Combined large-scale tropical and subtropical forcing on severe soil desiccation in South America

doi:10.21203/rs.3.rs-2732119/v1

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Combined large-scale tropical and subtropical forcing on severe soil desiccation in South America

https://doi.org/10.21203/rs.3.rs-2732119/v1

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published 10 Nov, 2023

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Changes in the frequency and magnitude of dry spells have been recorded over the past few decades due to an intensification of the global water cycle. A long-term soil drying trend resulting from a joint contribution of natural decadal variability in precipitation levels and increasing temperatures, predisposed southeast South America (SESA) to experience during the 2019–2021 period outstanding soil desiccation. Flash droughts led to large areas (~100.000 ) in SESA covered by record-breaking soil dryness. April 2020 witnessed the most severe conditions when over 30% of the region experienced negative soil moisture anomalies larger than two standard deviations. Internal variability, particularly El Niño–Southern Oscillation, explained the dynamical forcing of this extreme drought. Large precipitation deficits and enhanced evaporation in SESA resulted from a coupled tropical and subtropical forcing associated to pronounced changes in the normal Walker and Hadley Cells' circulation and to the establishment of a Rossby wave extending from west south Pacific towards South America. These findings highlight the intricate interplay between different processes in the occurrence of climate extremes, stressing the need for an adequate representation of the dynamical processes associated with daily-to-multiyear timescales in climate models.

Earth and environmental sciences/Climate sciences/Hydrology

Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric dynamics

Earth and environmental sciences/Climate sciences/Climate change

Climate change is expected to cause, through both thermodynamic and dynamical mechanisms, a strengthening of the global water cycle and thus profound changes in the frequency and magnitude of dry and wet spells^1–3. This poses great pressure on the global freshwater availability^4,5, food security⁶ and sustainability of natural ecosystems⁷. Higher levels of lower-tropospheric water vapor promoted by increasing temperatures, are linked to changes in the balance between evaporation and precipitation (E–P) that ultimately determines soil moisture, groundwater recharge and total water available for runoff^8–10. Long-term drying trends can influence the atmospheric water budget, particularly over drylands, by limiting evaporation and reducing moisture recycling for precipitation¹⁰. On the other hand, reduced soil moisture conditions may also enhance, through convective instability, the atmospheric moisture convergence contributing to a higher frequency of extreme precipitation episodes from convective storms^10–13. In parallel, under a large atmospheric evaporative demand, a strong soil moisture imbalance may also play a major role in the surface energy partitioning by constraining surface latent heat fluxes, leading to an above-normal accumulation of sensible heat in the atmosphere and to the escalation of temperatures^14,15. Thus, the interplay between soil moisture, atmospheric water budget and temperature is part of a large and complex framework of feedbacks that are modulated by climate change (i.e. increasing temperatures, changes in vegetation and in atmospherics dynamics), and that may also affect the climate trends themselves^10,11,16.

So far, relatively little attention has been given to the simultaneous assessment of temperature, precipitation and soil moisture trends over South America (SA)¹⁷. According to the Intergovernmental Panel on Climate Change (IPCC)¹⁸, the continent warmed up by around 1 ºC during the 1901–2012 period, with central Brazil experiencing the largest warming trends (> 1.75 ºC). Previous studies have also pointed to a significant increase in the frequency of warm extremes during the 1961–2018 period^19,20. Regarding precipitation, the conclusions are less spatially coherent. Globally, when considering future E–P projections, the subtropics are expected to display an overall drying trend, while high-latitude regions are expected to witness a robust wetting trend²¹. Regarding SA, a likely E–P decrease is expected in the southeastern section while regions in the north and central-east Amazon are estimated to experience a very likely E–P increase ²¹. This agrees with the precipitation projections for SA that indicate a dipole pattern of drier conditions centered in the Amazon, while wetter conditions prevail in central-eastern parts of the continent, namely over La Plata Basin^22,23. The evaporation from the Amazon and La Plata basins (Fig. 1a), the two largest hydrological basins of SA, is crucial for the regional hydroclimate and for precipitation through local moisture recycling and moisture transport between both basins^24–26. Other external moisture sources are also important for the atmospheric water budget over these two watersheds, namely the large quantities of water vapor transported from the Atlantic into the Amazon Basin by the trade winds^24,25. All these hydrological processes are modulated by a complex interplay between large-scale oceanic–atmospheric modes of variability such as the El Niño–Southern Oscillation (ENSO)^27,28, the Madden–Julian Oscillation^29,30 and the Atlantic Meridional and Zonal Modes²⁶. These climate variability modes influence semi-permanent and transient systems that play a key role for precipitation and evaporation in SA, namely the Inter-Tropical Convergence Zone, the South Atlantic Convergence Zone, the South America Low Level Jet (SALLJ), mid-latitude cold fronts, cyclones and quasy-stationary anticyclonic systems^27,31.

