atmosphere

Article Mechanisms for Severe Drought Occurrence in the Balsas River Basin (Mexico)

Ana E. Melgarejo 1, Paulina Ordoñez 2,*, Raquel Nieto 3 , Cristina Peña-Ortiz 4, Ricardo García-Herrera 5,6 and Luis Gimeno 3

1 Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Mexico City 04510, Mexico; [email protected] 2 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City 04510, Mexico 3 Environmental Physics Laboratory (EPhysLab), CIM–UVigo, Universidade de Vigo, 32004 Ourense, Spain; [email protected] (R.N.); [email protected] (L.G.) 4 Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, 41013 Sevilla, Spain; [email protected] 5 Departamento de Ciencias de la Tierra y Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain; rgarciah@fis.ucm.es 6 Instituto de Geociencias (CSIC-UCM), 28040 Madrid, Spain * Correspondence: [email protected]

Abstract: This work provides an assessment of the two most intense seasonal droughts that occurred over the Balsas River Basin (BRB) in the period 1980–2017. The detection of the drought events was performed using the 6 month scale standardized precipitation–evapotranspiration index (SPEI-6)



and the 6 month standardized precipitation index (SPI-6) in October. Both indices were quite
 similar during the studied period, highlighting the larger contribution of precipitation deficits vs. temperature excess to the drought occurrence in the basin. The origin of the atmospheric water Citation: Melgarejo, A.E.; Ordoñez, P.; Nieto, R.; Peña-Ortiz, C.; García- arriving to the BRB (1 May 1980–31 October 2017) was investigated by using a Lagrangian diagnosis Herrera, R.; Gimeno, L. Mechanisms method. The BRB receives moisture from the Caribbean Sea and the rest of the tropical Atlantic, the for Severe Drought Occurrence in the Gulf of Mexico, the eastern north Pacific and from three terrestrial evaporative sources: the region Balsas River Basin (Mexico). north of BRB, the south of BRB and the BRB itself. The terrestrial evaporative source of the BRB itself Atmosphere 2021, 12, 368. https:// is by far the main moisture source. The two most intense drought events that occurred in the studied doi.org/10.3390/atmos12030368 period were selected for further analysis. During the severe drought of 2005, the summertime sea surface temperature (SST) soared over the Caribbean Sea, extending eastward into a large swathe Academic Editor: Alexander of tropical North Atlantic, which was accompanied by the record to date of hurricane activity. This V. Chernokulsky heating generated a Rossby wave response with westward propagating anticyclonic/cyclonic gyres in the upper/lower troposphere. A cyclonic low-level circulation developed over the Gulf of Mexico Received: 7 January 2021 and prevented the moisture from arriving to the BRB, with a consequent deficit in precipitation. Accepted: 2 March 2021 Published: 11 March 2021 Additionally, subsidence also prevented convection in most of the months of this drought period. During the extreme drought event of 1982, the Inter Tropical Convergence Zone (ITCZ) remained

Publisher’s Note: MDPI stays neutral southern and stronger than the climatological mean over the eastern tropical Pacific, producing an with regard to jurisdictional claims in intense regional Hadley circulation. The descent branch of this cell inhibited the development of published maps and institutional affil- convection over the BRB, although the moisture sources increased their contributions; however, these iations. were bounded to the lower levels by a strong trade wind inversion.

Keywords: drought; SPEI; SPI; moisture transport; FLEXPART; trade wind inversion

Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article 1. Introduction distributed under the terms and Numerous severe drought events have been witnessed around the world during the conditions of the Creative Commons last decades [1], such as the 2007–2009 drought in the Fertile Crescent [2], the 2010 Amazon Attribution (CC BY) license (https:// drought [3], the long drought of the first decade of the current century in Australia [4] or the creativecommons.org/licenses/by/ 2011 record-setting Texas drought [5]. In the case of Mexico, the frequency and extension of 4.0/).

Atmosphere 2021, 12, 368. https://doi.org/10.3390/atmos12030368 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 368 2 of 16

drought conditions have been evaluated during the period 1950–1999 by using the Palmer Drought Severity Index (PDSI) [6], with more frequent episodes found in north-western Mexico [7]. However, it is important to mention that the time scale was not defined in PDSI, and this method was replaced by others that better detect the start and end of a drought event [8]. Regarding future drought conditions, the climate model outputs archived in the Coupled Model Intercomparison Project phase 5 (CMIP5) revealed increased drought in a few regions around the world, with one of them being Central America and Mexico [9]. Droughts would affect forestry and hydropower sectors in Mexico, but the biggest impacts are projected to occur in agriculture [10]. In fact, in the Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation by the Intergovernmental Panel on Climate Change (IPCC) [11], the Central America/Mexico region is one of the key regions projected to be more strongly affected by droughts. Overall, there is a lack of scientific studies about the severity, propagation and socio- economic impacts of droughts in Mexico [12]. Furthermore, the attention given to drought events is focused on the response to these events but not their risk management, which makes certain locations very vulnerable to droughts. Information about the origin of droughts is of the utmost importance in order to be able to manage the risks and assess the potential impacts and in general terms for proper decision-making in relation to droughts [13]. The Hydrological Region “Balsas”—hereafter referred to as Balsas River Basin (BRB)— is one of the 13 so-called Hydrological Administrative Regions into which Mexico is divided [14]. The BRB region (Figure1a) comprises eight states partially or totally (Jalisco, Michoacán, Tlaxcala, Morelos, Puebla, Guerrero, Estado de México and Oaxaca). The BRB in general gets around 95% of its annual precipitation between May and October (Figure1 b) [15], but due to the orography, the basin has a variety of climatic character- istics [16]. The BRB has an extension of 116,436 km2 [17], and it is the most important Pacific slope river in the country, supplying water to big conurbation areas such as Puebla, Tlaxcala and Cuernavaca-Cuautla, among others. Additionally, the hydroelectric installed capacity in Mexico, which contributes 48% of the annual clean energy generation, is mainly concentrated in the Balsas, Lerma Santiago and Grijalva basins, with the former being the most vulnerable to suffer a lack of water availability [18]. The BRB is a drought-prone region [19]. For example, during 2015, a strong drought caused losses of up to 80% in the sowing of jamaica, sesame, sorghum and corn [20]. According to the climate change vulnerability index [21], the Balsas region presents a high degree of hydric vulnerability. Heavy erosion processes caused by the combination of heavy rainfall and deforestation have also had an impact on the local hydrological cycle [22]. For example, in the state of Morelos, which belongs completely to the BRB, close to 60% of the original vegetation (seasonally dry tropical forests) has been lost [23]. On the other hand, rainfall is generally more directly related to moisture supply in the tropics, whereas in the extratropics, the role of stability and dynamical forcing mech- anisms normally gain relative importance [24,25]. To investigate the relative importance of moistening in the BRB, we take advantage of the use of the Lagrangian dispersion model FLEXPART (flexible particle dispersion model) [26,27]. This Lagrangian approach allows us to determine large-scale water vapor advection by tracking evaporation minus precipitation (E–P) from a region forwards or backwards in time, thereby facilitating the determination of the source–receptor relationships of water. The main goal of this work is to analyze the origin of severe drought events over the BRB since 1980. For this purpose, this study has three specific objectives: first, to rank the hydrological droughts that occurred over the BRB during the period 1980–2017; second, to implement a dynamical analysis of moisture transport over the BRB for the selected severe or extreme droughts; and finally, to assess the role played by the sea surface temperature (SST) and atmospheric stability in these droughts. Atmosphere 2021, 12, x FOR PEER REVIEW 2 of 16

