scholarly journals North Atlantic Eddy-Driven Jet in Interglacial and Glacial Winter Climates

2015 ◽  
Vol 28 (10) ◽  
pp. 3977-3997 ◽  
Author(s):  
Niklaus Merz ◽  
Christoph C. Raible ◽  
Tim Woollings
Keyword(s):  
Boundary Conditions ◽  
North Atlantic ◽  
Momentum Flux ◽  
Climate Models ◽  
Climate Model ◽  
Stationary Waves ◽  
Winter Climate ◽  
The North ◽  
Northern Areas ◽  
Westerly Flow

Abstract The atmospheric westerly flow in the North Atlantic (NA) sector is dominated by atmospheric waves or eddies generating via momentum flux convergence, the so-called eddy-driven jet. The position of this jet is variable and shows for the present-day winter climate three preferred latitudinal states: a northern, central, and southern position in the NA. Here, the authors analyze the behavior of the eddy-driven jet under different glacial and interglacial boundary conditions using atmosphere–land-only simulations with the CCSM4 climate model. As state-of-the-art climate models tend to underestimate the trimodality of the jet latitude, the authors apply a bias correction and successfully extract the trimodal behavior of the jet within CCSM4. The analysis shows that during interglacial times (i.e., the early Holocene and the Eemian) the preferred jet positions are rather stable and the observed multimodality is the typical interglacial character of the jet. During glacial times, the jet is strongly enhanced, its position is shifted southward, and the trimodal behavior vanishes. This is mainly due to the presence of the Laurentide ice sheet (LIS). The LIS enhances stationary waves downstream, thereby accelerating and displacing the NA eddy-driven jet by anomalous stationary momentum flux convergence. Additionally, changes in the transient eddy activity caused by topography changes as well as other glacial boundary conditions lead to an acceleration of the westerly winds over the southern NA at the expense of more northern areas. Consequently, both stationary and transient eddies foster the southward shift of the NA eddy-driven jet during glacial winter times.

scholarly journals Elevated dust depositions in West Asia linked to ocean–atmosphere shifts during North Atlantic cold events

2020 ◽  
Vol 117 (31) ◽  
pp. 18272-18277 ◽  
Author(s):  
Reza Safaierad ◽  
Mahyar Mohtadi ◽  
Bernd Zolitschka ◽  
Yusuke Yokoyama ◽  
Christoph Vogt ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Africa ◽  
North Atlantic ◽  
Ice Sheets ◽  
Stationary Waves ◽  
Last Deglaciation ◽  
Winter Climate ◽  
The North ◽  
Cold Events ◽  
The Last Deglaciation

Rapid North Atlantic cooling events during the last deglaciation caused atmospheric reorganizations on global and regional scales. Their impact on Asian climate has been investigated for monsoonal domains, but remains largely unknown in westerly wind-dominated semiarid regions. Here we generate a dust record from southeastern Iran spanning the period 19 to 7 cal. ka B.P. We find a direct link between frequent occurrences of dust plumes originating from the Arabian Peninsula and North Africa and rapid southward shifts of the westerlies associated with changes of the winter stationary waves during Heinrich Stadial 1, the Younger Dryas, the Preboreal Oscillation, and the 8.2-ka event. Dust input rises and falls abruptly at the transitions into and out of these cooling events, which we attribute to changes in the ocean circulation strength that are modulated by the North Atlantic winter sea-ice cover. Our findings reveal that waxing and waning of North American ice sheets have a stronger influence than those of European ice sheets on the winter climate over West Asia.

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.

Reconciling the role of the Labrador Sea overturning circulation in OSNAP and climate models

2020 ◽  
Author(s):  
Susan Lozier ◽  
Matthew Menary ◽  
Laura Jackson
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Climate Model ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Overturning Circulation ◽  
Important Indicator ◽  
The North ◽  
North Atlantic Subpolar Gyre ◽  
Meridional Overturning

