Weak Wind Research Articles

Regional and remote influences of ocean‐atmospheric processes on northeast monsoon rainfall during 2021 over India

AbstractSevere rainfall during the recent northeast monsoon season (October, November and December) in 2021 caused unexpected disasters (triggered flooding and landslides) in the pilgrim city of Tirupati and the metropolitan city of Chennai over the peninsular of India (PI). The remote and regional influence of several ocean‐atmospheric climate indices on northeast monsoon rainfall (NEMR) during the OND season is investigated to understand the contributory mechanisms of extreme rainfall events over the PI during 2021. The interannual variability of NEMR and its relationship with the OND sea surface temperature (SST) anomaly over the Niño3.4 and Indian Ocean Dipole (IOD) regions are considered. In addition, SST anomalies over the Bay of Bengal (BoB), such as east BoB (EBoB), east equatorial Indian Ocean (EEIO) and their gradients (GSST) connections with NEMR, are also examined. The impact of Siberian High (SH), Western Pacific High (WPH), Arabian Sea Surface Pressure (ASSP), easterly waves (EW) and land surface temperature contrast (LSTC) is also analysed and incorporated to find a specific linkage with NEM 2021. The results show that excess NEMR during OND 2021 is suggestively associated with favourable large‐scale conditions like negative IOD and the La Niña phase over the Indo‐Pacific region. In November 2021, two deep depressions made landfall over Tamil Nadu (within the latitude of 11°–13°N) and over adjoining areas, which caused copious amounts of rainfall (maximum ~90 mm·day−1), leading to flooding conditions over Tirupati and Chennai. During the NEM season, abundant warming of BoB and weak vertical wind shear, along with negative anomalies of ASSP, modulated the cyclonic activities that enhanced the sufficient moisture transports from BoB to the PI. The MSLP anomalies over the ASSP region depict a significant correlation (−0.47) with the interannual variability of NEM as compared to SH and WPH during the OND season of 2021.

Fire classification in natural ecosystems by physical and environmental characteristics

To qualitatively and quantitatively characterize and classify the intensity of forest fires and their environmental consequences, it is necessary to develop a special scale similar to the scale of wind strength, sea storms, earthquakes, geomagnetic storms, etc. Purpose. To describe the scales developed for the classification of forest fires according to various parameters characterizing physicochemical processes, environmental consequences and the level of danger from pyrogenic factors. Methods. System analysis, multifactorial analysis, mathematical modeling. Results. A seven-magnitude scale for classifying forest fires by intensity, energy characteristics, mass of emissions of the main combustion products and related chemical elements, as well as by environmental consequences and hazard level is proposed. It is substantiated that with moderate and weak winds, the intensity and energy of forest fires in Ukraine usually do not exceed 4-5 magnitudes, i.e., a moderate or high level. Fires of this level occurred, for example, in the spring, summer, and fall of 2020 in a number of regions of Ukraine. Conclusions. The developed special scales for classifying forest fires according to various parameters are an effective tool for qualitative and quantitative characterization of the intensity of forest fires and their environmental consequences. The obtained results can also be used to assess environmental impacts, material damage and social losses.

Detection of urban effects on precipitation in the Seoul metropolitan area, South Korea

With growing urban population and expanding urban areas, the importance of understanding urban effects on precipitation keeps increasing. This study attempts to detect urban effects on precipitation in the Seoul Metropolitan Area (SMA), South Korea by analyzing hourly rain gauge data during 2005–2020. Precipitation events are categorized according to 850-hPa wind directions, and the precipitation increase from the upwind to downwind regions are examined for different duration and intensity classes of precipitation events. The downwind precipitation increase is largest in summer (39%), especially in August (64%). The August precipitation is analyzed in detail. Precipitation statistically significantly increases in Seoul for weak winds and 25–50 km downwind of the center of Seoul for westerly winds, and the precipitation increases are largest in the afternoon. For the precipitation increases, the increases in frequency and intensity of precipitation events are responsible. Short-duration and heavy precipitation events associated with small-sized precipitation systems initiated within the SMA are mainly responsible for the precipitation increases. The downwind precipitation increase also occurs for southwesterly, southerly, and southeasterly winds, but the increases are associated with large-sized precipitation systems.

