scholarly journals Atmospheric Low Frequency Variability: The Examples of the North Atlantic and the Indian Monsoon

10.5772/33620 ◽  
2012 ◽  
Author(s):  
Abdel Hannachi ◽  
Tim Woollings ◽  
Andy Turner
Keyword(s):  
North Atlantic ◽  
Indian Monsoon ◽  
Low Frequency ◽  
The North ◽  
Low Frequency Variability ◽  
The North Atlantic

scholarly journals Ocean–Atmosphere Influences on Low-Frequency Warm-Season Drought Variability in the Gulf Coast and Southeastern United States

2011 ◽  
Vol 50 (6) ◽  
pp. 1177-1186 ◽  
Author(s):  
Jason T. Ortegren ◽  
Paul A. Knapp ◽  
Justin T. Maxwell ◽  
William P. Tyminski ◽  
Peter T. Soulé
Keyword(s):  
United States ◽  
North Atlantic ◽  
Low Frequency ◽  
Warm Season ◽  
Spatiotemporal Variability ◽  
Drought Variability ◽  
The North ◽  
Low Frequency Variability ◽  
Ocean Atmosphere ◽  
The North Atlantic

AbstractFrom the 344 state climate divisions in the conterminous United States, nine distinct regions of warm-season drought variability are identified using principal component analysis. The drought metric used is the Palmer hydrological drought index for the period 1895–2008. The focus of this paper is multidecadal drought variability in the Southeast (SEUS) and eastern Gulf South (EGS) regions of the United States, areas in which the low-frequency forcing mechanisms of warm-season drought are still poorly understood. Low-frequency drought variability in the SEUS and EGS is associated with smoothed indexed time series of major ocean–atmosphere circulation features, including two indices of spatiotemporal variability in the North Atlantic subtropical anticyclone (Bermuda high). Long-term warm-season drought conditions are significantly out of phase between the two regions. Multidecadal regimes of above- and below-average moisture in the SEUS and EGS are closely associated with slow variability in sea surface temperatures in the North Atlantic Ocean and with the summer mean position and mean strength of the Bermuda high. Multivariate linear regression indicates that 82%–92% of the low-frequency variability in warm-season moisture is explained by two of the three leading principal components of low-frequency variability in the climate indices. The findings are important for water resource managers and water-intensive industries in the SEUS and EGS. The associations identified in the paper are valuable for enhanced drought preparedness and forecasting in the study area and potentially for global models of coupled ocean–atmosphere variability.

scholarly journals Effect of Synoptic Systems on the Variability of the North Atlantic Oscillation

2005 ◽  
Vol 133 (10) ◽  
pp. 2894-2904 ◽  
Author(s):  
Ulrike Löptien ◽  
Eberhard Ruprecht
Keyword(s):  
North Atlantic Oscillation ◽  
Sea Level ◽  
North Atlantic ◽  
Low Frequency ◽  
Sea Level Pressure ◽  
Level Pressure ◽  
The North ◽  
Low Frequency Variability ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract The North Atlantic Oscillation (NAO) represents the dominant mode of atmospheric variability in the North Atlantic region. In the present study, the role of the synoptic systems (cyclones and anticyclones) in generating the NAO pattern is investigated. To study the intermonthly variations of the NAO, NCEP–NCAR reanalysis data are used, and for the interdecadal variations the results of a 300-yr control integration under present-day conditions of the coupled model ECHAM4/OPYC3 are analyzed. A filtering method is developed for the sea level pressure anomalies. Application of this method to each grid point yields the low-frequency variability in the sea level pressure field that is due to the synoptic systems. The low-frequency variability of the filtered and the original data are in high agreement. This indicates that the low-frequency pressure variability, and with it the variability of the NAO, is essentially caused by the distribution of the synoptic systems. The idea that the distribution of the synoptic systems is the cause of the variation of the NAO is confirmed by high correlation between the latitudinal position of the polar front over the North Atlantic and the NAO index. Since most of the low-frequency variability in sea level pressure can be explained through the distribution of the synoptic systems, the NAO seems to be a reflection of the distribution of the synoptic systems, rather than the source for variations in the cyclone tracks.

scholarly journals Climate Signals in the Mid- to High-Latitude North Atlantic from Altimeter Observations