<p>The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate change and variability. Since continuous, direct measurements of the overturning strength in the North Atlantic subpolar gyre (SPG) have been unavailable until recently, the understanding, based largely on climate models, is that the Labrador Sea has an important role in shaping the evolution of the AMOC. However, a recent high profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence of the credibility of the AMOC representation in climate models. Here, we reconcile these viewpoints by comparing the OSNAP data with a new, high-resolution coupled climate model: HadGEM3-GC3.1-MM. Unlike many previous models, we find our model compares well to the OSNAP overturning observations. Furthermore, overturning variability across the eastern OSNAP section (OSNAP-E), and not in the Labrador Sea region, appears linked to AMOC variability further south. Labrador Sea densities are shown to be an important indicator of downstream AMOC variability, but these densities are driven by upstream variability across OSNAP-E rather than local processes in the Labrador Sea.</p>

scholarly journals Comparing climate time series – Part 2: A multivariate test

2021 ◽  
Vol 7 (2) ◽  
pp. 73-85
Author(s):  
Timothy DelSole ◽  
Michael K. Tippett
Keyword(s):  
Time Series ◽  
North Atlantic ◽  
Climate Models ◽  
Climate Model ◽  
Multivariate Time Series ◽  
Internal Variability ◽  
Var Model ◽  
Anthropogenic Aerosols ◽  
The North ◽  
The North Atlantic

Abstract. This paper proposes a criterion for deciding whether climate model simulations are consistent with observations. Importantly, the criterion accounts for correlations in both space and time. The basic idea is to fit each multivariate time series to a vector autoregressive (VAR) model and then test the hypothesis that the parameters of the two models are equal. In the special case of a first-order VAR model, the model is a linear inverse model (LIM) and the test constitutes a difference-in-LIM test. This test is applied to decide whether climate models generate realistic internal variability of annual mean North Atlantic sea surface temperature. Given the disputed origin of multidecadal variability in the North Atlantic (e.g., some studies argue it is forced by anthropogenic aerosols, while others argue it arises naturally from internal variability), the time series are filtered in two different ways appropriate to the two driving mechanisms. In either case, only a few climate models out of three dozen are found to generate internal variability consistent with observations. In fact, it is shown that climate models differ not only from observations, but also from each other, unless they come from the same modeling center. In addition to these discrepancies in internal variability, other studies show that models exhibit significant discrepancies with observations in terms of the response to external forcing. Taken together, these discrepancies imply that, at the present time, climate models do not provide a satisfactory explanation of observed variability in the North Atlantic.

scholarly journals A Mechanism of Internal Decadal Atlantic Ocean Variability in a High-Resolution Coupled Climate Model

2015 ◽  
Vol 28 (19) ◽  
pp. 7764-7785 ◽  
Author(s):  
Matthew B. Menary ◽  
Daniel L. R. Hodson ◽  
Jon I. Robson ◽  
Rowan T. Sutton ◽  
Richard A. Wood
Keyword(s):  
High Resolution ◽  
Atlantic Ocean ◽  
North Atlantic ◽  
Climate Models ◽  
Climate Model ◽  
Heat Content ◽  
Coupled Climate Model ◽  
North Atlantic Current ◽  
The North ◽  
The North Atlantic

Abstract The North Atlantic Ocean subpolar gyre (NA SPG) is an important region for initializing decadal climate forecasts. Climate model simulations and paleoclimate reconstructions have indicated that this region could also exhibit large, internally generated variability on decadal time scales. Understanding these modes of variability, their consistency across models, and the conditions in which they exist is clearly important for improving the skill of decadal predictions—particularly when these predictions are made with the same underlying climate models. This study describes and analyzes a mode of internal variability in the NA SPG in a state-of-the-art, high-resolution, coupled climate model. This mode has a period of 17 yr and explains 15%–30% of the annual variance in related ocean indices. It arises because of the advection of heat content anomalies around the NA SPG. Anomalous circulation drives the variability in the southern half of the NA SPG, while mean circulation and anomalous temperatures are important in the northern half. A negative feedback between Labrador Sea temperatures/densities and those in the North Atlantic Current (NAC) is identified, which allows for the phase reversal. The atmosphere is found to act as a positive feedback on this mode via the North Atlantic Oscillation (NAO), which itself exhibits a spectral peak at 17 yr. Decadal ocean density changes associated with this mode are driven by variations in temperature rather than salinity—a point which models often disagree on and which may affect the veracity of the underlying assumptions of anomaly-assimilating decadal prediction methodologies.

