scholarly journals The remote influence of the extra-tropical stratosphere and tropical troposphere on North Atlantic-European winter circulation

2021 ◽  
Author(s):  
Anna Maidens ◽  
Jeff R Knight ◽  
Adam A Scaife
Keyword(s):  
Surface Pressure ◽  
North Atlantic ◽  
Tropical Atlantic ◽  
Prediction System ◽  
The North ◽  
Winter Conditions ◽  
Winter Circulation ◽  
Tropical Troposphere ◽  
The North Atlantic ◽  
European Sector

<p>Many seasonal forecast systems show skill at monthly to seasonal timescales in predicting the winter North Atlantic Oscillation (NAO), the primary mode of variability in surface pressure over the North Atlantic and European sector.  This skill has practical benefit for prediction of winter conditions over Northern Europe, and arises from the representation of remote teleconnections within the prediction system, such as from the stratosphere or the tropical troposphere.  Despite skill in the NAO, most prediction systems have little skill in other patterns of North Atlantic winter circulation variability, such as East Atlantic Pattern (EAP – the second mode of regional winter surface pressure variability). This is despite the clear contribution that patterns such as the EAP make to European winter climate variability and their demonstrated role in the generation of extreme winter conditions.</p><p> </p><p>We examine the role of the tropical troposphere and extra-tropical stratosphere in driving North Atlantic and European winter circulation patterns, with a focus on teleconnections to the EAP.  We use relaxation experiments, in which a set of seasonal-length hindcasts are run with the atmospheric conditions within the relaxation region constrained to be similar to reanalysis.  These are then compared with an initialised, but otherwise freely evolving, hindcast set, and with reanalysis, in regions outside the relaxation region. The aim is to assess how better prediction of the relaxation regions would influence the skill in prediction of winter atmospheric circulation in the North Atlantic-European sector.</p><p> </p><p>We find that both regions play a role in influencing regional circulation. Tropical tropospheric relaxation in particular increases the reproduction of winter surface pressure anomalies. A key part of this improvement is in the EAP, which is very well reproduced. It is shown that forcing of the EAP occurs via propagating Rossby waves linked to convective anomalies in the tropical Atlantic. In addition, we find that teleconnections from either the tropics or stratosphere lead to reproduction of observed large-scale surface pressure patterns in most winters.  In contrast, the diagnosed response to tropical forcing is rarely matched in the hindcast without relaxation, despite a similar rate of matches with the response to stratospheric forcing. This suggests that while winter stratospheric influences are well represented in the prediction system, tropical influences are under-represented.  The results suggest that the improvement of tropical Atlantic predictability could lead to improvements for European winter predictability, and should be an important focus for future work.</p>

scholarly journals The link between eddy-driven jet variability and weather regimes in the North Atlantic-European sector

2017 ◽  
Vol 143 (708) ◽  
pp. 2960-2972 ◽  
Author(s):  
Erica Madonna ◽  
Camille Li ◽  
Christian M. Grams ◽  
Tim Woollings
Keyword(s):  
North Atlantic ◽  
Weather Regimes ◽  
The North ◽  
The North Atlantic ◽  
Jet Variability ◽  
European Sector

Tropical Air–Sea Interactions Accelerate the Recovery of the Atlantic Meridional Overturning Circulation after a Major Shutdown

2007 ◽  
Vol 20 (19) ◽  
pp. 4940-4956 ◽  
Author(s):  
Uta Krebs ◽  
A. Timmermann
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Tropical Atlantic ◽  
Trade Winds ◽  
The North ◽  
Salinity Anomaly ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract Using a coupled ocean–sea ice–atmosphere model of intermediate complexity, the authors study the influence of air–sea interactions on the stability of the Atlantic Meridional Overturning Circulation (AMOC). Mimicking glacial Heinrich events, a complete shutdown of the AMOC is triggered by the delivery of anomalous freshwater forcing to the northern North Atlantic. Analysis of fully and partially coupled freshwater perturbation experiments under glacial conditions shows that associated changes of the heat transport in the North Atlantic lead to a cooling north of the thermal equator and an associated strengthening of the northeasterly trade winds. Because of advection of cold air and an intensification of the trade winds, the intertropical convergence zone (ITCZ) is shifted southward. Changes of the accumulated precipitation lead to the generation of a positive salinity anomaly in the northern tropical Atlantic and a negative anomaly in the southern tropical Atlantic. During the shutdown phase of the AMOC, cross-equatorial oceanic surface flow is halted, preventing dilution of the positive salinity anomaly in the North Atlantic. Advected northward by the wind-driven ocean circulation, the positive salinity anomaly increases the upper-ocean density in the deep-water formation regions, thereby accelerating the recovery of the AMOC considerably. Partially coupled experiments that neglect tropical air–sea coupling reveal that the recovery time of the AMOC is almost twice as long as in the fully coupled case. The impact of a shutdown of the AMOC on the Indian and Pacific Oceans can be decomposed into atmospheric and oceanic contributions. Temperature anomalies in the Northern Hemisphere are largely controlled by atmospheric circulation anomalies, whereas those in the Southern Hemisphere are strongly determined by ocean dynamical changes and exhibit a time lag of several decades. An intensification of the Pacific meridional overturning cell in the northern North Pacific during the AMOC shutdown can be explained in terms of wind-driven ocean circulation changes acting in concert with global ocean adjustment processes.