[4] or the 2011 record-setting Texas drought [5]. In the case of Mexico, the frequency and extension of drought conditions have been evaluated during the period 1950–1999 by us- ing the Palmer Drought Severity Index (PDSI) [6], with more frequent episodes found in north-western Mexico [7]. However, it is important to mention that the time scale was not defined in PDSI, and this method was replaced by others that better detect the start and end of a drought event [8]. Regarding future drought conditions, the climate model out- puts archived in the Coupled Model Intercomparison Project phase 5 (CMIP5) revealed increased drought in a few regions around the world, with one of them being Central America and Mexico [9]. Droughts would affect forestry and hydropower sectors in Mex- ico, but the biggest impacts are projected to occur in agriculture [10]. In fact, in the Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation by the Intergovernmental Panel on Climate Change (IPCC) [11], the Central America/Mexico region is one of the key regions projected to be more strongly affected by droughts. Overall, there is a lack of scientific studies about the severity, propagation and socio- economic impacts of droughts in Mexico [12]. Furthermore, the attention given to drought events is focused on the response to these events but not their risk management, which makes certain locations very vulnerable to droughts. Information about the origin of droughts is of the utmost importance in order to be able to manage the risks and assess the potential impacts and in general terms for proper decision-making in relation to droughts [13]. The Hydrological Region “Balsas”—hereafter referred to as Balsas River Basin (BRB)—is one of the 13 so-called Hydrological Administrative Regions into which Mexico is divided [14]. The BRB region (Figure 1a) comprises eight states partially or totally (Ja- lisco, Michoacán, Tlaxcala, Morelos, Puebla, Guerrero, Estado de México and Oaxaca). The BRB in general gets around 95% of its annual precipitation between May and October (Figure 1b) [15], but due to the orography, the basin has a variety of climatic characteristics [16]. The BRB has an extension of 116,436 km2 [17], and it is the most important Pacific slope river in the country, supplying water to big conurbation areas such as Puebla, Tlax- cala and Cuernavaca-Cuautla, among others. Additionally, the hydroelectric installed ca- pacity in Mexico, which contributes 48% of the annual clean energy generation, is mainly Atmosphere 2021, 12, 368 3 of 16 concentrated in the Balsas, Lerma Santiago and Grijalva basins, with the former being the most vulnerable to suffer a lack of water availability [18].

Figure 1. (a) Map of the study region showing the Balsas River Basin (BRB) contoured in a black line. The elevation of the Figureregion 1. ( isa) indicated Map of the in study colors region (units showing in meters). the ( bBalsas) Monthly River mean Basin precipitation (BRB) contoured for the in a average black line. region The ofelevation the Balsas of the River region is indicated in colors (units in meters). (b) Monthly mean precipitation for the average region of the Balsas River Basin for the period from 1980–2017 from the University of East Anglia Climate Research Unit (CRU) database [15]. Basin for the period from 1980–2017 from the University of East Anglia Climate Research Unit (CRU) database [15]. 2. Data and Methods 2.1. Drought Indices The standardized precipitation index (SPI) relies on the difference of precipitation from the mean for a specified time period divided by the standard deviation [28]. This

method takes into account the fact that the time period from the arrival of precipitation until water is available in each useable form (soil moisture, ground water, snowpack, streamflow and reservoir storage) differs greatly [28]. The SPI can be calculated over different precipitation accumulation periods, and the averaging period (1, 3, 6, 12, 24 or 48 months) is selected depending on the type of drought in which user is interested (i.e., meteorological, agricultural, hydrological or ecological). A disadvantage of this simple method is that precipitation is typically not normally distributed for accumulation periods of 12 months or less, but this is overcome by applying a transformation to the distribution [28,29]. Many studies using the SPI have been undertaken, as the SPI provides information on precipitation deficit, average percentage and probability. However, in recent years, some shortcomings in the index have become apparent due to the fact that it does not take into account variables such as evapotranspiration or temperature [30,31], which can be crucial in order to assess the occurrence of droughts. The standardized precipitation–evapotranspiration index (SPEI) takes into account the potential evapotranspiration (PET), besides the precipitation, in order to determine the occurrence of droughts [32]. The SPEI is able to capture the impact of temperature on the atmospheric evaporative demand of water [33]. The method used to compute the SPEI is similar to that for the SPI. However, while the SPI adjusts precipitation to a certain distribution, in the case of the SPEI, it is the difference between precipitation and PET that is adjusted. The SPEI can also be calculated over different time-scales for the estimation of the diverse potential impacts of water deficit accumulation occurring over time periods of different lengths. The response of the hydrological system to precipitation is time-lagged and not known a priori [34], but in general the SPEI and SPI average accumulation periods (e.g., 3 to 12 months) can be used as an indicator for reduced stream flow and reservoir storage [35,36]. We computed 6 month scale indices in October (SPI-6 and SPEI-6) to analyze the seasonal scale droughts. The 6 month indices at the end of October provide a very good indication of the amount of precipitation that has fallen during the wet season for the BRB. In this way, we can even take into account the impact in the early stages on water reservoirs. In this work, 1 month indices (SPEI-1 and SPI-1) were also used to determine the months within the drought period with the most severe meteorological drought. Atmosphere 2021, 12, 368 4 of 16

Monthly rainfall data from the University of East Anglia Climate Research Unit (CRU) database [15] were used to generate the SPEI and SPI for a 38 year period (1980–2017) with a 0.5◦ resolution in longitude and latitude. The PET from the CRU database, which employs the FAO (Food and Agricultural Organization) Penman–Monteith formula [15], was also used to compute SPEI.

2.2. E–P Backward Tracking FLEXPART v9 (flexible particle dispersion model) was used here to identify the large- scale water vapor advection toward the region of study. Although the model was originally created to determine pollutant dispersion, it was adapted also to diagnose any change of different variables along the trajectories. Over the last decades, the model has largely been used to determinate moisture transport that generates precipitation over continents based on the methodology by Stohl and James [37,38]. FLEXPART is supported by a large number of peer-reviewed publications [39] and has been validated for the atmospheric branch of the water cycle of major river catchments [38]. The model has also been used to study water vapor transport variability toward several hydrologic basins, such as the Amazon [40], Niger River basin [41] or the Danube river basin [42], among others. FLEXPART generates the trajectories of a large number of particles (2 million) into which the atmosphere is divided; each particle moves by using a three-dimensional wind field with a constant mass. FLEXPART uses meteorological data from the European Centre for Medium-Range Weather Forecasts (ECMWF) [43] every 6 h with a 1◦ × 1◦ resolution on 60 vertical levels to calculate the grid-scale advection. To determine moisture sources and sinks, the position of each particle is tracked backward or forward, respectively, in combination with the particle loss of specific humidity by precipitation (p) or gain by evaporation (e). The (E–P) differences of all particles in a column are amassed, and then the total (E–P) field is obtained, where (E) is the evaporation rate and (P) the precipitation rate per unit area. This method therefore diagnoses E–P, but not E or P individually [37]. In this work, the trajectories were followed in backward mode, which means that we were interested in determining the origin of the humidity over the BRB. By integrating the changes of all particles aimed towards the BRB region, we found the areas where those particles had either gained (E–P > 0) or lost moisture (E–P < 0) along their path. We limited the transport to 10 days as this is the optimal particle travel time for the BRB region during our study period using the database by Nieto and Gimeno [44]. Then, a general view of the moisture sources was attained by adding the net freshwater flux from day −1 to day −10, ((E–P)1–10 from now on). When a long enough period is analyzed, the mean moisture sources for the region can be described from a climatological perspective.

