Tropical Rain Belt Research Articles

Multimodel precipitation responses to removal of U.S. sulfur dioxide emissions.

Emissions of aerosols and their precursors are declining due to policies enacted to protect human health, yet we currently lack a full understanding of the magnitude, spatiotemporal pattern, statistical significance, and physical mechanisms of precipitation responses to aerosol reductions. We quantify the global and regional precipitation responses to U.S. SO2 emission reductions using three fully coupled chemistry-climate models: Community Earth System Model version 1, Geophysical Fluid Dynamics Laboratory Coupled Model 3, and Goddard Institute for Space Studies ModelE2. We contrast 200 year (or longer) simulations in which anthropogenic U.S. sulfur dioxide (SO2) emissions are set to zero with present-day control simulations to assess the aerosol, cloud, and precipitation response to U.S. SO2 reductions. In all three models, reductions in aerosol optical depth up to 70% and cloud droplet number column concentration up to 60% occur over the eastern U.S. and extend over the Atlantic Ocean. Precipitation responses occur both locally and remotely, with the models consistently showing an increase in most regions considered. We find a northward shift of the tropical rain belt location of up to 0.35° latitude especially near the Sahel, where the rainy season length and intensity are significantly enhanced in two of the three models. This enhancement is the result of greater warming in the Northern versus Southern Hemispheres, which acts to shift the Intertropical Convergence Zone northward, delivering additional wet season rainfall to the Sahel. Two of our three models thus imply a previously unconsidered benefit of continued U.S. SO2 reductions for Sahel precipitation.

Coupling of equatorial Atlantic surface stratification to glacial shifts in the tropical rainbelt

The modern state of the Atlantic meridional overturning circulation promotes a northerly maximum of tropical rainfall associated with the Intertropical Convergence Zone (ITCZ). For continental regions, abrupt millennial–scale meridional shifts of this rainbelt are well documented, but the behavior of its oceanic counterpart is unclear due the lack of a robust proxy and high temporal resolution records. Here we show that the Atlantic ITCZ leaves a distinct signature in planktonic foraminifera assemblages. We applied this proxy to investigate the history of the Atlantic ITCZ for the last 30,000 years based on two high temporal resolution records from the western Atlantic Ocean. Our reconstruction indicates that the shallowest mixed layer associated with the Atlantic ITCZ unambiguously shifted meridionally in response to changes in the strength of the Atlantic meridional overturning with a southward displacement during Heinrich Stadials 2–1 and the Younger Dryas. We conclude that the Atlantic ITCZ was located at ca. 1°S (ca. 5° to the south of its modern annual mean position) during Heinrich Stadial 1. This supports a previous hypothesis, which postulates a southern hemisphere position of the oceanic ITCZ during climatic states with substantially reduced or absent cross-equatorial oceanic meridional heat transport.

Fast and slow shifts of the zonal‐mean intertropical convergence zone in response to an idealized anthropogenic aerosol

AbstractPrevious modeling work showed that aerosol can affect the position of the tropical rain belt, i.e., the intertropical convergence zone (ITCZ). Yet it remains unclear which aspects of the aerosol impact are robust across models, and which are not. Here we present simulations with seven comprehensive atmosphere models that study the fast and slow impacts of an idealized anthropogenic aerosol on the zonal‐mean ITCZ position. The fast impact, which results from aerosol atmospheric heating and land cooling before sea‐surface temperature (SST) has time to respond, causes a northward ITCZ shift. Yet the fast impact is compensated locally by decreased evaporation over the ocean, and a clear northward shift is only found for an unrealistically large aerosol forcing. The local compensation implies that while models differ in atmospheric aerosol heating, this does not contribute to model differences in the ITCZ shift. The slow impact includes the aerosol impact on the ocean surface energy balance and is mediated by SST changes. The slow impact is an order of magnitude more effective than the fast impact and causes a clear southward ITCZ shift for realistic aerosol forcing. Models agree well on the slow ITCZ shift when perturbed with the same SST pattern. However, an energetic analysis suggests that the slow ITCZ shifts would be substantially more model‐dependent in interactive‐SST setups due to model differences in clear‐sky radiative transfer and clouds. We also discuss implications for the representation of aerosol in climate models and attributions of recent observed ITCZ shifts to aerosol.

