Large-scale circulation changes over South America are impacting synoptic-scale tropical-extratropical interactions and altering rainfall seasonality

Mapping Intimacies ◽

10.5194/egusphere-egu21-10393 ◽

2021 ◽

Author(s):

Marcia Zilli ◽

Neil Hart

Keyword(s):

Global Warming ◽

South America ◽

Rainy Season ◽

Rossby Waves ◽

Total Precipitation ◽

Synoptic Scale ◽

The South ◽

Subtropical Jet ◽

Upper Level ◽

Extratropical Disturbances

<p>During austral summer, persistent tropical-extratropical (TE) cloud bands, such as the South Atlantic Convergence Zone (SACZ) over South America, link tropical humid areas to the subtropics. Changes in circulation due to global warming is already impacting the location and duration of these TE cloud bands, affecting the hydrological regime of the subtropics. In this study, we present an automatic object-based identification of TE cloud bands which we utilize to obtain an event set of TE cloud bands over South America. This approach and our newly-identified sample base are ideal for understanding interactions between the variability and change in the regional mean state and synoptic-scale weather systems. TE cloud bands are responsible for almost 60% of the subtropical precipitation during the South America rainy season (November to March), mostly produced by SACZ events, a TE cloud band persisting for four or more days. Their location and persistence are modulated by the propagation of synoptic-scale extratropical disturbances interacting with intraseasonal variability in the basic state upper-level zonal wind. The persistent SACZ events (i.e., lasting four or more days) are supported by upper-level westerly anomalies over the subtropics caused by an anomalous trough in the subtropical jet which favours the propagation extratropical disturbances deeper into the tropics. Conversely, transient events occur when the Bolivian High is displaced/expanded southeastward, resulting in upper-level easterly winds occurring over subtropical latitudes and blocking the equatorward propagation of Rossby waves.</p><p>In recent decades, changes in circulation due to global warming has affected the basic-state circulation, resulting in different impacts in transient and persistent TE events throughout the rainy season. Over South America, the number of days with TE events has decreased during the rainy season peak but increased during onset and cessation months, resulting in the displacement of accumulated precipitation into early and late summer. These results are obtained by comparing two periods: 1979-1996 and 1997-2018, excluding ENSO years. These synoptic-scale changes are related to changes in the position of the subtropical jet and its trough, impacting on the propagation of RW towards South America. In the beginning (November) and end (February) of the rainy season, the westerlies have become stronger over subtropical South America, favouring the development of more persistent events and resulting in an increase in the total precipitation during TE events. During the peak of the rainy season (December and January), changes in upper-level circulation have reduced the conditions necessary to the development of TE events, affecting the total precipitation during these months. We show that anomalous subtropical convection from the cloud bands is a source of Rossby waves that interact with the basic flow, resulting in downwind enhancement or damping of the extratropical disturbances. Therefore, these contemporary changes over South America are likely to have implications for changes Rossby Wave spectra in the Southern Hemisphere, especially downstream from the SACZ.</p>

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Rossby wave dynamics over South America explored with automatic Tropical-Extratropical cloud band identification framework

Journal of Climate ◽

10.1175/jcli-d-21-0020.1 ◽

2021 ◽

pp. 1-58

Author(s):

Marcia T. Zilli ◽

Neil C. G. Hart

Keyword(s):

South America ◽

Rossby Waves ◽

Intraseasonal Variability ◽

Austral Summer ◽

South Atlantic Convergence Zone ◽

Cloud Band ◽

Transient Events ◽

Upper Level ◽

Extratropical Disturbances ◽

The Tropics

AbstractDuring austral summer, persistent tropical-extratropical cloud bands, such as the South Atlantic Convergence Zone (SACZ) over South America (SAm), link the tropical humid areas to the subtropics. In this study, we utilize an automatic object-based methodology to identify synoptic cloudband events occurring over SAm which are responsible for almost 60% of the precipitation during the rainy season (November to March). In addition to identifying SACZ events as cloud bands persisting four or more days, the framework also highlights the relevance of transient events (i.e., events persisting for three days or less) to the climatology. The location and persistence of the cloudband events are modulated by the propagation of synoptic-scale extratropical disturbances interacting with intraseasonal variability in the basic state upper-level zonal wind. During persistent events (i.e., lasting four or more days), upper-level westerly anomalies over the subtropics favour the propagation extratropical disturbances deeper into the tropics. Conversely, transient events occur when the Bolivian High is displaced/expanded southeastward, bringing upper-level easterly winds into subtropical latitudes and blocking the propagation of Rossby waves into lower latitudes. Subsequent anomalous subtropical convection from the cloud bands result in sources of Rossby waves that interact with the basic flow, resulting in downwind enhancement or damping of the extratropical disturbances. The adopted methodology proved to be a powerful framework in demonstrating this interaction between scales, with the basic state influencing and being modified by the synoptic disturbances.