2.3. Climatic Fields Data The COBE (Centennial in situ Observation-Based Estimates) SST v2 monthly dataset was used from 1980 to 2017 [45,46] at 1◦ latitude × 1◦ longitude spatial resolution. COBE incorporates ICOADS (International Comprehensive Ocean-Atmosphere Data Set) data [47] and includes some additional observational datasets mainly for recent periods. ERA-Interim reanalysis monthly means of horizontal wind, vertically integrated moisture flux, geopotential height and vertical velocity (omega) at several tropospheric levels [43] were used for the period 1980 to 2017 at a 1◦ resolution in longitude and latitude. In addition, temperature and relative humidity were obtained from the same reanalysis database. These variables were employed to compute the equivalent potential temperature (θE) according to Equation (38) in [48], allowing us to assess the moisture content and thermal properties of the air column. Seasonal anomalies were computed with respect to the climatology of May 1981– October 2010, referred to hereafter as 1981–2010 climatology. Atmosphere 2021, 12, x FOR PEER REVIEW 5 of 16

Atmosphere 2021, 12, 368 Seasonal anomalies were computed with respect to the climatology of May 1981–5 of 16 October 2010, referred to hereafter as 1981–2010 climatology.

3. Results 3.1. Drought Characteristics during thethe PeriodPeriod ofof 1980–20171980–2017 The temporaltemporal variationvariation of SPI-6SPI-6 andand SPEI-6SPEI-6 inin OctoberOctober forfor thethe periodperiod 1980–20171980–2017 isis depicted inin Figure2 2.. BothBoth indicesindices areare veryvery similar,similar, with with the the PearsonPearson correlationcorrelation coefficient coefficient between them being ρ = = 0.99, 0.99 and, and both both match match the the same same dry dry periods. periods. ThisThis indicatesindicates thatthat thethe contributioncontribution ofof precipitationprecipitation deficitsdeficits toto thethe droughtdrought severityseverity inin thethe BRBBRB prevailsprevails overover thethe temperaturetemperature excess,excess, unlikeunlike otherother regions,regions, suchsuch asas thethe MediterraneanMediterranean region,region, wherewhere thethe increaseincrease inin thethe atmosphericatmospheric evaporativeevaporative demanddemand drivendriven byby increasingincreasing temperaturestemperatures inin thethe lastlast decadesdecades hashas enhancedenhanced droughts,droughts, independentlyindependently ofof thethe precipitationprecipitation evolutionevolution inin thethe regionregion [[49,50].49,50]. InIn fact,fact, thethe linklink betweenbetween droughtsdroughts andand heatheat waveswaves isis widelywidely known:known: highhigh temperaturestemperatures accelerateaccelerate soil soil drying drying and and dry dry soils soils in in turn turn warm warm the the atmosphere atmosphere by by retaining retain- lessing less water water for evapotranspiration for evapotranspiration [51]. [51]. Studies Studies examining examining the coupling the coupling between between the land the surfaceland surface and extremeand extreme temperature temperature have have shown shown that that the the soil soil moisture/temperature moisture/temperature rela- re- tionshiplationship is is geographically geographically widespread widespread [52 [52].]. This This area area of of research research has has held held aa predominantpredominant European focusfocus [[51].51]. For example,example, itit isis knownknown thatthat thethe intensityintensity ofof thethe 20032003 heatwaveheatwave waswas increasedincreased byby drydry conditionsconditions byby upup toto 40%40% [[53,54],53,54], whilewhile thethe record-breakingrecord-breaking droughtdrought thatthat affected EuropeEurope inin 2016/20172016/2017 has has been found to have been caused by decreased precipita- tiontion andand increasedincreased sunshinesunshine durationduration inin thethe northernnorthern part,part, whereaswhereas thethe mainmain contributorscontributors toto thethe droughtdrought inin thethe southsouth werewere thermodynamicthermodynamic processes,processes, mostlymostly associatedassociated withwith highhigh temperaturestemperatures [[55].55].

Figure 2.2. Time seriesseries ofof October’sOctober’s standardizedstandardized precipitation–evapotranspiration precipitation–evapotranspiration index index (SPEI-6) (SPEI-6) and standardizedand standardized precipitation precipitat indexion index (SPI-6) (SPI-6) averaged averaged over over the BRB the duringBRB during 1980–2017. 1980–2017.

According to [28], [28], the the intensity intensity of of a a drought drought event event is is arbitrarily arbitrarily defined defined for for values values of ofthe the SPI SPI with with the thefollowing following categories: categories: mild milddrought drought if 0 ≤ SPI if 0 ≤≤ −0.99,SPI moderate≤ −0.99, moderatedrought if drought−1.00 ≤ SPI if − ≤1.00 −1.49,≤ SPI severe≤ − drought1.49, severe if 1.50 drought ≤ SPI if≤ 1.50−1.99≤ andSPI ≤extreme −1.99 anddrought extreme for SPI drought ≤ −2. forRecent SPI ≤studies—e.g., −2. Recent [42,56–59]—classified studies—e.g., [42,56–59 dr]—classifiedought events drought with identical events withthresholds identical for thresholdsSPEI and SPI. for Therefore, SPEI and SPI. we identified Therefore, seven we identified moderate seven to extreme moderate events to extreme in the years events 1982, in the1986, years 1987, 1982, 1994, 1986, 1997, 1987, 2005 1994, and 1997,2009. 2005 and 2009. Table1 1 summarizes summarizes the the main main characteristics characteristics of of these these drought drought events, events, including including the the yearyear andand averageaverage intensity, intensity, based based on on SPEI-6 SPEI-6 and and the the number number of of months months with with meteorological meteorolog- drought and the driest months based on SPEI-1 (in bold letter). As mentioned before, SPI ical drought and the driest months based on SPEI-1 (in bold letter). As mentioned before, values are very similar and they are not shown. The year 1982 is identified as the year with SPI values are very similar and they are not shown. The year 1982 is identified as the year the most intense drought episode for the study period. All episodes had meteorological with the most intense drought episode for the study period. All episodes had meteorolog- droughts with a duration of roughly 4 months. June was the peak month for 2005 and July ical droughts with a duration of roughly 4 months. June was the peak month for 2005 and was the peak month for 1986, 1994 and 2009. The droughts of 1982 and 1997 peaked in July was the peak month for 1986, 1994 and 2009. The droughts of 1982 and 1997 peaked August, and that of 1987 peaked in October. It is interesting to mention that five of these in August, and that of 1987 peaked in October. It is interesting to mention that five of these top-seven drought events occurred during El Niño - Southern Oscillation (ENSO) warm phases (1982, 1987, 1994, 1997, 2009) being three of them during strong El Niño years (1982, 1987, 1997). The other two drought events developed under ENSO neutral conditions (1986, 2005). It is widely accepted that dry conditions prevail in the southern part of Mexico Atmosphere 2021, 12, x FOR PEER REVIEW 6 of 16

