Sensitivity of the Present-Day Climate to Freshwater Forcing Associated with Antarctic Sea Ice Loss

2008 ◽  
Vol 21 (15) ◽  
pp. 3936-3946 ◽  
Author(s):  
Christopher M. Aiken ◽  
Matthew H. England
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Southern Hemisphere ◽  
Climate Model ◽  
Global Climate ◽  
Climate System ◽  
Climatic Effects ◽  
The North ◽  
Antarctic Sea Ice ◽  
Freshwater Forcing

Abstract The role played by Southern Hemisphere sea ice in the global climate system is explored using an earth system climate model of intermediate complexity. An ensemble of experiments is analyzed in which freshwater forcing equivalent to a complete 100-yr meltback of Southern Hemisphere sea ice is applied to a model run that simulates the present climate. This freshwater forcing acts to mildy subdue Southern Ocean deep overturning, reducing mean Antarctic Bottom Water (AABW) export by 0.5 Sv (1 Sv ≡ 106 m3 s−1) in the ensemble average. The decreased convective overturning cools the surface waters, thereby increasing sea ice volume and thus forming a negative feedback that stabilizes Antarctic sea ice. In contrast, the reduced convective overturn warms subsurface waters in the Southern Ocean, which, combined with the imposed freshening, results in a reduction in the meridional steric height gradient and hence a slowdown of the Antarctic Circumpolar Current (ACC). The reduction in ACC strength is, however, only modest at 1.5 Sv. These responses are thus of only weak magnitude, and the system recovers to its original state over time scales of decades. An extreme scenario experiment with essentially instantaneous addition of this meltwater load shows similar results, indicating the limited response of the climate system to the freshening implied by Antarctic sea ice melt. An additional experiment in which a much larger freshwater forcing of approximately 0.4 Sv is applied over 100 yr confirms the relatively weak response of the model’s climate state to such forcing, relative to the well-documented climatic effects of freshwater forcing added to the North Atlantic.

Sea Ice in the Greenland Polynya in 2018 - A Study with CryoSat-2 and SMOS

2020 ◽  
Author(s):  
Weixin Zhu ◽  
Lu Zhou ◽  
Shiming Xu
Keyword(s):  
Sea Ice ◽  
Global Climate ◽  
Arctic Sea Ice ◽  
Climate System ◽  
The Arctic ◽  
Ice Thickness ◽  
Sea Ice Thickness ◽  
The North ◽  
Ice Surface ◽  
Snow Conditions

<p><strong>Abstract</strong></p><p>Arctic sea ice is a critical component in the global climate system. It affects the climate system by radiating incident heat back into space and regulating ocean-atmosphere heat and momentum. Satellite altimetry such as CryoSat-2 serves as the primary approach for observing sea ice thickness. Nevertheless, the thickness retrieval with CryoSat-2 mainly depends on the height of the ice surface above the sea level, which leads to significant uncertainties over thin ice regimes. The sea ice at the north of Greenland is considered one of the oldest and thickest in the Arctic. However, during late February - early March 2018, a polynya formed north to Greenland due to extra strong southern winds. We focus on the retrieval of sea ice thickness and snow conditions with CryoSat-2 and SMOS during the formation of the polynya. Specifically, we investigate the uncertainty of CryoSat-2 and carry out inter- comparison of sea ice thickness retrieval with SMOS and CryoSat-2/SMOS synergy. Besides, further discussion of retrieval with CryoSat-2 is provided for such scenarios where the mélange of thick ice and newly formed thin ice is present.</p>

scholarly journals The Role of Stratification-Dependent Mixing for the Stability of the Atlantic Overturning in a Global Climate Model*

2007 ◽  
Vol 37 (11) ◽  
pp. 2672-2681 ◽  
Author(s):  
Ben Marzeion ◽  
Anders Levermann ◽  
Juliette Mignot
Keyword(s):  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Buoyancy Frequency ◽  
Freshwater Flux ◽  
The North ◽  
Vertical Diffusivity ◽  
Freshwater Forcing

