Our analysis reveals that the QBO is a significant driver of early spring SAT over SA. The correlation map between the 50 hPa QBO index and September 2-meter temperatures establishes a clear statistical link (Figure 1(a)). A coherent region of positive correlation is evident over the SWC area, while a negative correlation dominates SEA. This dipole pattern suggests that during the EQBO, SEA experiences warmer conditions, whereas the SWC area experiences cooler conditions. This dipole pattern is a robust feature evident in the composite analysis of all 12 EQBO years. While interannual variability is naturally present, the coherence of the pattern across the composite mean and its alignment with the significant correlation map in Figure 1(a) confirm that it represents a systematic response to the EQBO phase, rather than an artifact of a few outlier years.

This statistical relationship is further substantiated by composite analyses, which illustrate the QBO’s distinct influence during its different phases. The EQBO composite (Figure 1(b)) confirms the presence of significant positive temperature anomalies over SEA and negative anomalies along the SWC. Conversely, during the WQBO (Figure 1(c)), the temperature anomalies are generally weaker and statistically insignificant over much of the region, which nevertheless confirms the same broad relationship observed in Figure 1(a). The difference between the EQBO and WQBO composites effectively isolates the QBO’s specific influence, highlighting that the warming over SEA and the cooling over the SWC area are substantial and robust signals associated with the EQBO phase (Figure 1(d)).

The observed surface temperature pattern can be understood as part of a dynamical chain reaction with downstream effects. One plausible mechanism which involves the QBO’s influence is the Southern Annular Mode (SAM), whereby during the EQBO phase, the unique wind structure in the tropical stratosphere refracts planetary waves, ultimately contributing to a weakening of the Antarctic stratospheric polar vortex ([4]). A weaker polar vortex often promotes a more negative phase of the SAM, which can alter pressure patterns and favor enhanced warm air advection over parts of SA, resulting in the

Figure 1.(a) The correlation coefficient between 2-meter temperature (T2m) in Kelvin (K) and quasi-biennial oscillation (QBO) at 50 hPa in September. (b), (c), and (d) are T2m composite anomalies in September during EQBO, WQBO, EQBO-WQBO respectively. The dotted areas are those which are significant at the 95% confidence level.

surface warming documented here ([35]). While this pathway is supported by established literature on stratosphere-troposphere coupling, a detailed analysis of the QBO-SAM relationship is beyond the scope of this study. Nonetheless, this pathway underscores that the QBO is not merely a tropical phenomenon but a key remote influencer of regional climate variability, highlighting the deep interconnections within the global atmospheric system ([12]).

The sensitivity of regional surface climate to the QBO phase, as demonstrated in Figure 1, aligns with growing evidence that the QBO’s influence extends beyond the tropics into the subtropics and mid-latitudes, and can be sensitive to its vertical structure ([12]; [25]). Our findings thus identify the QBO as a previously overlooked stratospheric precursor for seasonal temperature variability over SA during the critical early spring season.

To explore the physical mechanism by which QBO influences September SAT in SA, we assessed: the horizontal wind circulation and geopotential height, vertical wind velocity, OLR, and cloud cover as presented below. Figure 2 presents the composite differences in horizontal wind and geopotential height anomalies between EQBO and WQBO years at 50, 200, 500, and 850 hPa. The signal originates in the lower stratosphere, at the 50 hPa level, a pronounced dipole of geopotential height anomalies (Figure 2(a)). Negative geopotential height anomalies are centered over the equatorial (mid-latitude) region, characteristic of the thermal wind balance associated with the EQBO’s easterly (westerly) shear zone ([4]). Near these equatorial anomalies are nonsignificant positive geopotential height anomalies in the subtropics of the Southern Hemisphere. This anomalous wave-like pattern shows a quasi-barotropic vertical structure, propagating clearly downward through the tropospheric levels (Figures 2(b)-(d)). By the 850 hPa level (Figure 2(d)), the region of SEA (red box) is dominated by significant negative geopotential height anomalies, with a center of positive values situated over the southwestern Indian Ocean (SWIO).

Correspondingly, anomalous easterly wind prevails in the deep tropics across the lower stratosphere, while anomalous westerlies border the subtropical ridges. This configuration suggests an alteration of the subtropical jet stream and a modulation of large-scale circulation patterns ([18]; [25]). The persistent positive geopotential height anomalies over and to the SEA during EQBO years link to the observed surface warming, as they are typically associated with sinking motion and adiabatic warming ([12]). This mechanism confirms the stratosphere-troposphere pathway for the QBO’s distant influence on the region.

The thermodynamic link between the large-scale circulation and surface temperatures is further described by examining mid-tropospheric vertical motion. Figure 3 presents the composite vertical velocity at 500 hPa for the (a) EQBO and

Figure 2.Composite differences; wind in meter per second (vectors, ms⁻¹) and geopotential height in geopotential-meter (shadings, gpm) anomalies between easterly phase (EQBO) and westerly phase (WBQO) at (a) 50 hPa, (b) 200 hPa, (c) 500 hPa, and (d) 850 hPa, respectively. Dotted areas and green vectors indicate anomalies that are significant at the 95% confidence level. The red box is the key area of study.