Hemispherically in-phase precipitation variability over the last 1700 years in a Madagascar speleothem record

Paleoclimate studies of tropical rainfall have led to a recognition of a predominant pattern of anti-phase behavior between the Northern and Southern hemispheres at both orbital and millennial timescales. Less certain is how regional tropical rainfall patterns have changed in the late Holocene, under boundary conditions and on timescales which are most relevant to the tropics' response to a warming world. Several high-resolution southern hemisphere rainfall records are at odds with meridional movement of the mean Inter-Tropical Convergence Zone location as the major driver of Holocene tropical rainfall variability, with regional precipitation patterns resembling modern day El-Niño Southern Oscillation end members. To test emerging ideas on sub-millennial tropical rainfall variability, additional records from the southern hemisphere are required.We present a new speleothem δ18O record from Anjohibe Cave, northwestern Madagascar, which provides a quasi-annual record of monsoonal strength and precipitation amount for the last 1700 years. The majority of δ18O variability in the record is at the decadal scale, and shows little to no correlation with major climate indices or cyclical climate drivers. At lower frequencies, changes in mean speleothem δ18O show good correlation with other regional precipitation records both north and south of the equator. The regional coherency of tropical rainfall across the west Indian Ocean resembles expansion and contraction of the tropical rain belt and positive-Indian Ocean Dipole-like conditions at different timescales. The cause of this coherency could be related to symmetrical changes in continental sensible heating, or to a low frequency sea surface temperature climate mode.

Sensitivity of the summertime tropical Atlantic precipitation distribution to convective parameterization and model resolution in ECHAM6

AbstractA realistic simulation of the tropical Atlantic precipitation distribution remains a challenge for atmospheric general circulation models, owing to their too coarse resolution that makes it necessary to parameterize convection. During boreal summer, models tend to underestimate the northward shift of the tropical Atlantic rain belt, leading to deficient precipitation over land and an anomalous precipitation maximum over the west Atlantic ocean. In this study, the model ECHAM6 is used to test the sensitivity of the precipitation biases to convective parameterization and horizontal resolution. Two sets of sensitivity experiments are performed. In the first set of experiments, modifications are applied to the convection scheme in order to investigate the relative roles of the trigger, entrainment, and closure formulations. In the second set, the model is run at high resolution with low‐resolution boundary conditions in order to identify the relative contributions of a high‐resolution atmosphere, orography, and surface. Results show that the dry bias over land in the model can be reduced by weakening the entrainment rate over land. Over ocean, it is found that the anomalous precipitation maximum occurs because of model choices that decrease the sensitivity of convection to the monsoon circulation in the east Atlantic. A reduction of the west Atlantic precipitation bias can be achieved by (i) using a moisture convergence closure, (ii) increasing the resolution of orography, or (iii) enhancing the production of deep convection in the east Atlantic. The biases over land and over ocean do not impact each other.

Study of the global and regional climatic impacts of ENSO magnitude using SPEEDY AGCM