top-seven drought events occurred during El Niño - Southern Oscillation (ENSO) warm phases (1982, 1987, 1994, 1997, 2009) being three of them during strong El Niño years Atmosphere 2021, 12, 368 6 of 16 (1982, 1987, 1997). The other two drought events developed under ENSO neutral condi- tions (1986, 2005). It is widely accepted that dry conditions prevail in the southern part of Mexico during El Niño summers, and wetter conditions dominate during La Niña duringthroughout El Niño the summers,American andtropical wetter regions conditions including dominate southern during parts La of Niña Mexico throughout [60–62]. the American tropical regions including southern parts of Mexico [60–62]. TableSevere 1. Drought and seasonal extreme episodes drought observed events over that the occurred BRB for in the the period period 1980–2017. 1980–2017 were se- lected for analysis in this study, corresponding to events in 1982 and 2005. The spatial SPEI-1 representations of SPEI-6Drought values SPEI-1 in October SPEI-1 for these SPEI-1 two episodes SPEI-1 show different patternsSPEI-1 Year SPEI-6 Septem- (Figure3). During 1982Intensity (Figure 3a),May drought June conditions July covered August an extensive areaOctober over central Mexico, and wet conditions remained confined over Baja Californiaber and a portion of the1982 Yucatan −2.15 Peninsula. Extreme By contrast, 0.90 in 2005− (Figure1.39 3b),−1.35 the eastern−1.96 coast of−1.95 central Mexico0.32 and1986 the north–central−1.36 Moderate part of the country 0.51 also 0.36 showed −1.45 wet conditions.−0.85 Both−1.40 episodes −0.20 were particularly1987 −1.41 severe Moderate over the BRB. −0.93 −0.55 1.02 −1.03 −0.15 −1.86 1994 −1.11 Moderate −0.43 −0.86 −1.71 0.40 −0.93 1.02 Table 1. Drought1997 seasonal−1.40 episodesModerate observed over 0.03 the BRB−0.65 for the period−0.76 1980–2017.−1.41 −0.92 0.79 Drought2005 −1.61SPEI-1 SevereSPEI-1 −1.04 SPEI-1−1.89 SPEI-10.38 0.29SPEI-1 −1.84 SPEI-1−0.17 Year SPEI-6 Intensity2009 −1.30May ModerateJune −0.16 July−0.50 August−1.86 −September1.36 0.44 October 1.05 1982 −2.15 Extreme 0.90 −1.39 −1.35 −1.96 −1.95 0.32 Severe and extreme drought events that occurred in the period 1980–2017 were se- 1986 −1.36 Moderate 0.51 0.36 −1.45 −0.85 −1.40 −0.20 lected for analysis in this study, corresponding to events in 1982 and 2005. The spatial 1987 −1.41 Moderaterepresentations−0.93 of SPEI-6 −values0.55 in October 1.02 for these− two1.03 episodes −show0.15 different− patterns1.86 1994 −1.11 Moderate(Figure 3). During−0.43 1982 (Figure−0.86 3a), drought−1.71 conditions0.40 covered −an0.93 extensive area 1.02 over 1997 −1.40 Moderatecentral Mexico, 0.03 and wet conditions−0.65 remained−0.76 confined−1.41 over Baja California−0.92 and a 0.79 portion of the Yucatan Peninsula. By contrast, in 2005 (Figure 3b), the eastern coast of central Mex- 2005 −1.61 Severe −1.04 −1.89 0.38 0.29 −1.84 −0.17 ico and the north–central part of the country also showed wet conditions. Both episodes − − − − − 2009 1.30 Moderatewere particularly0.16 severe over0.50 the BRB. 1.86 1.36 0.44 1.05

FigureFigure 3.3. SPEI-6 valuesvalues inin OctoberOctober forfor ((aa)) 19821982 andand ((bb)) 2005.2005.

3.2. Water Vapor Transport towardtoward thethe BalsasBalsas RiverRiver BasinBasin To betterbetter understandunderstand the the origin origin of of the the precipitation precipitation deficits, deficits, we we analyzed analyzed the the anomalies anoma- oflies moisture of moisture transport transport toward toward the BRB the BRB during du thering drought the drought events. events. Figure Figure4 shows 4 shows the sum the of (E–P) > 0 up to 10 days (n = 1, ... , 10) before an air mass that moved towards the sum of (E–P)n n > 0 up to 10 days (n = 1, …, 10) before an air mass that moved towards the BRB reached the area ((E–P) > 0 from now on) from May to October for the study BRB reached the area ((E–P)1–101–10 > 0 from now on) from May to October for the study period period1980–2017. 1980–2017. Figure Figure4 also 4depicts also depicts the vertically the vertically integrated integrated moisture moisture flux flux(VIMF) (VIMF) and andgeo- geopotential height isolines at 925 hPa for the same study period and the same months. potential height isolines at 925 hPa for the same study period and the same months. In In May, positive water vapor values were found in the northern and north-western BRB May, positive water vapor values were found in the northern and north-western BRB along the Pacific coast (Figure4a), linked to the circulation of the North Pacific Subtropical along the Pacific coast (Figure 4a), linked to the circulation of the North Pacific Subtropical High (NPSH). A smaller amount of moisture arrived following the Caribbean Low-Level High (NPSH). A smaller amount of moisture arrived following the Caribbean Low-Level Jet (CLLJ) region. From June to September (JJAS) the distribution of water vapor fluxes during each month was fairly similar; therefore, the 4 month average is shown in Figure4b. The NPSH system moved northwestward, the influence of the North Atlantic Subtropical High (NASH) on the Caribbean was stronger and the trade winds intensified along its southern flank. Particles transporting moisture spread from the Yucatan Peninsula to the Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 16

Jet (CLLJ) region. From June to September (JJAS) the distribution of water vapor fluxes during each month was fairly similar; therefore, the 4 month average is shown in Figure Atmosphere 2021, 12, 368 7 of 16 4b. The NPSH system moved northwestward, the influence of the North Atlantic Subtrop- ical High (NASH) on the Caribbean was stronger and the trade winds intensified along its southern flank. Particles transporting moisture spread from the Yucatan Peninsula to Caribbeanthe Caribbean Sea, extending Sea, extending to the tropicalto the tropical North AtlanticNorth Atlantic Ocean. OurOcean. results Our confirm results thatconfirm the CLLJthat the core CLLJ south core of thesouth Greater of the Antilles Greater [ 63Antilles,64] is an[63,64] important is an important moisture sourcemoisture for source the BRB for duringthe BRB boreal during summer boreal (Figure summer4b), as(Figure pointed 4b), out as by pointed other previous out by other research previous works [research62,65]. Duringworks [62,65]. October, During the NASH October, migrates the NASH north-eastward, migrates north-eastward, and its western and flank its causeswestern more flank watercauses vapour more water to arrive vapour to the to BRB arrive from to the BR northB from of the the Gulf north of Mexicoof the Gulf (Figure of Mexico4c). From (Fig- Mayure 4c). to October From May (Figure to October4a–c), evaporation (Figure 4a–c), clearly evaporation dominates clearly over precipitationdominates over in theprecipi- air massestation in located the air over masses the basinlocated itself over and the the basin surrounding itself and areas.the surrounding areas.

−1−1 FigureFigure 4. 4.Averaged Averaged values values of of (E–P) (E–P)1−110−10> >0 0 forfor allall thethe particlesparticles moving moving towards towards the the BRB BRB region region (color (color shades, shades, mm mm·day∙day ),), verticallyvertically integrated integrated moisturemoisture fluxflux (vectors,(vectors, kg ∙·m−−1∙1s·−s1)− and1) and geopotential geopotential isolines isolines at 925 at 925hPa hPa(solid (solid lines, lines, m−2∙s m−2)− in2· s(a−)2 May,) in (a()b May,) June–September (b) June–September (JJAS) and (JJAS) (c) andOctober. (c) October. Study period: Study period: 1980–2017. 1980–2017.