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The role of the observed tropical convection in the generation of frost events in the southern cone of South America

Annales Geophysicae ◽

10.5194/angeo-26-1379-2008 ◽

2008 ◽

Vol 26 (6) ◽

pp. 1379-1390 ◽

Cited By ~ 4

Author(s):

G. V. Müller ◽

T. Ambrizzi ◽

S. E. Ferraz

Keyword(s):

South America ◽

Large Scale ◽

Rossby Waves ◽

Tropical Convection ◽

The Other ◽

Linear Wave ◽

Frequency Of Occurrence ◽

The South ◽

Cold Events ◽

Extreme Cold

Abstract. Based on previous results obtained from observations and linear wave theory analysis, the hypothesis that large-scale patterns can generate extreme cold events in southeast South America through the propagation of remotely excited Rossby waves was already suggested. This work will confirm these findings and extend their analysis through a series of numerical experiments using a primitive equation model where waves are excited by a thermal forcing situated in positions chosen according to observed convection anomalies over the equatorial region. The basic state used for these experiments is a composite of austral winters with maximum and minimum frequency of occurrence of generalized frosts that can affect a large area known as the Wet Pampas located in the central and eastern part of Argentina. The results suggest that stationary Rossby waves may be one important mechanism linking anomalous tropical convection with the extreme cold events in the Wet Pampas. The combination of tropical convection and a specific basic state can generate the right environment to guide the Rossby waves trigged by the tropical forcing towards South America. Depending on the phase of the waves entering the South American continent, they can favour the advection of anomalous wind at low levels from the south carrying cold and dry air over the whole southern extreme of the continent, producing a generalized frost in the Wet Pampa region. On the other hand, when a basic state based on the composites of minimum frosts is used, an anomalous anticyclone over the southern part of the continent generates a circulation with a south-southeast wind which brings maritime air and therefore humidity over the Wet Pampas region, creating negative temperature anomalies only over the northeastern part of the region. Under these conditions even if frosts occur they would not be generalized, as observed for the other basic state with maximum frequency of occurrence of generalized frosts.

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Ocean Global Warming Impacts on the South America Climate

Frontiers in Earth Science ◽

10.3389/feart.2016.00030 ◽

2016 ◽

Vol 4 ◽

Cited By ~ 3

Author(s):

Renato Ramos da Silva ◽

Reinaldo Haas

Keyword(s):

Global Warming ◽

South America ◽

The South ◽

South America Climate

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Tropical–Extratropical Interactions Associated with an Atlantic Tropical Plume and Subtropical Jet Streak

Monthly Weather Review ◽

10.1175/mwr2999.1 ◽

2005 ◽

Vol 133 (9) ◽

pp. 2759-2776 ◽

Cited By ~ 31

Author(s):

Peter Knippertz

Keyword(s):

