Mid-summer Drought Research Articles

A wavelet transform method to determine monsoon onset and retreat from precipitation time‐series

AbstractA new method to determine monsoon onset and retreat timings using wavelet transform methodology applied to precipitation time‐series at the pentad scale is described. The principal advantage of this method is its portability, since it can be easily adapted for any region and dataset. The application of the method is illustrated for the North American Monsoon and the Indian Monsoon using four different precipitation datasets and climate model output. The method is shown to be robust across all the datasets and both monsoon regions. The mean onset and retreat dates agree well with previous methods. Spatial distributions of the precipitation and circulation anomalies identified around the onset and retreat dates are also consistent with previous work and illustrate that this method may be used at the grid‐box scale, not just over large area‐averaged regions. The method is also used to characterize the strength and timing of the Midsummer drought (MSD) in southern Mexico and Central America. A two peak structure is found to be a robust structure in only in 33% of the years, with other years showing only one peak or no signs of a bimodal distribution. The two‐peak structure analysed at the grid‐box scale is shown to be a significant signal in several regions of Central America and southern Mexico. The methodology is also applied to climate model output from the Met Office Hadley Centre UKESM1 and HadGEM3 CMIP6 experiments. The modelled onset and retreat dates agree well with observations in the North American Monsoon but not in the Indian Monsoon. The start and end of the modelled MSD in southern Mexico and Central America is delayed by one pentad and has a stronger bimodal signal than observed.

The mid-summer drought spatial variability over Mesoamerica

The region that includes southern Mexico and Central America is known as Mesoamerica. The annual cycle of precipitation characteristic of the Pacific slope of this region presents a bimodal distribution on summertime, with two maxima and a relative intraseasonal minimum, known as the Mid-Summer Drought (MSD). In this study, the small-scale (tens of kilometers) variability of the MSD is analyzed. Numerical simulations are performed using the regional climate model RegCM4 over a Mesoamerican domain for a six-year period. ERA Interim reanalysis data is used as initial and lateral boundary conditions. The domain is subdivided into smaller areas and the average annual cycle of precipitation distribution is computed for each of them. The MSD pattern is found to present a high spatial variability in the intensity of its two maxima and even the total absence of its characteristic minimum. Such behavior is attributed to soil-atmosphere and terrain topography interactions that are better resolved with a regional climate model.

Spatio-Temporal Distribution of Deep Convection Observed along the Trans-Mexican Volcanic Belt

Complex terrain features—in particular, environmental conditions, high population density and potential socio-economic damage—make the Trans-Mexican Volcanic Belt (TMVB) of particular interest regarding the study of deep convection and related severe weather. In this research, 10 years of Moderate-Resolution Imaging Spectroradiometer (MODIS) cloud observations are combined with Climate Hazards Group Infrared Precipitation with Station (CHIRPS) rainfall data to characterize the spatio-temporal distribution of deep convective clouds (DCCs) and their relationship to extreme precipitation. From monthly distributions, wet and dry phases are identified for cloud fraction, deep convective cloud frequency and convective precipitation. For both DCC and extreme precipitation events, the highest frequencies align just over the higher elevations of the TMVB. A clear relationship between DCCs and terrain features, indicating the important role of orography in the development of convective systems, is noticed. For three sub-regions, the observed distributions of deep convective cloud and extreme precipitation events are assessed in more detail. Each sub-region exhibits different local conditions, including terrain features, and are known to be influenced differently by emerging moisture fluxes from the Gulf of Mexico and the Pacific Ocean. The observed distinct spatio-temporal variabilities provide the first insights into the physical processes that control the convective development in the study area. A signal of the midsummer drought in Mexico (i.e., “canícula”) is recognized using MODIS monthly mean cloud observations.