ToTo quantify quantify water water vapor vapor contributions, contributions, it it is is necessary necessary to to define define the the moisture moisture source source regionsregions independently. independently. TheThe thresholdthreshold of the 95th 95th percentile percentile of of the the (E–P) (E–P)1–101–10 > 0> values 0 values av- averagederaged from from May May to to October October during during the the stud studyy period (1980–2017)(1980–2017) waswas usedused to to delimit delimit the the sourcesource regions regions in in which which the the water water vapor vapor recharge recharge was was most most intense. intense. Seven Seven main main moisture moisture sourcessources thatthat werewere potentially important important for for precipitation precipitation development development over over BRB BRB were were de- definedfined (Figure (Figure 5a),5a), comprising comprising four four oceanic oceanic an andd three three terrestrial terrestrial sources. sources. The The moisture moisture ar- arrivingriving from from the the ocean ocean was was divided divided into into four four regions: regions: the Caribbean the Caribbean Sea (CAR), Sea (CAR), the Gulf the of GulfMexico of Mexico (GOM), (GOM), the rest the of the rest tropical of the tropical Atlantic Atlantic (ATL) and (ATL) the andPacific the (PAC) Pacific from (PAC) the from west. theThe west. terrestrial The terrestrial evaporative evaporative moisture moisturesource regions source were regions classified were classifiedinto three intocategories: three categories:north of the north BRB of (NORTH), the BRB (NORTH), the southern the BR southernB region BRB including region includingthe Yucatan the Peninsula Yucatan Peninsula(SOUTH), (SOUTH), and finally and the finally BRB the itself BRB (BRB). itself (BRB).The monthly The monthly evolution evolution of (E–P) of (E–P)1–10 >1–10 0 inte-> 0 integratedgrated over over these these seven seven areas areas is shown is shown in Fi ingure Figure 5b. As5b. expected, As expected, the evaporative the evaporative source sourceof BRB of itself BRB itselfis highly is highly significant, significant, representing representing by by far far the the main main atmosphericatmospheric moisturemoisture sourcesource during during the the wet wet season season (note (note that that the the y-axis y-axis of of Figure Figure6 b6b is is in in a a logarithmic logarithmic scale). scale). NORTHNORTH is is the the second second moisture moisture source source during during the the beginning beginning (May) (May) and and the the end end (October) (October) ofof the the wet wet season. season. However,However, from from June June to to September, September, the the contributions contributions of of SOUTH SOUTH and and CARCAR are are slightly slightly higher. higher. AccordingAccording to to these these results, results, the the importance importance of the of availablethe available moisture moisture in the in atmosphere the atmos- ofphere the BRBof the itself BRB must itself be must decisive be decisive compared comp withared the with rest the of rest the of moisture the moisture sources. sources. For simplicity,For simplicity, we limited we limited the analysis the analysis to the to source the source regions regions delineated delineated by the by 99.5th the percentile99.5th per- of the (E–P) > 0 values (black contours in Figure5a). Three moisture sources were centile of the1–10 (E–P)1–10 > 0 values (black contours in Figure 5a). Three moisture sources obtained:were obtained: the terrestrial the terrestrial evaporative evaporative source ofsour thece BRB, of the the BRB, Yucatan the PeninsulaYucatan Peninsula inner SOUTH inner andSOUTH the central and the area central of CAR. area of CAR.

Atmosphere 2021, 12, x FOR PEER REVIEW 8 of 16

Figure 5. (a) Name and geographical limits of the moisture sources defined for the BRB region (in colors). Black solid lines delimit the regions whose contributions exceed the 99.5th percentile. (b) Monthly (E–P)1–10 > 0 values for the seven areas defined as moisture sources: Balsas River Basin (BRB), Caribbean Sea (CAR), Gulf of Mexico (GOM), tropical Atlantic (ATL), Pacific (PAC), north of the BRB (NORTH) and southern BRB (SOUTH).

Atmosphere 2021, 12, 368 8 of 16 Atmosphere 2021, 12, x FOR PEER REVIEWThe above moisture sources show interannual variability. The bars in Figure 6 repre-8 of 16 sent the accumulated monthly anomalies of the water vapor supplied by these three mois- ture sources (BRB: blue, SOUTH: pink and CAR: red; black solid lines in Figure 5a) for the severe and the extreme drought events that occurred in the period 1980–2017. In 1982 (Figure 6a), unexpected positive anomalies of moisture supplies were found for the three sources during the 6 months, including the month of August, when the peak value of SPEI-1 occurred (see Table 1). In contrast, during 2005 (Figure 6b), all source regions showed lower contributions compared with the climatological mean; therefore, the lack of water vapor was associated with precipitation deficits. It is worth pointing out that the large-scale circulation variability in local BRB conditions was far more important than the moisture transport variability from the other two top-contributing external sources, SOUTH and CAR. To explain the drought signal in 1982 despite the positive anomalies found in moisture transport, we also analyzed the evolution of omega anomalies at 500- hPa averaged over the BRB area (Figure 6a). This drought event was associated with pos-

Figure 5. (a) Name and geographicalitive anomalies limits of of theomega moisture from sources June to defined October, for the indicating BRB region a general (in colors). meteorological Black solid lines pat- Figure 5. (a) Name and geographical limits of the moisture sources defined for the BRB region (in colors). Black solid lines delimit the regions whosetern contributions of subsidence exceed that the 99.5thinhibits percentile. the upward (b) Monthly movement (E–P) needed1–10 > 0 values to generate for the precipitation. seven areas delimit the regions whose contributions exceed the 99.5th percentile. (b) Monthly (E–P)1–10 > 0 values for the seven areas defined as moisture sources:In Balsas2005, besides River Basin the (BRB), negative Caribbean anomalies Sea (CAR), of water Gulf ofvapor Mexico transport (GOM), tropicalfound, Atlanticsubsidence (ATL), was defined as moisture sources: Balsas River Basin (BRB), Caribbean Sea (CAR), Gulf of Mexico (GOM), tropical Atlantic also present at a 500 hPa level over the basin during most of the months. Pacific(ATL), (PAC), Pacific north (PAC), of thenorth BRB of (NORTH)the BRB (NORTH) and southern and southern BRB (SOUTH). BRB (SOUTH).

The above moisture sources show interannual variability. The bars in Figure 6 repre- sent the accumulated monthly anomalies of the water vapor supplied by these three mois- ture sources (BRB: blue, SOUTH: pink and CAR: red; black solid lines in Figure 5a) for the severe and the extreme drought events that occurred in the period 1980–2017. In 1982 (Figure 6a), unexpected positive anomalies of moisture supplies were found for the three sources during the 6 months, including the month of August, when the peak value of SPEI-1 occurred (see Table 1). In contrast, during 2005 (Figure 6b), all source regions showed lower contributions compared with the climatological mean; therefore, the lack of water vapor was associated with precipitation deficits. It is worth pointing out that the large-scale circulation variability in local BRB conditions was far more important than the moisture transport variability from the other two top-contributing external sources, SOUTH and CAR. To explain the drought signal in 1982 despite the positive anomalies found in moisture transport, we also analyzed the evolution of omega anomalies at 500- Figure 6. Monthly accumulated anomalies of (E–P) > 0 for the three moisture sources contoured FigurehPa 6. averaged Monthly accumulatedover the BRB anomalies area (Figure of (E–P) 6a).1 - 1This10− >010 fordrought the three event moisture was associatedsources contoured with pos- by black solid lines in the Figure5a (colored bars, left black y-axis) and omega anomalies (blue lines; by blackitive anomaliessolid lines in of the omega Figure from 5a (colored June toba October,rs, left black indicating y-axis) and a generalomega anomalies meteorological (blue pat- right, blue y-axis) during the drought episodes of (a) 1982 and (b) 2005. lines;tern right, of subsidenceblue y-axis) durithatng inhibits the drought the upward episodes movement of (a) 1982 and needed (b) 2005. to generate precipitation. In 2005, besides the negative anomalies of water vapor transport found, subsidence was The above moisture sources show interannual variability. The bars in Figure6 rep- resentalso present the accumulated at a 500 hPa monthly level over anomalies the basin of during the water most vapor of the suppliedmonths. by these three moisture sources (BRB: blue, SOUTH: pink and CAR: red; black solid lines in Figure5a) for the severe and the extreme drought events that occurred in the period 1980–2017. In 1982 (Figure6a), unexpected positive anomalies of moisture supplies were found for the three sources during the 6 months, including the month of August, when the peak value of SPEI-1 occurred (see Table1). In contrast, during 2005 (Figure6b), all source regions showed lower contributions compared with the climatological mean; therefore, the lack of water vapor was associated with precipitation deficits. It is worth pointing out that the large-scale circulation variability in local BRB conditions was far more important than the moisture transport variability from the other two top-contributing external sources, SOUTH and CAR. To explain the drought signal in 1982 despite the positive anomalies found in moisture transport, we also analyzed the evolution of omega anomalies at 500-hPa averaged over the BRB area (Figure6a). This drought event was associated with positive anomalies of omega from June to October, indicating a general meteorological pattern of subsidence that inhibits the upward movement needed to generate precipitation. In

2005,Figure besides 6. Monthly the negativeaccumulated anomalies anomalies of waterof (E–P) vapor1 - 10 >0transport for the three found, moisture subsidence sources contoured was also presentby black at solid a 500 lines hPa in levelthe Figure over 5a the (colored basin during bars, left most black of y-axis) the months. and omega anomalies (blue lines; right, blue y-axis) during the drought episodes of (a) 1982 and (b) 2005.

