Air Mass Analysis




Origin of air masses in Andalucia
(v. 20-8-25) 




Metodology
Five–day backward air mass trajectories were obtained using the HYSPLIT model at a fixed arrival time of 12:00 UTC, covering the period 1996–2025. Trajectories were classified into seven source regions: Atlantic West (AW), Atlantic Northwest (ANW), Atlantic Southwest (ASW), Regional (REG), Mediterranean (MED), Europe (EU), and North African (NAF). Daily classifications were performed separately for Western Andalusia and Eastern Andalusia.

The original dataset was harmonized by grouping minor variants into the main categories (e.g., NAF SIN, NAF IND, NAF sin → NAF; ME and RE → MED and REG, respectively). The dataset was then aggregated at different temporal scales:
• Monthly climatology: average proportion of days classified as NAF for each month.
• Seasonal means: winter (DJF), spring (MAM), summer (JJA), autumn (SON).
• Summer averages by year (1996–2025): mean fraction of NAF trajectories for June–August, used to assess long–term trends.

To evaluate long–term changes in NAF trajectories, a simple linear regression model was fitted:
where NAFsummer is the proportion of days with NAF trajectories in summer (JJA). Separate models were fitted for Western and Eastern Andalusia. Model performance and statistical significance of the slope (β1) were assessed using the coefficient of determination (R²) and p–values.

• In Eastern Andalusia, the model indicated a statistically significant positive trend (~+0.94% per year, p = 0.001).

• In Western Andalusia, the slope was positive but not statistically significant (~+0.2% per year, p = 0.50), reflecting stronger interannual variability.

The fitted regression models were extrapolated to 2045 to provide a statistical projection of NAF trajectory frequency. Predictions include both central estimates and 95% confidence intervals.

For the monthly climatology, the observed cycle (1996–2025) was used as a baseline. The projected increase in NAF frequency (2045 vs. 2025) derived from the regression was applied to the summer months, under the assumption that the annual cycle persists but with amplified summer maxima.

The projections are statistical and rely on linear extrapolation of past behavior. They do not explicitly account for external climate drivers (e.g., NAO, subtropical circulation shifts, dust availability in North Africa). Consequently, the Western Andalusia projections should be interpreted with caution, while the Eastern Andalusia trend appears robust and consistent with recent literature projecting enhanced North African dust intrusions over the western Mediterranean under climate change scenarios.

Annual Analysis

Results
Backward trajectory analysis for Western Andalusia indicates a predominance of Atlantic air masses (AW and ANW), with NAF intrusions representing a comparable fraction (Figure 1). 
In contrast, Eastern Andalusia is consistently dominated by NAF trajectories, which outnumber Atlantic contributions. Other origins (MED, REG, EU, ASW) represent minor fractions (<10%). 


Figure 1.- Distribution of air mass origins in Western (blue) and Eastern Andalusia (red) for the period 1996–2025. Bars represent the mean proportion of trajectories arriving from each sector, with 95% confidence intervals shown as error bars. Western Andalusia is dominated by Atlantic inflows (AW and ANW), with NAF trajectories contributing a comparable fraction. In contrast, Eastern Andalusia is consistently dominated by NAF trajectories, which outnumber Atlantic contributions. Other origins (MED, REG, EU, ASW) remain minor, each contributing less than 10%.

A clear annual cycle of NAF intrusions was identified in both regions (Figure 2).
• Winter (DJF): NAF trajectories represent less than 20% of cases.
• Spring (MAM) and Autumn (SON): intermediate frequencies (~25–30%).
• Summer (JJA): maximum occurrence, reaching ~40% in Western Andalusia and up to ~53% in Eastern Andalusia on average (Figure 3a and 3b).


Figure 2. Monthly climatology of NAF trajectories (1996–2025), with shaded bands showing the 95% CI around the mean values for Western (blue) and Eastern Andalusia (red).

Figure 3b. Interannual variability of summer (JJA) NAF trajectories (1996–2025), with error bars indicating the 95% CI.


Figure 3b. Heatmaps of the monthly proportion of NAF trajectories by year in (a) Western Andalusia and (b) Eastern Andalusia (1996–2025). Warm colors indicate higher NAF frequency. The recurring summer peaks and extreme years (e.g., 2003, 2017, 2020) are evident.

Monthly climatologies (1996–2025) confirm this cycle, with minimum values in December–January (~13–16%) and maxima in July–August (>55% in Eastern Andalusia) (Figure 4).


Figure 4.- Seasonal averages of NAF trajectory proportions in Western (blue) and Eastern Andalusia (red) for the period 1996–2025. Bars represent mean values, and error bars indicate 95% confidence intervals. Summer shows the highest frequencies, particularly in Eastern Andalusia, while winter presents the lowest values.

The summer proportion of NAF trajectories exhibits pronounced interannual variability. The extreme summers of 2003 and 2017 stand out, with NAF exceeding 70% of summer days in Eastern Andalusia and ~60% in Western Andalusia. More recently, summers 2015, 2020, and 2025 also registered unusually high NAF frequencies in Eastern Andalusia (>70%).

