scholarly journals On the Dynamical Mechanism of the Southern Annular Mode, Including Seasonality: Inter-Annual Variability: and Trends

2021 ◽  
Author(s):  
◽  
Joseph Kidston
Keyword(s):  
Storm Track ◽  
South Pacific ◽  
La Niña ◽  
Pacific Region ◽  
Mean Flow ◽  
The South ◽  
Annular Modes ◽  
La Nina ◽  
Poleward Shift ◽  
Leading Mode

<p>This thesis considers the dynamics of the leading mode of extratropical atmospheric variability, the so-called annular modes, with a focus on the Southern Hemisphere (SH). Various aspects of the annular modes are addressed, from the underlying mechanism, to variability at progressively longer time-scales; ranging from the seasonality; to inter-annual variability; to the observed and predicted trends. The underlying mechanism of the annular modes is approached in the context of the recent theory that eddy-driven jets may be self-maintaining. We show that the leading mode of variability is associated with changes in the eddy source latitude, and that the latitude of the eddy source region is organised by the mean flow. This is consistent with the idea that the annular modes should be thought of as the meridional wandering of a self-maintaining jet, and that a positive baroclinic feedback prolongs these vacillations. Further, the degree to which the eddy-driven flow is self-maintaining determines the time-scale of the leading mode in a simplified general circulation model (GCM). Preliminary results indicate that the same dynamics are important in the real atmosphere. Secondly the seasonality of the southern annular mode (SAM) is investigated. As with previous studies, during summer the SAM is found to be largely zonally symmetric, whereas during winter it exhibits increased zonal wave number 2-3 variability. This is consistent with seasonal variations in the mean-state, and it is argued that the seasonal cycle of near-surface temperature over the Australian continent plays an important role, making the eddy driven jet, and hence the SAM, more zonally symmetric during summer than winter. During winter, the SAM exhibits little variability over the South Pacific and southeast of Australia. Dynamical reasons for this behaviour are discussed. This seasonality is discussed in the context of New Zealand climate, where it is shown that the variability in rainfall and temperature data are impacted by the large-scale seasonality of the SAM. Thirdly the zonally symmetric response of the SH to the El Nino Southern Oscillation (ENSO) is examined. Such a response is only observed in the mid-latitudes during austral summer and autumn, the same period when the climatological mean flow and storm-track is most zonally symmetric. During all seasons the ENSO stationary wave, or Pacific South American mode affects the baroclinicity at 850 hPa in the South Pacific region, so that during La Nina (El Nino) events the baroclinicity is increased (reduced). During summer La Nina events the anomalous transient eddy activity is increased over the entire meridional extent of the storm-track in the South Pacific region, whereas down-stream, over the Atlantic and Indian Oceans, the storm track moves poleward. It is suggested that during La Nina events, more vigorous eddy activity in the South Pacific leads to a poleward shift of the storm-track immediately down-stream, in the East Pacific. During summer and autumn the location of the storm-track in the Pacific region may be communicated around the hemisphere because there is a single climatological storm track, and so eddies can propagate from the Pacific region to the Atlantic region. There is some evidence of these dynamics in that the anomalous eddy activity associated with La Nina events begins in the South Pacific region and subsequently propagates zonally. Finally the cause of the poleward shift of the mid-latitude eddy-driven jet streams under global warming is considered. GCMs indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st Century. Here it is shown that the shift is associated with an increase in the eddy length-scale. The cause of the increase in eddy length-scale is discussed. Larger eddies are shown to propagate preferentially poleward, and it is argued that this may induce a corresponding shift in the mean flow that they maintain. The mechanism is investigated using a simplified GCM.</p>

scholarly journals On the Dynamical Mechanism of the Southern Annular Mode, Including Seasonality: Inter-Annual Variability: and Trends

2021 ◽  
Author(s):  
◽  
Joseph Kidston
Keyword(s):  
Storm Track ◽  
South Pacific ◽  
La Niña ◽  
Pacific Region ◽  
Mean Flow ◽  
The South ◽  
Annular Modes ◽  
La Nina ◽  
Poleward Shift ◽  
Leading Mode