scholarly journals Climatology and Interannual Variability of North Atlantic Hurricane Tracks

2005 ◽  
Vol 18 (24) ◽  
pp. 5370-5381 ◽  
Author(s):  
Lian Xie ◽  
Tingzhuang Yan ◽  
Leonard J. Pietrafesa ◽  
John M. Morrison ◽  
Thomas Karl
Keyword(s):  
North Atlantic ◽  
The United States ◽  
Track Density ◽  
Spatial And Temporal Variability ◽  
Tropical Atlantic ◽  
Atlantic Hurricane ◽  
The North ◽  
Hurricane Track ◽  
The North Atlantic ◽  
Hurricane Tracks

Abstract The spatial and temporal variability of North Atlantic hurricane tracks and its possible association with the annual hurricane landfall frequency along the U.S. East Coast are studied using principal component analysis (PCA) of hurricane track density function (HTDF). The results show that, in addition to the well-documented effects of the El Niño–Southern Oscillation (ENSO) and vertical wind shear (VWS), North Atlantic HTDF is strongly modulated by the dipole mode (DM) of Atlantic sea surface temperature (SST) as well as the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO). Specifically, it was found that Atlantic SST DM is the only index that is associated with all top three empirical orthogonal function (EOF) modes of the Atlantic HTDF. ENSO and tropical Atlantic VWS are significantly correlated with the first and the third EOF of the HTDF over the North Atlantic Ocean. The second EOF of North Atlantic HTDF, which represents the “zonal gradient” of North Atlantic hurricane track density, showed no significant correlation with ENSO or with tropical Atlantic VWS. Instead, it is associated with the Atlantic SST DM, and extratropical processes including NAO and AO. Since for a given hurricane season, the preferred hurricane track pattern, together with the overall basinwide hurricane activity, collectively determines the hurricane landfall frequency, the results provide a foundation for the construction of a statistical model that projects the annual number of hurricanes striking the eastern seaboard of the United States.

Atmospheric Teleconnections of Tropical Atlantic Variability: Interhemispheric, Tropical–Extratropical, and Cross-Basin Interactions

2007 ◽  
Vol 20 (5) ◽  
pp. 856-870 ◽  
Author(s):  
Lixin Wu ◽  
Feng He ◽  
Zhengyu Liu ◽  
Chun Li
Keyword(s):  
North Atlantic ◽  
Southern Oscillation ◽  
Wave Train ◽  
Seasonal Migration ◽  
Atmospheric Response ◽  
Tropical Atlantic ◽  
The North ◽  
Atmospheric Teleconnections ◽  
Ocean Atmosphere ◽  
The North Atlantic

Abstract In this paper, the atmospheric teleconnections of the tropical Atlantic SST variability are investigated in a series of coupled ocean–atmosphere modeling experiments. It is found that the tropical Atlantic climate not only displays an apparent interhemispheric link, but also significantly influences the North Atlantic Oscillation (NAO) and the El Niño–Southern Oscillation (ENSO). In spring, the tropical Atlantic SST exhibits an interhemispheric seesaw controlled by the wind–evaporation–SST (WES) feedback that subsequently decays through the mediation of the seasonal migration of the ITCZ. Over the North Atlantic, the tropical Atlantic SST can force a significant coupled NAO–dipole SST response in spring that changes to a coupled wave train–horseshoe SST response in the following summer and fall, and a recurrence of the NAO in the next winter. The seasonal changes of the atmospheric response as well as the recurrence of the next winter’s NAO are driven predominantly by the tropical Atlantic SST itself, while the resulting extratropical SST can enhance the atmospheric response, but it is not a necessary bridge of the winter-to-winter NAO persistency. Over the Pacific, the model demonstrates that the north tropical Atlantic (NTA) SST can also organize an interhemispheric SST seesaw in spring in the eastern equatorial Pacific that subsequently evolves into an ENSO-like pattern in the tropical Pacific through mediation of the ITCZ and equatorial coupled ocean–atmosphere feedback.

Sensitivity of the Atlantic meridional overturning circulation (AMOC) to the tropical Indian Ocean warming.