The linear regression analysis reveals divergent trends between the two regions (Figure 5):
• In Eastern Andalusia, the positive trend is statistically significant (p = 0.001), suggesting an increase of ~0.94% per year. Projections indicate that by 2045, the mean summer proportion of NAF trajectories could reach ~60%, with individual years exceeding 70%.
• In Western Andalusia, the slope is not statistically significant (p = 0.50), and the wide confidence intervals reflect strong interannual variability. Nevertheless, occasional summers may still experience high NAF frequencies (~50–55%).

Figure 5.- Observed summer (JJA) NAF trajectory proportions (1996–2025) and linear regression projections to 2045 for Western (blue) and Eastern Andalusia (red). Shaded areas represent 95% confidence intervals. The significant upward trend in Eastern Andalusia contrasts with the non-significant trend in Western Andalusia.


Figure 6 illustrates the evolution and climatological cycle for 1996–2025 compared with the 2045 projection. In both regions, the seasonality remains similar, with winter minima and summer maxima, but the intensification of summer peaks is evident in Eastern Andalusia, consistent with climate change projections of enhanced Saharan dust intrusions over the western Mediterranean.


Figure 6. Monthly climatology of NAF trajectories in Western (a) and Eastern Andalusia (b). Solid lines represent the observed mean climatology (1996–2025), with shaded areas indicating 95% confidence intervals. The projected climatology for 2045 is shown as colored markers (cyan for Western, orange for Eastern Andalusia). The seasonal cycle is preserved, but summer peaks are projected to intensify, particularly in Eastern Andalusia. 

Discussion

The analysis of five–day backward trajectories over nearly three decades (1996–2025) demonstrates a clear spatial and temporal differentiation in air mass origins over Andalusia. Western Andalusia is primarily influenced by Atlantic airflows (AW, ANW), whereas Eastern Andalusia is consistently more affected by North African intrusions (NAF). This east–west contrast is consistent with the geographical setting: Western Andalusia is more exposed to Atlantic inflows, while Eastern Andalusia lies closer to the North African coast and is more directly influenced by Saharan advections.

The seasonal cycle of NAF intrusions is robust, with summer maxima and winter minima. This pattern reflects the dynamics of the subtropical high and thermal low over the Iberian Peninsula, which favor the northward transport of Saharan air masses during the warm season. The high summer frequencies (up to 50–60% of days in Eastern Andalusia) are consistent with previous observational studies reporting enhanced dust transport episodes during late spring and summer over the western Mediterranean.

The analysis also reveals marked interannual variability. Extreme summers such as 2003 and 2017 registered exceptionally high NAF contributions, with more than 70% of summer days affected in Eastern Andalusia. These events coincide with well–documented heatwaves and intense Saharan dust outbreaks over Europe. Other recent summers (2015, 2020, 2025) also showed above–average NAF influence, suggesting a clustering of intense episodes in the last decade.

Projections to 2045 based on linear regression indicate a statistically significant increase in Eastern Andalusia, with an expected rise of approximately +0.9% per year in summer NAF trajectories. This would result in ~60% of summer days being affected by NAF air masses by mid–century, with individual years potentially exceeding 70%. In contrast, the trend in Western Andalusia is not statistically significant, reflecting stronger Atlantic influence and higher interannual variability. Nevertheless, occasional summers in Western Andalusia are still projected to experience high NAF frequencies (~50%).

These findings are in line with climate projections of enhanced Saharan dust emissions and transport under warmer and drier conditions in North Africa, coupled with the poleward expansion of the Hadley circulation and strengthening of subtropical ridges. Such changes may not only increase the frequency but also the duration of the dust season, potentially extending from late spring to early autumn.

From an air quality perspective, the projected intensification of NAF intrusions implies more frequent exceedances of particulate matter (PM₁₀ and PM₂.₅) thresholds, particularly in Eastern Andalusia. This has direct implications for public health and regulatory compliance under European air quality directives. Moreover, the interaction of Saharan dust with local pollution sources may exacerbate urban air quality problems in densely populated areas such as Seville, Granada, and Málaga.

Conclusions

Eastern Andalusia is consistently more influenced by NAF trajectories than Western Andalusia, with summer dominance exceeding 50% of days. The intrusions display a robust seasonal cycle, characterized by maxima in summer and minima in winter.
 
Several summers, such as 2003, 2017, 2015, 2020, and 2025, stand out as extreme events with exceptionally high NAF contributions. Future projections indicate a statistically significant increase in Eastern Andalusia, with the proportion of summer days affected by NAF reaching approximately 60% by 2045. In contrast, 

Western Andalusia exhibits weaker, non–significant changes, although occasional summers may still experience strong NAF influence. Overall, these findings highlight the potential intensification of Saharan dust episodes under climate change, with important implications for air quality, public health, and policy frameworks in southern Spain.

Summary

  1. Eastern Andalusia is the primary receptor of NAF trajectories, with consistently higher frequencies than Western Andalusia.
  2. NAF intrusions display a strong seasonal cycle, with summer maxima and winter minima.
  3. Several summers (2003, 2017, 2015, 2020, 2025) recorded exceptionally high NAF influence.
  4. Projections suggest a future intensification of summer NAF frequencies, especially in Eastern Andalusia, where up to 60–70% of summer days may be affected by 2045.

Web links
Real analysis
Atmospheric Dispersion Modelling
 
SPATIAL SYNOPTIC CLASSIFICATION


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