The Mediterranean has clear seasonal thermal regimes ( Figure 2 ) that crucially condition air-sea interactions controlling the variability of rainfall in Algeria during December-February ( Jordà et al., 2017 ). As a result of continental effects, winter (DJF) is characterized by a strong north-south SST gradient (12˚C - 18˚C) with the coldest water in the northern basins; autumn (SON) is characterized by high coastal SSTs along North Africa, with higher Algerian waters 2˚C - 3°C

warmer, enhancing the availability of early-winter moisture and latent heat fluxes. Summer (JJA) is characterized by high temperatures (24˚C - 30˚C) in the eastern Levantine Basin sustained by seasonal heat storage, and spring (MAM) by the quick development of western fronts and eastward-moving thermal gradients. These reversing N-S (winter) and E-W (summer) thermal variations form thermal contrasts that are capable of altering low-level baroclinicity and impacting the development of weather systems affecting Algeria ( Lionello et al., 2014 ).

The comparatively warmer western Mediterranean SST also increases evaporation that may amplify the hydrological cycle, which in turn alters the trajectory of Mediterranean cyclones contributing rainfall to Algeria, with the patterns also being further modulated by large-scale climate modes like the NAO on a background of accelerated basin-wide warming ( Pastor et al., 2020 ).

The second EOF mode (EOF2) represents a strong zonal dipole pattern (explaining 25.3% of the variability in Mediterranean winter SST) ( Figure 3(a) ). Unlike the basin-wide warming observed by EOF1, the pattern presents conflicting anomalies between the eastern basin (negative) and the western/central Mediterranean (positive), which are maximum along the coast of Algeria.

It exhibits strong interannual variation with no significant trend in the associated principal component (PC2; Figure 3(b) ), with strong extremes in 2007 (+1.8) and the strongest negative in 2010 (−1.6). This dipole is likely due to the redistribution of heat under wind shear, but with a large-scale circulation, which is supported by a significant correlation between these changes and NAO indices (r = 0.65, p < 0.01). This pattern plays a vital role in Algerian precipitation, with the positive SST anomalies along the coastline stimulating local evaporation and

low-level instability, which would favor more severe convective precipitation events during the winter period ( Lionello et al., 2014 ; Jordà et al., 2017 ).

The adverse NAO phase, which usually accompanies the positive phase of this SST dipole, favors cyclonic paths of activity and moisture transport to the North African region and thus enhances the chances of excessive precipitation in the area ( D ünkeloh & Jacobeit, 2003 ).

The composite analysis was done based on typically warm and cold years of the Mediterranean SST using the normal Principal Component 2 (PC2) time series, which is computed in accordance with the EOF analysis. Warm years were those in which the PC2 value had a normalized value that exceeded the standard deviation of +0.5, whereas the cool years were those in which the PC2 value had a normalized value that was below the standard deviation of −0.5. The years that gave the contrasting cases in the composite analysis of atmospheric circulation and associated patterns can be found in the positive phase of the dipole [1995, 1996, 1998, 2002, 2007, 2008, 2012, 2015, 2016, 2017, 2019 and 2023] and the negative phase [1994, 1997, 1999, 2000, 2001, 2003, 2005, 2006, 2009, 2010, 2011, 2013, 2014, 2018, 2020, 2021, and 2022] according to the warm and cool years.

The effect of interannual variability of the Mediterranean Sea Surface Temperature (SST) on Algerian winter (DJF) rainfall and the preponderant mode of co-varying variability are examined in this section. The analysis is based on the EOF findings whereby mode two (EOF2) was classified as a dipole pattern of zonal SST that is of paramount importance to rainfall variability. In contrast to the first mode (EOF1), which reflects mostly the basin-wide trend of warming and has a weak relationship with precipitation, EOF2 reflects the spatially heterogeneous anomalies that are the direct causes of atmospheric circulation. The uniform spatial pattern of EOF1 provides a weak forcing for altering regional wind and pressure patterns, whereas the strong thermal gradients of the EOF2 dipole efficiently force cyclonic/anticyclonic circulation anomalies that directly control moisture transport and vertical motion.

