Sunlight-absorbing aerosol amplifies the seasonal cycle in low cloud fraction over the southeast Atlantic

Abstract. The mean altitude of the smoke loading over the southeast Atlantic moves from the boundary layer in July to the free troposphere by October. This study details the month-by-month changes in cloud properties and the large-scale environment as a function of the biomass-burning aerosol loading at Ascension Island (8◦S, 14.5◦W) from July to October, based on island measurements, satellite retrievals and reanalysis. In July and August, the smoke loading predominantly varies within the boundary layer. During both months, the low-cloud fraction is less and is increasingly cumuliform when more smoke is present, with 5 the exception of a late morning boundary layer deepening that encourages a short-lived cloud development. The meteorology varies little, suggesting aerosol-cloud interactions explain the cloudiness changes. September marks a transition month during which mid-latitude disturbances can intrude into the Atlantic subtropics, constraining the free-tropospheric aerosol closer to the African coast. Stronger boundary layer winds on cleaner days help deepen, dry, and cool much of the marine boundary layer compared to that on days with high smoke loadings, with stratocumulus reducing everywhere but at the northern deck 10 edge. The September free troposphere is better-mixed on smoky days compared to October. Longwave cooling rates generated by a sharp water vapor gradient at the aerosol layer top encourages a small-scale vertical mixing that could help maintain the well-mixed smoky September free troposphere. The October meteorology primarily varies as a function of the strength of the free-tropospheric winds advecting aerosol offshore. The free-tropospheric aerosol loading is less than in September, and the moisture variability is greater. Low-level clouds increase and are more stratiform in October when the smoke loadings are 15 higher. The increased free-tropospheric moisture can help sustain the clouds through reducing evaporative drying during cloudtop entrainment. Enhanced subsidence above the coastal upwelling region increasing cloud droplet number concentrations may further prolong cloud lifetime through microphysical interactions. Reduced subsidence underneath stronger free-tropospheric winds at Ascension supports slightly higher cloud tops during smokier conditions. Overall the monthly changes in the largescale aerosol and moisture vertical structure act to amplify the seasonal cycle in low-cloud amount and morphology. This is 20 climatically important as cloudiness changes dominate changes in the top-of-atmosphere radiation budget.

The impact of absorbing aerosol on marine boundary layer clouds is governed by the relative location of the aerosol layer to the cloud layer, with aerosol embedded within the cloud layer giving rise to local aerosol-cloud microphysical and radiative 25 interactions, while aerosol above a cloud layer can only be radiatively active until it is entrained into the cloud (Johnson et al., 2004;Johnson, 2005;Costantino and Bréon, 2013;Yamaguchi et al., 2015;Zhou et al., 2017;Zhang and Zuidema, 2019;Kacarab et al., 2020;Herbert et al., 2020;Che et al., 2021). Many studies focusing on the southeast Atlantic region apply a seasonal-averaging to improve the robust detection of absorbing aerosol impacts (e.g., Wilcox, 2010Wilcox, , 2012Adebiyi and Zuidema, 2018;Mallet et al., 2020). This averages over a noticeable rise in the smoke layer, from mostly within the 30 boundary layer in July , to a mixture of boundary layer and free-tropospheric smoke in August (Zhang and Zuidema, 2019;Haywood et al., 2021), to mostly above and distinctly separated from the cloud layer by September and October (Shinozuka et al., 2020;Haywood et al., 2021). Zhang and Zuidema (2019, hereafter  and Zuidema (2011). MODIS-Meyer cloud and aerosol retrievals are aggregated to 1 • resolution to match the Level-3 MODIS retrievals, if the former can provide an areal coverage of at least 20%. Daily-mean MODIS-based retrievals rely on averages 90 between the Terra and Aqua retrievals weighted by their frequency, subsequently averaged spatially over 2 • by 2 • , 3 • by 3 • , and 4 • by 4 • domains centered on Ascension. Low-cloud fractions across the diurnal cycle are retrieved using the Visible Infrared Solar-Infrared Split Window Technique (VISST; Minnis et al., 2008) from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board the geostationary Meteosat10 satellite. These are averaged over a 4 • by 4 • domain latitudinally centered on Ascension but centered slightly to the island's east (6 • -10 • S, 15 • -11 • W) to better capture the upwind clouds more typical 95 of the island. All-sky albedos at the top-of-atmosphere (TOA), at Terra and Aqua overpass times, are the ratio of the reflected shortwave fluxes at TOA to the incoming solar radiation measured by the Clouds and the Earth's Radiant Energy Systems (CERES; Wielicki et al., 1996) sensors, drawing on the CERES Single Scanner Footprint (resolution of 20 km) product Edition 4 (Su et al., 2015).