Correction to: Hydraulic traits of co-existing conifers do not correlate with local hydroclimate condition: a case study in the northern Rocky Mountains, U.S.A

In this study, we examined the inter- and intra-specific variation of hydraulic traits of three conifers of the Northern Rockies: Pinus ponderosa, Picea engelmannii, and Pseudotsuga menziesii to understand the mechanisms that allow different plant species to co-exist across a watershed. We quantified differences in plant xylem water potential (ψx), xylem tissue vulnerability to cavitation (P50, or ψ causing 50% loss of hydraulic conductivity), and safety margins for co-occurring trees from low and high elevations. We then investigated xylem vulnerability to cavitation with rooting depth. We found that xylem vulnerability to cavitation did not correspond to where tree species were found in the landscape. For example, P. ponderosa grew in more xeric locations, while P. engelmannii were largely confined to more mesic locations, yet P. engelmannii had more negative P50 values. P. menziesii had the lowest P50 value, but displayed little variation in vulnerability to cavitation across the hydroclimatic gradient. These patterns were also reflected in the safety margins; P. menziesii had the widest safety margin, P. engelmannii was intermediate, and P. ponderosa displayed the narrowest safety margin. All three species were also using water sources deeper than 30 cm in depth, allowing them to persist throughout the mid-summer drought. Overall, species-specific hydraulic traits did not necessarily follow a predictable response to the environment; instead, a combination of physiological and morphological traits likely allow trees to persist across a broader hydroclimatic gradient than would be otherwise expected from hydraulic trait measurements alone.

Stocking rate of extensive land-reform livestock farmers during 2018/2019 drought: Bloemfontein grassland biome case study

The study aimed to determine the average stocking rate among land reform beneficiary farmers specialising in livestock production in order to establish differences between calving percentage, fodder availability, and mortality rate of sampled farms, as well as to compare forage scarcities of Land Reform farms with their neighbouring farms during the midsummer drought of 2018/2019 in the Bloemfontein area. The average stocking rate was 5.9 ha/LSU in comparison with the Departmental grazing capacity norm of 6 ha/LSU for rangeland in good condition. However, 31% of the sampled farms were found to be severely overstocked, and the mortality rate on these farms, in relation to grazing capacity of 6 ha/LSU, was significantly higher (P < 0.05) than the mortalities on the other remaining farms. Naturally available fodder was found to be heterogeneous, with 37.9% of the respondents observing their available fodder as worse than that of their neighbours. The total mortality of 176.77 LSUs was recorded for the 29 sampled farms. These findings will assist the local extension personnel prevent future rangeland condition degradation and increase land reform farmers’ productivity. The study concluded that training is paramount to farmers’ development and further recommends more research undertakings.

Precipitation and Temperature in Costa Rica at the End of the Century Based on NEX-GDDP Projected Scenarios

The evaluation of intraseasonal, seasonal, and annual variability of rainfall and temperature extremes, while using climate change scenarios data, is extremely important for socio-economic activities, such as water resources management. Costa Rica, a climate change hotspot, is largely dependent on rainfall for socioeconomic activities; hence, the relevance of this study. Based on the NEX-GDDP, rainfall and temperature range were analyzed for Costa Rica at the end of the century (2070–2099), while using 1970–1999 as a baseline for six available meteorological stations. Differences between the multimodel ensembles of two prospective scenarios (RCP 4.5 and 8.5) and the historical information were computed. This study highlights Costa Rica as an inflexion region for climate change impacts in Central America, for which projected scenarios suggest an early onset of the rainy season, and a decline in the mid-summer drought (MSD) minimum. The assessment of model data in some regions of Costa Rica, for which historical data were available, suggests that the latter does not capture a well-known regional climate feature, the MSD, in both precipitation and temperature range well. The availability of observed past data sources is a major limitation of this research; however, with the station data used, it is still possible to draw some conclusions regarding future climate in some regions of Costa Rica, especially in the northwest side of the country, where past data are consistent with model information, providing a more reliable picture of changes in climate there that has potential implications for socioeconomic sectors.