ENSO is considered as a strong atmospheric teleconnection that has pronounced global and regional circulation effects. It modifies global monsoon system, especially, Asian and African monsoons. Previous studies suggest that both the frequency and magnitude of ENSO events have increased over the last few decades resulting in a need to study climatic impacts of ENSO magnitude both at global and regional scales. Hence, to better understand the impact of ENSO amplitude over the tropical and extratropical regions focussing on the Asian and African domains, ENSO sensitivity experiments are conducted using ICTPAGCM (‘SPEEDY’). It is anticipated that the tropical Pacific SST forcing will be enough to produce ENSO-induced teleconnection patterns; therefore, the model is forced using NINO3.4 regressed SST anomalies over the tropical Pacific only. SPEEDY reproduces the impact of ENSO over the Pacific, North and South America and African regions very well. However, it underestimates ENSO teleconnection patterns and associated changes over South Asia, particularly in the Indian region, which suggests that the tropical Pacific SST forcing is not sufficient to represent ENSO-induced teleconnection patterns over South Asia. Therefore, SST forcing over the tropical Indian Ocean together with air–sea coupling is also required for better representation of ENSO-induced changes in these regions. Moreover, results obtained by this pacemaker experiment show that ENSO impacts are relatively stronger over the Inter-Tropical Convergence Zone (ITCZ) compared to extratropics and high latitude regions. The positive phase of ENSO causes weakening in rainfall activity over African tropical rain belt, parts of South and Southeast Asia, whereas, the La Niña phase produces more rain over these regions during the summer season. Model results further reveal that ENSO magnitude has a stronger impact over African Sahel and South Asia, especially over the Indian region because of its significant impact over the tropical Atlantic and the Indian Ocean through Walker circulation. ENSO-induced negative (positive) NAO-like response and associated changes over Southern Europe and North Africa get significantly strong following increased intensity of El Niño (La Niña) in the northern (southern) hemisphere in the boreal winter (summer) season. We further find that ENSO magnitude significantly impacts Hadley and Walker circulations. The positive phase of ENSO (El Niño) overall strengthens Hadley cell and a reverse is true for the La Niña phase. ENSO-induced strengthening and weakening of Hadley cell induces significant impact over South Asian and African ITCZ convective regions through modification of ITCZ/monsoon circulation system.

Intensity of African Humid Periods Estimated from Saharan Dust Fluxes.

North Africa experienced dramatic changes in hydrology and vegetation during the late Quaternary driven by insolation-induced shifts of the tropical rain belt and further modulated by millennial-scale droughts and vegetation-climate feedbacks. While most past proxy and modelling studies concentrated on the temporal and spatial dynamics of the last African humid period, little is known about the intensities and characteristics of pre-Holocene humid periods. Here we present a high-resolution record of fine-grained eastern Saharan dust from the Eastern Mediterranean Sea spanning the last 180 kyr, which is based on the clay mineral composition of the marine sediments, especially the kaolinite/chlorite ratio. Minimum aeolian kaolinite transport occurred during the African Humid Periods because kaolinite deflation was hampered by increased humidity and vegetation cover. Instead, kaolinite weathering from kaolinite-bearing Cenozoic rocks was stored in lake basins, river beds and soils during these periods. During the subsequent dry phases, fine-grained dust was mobilised from the desiccated lakes, rivers and soils resulting in maximum aeolian uptake and transport of kaolinite. The kaolinite transport decreased again when these sediment sources exhausted. We conclude that the amount of clay-sized dust blown out of the Sahara into the Eastern Mediterranean Sea is proportional to the intensity of the kaolinite weathering and accumulation in soils and lake sediments, and thus to the strength of the preceding humid period. These humid periods provided the windows for the migration of modern humans out of Africa, as postulated previously. The strongest humid period occurred during the Eemian and was followed by two weaker phases centred at ca. 100 ka and ca. 80 ka.

