The Effect of Oceanic South Atlantic Convergence Zone Episodes on Regional SST Anomalies: the Roles of Heat Fluxes and Upper-Ocean Dynamics

The South Atlantic Convergence Zone (SACZ) is an atmospheric system occurring in austral summer on the South America continent and sometimes extending over the adjacent South Atlantic. It is characterized by a persistent and very large, northwest-southeast-oriented, cloud band. Its presence over the ocean causes sea surface cooling that some past studies indicated as being produced by a decrease of incoming solar heat flux induced by the extensive cloud cover. Here we investigate ocean-atmosphere interaction processes in the Southwestern Atlantic Ocean (SWA) during SACZ oceanic episodes, as well as the resulting modulations occurring in the oceanic mixed layer and their possible feedbacks on the marine atmospheric boundary layer. Our main interests and novel results are on verifying how the oceanic SACZ acts on dynamic and thermodynamic mechanisms and contributes to the sea surface thermal balance in that region. In our oceanic SACZ episodes simulations we confirm an ocean surface cooling. Model results indicate that surface atmospheric circulation and the presence of an extensive cloud cover band over the SWA promote sea surface cooling via a combined effect of dynamic and thermodynamic mechanisms, which are of the same order of magnitude. The sea surface temperature (SST) decreases in regions underneath oceanic SACZ positions, near Southeast Brazilian coast, in the South Brazil Bight (SBB) and offshore. This cooling is the result of a complex combination of factors caused by the decrease of solar shortwave radiation reaching the sea surface and the reduction of horizontal heat advection in the Brazil Current (BC) region. The weakened southward BC and adjacent offshore region heat advection seems to be associated with the surface atmospheric circulation caused by oceanic SACZ episodes, which rotate the surface wind and strengthen cyclonic oceanic mesoscale eddy. Another singular feature found in this study is the presence of an atmospheric cyclonic vortex Southwest of the SACZ (CVSS), both at the surface and aloft at 850 hPa near 24°S and 45°W. The CVSS induces an SST decrease southwestward from the SACZ position by inducing divergent Ekman transport and consequent offshore upwelling. This shows that the dynamical effects of atmospheric surface circulation associated with the oceanic SACZ are not restricted only to the region underneath the cloud band, but that they extend southwestward where the CVSS presence supports the oceanic SACZ convective activity and concomitantly modifies the ocean dynamics. Therefore, the changes produced in the oceanic dynamics by these SACZ events may be important to many areas of scientific and applied climate research. For example, episodes of oceanic SACZ may influence the pathways of pollutants as well as fish larvae dispersion in the region.

Dynamic Characteristics of the Circulation and Diurnal Spatial Cycle of Outgoing Longwave Radiation in the Different Phases of the Madden–Julian Oscillation during the Formation of the South Atlantic Convergence Zone

In this work, we verified the formation of the South Atlantic Convergence Zone (SACZ) during the active, unfavorable, and transition phases of the Madden–Julian Oscillation (MJO), as well as the diurnal spatial variability in the estimated Outgoing Longwave Radiation (OLR) data. The real-time multivariate index (RMM) and the composites of meteorological variables were used, along with the temporal average of the estimated OLR data. All the different patterns for the average period of SACZ showed classic behavior: well-organized and with meteorological variables in phases throughout the troposphere. However, some differences were evident in the organization of each phase of the MJO: at 200 hPa, the Bolivian High (BH) was more flattened during the active phase pattern than in the unfavorable and transition phases, being wider and with a wavier trough embedded in the western flow; at medium levels, the subtropical highs appeared more defined and with a very wide trough; the trough supported the frontal systems on the surface and, together with the subtropical highs, concentrated all the moisture in this layer. In the OLR dataset, the formation of the Coast Squall Line (CSL) occurred during SACZ events in the active phase and MJO transition, whereas in the unfavorable phase, this system was not observed.

Water vapor transport in cases of South Atlantic Convergence Zone (SACZ)

This case study analyzes water vapor flux that is vertically integrated into the atmosphere in episodes of the South Atlantic Convergence Zone (SACZ). The scope of this study is two cases that occurred between January and February 2018. We use the ERA-Interim reanalysis data from the European Center for Medium-Range Weather Forecasts (ECMWF) to build the maps of vertically integrated water vapor flux and its divergence. We use two 5º by 5º sub-areas, centralized over Belo Horizonte and São Paulo, as control for water vapor balance. The results point to the existence of water vapor transport from the Amazon region to Southeastern Brazil in association to the SACZ. Convergence areas of vertically integrated water vapor flux predominate along the Northwest-Southeast line. The two cases over the Belo Horizonte area presented an average of water vapor balance of -1.8 and -12.9 mm/day. The average at the São Paulo area was -3.6 and 2.0 mm/day. The negative values indicate that precipitation exceeded evapotranspiration, that is, the area served as a water vapor sink.

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

During 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.

