scholarly journals Dynamical drivers of Greenland blocking in climate models

2021 ◽  
Vol 2 (4) ◽  
pp. 1131-1148
Author(s):  
Clio Michel ◽  
Erica Madonna ◽  
Clemens Spensberger ◽  
Camille Li ◽  
Stephen Outten
Keyword(s):  
North Atlantic ◽  
Wave Breaking ◽  
Climate Models ◽  
Rare Event ◽  
Historical Period ◽  
The North ◽  
Strong Surface ◽  
Correct Representation ◽  
The North Atlantic ◽  
Projected Changes

Abstract. Blocking over Greenland is known to lead to strong surface impacts, such as ice sheet melting, and a change in its future frequency can have important consequences. However, as previous studies demonstrated, climate models underestimate the blocking frequency for the historical period. Even though some improvements have recently been made, the reasons for the model biases are still unclear. This study investigates whether models with realistic Greenland blocking frequency in winter have a correct representation of its dynamical drivers, most importantly, cyclonic wave breaking (CWB). Because blocking is a rare event and its representation is model-dependent, we use a multi-model large ensemble. We focus on two models that show typical Greenland blocking features, namely a ridge over Greenland and an equatorward-shifted jet over the North Atlantic. ECHAM6.3-LR has the best representation of CWB of the models investigated but only the second best representation of Greenland blocking frequency, which is underestimated by a factor of 2. While MIROC5 has the most realistic Greenland blocking frequency, it also has the largest (negative) CWB frequency bias, suggesting that another mechanism leads to blocking in this model. Composites over Greenland blocking days show that the present and future experiments of each model are very similar to each other in both amplitude and pattern and that there is no significant change in Greenland blocking frequency in the future. However, these projected changes in blocking frequency are highly uncertain as long as the mechanisms leading to blocking formation and maintenance in models remain poorly understood.

scholarly journals Dynamical drivers of Greenland blocking in climate models

2021 ◽  
Author(s):  
Clio Michel ◽  
Erica Madonna ◽  
Clemens Spensberger ◽  
Camille Li ◽  
Stephen Outten
Keyword(s):  
North Atlantic ◽  
Wave Breaking ◽  
Climate Models ◽  
Rare Event ◽  
Historical Period ◽  
Future Projections ◽  
The North ◽  
Strong Surface ◽  
Correct Representation ◽  
The North Atlantic

Abstract. Blocking over Greenland is known to lead to strong surface impacts, such as ice sheet melting, and a change in its future frequency can have important consequences. However, as previous studies demonstrated, climate models underestimate the blocking frequency for the historical period. Even though some improvements have recently been made, the reasons for the model biases are still unclear. This study investigates whether models with realistic Greenland blocking frequency have a correct representation of its dynamical drivers, most importantly, cyclonic wave breaking (CWB). Because blocking is a rare event and its representation is model-dependent, we here use a multi-model large ensemble. All of the models underestimate CWB frequency and four out of five models underestimate the frequency of Greenland blocking. Nevertheless, they all show the typical Greenland blocking features, namely a ridge with anticyclonic anomaly over Greenland and an equatorward-shifted jet over the North Atlantic. However, we find that the model with the most realistic Greenland blocking frequency, MIROC5, has the most negative CWB frequency bias. While in reanalysis CWB is an important mechanism leading to blocking formation, the link between blocking and CWB is much weaker in MIROC5, suggesting that another mechanism leads to blocking in this model. Composites over Greenland blocking days show that the present and future experiments of each model are very similar to each other in both amplitude and pattern and that there is no significant change of Greenland blocking frequency in the future. However, this result must be taken with caution since the Greenland blocking driver is not well represented in all models. This highlights the need to accurately understand and represent the mechanisms leading to blocking formation and maintenance in models to get more reliable future projections.

scholarly journals Tropical cyclones and climate change: Recent results and uncertainties

2020 ◽  
Author(s):  
Suzana Camargo ◽  
Chia-Ying Lee ◽  
Adam Sobel ◽  
Michael Tippett
Keyword(s):  
Climate Change ◽  
Tropical Cyclones ◽  
North Atlantic ◽  
Climate Models ◽  
Dynamical Downscaling ◽  
Current Knowledge ◽  
Historical Period ◽  
The North ◽  
The North Atlantic ◽  
High Resolution Models