North America ◽

South America ◽

Atlantic Ocean ◽

North Atlantic Ocean ◽

Cloud Formation ◽

Cloud Band ◽

The North ◽

Northwest Africa ◽

Subtropical Jet ◽

Upper Level

Abstract Tropical plumes (TPs) are elongated bands of upper- and midlevel clouds stretching from the Tropics poleward and eastward into the subtropics, typically accompanied by a subtropical jet (STJ) streak and a trough on their poleward side. This study uses ECMWF analyses and high-resolution University of Wisconsin–Nonhydrostatic Modeling System trajectories to analyze the multiscale complex tropical–extratropical interactions involved in the genesis of a pronounced TP and STJ over the NH Atlantic Ocean in late March 2002 that was associated with extreme precipitation in arid northwest Africa. Previous concepts for TP genesis from the literature are discussed in the light of this case study. Analysis of the upper-level flow prior to the TP formation shows a northeastward propagation and a continuous acceleration of the STJ over the Atlantic Ocean equatorward of a positively tilted upper-level trough to the west of northwest Africa. Both dynamic and advective processes contribute to the generation of the accompanying cloud band. The northern portion of the TP consists of parcels that exit a strong STJ streak over North America, enter the deep Tropics over South America, and then accelerate into the Atlantic STJ, accompanied by strong cross-jet ageostrophic motions, rising, and cloud formation. The southern portion is formed by parcels originating in the divergent outflow from strong near-equatorial convection accompanying the TP genesis. A local increase in the Hadley overturning is found over the tropical Atlantic and east Pacific/South America and appears to be related to low inertial stability at the outflow level and to low-level trade surges associated with the cold advection, sinking, and lower-level divergence underneath two strong upper-level convergence centers in the eastern portions of both a subtropical ridge over North America and an extratropical ridge over the North Atlantic Ocean. Evidence is presented that the convective response lags the trade surge by several days.

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Recurrent Rossby waves during Southeast Australian heatwaves and links to quasi-resonant amplification and atmospheric blocks

10.5194/wcd-2022-1 ◽

2022 ◽

Author(s):

S. Mubashshir Ali ◽

Matthias Röthlisberger ◽

Tess Parker ◽

Kai Kornhuber ◽

Olivia Martius

Keyword(s):

Rossby Waves ◽

Weather Conditions ◽

The South ◽

Reanalysis Dataset ◽

South Pacific Ocean ◽

Weather Events ◽

Mean Fields ◽

High Impact Weather ◽

Upper Level ◽

The Relationship

Abstract. In the Northern Hemisphere, recurrence of transient Rossby wave packets over periods of days to weeks, termed RRWPs, may repeatedly create similar weather conditions. This recurrence leads to persistent surface anomalies and high-impact weather events. Here, we demonstrate the significance of RRWPs for persistent heatwaves in the Southern Hemisphere (SH). We investigate the relationship between RRWPs, atmospheric blocking, and amplified quasi-stationary Rossby waves with two cases of heatwaves in Southeast Australia (SEA) in 2004 and 2009. This region has seen extraordinary heatwaves in recent years. We also investigate the importance of transient systems such as RRWPs and two other persistent dynamical drivers: atmospheric blocks and quasi-resonant amplification (QRA). We further explore the link between RRWPs, blocks, and QRA in the SH using the ERA-I reanalysis dataset (1979–2018). We find that QRA and RRWPs are strongly associated: 40 % of QRA days feature RRWPs, and QRA events are 13 times more likely to occur with an RRWPs event than without it. Furthermore, days with QRA and RRWPs show high correlations in the composite mean fields of upper-level flows, indicating that both features have a similar hemispheric flow configuration. Blocking frequencies for QRA and RRWP conditions both increase over the south Pacific Ocean but differ substantially over parts of the south Atlantic and Indian Ocean.

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Future Changes in the Tropical Cyclone Intensity and Frequency over the Western North Pacific Based on 20-km HiRAM and MRI Models

Journal of Climate ◽

10.1175/jcli-d-20-0417.1 ◽

2020 ◽

pp. 1-47

Author(s):

Chi-Cherng Hong ◽

Chih-Hua Tsou ◽

Pang-Chi Hsu ◽

Kuan-Chieh Chen ◽

Hsin-Chien Liang ◽

...

Keyword(s):