<p>This thesis considers the dynamics of the leading mode of extratropical atmospheric variability, the so-called annular modes, with a focus on the Southern Hemisphere (SH). Various aspects of the annular modes are addressed, from the underlying mechanism, to variability at progressively longer time-scales; ranging from the seasonality; to inter-annual variability; to the observed and predicted trends. The underlying mechanism of the annular modes is approached in the context of the recent theory that eddy-driven jets may be self-maintaining. We show that the leading mode of variability is associated with changes in the eddy source latitude, and that the latitude of the eddy source region is organised by the mean flow. This is consistent with the idea that the annular modes should be thought of as the meridional wandering of a self-maintaining jet, and that a positive baroclinic feedback prolongs these vacillations. Further, the degree to which the eddy-driven flow is self-maintaining determines the time-scale of the leading mode in a simplified general circulation model (GCM). Preliminary results indicate that the same dynamics are important in the real atmosphere. Secondly the seasonality of the southern annular mode (SAM) is investigated. As with previous studies, during summer the SAM is found to be largely zonally symmetric, whereas during winter it exhibits increased zonal wave number 2-3 variability. This is consistent with seasonal variations in the mean-state, and it is argued that the seasonal cycle of near-surface temperature over the Australian continent plays an important role, making the eddy driven jet, and hence the SAM, more zonally symmetric during summer than winter. During winter, the SAM exhibits little variability over the South Pacific and southeast of Australia. Dynamical reasons for this behaviour are discussed. This seasonality is discussed in the context of New Zealand climate, where it is shown that the variability in rainfall and temperature data are impacted by the large-scale seasonality of the SAM. Thirdly the zonally symmetric response of the SH to the El Nino Southern Oscillation (ENSO) is examined. Such a response is only observed in the mid-latitudes during austral summer and autumn, the same period when the climatological mean flow and storm-track is most zonally symmetric. During all seasons the ENSO stationary wave, or Pacific South American mode affects the baroclinicity at 850 hPa in the South Pacific region, so that during La Nina (El Nino) events the baroclinicity is increased (reduced). During summer La Nina events the anomalous transient eddy activity is increased over the entire meridional extent of the storm-track in the South Pacific region, whereas down-stream, over the Atlantic and Indian Oceans, the storm track moves poleward. It is suggested that during La Nina events, more vigorous eddy activity in the South Pacific leads to a poleward shift of the storm-track immediately down-stream, in the East Pacific. During summer and autumn the location of the storm-track in the Pacific region may be communicated around the hemisphere because there is a single climatological storm track, and so eddies can propagate from the Pacific region to the Atlantic region. There is some evidence of these dynamics in that the anomalous eddy activity associated with La Nina events begins in the South Pacific region and subsequently propagates zonally. Finally the cause of the poleward shift of the mid-latitude eddy-driven jet streams under global warming is considered. GCMs indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st Century. Here it is shown that the shift is associated with an increase in the eddy length-scale. The cause of the increase in eddy length-scale is discussed. Larger eddies are shown to propagate preferentially poleward, and it is argued that this may induce a corresponding shift in the mean flow that they maintain. The mechanism is investigated using a simplified GCM.</p>

scholarly journals Dynamical Causes of the 2010/11 Texas–Northern Mexico Drought*

2014 ◽  
Vol 15 (1) ◽  
pp. 39-68 ◽  
Author(s):  
Richard Seager ◽  
Lisa Goddard ◽  
Jennifer Nakamura ◽  
Naomi Henderson ◽  
Dong Eun Lee
Keyword(s):  
North American ◽  
Climate Models ◽  
Storm Track ◽  
La Niña ◽  
Transient Eddy ◽  
Mean Flow ◽  
Seasonal Forecasts ◽  
Atmospheric Variability ◽  
Northern Mexico ◽  
La Nina

Abstract The causes of the Texas–northern Mexico drought during 2010–11 are shown, using observations, reanalyses, and model simulations, to arise from a combination of ocean forcing and internal atmospheric variability. The drought began in fall 2010 and winter 2010/11 as a La Niña event developed in the tropical Pacific Ocean. Climate models forced by observed sea surface temperatures (SSTs) produced dry conditions in fall 2010 through spring 2011 associated with transient eddy moisture flux divergence related to a northward shift of the Pacific–North American storm track, typical of La Niña events. In contrast the observed drought was not associated with such a clear shift of the transient eddy fields and instead was significantly influenced by internal atmospheric variability including the negative North Atlantic Oscillation of winter 2010/11, which created mean flow moisture divergence and drying over the southern Plains and southeast United States. The models suggest that drought continuation into summer 2011 was not strongly SST forced. Mean flow circulation and moisture divergence anomalies were responsible for the summer 2011 drought, arising from either internal atmospheric variability or a response to dry summer soils not captured by the models. The summer of 2011 was one of the two driest and hottest summers over recent decades but it does not represent a clear outlier to the strong inverse relation between summer precipitation and temperature in the region. Seasonal forecasts at 3.5-month lead time did predict onset of the drought in fall and winter 2010/11 but not intensification into summer 2011, demonstrating the current, and likely inherent, inability to predict important aspects of North American droughts.