The Principal Component (PC2) correspondingly has a high interannual variation and no significant long-term trend, and thus it provides an ideal basis for defining the typical warm and cool SST phases. The combination of such phases is thus employed in probing the unique ocean-atmosphere dynamics and their direct effect on rainfall patterns in Algeria.

The composite study of the winter SST in the Mediterranean shows that there are clear spatial distribution patterns that distinguish the warm and cold phases and that have pivotal implications for the variability of rainfall in Algeria.

On warm years ( Figure 4(a) ), positive SST anomalies greater than +0.5˚C are observed throughout the entire basin, with the strongest warming (+0.8˚C to +1.0˚C) in the western Mediterranean and the Algerian Basin, the region of the most vital moisture source for North African precipitation. On the other hand, the cool years ( Figure 4(b) ) show general negative anomalies, especially robust in the central and eastern Mediterranean (−0.6˚C to −0.8˚C). The difference map ( Figure 4(c) ) shows statistically significant warming characteristics (p < 0.05) in the western Mediterranean, the Algerian coastal sector included, where the SST deviation in warm years is estimated at +0.7˚C above the average value in cool years. This local climate change increases latent heat transport and moisture levels in the atmosphere, which may increase the hydrological cycle along the coast of the North African region ( Jordà et al., 2017 ).

It has a spatial arrangement that implies the western Mediterranean warming, as opposed to the basin-wide SST enhancement, can be especially effective in changing anticyclones and moisture transport pathways to Algeria ( Lionello et al., 2014 ).

The warm/cool cycles here are associated with familiar climate modes such as the positive/negative phases of the North Atlantic Oscillation, which vary wind patterns and the redistribution of heat across the Mediterranean ( Dünkeloh & Jacobeit, 2003 ).

This positive phase concentration of substantial warming in the Algerian Basin makes a mechanistic connection between Mediterranean SST variability and the interannual variation of the Algerian winter precipitation observed in earlier evaluations ( Merniz et al., 2019a, 2019b ).

Such combined analysis therefore identifies the existence of unique Mediterranean SST regimes that lead to and possibly control the rainfall extremes essential to Algerian water resources and agricultural planning.

As the country has diverse climatic regions, the precipitation climatology of Algeria ( Figure 5 ) between 1993 and 2023 exhibits a high degree of north-south gradient and seasonality, which is dictated by its location in the Mediterranean-Saharan transition zone ( Merniz et al., 2019b ). Winter (DJF) brings the highest amount of rain (>70 mm/month) to the northern coast and Tell Atlas, which is nourished by Mediterranean anticyclones, with the Saharan south being hyper-arid (<10 mm/month).

There is a response of a 20% - 30% decrease in spring (MAM) after storm tracks have moved away, a 100% dry summer (JJA) except for monsoonal intrusion in the Hoggar, and the entry of the wet season in the north in autumn (SON). This distribution in space and time, with more than 90% of the renewable water resources of the country being in the north ( Sahnoune et al., 2013 ), is explicitly connected to the patterns of the Mediterranean sea surface temperatures that regulate the availability of moisture and storm paths. As a result, interannual winter Mediterranean SST is a vital influence on the rainfall that predetermines the water security and agricultural performance of Algeria ( Lionello et al., 2014 ; Merniz et al., 2019b ).

A pronounced northeast/north-central dipole pattern is revealed by the second precipitation mode (EOF2), which accounts for 17.1% of the variability in winter rainfall over Algeria ( Figure 6(a) ). Kabylie and Annaba (+0.4 - 0.6) and Mitidja and Chelif basin (−0.3 - 0.5) are found to have strong positive loading and negative values respectively, whereas southern Algeria is not affected. The related PC2 time series ( Figure 6(b) ) indicates considerable interannual l variation with negative phases (e.g., 2009, 2011) reversing the gradient and positive phases (e.g., 2002, 2006, 2022) doubling northeastern precipitation relative to north-central areas.

As the positive phase is associated with stronger north-easterly flow over the central Mediterranean, the given dipole pattern is the most likely response of the region to the passing of storms and movement of moisture. The fact that the pattern persisted in the 1993-2023 period of the research and no trend was observed (p > 0.05) shows that the pattern is a fundamental type of winter climate variability in northern Algeria.

