scholarly journals Local versus Tropical Diabatic Heating and the Winter North Atlantic Oscillation

2007 ◽  
Vol 20 (10) ◽  
pp. 2058-2075 ◽  
Author(s):  
Richard J. Greatbatch ◽  
Thomas Jung
Keyword(s):  
Boundary Layer ◽  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Diabatic Heating ◽  
Tropical Pacific ◽  
Atlantic Sector ◽  
The North ◽  
Model Response ◽  
The North Atlantic ◽  
The Tropical Pacific

Abstract In this paper, a version of the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model is used to (i) diagnose the diabatic heating associated with the winter North Atlantic Oscillation (NAO) and (ii) assess the role of this heating in the dynamics of the NAO in the model. Over the North Atlantic sector, the NAO-related diabatic heating is dominated above the planetary boundary layer by the latent heat release associated with precipitation, and within the boundary layer by vertical diffusion associated with sensible heat flux from the ocean. An association between La Niña–El Niño–type conditions in the tropical Pacific and the positive/negative NAO is found in model runs using initial conditions and sea surface temperature (SST) lower boundary conditions from the period 1982–2001, but not in a companion set of model runs for the period 1962–81. Model experiments are then described in which the NAO-related diabatic heating diagnosed from the 1982–2001 control run is applied as a constant forcing in the model temperature equation using both 1982–2001 and 1962–81 model setups. To assess the local feedback from the diabatic heating, the specified forcing is first restricted to the North Atlantic sector alone. In this case, the model response (in an ensemble mean sense) is suggestive of a weak negative feedback, but exhibits more baroclinic structure and has its centers of action shifted compared to those of the NAO. On the other hand, forcing with only the tropical Pacific part of the diabatic heating leads to a robust model response in both the 1982–2001 and 1962–81 model setups. The model response projects on to the NAO with the same sign as that used to diagnose the forcing, arguing that the link between the tropical Pacific and the NAO is real in the 1982–2001 control run. The missing link in the corresponding run for 1962–81 is a result of a change in the tropical forcing between the two periods, and not the extratropical flow regime.

Origins of variability and predictability in the North Atlantic region

2020 ◽  
Author(s):  
Daniela Domeisen
Keyword(s):  
North Atlantic ◽  
Tropical Pacific ◽  
Upper Atmosphere ◽  
Indirect Pathway ◽  
North Atlantic Region ◽  
Atlantic Region ◽  
Atlantic Sector ◽  
The North ◽  
The North Atlantic ◽  
The Tropical Pacific

<p>The atmosphere over the North Atlantic sector exhibits significant interannual and interdecadal variability, as well as long-term trends due to global change. This variability is accompanied by changes in predictability. The origins of North Atlantic variability can to a large extent be traced back to the ocean and the land surface, the upper atmosphere, the tropics, as well as circum-global patterns. In particular, the tropical Pacific and the upper atmosphere have a strong influence on interannual and decadal variability in the North Atlantic region. As an example, the tropical Pacific affects the North Atlantic both through a tropospheric pathway across North America and through an indirect pathway through the stratosphere. Hence, due to the large number of factors influencing the North Atlantic region, their inter-dependence and their non-stationarity, the influence of these different factors is difficult to disentangle. Furthermore, models are often not able to capture the inter-dependence and superposition of these factors, which affects to what extent models are able to predict the North Atlantic region. This submission will explore the contribution to variability and predictability for several of these remote influences.</p><p> </p>

scholarly journals Dynamical analysis of a reduced model for the North Atlantic Oscillation

2021 ◽  
Author(s):  
Courtney Quinn ◽  
Dylan Harries ◽  
Terence J. O’Kane
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Growth Rates ◽  
Time Dependent ◽  
Unstable State ◽  
Time Step ◽  
Atlantic Sector ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