More recently, La Plata basin and Pantanal in southeast South America (SESA), have been affected by a severe drought whose first signs were noticed during mid-2018 in Southeast Brazil³². However, soil dryness rapidly extended to adjacent areas in Paraguay, Bolivia, and later over northern Argentina, where millions of hectares of wheat production were affected³³. In Paraguay, Uruguay and Southern Brazil, huge agricultural losses in maize, wheat and soybean production were also recorded and many Brazilian farmers were forced to request economic assistance³³. Alongside, extremely low levels in discharge in both Paraná and Paraguay rivers were observed, leading to serious constraints in the hydropower generation and water supply, and to the interruption of waterways that are essential for the fluvial transport and economy of these countries^33,34. This prolonged and intense drought also affected, particularly during 2020, the Pantanal biome, which is the largest contiguous wetland on the planet and home to a large variety of animal and plant species³⁵. Pronounced soil dry-out in the region coincided with several heatwaves, leading to periods characterized by a rapid and sudden intensification of the drought conditions and a large evaporative stress. This short-term (weekly to monthly) periods, marked by a rapid rate of intensification of the already established dry conditions, are usually referred as flash droughts and are often linked to more severe impacts that the slowly evolving droughts^36,37. In Pantanal, these flash droughts raised fire danger to levels not seen since 1980 and led to devastating fires³⁸, resulting in one-third of the biome burnt and economic losses of around USD 3.6 billion³⁸. The extreme drought lasted until the end of 2021, spreading from Pantanal to wider areas in SESA³⁹.

This study seeks to provide a detailed spatiotemporal characterization of the unprecedented 2019–2021 drought in SESA, exploring the historical context of the event, assessing the exceptionality of the soil moisture anomalies and describing the atmospheric mechanisms involved from a daily to multiyear timescale perspective. Overall, we aim to contribute to a better representation of the dynamical processes associated with internal variability in climate models, and thus to improve the predictability of climate extremes in future.

The 2019–2021 drought in a historical context

Most of SA has been witnessing a land surface drying trend since 1951 that is in general statistically significant, with SESA (limited by the red box show in Fig. 1a), revealing the steepest soil moisture decrease, particularly over the Pantanal, Southeast Brazil, central Paraguay and northern Argentina (Fig. 1a). When analyzing long-term changes in temperature and precipitation distributions for SESA (Fig. 1b), it is possible to observe a pronounced shift towards higher temperatures from the first half of the analysis period (1959–1989) into the second half (1990–2021) – see blue and orange ellipses in Fig. 1b. Regarding precipitation, there is a less pronounced contrast in the distribution between both halves. However, a skewed distribution is observed during the second half of the period, indicating the occurrence of less pronounced wet and dry extremes during recent decades. The correlation between precipitation and temperature is stronger for the second half (R= − 0.63) than for the first half (R= − 0.43), indicating that during the last three decades, dry years were more often associated with extremely warm conditions. This recent warmer and drier conditions explain a continuous decrease in soil moisture levels (Fig. 1c), particularly after 1990’s decade (–0.010 per decade, statistically significant at a 5% level), paving the way for the outstanding 2019–2021 drought. In fact, this 3-year period finds no parallel with any other period in the historical record, being unprecedented in terms of dryness intensity and duration.

When analyzing the total precipitation anomalies (grey and black lines in Fig. 2a) as well as the precipitation anomalies caused only by the vertically integrated moisture convergence (VIMC) (blue bars and line in Fig. 2a) in SESA for the 1959–2021 historical period, one can observed that they were characterized by a marked decadal variability and by the occurrence of a drier period during the first and last two decades and a wetter period during the 1980’s and 1990’s decades. These two variables present a correlation coefficient of 0.94 over the entire period, statistically significant at a 5% level, showing that long-term disturbances in moisture convergence in SESA are strongly reflected in the total amount of precipitation in the region. On the other hand, moisture recycling seems to be less important in explaining inter-annual changes of precipitation anomalies as it shows a less pronounced decadal variability and a lower correlation coefficient (R = 0.56, statistically significant at a 5% level). The precipitation anomalies caused by moisture recycling show a decreasing trend after the 1980’s decade, which likely results from the previously observed progressive reduction of soil moisture in the region for this period (see Fig. 1c). In particular, the drought years of 2019 and 2020 experienced the two lowest ever recorded VIMC levels, and 2021 the lowest ever recorded contribution of moisture recycling for total precipitation.