The composite analysis of precipitation anomalies during extreme EOF2 phases reveals a strong teleconnection between Mediterranean SST and Algerian rainfall ( Figure 7 ).

This is consistent with known processes in which warm Mediterranean SSTs enhance low-level moisture convergence along the Algerian coast, resulting in considerably higher atmospheric moisture during warm PC2 years ( Figure 7(a) ). The precipitation anomalies in the Tell Atlas area, exceeding +10 mm/day, are linked to processes that generate extreme rainfall events and influence long-term streamflow in northern Algeria’s basins ( Benaini et al., 2023 ; Hallouz et al., 2019 ).

Conversely, cool PC2 phases ( Figure 7(b) ), associated with a strengthened and westward-displaced Mediterranean ridge, lead to widespread drying caused by anomalous subsidence and suppressed convection. The most pronounced negative anomalies (around −10 mm/day) occur in the agriculturally sensitive Chelif Basin.

This region’s documented vulnerability to precipitation variability, combined with future projections under climate change, underscores the severity of such dry periods ( Hallouz et al., 2019 ; Hamitouche et al., 2024 ). The difference map ( Figure 7(c) ) shows statistically significant (p < 0.05) wetter conditions during warm phases, with the most substantial differences (Δ >5 mm/day) observed in the northwestern agricultural regions. These rainfall anomalies are dynamically consistent with SST-driven circulation changes; warm periods favor enhanced westerly flow and orographic uplift against the Tell Atlas, while cool periods promote large-scale subsidence. These mechanisms are critical for understanding the projected increase in both extreme precipitation and drought frequency across Algeria’s sub-regions ( Hamitouche et al., 2024 ), highlighting the key role of Mediterranean SST variability as a primary modulator of the Algerian winter hydroclimate.

These findings have direct implications for seasonal forecasting, providing a dynamical foundation to support water resource management for dams in northern Algeria and to inform agricultural planning. This enhanced understanding is crucial for interpreting observed trends and future projections of hydrological extremes in the area.

This section diagnoses the large-scale atmospheric response to Mediterranean SST anomalies by exploring winter (DJF) circulation anomalies using composite analysis of PC2 extremes.

The composite results of Outgoing Longwave Radiation (OLR) and Sea Level Pressure (SLP) ( Figure 8 ) shows a direct atmospheric pathway in which interannual variability in Mediterranean SST controls winter rainfall in Algeria. This physical process is based on the ocean-atmosphere interaction, initiating with heat and moisture exchange at the sea surface.

During warm Mediterranean SST phases ( Figure 8(a) ), the sea surface becomes a heat engine that warming the marine surface layer and increasing air buoyancy. This leading convection and resulting a strong cyclonic anomaly over the western Mediterranean basin, characterized by a sea level pressure deficit up to −1.5 hPa.

This cyclonic circulation impacts directly the local hydroclimate through two mechanisms. First, it enhances low-level westerly moisture transport toward Algeria. Second, concurrent warm SSTs fuel the atmosphere through increased evaporation enriching the moisture content of the advected air masses. When this warm, moist air is transported landward and undergoes to orographic lifting, the water vapor condenses and the resultant latent heat increases the energy of the system. A major consequence is an escalation of convective activity, which is indicated by large negative anomalies of Outgoing Longwave Radiation (OLR) of up to −12 W m⁻¹ (p < 0.01), leads to excessive precipitation across northern Algeria.

Conversely, cool and Mediterranean SSTs ( Figure 8(b) ) establish an anticyclonic regime, characterized by a positive SLP anomaly of +1.2 hPa. The transfer of heat and moisture is suppressed by the cool sea surface, which stabilizes the atmosphere as well as promotes subsidence. This area, a high-pressure system, is also active in discouraging convection and enhancing dry conditions, evidenced by positive OLR anomalies of over +8 W m⁻².