Fine particulate matter (PM2.5) exposure assessment among active daily commuters to induce behaviour change to reduce air pollution

Fine particulate matter (PM2.5), a detrimental urban air pollutant primarily emitted by traffic and biomass burning, poses disproportionately significant health risks at relatively limited exposure during commuting. Previous studies have mainly focused on fixed locations when assessing PM2.5 exposure, while neglecting pedestrians and cyclists, who often experience higher pollution levels. In response, this research aimed to independently validate the effectiveness of bicycle-mounted low-cost sensors (LCS) adopted by citizens, evaluate temporal and spatial PM2.5 exposure, and assess associated health risks in Ljubljana, Slovenia. The LCS quality assurance results, verified by co-location field tests by air quality monitoring stations (AQMS), showed comparable outcomes with an average percentage difference of 21.29 %, attributed to humidity-induced nucleation effects. The colder months exhibited the highest air pollution levels (μ = 32.31 μg/m3) due to frequent thermal inversions and weak wind circulation, hindering vertical air mixing and the adequate dispersion of pollutants. Additionally, PM2.5 levels in all sampling periods were lowest in the afternoon (μ = 12.09 μg/m3) and highest during the night (μ = 61.00 μg/m3) when the planetary boundary layer thins, leading to the trapping of pollutants near the surface, thus significantly affecting diurnal and seasonal patterns. Analysis of exposure factors revealed that cyclists were approximately three times more exposed than pedestrians. However, the toxicological risk assessment indicated a minimal potential risk of PM2.5 exposure. The collaborative integration of data from official AQMS and LCS can enhance evidence-based policy-making processes and facilitates the realignment of effective regulatory frameworks to reduce urban air pollution.

Quantification of the extremely intensified East Korea Warm Current in the summer of 2021: offshore and coastal variabilities

In this study, we investigate the record-breaking intensification and abrupt weakening of the East Korea Warm Current (EKWC) in the summer of 2021. We analyzed the ocean data assimilation products resolving this event to examine the association between the abrupt changes in the EKWC and various oceanic/atmospheric factors. The results indicate that during the summer of 2021, the EKWC extended northward beyond its climatology, reaching up to 40°N with the maximum speed of 1.16 m s-1 on August 1. In mid-August, the EKWC underwent a rapid weakening, returning to its climatological level. We could attribute the temporal variability in the anomalous EKWC in 2021 to the distinct temporal variability in the dynamic height anomalies between coastal and offshore regions. The offshore variability in the dynamic height anomaly, which is related to warm eddy variability, led to an anomalously increased EKWC velocity (up to 0.59 m s-1) during the EKWC peak velocity period in 2021. However, anomalous coastal downwelling induced by a weak northerly wind anomaly decelerated the EKWC by -0.06 m s-1 in the same period. In mid-August, a typhoon-related northerly wind induced a sudden rise in the coastal dynamic height anomaly, resulting in a rapid weakening of the EKWC. Our findings suggest that changes in geostrophic current related to warm eddies and typhoons have substantially contributed to the temporal variability in the EKWC, improving our understanding of the temporal variability in the western boundary currents.

