Mexico, a vibrant country known for its rich culture, diverse landscapes, and significant historical impact, often sparks geographical questions, particularly concerning its location relative to Central America. While frequently associated with its southern neighbors, the geographical positioning of Mexico is more nuanced. This article delves into the complexities of Mexico’s geography, exploring its place within North America and its intricate relationship with Central America, especially through the lens of climate patterns and regional dynamics. Understanding Mexico’s location is not merely about drawing lines on a map; it’s about grasping the environmental and climatic factors that shape this pivotal region of the Americas.
Persistent and prolonged droughts, lasting over a year, have historically cast a long shadow over Mexico’s socioeconomic landscape. Evidence suggests that these climatic events have even played a role in significant historical shifts, such as the decline of the Mayan empire in the eighth century, and the devastating “Year of Hunger” in the late eighteenth century. In the twentieth century, prolonged droughts continued to impact Mexican society, even leading to international water disputes. Therefore, understanding the underlying mechanisms that trigger and sustain these droughts remains a critical concern for the region.
Mexico’s climate exhibits dramatic variations, ranging from the arid conditions of the Sonoran Desert in the northwest, receiving less than 100 mm of annual rainfall, to the tropical rainforests in the south, where rainfall can exceed 3000 mm annually. During the winter months, high pressure systems linked to the Intertropical Convergence Zone (ITCZ) in the eastern Pacific Ocean bring stable, dry conditions to much of Mexico. While studies using tree-ring chronologies provide insights into past climate conditions, they primarily reflect winter and spring patterns. Crucially, the majority of Mexico’s annual rainfall, over 60%, occurs during the Northern Hemisphere summer, from May-June to September-October. Consequently, any comprehensive analysis of prolonged drought in Mexico must prioritize these summer rains. During this season, trade winds and easterly waves transport moisture from the warm waters of the Americas into Mesoamerica, a region geographically spanning from central Mexico through Central America. While northern Mexico experiences persistent high pressure for much of the year, easterly waves and tropical cyclones can occasionally bring substantial rainfall, particularly to the northeastern states. In northwestern Mexico, the North American monsoon system, active from July to September, generates mesoscale convective systems and significant rainfall events.
Large-scale climate phenomena like El Niño-Southern Oscillation (ENSO) introduce interannual climate variability across the globe, including Mexico. During summer El Niño conditions, Mexico typically experiences below-average rainfall across most of the country. Conversely, La Niña summers tend to bring above-average precipitation. This increased rainfall during La Niña is likely due to a northward shift of the eastern Pacific ITCZ, weaker trade winds that favor easterly wave activity, and an increase in tropical cyclones in the Intra-Americas Seas (IAS), encompassing the Gulf of Mexico and the Caribbean Sea. In contrast, El Niño summers often suppress rainfall in central and southern Mexico due to an equatorward shift of the ITCZ, increased atmospheric subsidence, and reduced easterly wave and tropical cyclone activity in the IAS. Unlike the southwestern United States, El Niño does not typically result in increased rainfall in Mexico.
Water scarcity is a significant concern in northern Mexico, making the impacts of droughts particularly severe and prompting extensive socioeconomic analysis. However, both dry and wet periods are inherent aspects of climate variability throughout central and southern Mexico as well. Historical records, meteorological data, and climate proxies like marine and lake sediments and tree-ring data have been used to study prolonged dry periods in recent centuries. These studies have documented severe droughts in northern Mexico during the latter half of the sixteenth century (1545–1600) and periods such as 1752–68, 1801–13, 1859–68, the 1910s, 1930s, 1950s, and 1990s. Interestingly, the 1940s, 1970s, and mid-1980s saw relatively dry conditions in central-southern Mexico, contrasting with wetter conditions in the north, illustrating a seesaw pattern in precipitation anomalies. This contrasting pattern is a hallmark of decadal climate variability across Mexico and Central America. Tree-ring reconstructions of North American precipitation further reveal that prolonged droughts in northern Mexico, like the sixteenth-century event, often coincide with wetter conditions in the south, and vice versa, as seen with the 1630s drought in southern Mexico occurring during a wet period in the north.