<p>Here I will describe recent results on the influence of climate change on tropical cyclones (TC) using the Columbia Hazard (CHAZ) model. Using environmental conditions from reanalysis and climate models and a statistical-dynamical downscaling methodology (Lee et al. 2018), CHAZ generates synthetic TCs that can be used to analyze TC risk.  I will first discuss the current knowledge and uncertainties in TC frequency projections. Then I will present our recent projections on TC frequency using CHAZ. Focusing on the North Atlantic, I will finish by discussing how we can use a combination of observations, high-resolution models and CHAZ synthetic TCs in the historical period to inform the reliability of the models' TC frequency projections. </p><p>Reference:</p><p>Lee, C.-Y., M.K. Tippett, A.H. Sobel, and S.J. Camargo, 2018. An environmentally forced tropical cyclone hazard model. J. Adv. Model. Earth Sys., 10, doi: 10.1002/2017MS001186.</p>

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 On the Interpretation of the North Atlantic Averaged Sea Surface Temperature

2020 ◽  
Vol 33 (14) ◽  
pp. 6025-6045
Author(s):  
Jing Sun ◽  
Mojib Latif ◽  
Wonsun Park ◽  
Taewook Park
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Climate Models ◽  
Ocean Dynamics ◽  
Sea Surface ◽  
The North ◽  
Sst Variability ◽  
The North Atlantic ◽  
Different Time Scales

AbstractThe North Atlantic (NA) basin-averaged sea surface temperature (NASST) is often used as an index to study climate variability in the NA sector. However, there is still some debate on what drives it. Based on observations and climate models, an analysis of the different influences on the NASST index and its low-pass filtered version, the Atlantic multidecadal oscillation (AMO) index, is provided. In particular, the relationships of the two indices with some of its mechanistic drivers including the Atlantic meridional overturning circulation (AMOC) are investigated. In observations, the NASST index accounts for significant SST variability over the tropical and subpolar NA. The NASST index is shown to lump together SST variability originating from different mechanisms operating on different time scales. The AMO index emphasizes the subpolar SST variability. In the climate models, the SST-anomaly pattern associated with the NASST index is similar. The AMO index, however, only represents pronounced SST variability over the extratropical NA, and this variability is significantly linked to the AMOC. There is a sensitivity of this linkage to the cold NA SST bias observed in many climate models. Models suffering from a large cold bias exhibit a relatively weak linkage between the AMOC and AMO and vice versa. Finally, the basin-averaged SST in its unfiltered form, which has been used to question a strong influence of ocean dynamics on NA SST variability, mixes together multiple types of variability occurring on different time scales and therefore underemphasizes the role of ocean dynamics in the multidecadal variability of NA SSTs.

scholarly journals Dynamical Properties of the North Atlantic Atmospheric Circulation in the Past 150 Years in CMIP5 Models and the 20CRv2c Reanalysis

2018 ◽  
Vol 31 (15) ◽  
pp. 6097-6111 ◽  
Author(s):  
David Rodrigues ◽  
M. Carmen Alvarez-Castro ◽  
Gabriele Messori ◽  
Pascal Yiou ◽  
Yoann Robin ◽  
...  
Keyword(s):  
Dynamical Systems ◽  
Atmospheric Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
Cmip5 Models ◽  
Historical Period ◽  
Local Dimension ◽  
The North ◽  
The North Atlantic ◽  
Over Time

It is of fundamental importance to evaluate the ability of climate models to capture the large-scale atmospheric circulation patterns and, in the context of a rapidly increasing greenhouse forcing, the robustness of the changes simulated in these patterns over time. Here we approach this problem from an innovative point of view based on dynamical systems theory. We characterize the atmospheric circulation over the North Atlantic in the CMIP5 historical simulations (1851–2000) in terms of two instantaneous metrics: local dimension of the attractor and stability of phase-space trajectories. We then use these metrics to compare the models to the Twentieth Century Reanalysis version 2c (20CRv2c) over the same historical period. The comparison suggests that (i) most models capture to some degree the median attractor properties, and models with finer grids generally perform better; (ii) in most models the extremes in the dynamical systems metrics match large-scale patterns similar to those found in the reanalysis; (iii) changes in the attractor properties observed for the ensemble-mean 20CRv2c are artifacts resulting from inhomogeneities in the standard deviation of the ensemble over time; and (iv) the long-term trends in local dimension observed among the 56 members of the 20CR ensemble have the same sign as those observed in the CMIP5 multimodel mean, although the multimodel trend is much weaker.