scholarly journals Tropical Pacific relationships with Southern Hemisphere atmospheric circulation and Antarctic climate

2021 ◽  
Author(s):  
◽  
Kyle Clem
Keyword(s):  
Antarctic Peninsula ◽  
Deep Convection ◽  
South Pacific ◽  
Tropical Pacific ◽  
La Niña ◽  
Eastern Tropical Pacific ◽  
The South ◽  
La Nina ◽  
Surface Climate ◽  
The Tropics

<p>Significant trends in high-latitude Southern Hemisphere atmospheric circulation and surface climate have been observed over recent decades, which are likely linked to teleconnections from the tropics. This study investigates how a recent shift in tropical Pacific climate toward increased La Niña conditions has influenced the atmospheric circulation and surface climate across the high southern latitudes, and how variations in the El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM) influence the surface climate of Antarctica.  Over 1979-2014, significant cooling of eastern tropical Pacific sea surface temperatures (SSTs) is detected in all seasons. The eastern tropical Pacific cooling is associated with: (1) an intensified Walker Circulation during austral summer and autumn; (2) a weakened South Pacific Hadley cell and sub-tropical jet during autumn; and (3) a strengthening of the circumpolar westerlies between 50 and 60°S during both summer and autumn. Observed cooling in the eastern tropical Pacific is linearly congruent with 60-80% of the observed positive zonal-mean zonal wind trend between 50 and 60°S during summer (~35% of the interannual variability), and around half of the positive zonal-mean zonal wind trend during autumn (~15% of the interannual variability), the latter being most marked over the South Pacific. Although previous studies have linked the strengthening of the tropospheric westerlies during summer and autumn to ozone depletion, results from this study indicate poleward momentum fluxes and strengthened lower-tropospheric baroclinicity associated with eastern tropical Pacific cooling also help to maintain a strengthened mid-latitude jet through the 21st century, especially across the South Pacific.  The La Niña shift in tropical Pacific SSTs is also significantly related to several changes in Antarctic surface climate. During autumn, a regional pattern of cooling occurred along coastal East Antarctica after 1979, with the rate of cooling increasing at Novolazarevskaya, Syowa, Casey, and Dumont d’Urville stations, while the rate of cooling decreased at Mawson and Davis stations. It is shown that regional circulation changes associated with tropical Pacific teleconnections project strongly onto the regional nature of the cooling trends, with 40% of the cooling at Novolazarevskaya and Syowa linearly congruent with the increased La Niña conditions, and more than 60% of the cooling at Casey and Dumont d’Urville linearly congruent with increased SSTs over the western tropical Pacific. The autumn La Niña pattern is associated with an anomalous anticyclone over the high-latitude South Atlantic that strengthens southwesterly winds and cold air advection across Novolazarevskaya and Syowa. Meanwhile, warming over the western tropical Pacific is associated with a meridional wavetrain stretching from southwest Australia to eastern East Antarctica and anomalous poleward momentum fluxes that locally strengthen westerly / southwesterly winds along and offshore of Casey and Dumont d’Urville, amplifying the cooling seen there.  During spring, a physical mechanism linking the West Antarctic warming to the tropical Pacific is identified. Spring warming of West Antarctica and the Antarctic Peninsula is associated with a significant increase in tropical deep convection on the poleward side of the South Pacific Convergence Zone. The increase in deep convection is strongest during September, during which a meridional wavetrain is seen over the western South Pacific with anomalous cyclonic circulation over the Ross Sea and warm, northerly flow to western West Antarctica. During October, the wavetrain propagates east toward the Antarctic Peninsula as the climatological background westerlies strengthen, which leads to increased warm, northerly flow to the western Antarctic Peninsula. Observed increases in deep convection along the South Pacific Convergence Zone during September are linearly congruent with over half of the observed circulation and surface warming trends seen across the West Antarctic region during September and October.  Lastly, this study finds a spatial dependency of the ENSO and SAM impact on Antarctic Peninsula climate. Variability in ENSO has a persistent and statistically significant relationship with western Peninsula climate only, which is strongest during the winter and spring seasons. Meanwhile, variability in the SAM dominates climate across the northeastern Peninsula during all seasons through the Föhn effect, and northeast Peninsula relationships with the tropics are relatively weak. In autumn, when widespread warming of the Antarctic Peninsula has been linked to the tropics, this study finds the tropical connection to be weak and statistically insignificant on interannual timescales, and regional circulation associated with the SAM dominates climate variability across the Peninsula during autumn.</p>