(b) WQBO phases, along with (c) their difference (EQBO minus WQBO). In the mid-troposphere, EQBO (WQBO) corresponds to an anomalous negative (positive) vertical velocity over the SWC area which favors ascending motion (Figure 3(a)-(b)). The composite difference (Figure 3(c)) reveals a statistically significant dipole pattern in vertical velocity anomalies across SEA and the SWC area, which is characterized by pronounced negative anomalies which cause upward motion. This pattern is dynamically consistent with the geopotential height anomalies observed in Figure 2. The existence of negative geopotential height over the SWC area at 50 hPa creates a region of low (high) pressure during EQBO (WQBO) years. This anomalous low (high) pressure in the low stratosphere causes upper-level convergence, which then favors sinking motion leading to surface warming. This suppresses convection at the surface over SWA ([4]).

This dipole in vertical motion provides a direct physical explanation for the surface temperature patterns described in Figure 1. The enhanced upward motion over the SWA area during EQBO favors increased cloud formation, which will block incoming solar radiation, which typically results in net cooling at the surface ([35]). Conversely, the descending motion over SEA suppresses cloud formation, allowing for greater incoming solar radiation, leading to warmer surface temperatures. The consistency between the geopotential height, vertical velocity, and temperature fields underscores a robust tropospheric pathway for the QBO’s influence.

Figure 3.Composite 500 hPa vertical velocity in Pascal per second (Pas⁻¹) for the (a) easterly phase (EQBO), (b) westerly phase (WBQO), and (c) the difference between EQBO and WQBO, respectively. Dotted areas are significant at 95% confidence level.

The modulation of atmospheric convection and cloudiness by the QBO is quantitatively assessed through OLR, a reliable representation of deep convective activity and high-level cloud cover. At the surface, EQBO (WQBO) corresponds to an anomalous positive (negative) OLR over SEA, which favors lower-level divergence (convergence) (Figure 4(a)-(b)). Increased OLR is a well-established indicator of suppressed deep convection and decreased upper-level cloud cover ([25]). The increase (decrease) of OLR during EQBO in the SEA (SWC area) is a direct consequence of the dynamic forcing of sinking (rising), which stimulates divergence (convergence). This enhances (suppresses) latent heat release within these systems and contributes directly to the tropospheric warming (cooling) and the positive (negative) temperature anomalies observed at the surface ([35]). This OLR pattern provides critical evidence linking the QBO-driven circulation anomalies to changes in cloud and convection that ultimately drive the surface temperature response.

Figure 4. Composite outgoing longwave radiation in Watt per squared meter (Wm⁻²) for the (a) easterly phase (EQBO), (b) westerly phase (WBQO), and (c) the difference between EQBO and WQBO, respectively. Dotted areas are significant at 95% confidence level.

A significant decrease in total cloud cover is observed over SEA during the EQBO (Figure 5(a)). This decrease in cloudiness directly explains the increased OLR values documented in Figure 4. Decreased cloud cover alters the local radiative budget by increasing the shortwave radiation (warming effect) ([14]). The co-location of decreased cloud cover (Figure 5(c)) and positive OLR anomalies (Figure 4(c)) forms a physically consistent and robust explanation that conclusively explains the mechanism behind the surface warming over the continental interior.

Figure 5.Composite cloud cover in percentage (%) for the (a) easterly phase (EQBO), (b) westerly phase (WBQO), and (c) the difference between EQBO and WQBO, respectively. Dotted areas are significant at 95% confidence level.

To consolidate the stratospheric initiation of the teleconnection and illustrate the vertical coupling into the troposphere, Figure 6 shows the latitude-pressure cross-section of the zonal wind composite difference (EQBO minus WQBO), averaged over the longitudinal domain of SA (0˚E - 40˚E). The cross-section reveals the canonical QBO wind signal in the tropical lower stratosphere, with a core of significant easterly anomalies centered at the equator, consistent with the definition of the EQBO phase at 50 hPa ([4]). Figure 6 demonstrates that this stratospheric anomaly is not confined to the tropics

Figure 6.Composite difference of 50 hPa QBO and uwind (ms⁻¹) during September for the (a) easterly phase (EQBO), (b) westerly phase (WBQO), and (c) the difference between EQBO and WQBO, respectively. Dotted areas are significant at 95% confidence level. Latitudinal values were averaged between 0˚E and 40˚E longitudes.

but extends its influence both poleward and downward. A pronounced dipole pattern emerges, with the equatorial easterly anomalies bordered by significant westerly anomalies in the mid-latitudes. This meridional dipole structure descends from the stratosphere deep into the troposphere, creating a distinctive vertical wind shear pattern that reaches the mid-troposphere around 20˚S - 30˚S.

This deep vertical structure is consistent with a mechanism whereby the QBO, by altering the background wind and thermal structure in the stratosphere, diverts planetary waves and modulates the subtropical tropospheric jet stream ([10]; [12]). The resulting anomalous circulation, characterized by the descending westerly margin over the subtropics, directly projects onto the tropospheric circulation over the African sector. Therefore, this cross-section confirms the existence of a coherent vertical pathway, establishing how the tropical QBO can distantly force the circulation, cloud, and temperature anomalies over SA during the early austral spring.