scholarly journals Relative Importance of the Austral Summer and Autumn SAM in Modulating Southern Hemisphere Extratropical Autumn SST*

2015 ◽  
Vol 28 (20) ◽  
pp. 8003-8020 ◽  
Author(s):  
Fei Zheng ◽  
Jianping Li ◽  
Juan Feng ◽  
Yanjie Li ◽  
Yang Li
Keyword(s):  
Southern Hemisphere ◽  
North Atlantic ◽  
Large Scale ◽  
Austral Summer ◽  
Relative Importance ◽  
Seasonal Influence ◽  
New Approach ◽  
The North ◽  
Sst Variability ◽  
Sst Anomalies

Abstract Sea surface temperature (SST) variability in the extratropical Southern Hemisphere is mainly determined by physical processes at the air–sea interface associated with the Southern Hemisphere annular mode (SAM). Both the austral summer [December–February (DJF)] and autumn [March–May (MAM)] SAM can imprint their signals on southern extratropical MAM SST. Here three approaches are employed to determine the relative importance of the DJF and MAM SAM in modulating southern extratropical MAM SST: a simple lead–lag correlation without SST decomposition, a decomposition method based on linear regression, and a new approach named the persistent signal decomposition (PSD). The results show that the DJF SAM plays a more important role than the MAM SAM in driving MAM large-scale southern extratropical SST anomalies, implying that MAM SST anomalies caused by the preceding DJF SAM would not be largely perturbed by the MAM SAM, and thus the DJF SAM can be regarded as an effective predictor for the following season’s climate. The PSD also provides an estimation of the contribution of atmospheric persistence and SST persistence toward cross-seasonal influence of the DJF SAM on MAM southern extratropical SST. The results show that this cross-seasonal influence is mainly caused by the SST persistence. The detection of the relative importance of the preceding and contemporaneous atmospheric signal in driving SSTA contributes to the understanding of air–sea interactions and helps to obtain better SST-based statistical predictions. The PSD has the potential to be employed in the North Atlantic and other extratropical oceans.

Causes of Late Twentieth-Century Trends in New Zealand Precipitation

2009 ◽  
Vol 22 (1) ◽  
pp. 3-19 ◽  
Author(s):  
Caroline C. Ummenhofer ◽  
Alexander Sen Gupta ◽  
Matthew H. England
Keyword(s):  
Twentieth Century ◽  
New Zealand ◽  
Southern Hemisphere ◽  
Large Scale ◽  
Austral Summer ◽  
Climate Modes ◽  
The North ◽  
Late Twentieth ◽  
Late Twentieth Century ◽  
Large Scale Atmospheric Circulation

Abstract Late twentieth-century trends in New Zealand precipitation are examined using observations and reanalysis data for the period 1979–2006. One of the aims of this study is to investigate the link between these trends and recent changes in the large-scale atmospheric circulation in the Southern Hemisphere. The contributions from changes in Southern Hemisphere climate modes, particularly the El Niño–Southern Oscillation (ENSO) and the southern annular mode (SAM), are quantified for the austral summer season, December–February (DJF). Increasingly drier conditions over much of New Zealand can be partially explained by the SAM and ENSO. Especially over wide parts of the North Island and western regions of the South Island, the SAM potentially contributes up to 80% and 20%–50% to the overall decline in DJF precipitation, respectively. Over the North Island, the contribution of the SAM and ENSO to precipitation trends is of the same sign. In contrast, over the southwest of the South Island the two climate modes act in the opposite sense, though the effect of the SAM seems to dominate there during austral summer. The leading modes of variability in summertime precipitation over New Zealand are linked to the large-scale atmospheric circulation. The two dominant modes, explaining 64% and 9% of the overall DJF precipitation variability respectively, can be understood as local manifestations of the large-scale climate variability associated with the SAM and ENSO.

scholarly journals Predictability of Recurrent Weather Regimes over North America during Winter from Submonthly Reforecasts

