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Abstract
Meteorological conditions along the west coast of Africa between 30° and 35°S were monitored by means of an instrumented aircraft flown at a height of 150 m over the continental shelf and coastal margin. Case studies, selected from 600 h of aerial survey data encompassing a variety of weather conditions, are used to illustrate mesoscale gradients in the wind, air and sea temperature fields forced by local topographic features. These results focus on the summer season, when the periodic eastward ridging of the South Atlantic high pressure cell gives rise to pulses in SE trade wind flow of variable depth. A composite of four grids describes many of the summer mesoclimatic features found along the west coast, including: divergence of southerly wind flow over the continental shelf in response to friction and seabreezes over the coast; low-level wind jets off the mountainous capes at 33° and 34°S; and wind shadows leeward of the capes where subsidence is enhanced. Topographic channeling is contrasted under inversion heights of 1800 and 600 m using two aerial surveys at the beginning and end of a SE wind event. In the deep flow case, weak meteorological gradients were recorded, while the shallow wind field exhibited an alongshore variability which perturbed the air/sea temperature fronts over the continental shelf. Analyses of wind vorticity, divergence and dewpoint temperature provide evidence of topographically anchored circulations during shallow SE trades. Over a smaller domain encompassing the Cape Peninsula (34°S, 18°E), aerial survey results define mesoscale meteorological conditions during four discrete phases in the subtropical/midlatitude weather cycle. Influences of the synoptic-scale climatology on local topographic and thermal circulations are described.
Abstract
Meteorological conditions along the west coast of Africa between 30° and 35°S were monitored by means of an instrumented aircraft flown at a height of 150 m over the continental shelf and coastal margin. Case studies, selected from 600 h of aerial survey data encompassing a variety of weather conditions, are used to illustrate mesoscale gradients in the wind, air and sea temperature fields forced by local topographic features. These results focus on the summer season, when the periodic eastward ridging of the South Atlantic high pressure cell gives rise to pulses in SE trade wind flow of variable depth. A composite of four grids describes many of the summer mesoclimatic features found along the west coast, including: divergence of southerly wind flow over the continental shelf in response to friction and seabreezes over the coast; low-level wind jets off the mountainous capes at 33° and 34°S; and wind shadows leeward of the capes where subsidence is enhanced. Topographic channeling is contrasted under inversion heights of 1800 and 600 m using two aerial surveys at the beginning and end of a SE wind event. In the deep flow case, weak meteorological gradients were recorded, while the shallow wind field exhibited an alongshore variability which perturbed the air/sea temperature fronts over the continental shelf. Analyses of wind vorticity, divergence and dewpoint temperature provide evidence of topographically anchored circulations during shallow SE trades. Over a smaller domain encompassing the Cape Peninsula (34°S, 18°E), aerial survey results define mesoscale meteorological conditions during four discrete phases in the subtropical/midlatitude weather cycle. Influences of the synoptic-scale climatology on local topographic and thermal circulations are described.
Abstract
Southwest cloud bands during spring (April and May) bring rains to central Caribbean islands at the end of the dry season. A cluster analysis of daily 500-hPa geopotential height fields for 1980–2021 identifies a low-west–high-east dipole pattern related to Pacific–North America response to El Niño–Southern Oscillation (ENSO) and springtime wet spells over the Dominican Republic and Puerto Rico. The regional dipole and local rain time series are ranked to identify the top 10 cases for analysis of meteorological conditions. Hovmöller plots of midlevel meridional wind and specific humidity during April 1983 and May 1986 wet spells reveal a standing Rossby wave train pattern that converges moisture onto the leading edge of a subtropical trough. Composite vertical sections during Caribbean Sea wet spells reveal lower easterly–upper westerly wind anomalies over South America associated with the equatorial Madden–Julian oscillation. Thunderstorm clusters within the southwesterly airflow induce multiday wet spells and flash floods. A second statistical method demonstrated how ENSO underpins Caribbean spring climate anomalies via tropical ocean–atmosphere Rossby wave coupling. Historical trends and long-range projections indicate that springtime tropical–midlatitude interactions may diminish due to an accelerating Hadley cell and retreating jet stream, leading to a delayed onset of the wet season across the Antilles Islands.