The prevailing explanations for prolonged North American droughts often point to persistent sea surface temperature (SST) anomalies, specifically in the tropical Pacific Ocean, linked to the Pacific Decadal Oscillation (PDO), or in the subtropical North Atlantic Ocean, associated with the Atlantic Multidecadal Oscillation (AMO). The droughts of the 1930s and 1950s, for example, are linked to cooler tropical Pacific and warmer subtropical Atlantic conditions. Interdecadal shifts in the PDO have been shown to significantly alter the relationship between rainfall in Mexico and large-scale climate patterns. Northwestern Mexico’s winter rainfall fluctuations are typically in sync with the PDO, with positive PDO phases correlating with wetter winters and negative phases with drier ones. While winter climate across Mexico is primarily influenced by the tropical Pacific, with a lesser Atlantic influence, some research suggests that historical droughts in southern Mexico coincide with the negative AMO phase but can also be influenced by the positive PDO phase or El Niño conditions. Recurrent droughts are anticipated in central and southern Mexico during this PDO phase. Northwestern Mexico tends to be drier, experiencing reduced extreme daily precipitation during the positive AMO phase. However, the precise mechanisms by which AMO affects central and southern Mexico remain less understood.
Atmospheric subsidence from the Hadley cell is a mechanism that can modulate precipitation over Mexico. The meridional position of the Hadley cell is influenced by SST anomalies in the eastern Pacific. Consequently, ENSO and PDO impact the latitudinal position of subsidence and negative precipitation anomalies. However, this mechanism doesn’t fully account for the Atlantic SST influence on summer precipitation. Given the correlation between Atlantic SST anomalies and precipitation levels, it’s crucial to investigate how these anomalies affect rainfall. The tropical Atlantic can influence Mexico through easterly wave activity, moisture fluxes from the Caribbean Sea, and other atmospheric processes.
During the Northern Hemisphere summer, several dynamic factors are essential for understanding drought in Mexico in relation to persistent SST anomalies. This study aims to examine the regional spatial patterns of prolonged droughts in Mexico and Central America during the instrumental period (1903–2002) and their connection to low-frequency variability in the Pacific and Atlantic Oceans, represented by the PDO and AMO. It also explores regional drought characteristics, including the north-south extent of negative precipitation anomalies and east-west precipitation contrasts in northern Mexico. Furthermore, the study discusses the applicability of PDO and AMO in explaining nineteenth-century droughts in Mexico.
The study is structured into five sections, beginning with data and methodology, followed by an analysis of the relationships between PDO, AMO, and persistent regional precipitation anomalies in Mexico. The subsequent section discusses the primary findings in terms of the dynamic mechanisms that may link SST anomalies to regional climate in Mexico, culminating in a summary and conclusions.
Data and Methodology
This research utilizes drought indices derived from precipitation records. While drought indices can also be calculated using other meteorological variables like soil moisture, temperature, or hydrological data, this analysis focuses on precipitation deficits as the primary driver of meteorological droughts. The standardized precipitation index (SPI) is employed to analyze meteorological droughts, as it is a highly regarded measure of drought severity. SPI quantifies precipitation anomalies across different time scales (3, 6, 12, 24 months), tailored to the specific process under investigation. For instance, soil moisture responds to short-term precipitation anomalies, while groundwater, streamflow, and reservoir levels reflect longer-term precipitation patterns. SPI offers several advantages, including its simplicity, requiring only precipitation data and two parameters for calculation, its temporal flexibility for analyzing drought dynamics across various time scales, and its standardized format for characterizing the frequency and intensity of extreme wet or dry conditions. However, SPI also has limitations, such as the assumption that precipitation data can always be modeled by a suitable probability distribution, and its potential to misrepresent extreme wet and dry conditions over very long periods due to the equal frequency of droughts across locations over extended timeframes. Despite these limitations, SPI remains a valuable tool for characterizing prolonged droughts.