2018 ◽  
Vol 146 (8) ◽  
pp. 2559-2577 ◽  
Author(s):  
N. Vigaud ◽  
A.W. Robertson ◽  
M. K. Tippett
Keyword(s):  
North America ◽  
North Atlantic ◽  
Large Scale ◽  
Wave Train ◽  
Spatial Structures ◽  
Weather Regimes ◽  
The North ◽  
Sst Variability ◽  
Cluster Partition ◽  
Rossby Wave Train

Abstract Four recurrent weather regimes are identified over North America from October to March through a k-means clustering applied to MERRA daily 500-hPa geopotential heights over the 1982–2014 period. Three regimes resemble Rossby wave train patterns with some baroclinicity, while one is related to an NAO-like meridional pressure gradient between eastern North America and western regions of the North Atlantic. All regimes are associated with distinct rainfall and surface temperature anomalies over North America. The four-cluster partition is well reproduced by ECMWF week-1 reforecasts over the 1995–2014 period in terms of spatial structures, daily regime occurrences, and seasonal regime counts. The skill in forecasting daily regime sequences and weekly regime counts is largely limited to 2 weeks. However, skill relationships with the MJO, ENSO, and SST variability in the Atlantic and Indian Oceans suggest further potential for subseasonal predictability based on wintertime large-scale weather regimes.

Atlantic Multidecadal Oscillation (AMO) and sea surface temperature in the Bay of Biscay and adjacent regions

2011 ◽  
Vol 92 (2) ◽  
pp. 213-234 ◽  
Author(s):  
Carlos Garcia-Soto ◽  
Robin D. Pingree
Keyword(s):  
North Atlantic ◽  
Atlantic Multidecadal Oscillation ◽  
Sea Surface ◽  
Bay Of Biscay ◽  
The North ◽  
Sst Variability ◽  
The North Atlantic ◽  
Climate Mode ◽  
Sst Anomalies

The sea surface temperature (SST) variability of the Bay of Biscay and adjacent regions (1854–2010) has been examined in relation to the evolution of the Atlantic Multidecadal Oscillation (AMO), a major climate mode. The AMO index explains ~25% of the interannual variability of the annual SST during the last 150 years, while different indices of the North Atlantic Oscillation (NAO) explain ≤1% of the long-term record. NAO is a high frequency climate mode while AMO can modulate low frequency changes. Sixty per cent of the AMO variability is contained in periods longer than a decade. The basin-scale influence of NAO on SST over specific years (1995 to 1998) is presented and the SST anomalies explained. The period analysed represents an abrupt change in NAO and the North Atlantic circulation state as shown with altimetry and SST data. Additional atmospheric climate data over a shorter ~60 year period (1950–2008) show the influence on the Bay of Biscay SST of the East Atlantic (EA) pattern and the Scandinavia (SCA) pattern. These atmospheric teleconnections explain respectively ~25% and ~20% of the SST variability. The winter SST in the shelf-break/slope or poleward current region is analysed in relation to AMO. The poleward current shows a trend towards increasing SSTs during the last three decades as a result of the combined positive phase of AMO and global warming. The seasonality of this winter warm flow in the Iberian region is related to the autumn/winter seasonality of south-westerly (SW) winds. The SW winds are strengthened along the European shelf-break by the development of low pressure conditions in the region to the north of the Azores and therefore a negative NAO. AMO overall modulates multidecadal changes (~60% of the AMO variance). The long-term time-series of SST and SST anomalies in the Bay of Biscay show AMO-like cycles with maxima near 1870 and 1950 and minima near 1900 and 1980 indicating a period of 60–80 years during the last century and a half. Similar AMO-like variability is found in the Russell cycle of the Western English Channel (1924–1972). AMO relates at least to four mesozooplankton components of the Russell cycle: the abundance of the chaetognaths Parasagitta elegans and Parasagitta setosa (AMO −), the amount of the species Calanus helgolandicus (AMO −), the amount of the larvae of decapod crustaceans (AMO −) and the number of pilchard eggs (Sardine pilchardus; AMO +). In addition to AMO, the decadal to multidecadal (D2M) variability in the number of sunspots is analysed for the last 300 years. Several periodicities and a multi-secular linear increase are presented. There are secular minima near 1710, 1810, 1910 and 2010. The long term variability (>11 years) of the solar sunspot activity explains ~50% of the variance of the SST of the Bay of Biscay with periods longer than 11 years. AMO is finally compared with the Pacific Decadal Oscillation, the leading principal component of North Pacific SST anomalies.