Global Warming ◽

Tropical Cyclone ◽

Energy Conversion ◽

North Pacific ◽

Western North Pacific ◽

Synoptic Scale ◽

Steering Flow ◽

The Future ◽

Budget Equation ◽

Upper Level

AbstractThe future changes in the tropical cyclone (TC) intensity and frequency over the western North Pacific (WNP) under global warming remain uncertain. In this study, we investigated such changes using 20-km resolution HiRAM and MRI models, which can realistically simulate the TC activity in the present climate. We found that the mean intensity of TCs in the future (2075−2099) would increase by approximately 15%, along with an eastward shift of TC genesis location in response to the El-Niño like warming. However, the lifetime of future TCs would be shortened because the TCs tend to have more poleward genesis locations and move faster due to a stronger steering flow related to the strengthened WNP subtropical high in a warmer climate. In other words, the enhancement of TC intensity in future is not attributable to the duration of TC lifetime.To understand the processes responsible for the change in TC intensity in a warmer climate, we applied the budget equation of synoptic-scale eddy kinetic energy along the TC tracks in model simulations. The diagnostic results suggested that both the upper level baroclinic energy conversion (CE) and lower-level barotropical energy conversion (CK) contribute to the intensified TCs under global warming. The increased CE results from the enhancement of TC-related perturbations of temperature and vertical velocity over the subtropical WNP, whereas the increased CK mainly comes from synoptic-scale eddies interacting with enhanced zonal-wind convergence associated with seasonal mean and intraseasonal flows over Southeast China and the northwestern sector of WNP.

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Phase coherence between precipitation in South America and Rossby waves

Science Advances ◽

10.1126/sciadv.aau3191 ◽

2018 ◽

Vol 4 (12) ◽

pp. eaau3191 ◽

Cited By ~ 5

Author(s):

Maximilian Gelbrecht ◽

Niklas Boers ◽

Jürgen Kurths

Keyword(s):

South America ◽

Southern Hemisphere ◽

Rossby Waves ◽

Phase Synchronization ◽

Monsoon Season ◽

South Atlantic Convergence Zone ◽

Dominant Mode ◽

Precipitation Pattern ◽

The South ◽

Regional Food

The dominant mode of intraseasonal precipitation variability during the South American monsoon is the so-called precipitation dipole between the South Atlantic convergence zone (SACZ) and southeastern South America (SESA). It affects highly populated areas that are of substantial importance for the regional food supplies. Previous studies using principal components analysis or complex networks were able to describe and characterize this variability pattern, but crucial questions regarding the responsible physical mechanism remain open. Here, we use phase synchronization techniques to study the relation between precipitation in the SACZ and SESA on the one hand and southern hemisphere Rossby wave trains on the other hand. In combination with a conceptual model, this approach demonstrates that the dipolar precipitation pattern is caused by the southern hemisphere Rossby waves. Our results thus show that Rossby waves are the main driver of the monsoon season variability in South America, a finding that has important implications for synoptic-scale weather forecasts.

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Relationships between Upper-Level Circulation over South America and Rainfall over Southeastern South America: A Physical Base for Seasonal Predictions

Journal of Climate ◽

10.1175/2009jcli3129.1 ◽

2010 ◽

Vol 23 (12) ◽

pp. 3300-3315 ◽

Cited By ~ 13

Author(s):

Laura Zamboni ◽

Carlos R. Mechoso ◽

Fred Kucharski

Keyword(s):

South America ◽

Southern Oscillation ◽

South Atlantic Convergence Zone ◽

Empirical Method ◽

Eastern Coast ◽

The South ◽

Modes Of Variability ◽

Continental Scale ◽

Precipitation Anomalies ◽

Upper Level

Abstract The existence of a significant simultaneous correlation between bimonthly mean precipitation anomalies over southeastern South America (SESA) and either the first or the second (depending on season) leading mode of interannual variability of upper-level wind over South America (SA) is demonstrated during all seasons except winter. The pattern associated with these modes of variability is similar during all seasons and consists of a continental-scale vortex centered over the eastern coast of subtropical SA. The vortex has a quasi-barotropic structure during all seasons, and its variability modifies moisture transport from the South American low-level jet and the western tropical Atlantic to SESA thus creating precipitation anomalies in this region. During spring (October–November) and summer (January–February) the circulation creates a second center of precipitation anomalies over the South Atlantic convergence zone that are of opposite sign to those over SESA, while during fall (April–May) precipitation anomalies are primarily confined to SESA. On the basis of the correlation between upper-level winds and precipitation, an empirical method to produce long-range forecasts of bimonthly mean precipitation over SESA is developed. Method tests in hindcast mode for the period 1959–2001 show a potential for reliable predictions during the southern spring, summer, and fall. The method is further tested in an experimental mode by using Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) wind hindcasts. Forecasts obtained in this way are skillful during spring only, with highest skill during El Niño–Southern Oscillation years. During summer and fall, the DEMETER forecasts of wind anomalies limit the method’s ability to make reliable real predictions.

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