Correlations among high-order statistics and low-occurrence wind speeds within a simplified urban canopy based on particle image velocimetry datasets

Predicting infrequent and extreme wind speeds in a built environment is essential for ensuring comfortable and safe pedestrian spaces. Recent studies have employed statistical methods that assume a distribution function for wind speeds at pedestrian levels. Fundamental studies on the relationship between canopy flow and statistics are required to further develop statistical models. Therefore, this study aimed to understand the characteristics of strong and weak wind events within a simplified urban canopy and to scrutinize the relationship between high-order moments and extreme wind events. Particle image velocimetry (PIV) was employed to capture the velocities within a canopy consisting of cubes arranged in a staggered layout with a packing density of 25 %. The probability density functions (PDFs) of the velocity components classified by the mean flow patterns revealed that the PDF shapes were altered by the reverse and spanwise flows. In addition, strong correlations were verified between the gust or peak factor (PF) and high-order moments such as skewness, kurtosis, fifth-order, and sixth-order moments. Accordingly, the PF of the velocity components and wind speed were compared with the predictions by statistical methods based on the Weibull and Gram–Charlier series (GCS). These observations validate the previous statistical methods based on Weibull or GCS distributions. Although the physical interpretation of these statistics is ambiguous, the present analyses indicate that PF can be predicted by high-order moments, especially in particular, by skewness and kurtosis.

Identifying the Drivers of Caribbean Severe Weather Impacts

Severe weather impacts in the central Caribbean are quantified by an objective index of daily maximum wind and rainfall (W•R) in the area 16–19°N, 63–69°W over the period 1970–2021. The index, based on ERA5 hindcast assimilation of satellite and in situ data, peaks from the July to October season as high sea temperatures and weak wind shear promote tropical cyclogenesis. Climate forcing is studied by reducing the W•R index to seasonal values and regressing the time series onto reanalysis fields 10°S–25°N, 180°W–20°E. The outcome reflects Jul–Oct warming in the tropical Atlantic, cooling in the tropical east Pacific (cold tongue), decreased/increased convection over the Pacific/Atlantic, and tropical upper easterly winds. New findings emerge in the Mar–Jun season preceding higher W•R: reduced SW-cloud bands in the northeast Pacific, a convective trough over the equatorial Atlantic, and Caribbean cold-air outbreaks. The multivariate El Niño Southern Oscillation index correlates with Jul–Oct Caribbean W•R at 2-month lead time and shows growing influence. Composite analysis of the top-10 years identifies an anomalous Pacific–Atlantic Walker Circulation favoring higher Caribbean W•R. Salinity is below normal and heat flux is downward across the Atlantic. Anomalous low-level airflow inhibits upwelling in the SW Caribbean, deepening atmospheric moisture. A leading case (TC Fiona 2022) demonstrates the environmental conditions underpinning storm intensification. The key drivers of severe weather impacts yield guidance in strategic planning, risk management and disaster preparedness. New insights are gained from a localized index of severe weather.

Case Study of Coastal Fog Events in Senegal Using LIDAR Ceilometer

This study aims to examine the atmospheric conditions characterising fog phenomena on the Senegalese coast focusing on two specific instances that occurred on April 3 and April 30, 2023. These events were detected by the LIDAR Ceilometer installed at LPAOSF/ESP/UCAD and confirmed on the METARs of the meteorological stations at Dakar and Diass airports. The LIDAR's backscatter signal showed that the fog of April 3 started around midnight with a vertical extension at 100 m altitude and dissipated around 10 a.m. The April 30 event characterized by a good vertical extension from the surface up to 300 m above sea level, was triggered just after 2 a.m. and lasted around 3 hours. The results showed that a decrease in temperature, accompanied by an increase in humidity and light wind, is favorable for the triggering and persistence of fog. Sea Level Pressure (SLP) anomaly fields show two distinct configurations. The April 3 event was characterized by a zonal dipole of SLP anomalies between the Sahara and the northern Senegalese coast, while the April 30 event was characterized by a meridional dipole between the Sahara and the Gulf of Guinea area as far as the equatorial Atlantic. A weakening of the pressure around the study area was observed in both cases, allowing moisture advection to favor the onset of fog. The hovmoller diagrams of relative humidity and wind show that a good vertical extension of humidity associated with a westerly wind in the lower layers plays an important role in the formation and persistence of fog. The presence of dry air associated with a weak easterly wind in the middle layers could explain the low vertical extension of the fog on April 3. A strong wind in the lower layers would be responsible for the premature dissipation of the April 30 fog.