The boxplots shown in Fig. S1 highlight the statistical distribution of the mean annual values of daily vertically integrated water vapor transport (IVT) across each one of the four SESA borders depicted in Fig. 1a, with positive (negative) values indicating moisture inflow (outflow) to (from) SESA. The inflow of moisture occurs mostly throughout the northern and western borders while the outflow occurs mainly throughout the eastern and southern borders. The 2019–2021 drought years were defined by a lower than normal northern and western moisture inflow, which clearly contrasts with the higher-than-normal moisture inflow throughout these two limits during the three wettest years (1965, 1983, 1992, see Fig. 2a). In fact, the correlation coefficients for the 1959–2021 period between the mean annual IVT over each one of the four frontiers and the mean annual VIMC over SESA (inset table in Fig. S1), are positive and statistically significant for the northern (R = 0.74) and western borders (R = 0.56), indicating that the moisture convergence and precipitation anomalies are mostly determined by the amount

of moisture inflow from Amazon. In contrast, the amount of moisture outflow through the western and southern borders do not play a significant role in moisture convergence in SESA. These results are corroborated when analyzing from a spatial perspective the anomalous IVT over SA that shows an anomalous southeast–northwest orientation pattern during the 2019–2021 drought period (Fig. 2b), and an anomalous northwest–southeast orientation during the three wettest years (Fig. S2a). Comparing with climatological conditions (see Fig. S3), the IVT configuration observed during the 2019–2021 period points to a weakening of the expected northwest–southeast moisture transport from the Amazon basin towards SESA that is mostly supported by the SALLJ⁴⁰. Precipitation anomalies are determined not only by moisture availability but also by convergence patterns in the atmosphere. Accordingly, the positive anomalies of the vertical integral of divergence of moisture flux in SESA reveal that during the 2019–2021 period the region was characterized by a lower-than-normal moisture convergence (Fig. 2c). The anomalous low tropospheric (850hPa) wind field shows enhanced divergence and air spread from SESA towards the surrounding regions, highlighting pronounced subsidence and clear sky conditions. These conditions were responsible for large precipitation deficits in SESA and thus for positive anomalies in evaporation minus precipitation (E–P) in the region (see blue contours in Fig. 2c). Moreover, this divergence implied anomalous advection from SESA towards north SA, where pronounced moisture convergence conditions and negative anomalies in E–P prevailed. The mean observed conditions during the wettest years show a clear contrasting pattern (Fig. S2b).

A closer insight into the 2019–2021 drought: evolution, exceptionality and spatial extent

The time series for the 2019–2021 period of the R-index, a metric defined to rank extreme and widespread drought events using soil moisture anomalies (see “Data and Methods”), presents a relatively large variability resulting from pronounced fluctuations of both the drought intensity and spatial extent. At the beginning of 2019, marked dry conditions started to affect SESA (Fig. 3a). The R-index peaks for the first time on January 26th (7th and 18th in the ranking classification considering the short 2019–2021 and the long 1951–2021 periods, respectively), highlighting a rapid and pronounced soil desiccation and the occurrence of a flash drought over southeast Brazil and the Pantanal biome (see left panel of Fig. 3c). Analyzing Fig. 3b and the color shading level intercepted by the purple line which depicts the 31-day time scale (the time-scale used to obtain the running mean filtered soil moisture anomalies that were input for the R-index), one can observe that this period was characterized by several areas within SESA experiencing record-breaking low soil moisture levels (with a total extension of around 100 000 ${ km}^{2}$). Later, during the following months, there was a clear amelioration of the drought, with R-index reaching values closer to zero. However, the year 2020 witnessed the most critical conditions with the occurrence of several flash drought episodes, particularly during the months of March, April, October, November and December, when many regions in SESA experienced record-breaking low soil moisture levels (Fig. 3b). In fact, the five most severe R-index peaks occurred within each of these particular months, with April 26th (peak #1) witnessing the lowest R-index on record (since 1951). More than 30% of SESA experienced soil moisture anomalies larger than two standard deviations on that day (see purple shaded area in Fig. 3a) and a total area of around 100 000 ${km}^{2}$ was affected by record-breaking drying conditions (Fig. 3b). When considering longer temporal scales (seasonal and annual) the amount of SESA covered by unprecedented low soil moisture increases, underlying the exceptional duration of the event. For instance, for temporal scales between 300 and 365 days, more than 700