2016 ◽  
Vol 29 (13) ◽  
pp. 4905-4925 ◽  
Author(s):  
Feili Li ◽  
Young-Heon Jo ◽  
Xiao-Hai Yan ◽  
W. Timothy Liu
Keyword(s):  
Sea Level ◽  
Time Scales ◽  
North Atlantic ◽  
High Latitude ◽  
Low Frequency ◽  
Height Anomaly ◽  
The North ◽  
Low Frequency Variability ◽  
The North Atlantic ◽  
Annual Time

Abstract The variability of the sea surface height anomaly (SSHA) in the mid- to high-latitude North Atlantic for the period of 1993–2010 was investigated using the ensemble empirical mode decomposition to identify the dominant time scales. Sea level variations in the North Atlantic subpolar gyre (SPG) are dominated by the annual cycle and the long-term increasing trend. In comparison, the SSHA along the Gulf Stream (GS) is dominated by variability at intraseasonal and annual time scales. Moreover, the sea level rise in the SPG developed at a reduced rate in the 2000s compared to rates in the 1990s, which was accompanied by a rebound in SSHA variability following a period of lower variability in the system. These changes in both apparent trend and low-frequency SSHA oscillations reveal the importance of low-frequency variability in the SPG. To identify the possible contributing factors for these changes, the heat content balance (equivalent variations in the sea level) in the subpolar region was examined. The results indicate that horizontal circulations may primarily contribute to the interannual to decadal variations, while the air–sea heat flux is not negligible at annual time scale. Furthermore, the low-frequency variability in the SPG relates to the propagation of Atlantic meridional overturning circulation (AMOC) variations from the deep-water formation region to midlatitudes in the North Atlantic, which might have the implications for recent global surface warming hiatus.

scholarly journals Glacial fluctuations of the Indian monsoon and their relationship with North Atlantic climate: new data and modelling experiments

2013 ◽  
Vol 9 (5) ◽  
pp. 2135-2151 ◽  
Author(s):  
C. Marzin ◽  
N. Kallel ◽  
M. Kageyama ◽  
J.-C. Duplessy ◽  
P. Braconnot
Keyword(s):  
North Atlantic ◽  
Bay Of Bengal ◽  
Indian Monsoon ◽  
Hydrological Cycle ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The North ◽  
North Atlantic Climate ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. Several paleoclimate records such as from Chinese loess, speleothems or upwelling indicators in marine sediments present large variations of the Asian monsoon system during the last glaciation. Here, we present a new record from the northern Andaman Sea (core MD77-176) which shows the variations of the hydrological cycle of the Bay of Bengal. The high-resolution record of surface water δ18O dominantly reflects salinity changes and displays large millennial-scale oscillations over the period 40 000 to 11 000 yr BP. Their timing and sequence suggests that events of high (resp. low) salinity in the Bay of Bengal, i.e. weak (resp. strong) Indian monsoon, correspond to cold (resp. warm) events in the North Atlantic and Arctic, as documented by the Greenland ice core record. We use the IPSL_CM4 Atmosphere-Ocean coupled General Circulation Model to study the processes that could explain the teleconnection between the Indian monsoon and the North Atlantic climate. We first analyse a numerical experiment in which such a rapid event in the North Atlantic is obtained under glacial conditions by increasing the freshwater flux in the North Atlantic, which results in a reduction of the intensity of the Atlantic meridional overturning circulation. This freshwater hosing results in a weakening of the Indian monsoon rainfall and circulation. The changes in the continental runoff and local hydrological cycle are responsible for an increase in salinity in the Bay of Bengal. This therefore compares favourably with the new sea water δ18O record presented here and the hypothesis of synchronous cold North Atlantic and weak Indian monsoon events. Additional sensitivity experiments are produced with the LMDZ atmospheric model to analyse the teleconnection mechanisms between the North Atlantic and the Indian monsoon. The changes over the tropical Atlantic are shown to be essential in triggering perturbations of the subtropical jet over Africa and Eurasia, that in turn affect the intensity of the Indian monsoon. These relationships are also found to be valid in additional coupled model simulations in which the Atlantic meridional overturning circulation (AMOC) is forced to resume.

scholarly journals Glacial climate sensitivity to different states of the Atlantic Meridional Overturning Circulation: results from the IPSL model