AbstractThe dynamics of the North Atlantic Oscillation (NAO) are analyzed through a data-driven model obtained from atmospheric reanalysis data. We apply a regularized vector autoregressive clustering technique to identify recurrent and persistent states of atmospheric circulation patterns in the North Atlantic sector (110°W-0°E, 20°N-90°N). In order to analyze the dynamics associated with the resulting cluster-based models, we define a time-dependent linear delayed map with a switching sequence set a priori by the cluster affiliations at each time step. Using a method for computing the covariant Lyapunov vectors (CLVs) over various time windows, we produce sets of mixed singular vectors (for short windows) and approximate the asymptotic CLVs (for longer windows). The growth rates and alignment of the resulting time-dependent vectors are then analyzed. We find that the window chosen to compute the vectors acts as a filter on the dynamics. For short windows, the alignment and changes in growth rates are indicative of individual transitions between persistent states. For long windows, we observe an emergent annual signal manifest in the alignment of the CLVs characteristic of the observed seasonality in the NAO index. Analysis of the average finite-time dimension reveals the NAO− as the most unstable state relative to the NAO+, with persistent AR states largely stable. Our results agree with other recent theoretical and empirical studies that have shown blocking events to have less predictability than periods of enhanced zonal flow.

scholarly journals Hydrological variations in central China over the past millennium and their links to the tropical Pacific and North Atlantic oceans

2020 ◽  
Vol 16 (2) ◽  
pp. 475-485
Author(s):  
Fucai Duan ◽  
Zhenqiu Zhang ◽  
Yi Wang ◽  
Jianshun Chen ◽  
Zebo Liao ◽  
...  
Keyword(s):  
North Atlantic ◽  
Atmospheric Precipitation ◽  
Tropical Pacific ◽  
Meridional Overturning Circulation ◽  
Central China ◽  
Ice Age ◽  
The North ◽  
Future Global Warming ◽  
The North Atlantic ◽  
The Tropical Pacific

Abstract. Variations of precipitation, also called the Meiyu rain, in the East Asian summer monsoon (EASM) domain during the last millennium could help enlighten the hydrological response to future global warming. Here we present a precisely dated and highly resolved stalagmite δ18O record from the Yongxing Cave, central China. Our new record, combined with a previously published one from the same cave, indicates that the Meiyu rain has changed dramatically in association with the global temperature change. In particular, our record shows that the Meiyu rain was weakened during the Medieval Climate Anomaly (MCA) but intensified during the Little Ice Age (LIA). During the Current Warm Period (CWP), our record indicates a similar weakening of the Meiyu rain. Furthermore, during the MCA and CWP, our records show that the atmospheric precipitation is similarly wet in northern China and similarly dry in central China, but relatively wet during the CWP in southern China. This spatial discrepancy indicates a complicated localized response of the regional precipitation to the anthropogenic forcing. The weakened (intensified) Meiyu rain during the MCA (LIA) matches well with the warm (cold) phases of Northern Hemisphere surface air temperature. This Meiyu rain pattern also corresponds well to the climatic conditions over the tropical Indo-Pacific warm pool. On the other hand, our record shows a strong association with the North Atlantic climate as well. The reduced (increased) Meiyu rain correlates well with positive (negative) phases of the North Atlantic Oscillation. In addition, our record links well to the strong (weak) Atlantic meridional overturning circulation during the MCA (LIA) period. All abovementioned localized correspondences and remote teleconnections on decadal to centennial timescales indicate that the Meiyu rain was coupled closely with oceanic processes in the tropical Pacific and North Atlantic oceans during the MCA and LIA.

scholarly journals Sensitivity study of the tropical Pacific precipitation anomalies

2016 ◽  
Author(s):  
Shouwen Zhang ◽  
Hua Jiang ◽  
Hui Wang ◽  
Ling Du ◽  
Dakui Wang
Keyword(s):  
Pacific Ocean ◽  
Sea Ice ◽  
North Atlantic ◽  
Climate Model ◽  
Tropical Pacific ◽  
Tropical Pacific Ocean ◽  
The North ◽  
Precipitation Anomalies ◽  
The North Atlantic ◽  
The Tropical Pacific

Abstract. Climate model results have shown that precipitation in the tropical Pacific Ocean will change up to 15 % and 25 % in one century. In this paper, both reanalysis data and climate model are used to study the response of global ocean and atmosphere to precipitation anomalies in the tropical Pacific Ocean. It shows that positive precipitation anomalies could trigger an El Nino-like SSTA response, with warmer SST in the east tropical Pacific Ocean and slightly cooler SST in the west tropical Pacific Ocean. The zonal tropical ocean currents change significantly, of which the magnitudes and directions are mainly relying on the intensity of the precipitation anomalies. Through a wave train encompassing the whole Northern Hemisphere named as the Circumglobal Waveguide Pattern (CWP), the North Atlantic atmospheric circulation responds to the freshwater anomalies in a NAO-like pattern. The anomalous atmospheric circulation transport sea ice to the North Atlantic Ocean. The sea ice melts in summer and freshen the upper ocean, which makes the ocean more stable. It thus constrains vertical heat transport and makes the upper water cooler, forming a significant positive feedback mechanism.