Abstract Using the “CLIMBER-3α” coupled climate model of intermediate complexity, the effect of a stratification-dependent vertical diffusivity on the sensitivity of the Atlantic Ocean meridional overturning circulation to perturbations in freshwater forcing is investigated. The vertical diffusivity κ is calculated as κ ∼ N−α, where N is the local buoyancy frequency and the parameter α is a measure of the sensitivity of the vertical diffusivity to changes in stratification. Independent of α, the stratification of the deep ocean is weakly increased as a response to an anomalous freshwater flux in the North Atlantic in these experiments. In the region of freshwater forcing and north of it this is caused by the freshwater anomaly itself, but farther south it is associated with anomalously warm surface waters caused by a reduction of the northward oceanic heat transport. Subsequently, and in opposition to results from previous studies, the overturning is reduced by the anomalous freshwater flux, independent of the choice of α. However, the amount of reduction in overturning following a freshwater perturbation is found to depend critically on the choice of the mixing sensitivity α. If α < αcr, the response is similar to the model’s response using constant vertical diffusivity (α = 0). For α > αcr, a sharp increase of the sensitivity is found. The value of αcr is found to be between 0.5 and 1. A general feedback is proposed explaining this threshold behavior: if α is large, both positive and negative perturbations of stratification are amplified by associated changes in diffusivity. In the experiments presented here, this enhances the initial positive stratification anomaly in northern high latitudes, which is created by the anomalous freshwater flux. As a result, convection is strongly reduced, and the overturning is significantly weakened.

scholarly journals The Roles of Wind and Sea Ice in Driving the Deglacial Change in the Southern Ocean Upwelling: A Modeling Study

2021 ◽  
Vol 13 (1) ◽  
pp. 353
Author(s):  
Gagan Mandal ◽  
Shih-Yu Lee ◽  
Jia-Yuh Yu
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Southern Hemisphere ◽  
Wind Stress ◽  
Deep Ocean ◽  
Last Deglaciation ◽  
Buoyancy Forcing ◽  
Antarctic Sea Ice ◽  
Freshwater Fluxes ◽  
Deep Ocean Water

The Southern Ocean (SO) played a fundamental role in the deglacial climate system by exchanging carbon-rich deep ocean water with the surface. The contribution of the SO’s physical mechanisms toward improving our understanding of SO upwelling’s dynamical changes is developing. Here, we investigated the simulated transient SO atmosphere, ocean, and sea ice evolution during the last deglaciation in a fully coupled Earth system model. Our results showed that decreases in SO upwelling followed the weakening of the Southern Hemisphere surface westerlies, wind stress forcing, and Antarctic sea ice coverage from the Last Glacial Maximum to the Heinrich Stadial 1 and the Younger Dryas. Our results support the idea that the SO upwelling is primarily driven by wind stress forcing. However, during the onset of the Holocene, SO upwelling increased while the strength of the wind stress decreased. The Antarctic sea ice change controlled the salt and freshwater fluxes, ocean density, and buoyancy flux, thereby influencing the SO’s dynamics. Our study highlighted the dynamic linkage of the Southern Hemisphere westerlies, ocean, and sea ice in the SO’s latitudes. Furthermore, it emphasized that zonal wind stress forcing and buoyancy forcing control by sea ice together regulate the change in the SO upwelling.

Evaluation of the numerical wave model (WaveWatch III) for wave simulation in the Antarctic marginal ice zone

2020 ◽  
Author(s):  
Marzieh H. Derkani ◽  
Katrin Hessner ◽  
Stefan Zieger ◽  
Filippo Nelli ◽  
Alberto Alberello ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Southern Hemisphere ◽  
Global Climate ◽  
Wave Model ◽  
Open Ocean ◽  
Wavewatch Iii ◽  
Marginal Ice Zone ◽  
The Antarctic ◽  
The Impact