2009 ◽  
Vol 5 (3) ◽  
pp. 551-570 ◽  
Author(s):  
M. Kageyama ◽  
J. Mignot ◽  
D. Swingedouw ◽  
C. Marzin ◽  
R. Alkama ◽  
...  
Keyword(s):  
Atmospheric Circulation ◽  
North Atlantic ◽  
Indian Monsoon ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Climate Variations ◽  
Climatic Response ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. Paleorecords from distant locations on the globe show rapid and large amplitude climate variations during the last glacial period. Here we study the global climatic response to different states of the Atlantic Meridional Overturning Circulation (AMOC) as a potential explanation for these climate variations and their possible connections. We analyse three glacial simulations obtained with an atmosphere-ocean coupled general circulation model and characterised by different AMOC strengths (18, 15 and 2 Sv) resulting from successive ~0.1 Sv freshwater perturbations in the North Atlantic. These AMOC states suggest the existence of a freshwater threshold for which the AMOC collapses. A weak (18 to 15 Sv) AMOC decrease results in a North Atlantic and European cooling. This cooling is not homogeneous, with even a slight warming over the Norwegian Sea. Convection in this area is active in both experiments, but surprisingly stronger in the 15 Sv simulation, which appears to be related to interactions with the atmospheric circulation and sea-ice cover. Far from the North Atlantic, the climatic response is not significant. The climate differences for an AMOC collapse (15 to 2 Sv) are much larger and of global extent. The timing of the climate response to this AMOC collapse suggests teleconnection mechanisms. Our analyses focus on the North Atlantic and surrounding regions, the tropical Atlantic and the Indian monsoon region. The North Atlantic cooling associated with the AMOC collapse induces a cyclonic atmospheric circulation anomaly centred over this region, which modulates the eastward advection of cold air over the Eurasian continent. This can explain why the cooling is not as strong over western Europe as over the North Atlantic. In the Tropics, the southward shift of the Inter-Tropical Convergence Zone appears to be strongest over the Atlantic and Eastern Pacific and results from an adjustment of the atmospheric and oceanic heat transports. Finally, the Indian monsoon weakening appears to be connected to the North Atlantic cooling via that of the troposphere over Eurasia. Such an understanding of these teleconnections and their timing could be useful for paleodata interpretation.

The Role of Extratropical Air–Sea Interaction in the Autumn Subseasonal Variability of the North Atlantic Oscillation

2019 ◽  
Vol 32 (22) ◽  
pp. 7697-7712 ◽  
Author(s):  
Yu Nie ◽  
Hong-Li Ren ◽  
Yang Zhang
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Gulf Stream ◽  
Low Frequency ◽  
Mean Flow ◽  
The North ◽  
Subseasonal Variability ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract Considerable progress has been made in understanding the internal eddy–mean flow feedback in the subseasonal variability of the North Atlantic Oscillation (NAO) during winter. Using daily atmospheric and oceanic reanalysis data, this study highlights the role of extratropical air–sea interaction in the NAO variability during autumn when the daily sea surface temperature (SST) variability is more active and eddy–mean flow interactions are still relevant. Our analysis shows that a horseshoe-like SST tripolar pattern in the North Atlantic Ocean, marked by a cold anomaly in the Gulf Stream and two warm anomalies to the south of the Gulf Stream and off the western coast of northern Europe, can induce a quasi-barotropic NAO-like atmospheric response through eddy-mediated processes. An initial southwest–northeast tripolar geopotential anomaly in the North Atlantic forces this horseshoe-like SST anomaly tripole. Then the SST anomalies, through surface heat flux exchange, alter the spatial patterns of the lower-tropospheric temperature and thus baroclinicity anomalies, which are manifested as the midlatitude baroclinicity shifted poleward and reduced baroclinicity poleward of 70°N. In response to such changes of the lower-level baroclinicity, anomalous synoptic eddy generation, eddy kinetic energy, and eddy momentum forcing in the midlatitudes all shift poleward. Meanwhile, the 10–30-day low-frequency anticyclonic wave activities in the high latitudes decrease significantly. We illustrate that both the latitudinal displacement of midlatitude synoptic eddy activities and intensity variation of high-latitude low-frequency wave activities contribute to inducing the NAO-like anomalies.

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