Geopotential height, temperature and wind velocity maps are based on the European Centre for Medium-Range Weather 100 Forecasts (ECMWF) fifth-generation atmospheric reanalysis (ERA5; Hersbach et al., 2020), available every hour and gridded to 0.25 • spatial resolution. Back trajectories from Ascension Island at 2000 m, or just above the cloud tops, help indicate the transport of aerosol most likely to entrain into the boundary layer near Ascension. The back trajectories rely on the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT; Draxler and Hess, 1998)  The basic approach is to construct composites of the more and less smoky conditions for each month, and to analyze the differences in cloud properties with an eye on the accompanying meteorology as well as aerosol. Composites can identify the northwest, but the cloud fraction evolutions agree between the domains. The amplitude of the diurnal cycle (Fig. 5a) is mostly unaffected by the smoke loading, except in August, when a more pronounced diurnal amplitude can be related to the afternoon clearing of stratiform clouds under smokier conditions (ZZ19). Overall the modulation of the cloudiness seasonal cycle by the presence (or lack of) smoke is important because the cloudiness changes ultimately dominate the change to the top-of-atmosphere shortwave radiation balance (Fig. 5c). The all-sky albedo decreases or increases, depending to first order on the changes in the cloudiness fraction.
4 July: smoke reduces cloud fraction For more smoky conditions in July, low-cloud is less frequent throughout the day (Fig. 6a), cloud bases are higher by 50-90 200 m, and cloud tops are typically lower, by up to 150 m, compared to less smoky conditions. An exception is the morning (6-12 LST), when the cloud top heights and liquid water paths don't vary with the smoke loading ( Fig. 6a-b) and precipitation frequencies almost match (Fig. 6c). This is reminiscent of the morning cumulus invigoration documented for August (see Fig.   8b in ZZ19) when more boundary-layer smoke is present. Rain frequency is otherwise reduced throughout the day (Fig. 6c), in smokier July conditions, most pronounced in the afternoon, when cloud LWP is also substantially reduced (Fig. 6b). The 205 low-cloud fraction is reduced over a larger area than just at Ascension when the boundary layer is smokier (Fig. 6d).
When more smoke is present, the entire boundary layer is warmer by ∼0.3 K (Fig. 6e). The boundary layers are more decoupled, with a more moist sub-cloud layer and a drier cloud layer (Fig. 6e), consistent with the reduction in cloudiness. The cloud-top inversions are weaker (by ∼1 K), lower (by ∼200 m), and thinner (by ∼40 m), compared to less smoky conditions ( Fig. 6e and Fig. S1). Given that smokier conditions last for a few days (Fig. 3), the shortwave absorption can warm the sub-210 cloud layer over multiple days, with the warmer sub-cloud layer persisting through the night (shown for August in ZZ19), supporting a boundary-layer semi-direct effect. An aerosol-cloud microphysical interaction is also apparent in the doubling of the satellite-derived N d (see values printed on Fig. 6e, left panel). The radiosonde-derived wind speeds indicate slightly weaker free-tropospheric winds when the boundary layer is more smoky, but the atmospheric circulation patterns are not significantly different (not shown). The lack of strong synoptic variations suggests the observed low-cloud variability is mostly driven by 215 the presence of the shortwave-absorbing smoke.
5 September: mid-latitude disturbances reduce stratocumulus cloud and raise boundary layer heights on cleaner days Previous studies assessing the impact of above-cloud absorbing aerosol on the boundary layer height are not in full agreement.
