric Dynamics ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
The North ◽  
The North Atlantic ◽  
The Impact

Abstract. Using a highly resolved atmospheric general circulation model the impact of different glacial boundary conditions on precipitation and atmospheric dynamics in the North Atlantic region is investigated. Seven 30-yr time slice experiments of the Last Glacial Maximum (21 ka ago) and of a less pronounced glacial state – the Middle Weichselian (65 ka ago) – are compared to analyse the sensitivity to changes in the ice sheet distribution, in the radiative forcing, and in the prescribed time-varying lower boundary conditions, which are taken from a lower-resolved but fully-coupled atmosphere-ocean general circulation model. The strongest differences are found for simulations with different heights of the Laurentide ice sheet. A large altitude of this ice sheet leads to a southward displacement of the jet stream and the storm track in the North Atlantic region. These changes in the atmospheric dynamics generate a band of increased precipitation in the mid-latitudes across the Atlantic to southern Europe in winter, while the precipitation pattern in summer is only marginally affected. The impact of the radiative forcing differences between the two glacial periods and of the prescribed time-varying lower boundary conditions – evaluated using two simulations of the Last Glacial Maximum with a global mean temperature difference of 1.1 °C – are of second order compared to the one of the Laurentide ice sheet. They affect the atmospheric dynamics and precipitation in a similar but less pronounced manner as the topographic changes.

scholarly journals The impact of different glacial boundary conditions on atmospheric dynamics and precipitation in the North Atlantic region

2012 ◽  
Vol 8 (3) ◽  
pp. 935-949 ◽  
Author(s):  
D. Hofer ◽  
C. C. Raible ◽  
A. Dehnert ◽  
J. Kuhlemann
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Ice Sheet ◽  
Atmospheric Dynamics ◽  
Laurentide Ice Sheet ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
The North ◽  
The North Atlantic ◽  
The Impact

Abstract. Using a highly resolved atmospheric general circulation model, the impact of different glacial boundary conditions on precipitation and atmospheric dynamics in the North Atlantic region is investigated. Six 30-yr time slice experiments of the Last Glacial Maximum at 21 thousand years before the present (ka BP) and of a less pronounced glacial state – the Middle Weichselian (65 ka BP) – are compared to analyse the sensitivity to changes in the ice sheet distribution, in the radiative forcing and in the prescribed time-varying sea surface temperature and sea ice, which are taken from a lower-resolved, but fully coupled atmosphere-ocean general circulation model. The strongest differences are found for simulations with different heights of the Laurentide ice sheet. A high surface elevation of the Laurentide ice sheet leads to a southward displacement of the jet stream and the storm track in the North Atlantic region. These changes in the atmospheric dynamics generate a band of increased precipitation in the mid-latitudes across the Atlantic to southern Europe in winter, while the precipitation pattern in summer is only marginally affected. The impact of the radiative forcing differences between the two glacial periods and of the prescribed time-varying sea surface temperatures and sea ice are of second order importance compared to the one of the Laurentide ice sheet. They affect the atmospheric dynamics and precipitation in a similar but less pronounced manner compared with the topographic changes.

scholarly journals How did Marine Isotope Stage 3 and Last Glacial Maximum climates differ? – Perspectives from equilibrium simulations

2009 ◽  
Vol 5 (1) ◽  
pp. 33-51 ◽  
Author(s):  
C. J. Van Meerbeeck ◽  
H. Renssen ◽  
D. M. Roche
Keyword(s):  
Boundary Conditions ◽  
Last Glacial Maximum ◽  
North Atlantic ◽  
Deep Water ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
The North ◽  
Freshwater Forcing ◽  
The North Atlantic ◽  
Marine Isotope Stage 3