scholarly journals Dynamical Evolution of North Atlantic Ridges and Poleward Jet Stream Displacements

2011 ◽  
Vol 68 (5) ◽  
pp. 954-963 ◽  
Author(s):  
Tim Woollings ◽  
Joaquim G. Pinto ◽  
João A. Santos
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
Wave Breaking ◽  
Large Scale ◽  
Wave Train ◽  
Jet Stream ◽  
Rossby Wave Breaking ◽  
The North ◽  
The North Atlantic ◽  
Circulation Anomalies

Abstract The development of a particular wintertime atmospheric circulation regime over the North Atlantic, comprising a northward shift of the North Atlantic eddy-driven jet stream and an associated strong and persistent ridge in the subtropics, is investigated. Several different methods of analysis are combined to describe the temporal evolution of the events and relate it to shifts in the phase of the North Atlantic Oscillation and East Atlantic pattern. First, the authors identify a close relationship between northward shifts of the eddy-driven jet, the establishment and maintenance of strong and persistent ridges in the subtropics, and the occurrence of upper-tropospheric anticyclonic Rossby wave breaking over Iberia. Clear tropospheric precursors are evident prior to the development of the regime, suggesting a preconditioning of the Atlantic jet stream and an upstream influence via a large-scale Rossby wave train from the North Pacific. Transient (2–6 days) eddy forcing plays a dual role, contributing to both the initiation and then the maintenance of the circulation anomalies. During the regime there is enhanced occurrence of anticyclonic Rossby wave breaking, which may be described as low-latitude blocking-like events over the southeastern North Atlantic. A strong ridge is already established at the time of wave-breaking onset, suggesting that the role of wave-breaking events is to amplify the circulation anomalies rather than to initiate them. Wave breaking also seems to enhance the persistence, since it is unlikely that a persistent ridge event occurs without being also accompanied by wave breaking.

scholarly journals Quantifying the rarity of extreme multi-decadal trends: how unusual was the late twentieth century trend in the North Atlantic Oscillation?

2021 ◽  
Author(s):  
R. Eade ◽  
D. B. Stephenson ◽  
A. A. Scaife ◽  
D. M. Smith
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Serial Correlation ◽  
Climate Models ◽  
Climate Indices ◽  
Positive Trend ◽  
Climate Trends ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

AbstractClimate trends over multiple decades are important drivers of regional climate change that need to be considered for climate resilience. Of particular importance are extreme trends that society may not be expecting and is not well adapted to. This study investigates approaches to assess the likelihood of maximum moving window trends in historical records of climate indices by making use of simulations from climate models and stochastic time series models with short- and long-range dependence. These approaches are applied to assess the unusualness of the large positive trend that occurred in the North Atlantic Oscillation (NAO) index between the 1960s to 1990s. By considering stochastic models, we show that the chance of extreme trends is determined by the variance of the trend process, which generally increases when there is more serial correlation in the index series. We find that the Coupled Model Intercomparison Project (CMIP5 + 6) historical simulations have very rarely (around 1 in 200 chance) simulated maximum trends greater than the observed maximum. Consistent with this, the NAO indices simulated by CMIP models were found to resemble white noise, with almost no serial correlation, in contrast to the observed NAO which exhibits year-to-year correlation. Stochastic model best fits to the observed NAO suggest an unlikely chance (around 1 in 20) for there to be maximum 31-year NAO trends as large as the maximum observed since 1860. This suggests that current climate models do not fully represent important aspects of the mechanism for low frequency variability of the NAO.

scholarly journals Are the Transient and Equilibrium Climate Change Patterns Similar in Response to Increased CO2?