The regional modeling studies of Sakaeda et al. (2011) andLu et al. (2018) report an increase in cloud-top heights when 220 biomass burning aerosols are present above clouds, attributed to a reduced free-tropospheric subsidence caused by aerosol heating. This can increase the contact with the smoke layer, enhancing entrainment of aerosol into the cloud, increasing N d , further increasing the cloud-top height (Lu et al., 2018). In contrast, observational studies report a reduction in the cloud top height (Wilcox, 2010(Wilcox, , 2012Adebiyi et al., 2015) which could be because an enhanced lower-tropospheric stability reduces cloud-top entrainment. A climate-scale modeling study (Gordon et al., 2018) also produces a decrease in boundary layer depth 225 under a plume of biomass burning smoke, when the model free-tropospheric conditions are nudged to reanalysis. Most higherresolution process modeling studies (Johnson et al., 2004;Herbert et al., 2020;Yamaguchi et al., 2015;Zhou et al., 2017) impose a free-tropospheric model velocity, disallowing an aerosol feedback. The change in boundary layer height accompanying free-tropospheric aerosol is important to clarify, because more shallow boundary layer heights tend to be better coupled to the surface (Zuidema et al., 2009), with the surface moisture fluxes better able to sustain higher cloud fractions. On the other 230 hand, if the cloud base remains invariant while the MBL shoals, the clouds should thin.
The radar-derived cloud top height varies little with smoke loading, with a slight increase in the afternoon and after sunset, by up to 60 m, on days with more smoke (Fig. 7a). More clear is that cloud frequencies, particularly in the lower levels, increase with the smoke loading, by up to ∼20%. This is because the cloud bases lower, by up to 230 m, when more smoke is present ( Fig. 7a). The island-based cloud frequency profiles can be limited in interpreting cloud cover over a larger area, due to a 235 systemic island orographic effect and subsampling by the relatively short time series of point measurements, but diurnal cycle composites of SEVIRI-derived low-cloud fraction also indicate an increase in afternoon cloud cover, if weak (Fig. S2). Cloud occurrence increases just above lifting condensation level when more smoke is present, more pronounced in the morning (Fig.   7a), while surface observers only report a subtle shift in low cloud type as a function of smoke loading (Fig. 5b).
A focus on the cleaner conditions provides a useful alternative perspective. When the free troposphere is less aerosol-laden, 240 the boundary layer is less humid (q v decreases by 1 g kg -1 , RH by ∼5%), the cloud layer is cooler (∼1K at inversion base), with a stronger and slightly higher cloud-top inversion (1.8 K and 70 m) ( Fig. 7b and Fig. S3). The changes in the free troposphere are equally dramatic: much weaker winds, less moisture, and more stable thermodynamic structure. Differences between the composite-mean N d s and rBC mass concentrations are statistically insignificant (numbers printed on Fig. 7b), indicating negligible aerosol-cloud microphysical interactions (as expected, given the small amount of boundary layer aerosol).

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The 700-hPa atmospheric circulation differs significantly between days with low and high free-tropospheric smoke loadings at Ascension ( Fig. 7c and d). On days with more smoke, the AEJ-S extends further westward, and backtrajectories from Ascension near cloud top clearly trace back to continental Africa (Fig. 7c). On days with little smoke, the circulation is anticyclonic about a deeper land-based pressure high, with the aerosol remaining closer to the coast and further south. The above-cloud air at Ascension is more likely to come from the north and west on these days (Fig. 7d).

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The primary distinction between the two composite circulations is a disruption of the mid-latitude eastward flow, in which a high-pressure ridge at 700 hPa associated with baroclinic activity from further south counteracts the free-tropospheric zonal jet at 10 • S. The subsidence above the cloud top is stronger on the less-smoky days when the boundary layer at Ascension is also higher (Fig. 7a, b , e and g). The increased subsidence also reflects the mid-latitude intrusion: the anomalous westerlies weakening the free-tropospheric winds also create an anomalous convergence, supporting an anomalous subsidence (Fig. 7e).

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This is most pronounced east of the 700 hPa pressure ridge (right above the region bounded by Ascension and St. Helena), where the flow shifts from cyclonic to anti-cyclonic and the AEJ-S is most weakened (Fig. 7d). At the surface, the mid-latitude disturbance strengthens the south Atlantic high and shifts it slightly to the southwest (not shown), strengthening the southerlies in the boundary layer, although weakly felt over the Ascension region (Fig. 7g, cyan vectors). Closer to St. Helena, the prevailing southeasterly boundary layer flow is weakened by the anomalous westerlies, corresponding to the upper-level (700 hPa) 260 mid-latitude disturbance. These changes in the regional atmospheric circulation correlate with a pronounced cloudiness reduction within the main southeast Atlantic stratocumulus deck, except at the northern edge of the deck encompassing Ascension (Fig. 7f).