Abstract. Dansgaard-Oeschger events occurred frequently during Marine Isotope Stage 3 (MIS3), as opposed to the following MIS2 period, which included the Last Glacial Maximum (LGM). Transient climate model simulations suggest that these abrupt warming events in Greenland and the North Atlantic region are associated with a resumption of the Thermohaline Circulation (THC) from a weak state during stadials to a relatively strong state during interstadials. However, those models were run with LGM, rather than MIS3 boundary conditions. To quantify the influence of different boundary conditions on the climates of MIS3 and LGM, we perform two equilibrium climate simulations with the three-dimensional earth system model LOVECLIM, one for stadial, the other for interstadial conditions. We compare them to the LGM state simulated with the same model. Both climate states are globally 2°C warmer than LGM. A striking feature of our MIS3 simulations is the enhanced Northern Hemisphere seasonality, July surface air temperatures being 4°C warmer than in LGM. Also, despite some modification in the location of North Atlantic deep water formation, deep water export to the South Atlantic remains unaffected. To study specifically the effect of orbital forcing, we perform two additional sensitivity experiments spun up from our stadial simulation. The insolation difference between MIS3 and LGM causes half of the 30–60° N July temperature anomaly (+6°C). In a third simulation additional freshwater forcing halts the Atlantic THC, yielding a much colder North Atlantic region (−7°C). Comparing our simulation with proxy data, we find that the MIS3 climate with collapsed THC mimics stadials over the North Atlantic better than both control experiments, which might crudely estimate interstadial climate. These results suggest that freshwater forcing is necessary to return climate from warm interstadials to cold stadials during MIS3. This changes our perspective, making the stadial climate a perturbed climate state rather than a typical, near-equilibrium MIS3 climate.

scholarly journals Synoptic Forcing of Precipitation over Greenland: Climatology for 1961–99

2009 ◽  
Vol 10 (1) ◽  
pp. 60-78 ◽  
Author(s):  
Keah C. Schuenemann ◽  
John J. Cassano ◽  
Joel Finnis
Keyword(s):  
North Atlantic ◽  
Winter Season ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Synoptic Climatology ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
Synoptic Patterns ◽  
The North ◽  
The North Atlantic

Abstract Analysis of the synoptic climatology and precipitation patterns over the North Atlantic region allows for a better understanding of the atmospheric input to the mass balance of the Greenland ice sheet. The self-organizing map (SOM) technique was applied to the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) daily sea level pressure (SLP) data from 1961 to 1999 to objectively identify synoptic SLP patterns over the North Atlantic region. A total of 35 different SLP patterns were identified. Patterns common to the winter season are characterized by deep low pressure systems that approach Greenland through an active North Atlantic storm track, whereas patterns most common to the summer months are generally weaker and approach the ice sheet from the west through Baffin Bay. The blocking, splitting, and intensification of cyclones by the high elevations of the Greenland ice sheet were identified in this analysis. Analysis of ERA-40 precipitation associated with each SLP pattern revealed that the largest precipitation events were associated with passing cyclones that created onshore flow, allowing the air to be lifted orographically by the steep margins of the ice sheet. The ERA-40 annual mean precipitation over Greenland from 1961 to 1999 was 35.8 cm yr−1. Greenland was divided into five subregions, and the preferred synoptic patterns for receiving precipitation in each region include cyclones positioned to allow dynamic and orographic lift in each region. Annual precipitation contributions from each SLP pattern were isolated to reveal that half of the annual mean precipitation over Greenland comes from only 11 of the 35 identified synoptic patterns (31.4%), highlighting the importance of studying Greenland precipitation on an event-by-event basis on a daily time scale.

scholarly journals Sensitivity of the North Atlantic climate to Greenland Ice Sheet melting during the Last Interglacial

2012 ◽  
Vol 8 (3) ◽  
pp. 995-1009 ◽  
Author(s):  
P. Bakker ◽  
C. J. Van Meerbeeck ◽  
H. Renssen
Keyword(s):  
North Atlantic ◽  
Deep Convection ◽  
Greenland Ice Sheet ◽  
Last Interglacial ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
The North ◽  
Summer Temperatures ◽  
The North Atlantic ◽  
The Impact

Abstract. During the Last Interglacial (LIG; ~130 000 yr BP), part of the Greenland Ice Sheet (GIS) melted due to a warmer than present-day climate. However, the impact of this melting on the LIG climate in the North Atlantic region is relatively unknown. Using the LOVECLIM Earth system model of intermediate complexity, we have systematically tested the sensitivity of the LIG climate to increased freshwater runoff from the GIS. In addition, experiments have been performed to investigate the impact of an idealized reduction of both surface elevation and extent of the GIS on the LIG climate. Based on changes in the maximum sea-ice cover and the strength of the overturning circulation, three regimes have been identified, which are characterized by a specific pattern of surface temperature change in the North Atlantic region. By comparing the simulated deep ocean circulation with proxy-based reconstructions, the most realistic simulated climate could be discerned. The resulting climate is characterized by a shutdown of deep convection and a subsequent ~4 °C cooling in the Labrador Sea. Furthermore, a cooling of ~1 °C over the North Atlantic Ocean between 40° N and 70° N is seen. The prescribed reduction in surface elevation and extent of the GIS results in a local warming of up to 4 °C and amplifies the freshwater-forced reduction in deep convection and the resultant cooling in the Nordic Seas. A further comparison of simulated summer temperatures with both continental and oceanic proxy records reveals that the partial melting of the GIS during the LIG could have delayed maximum summer temperatures in the western part of the North Atlantic region relative to the insolation maximum.