2020 ◽  
Author(s):  
Brady Ferster ◽  
Alexey Fedorov ◽  
Juliette Mignot ◽  
Eric Guilyardi
Keyword(s):  
Indian Ocean ◽  
Atlantic Ocean ◽  
North Atlantic ◽  
Tropical Indian Ocean ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Tropical Atlantic ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

<p>The Arctic and North Atlantic Ocean play a fundamental role in Earth’s water cycle, distribution of energy (i.e. heat), and the formation of cold, dense waters. Through the Atlantic meridional overturning circulation (AMOC), heat is transported to the high-latitudes. Classically, the climate impact of AMOC variations has been investigated through hosing experiments, where anomalous freshwater is artificially added or removed from the North Atlantic to modulate deep water formation. However, such a protocol introduces artificial changes in the subpolar area, possibly masking the effect of the AMOC modulation. Here, we develope a protocol where AMOC intensity is modulated remotely through the teleconnection of the tropical Indian Ocean (TIO), so as to investigate more robustly the impact of the AMOC on climate. Warming in the TIO has recently been shown to strengthen the Walker circulation in the Atlantic through the propagation of Kelvin and Rossby waves, increasing and stabilizing the AMOC on longer timescales. Using the latest coupled-model from Insitut Pierre Simon Laplace (IPSL-CM6), we have designed a three-member ensemble experiment nudging the surface temperatures of the TIO by -2°C, +1°C, and +2°C for 100 years. The objectives are to better quantify the timescales of AMOC variability outside the use of hosing experiments and the TIO-AMOC relationship.  In each ensemble member, there are two distinct features compared to the control run. The initial changes in AMOC (≤20 years) are largely atmospherically driven, while on longer timescales is largely driven by the TIO teleconnection to the tropical Atlantic. In the northern North Atlantic, changes in sensible heat fluxes range from 15 to 20 W m<sup>-2 </sup>in all three members compared to the control run, larger than the natural variability. On the longer timescales, AMOC variability is strongly influenced from anomalies in the tropical Atlantic Ocean. The TIO teleconnection supports decreased precipitation in the tropical Atlantic Ocean during warming (opposite during TIO cooling) events, as well as positive salinity anomalies and negative temperature anomalies. Using lagged correlations, there are the strongest correlations on scales within one year and a delayed response of 30 years (in the -2°C ensembles). In comparing the last 20 years, nudging the TIO induces a 3.3 Sv response per 1°C change. In summary, we have designed an experiment to investigate the AMOC variability without directly changing the North Atlantic through hosing, making way for a more unbiased approach to analysing the AMOC variability in climate models.</p>

Importance of Stratospheric Polar Vortex Events for Seasonal Predictability of Sea Level Pressure over the North Atlantic and Europe

2020 ◽  
Author(s):  
Johanna Baehr ◽  
Simon Wett ◽  
Mikhail Dobrynin ◽  
Daniela Domeisen
Keyword(s):  
Sea Level ◽  
North Atlantic ◽  
Polar Vortex ◽  
Seasonal Prediction ◽  
Prediction System ◽  
Seasonal Predictability ◽  
Stratospheric Polar Vortex ◽  
Sea Level Pressure ◽  
The North ◽  
The North Atlantic

<p>The downward influence of the stratosphere on the troposphere can be significant during boreal winter when the polar vortex is most variable, when major circulation changes in the stratosphere can impact the tropospheric flow. These strong and weak vortex events, the latter also referred to as Sudden Stratospheric Warmings (SSWs), are capable of influencing the tropospheric circulation down to the sea level on timescales from weeks to months. Thus, the occurrence of stratospheric polar vortex events influences the seasonal predictability of sea level pressure (SLP), which is, over the Atlantic sector, strongly linked to the North Atlantic oscillation (NAO).<br>We analyze the influence of the polar vortex on the seasonal predictability of SLP in a seasonal prediction system based on the mixed resolution configuration of the coupled Max-Planck-Institute Earth System Model (MPI-ESM), where we investigate a 30 member ensemble hindcast simulation covering 1982 -2016. Since the state of the polar vortex is predictable only a few weeks or even days ahead, the seasonal prediction system cannot exactly predict the day of occurrence of stratospheric events. However, making use of the large number of stratospheric polar vortex events in the ensemble hindcast simulation, we present a statistical analysis of the influence of a correct or incorrect prediction of the stratospheric vortex state on the seasonal predictability of SLP over the North Atlantic and Europe.</p>

scholarly journals Transient Atmospheric Response to Interactive SST Anomalies

2008 ◽  
Vol 21 (3) ◽  
pp. 576-583 ◽  
Author(s):  
David Ferreira ◽  
Claude Frankignoul
Keyword(s):  
North Atlantic ◽  
Initial Conditions ◽  
Maximum Amplitude ◽  
Atmospheric Response ◽  
The North ◽  
Winter Conditions ◽  
Initial Evolution ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
Sst Anomalies

Abstract The transient atmospheric response to interactive SST anomalies in the midlatitudes is investigated using a three-layer QG model coupled in perpetual winter conditions to a slab oceanic mixed layer in the North Atlantic. The SST anomalies are diagnosed from a coupled run and prescribed as initial conditions, but are free to evolve. The initial evolution of the atmospheric response is similar to that obtained with a prescribed SST anomaly, starting as a quasi-linear baroclinic and then quickly evolving into a growing equivalent barotropic one. Because of the heat flux damping, the SST anomaly amplitude slowly decreases, albeit with little change in pattern. Correspondingly, the atmospheric response only increases until it reaches a maximum amplitude after about 1–3.5 months, depending on the SST anomaly considered. The response is similar to that at equilibrium in the fixed SST case, but it is 1.5–2 times smaller, and then slowly decays away.

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