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The importance of the Atlantic Multidecadal Oscillation (AMO) and Interdecadal Pacific Oscillation (IPO) in influencing zonally asymmetric changes in Antarctic surface air temperature (SAT) has been established. However, previous studies have primarily concentrated on examining the combined impact of the contrasting phases of the AMO and IPO, which have been dominant since the advent of satellite observations in 1979. This study utilizes long-term reanalysis data to investigate the impact of four combinations of +AMO+IPO, −AMO−IPO, +AMO−IPO, and −AMO+IPO on Antarctic SAT over the past 115 years. The +AMO phase is characterized by a spatial mean temperature amplitude of up to 0.5°C over the North Atlantic Ocean, accompanied by positive sea surface temperature (SST) anomalies in the tropical eastern Pacific and negative SST anomalies in the extratropical-mid-latitude western Pacific, which are indicative of the +IPO phase. The Antarctic SAT exhibits contrasting spatial patterns during the +AMO+IPO and +AMO−IPO periods. However, during the −AMO+IPO period, apart from the Antarctic Peninsula and the vicinity of the Weddell Sea, the entire Antarctic region experiences a warming trend. The most pronounced signal in the SAT anomalies is observed during the austral autumn, whereas the combination of −AMO and −IPO exhibits the smallest magnitude across all the combinations. The wavetrain excited by the SST anomalies associated with the AMO and IPO induces upper-level and surface atmospheric circulation anomalies, which alter the SAT anomalies. Furthermore, downward longwave radiation anomalies related to anomalous cloud cover play a crucial role. In the future, if the phases of AMO and IPO were to reverse (AMO transitioning to a negative phase and IPO transitioning to a positive phase), Antarctica could potentially face more pronounced warming and accelerated melting compared to the current observations. [ABSTRACT FROM AUTHOR]

The tropical climate variabilities, such as Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO), are accompanied by changes in the tropical deep convection which can influence the atmospheric circulation in the Southern Hemisphere (SH). To investigate each role of IOD and ENSO in the September‐November (SON) circulation, we examine teleconnection patterns associated with IOD and ENSO events using the ERA5 monthly averaged data from 1979 to 2020. Our approach is to calculate the power spectral density (PSD) of the sea level pressure (SLP) and meridional wind and geopotential height at 300 hPa that are decomposed by zonal wave numbers (ZWNs), and to compute their correlations with IOD and ENSO at each latitudinal band. The main results are that IOD (ENSO) is negatively (positively) correlated with PSDs of ZWN2 and ZWN3 (ZWN1) at 300 hPa in the SH middle latitudes. Considering the Rossby wave train, IOD (ENSO) considerably affects the variability of the ZWN3 (ZWN1) pattern, which influences the meridional exchange of momentum. Additionally, the relationship between IOD and ZWN3 has become tighter in recent years, which is not seen in that with ENSO. The IOD and ENSO events also modify the SLP patterns and meridional surface winds, modulating the sea ice extent in the Southern Ocean. During the highly positive 2019 IOD event, the variability of the middle latitudes atmospheric circulation was considerably larger than climatology, suggesting a higher chance of more extreme weather patterns associated with more frequent intense IOD events in the warming climate. Plain Language Summary: The two tropical climate variabilities, Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO), can change the location and intensity of tropical convection, generate anomalous Rossby wave trains heading to the Southern Hemisphere (SH), and the zonal wave pattern of the middle latitudes' atmospheric circulation and further the Antarctic climate. We analyzed this low‐high latitudes teleconnection in the austral spring when IOD and ENSO strongly affect the wave patterns using atmospheric reanalysis data. The results show that ENSO is positively correlated with zonal wave number 1 (ZWN1) activity, but IOD negatively correlates with ZWN2 and ZWN3 activities in the SH middle latitudes. The relationship between IOD and ZWN3 has become tighter in recent years, which is not seen in that with ENSO. The IOD and ENSO events also modify the sea level pressure patterns and meridional surface winds in the Southern Ocean, leaving the ZWN3 features. The surface wind patterns affect the sea ice concentration, but with a difference between IOD and ENSO. Key Points: Indian Ocean Dipole (IOD)'s Rossby wave train is partially similar to the climatology; a negative IOD enormously increases zonal wavenumber 3IOD (El Niño Southern Oscillation) is negatively (positively) correlated with zonal wavenumber 2 and 3 (1) in austral springThe extremely positive IOD in 2019 exhibited more significant increases and unusual changes in wave patterns [ABSTRACT FROM AUTHOR]