Techno-economic analysis of hybrid solar-biomass-wind for water pumping considering carbon tax and renewable energy rebate

There is a need for increased penetration of renewable energy resources (RES) in the energy mix of South Africa that is currently bedevilled by recurring load shedding thus affecting sustainable critical infrastructures such as water pumping and water use. This paper assesses the viability of three RES of solar, biomass, and wind for water pumping at two locations (University of Johannesburg, APK campus – L1 and Zuikerbosch pumping station – L2) in Johannesburg, South Africa by utilising daily data from 2003 to 2020. A detailed feasibility analysis of the three RES was conducted, after which a rebate-based novel model for reducing the unit cost with zero greenhouse gas (GHG) was implemented for solar PV. Key findings reveal that the power requirement in L1 and L2 booster stations are 91 kW and 71,331 kW, respectively while the produced power from a 350 W monocrystalline PV-module utilized is 0.103 kW in L1 and L2 using 882 and 692,534 modules. The biomass power of 47.68 GW in L1 and 99.42 GW in L2 was produced from an average amount of food waste estimate of 1641 and 1248,800 tonnes evaluated from 3729 and 2838,100 tonnes of putrescible waste using a population model. The power density results depict a very weak wind energy application capability in the two locations based on National Renewable Energy Laboratory indices after using 11 differently sized wind turbines in the two locations. Finally, the result from the rebated unit-cost model reveals that the unit cost of water dropped by 17.09$/kWhr from 0.33$/kWhr in L1 and 21.20$/kWhr from 0.66$/kWhr in L2 using the solar PV modules. The study aligns with similar study done in Qatar, and the Springer report on Energy Democracy Evaluation for 100% Renewable Energy Policies and Projects showing an increased penetration of affordable and carbon-free RES which will in turn reduce climate change effects globally.

Characterization of the marine‐air intrusion Marinada in the eastern Ebro sub‐basin

AbstractThe eastern Ebro basin is composed of an extensive irrigated plain, surrounded by rainfed slopes and wooden mountain ranges and open to the west to the agricultural western Ebro basin. The sea breeze generated at the coast is able to surmount the Catalan prelitoral range through its lowest heights, reaching the basin by its easternmost part. It is a well‐known feature in the region, called Marinada. A network of Automatic Weather Stations is used here to analyse a period of 19 years (2003–2021). A filtering procedure is developed which selects the events when the Marinada is present, based on detecting clear sky, weak wind conditions and the wind direction from the coast in the afternoon. The analysis of these days show that the Marinada propagates along the basin in the afternoon meanwhile observations of the specific humidity show a sudden increase as the temperature cools down, resulting on a cold and humid advection. It is also found that the timing of the arrival of the Marinada depends on the mesoscale/synoptical circulations already present in the region (westerlies or a thermal low).

Contribution of local erosion enhanced by winds to sediment transport in intertidal flat

Field work combining in-situ mooring and erosion experiments was conducted in the intertidal flat of Asan Bay to reveal the relationship between sediment erodibility and external forcing and the contribution of local erosion to sediment transport. A total of 25 tidal cycles occurred during the mooring period with 6-h submergence and 5.7-h air exposure. The erosion rate (E) for sediment bed during air exposure was 0.007 g m−2 s−1, which was 36% of E (0.019 g m−2 s−1) during submergence. Although the sediment bed was influenced by consolidation during air exposure, the erodibility of the submerged bed was closely associated with the unconsolidated deposits of sediments newly supplied from tidal channel and bed shear stress (τcw) induced by current-wave interactions. In particular, τcw could be enhanced by strong winds, which depended on the ratio (Hs/h) of significant wave height and water level, directly disturbing the sediment bed. Under weak winds (Hs/h < 0.06), the newly deposited sediments with high erodibility increased the bed elevation. A small amount of the sediment bed was eroded (E: 0.006 g m−2 s−1), as a floc erosion, with a slight decrease in suspended sediment concentration (SSCT). This increased the contribution (57.2%) of eroded sediments to SSCT. Under strong winds (Hs/h > 0.06) with enhanced τcw, meanwhile, the erodible sediment in the bed was depleted accompanied by a decrease in bed elevation. A large amount of sediment bed was eroded (E: 0.025 g m−2 s−1), as a surface erosion, resulting in the rapid increase in SSCT and the enhanced seaward sediment flux. The sediments eroded from the bed were supplied to water column accounting for 86.8% of SSCT. This suggests that seaward sediment transport under strong winds could be caused by surface erosion shifting from floc erosion in the intertidal flat.