Abstract
Southwest cloud bands during spring (April and May) bring rains to central Caribbean islands at the end of the dry season. A cluster analysis of daily 500-hPa geopotential height fields for 1980–2021 identifies a low-west–high-east dipole pattern related to Pacific–North America response to El Niño–Southern Oscillation (ENSO) and springtime wet spells over the Dominican Republic and Puerto Rico. The regional dipole and local rain time series are ranked to identify the top 10 cases for analysis of meteorological conditions. Hovmöller plots of midlevel meridional wind and specific humidity during April 1983 and May 1986 wet spells reveal a standing Rossby wave train pattern that converges moisture onto the leading edge of a subtropical trough. Composite vertical sections during Caribbean Sea wet spells reveal lower easterly–upper westerly wind anomalies over South America associated with the equatorial Madden–Julian oscillation. Thunderstorm clusters within the southwesterly airflow induce multiday wet spells and flash floods. A second statistical method demonstrated how ENSO underpins Caribbean spring climate anomalies via tropical ocean–atmosphere Rossby wave coupling. Historical trends and long-range projections indicate that springtime tropical–midlatitude interactions may diminish due to an accelerating Hadley cell and retreating jet stream, leading to a delayed onset of the wet season across the Antilles Islands.
Abstract
This study considers tropical cyclones Irene in Puerto Rico from 2011 and Isaac in the Dominican Republic from 2012. Impacts trailed more than a day after the storm in both cases. Irene passed Puerto Rico on 22 August 2011, yet bands of heavy rainfall caused floods and disruption on 23 August. In the second case, Isaac passed Hispaniola on 24 August 2012, but stormy weather continued on 25 August. Onshore winds, 4-m waves, and associated tides and river outflow closed the harbor of Santo Domingo. Emergency managers and maritime operators should be aware of the delayed impacts of tropical cyclones in the Caribbean Sea region.
Abstract
This study considers tropical cyclones Irene in Puerto Rico from 2011 and Isaac in the Dominican Republic from 2012. Impacts trailed more than a day after the storm in both cases. Irene passed Puerto Rico on 22 August 2011, yet bands of heavy rainfall caused floods and disruption on 23 August. In the second case, Isaac passed Hispaniola on 24 August 2012, but stormy weather continued on 25 August. Onshore winds, 4-m waves, and associated tides and river outflow closed the harbor of Santo Domingo. Emergency managers and maritime operators should be aware of the delayed impacts of tropical cyclones in the Caribbean Sea region.
Abstract
This study compares different methods of predicting crop-related climate in the Ethiopian highlands for the period 1979–2009. A target index (ETH4) is developed as an average of four variables in the June–September season—rainfall, rainfall minus evaporation, estimated latent heat flux, and vegetation, following correlation with crop yields at Melkassa, Ethiopia (8.4°N, 39.3°E, 1550 m elevation). Predictors are drawn from gridded near-global fields of surface temperature, surface air pressure, and 200-hPa zonal wind in the preceding December–March season. Prediction algorithms are formulated by stepwise multivariate regression. The first set of predictors derive from objective principal component (PC) time scores with tropical loading patterns, and the second set is based on key areas determined from correlation with the target index. The second PC of upper zonal wind reveals a tropical–subtropical dipole that is correlated with ETH4 at two-season lead time (correlation coefficient r = −0.53). Point-to-field regression maps show high-latitude signals in surface temperature (positive in North America and negative in Eurasia) and air pressure (negative in the North Pacific Ocean and positive in the South Pacific). Upper zonal winds are most strongly related with ETH4 over the tropical Pacific and Africa at two-season lead time. The multivariate algorithm that is based on PC predictors has an adjusted r 2 fit of 0.23, and the algorithm using key-area predictors achieves r 2 = 0.37. In comparison, numerical model forecasts reach r 2 = 0.33 for ECMWF simulations but are low for other models. The statistical results are specific to the ETH4 index, which is a climate proxy for crop yields in the Ethiopian highlands.