In this study, the onset and cessation of rainy season over the Hengduan Mountains (HM) are determined by an objective method. The onset and cessation of climatological rainy season are largely controlled by large-scale circulation systems, such as the Indian summer monsoon (ISM), 200 hPa subtropical jet, and the South Asian high. Both onset and cessation dates exhibit strong interannual variability. For late onset, the lower-level northeasterly anomalies over the north Indian Ocean (IO) delay the advance of moisture transported by ISM, and are excited by the positive SST anomalies over the southwest IO. In accord with late cessation, the equatorial west IO cooling adjusts the zonal overturn circulation and causes the west side wet anomalies, which trigger the north IO cyclonic wind anomalies and the northwest Pacific anticyclonic wind anomalies following Gill response. These two anomalous flows merge, turn southerly, and continuously transport moisture to the HM, prolonging the rainy season. At upper levels, the late onset and cessation related circulation anomalies are almost mirror images and highly resemble the Silk Road pattern. The anomalous upper and lower-level circulation cooperate and favor the dynamical and thermodynamical processes, which contribute to the vertical motion anomalies and finally affect the onset and cessation dates. Especially, the 200 hPa geopotential height anomaly center on the north side of the Arabian Sea (AS) is stronger than the other centers both for late onset and cessation, and coincides well with the nearby Rossby wave source, which are reinforced via the planetary vorticity stretching term due to the upper-level wind anomalies induced by the IO SST anomalies. [ABSTRACT FROM AUTHOR]

Based on the fifth-generation reanalysis dataset from the European Centre for Medium-Range Weather Forecasts for 1979–2019, we investigated the effects of the circumglobal teleconnection (CGT) on the interdecadal variation of the March atmospheric heat source (AHS) over the Southeast Asian low-latitude highlands (SEALLH). The dominant mode of the March AHS over the SEALLH features a monopole structure with an 8–11-year period. Decadal variations in the AHS make an important contribution to the 11-year low-pass filtered component of the AHS index, whichexplains 54.3% of the total variance. The CGT shows a clear interdecadal variation, which explains 59.3% of the total variance. The March AHS over the SEALLH is significantly related to the CGT on interdecadal timescales. When the CGT is optimally excited by a significant cyclonic vorticity source near northern Africa (i.e., in its positive phase), the SEALLH is dominated by anomalous southerly winds and ascending motions on the east of the anomalous cyclone. The enhanced advection and upward transfer result in a high-enthalpy air mass that converges into and condenses over the SEALLH, leading to a larger-than-average March AHS over this region. The key physical processes revealed by this diagnostic analysis are supported by numerical experiments. [ABSTRACT FROM AUTHOR]

Distinct Pacific-North American (PNA) teleconnection patterns and their associated climate impacts in North America have been observed during single-year El Niño events, as well as during both the first and second years of multi-year El Niño events. The research highlights the critical role of PNA-related North Pacific circulation anomalies, which shift northwestward during multi-year events and reach their peak intensity in the second year. Multiple dynamical diagnostic methods are employed to elucidate the reasons behind the diverse subtropical circulation responses. Differences in the anomalous tropical heat sources influence the formation of Rossby wave sources through adjustments in divergent wind anomalies, subsequently modulating the position and intensity of the PNA patterns. Additionally, variations in the meridional range of sea surface temperature anomalies affect the edge of tropospheric temperature through moist-adiabatic adjustment, leading to distinct subtropical jet stream responses. This, in turn, modifies the position of North Pacific circulation anomalies through the advection of anomalous kinetic energy. Furthermore, synoptic-scale transient eddies act as a feedback mechanism, helping to maintain and intensify these diverse atmospheric anomalies.