St. Helena Island is located approximately 2 days upwind within the boundary layer flow, with Lagrangian forward trajectories from St. Helena placing boundary layer air near if slightly west of Ascension ( Fig. 7 within Zuidema et al., 2015). A height 265 cross-section between Ascension Island and St. Helena Island (16 • S, 6 • W; gray dashed line on Fig. 7e), indicates a consistent increase in the 700-800 hPa subsidence on days with less free-tropospheric smoke loadings (Fig. 7g). As such, the radiosondes at St. Helena can provide insight into the 24-48 hour adjustment of clouds to their large-scale environmental conditions (Klein et al., 1995;Mauger and Norris, 2010;Eastman et al., 2016) for the clouds characterized at Ascension.
A 2 day lead is incorporated into the St. Helena comparisons between low/high smoke days in Figure 8. Two days priori 270 to the less smoky days at Ascension, the boundary layer heights are much higher at St. Helena, by 320 m compared to more smoky days, with a weaker temperature and moisture cloud-top inversion gradient (1.6 K and 1.6 g kg -1 ; Fig. 8a and Fig.   S4). This indicates that the cloud tops at Ascension are higher, despite stronger subsidence, because the boundary layer is deeper upstream. The potential temperature, q v and RH vertical structure differences as a function of smoke loading are qualitatively similar to those at Ascension (Fig. 8a). The boundary layer is deeper and less humid near the surface (Fig. 8a), In October, the temperature gradient between the continental heat low in southern Africa and equatorial convection continues to encourage stronger free-tropospheric easterlies (Tyson et al., 1996;Nicholson and Grist, 2003;Adebiyi and Zuidema, 2016), capable of transporting biomass burning smoke far westward at altitudes reaching up to 5-6 km. This encourages smoke 295 to predominantly stay in the free-troposphere over the southeast Atlantic ( Fig. 1 and 3). Nevertheless, reduced burning and increased moist convection on the African continent reduces aerosol transport but increases moisture transport, compared to September.
At Ascension, the radar-derived cloud frequency profiles (October 2016 only) emphasize a more persistent stratiform cloud structure, through the linear increase in cloud frequency with height, lasting throughout the diurnal cycle and invariant of the 300 smoke loading (Fig. 9a). Cloud occurs more frequently when it is less smoky (Fig. 9a, confirmed through a Student's t-test), consistent with the satellite-derived low-cloud fraction covering a larger area (Fig. 5a), except in the afternoon (12-18 LST).
There is some indication that the cloud layer rises under smokier conditions, with higher cloud bases by up to 90 m in the late morning, a reduced sub-cloud relative humidity ( also less and rain is less frequent (Fig. 9b). Combined, these observations suggest smokier conditions correspond with thinner stratiform cloud layers near the trade-wind inversion. Figure 9c indicates slightly warmer and drier sub-cloud layers in smokier conditions. The moisture and wind profiles clearly differ, with more moisture overhead between 1.5-3.5 km and stronger winds from the surface to 4km on days with more free-tropospheric smoke. The increase in free-tropospheric moisture immediately above the cloud tops reduces the gradients of RH and q v across the inversion, by ∼2 g kg -1 ( Fig. 9c and Fig. S5). This should 310 help sustain the stratiform cloud layer through suppressing evaporative drying by cloud-top entrainment. Fig. 9d indicates a broad, zonally-oriented band of elevated τ af , also seen in ACAOD (not shown). The satellite-derived low-cloud fraction is enhanced west of 5 • W by up to 0.35 (including at Ascension), and slightly reduced to the south, east of 0 • E by at most 0.1 ( Fig. 9e), indicating a more zonally-oriented, westward extending cloud deck, when more smoke is present overhead.