Hydraulic traits of co-existing conifers do not correlate with local hydroclimate condition: a case study in the northern Rocky Mountains, U.S.A

In this study, we examined the inter- and intra-specific variation of hydraulic traits of three conifers of the Northern Rockies:Pinus ponderosa, Picea engelmannii, and Pseudotsuga menziesiito understand the mechanisms that allow different plant species to co-exist across a watershed. We quantified differences in plant xylem water potential (ψx), xylem tissue vulnerability to cavitation (P50, or ψ causing 50% loss of hydraulic conductivity), and safety margins for co-occurring trees from low and high elevations. We then investigated xylem vulnerability to cavitation with rooting depth. We found that xylem vulnerability to cavitation did not correspond to where tree species were found in the landscape. For example,P. ponderosagrew in more xeric locations, whileP. engelmannii were largely confined to more mesic locations, yet P. engelmanniihad more negative P50values.P. menziesiihad the lowest P50value, but displayed little variation in vulnerability to cavitation across the hydroclimatic gradient. These patterns were also reflected in the safety margins;P. menziesiihad the widest safety margin,P. engelmanniiwas intermediate, andP. ponderosadisplayed the narrowest safety margin. All three species were also using water sources deeper than 30cm in depth, allowing them to persist throughout the mid-summer drought. Overall, species-specific hydraulic traits did not necessarily follow a predictable response to the environment; instead, a combination of physiological and morphological traits likely allow trees to persist across a broader hydroclimatic gradient than would be otherwise expected from hydraulic trait measurements alone.

Influence of the Madden-Julian Oscillation on moisture transport by the Caribbean Low Level Jet during the Midsummer Drought in Mexico

This study aims to improve the understanding of the role of the Madden-Julian Oscillation (MJO) in modulating the midsummer drought (MSD) in Mexico. First, the main moisture sources for the MSD region in Mexico during the summer were identified using the Lagrangian particle dispersion model FLEXPART for the period 1979–2017. From this analysis, the Caribbean Sea was identified as one of the main moisture sources, particularly the area where the core of the Caribbean low-level jet (CLLJ) is located. Second, the water vapour flux transported by the CLLJ to Mexico was then analysed. It was found that the summer seasonal cycle of the CLLJ is associated with the bimodal precipitation pattern in Mexico through the seasonal variability of the moisture contribution from the CLLJ to precipitation over the MSD region, thereby confirming the CLLJ as a key element in the occurrence of the MSD in the Americas. Third, the influence of the MJO on this transport was examined. This analysis showed that the locally dry phases of the MJO decrease the contribution of moisture from the CLLJ core towards the MSD region, while the locally wet phases increase it. Moreover, phases 1 and 2 of the MJO were found to influence the first precipitation peak that occurs in the southwestern region of Mexico by increasing the contribution of moisture from the northeastern tropical Pacific. The study ends by proposing, for the first time, a mechanism by which the MJO modulates the MSD in Mexico.

The American monsoon system in HadGEM3 and UKESM1

Abstract. The simulated climate of the American monsoon system (AMS) in the UK models HadGEM3 GC3.1 (GC3) and the Earth system model UKESM1 is assessed and compared to observations and reanalysis. We evaluate the pre-industrial control, AMIP and historical experiments of UKESM1 and two configurations of GC3: a low (1.875∘×1.25∘) and a medium (0.83∘×0.56∘) resolution. The simulations show a good representation of the seasonal cycle of temperature in monsoon regions, although the historical experiments overestimate the observed summer temperature in the Amazon, Mexico and Central America by more than 1.5 K. The seasonal cycle of rainfall and general characteristics of the North American monsoon of all the simulations agree well with observations and reanalysis, showing a notable improvement from previous versions of the HadGEM model. The models reasonably simulate the bimodal regime of precipitation in southern Mexico, Central America and the Caribbean known as the midsummer drought, although with a stronger-than-observed difference between the two peaks of precipitation and the dry period. Austral summer biases in the modelled Atlantic Intertropical Convergence Zone (ITCZ), cloud cover and regional temperature patterns are significant and influence the simulated regional rainfall in the South American monsoon. These biases lead to an overestimation of precipitation in southeastern Brazil and an underestimation of precipitation in the Amazon. The precipitation biases over the Amazon and southeastern Brazil are greatly reduced in the AMIP simulations, highlighting that the Atlantic sea surface temperatures are key for representing precipitation in the South American monsoon. El Niño–Southern Oscillation (ENSO) teleconnections, of precipitation and temperature, to the AMS are reasonably simulated by all the experiments. The precipitation responses to the positive and negative phase of ENSO in subtropical America are linear in both pre-industrial and historical experiments. Overall, the biases in UKESM1 and the low-resolution configuration of GC3 are very similar for precipitation, ITCZ and Walker circulation; i.e. the inclusion of Earth system processes appears to make no significant difference for the representation of the AMS rainfall. In contrast, the medium-resolution HadGEM3 N216 simulation outperforms the low-resolution simulations due to improved SSTs and circulation.