The wave activity flux representing the energy propagation direction of planetary Rossby wave generally originates from a large wave source area. This study investigates the interdecadal variability and formation mechanism of Rossby wave source over the Tibetan Plateau (TP-RWS) and its impact on the atmospheric circulation and precipitation pattern in East Asia based on the ERA-20C reanalysis dataset in summer (June–July–August) during 1900 to 2010. Results show that the region with the maximum variabilities of Rossby wave source (RWS) in the past 110 years appears over the Tibetan Plateau (TP) during boreal summer, and the TP-RWS shows prominent characteristics of interdecadal oscillation. Secondly, the TP-RWS is mainly composed of the vortex stretching term (RWS-S1) and the absolute vorticity advection term (RWS-S2). The interdecadal TP-RWS is a synergistic result of the snow cover over northwestern TP associated with the RWS-S1, and the deep convection over southeastern TP associated with the RWS-S2. Furthermore, the interdecadal TP-RWS can lead to an alternatively positive and negative pattern of geopotential height anomalies from the northwestern TP to the North Pacific, which has a great climate effect on the precipitation in Huang-huai River Basin, South Korea and Japan Island. Under the guidance of the anomalous cyclonic circulation in East Asia, the prevailing southerly and easterly winds occur over the West Pacific and the Huang-huai River Basin, which lead to the water vapor convergence and upward movement at middle and lower troposphere. [ABSTRACT FROM AUTHOR]

El Niño/Southern Oscillation (ENSO) as the strongest tropical interannual signature has the most prominent impact on wintertime Pacific‐North American (PNA) teleconnection pattern. ENSO exhibits an increasing asymmetry in recent decades. This study quantifies its extratropical asymmetric impact on the amplitude of the PNA pattern, using a normalized asymmetry index defined as a ratio of asymmetric versus symmetric anomalies for El Niño and La Niña. Relative to the ENSO asymmetry, the extratropical asymmetric impact is largely amplified especially downstream by up to 82%. Such an amplification is attributed to noticeable increases, 83.6% (68.9%), in asymmetry of Rossby wave source (RWS) anomalies over two key regions of North Pacific, in which anomalous divergence induced by nonlinear condensational heating feedback and anomalous synoptic eddy forcing are two major contributors. The former contributes 39.3% (47.5%) over the western (eastern) North Pacific through increasing asymmetric RWS anomalies, while the latter contributes 29.5% (21.3%) through decreasing symmetric RWS anomalies. Plain Language Summary: El Niño/Southern Oscillation (ENSO) as the strongest tropical interannual signature can cause prominent wintertime North American climate anomalies through Pacific‐North American (PNA) teleconnection pattern. El Niño and La Niña events as two opposite phases of ENSO are not mirror images of each other, with El Niño significantly stronger than La Niña in recent decades, due to the nonlinearities in the tropical air‐sea system. Such an ENSO asymmetry tends to induce El Niño to have more serious impact than La Niña. This asymmetric impact of ENSO events on the PNA pattern is quantitatively found in this study to be largely amplified especially downstream due to the nonlinearities within the extratropical atmosphere, with using a normalized asymmetry index defined as a ratio of the asymmetric versus symmetric anomalies for El Niño and La Niña. The amplification is attributed to noticeable increases in asymmetry of the Rossby wave source anomalies over North Pacific, in which anomalous divergence induced by nonlinear condensational heating feedback and anomalous synoptic eddy vorticity forcing are two major contributors. Given that ENSO is the primary source of seasonal predictability, the amplified asymmetric impact on the PNA pattern discovered in this study would be of predictive value for the North American climate anomalies. Key Points: Relative to the El Niño/Southern Oscillation (ENSO) asymmetry, ENSO's asymmetric impact on the Pacific‐North American pattern is quantitatively found to be largely amplified by up to 82%The amplified asymmetric impact is due to the noticeable increases in asymmetry of Rossby wave source (RWS) anomalies over North PacificExtratropical nonlinearities in condensational heating and synoptic eddy vorticity feedbacks are responsible for the asymmetric RWS increase [ABSTRACT FROM AUTHOR]

In this study, we report significant biennial variability or oscillation (BO) in the boreal spring (March–May) Surface Air Temperature (SAT) over India and unravelled the causative mechanisms. The positive phase of the BO exhibit significant seasonal warming over India, whereas seasonal cooling is observed during the negative phase of BO. Heat wave days are more during the positive phase of BO compared to negative or neutral phase. The positive (negative) phase of BO is generally coherent with the central (eastern) Pacific warming (cooling) years. The anomalous low-level divergence associated with low-level anticyclonic circulation induces less cloudiness and intense surface solar radiation ovr India during the positive phase, favouring surface warming. The evolution of some positive and/or negative phases of BO without any large scale forcing from the equatorial Pacific suggested the possibility of alternate pathways. The strong anomalous upper-level (at 200 hPa) anticyclonic circulation provoked by mid-latitude Rossby waves is found contributing to the positive phase, thereby highlighting the role of dominant mid-latitude pathways in the biennial SAT variability in addition to El Niño forcing. The sinking motion associated with persistent high, and the associated adiabatic compression also supported surface heating during the positive phase of BO. On the other hand, the mid-latitude Rossby wave induced upper-level cyclonic circulation is found contributing to the negative phase. The sinking motion associated with persistent high, and the associated adiabatic compression also supported surface heating during the positive phase of BO. In contrast, negative soil temperature anomalies and high latent heat flux release to the atmosphere supported surface cooling during the negative phase. • Existence of biennial variability of surface air temperature (SAT) over India is reported. • Mechanisms for the biennial SAT variability are unravelled. • Impact of positive biennial SAT variability on the heat wave over India is highlighted. [ABSTRACT FROM AUTHOR]