An anomalous anti-cyclonic October circulation at 700 hPa offshore of continental Africa indicates a strengthening of the 315 dominating large-scale circulation on the days when the smoke loading is elevated over Ascension (Fig. 9f), consistent with the measured stronger winds. The free-tropospheric subsidence is reduced underneath the strengthened easterlies centered on 10 • S, consistent with a secondary circulation (Adebiyi and Zuidema, 2016) and explaining the slight increase in cloud top heights at Ascension on smokier days. Also notable in Fig. 9f is the enhancement in the subsidence just off of the coast of Namibia (17 • S -28 • S) to the southwest of the strengthened anticyclonic high, correlating with a local increase in N d on days with more 320 smoke (Fig. 9g). A broad expanse of increased N d , stretching from near the Namibian coast to beyond Ascension, is evident. At Ascension, the composite-mean MODIS-Meyer derived N d and surface-based rBC almost double between the high versus low smoke conditions (printed on Fig. 9c). Christensen et al. (2020) select days with enhanced clear-sky τ a to the south of the main stratocumulus deck, and find an increase in cloud fraction/lifetime far downwind within Lagrangian trajectories, consistent with the increased low-cloud fraction to the west in Fig. 9e. This, along with the rain suppression occurring on smokier days 325 and little change in the lower tropospheric stability ( Fig. 9b and c), supports the idea that an aerosol lifetime effect (Albrecht, 1989) is active, consistent with Christensen et al. (2020). To this we can add that the increase in free-tropospheric moisture also helps maintain the cloud against entrainment-driven cloud thinning. The elevated N d on more smoky days can also contribute to the significant brightening of the cloudy scene near Ascension in October, despite the reduction in cloud liquid water path (all told, a net ∼0.05 increase in TOA all-sky albedo; Fig. 5c). 330 We lack an explanation for the smaller reduction in cloud fraction to the south of the main stratocumulus deck. The contrasting decrease in N d over a narrow region confined within ∼2 • along the coast of Namibia on more smoky days (Fig. 9g) correlates with anomalous near-surface northerly winds (gray vectors on Fig. 9g). This circulation pattern advects moist, warm air along the coast of Namibia, encouraging an inland fog (Andersen et al., 2020). Perhaps this produces enough precipitation to reduce N d near-shore, although that remains a speculation. we show that longwave cooling at the top of the humidity layers can help maintain their vertical structure through encouraging downward small-scale mixing. The individual free-tropospheric humidity layers typically include a stability cap at the top, ensuring a sharp gradient to the water vapor mixing ratio, with q v reductions to near 0 g kg -1 above the aerosol layer reflecting the large-scale subsidence. The extremely dry overlying atmosphere provides a strong exposure of the underlying water vapor to outer space, creating a longwave radiative cooling profile that is maximized at the top of the moisture layer and helps maintain 345 a stability cap (Mapes and Zuidema, 1996). A negative buoyancy, generated at the top of these layers, can aid downward mixing. Although the longwave cooling from the additional water vapor transported within the aerosol layers is typically small compared to that from the aerosol shortwave absorption (Marquardt Collow et al., 2020), the vertical structure of the radiative heating is also altered, with most of the longwave cooling occurring above the maximum in the shortwave heating from aerosol.
It is this displacement that helps maintain a better-mixed aerosol/humidity layer.

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An example is made of a characteristic profile over Ascension from September 2 nd , 2017, in which well-mixed aerosol extinction (derived from the micro-pulse lidar according to Delgadillo et al., 2018, constrained to an aerosol optical depth of 0.35) and humidity vertical structures are clearly well colocated (Fig. 10). Instantaneous radiative transfer calculations are based on a noon solar zenith angle, a spectrally-dependent single scattering albedo (SSA) of 0.8 at 529 nm based on , and an asymmetry parameter of 0.67 loosely based on Cochrane et al. (2021). The spectral dependence of 355 SSA relies on an absorption angstrom exponent of 1 and a mean angstrom exponent of 1.9 , with no humidity dependence. A cloud layer consists of cloud water content calculated from the radiosonde profiles using the adiabatic assumption, with cloud optical properties calculated assuming a cloud droplet number concentration of 40 cm −3 following Painemal and Zuidema (2011). These yield a sharply-defined longwave cooling profile, maximized at ∼-28 K day -1 over a 50 m distance at the top of the free-tropospheric aerosol/moisture layer (Fig. 10). The noon-time shortwave heating produced 360 by the smoke is larger, with a maximum of ∼34 K day -1 over a 50 m layer. A key feature is that the maximum shortwave heating occurs lower in the atmosphere than does the maximum longwave cooling (Fig. 10, insert). As a result, a net cooling (∼-5 K day -1 50 m -1 ) pervades the top 100 m of the layer, even during the time of day when the shortwave warming is strongest. The net heating profile encourages a small-scale downward vertical mixing that can allow aerosol to move vertically more freely, regardless of time of day. Although such mixing is not deep, based on a simple diabatic heating/static stability 365 calculation, it does help explain why the free-troposphere is often stratified into individually well-mixed layer Pistone et al., 2021). In October, more of the convection over land is moist (Ryoo et al., 2021), which will produce more complex thermodynamic profiles from, e.g., microphysical melting and downdrafts. This may also help explain why the thermodynamic profiles are less well-mixed in October, also evident in Pistone et al. (2021), and do not reach as high (because surface land heating is reduced).