Elucidating the Impacts of Various Atmospheric Ventilation Conditions on Local and Transboundary Ozone Pollution Patterns: A Case Study of Beijing, China

AbstractThe ventilation condition in the atmospheric boundary layer, which varies with the synoptic pattern, is a crucial factor affecting the transport and dispersion of air pollutants. In this study, taking Beijing as an example, local ozone (O3) accumulation and the transboundary O3 pollution (TOP) patterns during the warm season were explored under different boundary layer ventilation conditions by using integrated objective weather classification, non‐negative matrix factorization, and backward trajectory model. Results show that local sources are a major contributor to O3 pollution in the whole of Beijing, accounting for ∼73.36% of the O3 concentration on average. The local accumulation is mainly facilitated by poor ventilation conditions with weak wind speeds (&lt;3 m/s) under favorable photochemical reaction conditions and abundant precursors, resulting in local O3 events. In contrast, the occurrence of synoptic patterns associated with TOP is more frequent, so the TOP contribution cannot be ignored, especially for the northern regions, where it may exceed 50%. Horizontal wind vector variations play a marked role in driving the TOP, affecting not only the location of the source region and transport pathway but also the transport distance and volume. In addition, a strong nocturnal low‐level jet (LLJ) results in horizontal transport and a temporary rise in O3 concentration, while a daytime LLJ stimulates a peak in O3 concentration over the downwind area, lagging by about 1 hr. Our findings provide new perspectives on the effects of boundary layer ventilation on the regulation of O3 pollution, as well as other air pollutants.

Mechanisms for Abrupt Summertime Circumpolar Surface Warming in the Southern Ocean

Abstract In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss—a stark reversal of the sea ice expansion and surface cooling that prevailed over the preceding decades. Here, we examine the mechanisms that led to the abrupt circumpolar surface warming events that occurred in late 2016 and 2019 and assess the role of internal climate variability. A mixed layer heat budget analysis reveals that these recent circumpolar surface warming events were triggered by a weakening of the circumpolar westerlies, which decreased northward Ekman transport and accelerated the seasonal shoaling of the mixed layer. We emphasize the underappreciated effect of the latter mechanism, which played a dominant role and amplified the warming effect of air–sea heat fluxes during months of peak solar insolation. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. A key insight from this analysis is that the seasonal phasing of springtime mixed layer depth shoaling is an important contributor to summertime SST variability in the Southern Ocean. Thus, future Southern Ocean summertime SST extremes will depend on the coevolution of mixed layer depth and surface wind variability. Significance Statement This study examines how reductions in the strength of the circumpolar westerlies can produce abrupt and extreme surface warming across the Southern Ocean. A key insight is that the mixed layer temperature is most sensitive to surface wind perturbations in late austral spring, when the regional mixed layer depth and solar insolation approach their respective seasonal minimum and maximum. This heightened surface temperature response to surface wind variability was realized during the austral spring of 2016 and 2019, when a dramatic weakening of the circumpolar westerlies triggered unprecedented warming across the Southern Ocean. In both cases, the anomalously weak circumpolar winds reduced the northward Ekman transport of cool subpolar waters and caused the mixed layer to shoal more rapidly in the spring, with the latter mechanism being more dominant. Using results from an ensemble of coupled climate simulations, we demonstrate that the 2016 and 2019 Southern Ocean warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM). These results suggest that future Southern Ocean surface warming extremes will depend on both the evolution of regional mixed layer depths and interannual wind variability.