scholarly journals Portraying the Impact of the Tibetan Plateau on Global Climate

2020 ◽  
Vol 33 (9) ◽  
pp. 3565-3583 ◽  
Author(s):  
Haijun Yang ◽  
Xingchen Shen ◽  
Jie Yao ◽  
Qin Wen
Keyword(s):  
Tibetan Plateau ◽  
North Atlantic ◽  
Thermohaline Circulation ◽  
Global Climate ◽  
Tropical Pacific ◽  
The Tibetan Plateau ◽  
The North ◽  
The North Atlantic ◽  
The Impact ◽  
The Tropical Pacific

AbstractAs the most extensive highland in the world, the Tibetan Plateau (TP) plays an important role in shaping the global climate. Quantifying the effect of the TP on global climate is the first step for a full understanding of the TP’s standing on planet Earth. Through coupled model sensitivity experiments, we draw a panorama of the TP’s global impact in this paper. Our model results show that the absence of the TP would result in a 4°C colder and 10% drier climate in the Northern Hemisphere (NH). The TP has a striking remote effect on the North Atlantic. Removing the TP would enhance the westerlies in the mid- to high latitudes of the NH and weaken the easterlies over the tropical Pacific. More moisture would be relocated from the tropical Pacific to the North Atlantic, shutting down the Atlantic thermohaline circulation, which would eventually result in more than 15°C colder and 20% drier climate over the North Atlantic. Our model results suggest that the presence of the TP may have contributed greatly to the hospitable modern climate in the NH, by promoting the establishment of the thermohaline circulation in the Atlantic, and therefore enhancing the northward ocean heat transport and atmosphere moisture transport across the equator.

Decadal Relationship between European Blocking and the North Atlantic Oscillation during 1978–2011. Part I: Atlantic Conditions

2015 ◽  
Vol 72 (3) ◽  
pp. 1152-1173 ◽  
Author(s):  
Dehai Luo ◽  
Yao Yao ◽  
Aiguo Dai
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Western Europe ◽  
Storm Track ◽  
Western Atlantic ◽  
Atlantic Sector ◽  
The North ◽  
Nao Index ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract Both the positive and negative phases of the North Atlantic Oscillation (NAO+ and NAO−, respectively) and atmospheric blocking in the Euro-Atlantic sector reflect synoptic variability over the region and thus are intrinsically linked. This study examines their relationship from a decadal change perspective. Since the winter-mean NAO index is defined as a time average of instantaneous NAO indices over the whole winter, it is unclear how the activity of European blocking (EB) events can be related to the variation of the positive mean NAO index. Here, this question is examined by dividing the winter period 1978–2011 into two decadal epochs: 1978–94 (P1) with an increasing and high NAO index and 1995–2011 (P2) with a decreasing and low NAO index. Using atmospheric reanalysis data, it is shown that there are more intense and persistent EB events in eastern Europe during P1 than during P2, while the opposite is true for western Europe. It is further shown that there are more NAO+ (NAO−) events during P1 (P2). The EB events associated with NAO+ events extend more eastward and are associated with stronger Atlantic mean zonal wind and weaker western Atlantic storm track during P1 than during P2, but EB events associated with NAO− events increase in western Europe under opposite Atlantic conditions during P2. Thus, the increase in the number of individual NAO+ (NAO−) events results in more EB events in eastern (western) Europe during P1 (P2). The EB change is also associated with the increased frequency of NAO− to NAO+ (NAO+ to NAO−) transition events.

scholarly journals The Development of a Statistical Forecast Model for Changma

2013 ◽  
Vol 28 (6) ◽  
pp. 1304-1321 ◽  
Author(s):  
Seung-Eon Lee ◽  
Kyong-Hwan Seo
Keyword(s):  
Wave Propagation ◽  
North Atlantic ◽  
North Pacific ◽  
Forecast Model ◽  
Tropical Pacific ◽  
The North ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
The North Pacific ◽  
The Tropical Pacific