Characteristic atmospheric states during mid-summer droughts over Central America and Mexico

Annual precipitation over Central America and large areas of Mexico is typically characterised by its bimodal distribution, with a precipitation minimum in July to August that occurs between two separate maxima from May to July and August to October. Several theories have been proposed to explain this phenomenon, which is often termed the mid-summer drought (MSD), but most fail to address the different characteristics associated with individual MSD events. Here, a regression-based approach is used to detect and quantify the annual and climatological MSD signature over Central America and Mexico. This approach has been evaluated and shown to be robust for various datasets with different spatial resolutions. It was found that in the southeast of the Mexico/Central America region, MSDs start earlier and end later than elsewhere, and are thus longer in duration. However, the coast of the Gulf of Mexico, Cuba, and large areas of Central America, exhibit climatologically stronger MSDs. Changes in precipitation, brought about by the interaction between reversals of the onshore/offshore winds and orographic forcing associated with the steep mountainous terrain, have also been shown to be significant factors in the timing of MSD occurrences, offering support for a combined theory of large-scale dynamics and regional forcing. Using self-organising maps (SOMs) as an analysis tool, it was found that MSD events over the domain display strong spatial variability. The MSDs over the domain also generate distinct signatures and may be forced by particular mechanisms. We found that El Nino-Southern Oscillation (ENSO) could be a potential classifier for the SOM identified atmospheric states, based on the correspondence of MSD occurrences with ENSO phases.

Community perception, adaptation and resilience to extreme weather in the Yucatan Peninsula, Mexico

Perceptions of climate change, the impacts of and responses to climatic variability and extreme weather are explored in three communities in the Yucatan Peninsula, Mexico, in relation to livelihood resilience. These communities provide examples of the most common livelihood strategies across the region: small-scale fisheries (San Felipe) and semi-subsistence small-holder farming (Tzucacab and Calakmul). Although the perception that annual rainfall is reducing is not supported by instrumental records, changes in the timing of vital summer rainfall and an intensification of the mid-summer drought (canicula) are confirmed. The impact of both droughts and hurricanes on livelihoods and crop yields was reported across all communities, although the severity varied. Changes in traditional milpa cultivation were seen to be driven by less reliable rainfall but also by changes in Mexico’s agricultural and wider economic policies. Diversification was a common adaptation response across all communities and respondents, resulting in profound changes in livelihood strategies. Government attempts to reduce vulnerability were found to lack continuity, be hard to access and too orientated toward commercial scale producers. Population growth, higher temperatures and reduced summer rainfall will increase the pressures on communities reliant on small-scale farming and fishing, and a more nuanced understanding of both impacts and adaptations is required for improved livelihood resilience. Greater recognition of such local-scale adaptation strategies should underpin the developing Mexican National Adaptation Policy and provide a template for approaches internationally as adaptation becomes an increasingly important part of the global strategy to cope with climate change.

Multiscale trends and precipitation extremes in the Central American Midsummer Drought

Anecdotal evidence suggests that the timing and intensity of the Central American Midsummer Drought (MSD) may be changing, while observations from limited meteorological station data and paleoclimate reconstructions show neither significant nor consistent trends in seasonal rainfall. Climate model simulations project robust future drying across the region, but internal variability is expected to dominate until the end of the century. Here we use a high-resolution gridded precipitation dataset to investigate these apparent discrepancies and to quantify the spatiotemporal complexities of the MSD. We detect spatially variable trends in MSD timing, the amount of rainy season precipitation, the number of consecutive and total dry days, and extreme wet events at the local scale. At the regional scale, we find a positive trend in the duration, but not the magnitude of the MSD, which is dominated by spatially heterogeneous trends and interannual variability linked to large-scale modes of ocean-atmosphere circulation. Although the current climate still reflects predominantly internal variability, some Central American communities are already experiencing significant changes in local characteristics of the MSD. A detailed spatiotemporal understanding of MSD trends and variability can contribute to evidence-based adaptation planning and help reduce the vulnerability of Central American communities to both natural rainfall variability and anthropogenic change.