Abstract
This study compares different methods of predicting crop-related climate in the Ethiopian highlands for the period 1979–2009. A target index (ETH4) is developed as an average of four variables in the June–September season—rainfall, rainfall minus evaporation, estimated latent heat flux, and vegetation, following correlation with crop yields at Melkassa, Ethiopia (8.4°N, 39.3°E, 1550 m elevation). Predictors are drawn from gridded near-global fields of surface temperature, surface air pressure, and 200-hPa zonal wind in the preceding December–March season. Prediction algorithms are formulated by stepwise multivariate regression. The first set of predictors derive from objective principal component (PC) time scores with tropical loading patterns, and the second set is based on key areas determined from correlation with the target index. The second PC of upper zonal wind reveals a tropical–subtropical dipole that is correlated with ETH4 at two-season lead time (correlation coefficient r = −0.53). Point-to-field regression maps show high-latitude signals in surface temperature (positive in North America and negative in Eurasia) and air pressure (negative in the North Pacific Ocean and positive in the South Pacific). Upper zonal winds are most strongly related with ETH4 over the tropical Pacific and Africa at two-season lead time. The multivariate algorithm that is based on PC predictors has an adjusted r 2 fit of 0.23, and the algorithm using key-area predictors achieves r 2 = 0.37. In comparison, numerical model forecasts reach r 2 = 0.33 for ECMWF simulations but are low for other models. The statistical results are specific to the ETH4 index, which is a climate proxy for crop yields in the Ethiopian highlands.
Abstract
The meteorological conditions associated with air pollution episodes on South Africa’s Highveld were studied using Ozone Monitoring Instrument (OMI) and Atmospheric Infrared Sounder (AIRS) satellite estimates, MERRA2 reanalysis model products, and in situ weather data. Surface-layer sulfur dioxide (SO2) and nitrogen dioxide (NO2) display high concentrations during winter (May–July) and provide a focus for statistical analysis of monthly and daily time series. Highveld area-averaged monthly model SO2 was temporally correlated with boundary layer height (correlation coefficient of −0.76) and temperature lapse rate (+0.65) for the period of 1980–2015, but relationships with winds were weak. Daily Highveld area-averaged satellite NO2 was related to dewpoint temperature (−0.59) and exhibited pulsing in the range of 7–24 days for 2005–15. High concentrations of these short-lived locally generated air pollutants were found over and southeast of Johannesburg as a result of urban and industrial emissions. The spatial regression of daily NO2 onto regional sea level air pressure fields for May–July over 2005–15 revealed the slow eastward movement of an anticyclone. At the climatic time scale, Pacific Ocean La Niña conditions favored an increase of May–July SO2 concentrations when sea surface temperatures in the equatorial Atlantic Ocean were warmer than normal. The meteorological pattern underlying the highest-ranked air pollution event of 18–25 July 2008 was characterized by sharp anticyclonic curvature of low-level winds that induce subsidence and consequently a stable lapse rate and low dewpoint temperature (−5°C). The wind vorticity exerted a stronger influence on dispersion than did the surface divergence. This new understanding will underpin better air-quality forecasts over the South African Highveld.
Abstract
The meteorological conditions associated with air pollution episodes on South Africa’s Highveld were studied using Ozone Monitoring Instrument (OMI) and Atmospheric Infrared Sounder (AIRS) satellite estimates, MERRA2 reanalysis model products, and in situ weather data. Surface-layer sulfur dioxide (SO2) and nitrogen dioxide (NO2) display high concentrations during winter (May–July) and provide a focus for statistical analysis of monthly and daily time series. Highveld area-averaged monthly model SO2 was temporally correlated with boundary layer height (correlation coefficient of −0.76) and temperature lapse rate (+0.65) for the period of 1980–2015, but relationships with winds were weak. Daily Highveld area-averaged satellite NO2 was related to dewpoint temperature (−0.59) and exhibited pulsing in the range of 7–24 days for 2005–15. High concentrations of these short-lived locally generated air pollutants were found over and southeast of Johannesburg as a result of urban and industrial emissions. The spatial regression of daily NO2 onto regional sea level air pressure fields for May–July over 2005–15 revealed the slow eastward movement of an anticyclone. At the climatic time scale, Pacific Ocean La Niña conditions favored an increase of May–July SO2 concentrations when sea surface temperatures in the equatorial Atlantic Ocean were warmer than normal. The meteorological pattern underlying the highest-ranked air pollution event of 18–25 July 2008 was characterized by sharp anticyclonic curvature of low-level winds that induce subsidence and consequently a stable lapse rate and low dewpoint temperature (−5°C). The wind vorticity exerted a stronger influence on dispersion than did the surface divergence. This new understanding will underpin better air-quality forecasts over the South African Highveld.