8 Concluding remarks
This study characterizes the sub-seasonal evolution of marine boundary layer clouds over the remote southeast Atlantic, from July to October during 2016 and 2017, as a function of the aerosol loading and its vertical distribution. We extend the work of ZZ19, which focused on August only, and distinguish this from previous studies that apply a longer-time-scale averaging over the biomass-burning season (e.g., Wilcox, 2010Wilcox, , 2012Costantino and Bréon, 2013;Adebiyi and Zuidema, 2018; Dea- 1. When smoke is present, the seasonal evolution in low cloud amount is amplified. The low cloud amount first reduces in July-August, but then increases and becomes more stratiform in October. The cloudiness changes dominate the top-of-385 atmosphere all-sky albedo changes associated with the smoke (Fig. 5), although the cloudiness changes are not necessarily attributable to the aerosol.
2. In July, the cloud cover, LWP and rain occurrence are reduced when more smoke is present, particularly in the afternoon.
The thermodynamic and wind vertical structures are similar regardless of the smoke loading, suggesting the variability in the cloud response is primarily driven by the aerosol rather than synoptics (Fig. 6). A morning increase in LWP, even un-390 der smokier conditions, is similar to a recoupling of the cloud layer to the sub-cloud layer detailed more comprehensively for August in ZZ19.
3. In September, the days with less free-tropospheric smoke over Ascension are distinguished by mid-latitude synoptic intrusions into the subtropics. An upper-level pressure ridge constrains the circulation around the land-based heat low to the coastal region, reducing the westward extent of the free-tropospheric zonal winds at 10 • S that normally disperse 395 the aerosol (Fig. 7). A stronger surface anticyclone over the Atlantic strengthens boundary layer southerlies more likely to advect cleaner Southern Ocean air. The lower tropospheric stability is reduced, despite stronger synoptically-aided subsidence, helping to raise the boundary layer top, particularly noticeable at St. Helena Island (Fig. 8). This provides an alternative explanation to why the observed cloud top heights are lower on the smokier days, despite weakened subsidence. 400 4. In October, the free-tropospheric zonal winds are stronger when more aerosol is present over Ascension. The same winds enhance the humidity above the cloud top, reducing entrainment-driven evaporative drying. This helps support the increased occurrence of stratiform clouds and satellite-derived low-cloud fraction. Cloud tops are slightly higher at Ascension when the smoke loading is higher, consistent with reduced subsidence from the secondary circulation induced by the strong zonal winds (Fig. 9). A possible aerosol indirect effect is indicated, in that the N d double when more smoke 405 is present overhead. Enhanced subsidence off of the coast of Namibia may provide another pathway for aerosol to enter the boundary layer and ultimately reach Ascension. The additional aerosol may help prolong the cloud lifetime and enhancing their brightness ( Fig. 5c; Christensen et al., 2020). These two effects (an additional moisture source and an aerosol cloud lifetime effect) help explain why the low-cloud fraction is higher, despite a lower liquid water path, compared to the southeast Pacific stratocumulus deck during this time of year . 410 5. The September free-tropospheric thermodynamic profile is better-mixed than in October. The sharp gradient in water vapor mixing ratio at the top of a September free-tropospheric aerosol layer generates a net cooling at the layer-top, even at solar noon, that is offset vertically from the larger shortwave warming occurring below through aerosol absorption. The negative buoyancy can facilitate a downward vertical mixing and vertical dispersion of the free-tropospheric aerosol, over small distances (Fig. 10). A greater prevalence of moist convection over land in October, for which micro-415 physical and dynamical processes produce more complex thermodynamic vertical structures, may help explain why the thermodynamic profiles are less well-mixed in October ( Fig. 9c; Ryoo et al. (2021)).