Atmosphere 2021, 12, x FOR PEER REVIEW 9 of 16

3.3. Climatic Fields Anomalies during the Droughts of 1982 and 2005 A more detailed examination of the underlying causes of moisture advection and subsidence anomalies during the droughts of 1982 and 2005 could be performed through the assessment of the atmospheric circulation and vertical motions.

3.3.1. Drought Event in 1982 Anomalies of horizontal wind at 850 hPa and SST for the drought of 1982 are shown in Figure 7a. The SST pattern shows El Niño-like conditions over the tropical Pacific Ocean (Figure 7a). SST anomalies over the eastern tropical Pacific and those over the tropical North Atlantic (TNA) showed opposite signs. The SST gradients between the tropical Pa- Atmosphere 2021, 12, 368 cific and the TNA favor a stronger southern branch of the CLLJ via Walker circulation9 of 16 [66]. Other authors have previously shown similar results; that is, a strong southern branch of the CLLJ occurring in conjunction with warm Pacific SST anomalies and cold SST anomalies in the TNA [64,66–73]. This configuration favors greater moisture transport 3.3.toward Climatic the BRB, Fields although Anomalies it duringis important the Droughts to keep of in 1982 mind and that 2005 CLLJ is a secondary mois- ture Asource more for detailed the BRB, examination as stated in of the the previous underlying section. causes of moisture advection and subsidenceNegative anomalies vertical duringvelocity the (upward droughts movement, of 1982 and in 2005green) could anomalies be performed at 600 hPa through (Fig- theure assessment7b) were found of the over atmospheric the southern circulation part of the and Pacific vertical Inter motions. Tropical Convergence Zone (ITCZ—the ITCZ is marked approximately by the blue line in Figure 7b). Positive anom- 3.3.1.alies (subsidence, Drought Event in pink) in 1982 were present in the Atlantic ITCZ and northern South America. OverAnomalies the northern of horizontalareas of the wind eastern at 850 Pacifi hPac ITCZ, and SST positive for the anomalies drought of were 1982 also are present shown inand Figure reached7a. the The BRB SST region, pattern which shows is El consiste Niño-likent with conditions a southward over thedisplacement tropical Pacific of the OceanPacific (FigureITCZ. Other7a). SST authors anomalies have shown over the similar eastern results tropical for El Pacific Niño and years. those For overexample, the tropicalglobal ENSO-altered North Atlantic Walker (TNA) and showed Hadley opposite circulations signs. have The been SST shown gradients by betweenanalyzing the at- tropicalmospheric Pacific reanalysis and the products TNA favor [74,75], a stronger with the southerneastern Pacific branch ITCZ of the moving CLLJ closer via Walker to the circulationequator during [66]. El Other Niño authorssummers. have Intense previously convection shown in the similar eastern results; tropical that Pacific is, a around strong southern5°N has branchbeen reported of the CLLJ to produce occurring an inintense conjunction regional with Hadley warm cell Pacific with SST anomalously anomalies andstrong cold subsidence SST anomalies over inMexico the TNA that [ 64inhibits,66–73 the]. This development configuration of deep favors convective greater moisture activity transport[43]. Nevertheless, toward the it BRB,is unclear although how it an is importantincrease of to moisture keep in mindand simultaneous that CLLJ is a subsidence secondary moistureover the BRB source can for be the compatible. BRB, as stated in the previous section.

FigureFigure 7.7. (a) SST anomalies anomalies (°C, (◦C, contours) contours) and and hori horizontalzontal wind wind anomalies anomalies at at850 850 hPa hPa (m (m∙s−1·,s arrows),−1, arrows), (b) vertical (b) vertical velocity ve- locityanomalies anomalies (Pa∙s−1 (Pa, contours)·s−1, contours) at 600 hPa. at 600Period: hPa. wet Period: season wet (May-Octob season (May-October)er) during 1982. during Blue lines 1982. show Blue the lines seasonal show mean the seasonal(May–October) mean (May–October) precipitation values precipitation in the 1981–2010 values in theperiod 1981–2010 greater period than 5 greatermm in thanthe ITCZ 5 mm (Inter in the Tropical ITCZ (Inter Convergence Tropical ConvergenceZone). Zone).

NegativeThe Atlantic vertical trade velocity wind (upwardareas are movement, sometimes in dominated green) anomalies by extensive at 600 hPainversions(Figure 7thatb) wereform foundin the overdescending the southern branches part of of the the Hadley Pacific Intercirculation, Tropical resulting Convergence from the Zone interaction (ITCZ— thebetween ITCZ this is marked large-scale approximately subsiding air by and the conv blueection-driven line in Figure rising7b). air Positive from the anomalies lower lev- (subsidence,els [76–79]. Trade in pink) wind were inversions present in (TWIs) the Atlantic limit ITCZvertical and development, northern South confining America. the Over hu- themidity northern in the areas lower of levels. the eastern Criteria Pacific for ITCZ,inversion positive identification anomalies have were been also presenttraditionally and reachedbased on the temperature, BRB region, relative which humidity is consistent and with mixing a southward ratio profiles displacement in tropical ofand the subtrop- Pacific ITCZ.ical areas Other [78]. authors Equivalent have shownpotential similar temperature results for(θE) El profiles Niño years.have been For example,recently used global to ENSO-alteredcapture the signature Walker and of TWIs Hadley in circulationsPuerto Rico have [25,80]. been Figure shown 8 bydepicts analyzing the 1982 atmospheric seasonal reanalysis products [74,75], with the eastern Pacific ITCZ moving closer to the equator during El Niño summers. Intense convection in the eastern tropical Pacific around 5◦N has been reported to produce an intense regional Hadley cell with anomalously strong subsidence over Mexico that inhibits the development of deep convective activity [43]. Nevertheless, it is unclear how an increase of moisture and simultaneous subsidence over the BRB can be compatible. The Atlantic trade wind areas are sometimes dominated by extensive inversions that form in the descending branches of the Hadley circulation, resulting from the interaction between this large-scale subsiding air and convection-driven rising air from the lower levels [76–79]. Trade wind inversions (TWIs) limit vertical development, confining the humidity in the lower levels. Criteria for inversion identification have been tradition- ally based on temperature, relative humidity and mixing ratio profiles in tropical and subtropical areas [78]. Equivalent potential temperature (θE) profiles have been recently used to capture the signature of TWIs in Puerto Rico [25,80]. Figure8 depicts the 1982 Atmosphere 2021, 12, x FOR PEER REVIEW 10 of 16