Analysis of Record-High Temperature over Southeast Coastal China in Winter 2018/19: The Combined Effect of Mid- to High-Latitude Circulation Systems and SST Forcing over the North Atlantic and Tropical Western Pacific

2020 ◽  
Vol 33 (20) ◽  
pp. 8813-8831
Author(s):  
Yuntao Jian ◽  
Xiaoxia Lin ◽  
Wen Zhou ◽  
Maoqiu Jian ◽  
Marco Y. T. Leung ◽  
...  
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
East Asian ◽  
Western Pacific ◽  
Asian Winter Monsoon ◽  
The North ◽  
Tropical Western Pacific ◽  
Circulation Patterns ◽  
Coastal China ◽  
Sst Anomalies

AbstractIn winter 2018/19, southeastern coastal China experienced extreme warm temperatures that were due to a weak East Asian winter monsoon. On the basis of observations from 10 meteorological stations and reanalysis data, the large-scale circulation patterns associated with this extreme warm winter and the possible driving mechanism of its related sea surface temperature (SST) anomalies are investigated in this study. During this winter, many places in this region reached their highest winter mean temperature record and had more extreme warm days and fewer extreme cold days relative to climatology. According to the circulation patterns during winter 2018/19, several large-scale circulation conditions associated mainly with the weak East Asian winter monsoon are identified: the eastward shift of the Siberian high and a shallower East Asian trough, which is related to the low blocking frequency over the Aleutian region, are both unfavorable for cold-air intrusion southward. Meanwhile, strong low-level southerly wind anomalies over southeastern China are related mainly to the 2018/19 El Niño event. Furthermore, the possible role of SST anomalies over the North Atlantic and tropical western Pacific Oceans is examined by using an atmospheric general circulation model, suggesting that both the “tripole pattern” of North Atlantic SST and tropical western Pacific SST anomalies in winter 2018/19 played a role in influencing the East Asian trough. The combined effect of all of these factors seems to be responsible for this extreme warm winter over southeastern coastal China.

scholarly journals North Atlantic Rossby Wave Breaking during the Hurricane Season: Association with Tropical and Extratropical Variability

2019 ◽  
Vol 32 (13) ◽  
pp. 3777-3801 ◽  
Author(s):  
Gan Zhang ◽  
Zhuo Wang
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
Wave Breaking ◽  
Large Scale ◽  
Heat Fluxes ◽  
Weather Regimes ◽  
Rossby Wave Breaking ◽  
The North ◽  
Sst Variability ◽  
Hurricane Season

Abstract This study explores the connection of Rossby wave breaking (RWB) with tropical and extratropical variability during the Atlantic hurricane season. The exploration emphasizes subtropical anticyclonic RWB events over the western North Atlantic, which strongly affect tropical cyclone (TC) activity. The first part of the study investigates the link between RWB and tropical sea surface temperature (SST) variability. Tropical SST variability affects tropical precipitation and modulates the large-scale atmospheric circulation over the subtropical Atlantic, which influences the behaviors of Rossby waves and the frequency of RWB occurrence. Meanwhile, RWB regulates surface heat fluxes and helps to sustain SST anomalies in the western North Atlantic. The second part of the study explores the connections between RWB and extratropical atmosphere variability by leveraging weather regime analysis. The weather regimes over the North Atlantic are closely associated with RWB over the eastern North Atlantic and western Europe, but show weak associations with RWB over the western North Atlantic. Instead, RWB over the western basin is closely related to the weather regimes in the North Pacific–North America sector. The finding helps clarify why the correlation between the Atlantic TC activity and the summertime North Atlantic Oscillation is tenuous. The relations between the extratropical weather regimes and tropical climate modes are also discussed. The findings suggest that both tropical and extratropical variability are important for understanding variations of RWB events and their impacts on Atlantic TC activity.