Abstract
This observational study examines zonal gradients in the lower atmosphere and upper ocean across the windward Antilles during midsummer 2012. While earlier work reported on meridional confluence, here the focus is on the zonal enrichment of trade winds by upstream shallow seas and islands. Intercomparisons along 18.2°N are made between modern reanalysis and satellite estimates, and coastal station, moored buoy, aircraft, and radiosonde observations for one case and as a June–July 2012 average. Mean gradients per 100 km east of Puerto Rico were 0.2 g kg−1 for 925-hPa specific humidity, 0.16°C for SST, 0.01 m for sea level, and 2.0 × 10−3 g kg−1 for cloud water. Zonal changes in the lower atmosphere occur in three stages: 1) Atlantic, where westward ocean currents, strong trade winds, and subsidence prevail; 2) Antilles, where ocean currents diminish and sea temperatures increase; and 3) island, where trade winds abate [−1 m s−1 (100 km)−1] and diurnal evapotranspiration greater than 10 mm day−1 offsets large-scale subsidence producing a localized convection.
Abstract
This observational study examines zonal gradients in the lower atmosphere and upper ocean across the windward Antilles during midsummer 2012. While earlier work reported on meridional confluence, here the focus is on the zonal enrichment of trade winds by upstream shallow seas and islands. Intercomparisons along 18.2°N are made between modern reanalysis and satellite estimates, and coastal station, moored buoy, aircraft, and radiosonde observations for one case and as a June–July 2012 average. Mean gradients per 100 km east of Puerto Rico were 0.2 g kg−1 for 925-hPa specific humidity, 0.16°C for SST, 0.01 m for sea level, and 2.0 × 10−3 g kg−1 for cloud water. Zonal changes in the lower atmosphere occur in three stages: 1) Atlantic, where westward ocean currents, strong trade winds, and subsidence prevail; 2) Antilles, where ocean currents diminish and sea temperatures increase; and 3) island, where trade winds abate [−1 m s−1 (100 km)−1] and diurnal evapotranspiration greater than 10 mm day−1 offsets large-scale subsidence producing a localized convection.
Abstract
An analysis of food and water supplies and economic growth in South Africa leads to the realization that climate variability plays a major role. Summer rainfall in the period of 1980–99 is closely associated (variance = 48%) with year-to-year changes in the gross domestic product (GDP). Given the strong links between climate and resources, statistical models are formulated to predict maize yield, river flows, and GDP directly. The most influential predictor is cloud depth (outgoing longwave radiation) in the tropical Indian Ocean in the preceding spring (September–November). Reduced monsoon convection is related to enhanced rainfall over South Africa in the following summer and greater economic prosperity during the subsequent year. Methodologies are outlined and risk-reduction strategies are reviewed. It is estimated that over U.S.$1 billion could be saved annually through uptake of timely and reliable long-range forecasts.
Abstract
An analysis of food and water supplies and economic growth in South Africa leads to the realization that climate variability plays a major role. Summer rainfall in the period of 1980–99 is closely associated (variance = 48%) with year-to-year changes in the gross domestic product (GDP). Given the strong links between climate and resources, statistical models are formulated to predict maize yield, river flows, and GDP directly. The most influential predictor is cloud depth (outgoing longwave radiation) in the tropical Indian Ocean in the preceding spring (September–November). Reduced monsoon convection is related to enhanced rainfall over South Africa in the following summer and greater economic prosperity during the subsequent year. Methodologies are outlined and risk-reduction strategies are reviewed. It is estimated that over U.S.$1 billion could be saved annually through uptake of timely and reliable long-range forecasts.