anomalies of θE, temperature and relative humidity profiles averaged over the BRB region. The value of θE sharply decreases with height from 850 to 500 hPa due to the strong de- crease in relative humidity with height compared with the subtle increase in temperature across the inversion. To the best of our knowledge, no previous studies of TWIs exist in Mexico, making it difficult to assess the intensity of this seasonal mean inversion. IN com- parison with the Caribbean, where the strength of the inversions during July ranges be- tween 0.5 to 1.0 °C [76], the mean value of ΔT = 0.2 °C (Figure 8) seems to be weak. There- fore, it could be possible that this TWI should allow some cloud development to produce moderate rains that weaken rapidly by the entrainment of dry air due to subsidence in the free troposphere [25]. Anomalies of relative humidity are positive below 700 hPa, which is consistent with the increase in the column content of moisture diagnosed by FLEXPART. Anomalous subsidence prevailed over the BRB in almost all vertical levels during almost all seasons (Figure 8) except May, as also shown in Figure 6a for the mid troposphere. In summary, in 1982, despite a higher moisture contribution, convection was inhib- ited further over the BRB, leading to precipitation deficits. The reason was the develop- ment of a southern and intense ITCZ over the eastern tropical Pacific, producing an anom- Atmosphere 2021, 12, 368 alous ascent that resulted in a strengthened regional Hadley circulation to the north.10 The of 16 anomalous Hadley circulation generated stronger subsidence over the BRB, inhibiting the development of deep convection. However, a greater moisture content was trapped below an anomalous trade wind inversion. seasonal anomalies of θE, temperature and relative humidity profiles averaged over the ENSO provides a clear connection with the position of the ITCZ in the equatorial BRB region. The value of θE sharply decreases with height from 850 to 500 hPa due to easternthe strong Pacific, decrease typically in relative migrating humidity southward with from height its comparednortherly posi withtion the during subtle El increase Niño eventsin temperature [74,81]. Five across of the the seven inversion. moderate To theto extreme best of ourdrought knowledge, events identified no previous in the studies pre- sentof TWIs work exist took inplace Mexico, under making El Niño it conditio difficultns, to assesssuggesting the intensity the ENSO of index this seasonal to be a useful mean predictorinversion. of IN droughts comparison over withthe BRB. the Caribbean, However, whereover the the past strength two decades, of the inversions El Niño events during haveJuly rangesweakened, between and their 0.5 to SST 1.0 anomalies◦C[76], the have mean shifted value westward of ∆T = 0.2 towards◦C (Figure the 8central) seems Pa- to cific,be weak. impeding Therefore, southward it could migration be possible toward that thisthe TWIEquator should of the allow ITCZ some [82]. cloud In fact, develop- alt- houghment tothe produce 2015 event moderate was classified rains that as canonica weaken rapidlyl El Niño, by this the event entrainment exhibited of several dry air fea- due turesto subsidence distinct from in the the freeprevious troposphere strong El [25 Niño,]. Anomalies and the eastern of relative Pacific humidity ITCZ failed are positiveto cross southbelow of 700 the hPa, Equator which even is consistent at the El Niño with peak. the increase Despite in the the large column magnitude content of moisturethe 2015 event,diagnosed the drought by FLEXPART. over theAnomalous BRB did not subsidence reach the category prevailed of over moderate the BRB (see in Figure almost 2). all Howvertical ENSO levels phenomenon during almost should all seasons change (Figurewith anthropogenic8) except May, warming as also shown is still inan Figure ongoing6a researchfor the mid [83]. troposphere.

− FigureFigure 8. 8. VerticalVertical profile profile of of monthly monthly vertical vertical velo velocitycity anomalies anomalies from from 925 925 hPa hPa to to 300 300 hPa hPa (hPa (hPa∙s−·1s, 1, contours)contours) over over the the BRB BRB during during the the wet wet season season (t (topop x-axis). Seasonal mean (May–October) anomaliesanoma- lies(bottom (bottom x-axis) x-axis) of profilesof profiles of equivalentof equivalent potential potential temperature temperature (K, (K, solid solid line), line), relative relative humidity humidity (%, (%,dotted dotted line) line) and and temperature temperature (K, (K, dashed dashed line). line).

3.3.2. InDrought summary, Event in 1982,in 2005 despite a higher moisture contribution, convection was inhibited further over the BRB, leading to precipitation deficits. The reason was the development of a Regarding the 2005 wet season (Figure 9a) the most significant signal was that SST southern and intense ITCZ over the eastern tropical Pacific, producing an anomalous ascent anomalies over the Caribbean Sea and the TNA were high (>0.8 °C). Negative anomalies that resulted in a strengthened regional Hadley circulation to the north. The anomalous Hadley circulation generated stronger subsidence over the BRB, inhibiting the development of deep convection. However, a greater moisture content was trapped below an anomalous trade wind inversion. ENSO provides a clear connection with the position of the ITCZ in the equatorial eastern Pacific, typically migrating southward from its northerly position during El Niño events [74,81]. Five of the seven moderate to extreme drought events identified in the present work took place under El Niño conditions, suggesting the ENSO index to be a useful predictor of droughts over the BRB. However, over the past two decades, El Niño events have weakened, and their SST anomalies have shifted westward towards the central Pacific, impeding southward migration toward the Equator of the ITCZ [82]. In fact, although the 2015 event was classified as canonical El Niño, this event exhibited several features distinct from the previous strong El Niño, and the eastern Pacific ITCZ failed to cross south of the Equator even at the El Niño peak. Despite the large magnitude of the 2015 event, the drought over the BRB did not reach the category of moderate (see Figure2). How ENSO phenomenon should change with anthropogenic warming is still an ongoing research [83].

3.3.2. Drought Event in 2005 Regarding the 2005 wet season (Figure9a) the most significant signal was that SST anomalies over the Caribbean Sea and the TNA were high (>0.8 ◦C). Negative anomalies Atmosphere 2021, 12, x FOR PEER REVIEW 11 of 16

of low-level wind over the CLLJ core and a cyclonic circulation over the Gulf of Mexico were evident, inhibiting the water transport from the CAR region, which is in agreement with the lower moisture over BRB, SOUTH and CAR found during this drought event in the previous section. High SST anomalies (Figure 9a) over the TNA increase the atmospheric water vapor content and are critical to promote disturbances that can develop into tropical storms. Consistently, positive vertical velocity anomalies (upward motion) over the Caribbean Sea and the TNA are found in Figure 9b. In fact, the 2005 Atlantic hurricane season was the most active on record until the 2020 season, which exceeded that season [84]. The year 2005 also reached the highest SST anomalies (0.9 °C) and the largest numbers of named Atmosphere 2021, 12, 368 storms and hurricanes recorded [85]. 11 of 16 Figure 9b also shows negative geopotential height anomalies at 850 hPa (red lines) and positive anomalies at 200 hPa (blue lines) north of the convective regions (green con- tours). These anomalies are consistent with Gill’s model of off-equatorial heating [86], ofwhose low-level solution wind is a over Rossby the wave CLLJ propagating core and a cyclonic westward circulation from the overforcing the region Gulf of with Mexico neg- wereative/positive evident, inhibitingpressure anomalies the water at transport the lower/upper from the levels. CAR region, Gill’s model which also is in explains agreement the withoccurrence the lower of subsidence moisture over elsewhere BRB, SOUTH around and the CARrising found motion during associated this drought with the event warm- in theing previousregion. section.

FigureFigure 9.9. ((aa)) SSTSST anomaliesanomalies ((°C,◦C, contours)contours) andand horizontalhorizontal windwind anomaliesanomalies atat 850850 hPahPa (arrows)(arrows) duringduring 2005,2005, ((bb)) verticalvertical velocityvelocity anomaliesanomalies (Pa(Pa·s∙s−−1,, contours) contours) at at 600 600 hPa hPa during 2005 with negativenegative geopotential height anomaliesanomalies atat 850850 hPahPa (red(red lines)lines) andand positivepositive atat 200200 hPa hPa (blue (blue lines). lines). Period:Period: wetwet seasonseason (May–October).(May–October).