SPI calculation involves creating a frequency distribution from precipitation data at each location for a given time period. A gamma probability density function is fitted to the empirical precipitation frequency distribution for the chosen time scale, and the cumulative distribution of precipitation is determined. This cumulative distribution is then transformed into a standard normal distribution to obtain SPI values. Positive SPI values indicate above-median precipitation, while negative values indicate below-median precipitation. SPI values near zero represent median precipitation, and values ranging from -1 to +1 indicate near-normal precipitation. SPI values between -2 and -1 signify moderate to severe drought, and values below -2 indicate extreme drought.
SPI’s temporal versatility allows for analysis at different time scales. SPI-1 reflects short-term conditions relevant to soil moisture, SPI-3 provides insights into short-to-medium-term moisture conditions and seasonal precipitation estimates, and SPI-6 or SPI-9 characterizes medium-term precipitation trends, sensitive to moisture conditions and potentially linked to streamflow and reservoir levels. SPI-12 is used to reflect long-term precipitation patterns related to river and reservoir volumes, while this study utilizes SPI-24 months to characterize persistent drought, capturing low-frequency climate variability associated with long-term water resources. SPI-24 helps to minimize the influence of year-to-year climate variability signals.
Precipitation data for the twentieth century is relatively reliable due to the Mexican Weather Service’s digitized historical records. Data availability increases significantly after the 1930s and 1950s. For regions outside Mexico, monthly precipitation data was obtained from the Global Historical Climatological Network (GHCN) version 2. These precipitation records underwent quality control, including duplicate station checks and spatial consistency checks. Data quality was also assessed by examining the coherence of monthly precipitation among neighboring stations. A gridded monthly precipitation dataset was developed using the Cressman objective analysis scheme, interpolating data from climatological stations across Mexico, the United States, Central America, and the Caribbean. This dataset, spanning 1901–2002 with a 0.5° × 0.5° resolution, is available at the International Research Institute for Climate and Society (IRI).
The Pacific Decadal Oscillation (PDO) is defined as the first empirical orthogonal function (EOF) of monthly SST anomalies in the North Pacific Ocean (20°–65°N, 120°E–100°W). The PDO index, representing its time evolution, characterizes decadal Pacific Ocean variability. A positive PDO index indicates a cooler north-central Pacific and warmer North American west coast, with the opposite holding true for a negative PDO index.
The Atlantic Multidecadal Oscillation (AMO) is characterized as the first EOF of SST anomalies in the North Atlantic region (20°–65°N, 100°W–0°). The AMO’s spatial pattern is defined by SST anomalies across the North Atlantic basin, exhibiting a decadal timescale that influences interannual climate variations in regions like the United States. AMO and PDO time series data for the twentieth century (1903–2002) are accessible through the NOAA Climate Diagnostics Center (CDC) website. Nineteenth-century dry period analyses in Mexico utilize PDO proxy climate reconstructions based on tree-ring chronologies from Southern and Baja California, and tree-ring-based AMO index reconstructions.
Tropospheric wind data from the NCEP–NCAR reanalysis (1948–2002) is used to examine dynamical processes influencing precipitation anomalies in Mexico during the latter half of the twentieth century. Specifically, easterly wave (EW) activity in the IAS region is analyzed using the 3–9 day high-frequency variance of meridional wind at 700 hPa during June-September in the central Caribbean Sea (17.5°N, 70°W). The Caribbean low-level jet (CLLJ) intensity is calculated by averaging 925-hPa zonal wind anomalies multiplied by -1 over the region 12.5°–17.5°N, 80°–70°W.
Persistent Droughts in Mexico During the Twentieth Century
Analyzing annual precipitation anomalies over the last century (1903–2002) reveals a frequent inverse relationship between northern and southern Mexico, where dry periods in the north often coincide with wet periods in the south, and vice versa. This out-of-phase pattern is also evident in earlier prolonged drought periods, with northern Mexico precipitation anomalies contrasting with those in Central America.
The twentieth century witnessed significant droughts in northern Mexico during the 1930s, 1950s, and late 1990s. An intense drought in the late 1910s also affected much of Mexico and Texas, but limited data availability makes it challenging to fully map its spatial extent. The 1930s “Dust Bowl” drought (1934–39) is perhaps the most studied due to its severe impacts across North America. During the summer months, this drought extended into northwestern Mexico, near the U.S. border. Concurrently, significant positive precipitation anomalies occurred across most of Mesoamerica and parts of the Caribbean, demonstrating the characteristic north-south seesaw drought pattern.