Tropical-extratropical teleconnection during boreal summer is characterized by a circumglobal teleconnection (CGT) pattern and is often considered as one of the major predictability sources of extratropical atmospheric circulation. The extent to which the summer circumglobal teleconnection pattern is remotely forced by tropical sea surface temperature (SST) variability remains to be elucidated. Here, we use a suite of pacemaker simulations from the Community Earth System Model (CESM1) to investigate the contribution of tropical sea surface temperature variability in the Eastern Tropical Pacific, Northern Atlantic, and Indo-Western Pacific to the summer circumglobal teleconnection circulation. We find that the variability of circumglobal teleconnection in the summer is dominated by the sea surface temperature variability in the Indo-Western Pacific and its remotely forced ENSO-like anomaly. As a result, the Indo-Western Pacific pacemaker experiment exhibits precipitation and surface temperature teleconnection patterns closest to the observed patterns in the Northern Hemisphere. A better understanding of such tropical-extratropical interactions can inform better seasonal forecasts and model development.

Abstract El Niño/Southern Oscillation (ENSO) as the strongest tropical interannual signature has the most prominent impact on wintertime Pacific‐North American (PNA) teleconnection pattern. ENSO exhibits an increasing asymmetry in recent decades. This study quantifies its extratropical asymmetric impact on the amplitude of the PNA pattern, using a normalized asymmetry index defined as a ratio of asymmetric versus symmetric anomalies for El Niño and La Niña. Relative to the ENSO asymmetry, the extratropical asymmetric impact is largely amplified especially downstream by up to 82%. Such an amplification is attributed to noticeable increases, 83.6% (68.9%), in asymmetry of Rossby wave source (RWS) anomalies over two key regions of North Pacific, in which anomalous divergence induced by nonlinear condensational heating feedback and anomalous synoptic eddy forcing are two major contributors. The former contributes 39.3% (47.5%) over the western (eastern) North Pacific through increasing asymmetric RWS anomalies, while the latter contributes 29.5% (21.3%) through decreasing symmetric RWS anomalies.

The wave activity flux representing the energy propagation direction of planetary Rossby wave generally originates from a large wave source area. This study investigates the interdecadal variability and formation mechanism of Rossby wave source over the Tibetan Plateau (TP-RWS) and its impact on the atmospheric circulation and precipitation pattern in East Asia based on the ERA-20C reanalysis dataset in summer (June–July–August) during 1900 to 2010. Results show that the region with the maximum variabilities of Rossby wave source (RWS) in the past 110 years appears over the Tibetan Plateau (TP) during boreal summer, and the TP-RWS shows prominent characteristics of interdecadal oscillation. Secondly, the TP-RWS is mainly composed of the vortex stretching term (RWS-S1) and the absolute vorticity advection term (RWS-S2). The interdecadal TP-RWS is a synergistic result of the snow cover over northwestern TP associated with the RWS-S1, and the deep convection over southeastern TP associated with the RWS-S2. Furthermore, the interdecadal TP-RWS can lead to an alternatively positive and negative pattern of geopotential height anomalies from the northwestern TP to the North Pacific, which has a great climate effect on the precipitation in Huang-huai River Basin, South Korea and Japan Island. Under the guidance of the anomalous cyclonic circulation in East Asia, the prevailing southerly and easterly winds occur over the West Pacific and the Huang-huai River Basin, which lead to the water vapor convergence and upward movement at middle and lower troposphere.