The Spatial Pattern of Midsummer Drought as a Possible Mechanistic Response to Lower-Tropospheric Easterlies over the Intra-Americas Seas

Abstract Following the idea that large-scale wind perturbations cause repeatable rainfall patterns over small tropical islands, the spatial pattern of the midsummer drought (MSD) is investigated as a repeatable rainfall pattern over the Intra-Americas Seas (IAS). For that, statistical techniques, including linear regressions, canonical correlation analysis, and variance budgets, were applied to the Tropical Rainfall Measuring Mission and ERA-Interim datasets to assess 1) the MSD pattern repeatability and 2) its explained variance in different time scales. As shown by the results, the MSD pattern is not a unique feature of the boreal summer intraseasonal variability in the IAS: it is more robust during summer but it exists in all rainy seasons on daily, intraseasonal, and interannual time scales. On diurnal time scales, the MSD pattern explains a negligible part of the total variance during summer (<2%), but on interannual scales it explains up to 20% and it captures the spatial features of “El Niño” rainfall anomalies. On all time scales, the MSD pattern is accompanied by repeatable wind and pressure patterns: anomalous lower-tropospheric (925 hPa) easterlies over a domain-wide meridional northward pressure gradient. These results provide evidence for the hypothesis that the MSD pattern manifests an underlying geographically determined, mechanistic pattern. Also, they suggest that the repeatable MSD-shaped rainfall and wind patterns could be extrapolated in time to better understand the climatic conditions behind droughts and pluvials, and to diagnose the causes behind rainfall trends.

The mid‐summer drought over Mexico and Central America in the 21st century

AbstractThe southern Mexico and Central America (SMCA) region shows a dominant well‐defined precipitation annual cycle. The rainy season usually begins in May and ends in October, with a relatively dry period in July and August known as the mid‐summer drought (MSD); notable exceptions are the Caribbean coast of Honduras and Costa Rica. This MSD phenomenon is expected to be affected as the SMCA experiences an enhanced differential warming between the Pacific and Atlantic Oceans (PO‐AO) towards the end of the 21st century. Previous studies have suggested that this differential warming will induce a strengthening of the westward Caribbean low‐level jet (CLLJ) and that this heightened CLLJ will shift precipitation westwards, falling on the PO instead that within the SMCA region causing a severe drought. In this work we examine this scenario with a new model, the Rossby Center Regional Climate Model (RCA4), for the COordinated Regional climate Downscaling EXperiment (CORDEX) Central America domain, forced with different general circulation models (GCMs) and for different representative concentration paths (RCPs). We consider 25‐year periods as “present conditions” (1981–2005) and “future scenario” (2071–2095), focusing on the “extended summer” season (May–October). Results suggest that in the future the spatial extension of the MSD will decrease and that in certain areas the MSD will be more intense but less frequent compared to present conditions. Also, the oceanic differential warming, the intensification of the CLLJ, and the reduction in regional precipitation in the future scenario, suggested by previous works, were verified in this study.

Climatic trends and regional climate models intercomparison over the CORDEX‐CAM (Central America, Caribbean, and Mexico) domain