2020 ◽  
Vol 33 (18) ◽  
pp. 8003-8023
Author(s):  
Danqing Huang ◽  
Aiguo Dai ◽  
Jian Zhu
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Climate Models ◽  
Surface Air Temperature ◽  
The Arctic ◽  
The North ◽  
Temperature And Precipitation ◽  
Time Periods ◽  
The North Atlantic ◽  
Subtropical Oceans

AbstractAfter a CO2 increase, whether the early transient and final equilibrium climate change patterns are similar has major implications. Here, we analyze long-term simulations from multiple climate models under increased CO2, together with the extended simulations from CMIP5, to compare the transient and equilibrium climate change patterns under different forcing scenarios. Results show that the normalized warming patterns (per 1 K of global warming) are broadly similar among different forcing scenarios (including abrupt 2 × CO2, 4 × CO2, and 1% CO2 increase per year) and during different time periods, except for the first 50 years or so when warming is weaker over the North Atlantic and Southern Ocean but stronger over most continents. During the first 200 years, this consistency is stronger over land than over ocean, but is lower in midlatitudes than other regions. Normalized precipitation change patterns are also similar, albeit to a lesser degree, among different forcing scenarios and across different time periods, although noticeable differences exist during the first few hundred years with smaller increases over the tropical Pacific. Precipitation over many subtropical oceans and land areas decreases consistently under different forcing scenarios and over all time periods. In particular, the transient and near-equilibrium change patterns for both surface air temperature and precipitation are similar over most of the globe, except for the North Atlantic warming hole, which is mainly a transient feature. The Arctic amplification and land–ocean warming contrast are largest during the first 100–200 years after CO2 quadrupling but they still exist in the equilibrium response.

scholarly journals Improvement in the decadal prediction skill of the northern hemisphere extra-tropical winter circulation through increased model resolution

2019 ◽  
Author(s):  
Mareike Schuster ◽  
Jens Grieger ◽  
Andy Richling ◽  
Thomas Schartner ◽  
Sebastian Illing ◽  
...  
Keyword(s):  
Northern Hemisphere ◽  
North Atlantic ◽  
Climate Models ◽  
Source Region ◽  
Prediction Skill ◽  
Decadal Prediction ◽  
North Atlantic Current ◽  
The North ◽  
The North Atlantic ◽  
Socio Economic Impact

Abstract. In this study the latest version of the MiKlip decadal hindcast system is analyzed and the effect of different horizontal and vertical resolutions on the prediction skill of the northern hemisphere extra-tropical atmospheric circulation is assessed. Four metrics – the stormtrack, blocking frequencies, cyclone frequencies and windstorm frequencies – are analyzed with respect to the anomaly correlation of their winter averages. The model bias and hindcast skill are evaluated in both, a lower resolution version (LR, atm: T63L47, ocean: 1.5° L40) and a higher resolution version (HR, atm: T127L95, ocean: 0.4° L40) of the MPI-ESM system, for the lead years 2–5 using initializations between 1978 and 2012. While the LR version shows common shortcomings of lower resolution climate models, e.g. a too zonal stormtrack and a negative bias of blocking frequencies over the eastern North Atlantic and Europe, the HR version works against these biases. As a result, a functional chain of significantly improved decadal prediction skill between all four metrics is found with the increase of the spatial resolution. While the stormtrack, is significantly improved primarily over the main source region of synoptic activity – the North Atlantic Current, the other extra-tropical measures experience a significant improvement downstream thereof. Thus, the skill of the cyclone frequencies is significantly improved over the central North Atlantic and Northern Europe, the skill of the blocking frequencies is significantly improved over the Mediterranean, Scandinavia and Eastern Europe and the skill of the windstorms is significantly improved over Newfoundland and Central Europe. Not only is the skill improved with the increase in resolution, but the HR system itself exhibits significant skill over large areas of the North Atlantic and European sector for all four circulation metrics. These results are particularly promising regarding the high socio-economic impact of European winter windstorms and blocking situations.

North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems

2021 ◽  
Author(s):  
Shenjie Zhou ◽  
Xiaoming Zhai ◽  
Ian Renfrew
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Ensemble Prediction ◽  
Subpolar Gyre ◽  
Atmospheric Forcing ◽  
The North ◽  
The North Atlantic

<p>The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (< 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization – based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems<sup> </sup>– to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges – an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.</p>

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