Abstract
This study reviews Kenya’s fluctuating hydroclimate (3°S–4°N, 35°–40°E) and evaluates products that describe its area-averaged daily rainfall during 2008–18, monthly evaporation during 2000–18, and catchment hydrology via gauge, satellite, and model hindcast/forecast. Using the correlation of rainfall as a metric of skill we found daily satellite versus model hindcasts achieved 75%, while model forecasts at 2–6-day lead achieved 55%–58%. The daily satellite versus model soil moisture had a significant correlation (84%), and model runoff versus gauge streamflow reached 61%. A 2-day delay was noted between rainfall and streamflow response in recent flood events; however, long-range predictability was found to be poor (35%). These outcomes were considered at a local workshop, and ways to sustainably improve the real-time reporting of key hydroclimate parameters for operational data assimilation were suggested as steps toward better monitoring and forecast services in Kenya.
Abstract
This study reviews Kenya’s fluctuating hydroclimate (3°S–4°N, 35°–40°E) and evaluates products that describe its area-averaged daily rainfall during 2008–18, monthly evaporation during 2000–18, and catchment hydrology via gauge, satellite, and model hindcast/forecast. Using the correlation of rainfall as a metric of skill we found daily satellite versus model hindcasts achieved 75%, while model forecasts at 2–6-day lead achieved 55%–58%. The daily satellite versus model soil moisture had a significant correlation (84%), and model runoff versus gauge streamflow reached 61%. A 2-day delay was noted between rainfall and streamflow response in recent flood events; however, long-range predictability was found to be poor (35%). These outcomes were considered at a local workshop, and ways to sustainably improve the real-time reporting of key hydroclimate parameters for operational data assimilation were suggested as steps toward better monitoring and forecast services in Kenya.
Abstract
This study examines the spatial variability of mean annual rainfall in the Caribbean in the satellite era 1979–2000. Intercomparisons of gridded rainfall fields from conventional stations, satellite estimators, reanalysis products, and coupled general circulation models (CGCMs) are made, with a focus on the Antilles island chain and their land–sea transitions. The rainfall products are rated for their ability to capture a number of key features, including (i) topographically enhanced precipitation over the larger western Antilles islands of Cuba, Jamaica, Hispanola, and Puerto Rico; (ii) the rain shadow west of Hispanola; (iii) the two dry zones where SSTs are low: north of Venezuela and north of the Lesser Antilles; and (iv) the wet axis extending north of Trinidad. The various monitoring and modeling systems produce gridded rainfall fields at resolutions from 50 to 280 km, from station reconstructions, satellite estimates, blended and reanalysis products, and CGCM climatologies with respect to surface forcing fields. Wet and dry biases were found in many of the reanalysis and satellite products, respectively—either over the whole Caribbean or in a certain sector. The intercomparison found some measure of consensus, but no single product is without discrepancy. High-resolution passive microwave satellite rainfall estimates [Climate Prediction Center’s multisaltellite passive microwave, IR morphed product (cMOR)] appear “most representative”; however, the climatology is short (2003–07) and the field is generally drier than the consensus. Of the conventional products, decadal variability of climate interpolated rain gauges (DEKL), World Climate Research Programme’s (WCRP) blended rain gauges, the Comprehensive Ocean–Atmosphere Data Set (COADS), and an operational climate anomaly monitoring system of NCEP (CAMS) perform well. Among the satellite estimators, the Global Precipitation Climatology Project’s blended gauge and IR satellite (GPCP) and outgoing longwave radiation (OLR) capture the key features and ocean–island transitions. The Center for Ocean–Land–Atmosphere Studies [COLA; the coupled model, part of the Coupled Model Intercomparison Project (CMIP, phase 3)] and the climate forecast system of the NCEP (CFS) models perform reasonably, but NCAR’s Parallel Climate Model (PCM; the CGCM’s historical run of CMIP3) fares poorly. The version 2 hindcast of the operational Medium-Range Forecast (MRF) weather prediction model (REAN) captures the smaller wet zones and topographically enhanced features, but it does not handle the broad oceanic dry zones well, as the input from the operational climate data assimilation system of NCEP (CDAS) has a wet bias. Of the various key rainfall features, high rainfall over southern Cuba and the rain shadow west of Hispanola are poorly handled by most products. The wet axis north of Trinidad and the dry zone north of Venezuela are well represented in many climatologies.