scholarly journals Tropical Pacific relationships with Southern Hemisphere atmospheric circulation and Antarctic climate

2021 ◽  
Author(s):  
◽  
Kyle Clem
Keyword(s):  
Antarctic Peninsula ◽  
Deep Convection ◽  
South Pacific ◽  
Tropical Pacific ◽  
La Niña ◽  
Eastern Tropical Pacific ◽  
The South ◽  
La Nina ◽  
Surface Climate ◽  
The Tropics

<p>Significant trends in high-latitude Southern Hemisphere atmospheric circulation and surface climate have been observed over recent decades, which are likely linked to teleconnections from the tropics. This study investigates how a recent shift in tropical Pacific climate toward increased La Niña conditions has influenced the atmospheric circulation and surface climate across the high southern latitudes, and how variations in the El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM) influence the surface climate of Antarctica.  Over 1979-2014, significant cooling of eastern tropical Pacific sea surface temperatures (SSTs) is detected in all seasons. The eastern tropical Pacific cooling is associated with: (1) an intensified Walker Circulation during austral summer and autumn; (2) a weakened South Pacific Hadley cell and sub-tropical jet during autumn; and (3) a strengthening of the circumpolar westerlies between 50 and 60°S during both summer and autumn. Observed cooling in the eastern tropical Pacific is linearly congruent with 60-80% of the observed positive zonal-mean zonal wind trend between 50 and 60°S during summer (~35% of the interannual variability), and around half of the positive zonal-mean zonal wind trend during autumn (~15% of the interannual variability), the latter being most marked over the South Pacific. Although previous studies have linked the strengthening of the tropospheric westerlies during summer and autumn to ozone depletion, results from this study indicate poleward momentum fluxes and strengthened lower-tropospheric baroclinicity associated with eastern tropical Pacific cooling also help to maintain a strengthened mid-latitude jet through the 21st century, especially across the South Pacific.  The La Niña shift in tropical Pacific SSTs is also significantly related to several changes in Antarctic surface climate. During autumn, a regional pattern of cooling occurred along coastal East Antarctica after 1979, with the rate of cooling increasing at Novolazarevskaya, Syowa, Casey, and Dumont d’Urville stations, while the rate of cooling decreased at Mawson and Davis stations. It is shown that regional circulation changes associated with tropical Pacific teleconnections project strongly onto the regional nature of the cooling trends, with 40% of the cooling at Novolazarevskaya and Syowa linearly congruent with the increased La Niña conditions, and more than 60% of the cooling at Casey and Dumont d’Urville linearly congruent with increased SSTs over the western tropical Pacific. The autumn La Niña pattern is associated with an anomalous anticyclone over the high-latitude South Atlantic that strengthens southwesterly winds and cold air advection across Novolazarevskaya and Syowa. Meanwhile, warming over the western tropical Pacific is associated with a meridional wavetrain stretching from southwest Australia to eastern East Antarctica and anomalous poleward momentum fluxes that locally strengthen westerly / southwesterly winds along and offshore of Casey and Dumont d’Urville, amplifying the cooling seen there.  During spring, a physical mechanism linking the West Antarctic warming to the tropical Pacific is identified. Spring warming of West Antarctica and the Antarctic Peninsula is associated with a significant increase in tropical deep convection on the poleward side of the South Pacific Convergence Zone. The increase in deep convection is strongest during September, during which a meridional wavetrain is seen over the western South Pacific with anomalous cyclonic circulation over the Ross Sea and warm, northerly flow to western West Antarctica. During October, the wavetrain propagates east toward the Antarctic Peninsula as the climatological background westerlies strengthen, which leads to increased warm, northerly flow to the western Antarctic Peninsula. Observed increases in deep convection along the South Pacific Convergence Zone during September are linearly congruent with over half of the observed circulation and surface warming trends seen across the West Antarctic region during September and October.  Lastly, this study finds a spatial dependency of the ENSO and SAM impact on Antarctic Peninsula climate. Variability in ENSO has a persistent and statistically significant relationship with western Peninsula climate only, which is strongest during the winter and spring seasons. Meanwhile, variability in the SAM dominates climate across the northeastern Peninsula during all seasons through the Föhn effect, and northeast Peninsula relationships with the tropics are relatively weak. In autumn, when widespread warming of the Antarctic Peninsula has been linked to the tropics, this study finds the tropical connection to be weak and statistically insignificant on interannual timescales, and regional circulation associated with the SAM dominates climate variability across the Peninsula during autumn.</p>