The 1950s drought (1953–57) stands as one of the most severe events in recent North American history, impacting both the United States and Mexico. A large area of precipitation deficit encompassed northern Mexico and much of the United States. During this period, positive precipitation anomalies were observed over southern Mexico, the Greater Antilles, and Central America, except for a small part of Honduras. This drought pattern extended across a significant portion of North America, including parts of the southeastern United States.
The most recent persistent drought in North America occurred in the late 1990s (1996–2002), affecting northern Mexico and parts of Texas, impacting crucial river basins shared by both countries. The negative precipitation anomaly during this period was concentrated in northern Mexico and the southwestern United States, with positive anomalies in southern Mexico and Central America, again highlighting the seesaw pattern.
In the context of climate variability, dry episodes are often followed by wet periods. Relatively wet episodes in northern Mexico contrast with negative precipitation anomalies over Mesoamerica and the Caribbean region, as seen during the 1940s (1941–43), 1970s (1972–79), and 1980s (1985–88). These periods reflect the decadal seesaw pattern in North American precipitation variability. At times, these wet episodes also show negative precipitation anomalies in the eastern United States, exhibiting a zonal contrast in the SPI-24 pattern. Similar to drought patterns, the transition zone between wet and dry conditions is roughly around 20°N latitude.
The north-south dipole of low-frequency summer precipitation variability is further highlighted by the first two EOFs of seasonal (June–September) SPI-24 for North America, Mesoamerica, and the Caribbean. The leading mode (EOF1) displays opposing precipitation patterns between the United States–northern Mexico and Mesoamerica–Caribbean regions, exhibiting a clear seesaw structure with a transition zone around 20°N. This mode accounts for approximately 15% of the total variance. The second EOF (EOF2) shows a contrasting SPI-24 structure between northern Mexico–southern United States and the eastern United States, and between the Caribbean and western Mesoamerica. This mode explains about 8% of the total variance, presenting a zonal contrast potentially described as a quadrupole pattern. The specific spatial characteristics of droughts vary between episodes, with individual drought patterns representable as a linear combination of EOF1 and EOF2, indicating the relative contribution of each mode. The amplitudes of EOF1 and EOF2 help to characterize the regional drought magnitudes.
Analyzing the principal components (PCs) of EOF1 and EOF2 for 1903–2002 allows for reconstructing regional dry or wet signals in SPI-24 for both northern and southern Mexico. The 1930s drought can be examined through the positive phase of EOF1, with EOF2 transitioning from negative to slightly positive. This condition results in positive precipitation anomalies along the U.S.–Mexico border, limiting the drought’s extent in the central and northeastern United States. Closer examination reveals EOF2 becoming negative to zero from 1933–1934, during which the SPI-24 anomaly expands across much of the upper United States. After 1935, the drought concentrates in the central United States, extending into Mexico, coinciding with PC for EOF2 nearing zero. The average SPI-24 pattern for the 1930s primarily reflects EOF1, with Mesoamerica and the Caribbean showing positive precipitation anomalies.
The severe 1950s drought pattern in Mexico is a result of the combined effect of positive EOF1 and EOF2, peaking during 1953–57. This leads to a significant negative precipitation anomaly extending across the southern and Midwest United States and northern Mexico. The positive EOF2 sign results in positive SPI-24 anomalies over the northeastern United States, somewhat mitigating the drought’s intensity in that region. Mesoamerica exhibits a positive SPI-24 pattern driven by a strong PC1 loading.
During the 1990s drought, EOF2 is in its negative phase, causing a positive drought anomaly in SPI-24 in northern Mexico. PC1 is in a negative-to-positive transition along the U.S.–Mexico border by the late twentieth century, amplifying the drought’s severity. This is evident in the large negative precipitation anomaly in Chihuahua around 2000.