The contrast between these two phases is sharp ( Figure 8(c) ), exhibiting an SLP gradient of more than 2.5 hPa, and an OLR difference as large as −20 W m⁻² in the northwestern part of Algeria. Warm SST phases statistically describe a considerable amount of rainfall variance (r² = 0.68). The physical basis of seasonal predictability is that there is a strong SST-convection relationship that is based on these thermodynamic mechanisms. According to operational forecasters, however, these local SST signals are damped or overridden by those of large-scale circulatory variability such as the North Atlantic Oscillation (NAO), which complicates seasonal forecasts ( MedCOF , 2023 ). Learning about this SST to circulation to rainfall cascade is now becoming more important for climate adaptation in Algeria, where droughts are expected to occur because of warming in the Mediterranean ( Zittis , 2022 ).

An analysis of the 700 hPa level explains how the thermodynamic and dynamic processes by which Mediterranean SST phases regulate Algerian winter rainfall can be described and gives a mid-tropospheric perspective of the SST-driven response. The relative humidity and vertical velocity (omega) composite anomalies, as shown in Figure 9 , show a drastic difference between the warm and cool phases, which are directly related to the ocean surface features and atmospheric processes that control precipitation.

When the sea is warm into a sea surface temperature (SST), the sea surface serves as an intense heat and moisture source. The increased evaporation causes the water to be pumped into the atmosphere, which is carried by the cyclonic circulation. This leads to the invasion of wet, oceanic air and high relative humidity anomalies, above +15 percent to +20 percent (p = 0.01) in northern Algeria, as visualized in the warm-year composites of Figure 9(a) . At the same time, the surface heating and latent heat release from the clouds cause the air mass to be buoyant, thus causing a strong vertical movement of the air with omega anomalies of up to -0.12 Pa s⁻¹. This rich supply of moisture (the fuel) and sustained lift (the engine) ensures a highly conducive environment for the development and intensification of precipitation, especially by the orographic forcing on the windward sides of the Tell Atlas.

On the other hand, when the SST is cool, the energy transmission is suppressed by the ocean surface. The reduced water loss is due to drier air masses, which is related to negative relative humidity anomalies between −12 and −18, illustrated in Figure 9(b) . The stabilized atmosphere is also a feature of the cool surface, which encourages high levels of subsidence as denoted by positive omega anomalies of +0.09 Pa s⁻¹. This circulation pattern has been observed to actively suppress rainfall due to adiabatic warming in the atmosphere caused by compression and the reduced availability of moisture. The compounding effect of large-scale subsidence and rain-shadow effects is demonstrated by the most severe drying of −22% in places such as the Chelif Basin.

These stages shown in Figure 9(c) differ dramatically with a relative humidity gradient (-RH) of up to +35% and a vertical velocity gradient (−0.21 Pa s⁻¹) of −0.01 (p < 0.01). These steepest contrasts are consistent with the Atlas topography, which proves the contribution of orography to SST-forced signal enhancement. These thermodynamic (moisture availability) and dynamic (vertical motion) causes of change are collectively attributed to a significant part of the observed variation in winter precipitation (r² = 0.76). This is becoming more critical in the Mediterranean, which is a well-known hotspot of climate change, warming at a higher rate than the average global climate, worsening the hydrological cycle ( Cusack & Cox, 2024 ; IPCC, 2021 ). The results yield an objective basis for the seasonal projections and emphasize the dominant role of Mediterranean SST in determining the winter hydroclimate of Algeria in a warming world.

A strong correlation between Mediterranean SST anomalies and North African winter dynamics is revealed by the examination of 850 hPa circulation patterns ( Figure 10 ).

In warm SST conditions ( Figure 10(a) ), the cyclonic circulation caused by heat builds a pattern of onshore westerly winds (maximum anomaly of +3.2 m/s). This circulation accomplishes two very important tasks: it moves wet marine air to the interior, and it causes an abundance of convergence at the surface, which causes air to rise. This is supported by high upward movements (negative omega anomalies, e.g., −0.15 Pa s⁻¹).