scholarly journals Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly

2021 ◽  
Author(s):  
Marius Årthun ◽  
Robert C. J. Wills ◽  
Helen L. Johnson ◽  
Léon Chafik ◽  
Helene R. Langehaug
Keyword(s):  
Atmospheric Circulation ◽  
North Atlantic ◽  
Time Scale ◽  
Low Frequency ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Sst Variability ◽  
Cold Anomaly ◽  
The North Atlantic ◽  
Sst Anomalies

<p>There has recently been a large focus on identifying the mechanisms responsible for Atlantic multidecadal variability (AMV). However, decadal-scale variability embedded within the AMV has received less attention, despite being a prominent feature of observed North Atlantic sea surface temperature (SST) and important for the climate of adjacent continents. These decadal fluctuations in the North Atlantic Ocean are also a key source of skill in decadal climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations and pre-industrial control simulations, we identify a decadal mode of atmosphere-ocean variability in the North Atlantic with a dominant time scale of 13-18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air-sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This strongly suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability, and that we might expect it to become less pronounced over the next few years.</p>

scholarly journals Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly

2020 ◽  
pp. 1-52
Author(s):  
Marius Ǻrthun ◽  
Robert C. J. Wills ◽  
Helen L. Johnson ◽  
Léon Chafik ◽  
Helene R. Langehaug
Keyword(s):  
Atmospheric Circulation ◽  
North Atlantic ◽  
Time Scale ◽  
Low Frequency ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Sst Variability ◽  
Cold Anomaly ◽  
The North Atlantic ◽  
Sst Anomalies

AbstractDecadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations and pre-industrial control simulations, we identify a decadal mode of atmosphere-ocean variability in the North Atlantic with a dominant time scale of 13-18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air-sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability.

Seasonal Predictability of Wintertime mid-latitude Cyclonic Activity over the North Atlantic and Europe

2021 ◽  
Author(s):  
Alvise Aranyossy ◽  
Sebastian Brune ◽  
Lara Hellmich ◽  
Johanna Baehr
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Jet Stream ◽  
Cyclonic Activity ◽  
Max Planck ◽  
Pressure System ◽  
Wind Speeds ◽  
Average Lifespan ◽  
The North ◽  
The North Atlantic

<p>We analyse the connections between the wintertime North Atlantic Oscillation (NAO), the eddy-driven jet stream with the mid-latitude cyclonic activity over the North Atlantic and Europe. We investigate, through the comparison against ECMWF ERA5 and hindcast simulations from the Max Planck Institute Earth System Model (MPI-ESM), the potential for enhancement of the seasonal prediction skill of the Eddy Kinetic Energy (EKE) by accounting for the connections between large-scale climate and the regional cyclonic activity. Our analysis focuses on the wintertime months (December-March) in the 1979-2019 period, with seasonal predictions initialized every November 1st. We calculate EKE from wind speeds at 250 hPa, which we use as a proxy for cyclonic activity. The zonal and meridional wind speeds are bandpass filtered with a cut-off at 3-10 days to fit with the average lifespan of mid-latitude cyclones. </p><p>Preliminary results suggest that in ERA5, major positive anomalies in EKE, both in quantity and duration, are correlated with a northern position of the jet stream and a positive phase of the NAO. Apparently, a deepened Icelandic low-pressure system offers favourable conditions for mid-latitude cyclones in terms of growth and average lifespan. In contrast, negative anomalies in EKE over the North Atlantic and Central Europe are associated with a more equatorward jet stream, these are also linked to a negative phase of the NAO.  Thus, in ERA5, the eddy-driven jet stream and the NAO play a significant role in the spatial and temporal distribution of wintertime mid-latitude cyclonic activity over the North Atlantic and Europe. We extend this connection to the MPI-ESM hindcast simulations and present an analysis of their predictive skill of EKE for wintertime months.</p>