Abstract Forecasting year-to-year variations in East Asian summer monsoon (EASM) precipitation is one of the most challenging tasks in climate prediction because the predictors are not sufficiently well known and the forecast skill of the numerical models is poor. In this paper, a statistical forecast model for changma (the Korean portion of the EASM system) precipitation is proposed that was constructed with three physically based predictors. A forward-stepwise regression was used to select the predictors that included sea surface temperature (SST) anomalies over the North Pacific, the North Atlantic, and the tropical Pacific Ocean. Seasonal predictions with this model showed high forecasting capabilities that had a Gerrity skill score of ~0.82. The dynamical processes associated with the predictors were examined prior to their use in the prediction scheme. All predictors tended to induce an anticyclonic anomaly to the east or southeast of Japan, which was responsible for transporting a large amount of moisture to the southern Korean Peninsula. The predictor in the North Pacific formed an SST front to the east of Japan during the summertime, which maintained a lower-tropospheric baroclinicity. The North Atlantic SST anomaly induced downstream wave propagation in the upper troposphere, developing anticyclonic activity east of Japan. Forcing from the tropical Pacific SST anomaly triggered a cyclonic anomaly over the South China Sea, which was maintained by atmosphere–ocean interactions and induced an anticyclonic anomaly via northward Rossby wave propagation. Overall, the model used for forecasting changma precipitation performed well (R = 0.85) and correctly predicted information for 16 out of 19 yr of observational data.

scholarly journals North Atlantic Oscillation Response to Anomalous Indian Ocean SST in a Coupled GCM

2005 ◽  
Vol 18 (24) ◽  
pp. 5382-5389 ◽  
Author(s):  
Jürgen Bader ◽  
Mojib Latif
Keyword(s):  
Indian Ocean ◽  
North Atlantic Oscillation ◽  
General Circulation Model ◽  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Atlantic Sector ◽  
The North ◽  
The Indian Ocean ◽  
The North Atlantic

Abstract The dominant pattern of atmospheric variability in the North Atlantic sector is the North Atlantic Oscillation (NAO). Since the 1970s the NAO has been well characterized by a trend toward its positive phase. Recent atmospheric general circulation model studies have linked this trend to a progressive warming of the Indian Ocean. Unfortunately, a clear mechanism responsible for the change of the NAO could not be given. This study provides further details of the NAO response to Indian Ocean sea surface temperature (SST) anomalies. This is done by conducting experiments with a coupled ocean–atmosphere general circulation model (OAGCM). The authors develop a hypothesis of how the Indian Ocean impacts the NAO.

scholarly journals Oscillatory Climate Modes in the Indian Monsoon, North Atlantic, and Tropical Pacific

2013 ◽  
Vol 26 (23) ◽  
pp. 9528-9544 ◽  
Author(s):  
Yizhak Feliks ◽  
Andreas Groth ◽  
Andrew W. Robertson ◽  
Michael Ghil
Keyword(s):  
North Atlantic ◽  
Indian Monsoon ◽  
Southern Oscillation ◽  
Singular Spectrum Analysis ◽  
Time Lag ◽  
Tropical Pacific ◽  
Climate Modes ◽  
The North ◽  
The North Atlantic ◽  
The Tropical Pacific

This paper explores the three-way interactions between the Indian monsoon, the North Atlantic, and the tropical Pacific. Four climate records were analyzed: the monsoon rainfall in two Indian regions, the Southern Oscillation index for the tropical Pacific, and the NAO index for the North Atlantic. The individual records exhibit highly significant oscillatory modes with spectral peaks at 7–8 yr and in the quasi-biennial and quasi-quadrennial bands. The interactions between the three regions were investigated in the light of the synchronization theory of chaotic oscillators. The theory was applied here by combining multichannel singular-spectrum analysis (M-SSA) with a recently introduced varimax rotation of the M-SSA eigenvectors. A key result is that the 7–8-yr and 2.7-yr oscillatory modes in all three regions are synchronized, at least in part. The energy-ratio analysis, as well as time-lag results, suggests that the NAO plays a leading role in the 7–8-yr mode. It was found therewith that the South Asian monsoon is not slaved to forcing from the equatorial Pacific, although it does interact strongly with it. The time-lag analysis pinpointed this to be the case in particular for the quasi-biennial oscillatory modes. Overall, these results confirm that the approach of synchronized oscillators, combined with varimax-rotated M-SSA, is a powerful tool in studying teleconnections between regional climate modes and that it helps identify the mechanisms that operate in various frequency bands. This approach should be readily applicable to ocean modes of variability and to the problems of air–sea interaction as well.

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