The Asian summer monsoon (ASM) affects more than one-third of the world's population due to its close connection with floods, droughts thus water resources in densely populated Asian countries. The effects of aerosols emitted in remote regions on the ASM, in contrast to local emissions, remain largely unclear. Here we demonstrate through a hierarchy of climate models that aerosol emissions from the central African wildfires could strengthen the circulation of the ASM (South Asian Monsoon in particular), increase precipitation over South Asia and reduce precipitation immediately north and south of it. The enhanced latent heating over South Asia provides a critical positive feedback to the initial strengthening of monsoon westerlies associated with wildfire-driven anomalous Rossby wave source. The atmospheric dynamical bridge discovered here effectively connects African biomass burning with hydroclimate variability over East Asia in boreal summer and offers a new source of monsoon predictability across a range of timescales. [ABSTRACT FROM AUTHOR]

Abstract The Rossby wave source (RWS) in the upper troposphere plays an important role in the tropical‐extratropical teleconnections. Using the daily outputs from the Phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, the overall model performances in simulating the climatological Rossby wave sources in both winter and summer are evaluated. The ensemble mean of the CMIP5 models can simulate the large‐scale geographical distributions of the RWS reasonably close to the observations, with the simulations of RWS in general better in the Southern Hemisphere. For the Northern Hemisphere, most models overestimate the subtropical RWS but underestimate the midlatitude RWS in both winter and summer. Many models even fail to simulate the seasonal source‐sink shift of RWS in East Asia. Greatest intermodel differences are shown in East Asia, western North America in both seasons, and in the subtropical belt in winter hemisphere. Possible reasons for the model biases in RWS are further investigated. In the Northern Hemisphere, our analysis shows that model performance in simulating the local divergence, which might relate to the overly smoothed topography in Asia and western North America in the model, is most responsible for the biases of the RWS simulations. In the Southern Hemisphere, the bias in subtropical divergence pattern and tropical convection all contribute to the intermodel divergence of RWS simulation.

The Rossby wave source (RWS) in the upper troposphere plays an important role in the tropical‐extratropical teleconnections. Using the daily outputs from the Phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, the overall model performances in simulating the climatological Rossby wave sources in both winter and summer are evaluated. The ensemble mean of the CMIP5 models can simulate the large‐scale geographical distributions of the RWS reasonably close to the observations, with the simulations of RWS in general better in the Southern Hemisphere. For the Northern Hemisphere, most models overestimate the subtropical RWS but underestimate the midlatitude RWS in both winter and summer. Many models even fail to simulate the seasonal source‐sink shift of RWS in East Asia. Greatest intermodel differences are shown in East Asia, western North America in both seasons, and in the subtropical belt in winter hemisphere. Possible reasons for the model biases in RWS are further investigated. In the Northern Hemisphere, our analysis shows that model performance in simulating the local divergence, which might relate to the overly smoothed topography in Asia and western North America in the model, is most responsible for the biases of the RWS simulations. In the Southern Hemisphere, the bias in subtropical divergence pattern and tropical convection all contribute to the intermodel divergence of RWS simulation. Key Points: Daily outputs show that most CMIP5 models overestimate subtropical RWS but underestimate midlatitude RWS in NH in both winter and summerGreatest intermodel differences are shown in East Asia, western North America in both seasons, and in subtropical belt in winter hemisphereModel performance in simulating the local convergence is most responsible for biases of NH RWS simulation and global teleconnection patterns [ABSTRACT FROM AUTHOR]

In previous work, a significant relationship was identified between the meridional displacement of the Asian westerly jet (JMD) and the Silk Road Pattern (SRP) in summer. The present study reveals that this relationship is robust in northward JMD years but absent in southward JMD years. In other words, the amplitude of the SRP increases with northward displacement of the jet but shows little change with southward displacement. Further analysis indicates that, in northward JMD years, the Rossby wave source (RWS) anomalies, which are primarily contributed by the planetary vortex stretching, are significantly stronger around the entrance of the Asian jet, i.e., the Mediterranean Sea-Caspian Sea area, with the spatial distribution being consistent with that related to the SRP. By contrast, in southward JMD years, the RWS anomalies are much weaker. Therefore, this study suggests that the RWS plays a crucial role in inducing the asymmetry of the JMD-SRP relationship. The results imply that climate anomalies may be stronger in strongly northward-displaced JMD years due to the concurrence of the JMD and SRP, and thus more attention should be paid to these years. [ABSTRACT FROM AUTHOR]