Data-model comparison of the Younger Dryas event

2000 ◽  
Vol 37 (5) ◽  
pp. 811-830 ◽  
Author(s):  
Nathaniel W Rutter ◽  
Andrew J Weaver ◽  
Dean Rokosh ◽  
Augustus F Fanning ◽  
Daniel G Wright
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Younger Dryas ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
Climate Proxy ◽  
Proxy Records ◽  
The North ◽  
The North Atlantic ◽  
Younger Dryas Event

The Younger Dryas cooling event is well established in the North Atlantic region through numerous climate proxy records. Although the climatological controls vary from site to site, it is considered to have taken place between about 10 000 and 11 000 BP (radiocarbon years) (~11 500-13 000 calendar years ago). Outside the North Atlantic region, climate proxy records and chronology commonly become problematic because of weaker signals and less dating control. In addition to this evidence, oceanic records reveal conflicting evidence for Younger Dryas forcing mechanisms and the timing of events. We compare proxy evidence with the results from an ocean general circulation model coupled to the energy-moisture balance atmospheric model. The model results reveal a global pattern and regional magnitude which generally agree with temperature changes interpreted from paleoclimate reconstructions. The model also supports the general duration of global cooling of the Younger Dryas. Although proxy data can be controversial outside of the North Atlantic region, the authors believe that there is enough evidence to support the Younger Dryas event on a global scale. They also recognize, however, that more concrete evidence is needed before the question can be unequivocally answered.

Major ice sheet response in eastern New England to a cold North Atlantic region, ca. 16–15 cal ka BP

2007 ◽  
Vol 68 (2) ◽  
pp. 280-283 ◽  
Author(s):  
Michael R. Kaplan
Keyword(s):  
New England ◽  
North Atlantic ◽  
Ice Sheet ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
The North ◽  
Time Period ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
Almost All

AbstractA large ice sheet still covered almost all of Maine and eastern New England until ca. 15 cal ka BP, reaching south of 45 °S, despite rising summer insolation intensity and major ice recession elsewhere outside the North Atlantic region. Furthermore, the well-studied moraine belt along eastern coastal Maine, including the prominent Pineo Ridge delta/moraine complex and Pond Ridge moraine, indicates repeated readvances and stillstands between ca. 16 and 15 cal ka BP. This moraine belt reflects a considerable ice sheet response over eastern North America during this time period, coeval with the latter half of the European Oldest Dryas period. Moraine deposition was concurrent with reduction or elimination of North Atlantic meridional overturning, starting with the earlier onset of peak IRD and Heinrich Event 1 (HE-1). The existing 14C chronology suggests that the coastal moraine belt and the persistence of the ice sheet until ∼ 15 cal ka BP was a response to the severe cooling of the North Atlantic region after ∼ 17 cal ka BP.

scholarly journals Sensitivity of the North Atlantic climate to Greenland Ice Sheet melting during the Last Interglacial

2011 ◽  
Vol 7 (4) ◽  
pp. 2763-2801 ◽  
Author(s):  
P. Bakker ◽  
C. J. Van Meerbeeck ◽  
H. Renssen
Keyword(s):  
North Atlantic ◽  
Deep Convection ◽  
Greenland Ice Sheet ◽  
Last Interglacial ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
The North ◽  
Summer Temperatures ◽  
The North Atlantic ◽  
The Impact