<p>The Southern Ocean is the birthplace of the fiercest waves on the Earth, which play a fundamental role in global climate by regulating momentum, heat and gas exchanges between the atmosphere and ocean. At high latitudes, waves interact with Antarctic sea ice, another crucial player of the Earth's climate system, modulating its expansion in the winter and its retreat in summer and hence affecting the global albedo. Despite the impact of waves on climate, global wave models are considerably biased in the Southern Hemisphere, due to the scarcity of observations in these remote waters. This is exacerbated in the marginal ice zone, the region of ice-covered water between the compact ice or land and the open ocean, where surface waves, upper ocean and atmosphere interact with sea ice but the dominant physics are still largely unknown. To improve our understanding of physical processes in Southern Ocean and model capabilities, the Antarctic Circumnavigation Expedition (ACE) sailed these waters from December 2016 to March 2017 to acquire wave data (among other climate variables) both in the open ocean and Antarctic marginal ice zone. Observations were gathered using a radar-based wave and surface current monitoring system (WaMoS-II) built on board of the research icebreaker Akademik Tryoshnikov. Here, we discuss how these observations underpin the set up, calibration and validation of the WaveWatch III wave model over a domain covering the entire Southern Hemisphere, therefore spanning from tropical waters to the edge of sea ice (open waters only). The calibrated model will then be used to carry out a thorough assessment of different sea ice modules, to evaluate accuracy of predictions in the marginal ice zone. Test cases of waves-in-ice recorded during the Antarctic Circumnavigation Expeditions will be discussed in details.</p>

Circumpolar Deep Water Circulation and Variability in a Coupled Climate Model

2006 ◽  
Vol 36 (8) ◽  
pp. 1523-1552 ◽  
Author(s):  
Agus Santoso ◽  
Matthew H. England ◽  
Anthony C. Hirst
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Time Scales ◽  
North Atlantic ◽  
Deep Water ◽  
Climate Model ◽  
Coupled Climate Model ◽  
Circumpolar Deep Water ◽  
The North ◽  
The North Atlantic

Abstract The natural variability of Circumpolar Deep Water (CDW) is analyzed using a long-term integration of a coupled climate model. The variability is decomposed using a standard EOF analysis into three separate modes accounting for 68% and 82% of the total variance in the upper and lower CDW layers, respectively. The first mode exhibits an interbasin-scale variability on multicentennial time scales, originating in the North Atlantic and flowing southward into the Southern Ocean via North Atlantic Deep Water (NADW). Salinity dipole anomalies appear to propagate around the Atlantic meridional overturning circulation on these time scales with the strengthening and weakening of NADW formation. The anomaly propagates northward from the midlatitude subsurface of the South Atlantic and sinks in the North Atlantic before flowing southward along the CDW isopycnal layers. This suggests an interhemispheric connection in the generation of the first CDW variability mode. The second mode shows a localized θ−S variability in the Brazil–Malvinas confluence zone on multidecadal to centennial time scales. Heat and salt budget analyses reveal that this variability is controlled by meridional advection driven by fluctuations in the strength of the Deep Western Boundary and the Malvinas Currents. The third mode suggests an Antarctic Intermediate Water source in the South Pacific contributing to variability in upper CDW. It is further found that NADW formation is mainly buoyancy driven on the time scales resolved, with only a weak connection with Southern Hemisphere winds. On the other hand, Southern Hemisphere winds have a more direct influence on the rate of NADW outflow into the Southern Ocean. The model’s spatial pattern of θ−S variability is consistent with the limited observational record in the Southern Hemisphere. However, some observations of decadal CDW θ−S changes are beyond that seen in the model in its unperturbed state.

scholarly journals Southern Ocean Sector Centennial Climate Variability and Recent Decadal Trends

2013 ◽  
Vol 26 (19) ◽  
pp. 7767-7782 ◽  
Author(s):  
Mojib Latif ◽  
Torge Martin ◽  
Wonsun Park
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Atmospheric Circulation ◽  
Climate Model ◽  
Deep Convection ◽  
Surface Air Temperature ◽  
Weddell Sea ◽  
Sea Ice Extent ◽  
Low Level ◽  
Antarctic Sea Ice