000${km}^{2}$ (20% of the total area of SESA) witnessed record-breaking soil dryness. Moreover, the spatial signature of the drought throughout 2020 was considerably variable (Fig. 3c). Marked dry conditions started to be recorded in March over Pantanal and southern Brazil (see the turquoise shading in the middle panel of Fig. 3c, associated with peak #4). Later, during April, the drought signal expanded south and eastwards, towards south Brazil, north Argentina and central Paraguay as shown by the red color shadings associated with peak #1 in the middle panel of Fig. 3c. In October and during the flash drought marked by the peak #5 (October 10th ), the soil dry-out pattern moved slightly northwards affecting more the central SESA. During the last two months of 2020, when peak #3 and #2 occurred, soil dryness dominated over the northern section of SESA and Pantanal. Finally, the year of 2021 started with a slight weakening of the drought signal (Fig. 3a), however a strong amplification occurred in April with pronounced soil desiccation being recorded over southern Brazil (red area in right panel of Fig. 3c), and later in December with the most extreme soil moisture deficits being witnessed over Pantanal and southeast Brazil (blue area in right panel of Fig. 3c).

The influence of large-scale tropical and subtropical atmospheric forcing on the 2019–2021 drought

Decadal-scale variability such as the one observed when analyzing the long-term inter-annual variability of precipitation and VIMC over SESA (Fig. 2a) is often associated with slowly varying atmospheric and/or oceanic conditions (e.g., sea surface temperature, atmospheric pressure^27,29,41) described by a particular large-scale atmospheric-oceanic variability mode. Figure 4a provides the spatial correlation between the mean annual SST’s and the mean annual IVT across the northern border of SESA, which controls the amount of moisture convergence and precipitation over SESA. The central and southeast tropical Pacific reveal pronounced positive and statistically significant correlations, indicating that mean annual wet conditions over SESA are associated with warmer SSTs in these areas of the Pacific Ocean. Statistically significant negative correlations were observed over the southwest and northwest Pacific and over the equatorial Atlantic Ocean. When analyzing the decadal variability of the Southern Oscillation Index (SOI) and of Oceanic Niño Index (ONI) (see Supplementary Material), we observe that multi-year dry periods in SESA were defined by positive values of SOI and negative levels of

ONI, thus to La Niña conditions (Fig. 4b). The correlation between these ENSO indicators and the IVT across the northern border of SESA filtered by a 10-year low pass Lanczos filter, was high and statistically significant during the autumn and winter seasons, while during summer, residual and non-statistically significant correlations were obtained (see inset table in Fig. 4). Regarding the Atlantic Ocean, the decadal variability of precipitation levels in SESA appears to be negatively correlated with the Atlantic Multidecadal Oscillation (AMO), although this relation is, in general, less pronounced compared to the other two modes of variability in Pacific.

The spatially distribution of the observed SST anomalies during the 2019–2021 drought period shows cold SSTs in tropical central and southeast Pacific associated to enhanced low tropospheric air divergence (Fig. 5). Such anomalous divergence pattern (see vectors in the lower panel of Fig. 5) is a signature of above-normal subsidence as shown by the positive anomalies of the vertical velocity over central tropical Pacific and northwest SA (middle panel in Fig. 5). The subsidence over northwest SA represents the descending branch of an eastward shifted Walker Cell (descending blue arrow), strongly connected to anomalous convergence at the top of the atmosphere, as shown by the positive anomalies of velocity potential (Fig. 5, top panel). The corresponding ascending branch, linked with divergence at the top of the atmosphere, air rising motion and convection, was located in the equatorial Atlantic near northeast SA (see ascending blue arrow in Fig. 5), supporting the low tropospheric moisture convergence patterns and the negative anomalies of E–P balance that were previously identified for this region (see Fig. 2d). Moreover, Fig. 5 shows that this large-scale anomalous zonal circulation was strongly connected with subtropical SA, through the establishment of an amplified meridional Hadley Cell (see purple lines in Fig. 5) with its descending branch associated with clear sky conditions, strong diabatic and adiabatic heating rates and moisture divergence, located over SESA. Accordingly, this descending branch promoted the low tropospheric spread of large amounts of moisture from SESA towards the surrounding regions and particularly towards northeast SA, where it converged, explaining the positive anomalies of the E–P balance that were previously identified in Fig. 2d. The three wettest years were characterized by a contrasting tropical zonal and meridional circulation, with the establishment over SESA of a Hadley cell’s ascending branch associated with strong moisture convergence and supply from the Amazon basin (Fig. S4).