The Rossby wave source ( RWS) and the corresponding extratropical wave response to tropical convection associated with different phases of the Madden-Julian Oscillation ( MJO) is investigated with the dynamical core of a climate model. The initial flow is specified to correspond to the boreal winter climatological flow and an imposed tropical heating that is derived from the observed precipitation for all eight MJO phases. One key question addressed here is why does the extratropical Rossby wave train depart the subtropics at a longitude well to the east of the RWS. For all eight MJO phases, it is found that the extratropical response over the North Pacific and North America is almost entirely due to the MJO convection over the western tropical Pacific. The RWS is excited within the first 24 h after the model heating is turned on. For MJO phases 1-3 and 8, the RWS leads to the development of a cyclonic anomaly over southeast Asia via advection of the climatological absolute vorticity by the anomalous divergent wind in the subtropics and by horizontal convergence in the Tropics. MJO phases 4-7 show opposite features. The resulting anomaly is then advected eastward by the climatological zonal wind toward the central Pacific, after which dispersion into the extratropics and the excitation of a Pacific/North American teleconnection pattern takes place. [ABSTRACT FROM AUTHOR]

Indian Ocean Dipole (IOD) is an air-sea coupled variability in the Tropical Indian Ocean (TIO), which has strong impact on the climate variability over the Indian Ocean rim countries. Though many IODs co-occurred with El Niño/Southern Oscillation (ENSO) in the past, IODs do evolve independently suggesting the possible role of internal dynamics of the Indian Ocean. In this study, the Subtropical IOD (SIOD) is reported as a trigger or a link for the formation of such pure IODs. The study further highlights the existence of a cyclic feedback between IOD and SIOD through tropical subtropical interaction when ENSO forcing is absent/weak. Through this feedback cycle, the positive (negative) SIODs favour positive (negative) IODs and positive (negative) IODs favour negative (positive) SIODs. The internal feedback between IOD and SIOD is found to be responsible for the observed biennial tendency in IOD and SIOD. The mechanisms associated with the feedback cycle and the causative factors are demonstrated in detail. The positive SIOD induced warming over the southwest subtropical Indian Ocean supports a meridional cell favouring the equatorward expansion of the subtropical anticyclone thereby initiating the equatorial easterlies during the pure IOD years. On the other hand, the southeastward extension of western warming associated with the positive phase of IOD initiates warm anomalies in the northeastern subtropics. The upper level divergence, high absolute vorticity gradient and the associated barotropic Rossby wave source in the southern subtropics then generate an equivalent barotropic Rossby wave pattern in the midlatitudes. The cyclonic circulation associated with this Rossby wave train supports the generation of negative SIOD through wind evaporation SST feedback. The cyclic feedback mechanism between IOD and SIOD through tropical and subtropical ocean-atmosphere interaction is shown. The black colour represents the processes during AM (SON) of IOD year in the left (right panels), and blue colour represents JJA (JFM following IOD). The solid red arrows indicate wind-evaporation-SST feedback mechanism, and upward arrows represent the convective heating. The solid (dashed) circles represent the anticyclonic (cyclonic) circulation. [Display omitted] • Existence of coupled cyclic feedback between IOD and SIOD is reported. • Mechanisms for the non ENSO IODs are established. • Tropical extratropical interaction leading to the generation of internal TIO variability is demonstrated. [ABSTRACT FROM AUTHOR]

This work is set out to quantify the contribution of tropical and extratropical atmospheric forcing mechanisms to the formation of the North Atlantic Oscillation (NAO) pattern. Although the NAO varies on a wide range of time scales, we focus on 10–60 d. At these time scales, mechanisms are at play in the atmosphere that can generate the characteristic dipole pattern. We focus on the tropical Rossby Wave Source (RWS) and extratropical eddy activity. Anomalous tropical and extratropical vorticity forcing associated with the NAO is derived from atmospheric reanalysis data and applied in an idealised barotropic model. Also, using winds from composites of the NAO, the vorticity forcing is derived inversely from the barotropic vorticity equation. Both types of forcing are imposed in the barotropic model in the tropics and extratropics, respectively. An important result is that the tropics dampen the NAO as a result of a negative feedback generated in the extratropics. The damping is strongest, about 30%, for the negative phase of the NAO. For the positive phase, the damping is about 50% smaller. The results show that the barotropic vorticity equation can represent the dynamics of both tropical and extratropical forcing related to the formation of the NAO patterns.