Data assimilation of oceanic proxies in the North Atlantic over the Common Era

2020 ◽  
Author(s):  
Hugues Goosse ◽  
Gaelle Gilson ◽  
François Klein ◽  
Guillaume Lenoir ◽  
Anne de Vernal ◽  
...  
Keyword(s):  
Data Assimilation ◽  
North Atlantic ◽  
Climate Models ◽  
Climate Model ◽  
Online Data ◽  
Sea Ice Extent ◽  
The North ◽  
The Common ◽  
The North Atlantic ◽  
Common Era

<p>The mismatch between oceanic proxy data and climate model results over the past millennia has been a long-lasting challenge. Although both are valuable sources of paleoclimate information, there is a strong discrepancy in variance between models and proxies, so that they cannot be compared directly. In addition, local sea-surface temperature (SST) reconstructions are often inconsistent among proxy types. We first performed several offline data assimilation experiments with different standardized SST proxy datasets using the climate models LOVECLIM and CESM in order to investigate the effect of proxy selection on local and regional reconstructions over the Common Era (0-2000 CE). All experiments work technically at the local scale, but the spatial pattern of the reconstructions vary with the type(s), number and density of proxies, and, where there is no proxy, the choice of the model. We then developed empirical scaling factors based on independent SST observations to correct for the discrepancy between model and proxy amplitude. While it is essential to scale proxies, scaling the model leads to complications because of the biases in the sea ice extent. Data assimilation of scaled proxies results in coherent SST reconstructions at the scale of the North Atlantic, with timing and amplitude that are in agreement with those given by forced models. Finally, results are compared to online data assimilation experiments.</p>

Observed seasonal regimes of North-South Atlantic Ocean interbasin exchange

2019 ◽  
Author(s):  
Hamed D. Ibrahim
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Volume Flux ◽  
The South ◽  
The North ◽  
Basin Scale ◽  
Interbasin Exchange ◽  
North And South

North and South Atlantic lateral volume exchange is a key component of the Atlantic Meridional Overturning Circulation (AMOC) embedded in Earth’s climate. Northward AMOC heat transport within this exchange mitigates the large heat loss to the atmosphere in the northern North Atlantic. Because of inadequate climate data, observational basin-scale studies of net interbasin exchange between the North and South Atlantic have been limited. Here ten independent climate datasets, five satellite-derived and five analyses, are synthesized to show that North and South Atlantic climatological net lateral volume exchange is partitioned into two seasonal regimes. From late-May to late-November, net lateral volume flux is from the North to the South Atlantic; whereas from late-November to late-May, net lateral volume flux is from the South to the North Atlantic. This climatological characterization offers a framework for assessing seasonal variations in these basins and provides a constraint for climate models that simulate AMOC dynamics.

scholarly journals Projected Changes in European and North Atlantic Seasonal Wind Climate Derived from CMIP5 Simulations

2019 ◽  
Vol 32 (19) ◽  
pp. 6467-6490 ◽  
Author(s):  
Kimmo Ruosteenoja ◽  
Timo Vihma ◽  
Ari Venäläinen
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Global Climate ◽  
Arctic Sea Ice ◽  
Global Climate Models ◽  
Near Surface ◽  
Wind Speeds ◽  
The North ◽  
Seasonal Wind ◽  
Projected Changes

Abstract Future changes in geostrophic winds over Europe and the North Atlantic region were studied utilizing output data from 21 CMIP5 global climate models (GCMs). Changes in temporal means, extremes, and the joint distribution of speed and direction were considered. In concordance with previous research, the time mean and extreme scalar wind speeds do not change pronouncedly in response to the projected climate change; some degree of weakening occurs in the majority of the domain. Nevertheless, substantial changes in high wind speeds are identified when studying the geostrophic winds from different directions separately. In particular, in northern Europe in autumn and in parts of northwestern Europe in winter, the frequency of strong westerly winds is projected to increase by up to 50%. Concurrently, easterly winds become less common. In addition, we evaluated the potential of the GCMs to simulate changes in the near-surface true wind speeds. In ocean areas, changes in the true and geostrophic winds are mainly consistent and the emerging differences can be explained (e.g., by the retreat of Arctic sea ice). Conversely, in several GCMs the continental wind speed response proved to be predominantly determined by fairly arbitrary changes in the surface properties rather than by changes in the atmospheric circulation. Accordingly, true wind projections derived directly from the model output should be treated with caution since they do not necessarily reflect the actual atmospheric response to global warming.

scholarly journals The use of a flow field correction technique for alleviating the North Atlantic cold bias with application to the Kiel Climate Model

2015 ◽  
Vol 65 (8) ◽  
pp. 1079-1093 ◽  
Author(s):  
Annika Drews ◽  
Richard J. Greatbatch ◽  
Hui Ding ◽  
Mojib Latif ◽  
Wonsun Park
Keyword(s):  
Flow Field ◽  
North Atlantic ◽  
Climate Model ◽  
Cold Bias ◽  
The North ◽  
Correction Technique ◽  
The North Atlantic
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