HighFinally, SST it anomaliesis also necessary (Figure to9a) mention over the th TNAat, in increasethis case, the we atmospheric found inter-monthly water vapor var- contentiations of and θE profiles, are critical with to TWI-like promote profiles disturbances in May, that June can and develop September into tropicaldriven by storms. large- Consistently,scale subsidence positive (Figure vertical 6b), velocityand these anomalies types of (upwardprofiles were motion) not over observed the Caribbean during July, Sea andAugust the TNAand October are found (figures in Figure not shown).9b. In fact, the 2005 Atlantic hurricane season was the mostAs active an onoverview, record until the 2005 the 2020 drought season, was which mainly exceeded associated that seasonwith a [ 84reduced]. The yearmoisture 2005 alsosupply reached from thethe highest main sources. SST anomalies A cyclonic (0.9 ◦circulationC) and the largestover the numbers Gulf of ofMexico named resulted storms andfrom hurricanes the atmospheric recorded response [85]. to a Caribbean Sea-persistent SST heating anomaly and preventedFigure the9b alsomoisture shows from negative reaching geopotential the BRB. Besides height anomaliesthis water vapor at 850 scarcity, hPa (red general lines) andsubsidence positive took anomalies place over at 200 thehPa BRB (blue during lines) the northmajority of theof the convective months, regionsalso inhibiting (green contours).potential convection. These anomalies are consistent with Gill’s model of off-equatorial heating [86], whoseData solution from observations is a Rossby waveand model propagating simulation westwards suggest from that the there forcing is little region historical with negative/positiveprecedent for the pressure2005 SST anomaliesevent [87]. at The the cont lower/upperribution of levels. natural Gill’s climate model variability also explains and theanthropogenic occurrence forcing of subsidence to this key elsewhere event has around been evaluated. the rising The motion interaction associated of three with inde- the warmingpendent climate region. teleconnections has been related to this anomalously high SST in the southernFinally, Caribbean it is also [88], necessary tropical toNorth mention Atlantic that, (TNA), in this Atlantic case, we multidecadal found inter-monthly oscillation variations(AMO) and of AtlanticθE profiles, meridional with TWI-like mode (AMM) profiles climate in May, indices. June and Donner September et al. [88] driven found by large-scalethat thermal subsidence stress reaching (Figure the6b), 2005 and level these wo typesuld be of extremely profiles were rare not without observed any duringanthro- July,pogenic August forcing. and In October this case, (figures in contrast not shown). with the mechanism promoting the 1982 drought, the influenceAs an overview, of global the warming 2005 drought provides was amainly background associated level withthat increases a reduced the moisture risk of supplyfuture drought from the events main similar sources. to Athat cyclonic in 2005. circulation over the Gulf of Mexico resulted from the atmospheric response to a Caribbean Sea-persistent SST heating anomaly and

prevented the moisture from reaching the BRB. Besides this water vapor scarcity, general subsidence took place over the BRB during the majority of the months, also inhibiting potential convection. Data from observations and model simulations suggest that there is little historical precedent for the 2005 SST event [87]. The contribution of natural climate variability and anthropogenic forcing to this key event has been evaluated. The interaction of three independent climate teleconnections has been related to this anomalously high SST in the southern Caribbean [88], tropical North Atlantic (TNA), Atlantic multidecadal oscillation (AMO) and Atlantic meridional mode (AMM) climate indices. Donner et al. [88] found that thermal stress reaching the 2005 level would be extremely rare without any anthropogenic forcing. In this case, in contrast with the mechanism promoting the 1982 drought, the influence of global warming provides a background level that increases the risk of future drought events similar to that in 2005. Atmosphere 2021, 12, 368 12 of 16

4. Summary and Conclusions In this paper, the 6 month standardized precipitation–evapotranspiration index (SPEI-6) in October and the 6 month standardized precipitation index (SPI-6) in the same month were used to identify and rank drought episodes over the Balsas River Basin (BRB) from 1980 to 2017. According to the classification of McKee et al. [28], drought events were classified into mild, moderate, severe and extreme, and the most intense events (severe and extreme) were selected for study. The anomalies in moisture supply during the selected drought periods were evaluated by using a Lagrangian approach. Finally, climatic fields at several levels were analyzed in order to investigate the physical mechanisms of severe drought occurrence. The main conclusions of this study are summarized as follows: 1. The evolution of SPEI-6 and SPI-6 averaged over the BRB are quite similar during the studied period, suggesting a larger contribution of precipitation deficit than temperature excess to the drought occurrence. 2. Two major drought events—one severe (2005) and another of extreme intensity (1982)— were found in the period 1980–2017. The differences between the spatial patterns obtained by both indices (SPEI-6 and SPI-6) were much reduced, confirming the important role of precipitation in these two events vs. atmospheric evaporative demand driven by temperature increase in the Mexican territory. 3. The main moisture sources during the wet season (May–October) for the BRB were identified. The terrestrial evaporative source of BRB itself was shown to be by far the main moisture source during the entire wet season, followed by the southern BRB terrestrial region and the Caribbean Low-Level Jet (CLLJ) core region, which are more active from June to September. 4. During the drought event of 2005, an anomalously warm SST emerged over the Caribbean Sea. A persistent elevated SST over the Atlantic warm pool favored hurricane activity during this year. As a response to this heating, lower-level negative pressure anomalies and upper tropospheric positive pressure anomalies developed to the northwest, consistent with Gill’s model. As a consequence of the lower level cyclonic circulation over the Gulf of Mexico, negative anomalies of the CLLJ winds arose, and the BRB itself, CLLJ and SOUTH provided a decreased water vapor contribution. Additionally, this drought event was associated with subsidence that also prevented convection over the BRB in four of the six months. 5. The mechanisms associated with the the top drought event, which occurred in 1982, were also determined. An intense and southward-shifted Inter Tropical Convergence Zone (ITCZ) over the eastern tropical Pacific produced an intense regional Hadley circulation that generated anomalously strong subsidence over the BRB. This descent branch of the Hadley cell inhibited the development of deep convection, although this drought episode was concurrent with an increase in the moisture supply. A strong trade wind inversion (TWI) driven by this large-scale subsiding air from the upper troposphere confined the moisture content in the lower layers. A better understanding of drought causes is clearly of major scientific and social value. This work identifies the underlying physical mechanisms behind severe droughts in the BRB and generates an understanding that may help to address the major chal- lenge of their prediction. Seasonal forecasts of severe droughts would give an advance warning to manage the detrimental impacts on crops, livestock farming and household food insecurity associated with a decline in production. An additional knowledge of the water-hydroelectric supply nexus in combination with seasonal forecasts of droughts in the BRB would also give the opportunity to manage drought impacts in the energy sector early. Finally, Mexico’s vulnerability to drought by climate change has been highlighted in the literature. This work presents a starting point to study the evolution of physical mechanisms causing droughts in a changing environment. Atmosphere 2021, 12, 368 13 of 16

Author Contributions: L.G., R.N. and P.O. conceived and designed the experiments. A.E.M. de- veloped the formal analysis. A.E.M. and P.O. prepared the original draft. R.N., C.P.-O., R.G.-H. reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the project PAPIIT (DGAPA-UNAM) IN116120 “Caracteri- zación climática y sinóptica de las olas de calor en México”. EPhysLab is partially funded by Xunta de Galicia under the project “Programa de Consolidación e Estructuración de Unidades de Investigación Competitivas (Grupos de Referencia Competitiva)” (no. ED431C 2017/64-GRC), co-funded by the ERDF, in the agenda of the Operational Program Galicia 2014–2020. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.

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