This work is set out to quantify the contribution of tropical and extratropical atmospheric forcing mechanisms to the formation of the North Atlantic Oscillation (NAO) pattern. Although the NAO varies on a wide range of time scales, we focus on 10-60 d. At these time scales, mechanisms are at play in the atmosphere that can generate the characteristic dipole pattern. We focus on the tropical Rossby Wave Source (RWS) and extratropical eddy activity. Anomalous tropical and extratropical vorticity forcing associated with the NAO is derived from atmospheric reanalysis data and applied in an idealised barotropic model. Also, using winds from composites of the NAO, the vorticity forcing is derived inversely from the barotropic vorticity equation. Both types of forcing are imposed in the barotropic model in the tropics and extratropics, respectively. An important result is that the tropics dampen the NAO as a result of a negative feedback generated in the extratropics. The damping is strongest, about 30%, for the negative phase of the NAO. For the positive phase, the damping is about 50% smaller. The results show that the barotropic vorticity equation can represent the dynamics of both tropical and extratropical forcing related to the formation of the NAO patterns. [ABSTRACT FROM AUTHOR]

Rossby waves (RW) propagation due to a local forcing is one of the mechanisms responsible for wave trains configurations known as teleconnections. The term teleconnection refers to anomalies patterns that are correlated in several regions of the world, causing large-scale changes in atmospheric waves patterns and temperature and precipitation regimes. The aim of teleconnections studies is to provide a better understanding of atmospheric variability and their mechanisms of action in order to identify patterns that can be tracked and predicted. The purpose of this study was to characterize seasonal and spatial variability of atmospheric RW sources. The RW source at 200 hPa was calculated for the four seasons with reanalysis data of zonal and meridional wind. In the Northern Hemisphere (NH), the RW sources were located on East Asia, North America, North Atlantic and Pacific. The main RW sources in the Southern Hemisphere (SH) were located over Intertropical, South Pacific, South Atlantic and South Indian Convergence Zones. Extratropical regions were also identified, mainly to the south of Australia. The vortex stretching term ( S1) and the advection of absolute vorticity by the divergent wind ( S2) were analyzed to discuss the physical mechanisms for RW generation. In the NH, the source at East Asia in DJF changed to a sink in JJA, related to the dominance of S1 term in DJF and S2 term in JJA. In the SH, the vortex stretching term had the dominant contribution for RW source located to the south of Australia. The main forcing for RW sources at east of Australia was the advection of absolute vorticity by divergent flow. Over South America, both terms contributed to the source in DJF. The main modes of RW source variability were discussed by using empirical orthogonal functions analysis. RW variability was characterized by wave trains configurations in both hemispheres over regions of jet streams and storm tracks, associated with favorable and unfavorable areas for RW generation. [ABSTRACT FROM AUTHOR]

Boreal wintertime extratropical circulation is studied in relation to the tropical convection during the 1982/83 El Niño and 1988/89 La Niña. The anomaly structure of 1982/83 and 1988/89 over the extratropics reveals remarkably different features as the longitudinal tropical forcing region changes. The Rossby wave source (Positive) shows the largest maximum over East Asia in both years due to the persistent heating from the western Pacific warm pool area. However, the sink term shows contrasting features over the subtropics and extratropics between the two years. In the El Niño year, enhanced tropical convection over the eastern Pacific produces the Rossby wave sink at 10°N and shifted eastward over the North Pacific, while in the La Niña year, the sink area is shifted westward over the North Pacific. The contrasting features between the two events in mean-eddy interaction appears especially over the downstream area of the East Asian Jet. The extension (retraction) of the meanflow eastward (westward) to the east (west) of the dateline is related with the effect of the westward (eastward) E-vector and the strengthening (weakening) of the negative anomalies of the barotropic growth of kinetic energy. Hence, almost opposite characteristics between the two events can explain the close relationship of tropical convection and the extratropical internal variability. [ABSTRACT FROM AUTHOR]

The large-scale dynamical processes associated with an exceptionally dry and warm autumn during 1993 in Greece were studied. On the regional scale, it was found that low-frequency eddies prevailed over Europe in the form of a block with large-scale descending motion persisting throughout the whole troposphere. Moreover, the subtropical jet was unusually positioned much farther north over the Balkan peninsula and oscillated between this position and one over the central mainland of Greece where it interacted with the southern part of the polar front jet. On the global scale, it was found that this type of atmospheric circulation over Europe was closely related with a region of latent heat release in convective clouds over the tropics between the longitudes 100° and 120°E, and the formation of a steady divergence system at 150 hPa over the same region, strongly affecting the global rotational flow. [Copyright &y& Elsevier]