Similarly, wet periods in northern Mexico can be analyzed. The 1940s period corresponds to a strong negative EOF2 phase and a weaker EOF1, resulting in a drought pattern resembling EOF2. The 1970s SPI-24 pattern shows a large negative PC1 and a large positive PC2, combining to produce positive SPI-24 over the eastern United States and northeastern Mexico. Over Mesoamerica, PC1 and PC2 together create significant negative precipitation anomalies. The 1980s SPI-24 pattern, a combination of large negative PC1 and PC2, clearly shows negative precipitation anomalies over the eastern United States, Central America, and the Caribbean.
The combination of the first two SPI-24 EOFs effectively captures the regional characteristics of persistent droughts across the United States, Mexico, Central America, and the Caribbean. These EOFs are linked to low-frequency modes in the Atlantic and Pacific Oceans. PC1 of SPI-24 shows a positive correlation with AMO, particularly after the 1920s when SPI-24 data becomes more reliable. Conversely, PC2 of SPI-24 is negatively correlated with PDO. This characteristic SPI-24 spatial structure, driven by persistent SST anomalies, provides a framework for exploring the mechanisms that link SST anomalies to regional precipitation patterns across North America, Central America, and the Caribbean. The dynamic elements connecting these persistent SST anomalies, such as during AMO events, are primarily associated with changes in quasi-stationary circulation anomalies, like the Hadley cell and the low-level jet in the Gulf of Mexico. Analyzing precipitation anomalies over Mexico also necessitates considering easterly wave activity over the IAS, which interacts with the Caribbean low-level jet and tropical cyclones, significantly influencing Mexican precipitation. Given wind data limitations prior to 1948, subsequent analysis focuses on droughts occurring after this year.
Fig. 1: Time series of reconstructed tree-ring PDSI (20-yr low pass) highlighting drought patterns across different regions within Mexico.
Fig. 2: Normalized annual precipitation anomalies in Chihuahua (northern Mexico) and Chiapas (southern Mexico), illustrating the contrasting precipitation patterns between these regions.
Fig. 3: SPI-24 maps for Northern Hemisphere summer droughts during the 1930s, 1950s, and 1990s, showcasing the spatial extent of these drought events.
Fig. 4: SPI-24 maps for Northern Hemisphere summer wet periods during the 1940s, 1970s, and 1980s, contrasting with drought patterns and demonstrating climate variability.
Fig. 5: EOF1 and EOF2 of SPI-24 during NH summer, illustrating the dominant spatial patterns of precipitation variability and their explained variance.
Fig. 6: Principal components of EOF1 and EOF2 of SPI-24 compared with AMO and PDO indices, showing the correlation between precipitation patterns and ocean oscillations.
Mechanisms That Produce Prolonged Droughts in Mexico
Numerous studies exploring the physical mechanisms behind prolonged droughts emphasize the role of anomalous Sea Surface Temperatures (SSTs). For Mexico, understanding prolonged droughts requires considering the interconnected influence of both the Pacific and Atlantic Oceans. Quasi-stationary circulations are often cited as a way to link geographically distant regions to local climate effects. However, shifts in mean circulations can also stem from transient atmospheric activity, where the cumulative effect over extended periods leads to significant climatic anomalies. The contrasting precipitation patterns observed between northern and southern Mexico could be attributed to shifts in meridional circulations, such as the Hadley cell, which modulates atmospheric subsidence and moisture convergence or divergence. This circulation pattern is closely linked to the average position and extent of the eastern Pacific ITCZ. Subsiding air motion tends to reduce precipitation, leading to drought conditions across northern or central Mexico and the southern United States. However, regional features, like the east-west SPI-24 contrast in northern Mexico, highlight the influence of Atlantic Ocean conditions, particularly within the IAS.
The positive phase of the AMO is strongly linked to the 1930s drought. During this phase, an intensified high-pressure system over the North Atlantic weakens the low-level jet over the Gulf of Mexico, reducing moisture transport into the U.S. Midwest. Conversely, a warmer IAS during a positive AMO phase promotes tropical convective activity over the Caribbean, increasing easterly wave activity. These waves travel across the Caribbean Sea, contributing significantly to summer rainfall, and generating numerous storms across Mesoamerica. Furthermore, under favorable conditions, easterly waves can intensify into tropical cyclones, whose activity is modulated by AMO, producing intense rainfall.