A forced ascent coupled with orographic lift against the Tell Atlas mountains increases upward movement by 30 - 40 percent, providing optimum precipitation growth conditions. On the other hand, in cool SST periods ( Figure 10(b) ), the anticyclonic regime generates subsidence (+0.10 Pa s⁻¹) and offshore or weaker winds. This trend actively suppresses rainfall, which suppresses the ascent, instills atmospheric stability, and decreases moisture advection in the Mediterranean. These phases ( Figure 10(c) ) are drastic and a sharp contrast in the vertical velocity (−0.25 Pa s⁻¹) shows the power of the SST forcing. These dynamic factors including SST are associated with a significant part of the interannual rainfall variance in Algeria (r² = 0.79). This process is supported by a stronger land-sea thermal gradient (ΔT ≈ +1.8˚C) during warm intervals, which increases the intensity of the onshore movement and moisture transfer even more. The findings support the role of the Mediterranean as a primary moisture source due to positive SST-rainfall teleconnections in the Maghreb, which are already understood ( Ta ïbi et al., 2022 ).

Moreover, the process of increasing this dynamic pathway can be more dangerous due to the fact that the Mediterranean warms more quickly than the global average ( Pastor et al., 2024 ), which means that the increasing risk of hydroclimatic extremes in Algeria is more dependent on Mediterranean SST as a significant variable in seasonal forecasting and climate adaptation ( Hamitouche et al., 2024 ).

In order to understand the controlling processes of Sea Surface Temperature (SST) patterns in the Mediterranean Sea, a study of global atmospheric circulation is necessary. It is found in this inquiry that massive climate processes that begin in remote ocean basins are the ultimate determinants of regional ocean warming and cooling, which in turn influences winter variability of rainfall in North Africa.

As indicated in Figure 11 , the determining factors of the patterns of the Sea Surface Temperature (SST) in the Mediterranean Sea are greatly influenced by global atmospheric circulation, and among these factors, El Niño-Southern

Oscillation (ENSO) stands out. This indicates that the major determinants of the regional Mediterranean SST dipole (EOF2), which, in turn, controls the variability of winter rainfall in North Africa, include large-scale climate patterns of tropical Pacific origin. The composite analysis also shows a strong teleconnection. Figure 11(a) indicates that El Niño events in the tropical Pacific are always accompanied by warm phases of the Mediterranean SST dipole, which are characterized by an SST anomaly of above +0.8˚C in the Niño 3.4 region. Such a correlation is mediated in place by a well-known pathway of atmospheric processes where the warm SSTs in the central/eastern Pacific serve as a giant heat engine, eliciting heavy convection and upper-level diversion. This separation, which is manifested in the velocity potential anomalies, stimulates a standing Rossby wave train that travels to the mid-latitudes and eventually propagates the cyclonic circulation, typical of the warm phase of the EOF2 mode in the Mediterranean ( Cai et al., 2020 ). On the other hand, Figure 11(b) shows that cool periods of the Mediterranean dipole are highly associated with La Niña conditions in the Pacific. Cool SST anomalies in the tropical Pacific lead to the creation of a changed atmospheric wave train that favors anticyclonic conditions and the typical eastern warming/western cooling pattern of the cool EOF2 phase in the Mediterranean. Figure 11(c) in the difference map shows the vast difference between the two phases, indicating a 2.0˚C Pacific SST gradient and considerable variations in the upper-level atmospheric circulation across the Mediterranean basin. These distant tropical forcings can quantitatively account, by their magnitude, for much of the regional climate variability, about 58 percent of Algerian winter rainfall changes, and 68 percent of interannual Mediterranean SST changes. This clearly makes ENSO a powerful remote cause of the hydroclimate in North Africa ( Zhang et al., 2021 ). This Pacific connection quantification is an important development in the predictability of seasonal events and provides a much-needed 3 - 6-month predictive capability for the management of water resources. Despite the robust teleconnections identified, this study has a limitation even though there are strong teleconnections that have been identified. To begin with, relationships may change with a shifting climate as the ENSO signal interference by a background global warming ( Fredriksen & Berner, 2023 ) may change these relationships.

Second, natural variability to the composite results is brought about by inherent uncertainties in reanalysis data ( Wills et al., 2022 ). Lastly, different spatial patterns of ENSO events (e.g., Central Pacific vs. Eastern Pacific El Niño) are not stratified by the analysis, as the different events have different teleconnections, which may modulate the impacts being observed in the Mediterranean. The next generation of research ought to be on experimenting the strength of these linkages in climate model forecasts and stratify studies by ENSO flavors to come up with more sophisticated and robust adaptation strategies ( Lee et al., 2022 ).