In addition to this anomalous tropical circulation responsible for the long-term precipitation deficits in SESA during the 2019–2021 drought, at synoptic scales, the subtropical dynamic may also have played a key role in explaining the occurrence of the flash droughts identified by the R-index peaks. Accordingly, we computed spatial anomaly composites considering the eight most extreme R-index peaks (Fig. 3a) and regarding several meteorological parameters (Fig. 6). Pronounced negative soil moisture anomalies are clearly visible all over SESA (Fig. 6a). The mid-level atmospheric circulation was defined by the occurrence of positive 500hPa geopotential height anomalies and by exceptional warm conditions in the low troposphere, likely promoted by strong air subsidence, enhanced adiabatic and diabatic heating rates (Fig. 6b). The occurrence of positive anomalies of the vertical integral of divergence of moisture flux (see color shading in Fig. 6c) and the spreading out of air masses from SESA to the surrounding regions, as indicated by the anomalous low tropospheric divergent wind field (see arrows in Fig. 6c), is evident. This points to the establishment over SESA of exceptional clear-sky conditions and strong shortwave radiation incidence at the surface that led to large evaporation rates. From a large-scale perspective and in agreement with Fig. 5, it is possible to observe the establishment of a meridional Hadley cell (Fig. 6d) with enhanced divergence at the top of the atmosphere over northeast SA (see the negative anomalies of potential velocity represented by the color shading in Fig. 6d), and its descending branch over SESA. In parallel, an eastward shifted zonal Walker circulation can be inferred with its descending branch over central-east equatorial Pacific and northwest SA. Regarding the mid-latitude atmospheric circulation, the anomalous meridional wind field at the 200 hPa level shows a clear sequence of F

divergence/convergence patterns in the top of the troposphere spanning from east Australia to south Atlantic (Fig. 6e). This is a clear signature of a Rossby Wave train (wavenumber 3) embedded in a jet stream that experienced an anomalous poleward shift over southeast South Pacific, near SA (see contours in Fig. 6e)⁴². A large Rossby wave source region was recorded in west Pacific, at east and south Australia, where the Rossby wave train was formed (Fig. 6f). At east of Australia, the Rossby wave forcing was mostly due to the advection of absolute vorticity by the divergent flow (see Fig. S5a) and to strong convection in the Indo-Pacific warm pool, while at south of Australia only dynamic factors were involved (Fig. S5b) which agrees with Shimizu et al⁴³.

Finally, from a long-term perspective, this 3-yr drought was characterized by a clear zonal expansion of the subtropical quasi-stationary high-pressure system in the south Atlantic, with a higher-than-normal continental penetration towards SA when compared to the mean climatology. This points for a higher influence of this system in modulating precipitation deficits mainly in the eastern section of SESA (Fig. S6). A similar zonal expansion of the south Pacific high-pressure system was observed, leading to pronounced positive mean sea level pressure anomalies in South Pacific, over 150ºW and 100ºW, in a latitudinal band between 35ºS and 45ºS (see color shading in Fig. S6). This suggests an increased ridging activity in the region⁴⁴. Consequently, the mean position of the subtropical Southern Hemisphere jet stream suffered a poleward shift over southeast south Pacific, near SA, reducing the passage of cyclones and frontal systems over SESA.

Despite the outstanding amplitude and length of the 2019–2021 drought, it is paramount to stress that it occurred within the context of a prolonged drying trend observed over the past three decades in SESA, that resulted from a joint contribution of natural decadal variability in precipitation levels and increasing temperatures. The observed decadal variability in precipitation was found to be mainly controlled by ENSO, with La Niña events, such as the recent one extending from 2020 to 2022⁴⁵, being associated to precipitation deficits over SESA. Similarly to previous studies, SST anomalies in eastern tropical Pacific were found to promote large-scale disturbances in the location and intensity of the Walker cell modulating the southwards moisture transport from the Amazon and the water vapor convergence over SA. Disturbances in these two hydrological processes are reflected in the amount of precipitation over SESA^24–26. On the other hand, deforestation has been associated to less moisture of Amazonian origin and to precipitation reductions in non-deforested parts of the western Amazon and in regions further downstream, mainly SESA^46,47. Therefore, recent deforestation may have enhanced the impact of La Niña on precipitation deficits over the last two decades in SESA, paving the way for the outstanding 2019–2021 drought. Soil moisture anomalies during this 3-yr period were defined by a large spatiotemporal variability. Flash droughts in 2020 affected multiple regions such as Pantanal, Bolivia, Paraguay and North Argentina, and led to large areas covered by record-breaking soil moisture anomalies (~ 100.000${ km}^{2}$). Such severe drought conditions can cause devastating losses in agriculture and livestock production, depletion of water resources, reduction of air quality and exacerbated wildfire risk^36,48. The Pantanal’s catastrophic wildfires recorded during October and November 2020 clearly expose this link^35,38.