Abstract
This study examines the spatial variability of mean annual rainfall in the Caribbean in the satellite era 1979–2000. Intercomparisons of gridded rainfall fields from conventional stations, satellite estimators, reanalysis products, and coupled general circulation models (CGCMs) are made, with a focus on the Antilles island chain and their land–sea transitions. The rainfall products are rated for their ability to capture a number of key features, including (i) topographically enhanced precipitation over the larger western Antilles islands of Cuba, Jamaica, Hispanola, and Puerto Rico; (ii) the rain shadow west of Hispanola; (iii) the two dry zones where SSTs are low: north of Venezuela and north of the Lesser Antilles; and (iv) the wet axis extending north of Trinidad. The various monitoring and modeling systems produce gridded rainfall fields at resolutions from 50 to 280 km, from station reconstructions, satellite estimates, blended and reanalysis products, and CGCM climatologies with respect to surface forcing fields. Wet and dry biases were found in many of the reanalysis and satellite products, respectively—either over the whole Caribbean or in a certain sector. The intercomparison found some measure of consensus, but no single product is without discrepancy. High-resolution passive microwave satellite rainfall estimates [Climate Prediction Center’s multisaltellite passive microwave, IR morphed product (cMOR)] appear “most representative”; however, the climatology is short (2003–07) and the field is generally drier than the consensus. Of the conventional products, decadal variability of climate interpolated rain gauges (DEKL), World Climate Research Programme’s (WCRP) blended rain gauges, the Comprehensive Ocean–Atmosphere Data Set (COADS), and an operational climate anomaly monitoring system of NCEP (CAMS) perform well. Among the satellite estimators, the Global Precipitation Climatology Project’s blended gauge and IR satellite (GPCP) and outgoing longwave radiation (OLR) capture the key features and ocean–island transitions. The Center for Ocean–Land–Atmosphere Studies [COLA; the coupled model, part of the Coupled Model Intercomparison Project (CMIP, phase 3)] and the climate forecast system of the NCEP (CFS) models perform reasonably, but NCAR’s Parallel Climate Model (PCM; the CGCM’s historical run of CMIP3) fares poorly. The version 2 hindcast of the operational Medium-Range Forecast (MRF) weather prediction model (REAN) captures the smaller wet zones and topographically enhanced features, but it does not handle the broad oceanic dry zones well, as the input from the operational climate data assimilation system of NCEP (CDAS) has a wet bias. Of the various key rainfall features, high rainfall over southern Cuba and the rain shadow west of Hispanola are poorly handled by most products. The wet axis north of Trinidad and the dry zone north of Venezuela are well represented in many climatologies.
Abstract
Ocean and atmosphere reanalysis fields are used to study environmental conditions and their relation to commercial fish catch in the central Benguela upwelling zone, using both targeted and objective techniques. Composite maps and sections indicate a 10%–20% weakening of southeasterly winds, a 0.5°C warming of sea temperatures over the shelf, and changes in currents and subsurface upwelling associated with higher fish catch. During periods of high fish catch, recirculating gyres form that may aid the retention of eggs and larvae. Offshore winds contribute to poleward Ekman transport in a 50-m-deep layer within 100 km of the coast.
In addition to composite analysis, the natural variability is studied by principal component analysis of wind stress, sea level, temperature, salinity, currents, and vertical motion in the period 1970–2007. Comparison of interannual time scores and fisheries data indicate that anomalous poleward winds and warmer temperatures in the Lüderitz plume, driven by an atmospheric trough in the South Atlantic, are associated with higher catch rates.
Abstract
Ocean and atmosphere reanalysis fields are used to study environmental conditions and their relation to commercial fish catch in the central Benguela upwelling zone, using both targeted and objective techniques. Composite maps and sections indicate a 10%–20% weakening of southeasterly winds, a 0.5°C warming of sea temperatures over the shelf, and changes in currents and subsurface upwelling associated with higher fish catch. During periods of high fish catch, recirculating gyres form that may aid the retention of eggs and larvae. Offshore winds contribute to poleward Ekman transport in a 50-m-deep layer within 100 km of the coast.
In addition to composite analysis, the natural variability is studied by principal component analysis of wind stress, sea level, temperature, salinity, currents, and vertical motion in the period 1970–2007. Comparison of interannual time scores and fisheries data indicate that anomalous poleward winds and warmer temperatures in the Lüderitz plume, driven by an atmospheric trough in the South Atlantic, are associated with higher catch rates.