Abstract Evidence is presented for the notion that some contribution to the recent decadal trends observed in the Southern Hemisphere, including the lack of a strong Southern Ocean surface warming, may have originated from longer-term internal centennial variability originating in the Southern Ocean. The existence of such centennial variability is supported by the instrumental sea surface temperatures (SSTs), a multimillennial reconstruction of Tasmanian summer temperatures from tree rings, and a millennial control integration of the Kiel Climate Model (KCM). The model variability was previously shown to be linked to changes in Weddell Sea deep convection. During phases of deep convection the surface Southern Ocean warms, the abyssal Southern Ocean cools, Antarctic sea ice extent retreats, and the low-level atmospheric circulation over the Southern Ocean weakens. After the halt of deep convection the surface Southern Ocean cools, the abyssal Southern Ocean warms, Antarctic sea ice expands, and the low-level atmospheric circulation over the Southern Ocean intensifies, consistent with what has been observed during the recent decades. A strong sensitivity of the time scale to model formulation is noted. In the KCM, the centennial variability is associated with global-average surface air temperature (SAT) changes of the order of a few tenths of a degree per century. The model results thus suggest that internal centennial variability originating in the Southern Ocean should be considered in addition to other internal variability and external forcing when discussing the climate of the twentieth century and projecting that of the twenty-first century.

scholarly journals Impact of meltwater on high-latitude early Last Interglacial climate

2016 ◽  
Vol 12 (9) ◽  
pp. 1919-1932 ◽  
Author(s):  
Emma J. Stone ◽  
Emilie Capron ◽  
Daniel J. Lunt ◽  
Antony J. Payne ◽  
Joy S. Singarayer ◽  
...  
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Climate Model ◽  
Model Data ◽  
Last Interglacial ◽  
Model Simulations ◽  
The North ◽  
Freshwater Forcing ◽  
The North Atlantic ◽  
Climate Model Simulations

Abstract. Recent data compilations of the early Last Interglacial period have indicated a bipolar temperature response at 130 ka, with colder-than-present temperatures in the North Atlantic and warmer-than-present temperatures in the Southern Ocean and over Antarctica. However, climate model simulations of this period have been unable to reproduce this response, when only orbital and greenhouse gas forcings are considered in a climate model framework. Using a full-complexity general circulation model we perform climate model simulations representative of 130 ka conditions which include a magnitude of freshwater forcing derived from data at this time. We show that this meltwater from the remnant Northern Hemisphere ice sheets during the glacial–interglacial transition produces a modelled climate response similar to the observed colder-than-present temperatures in the North Atlantic at 130 ka and also results in warmer-than-present temperatures in the Southern Ocean via the bipolar seesaw mechanism. Further simulations in which the West Antarctic Ice Sheet is also removed lead to warming in East Antarctica and the Southern Ocean but do not appreciably improve the model–data comparison. This integrated model–data approach provides evidence that Northern Hemisphere freshwater forcing is an important player in the evolution of early Last Interglacial climate.

scholarly journals Global atmospheric responses to Antarctic forcing

1998 ◽  
Vol 27 ◽  
pp. 521-527 ◽  
Author(s):  
David H. Bromwich ◽  
Biao Chen ◽  
Keith M. Hines ◽  
Richard I. Cullather
Keyword(s):  
Sea Ice ◽  
Southern Hemisphere ◽  
Climate Model ◽  
East Asian Monsoon ◽  
Southern China ◽  
Open Water ◽  
Northern China ◽  
Ice Shelves ◽  
Antarctic Sea Ice ◽  
Mean Fields

To evaluate the greatest impact that sea-ice anomalies around Antarctica could have on the global atmosphere, 15 year seasonal cycle simulations are conducted with the U.S. National Center for Atmospheric Research Community Climate Model version 2.1. Sensitivity simulations are performed with the following conditions: (1) all sea ice in the Southern Hemisphere is replaced by year-round open water, but the permanent ice shelves are retained (NSIS); and (2) all sea ice in the Southern Hemisphere and the major ice shelves are removed and replaced by open water (NISH). The results are compared to a standard run (CNT) with boundary conditions set for the present climate. The comparison shows that trains of positive and negative anomalies in zonal-mean fields extend into the tropical latitudes of the Northern Hemisphere. Anomalies are largest during April-October. The additional removal of the ice shelves in NISH enhances the response, as zonally averaged anomalies are similar in pattern to those in NSIS but are roughly twice as large poleward of 50° S, and only slightly larger farther north. Anomalies in the eddy fields are found in both hemispheres. in NISH, and to a lesser degree in NSIS. these anomalies appear to be related to a delayed northern advance over China during June of the rain front associated with the summer monsoon. Consequently, precipitation is enhanced in middle and southern China and decreased in northern China. Observational analyses have also found links between Antarctic sea-ice variations and modulations of the East Asian monsoon.

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