To summarize the tropical and mid-latitude mechanisms mentioned in the previous sections we propose a conceptual scheme (Fig. 7) that aims to capture the coupled large-scale tropical and subtropical atmospheric dynamics responsible for the historically unprecedented soil desiccation in SESA during the 2019–2021 drought. Enhanced surface air convergence over northeast SA and tropical Atlantic Ocean, linked to negative SST anomalies in tropical Pacific and to an anomalous ascending branch of an eastward shifted Walker Cell, was responsible for precipitation deficits and large evaporation rates in SESA via two distinct processes. First, enhanced deep convection in northern Brazil and tropical Atlantic led to a decrease in this region of moisture available to be advected and a weakening of the northeast trade winds, causing a reduction of the SALLJ activity⁴⁰. Secondly, this eastward shifted Walker Cell induced a meridional Hadley cell and enhanced subsidence over SESA, resulting in the suppression of moisture convergence and precipitation. The SALLJ is defined as a low tropospheric northerly jet along the eastern slopes of the Andes responsible for the transport of large amounts of moisture from the Amazon to the subtropics^40,49,50. In agreement with our results, previous studies have shown that on interannual timescales, this jet is strongly modulated by ENSO, and La Niña conditions are associated with weaker and less frequent jet days^26,49. Negative precipitation anomalies in SESA were also found to be related to AMO and to positive SST anomalies in tropical Atlantic. Although we did not explore in depth this association, our results corroborate the findings by Jones and Carvalho⁴¹ showing that positive AMO phases, such as the one that has been occurring in the last two decades, are linked to multidecadal regimes of low SALLJ activity and therefore to long-term precipitation deficits over SESA. During the flash drought episodes, when soil desiccation reached its maximum intensity and spatial extent, a Rossby wave pattern (wavenumber 3), reinforced the subsidence over SESA and the already established descending branch of the meridional Hadley Cell. This compound mechanism connecting the tropical and subtropical anomalous dynamics resulted in the establishment over SESA of a quasi-stationary anticyclonic circulation known to enhance evaporation and reduce soil moisture levels¹⁴. This Rossby wave train was forced at east and south of Australia in regions known to bring together the ideal conditions for the development of such

planetary wave trains, which have already been associated to precipitation and temperature extremes over SA^31,43,51,52.

The findings summarized in the conceptual model highlight the effect of compound internal variability in the occurrence of climate extremes, providing a guideline to characterize other historical drought events and also to predict the occurrence of future extreme dry spells together with climate models capable to simulate such dynamical processes in tropical to subtropical regions. Moreover, previous studies have identified SESA as a region with strong soil moisture–temperature and soil moisture–precipitation feedbacks^53,54, and this trend towards more water-limited conditions likely enhanced this land–atmosphere coupling as well as the influence of land conditions on local climate. In fact, recent hot extremes (e.g., 2020 in Pantanal and 2013/2014 in Southeast Brazil) resulted from a joint contribution of large-scale atmospheric circulation anomalies and a strong soil moisture imbalance^14,38.

Future climate change projections regarding land–atmosphere interactions in regions such as SESA where soil moisture is a limiting factor to these interactions, are shrouded in large uncertainty ^54,55. Even so, recent studies suggest that SESA will remain a land–atmosphere coupling hotspot, with some regions (namely the Pantanal and Brazilian Highlands) witnessing a change from energy-limited to water-limited regimes, and a higher concurrence of dry and hot spells^56,57. Accordingly, this study provides robust guidelines to better predict the occurrence of future dry extremes, stressing the urgent need to improve the representation of the dynamical processes associated with daily-to-multiyear timescales in climate models. We believe these findings will be of interest to the international community in the context of natural hazards characterization and prediction under a climate change scenario.