Unraveling atmospheric moisture transport linkages to extreme precipitation events and associated synoptic features over India

This study explores the physical mechanisms underlying extreme precipitation events linked to atmospheric moisture transport (IVT-P). It identifies regions where IVT influences extreme precipitation and investigates changes in synoptic-scale weather patterns over the Indian subcontinent. The study finds a strong association between daily IVT and precipitation over the core monsoon region and complex terrains of the Western Ghats and Himalayas. Event Coincidence Analysis reveals that extreme IVT can be used to forecast extreme precipitation in these regions. The dynamic component of moisture transport strongly influences daily and extreme precipitation over complex terrains, while the thermodynamic component influences precipitation over regions with abundant water vapor and weak horizontal winds. The study also identifies synoptic features and moisture transport ahead of IVT-P events and finds intense low-pressure anomalies, transition from ridge to trough patterns, and intense 700 hPa relative humidity in the specified regions. Overall, the study provides insights into the physical mechanisms underlying IVT linked extreme precipitation events.

The westerly winds control the zonal migration of rainy season over the Tibetan Plateau

Precipitation over the Tibetan Plateau is modulated by both the South Asian summer monsoon and the mid-latitude westerly winds. Using observations and numerical simulations, this study highlights the out-of-phase relationships between the mid-latitude westerly wind speeds and the west-east migration of Tibetan Plateau rainy season. When the westerly jet shifts northward before July, the weakening of westerly winds over the Tibetan Plateau leads to a westward shift of low-level warm air center and a westward extension of moist air convergence. Consequently, rainy season advances westward. Conversely, the southward shift of westerly jet after August leads to a strengthening of westerly winds and an eastward retreat of rainy season. Numerical simulations confirm the dominant role of mid-latitude westerly winds on the rain belt migration over the Tibetan Plateau, and further indicate that the timing of the westward extension of rain belt is determined by the weakening of mid-latitude westerly winds.

Stability of the Vertical Distribution of Dust Aerosol under Weak and Moderate Winds

Stability of the Vertical Distribution of Dust Aerosol under Weak and Moderate Winds

Snow algal blooms in Antarctic King George Island in 2017–2022 and their future trend based on CMIP6 projection

Snow algal blooms have a remarkable climatic or environmental effect through influencing the snow–albedo feedback, accelerating the melting of surface snow, and amplifying global warming. Snow algal blooms occurred frequently on King George Island, Antarctic, during the recent six austral summers (December to next February) through 2017–2022. Based on an assessment of satellite images, this study found that the range and amount of snow algal blooms in the summers of 2018, 2020, 2021 and 2022 are relatively larger than in the summers of 2017 and 2019. Whether meteorological conditions have shaped the year-to-year variation of algal bloom intensities is analyzed through observational composite. The results suggest that during the strong bloom summers there exist prevailing northerly or northwesterly wind anomalies which advect warm and humid airmass from the southern ocean into the island, increasing surface air temperature and humidity; the warmer and more humid surface favors melting of snow and an increase of low cloud cover, subsequently enhancing the atmospheric downward long-wave radiation and amplifying surface warmth; the increased low cloud cover reflects more ultraviolet rays back to space and weakens the short-wave radiation reaching the surface. All these factors together favor to a stronger bloom. In comparison, during 2017 and 2019 there exist weak southerly wind anomalies which induce the northward advection of cold and dry air from the Antarctic Continent and favor the cooler surface. Consequently, it is unfavorable for the snow algal bloom. Based on these results, a snow algal bloom potential index (API) integrating the meteorological conditions is constructed, and its future trend is projected based on the EC-Earth3 run attending the CMIP6 under SSP245 and SSP585. A significant increasing trend is projected especially under SSP585. Thus snow algal bloom on King George Island will become more frequent and stronger in future. This implies a potential accelerate melting of ice shelf over Antarctic Peninsula.