Conversely, persistent El Niño (La Niña) conditions, associated with the positive (negative) phase of the PDO, lead to prolonged dry (wet) conditions over Mesoamerica. El Niño is associated with a stronger CLLJ and reduced easterly wave activity. Easterly waves are crucial for generating intense precipitation events over Mesoamerica. A decrease in easterly wave activity results in negative precipitation anomalies across much of southern Mexico. Therefore, PDO also influences easterly wave activity. Easterly wave activity can be assessed by measuring the 3–9 day variance of the 700-hPa meridional wind during summer in the central Caribbean Sea. Easterly wave activity over the IAS is related to the intensity of the CLLJ. A strong CLLJ tends to suppress easterly wave development, while a weaker CLLJ favors increased easterly wave activity over the IAS, resulting in more precipitation over the Caribbean and Mesoamerica. During the 1970s and 1980s droughts in Central America, a strong CLLJ (positive CLLJ index) coincided with reduced easterly wave activity (negative variance anomaly). Generally, a strong (weak) CLLJ inhibits (allows) easterly wave activity, corresponding to less (more) precipitation in southern Mexico and Central America. This relationship aligns with changes in the barotropic instability of the CLLJ. Enhanced tropical convection over Central America can further reinforce subsidence over northern Mexico, promoting drier conditions.
An easterly wave guide around 20°N can also influence northern Mexico. The mean airflow over the Gulf of Mexico can affect the passage of these systems at these latitudes. Research suggests that tropical easterly waves north of 20°N can produce precipitation extending from northwestern Mexico into the west-central United States. Analyzing the 925-hPa wind field difference between drought periods (1950s and 1990s) and wet periods (1970s and 1980s) in northern Mexico reveals a reversed circulation in northeastern Mexico and the Gulf of Mexico, indicating reduced moisture flux and precipitation. In the Caribbean Sea, trade winds decelerate, increasing easterly wave activity and precipitation in southern Mexico. However, weaker trade winds over the Gulf of Mexico might reduce the number of northern tropical easterly waves reaching northern Mexico, contributing to drier conditions. Unlike the Caribbean, a weaker easterly flow at these higher latitudes does not enhance easterly wave activity, likely because the flow is not barotropically unstable.
The CLLJ is a key dynamic feature of the IAS climate. As a barotropically unstable circulation, it can trigger or amplify easterly waves and even tropical cyclones, leading to above-average precipitation during the Northern Hemisphere summer. The intensity of the CLLJ is linked to ENSO and PDO, with a warm eastern Pacific strengthening the CLLJ and reducing easterly wave formation and tropical convective activity over the Caribbean (south of 20°N). Reduced precipitation over the Caribbean and Mesoamerica can weaken the local Hadley cell and create less stable conditions over northern Mexico. A linear correlation between CLLJ intensity and summer SPI-24 shows that a strong (weak) CLLJ corresponds to positive (negative) precipitation anomalies in northern Mexico and negative (positive) anomalies in the south. In general, a strong (weak) CLLJ inhibits (allows) easterly wave activity, leading to less (more) precipitation in southern Mexico. An intense CLLJ results in negative precipitation anomalies over Mesoamerica and positive anomalies over northern Mexico and the south-central United States, resembling the spatial structure of EOF2. Therefore, CLLJ intensity plays a significant role in the relationship between PDO and PC2 of SPI-24. In summary, a strong (weak) CLLJ is associated with negative (positive) precipitation anomalies in southern Mexico due to decreased (increased) easterly wave activity.
The combined conditions in the tropical eastern Pacific and tropical Atlantic help explain the dynamics of IAS circulations and the processes driving dry and wet periods across the United States, Mexico, Central America, and the Caribbean. Moisture flux into the U.S. Midwest appears largely controlled by Atlantic conditions, as reflected by the first EOF of SPI-24 and AMO. Conversely, PDO is linked to the EOF2 of SPI-24. When PDO is in its positive phase and the Atlantic in its negative phase, as during the 1980s, conditions favor a rainy northern Mexico but a dry Mesoamerica and Caribbean. Combinations of positive or negative AMO and PDO phases can explain the spatial patterns of low (decadal) regional climate variability over Mexico, depending on the intensity of anomalies in each ocean. Drought events in North America can be largely explained by PDO and AMO. In Mexico, the relationship can be summarized as:
Fig. 7: Seasonal anomaly of variance of 3–9-day filtered meridional wind and anomaly of the mean seasonal magnitude of the Caribbean low-level jet, illustrating their inverse relationship.