Data

Daily data from several meteorological parameters at surface and at different pressure levels, such as precipitation, temperature, specific humidity, zonal and meridional wind, geopotential and vertical velocity were extracted from the European Centre of Medium-range Weather Forecast ERA-5 reanalysis datasets (Copernicus Climate Change Service, C2S, 2017) for the 1959–2021 period⁵⁸. Daily soil moisture levels for the 1951–2021 period were obtained from the ERA5-Land reanalysis which replaced the land component of the ERA5 model providing a better representation of the hydrological cycle⁵⁹. ERA5-Land shows a good performance in estimating soil moisture levels when compared to other datasets, including satellite products and other models with and without data assimilation⁶⁰. ERA5 and ERA5-Land show for SESA smaller biases in precipitation than other known reanalysis such as JRA-55 and MERRA-2⁶¹. The same is observed regarding the performance of ERA5 in estimating temperature extremes and the persistence of hot days^62,63

Methods

Bivariate Gaussian Distributions

In order to estimate temperature–precipitation distributions for the two halves of the total period of analysis (1959–2021) and thus assess long-term changes in the bivariate distribution of these variables, two probability density functions assuming a bivariate gaussian distribution were computed for pairs of mean annual temperature and precipitation averaged over the red box in Fig. 1 (hereafter SESA) and during the periods of 1959–1989 and 1990–2021. Similarly to Bevacqua et al.⁶⁴, we assume that a bivariate Gaussian distribution is acceptable for mean annual values of these two variables allowing for a simple understanding of their compound effect on soil moisture levels, with the hypothesis that in some particular grid-points both precipitation and temperature datasets may have a skewed (Gamma) distribution.

The Vertically Integrated Water Vapor Transport

The vertically integrated water vapor transport (IVT) quantifies the total horizontal moisture transport by integrating over the vertical column of the atmosphere the zonal (${Q}_{\lambda }$) and meridional (${Q}_{\varphi }$) transport of specific humidity, defined as follows⁶⁵:

$$IVT{\left[{\left(\frac{1}{g}{\int }_{SURF}^{TOA}qudp\right)}^{2}+{\left(\frac{1}{g}{\int }_{SURF}^{TOA}qvdp\right)}^{2}\right]}^{\frac{1}{2}}\left(1\right)$$

with,

$${Q}_{\lambda }=\frac{1}{g}{\int }_{SURF}^{TOA}qudp \left(2\right)$$

$${Q}_{\varphi }=\frac{1}{g}{\int }_{SURF}^{TOA}qvdp \left(3\right)$$

where q is the specific humidity, u and v are the zonal and meridional components of wind, respectively, g is the gravitational acceleration and dp an infinitesimal element of vertical pressure.

Considering this definition for the meridional and zonal components of the IVT, the vertically integrated moisture divergence can be computed using finite differences as follows:

$$\nabla .\overrightarrow{Q}=\frac{\partial {Q}_{\lambda }}{\partial x}+\frac{\partial {Q}_{\varnothing }}{dy} \left(4\right)$$

Accordingly, when the divergence is positive (negative) the atmospheric circulation is acting to decrease (increase) the water vapor transport over all the column of the atmosphere.

The Water Vapor Balance Over SESA

According to previous studies^65,66 the total (net) vertically integrated moisture convergence (VIMC) over SESA, whose area is geographically defined in Fig. 1 (see red box), can be obtained as a line integral that accounts for the IVT across the western (${F}_{w}$), the northern (${F}_{N}$), the eastern (${F}_{E}$) and the southern (${F}_{S}$) borders:

$$VIMC= {F}_{w}+ {F}_{N}+ {F}_{E}+ {F}_{S} \left(5\right)$$

with,

$${F}_{W}={\int }_{\varnothing =31^\circ S}^{\varnothing =15^\circ S}{Q}_{\lambda }d\varphi , \lambda =64^\circ W \left(6\right)$$

$${F}_{E}=-{\int }_{\varnothing =31^\circ S}^{\varnothing =15^\circ S}{Q}_{\lambda }d\varphi , \lambda =44^\circ W \left(7\right)$$

$${F}_{N}=-{\int }_{\lambda =64^\circ W}^{\lambda =44^\circ W}{Q}_{\varphi }d\lambda , \varphi =15^\circ S \left(8\right)$$

$${F}_{S}={\int }_{\lambda =64^\circ W}^{\lambda =44^\circ W}{Q}_{\varphi }d\lambda , \varphi =31^\circ S \left(9\right)$$