Evidence for a Rapid Global Climate Shift across the Late 1960s

2007 ◽  
Vol 20 (12) ◽  
pp. 2721-2744 ◽  
Author(s):  
Peter G. Baines ◽  
Chris K. Folland
Keyword(s):  
Southern Hemisphere ◽  
North Atlantic ◽  
Amazon Basin ◽  
Storm Track ◽  
Central Pacific ◽  
Atlantic Sector ◽  
The North ◽  
The North Atlantic ◽  
The 1950S ◽  
African Sahel

Abstract It is shown that a number of important characteristics of the global atmospheric circulation and climate changed in a near-monotonic fashion over the decade, or less, centered on the late 1960s. These changes were largest or commonest in tropical regions, the Southern Hemisphere, and the Atlantic sector of the Northern Hemisphere. Some, such as the decrease in rainfall in the African Sahel, are well known. Others appear to be new, but their combined extent is global and dynamical linkages between them are evident. The list of affected variables includes patterns of SST; tropical rainfall in the African Sahel and Sudan, the Amazon basin, and northeast Brazil; pressure and SST in the tropical North Atlantic and the west and central Pacific; various branches of the southern Hadley circulation and the southern subtropical jet stream; the summer North Atlantic Oscillation; south Greenland temperature; the Southern Hemisphere storm track; and, quite likely, the Antarctic sea ice boundary. These changes are often strongest in the June–August season; changes are also seen in December–February but are generally smaller. In Greenland, annual mean temperature seems to be affected strongly, reflecting similar changes in SST throughout the year in the higher latitudes of the North Atlantic. Possible causes for these coordinated changes are briefly evaluated. The most likely candidates appear to be a likely reduction in the northward oceanic heat flux associated with the North Atlantic thermohaline circulation in the 1950s to 1970s, which was nearly in phase with a rapid increase in anthropogenic aerosol emissions during the 1950s and 1960s, particularly over Europe and North America.

scholarly journals Simulated Relationships between Regional Temperatures and Large-Scale Circulation: 125 kyr BP (Eemian) and the Preindustrial Period

2005 ◽  
Vol 18 (19) ◽  
pp. 4032-4045 ◽  
Author(s):  
Nikolaus Groll ◽  
Martin Widmann ◽  
Julie M. Jones ◽  
Frank Kaspar ◽  
Stephan J. Lorenz
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Temperature Sensitive ◽  
Proxy Data ◽  
Large Scale Circulation ◽  
The North ◽  
Temperature Signal ◽  
Northern Winter ◽  
The Mean ◽  
Mean Circulation

Abstract To investigate relationships between large-scale circulation and regional-scale temperatures during the last (Eemian) interglacial, a simulation with a general circulation model (GCM) under orbital forcing conditions of 125 kyr BP is compared with a simulation forced with the Late Holocene preindustrial conditions. Consistent with previous GCM simulations for the Eemian, higher northern summer 2-m temperatures are found, which are directly related to the different insolation. Differences in the mean circulation are evident such as, for instance, stronger northern winter westerlies toward Europe, which are associated with warmer temperatures in central and northeastern Europe in the Eemian simulation, while the circulation variability, analyzed by means of a principal component analysis of the sea level pressure (SLP) field, is very similar in both periods. As a consequence of the differences in the mean circulation the simulated Arctic Oscillation (AO) temperature signal in the northern winter, on interannual-to-multidecadal time scales, is weaker during the Eemian than today over large parts of the Northern Hemisphere. Correlations between the AO index and the central European temperature (CET) decrease by about 0.2. The winter and spring SLP anomalies over the North Atlantic/European domain that are most strongly linearly linked to the CET cover a smaller area and are shifted westward over the North Atlantic during the Eemian. However, the strength of the connection between CET and these SLP anomalies is similar in both simulations. The simulated differences in the AO temperature signal and in the SLP anomaly, which is linearly linked to the CET, suggest that during the Eemian the link between the large-scale circulation and temperature-sensitive proxy data from Europe may differ from present-day conditions and that this difference should be taken into account when inferring large-scale climate from temperature-sensitive proxy data.

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