Previous studies applying a seasonal averaging successfully isolate a cloud thickening when more aerosol is present in the free troposphere, but typically overlook a cloud reduction when more smoke is present in the boundary layer. It may have required recent field measurements to better appreciate that the boundary layer can also be smoky. The cloudiness changes 420 are most dramatic over the main stratocumulus region in September (Fig. 7f), in part because of substantial cloud clearings during the less smoky time periods (e.g., Abel et al., 2020). Fig. 5c also indicates that over the July to October time frame, the all-sky albedo changes in October are the most dramatic near Ascension, consistent with higher cloud fractions and potentially an aerosol-induced cloud brightening effect . Thus, this study also suggests that seasonally-averaged changes in the regional radiation budget induced by biomass burning aerosols might be dominated by the contribution from 425 September-October, when the low-cloud fraction is large and more easily varied, which then helps explain why the boundary layer semi-direct effect has been difficult to isolate in previous studies over the southeast Atlantic.
Author contributions. JZ and PZ conceived this study. JZ analyzed the results, and PZ contributed to their interpretation. JZ wrote the manuscript with edits from PZ.
Competing interests. The authors declare that they have no conflict of interest.
Acknowledgements. This research is supported by the U.S. Department of Energy, Office of Science (grants DE-SC0018272 and DE-440 SC0021250). We are indebted to the LASIC scientists, instrument mentors, and logistics staff who made this analysis possible through their efforts in deploying and maintaining the instruments, and processing and calibrating the campaign datasets. We thank Kerry Meyer for providing the MODIS-derived above-cloud aerosol optical depth (ACAOD) product. We thank Michael Jensen and another anonymous reviewer for their constructive comments and suggestions that helped us improve the original manuscript and Franck Eckhardt for his editor- Exploring the elevated water vapor signal associated with the free-tropospheric biomass burning plume over the southeast Atlantic Ocean, Su, W., Corbett, J., Eitzen, Z., and Liang, L.: Next-generation angular distribution models for top-of-atmosphere radiative flux calculation from CERES instruments: methodology, Atmos. Meas. Tech., 8, 611-632, https://doi.org/10.5194/amt-8-611-2015 Tyson, P. D., Garstang, M., and Swap, R.: Large-Scale Recirculation of Air over Southern Africa, J. Appl. Meteorol., 35, 2218Meteorol., 35, -2236Meteorol., 35, , https://doi.org/10.1175Meteorol., 35, /1520Meteorol., 35, -0450(1996 Fig. 6a and 6e, but for September. Composite-mean cloud top heights and bases included in panel a) in km.

ASI-SHI cross-section [Low-High]
Composite-mean (median) rBC mass concentrations on left panel of b). c) and d): HYSPLIT 7-day back trajectories initialized at 2 km over Ascension at noon for September (red lines) for days with c) more and d) less smoke, overlaid on composite-mean ACAOD (colored contours), 700 hPa ERA5 geopotential heights (m, grey contours) and winds (purple vectors). e) Low-high smoke composite difference in 800 hPa geopotential heights (m, black countours), winds (blue vectors) and vertical velocity (hPa day −1 , colored background). f) Lowhigh smoke composite difference in MODIS daily liquid cloud fraction (LCF; filled-contours, overlaid with September-mean LCF (black contours). g) Height cross-section of the vertical velocity low-high smoke composite difference (colored background) and zonal/meridional winds (vectors; free-tropospheric differences < 2 m s −1 are omitted) between St. Helena and Ascension (red and blue stars respectively in panels c-g).        Figure 9. a) as in Fig. 7a, but for high-low smoke composite October 2016 only difference. b) as in Figs. 6b and 6c, but for October, with 3-hour rain frequencies derived from the tipping bucket. c) as in Fig. 7b, but for October. d) MODIS daily τ af (color-filled contours), overlaid with October-mean sea level pressure (hPa, purple), e) MODIS daily liquid cloud fraction (LCF; color-filled contours), overlaid with October-mean LCF (black), f) ERA5 geopotential heights (m, black contours), subsidence (color-filled contours), and horizontal winds (blue vectors) at 700 hPa, and g) daily MODIS-Meyer N d , overlaid on differences in sea level pressure (hPa, gray contours) and 10-m winds The insert zooms into the 4.2-4.5 km range, centered on the layer top. b) θ (red) and qv (blue) profiles from the noon sounding, and the MPL-derived extinction profile (black; following Delgadillo et al., 2018). Corresponding column-integrated AOD and cloud water path are indicated.