AbstractAn intercomparison of three regional climate models (RCMs) (PRECIS‐HadRM3P, RCA4, and RegCM4) was performed over the Coordinated Regional Dynamical Experiment (CORDEX)—Central America, Caribbean, and Mexico (CAM) domain to determine their ability to reproduce observed temperature and precipitation trends during 1980–2010. Particular emphasis was given to the North American monsoon (NAM) and the mid‐summer drought (MSD) regions. The three RCMs show negative (positive) temperature (precipitation) biases over the mountains, where observations have more problems due to poor data coverage. Observations from the Climate Research Unit (CRU) and ERA‐Interim show a generalized warming over the domain. The most significant warming trend (≥0.34°C/decade) is observed in the NAM, which is moderately captured by the three RCMs, but with less intensity; each decade from 1970 to 2016 has become warmer than the previous ones, especially during the summer (mean and extremes); this warming appears partially related to the positive Atlantic Multidecadal Oscillation (+AMO). CRU, GPCP, and CHIRPS show significant decreases of precipitation (less than −15%/decade) in parts of the southwest United States and northwestern Mexico, including the NAM, and a positive trend (5–10%/decade) in June–September in eastern Mexico, the MSD region, and northern South America, but longer trends (1950–2017) are not statistically significant. RCMs are able to moderately simulate some of the recent trends, especially in winter. In spite of their mean biases, the RCMs are able to adequately simulate inter‐annual and seasonal variations. Wet (warm) periods in regions affected by the MSD are significantly correlated with the +AMO and La Niña events (+AMO and El Niño). Summer precipitation trends from GPCP show opposite signs to those of CRU and CHIRPS over the Mexican coasts of the southern Gulf of Mexico, the Yucatan Peninsula, and Cuba, possibly due to data limitations and differences in grid resolutions.

Coupled Interannual Variability of Wind and Sea Surface Temperature in the Caribbean Sea and the Gulf of Mexico

AbstractThis work describes dominant patterns of coupled interannual variability of the 10-m wind and sea surface temperature in the Caribbean Sea and the Gulf of Mexico (CS&GM) during the period 1982–2016. Using a canonical correlation analysis (CCA) between the monthly mean anomalies of these fields, four coupled variability modes are identified: the dipole (March–April), transition (May–June), interocean (July–October), and meridional-wind (November–February) modes. Results show that El Niño–Southern Oscillation (ENSO) influences almost all the CS&GM coupled modes, except the transition mode, and that the North Atlantic Oscillation (NAO) in February has a strong negative correlation with the dipole and transition modes. The antisymmetric relationships found between the dipole mode and the NAO and ENSO indices confirm previous evidence about the competing remote forcings of both teleconnection patterns on the tropical North Atlantic variability. Precipitation in the CS and adjacent oceanic and land areas is sensitive to the wind–SST coupled variability modes from June to October. These modes seem to be strongly related to the interannual variability of the midsummer drought and the meridional migration of the intertropical convergence zone in the eastern Pacific. These findings may eventually lead to improving seasonal predictability in the CS&GM and surrounding land areas.

Intraseasonal Variability of Summer Precipitation in Mexico: MJO Influence on the Midsummer Drought

Abstract The aim of this study is to understand how the Madden–Julian oscillation (MJO) modulates the bimodal seasonal rainfall distribution across the regions in Mexico where the midsummer drought (MSD) occurs. The MSD is characterized by a precipitation decrease in the middle of the rainy season. Relative frequencies of each active phase of the Real-time Multivariate MJO index were calculated at each grid point in the high-resolution Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) rainfall dataset for the first (MAX1) and second (MAX2) rainfall peaks and the MSD minimum (MIN). In addition, standardized anomalies of precipitation (from the CHIRPS dataset) and 300-hPa omega, 500-hPa geopotential height, and 850-hPa u- and υ-wind components (from the Climate Forecast System Reanalysis) were calculated for each MJO phase and each month in the rainy season. Results show that the MIN (MAX2) occurs more frequently during the dry (wet) MJO phases, while the MJO seems not to influence MAX1 significantly. Anomalous anticyclonic (cyclonic) circulations at 850 hPa, positive (negative) 500-hPa geopotential height anomalies, northeast (southwest) 850-hPa wind anomalies over southern Mexico, and a low-level westward (eastward) flow in the northeastern tropical Pacific support the MIN (MAX2) pattern under the influence of the dry (wet) MJO phases. These features are more clearly observed in the MSDs of 1- and 2-month duration and over the southern half of Mexico. The results suggest that the bimodal distribution is less influenced by the MJO in regions of northeastern Mexico.