A case study of weather impact on air pollution during the COVID-19 pandemic in South Africa

This study investigates climate, weather characteristics, and air pollution during the third COVID-19 wave in South Africa. During this time, a decrease in of SO2 and NO2 column mass was observed. The investigation displays a positive trend in for tropospheric Ozone (O3) mass mixing in winter over the last two decades with a fitted slope of 1.0005. Despite the negative trend for the daily changes (for all 12 months) of the total column mass density for PM2.5 during the year before the pandemic and two years after for pandemic years, they have slightly increased specifically during winter COVID-19 third wave time. The temporal seasonal yearly time changes (1961–2021) of different meteorological parameters such as surface temperature, low-level vertical pressure velocity, low-level relative humidity, and precipitation rate over South Africa show sharp changes from 2020 to 2021. This analysis indicates rather colder surface temperatures, smaller but positive low-level vertical velocity, slightly higher low-level relative humidity, and higher total country precipitation rate in the study area during the COVID-19 pandemic years. Weather chart analysis displays a very stable atmospheric condition during the third wave of COVID-19 peak time. This is correlated with the extension of the atmospheric ridge associated with a blocking high pressure, downward motion, positive vorticity, and rather weak winds specific in the north, west, and central areas causing atmospheric stability and possible accumulation of the atmospheric pollution corresponding to the higher COVID-19 cases during the South Africa third wave.

New Observations of the Meteorological Conditions Associated with Particulate Matter Air Pollution Episodes in Santiago, Chile

The meteorological factors of the severe wintertime particulate matter (PM) air pollution problem of the city of Santiago, Chile, are investigated with newly available observations, including a 30 m tower measuring near-surface stability, winds and turbulence, as well as lower-tropospheric vertical profiles of temperature and winds measured by commercial airplanes operating from the Santiago airport (AMDAR database). Focusing on the cold season of the years 2017–2019, high-PM days are defined using an index of evening concentrations measured in the western part of the city. The diurnal cycles of the different meteorological variables computed over 25 PM episodes are compared against the overall diurnal cycles. PM episodes are associated with enhanced surface stability and weaker surface winds and turbulence during the evening and night. AMDAR vertical profiles of temperature and winds during episodes reveal a substantial lower-tropospheric warming attributed to enhanced regional subsidence, which is consistent with the shallower daytime boundary layer depth and the increased surface thermal amplitude observed during these days. An explanation for the weak surface winds during PM episodes was not evident, considering that these are clear days that would strengthen the local valley wind system. Two possible mechanisms are put forward to resolve this issue, which can be tested in the future using high-resolution numerical modeling validated with the new data described here.

Physical mechanisms affecting phytoplankton variability along the Chilean coast

Chile has high phytoplankton production due to being a classic example of an Eastern Boundary Upwelling System. Monthly averaged chlorophyll-α (Chl) and physical parameters (sea surface temperature, precipitation rate, southerly and westerly winds) were studied off the Chilean coast from 2002 to 2018, in order to understand the primary production along this important ocean margin. The coastal margin was split into three zones and ten sub-sections. The Northern Zone had a low phytoplankton production with small seasonal variability, except in its north. This pattern is due to a narrow shelf, weak winds, lack of precipitation and relatively stable weather conditions driven by the Southeast Pacific Subtropical Anticyclone (SPSA). The Central Zone presented a seasonally varying production, with a high Chl concentration in summer and early spring. This is linked to the SPSA movement and sunlight reduction during the winter. A high Chl activity is seen in the Southern Zone despite this Zone being at the limits of the SPSA effect, leading to weak longshore winds only during the warm season. Overall, this study has demonstrated the importance of shelf width and the upwelling driven by the presence or absence of the SPSA for ocean primary production. Thus, the most productive region is from 35°S to 45°S owing to both variables being present.