Summer dry events on synoptic and intraseasonal timescales in the Southeast Region of Brazil

Dry events occurring in the Southeast Region of Brazil (SEB) during the summer (rainfall season) have been in evidence in the last years, mainly due to previous extreme events in the 2013/14 and 2014/15 seasons. Drought analyses are usually carried out with monthly data. Here our methodology addresses the issue with daily data in order to generate a thorough analysis. Dry events were evaluated for different homogeneous precipitation sub-regions within the SEB, over 37 December-February (DJF) seasons and with two different timescales of duration: synoptic (5-9 days) and intraseasonal (≥10 days). Two main distinct dynamic patterns were found for dry events in southern and central-northern parts of SEB, respectively, but no major differences were identified for the different timescales of occurrence. Southern events were characterized by a stationary ridge acting over the whole southern South America, making the transient systems approximation to southern SEB difficult. At the same time, this pattern showed a northern-shifted South Atlantic Convergence Zone (SACZ) configuration. In the central-northern events, a high pressure centered between the South and Southeast regions of Brazil was associated with the dryness conditions. An anomalous southward shift of meteorological systems characteristic of the South American summer was also verified for these events. Over the South Atlantic, an opposite SST anomaly configuration was identified between southern and central-northern events.

Extreme precipitation and temperature events in the past decades and its future impacts in the São Paulo Macro Metropolis.

<p>The São Paulo Macro Metropolis (SPMM) is composed of five metropolitan regions (i.e., São Paulo, Campinas, Paraíba Valley and North Coast, Sorocaba, and Santos), two urban agglomerations (i.e., Piracicaba and Jundiaí) and a microregion of Bragança. With an estimated 33 million inhabitants over an area 53,000 km<sup>2</sup>. Urban areas take up 11,000 km<sup>2</sup> of this area, represented by 174 municipalities that account for nearly 73.9% of the state’s total population. The SPMM is already experiencing the impacts of severe climate events. It is situated along a transition climatic region between humid subtropical and tropical climates. Meteorological systems such as cold fronts, the South Atlantic Convergence Zone (ZCAS), sea breezes, urban heat islands, and other local circulations associated with topography favor the occurrence of severe weather at different time scales. The study aims at understanding climate change effects on the intensity and frequency of temperature and precipitation extremes in the SPMM, contributing to the establishment of policies to reduce the existing socioenvironmental risks in urban areas. For this, the ability of observation systems, reanalysis (ERA5, ERA40, NCEP/NCAR, and Terra Climate), and remote sensing (CMAP and CPC) were analyzed and used to show the decadal evolution of extreme events in SPMM. We also investigated the potential of CMIP5 models to simulate the extreme precipitation and temperature events in the SPMM that occurred between the 1950s and 2005. Future climate changes in the SPMM were analyzed based on CMIP5 models that best simulate past extreme events in the SPMM, considering the different carbon emission scenarios. This study is important to develop adaptive strategies to deal with climate change impacts on urban areas, such as droughts and floods.</p>

Atmospheric convergence zones stemming from large-scale mixing

Organised cloud bands are important features of tropical and subtropical rainfall. These structures are often regarded as convergence zones, alluding to an association with coherent atmospheric flow. However, the flow kinematics is not usually taken into account in classification methods for this type of event, as large-scale lines are rarely evident in instantaneous diagnostics such as Eulerian convergence. Instead, existing convergence zone definitions rely on heuristic rules of shape, duration and size of cloudiness fields. Here we investigate the role of large-scale turbulence in shaping atmospheric moisture in South America. We employ the finite-time Lyapunov exponent (FTLE), a metric of deformation among neighbouring trajectories, to define convergence zones as attracting Lagrangian coherent structures (LCSs). Attracting LCSs frequent tropical and subtropical South America, with climatologies consistent with the South Atlantic Convergence Zone (SACZ), the South American Low-Level Jet (SALLJ) and the Intertropical Convergence Zone (ITCZ). In regions under the direct influence of the ITCZ and the SACZ, rainfall is significantly positively correlated with large-scale mixing measured by the FTLE. Attracting LCSs in south and southeast Brazil are associated with significant positive rainfall and moisture flux anomalies. Geopotential height composites suggest that the occurrence of attracting LCSs in these regions is related with teleconnection mechanisms such as the Pacific–South Atlantic. We believe that this kinematical approach can be used as an alternative to region-specific convergence zone classification algorithms; it may help advance the understanding of underlying mechanisms of tropical and subtropical rain bands and their role in the hydrological cycle.

Connecting heavy precipitation events to outgoing longwave radiation variability scales: case analysis in Brazil

Spatial fields of outgoing longwave radiation (OLR) spectrum variance are analyzed in different time-scales: synoptic, sub-monthly, intra-seasonal, during the 1979-2016 austral summer months in southern Brazil. Variability fields differ both in intensity and location and highlight dominant convection cycles in the study area. The results show that the amplitude of sub-monthly variability is greater than the other scales in the southeastern region of Brazil, while the synoptic scale prevails in the southern region. The above-mentioned scales show higher amplitudes over the western Pacific Ocean where the Madden-Julian Oscillation plays an important role, along the South Pacific Convergence Zone, and over the storm track areas over the South Pacific Ocean. The influence of spectral OLR scale interaction is also analyzed, associated to the occurrence of two intense rainfall events over the southeastern Brazil in the austral summers of 2011 and 2014 when the South Atlantic Convergence Zone was involved in both events. The results obtained suggest that spectral OLR scale interaction takes place in such way that it strengthens the South Atlantic Convergence Zone, since the spatial pattern footprints of the 2 to 8-day timescale (synoptic), 10 to 30-day timescale (sub-monthly) and 30 to 60-day timescale (intra-seasonal) overlap in the study region.