Climate model projections for future seasonal rainfall cycle statistics in Northwest Costa Rica

Increasing water scarcity due to rising demand and changes in climate and land use are expected to exert significant stress on water resources in many parts of the world. In many areas, distinctive patterns of seasonal precipitation play an important role in regional ecosystems, economies, and food and energy supplies. Given the inconsistency among climate model precipitation projections, this study develops a statistical approach to better characterize seasonal patterns of precipitation and to assess the potential impact of climate change on these patterns. The method is applied to evaluate the bimodal seasonal pattern of precipitation in northwest Costa Rica as a case study. A Gaussian mixture model is used to describe the bimodal pattern and quantify changes in the seasonal precipitation cycle projected by 19 Coupled General Circulation Models. The model simulations for the current period (1979–2005) are compared with observed monthly precipitation data based on four goodness‐of‐fit metrics to select the best performing models. The monthly bias‐corrected and spatially disaggregated (BCSD) climate projections from the selected models are used to investigate the projected change in the bimodal seasonal pattern, seasonal mean precipitation and interannual variability at the late twenty‐first century. Under RCP8.5, the degree of consistency associated with these BCSD climate projections is found to be higher for the projections of the midsummer drought than that for the early portion of the wet season (early season). For the late portion of the wet season (late season) projections, the degree of consistency is found to be the lowest. The proposed method provides (a) an overall characterization of inconsistencies in climate model projections for the region, (b) more information about the possible changes in the seasonal cycle of precipitation, and (c) the possibility of performing detailed comparison tests among climate models over a set of simulations and projections.

Sediment Bypassing from the New Human-Induced Lobe to the Ancient Lobe of the Turbo Delta (Gulf of Urabá, Southern Caribbean Sea)

Alcantara-Carrio, J.; Caicedo, A.; Hernandez, J.C.; Jaramillo-Velez, A., and Manzolli, R.P., 2019. Sediment bypassing from the new human-induced lobe to the ancient lobe of the Turbo Delta (Gulf of Uraba, Southern Caribbean Sea). Journal of Coastal Research, 35(1), 196–209. Coconut Creek (Florida), ISSN 0749-0208.Artificial river diversions cause intense and unpredictable changes in the geomorphological evolution and sedimentary dynamics of deltas. This study analyzes the effects of the 1958 course diversion of the Turbo River delta, Gulf of Uraba, southern Caribbean Sea. Recent satellite images were analyzed and beach topography was monitored to obtain the shoreline migration and sediment budget; surface sediments were seasonally sampled, and their grain size parameters were determined; and seasonal sediment transport directions were deduced according to grain size trends. It was concluded that the bimodal wave regime controls the seasonal sedimentary patterns associated with southward longshore transport during both the dry and rainy seasons, and the northward longshore transport occurs in the midsummer drought. Beach morphodynamics of Yarumal Point, close to the present-day river mouth, show that this is the most exposed sector to waves, whereas fluvial sediment inputs are evidenced by a decrease in mean grain size and by poorly sorted sediments in the rainy season. A new spit is forming in Yarumal Point, with an intense increase in area and a westward progradation related to high fluvial sediment inputs from the La Nina event of 2010–12 and low erosion from the El Nino events of 2009–10 and 2015–16. In the central sector, longshore drift has formed the Yarumal barrier spit with a southeastward progradation that recently closed the interdistributary El Uno Bay. Consequently, a humaninduced sand barrier–lagoon system resulted with a stable or slightly regressive shoreline. Moreover, the Yarumal barrier spit has generated a new sediment bypass to Barajas Beach in the northern limit of the Las Vacas spit (i.e. the ancient delta lobe). Therefore, there is an unusual accretion of the abandoned delta lobe by sediment supply from the new lobe.

Influence of the Madden–Julian oscillation on Costa Rican mid‐summer drought timing

The Central American mid‐summer drought (MSD) is the decline of precipitation during the middle of the wet season (July and August) over Central America and southern Mexico. It affects agriculture and favours the initiation of bushfires in Costa Rica's national parks, particularly during El Niño years. The MSD is a seasonal phenomenon that varies in intensity and timing inter‐annually. The Madden–Julian oscillation (MJO) has been shown to influence Costa Rican rainfall on intra‐seasonal time scales, and therefore may be important to the MSD. In this study we use rainfall data from seven stations in Costa Rica to analyse the MJO's influence on the timing of the onset and end of the MSD. We find that the MSD is more likely to start and end in MJO Phases 1 and 8, respectively. Our findings indicate enhanced MSD predictability on intra‐seasonal time scales, which could be beneficial to agricultural planning in Costa Rica.