Analyzing Eq. (9) and neglecting the water storage changes $\left(\frac{\partial w}{\partial t}\right)$, when VIMC > 0, the region experiences a moisture flux convergence and acts as moisture sink if $P-E>0$, or $E-P<0$. On the other hand, when VIMC < 0, the region witnesses a moisture flux divergence and acts as moisture source if $P-E<0$, or $E-P>0$:

$$VIMC=P-E+\frac{\partial w}{\partial t} \left(10\right)$$

The area-averaged precipitation rate due to moisture convergence (${P}_{convergence}$) over SESA can be obtained dividing VIMC by the total area of the red box depicted in Fig. 1a (A):

$${P}_{convergence}=\frac{VIMC}{A} \left(11\right)$$

Accordingly, the contribution of moisture convergence to the total area averaged precipitation (${P}_{total}$) can be computed by $\frac{{P}_{convergence}}{{P}_{total}\times 100}$, while the contribution of local moisture recycling due to evaporation can be obtained by:

$$100\times \frac{\left({P}_{total}-{P}_{convergence}\right)}{{P}_{total}} \left(12\right)$$

R-index: ranking soil moisture anomalies

The R-index is defined here as an adaptation of a metric initially proposed by Hart and Grumm⁶⁷ and used in several studies^68,69 to rank precipitation events on a daily basis. The exact same rationale was adopted here but considering soil moisture levels in SESA as input. First, daily standardized soil moisture anomalies regarding the seasonal climatology (1981–2010) were computed for each grid point within SESA and then filtered using a 31-day running mean filter. Thus, for each day of the analysis period (1951–2021) and for each grid-point within SESA we attained a normalized soil moisture departure value from the climatology. Ultimately, the R-index quantifies and ranks, on a daily basis, the magnitude of the soil moisture anomalies over SESA by multiplying (1) the area of SESA (in percentage) that experiences soil moisture anomalies lower than two standard deviations (“Spatial Extend”) by (2) the mean soil moisture anomaly obtained by considering only the grid-points that recorded anomalies lower than two standard deviations (“Mean Anomaly”).

Therefore, severe drought conditions are diagnosed when R-index reaches extremely low levels. Weekly to monthly peaks of this metric are linked to a sudden and rapid onset development of dry conditions and thus to flash drought episodes.

Rossby Wave Source

The Rossby Wave Source (RWS) was computed considering the barotropic vorticity equation in pressure coordinates^70,71:

$$RWS=-\zeta D-{\nu }_{x}.\nabla \zeta \left(13\right)$$

Where $\zeta$ is the absolute vorticity, obtained by summing the relative vorticity with the Coriolis parameter, and ${\nu }_{x}$ is the non-rotational (i.e., divergent) wind vector component. The first term denotes the vortex stretching and accounts for the changing rate of vorticity, while the second term denotes the advection of vorticity by the divergent flow. Therefore, the Rossby Wave forcing tends to be higher in regions where divergence, divergent wind, absolute vorticity and the gradient of absolute vorticity is large. For instance, regions in the Western Pacific, at east and south of Australia, located on the edge of strong diabatic tropical heating and over the passage of the subtropical jet stream, where there are co-occurring conditions of high divergence, enhanced absolute vorticity and a high gradient of vorticity, reunite all the ideal conditions for a strong Rossby Wave forcing⁴³.

Data Availability

Data relevant to the analysis can be downloaded from the website listed below: ERA-5 reanalysis datasets is available at https://cds.climate.copernicus.eu/cdsapp#!/search?type=dataset. SOI, ONI and AMO index data was provide by NCAR (National Center for Atmospheric Research) / Climate Data Guide, at https://climatedataguide.ucar.edu/climate-data. Datasets generated and analyzed during the current study are available from 10the corresponding author on reasonable request.

Code Availability

The code used in this study is available on request from the authors.

Acknowledgments

JG and AR acknowledge FCT (Fundação para a Ciência e a Tecnologia) and I.P./MCTES for the PhD Grant 2020.05198.BD and for 2022.01167.CEECIND, respectively. This work was partially supported by national funds through FCT (PIDDAC) –UIDB/50019/2020, 2022.09185.PTDC (DHEFEUS). RL acknowledges FAPERJ (E26/200/329/2023) and CNPq (311487/2021-1).

Author Contributions

The study was conceptualized by all authors. JG collected the data, performed all the calculations and was responsible for the data curation and analysis. JG also designed the figures and wrote the manuscript. All authors analyzed the results, wrote and reviewed the final version of the manuscript.

Competing Interests

The authors declare no competing financial or non-financial interests.

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Combined large-scale tropical and subtropical forcing on severe soil desiccation in South America

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