Fig. 8: Composites of 925-hPa winds during dry and wet summers, showing the reversed circulation patterns associated with different precipitation regimes in northern Mexico.
Fig. 9: One-point correlation between CLLJ intensity and SPI-24 during NH summer, demonstrating the spatial relationship between jet strength and precipitation patterns.
Drought Variability During the Nineteenth Century Over Mexico
The established relationships between AMO, PDO, and regional prolonged droughts in Mexico can be used to analyze nineteenth-century droughts, serving as an independent validation. Historical documents indicate that the most severe droughts in north-central Mexico occurred during 1808–11, 1868, 1877, 1884–85, and 1892–96, with the latter being considered the most extreme. Conversely, the early twentieth century saw dry conditions across southern Mexico. Reconstructions of AMO and PDO show an intense negative AMO phase coinciding with frequent positive PDO episodes in the early nineteenth century. This combination would likely lead to dry conditions in Mesoamerica and the Caribbean. Most droughts in the latter half of the nineteenth century may be linked to positive PDO periods and relatively weaker AMO anomalies. For example, the 1890s coincided with a major drought across much of Mexico during a positive PDO phase. The late 1910s drought in Mexico, with large negative precipitation anomalies in central and southern Mexico, reflects a positive PDO phase combined with negative AMO conditions.
Given the link between tropical oceans, low-frequency variability, and Mexican precipitation anomalies, extrapolating AMO and PDO trends could offer insights into future persistent drought conditions at a regional level. Recent announcements from NASA indicate that the PDO has entered a negative phase, which, coupled with persistent positive AMO conditions, could potentially lead to significant drought in northern Mexico once again.
Fig. 10: Reconstructed AMO and PDO indices for the 1800–1900 period and observed indices for 1901–2008, illustrating long-term ocean oscillation patterns.
Table 1: Classification of drought severity based on SPI values and associated probabilities.
Table 2: Relationship between PDO and AMO index signs and regional drought patterns in Mexico.
Summary and Conclusions
The significant negative impacts of prolonged droughts in Mexico underscore the importance of understanding the factors that modulate regional climate variability at decadal timescales. Analysis of persistent precipitation anomalies and prolonged droughts in Mexico over the past century reveals a dominant north-south contrasting spatial pattern. EOF analysis of SPI-24 highlights this seesaw structure as the primary mode of variability (EOF1), while EOF2 indicates zonal contrasts in precipitation anomalies. This spatial characterization of persistent negative SPI-24 values provides valuable insights into the regional dynamics of drought. EOF1 and EOF2 for SPI-24 are modulated by low-frequency variability in the tropical oceans, with EOF1 linked to AMO and EOF2 associated with PDO. The combined influence of EOF1 and EOF2, or PDO and AMO, can reconstruct regional drought patterns across Mexico, the southern United States, Central America, and the Caribbean. The phase and amplitude of EOF1 and EOF2, or AMO and PDO, have proven useful in examining drought characteristics even during the nineteenth century.
Low-frequency climate variability in the tropics is significantly influenced by high-frequency transient atmospheric activity. In Mexico, easterly waves interacting with the mean flow are crucial for understanding regional precipitation extremes. Preliminary analysis indicates that the intensity of the CLLJ and easterly wave formation are key determinants of climate variability across the IAS and Mexico. Further research is needed to establish threshold values for CLLJ intensity that either inhibit or enhance easterly wave activity and precipitation in Mexico. Current understanding suggests that drought dynamics are shaped by the interplay between modulation of high-frequency transients and stationary circulations. Predicting future intense droughts in Mexico will depend on the ability to project PDO and AMO behavior. Understanding these climate variability modes will be crucial for developing regional climate change scenarios in the coming decades.