Abstract. During the Last Interglacial (LIG; ~130 thousand years BP), part of the Greenland Ice Sheet (GIS) melted under the influence of a warmer than present-day climate. However, the impact of this melting on the LIG climate in the North Atlantic region is unknown. Using the LOVECLIM earth system model of intermediate complexity we have systematically tested the sensitivity of the LIG climate to increased freshwater runoff from the GIS. Moreover, additional experiments have been performed to investigate the impact of an idealized reduction of both altitude and extent of the GIS on the LIG climate. By comparing the simulated deep ocean circulation with proxy-based reconstructions, the most realistic simulated climate could be discerned. The resulting climate is characterized by a shutdown of deep convection in the Labrador Sea and a subsequent cooling here by ~6 °C and ~2 °C over the southern part of Baffin Island and the North Atlantic Ocean between 40° N and 60° N. The reduction of altitude and extent of the GIS results in a local warming of up to 6 °C and a reduction in deep convection and accompanying cooling in the Nordic Seas. Combining model results and proxy-based reconstructions enabled us to constrain the possible melt rate of the GIS to a flux between 0.052 Sv and 0.13 Sv. A further comparison of simulated summer temperatures with both continental and oceanic proxy-records reveals that the partial melting of the GIS during the LIG could have delayed maximum summer temperatures in the western part of the North Atlantic region relative to the insolation maximum.

scholarly journals How did Marine Isotope Stage 3 and Last Glacial Maximum climates differ? Perspectives from equilibrium simulations

2008 ◽  
Vol 4 (5) ◽  
pp. 1115-1158 ◽  
Author(s):  
C. J. Van Meerbeeck ◽  
H. Renssen ◽  
D. M. Roche
Keyword(s):  
Boundary Conditions ◽  
Last Glacial Maximum ◽  
North Atlantic ◽  
Deep Water ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
The North ◽  
Freshwater Forcing ◽  
The North Atlantic ◽  
Marine Isotope Stage 3

Abstract. Dansgaard-Oeschger events occurred frequently during Marine Isotope Stage 3 (MIS3), as opposed to the following MIS2 period, which included the Last Glacial Maximum (LGM). Transient climate model simulations suggest that these abrupt warming events in Greenland and the North Atlantic region are associated with a resumption of the Thermohaline Circulation (THC) from a weak state during stadials to a relatively strong state during interstadials. However, those models were run with LGM, rather than MIS3 boundary conditions. To quantify the influence of different boundary conditions on the climates of MIS3 and LGM, we perform two equilibrium climate simulations with the three-dimensional earth system model LOVECLIM, one for stadial, the other for interstadial conditions. We compare them to the LGM state simulated with the same model. Both climate states are globally 2°C warmer than LGM. A striking feature of our MIS3 simulations is the enhanced Northern Hemisphere seasonality, July being 4°C warmer than in LGM. Also, despite some modification in the location of North Atlantic deep water formation, deep water export to the South Atlantic remains unaffected. To study specifically the effect of orbital forcing, we perform two additional sensitivity experiments spun up from our stadial simulation. The insolation difference between MIS3 and LGM causes half of the 30–60°N July temperature anomaly (+6°C). In a third simulation additional freshwater forcing halts the Atlantic THC, yielding a much colder North Atlantic region (−7°C). Comparing our simulation with proxy data, we find that the MIS3 climate with collapsed THC mimics stadials over the North Atlantic better than both control experiments, which might crudely estimate interstadial climate. These results suggest that freshwater forcing is necessary to return climate from warm interstadials to cold stadials during MIS3. This changes our perspective, making the stadial climate a perturbed climate state rather than a typical, near-equilibrium MIS3 climate.

Unstable Behavior of the Laurentide Ice Sheet over Deforming Sediment and Its Implications for Climate Change

1994 ◽  
Vol 41 (1) ◽  
pp. 19-25 ◽  
Author(s):  
Peter U. Clark
Keyword(s):  
North Atlantic ◽  
Thermohaline Circulation ◽  
Ice Sheet ◽  
Laurentide Ice Sheet ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
Climate Oscillations ◽  
Unstable Behavior ◽  
The North ◽  
The North Atlantic

AbstractGeologic records of fluctuations of the Laurentide ice sheet margin following the most recent glacial maximum (ca. 20,000 14C yr B.P.) identify fundamental differences in ice-sheet behavior depending on subglacial bed conditions. Rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment, indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet-deforming sediment dynamics. In contrast, largely uninterrupted ice-margin retreat with no evidence of significant readvance occurred over rigid-bed areas, indicating stable behavior. Unstable ice-sheet behavior was most pronounced from 15,000 until 10,000 14C yr B.P., by which time most of the ice margin had retreated onto a rigid bed. Unstable ice-sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater, as well as meltwater routing, to the North Atlantic, thus affecting thermohaline circulation. The abrupt climate oscillations in the North Atlantic region that ended at 10,000 14C yr B.P. may thus have their origin in the inherently unstable behavior of the Laurentide ice sheet overriding deforming sediment.

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