scholarly journals The role of the South Pacific in modulating Tropical Pacific variability

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Christine T. Y. Chung ◽  
Scott B. Power ◽  
Arnold Sullivan ◽  
François Delage
Keyword(s):  
El Niño ◽  
Decadal Variability ◽  
El Nino ◽  
South Pacific ◽  
Tropical Pacific ◽  
Equatorial Pacific ◽  
La Niña ◽  
The South ◽  
La Nina ◽  
Sst Anomalies

AbstractTropical Pacific variability (TPV) heavily influences global climate, but much is still unknown about its drivers. We examine the impact of South Pacific variability on the modes of TPV: the El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO). We conduct idealised coupled experiments in which we suppress temperature and salinity variability at all oceanic levels in the South Pacific. This reduces decadal variability in the equatorial Pacific by ~30% and distorts the spatial pattern of the IPO. There is little change to overall interannual variability, however there is a decrease in the magnitude of the largest 5% of both El Niño and La Niña sea-surface temperature (SST) anomalies. Possible reasons for this include: (i) reduced decadal variability means that interannual SST variability is superposed onto a ‘flatter’ background signal, (ii) suppressing South Pacific variability leads to the alteration of coupled processes linking the South and equatorial Pacific. A small but significant mean state change arising from the imposed suppression may also contribute to the weakened extreme ENSO SST anomalies. The magnitude of both extreme El Niño and La Niña SST anomalies are reduced, and the associated spatial patterns of change of upper ocean heat content and wind stress anomalies are markedly different for both types of events.

scholarly journals Southeast Asia and the South Pacific Region: current problems of development – 2019

2020 ◽  
pp. 494-514
Author(s):  
E.M. Astafieva ◽  
◽  
N.P. Maletin ◽  
Keyword(s):  
Southeast Asia ◽  
Academic Research ◽  
South Pacific ◽  
Educational Institutions ◽  
Pacific Region ◽  
The South ◽  
Russian Academy Of Sciences ◽  
Oriental Studies ◽  
Academy Of Sciences

The paper provides an overview of the reports presented at the conference "Southeast Asia and the South Pacific region: current problems of development", which was held in the Institute of Oriental Studies of the Russian Academy of Sciences on December 18, 2019. In the annual inter-institute conference of Orientalists organized by the Center for Southeast Asia, Australia and Oceania studies academics, as well as applicants and post-graduates from various academic, research and educational institutions, participated.

scholarly journals Development Assistance and Geopolitics in Australia-China-Taiwan Relations

2015 ◽  
Vol 16 (2) ◽  
pp. 1-23
Author(s):  
Joel Atkinson
Keyword(s):  
Development Assistance ◽  
Political Influence ◽  
South Pacific ◽  
Pacific Region ◽  
The South ◽  
Assistance Programs

The development assistance programs of Australia, China and Taiwan impact each other’s geopolitical interests in the South Pacific region. This “aid triangle” has recently undergone a significant transformation. Previously, the interests of Australia and China aligned in competing against Taiwan for political influence in the region. However, since 2008, China-Taiwan relations have warmed and their aid contest in the South Pacific has been largely put on hold. This has ameliorated Taiwan’s conflict with Australia, and the two countries have increased their development assistance cooperation. However, China’s role in undermining Australia’s policy towards Fiji, and the global deterioration in China’s relations with a US coalition (including Australia), have potentially increased the competitive aspects of the Sino-Australian side of the triangle.

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