Importance of background seasonality over the eastern equatorial Pacific in a coupled atmosphere-ocean response to westerly wind events

Coupled atmosphere-ocean response to westerly wind events (WWEs), which sometimes trigger El Niños, was investigated using a coupled general circulation model to clarify its dependence on the timing and location of WWEs. Twelve sets of 20-member ensembles were made with an idealized single WWE imposed in different months from January to July and at different longitudes from 160^circE to 160^circW. The initial ocean states are set to be near neutral to El Niño/La Niña so that the lagged response to WWEs can be isolated. The results show that sea surface temperature (SST) in the Niño3.4 region increases largely and persistently favorable for El Niño growth when a WWE is imposed in May, whereas a WWE in March increases SST only in the easternmost Pacific. In both cases, an oceanic Kelvin wave propagates eastward to warm the eastern Pacific. When forced by the WWE in May, a positive Niño3.4 SST anomaly appears in boreal summer according to the seasonal outcrop of the equatorial thermocline, and it can strongly interact with seasonally active tropical rain belt to amplify the SST response further. A favorable combination of timing and location that maximize the impact of WWEs on El Niños in the subsequent winter is suggested albeit its relevance to nature should severely be tested. Another experiment with initial ocean states in El Niño/La Niña years showed that the WWEs in May are efficient to amplify El Niño similar to the neutral case, but are not efficient to suppress La Niña.

The tropical rain belts with an annual cycle and a continent model intercomparison project: TRACMIP.

This paper introduces the Tropical Rain belts with an Annual cycle and a Continent Model Inter-comparison Project (TRACMIP). TRACMIP studies the dynamics of tropical rain belts and their response to past and future radiative forcings through simulations with 13 comprehensive and one simplified atmosphere models coupled to a slab ocean and driven by seasonally varying insolation. Five idealized experiments, two with an aquaplanet setup and three with a setup with an idealized tropical continent, fill the space between prescribed-SST aquaplanet simulations and realistic simulations provided by CMIP5/6. The simulations reproduce key features of present-day climate and expected future climate change, including an annual-mean intertropical convergence zone (ITCZ) that is located north of the equator and Hadley cells and eddy-driven jets that are similar to present-day climate. Quadrupling CO2 leads to a northward ITCZ shift and preferential warming in Northern high latitudes. The simulations show interesting CO2-induced changes in the seasonal excursion of the ITCZ and indicate a possible state dependence of climate sensitivity. The inclusion of an idealized continent modulates both the control climate and the response to increased CO2; for example, it reduces the northward ITCZ shift associated with warming and, in some models, climate sensitivity. In response to eccentricity-driven seasonal insolation changes, seasonal changes in oceanic rainfall are best characterized as a meridional dipole, while seasonal continental rainfall changes tend to be symmetric about the equator. This survey illustrates TRACMIP’s potential to engender a deeper understanding of global and regional climate and to address questions on past and future climate change.

Energy and precipitation

The latitude of the tropical rainbelt is constrained by the energy balance between hemispheres. An expansion of this theory that includes longitudinal variations of atmospheric heating can predict regional changes in tropical precipitation.

Expansion and Contraction of the Indo-Pacific Tropical Rain Belt over the Last Three Millennia.

The seasonal north-south migration of the intertropical convergence zone (ITCZ) defines the tropical rain belt (TRB), a region of enormous terrestrial and marine biodiversity and home to 40% of people on Earth. The TRB is dynamic and has been shown to shift south as a coherent system during periods of Northern Hemisphere cooling. However, recent studies of Indo-Pacific hydroclimate suggest that during the Little Ice Age (LIA; AD 1400–1850), the TRB in this region contracted rather than being displaced uniformly southward. This behaviour is not well understood, particularly during climatic fluctuations less pronounced than those of the LIA, the largest centennial-scale cool period of the last millennium. Here we show that the Indo-Pacific TRB expanded and contracted numerous times over multi-decadal to centennial scales during the last 3,000 yr. By integrating precisely-dated stalagmite records of tropical hydroclimate from southern China with a newly enhanced stalagmite time series from northern Australia, our study reveals a previously unidentified coherence between the austral and boreal summer monsoon. State-of-the-art climate model simulations of the last millennium suggest these are linked to changes in the structure of the regional manifestation of the atmosphere’s meridional circulation.

Sensitivity of the Middle East–North African Tropical Rainbelt to Dust Shortwave Absorption: A High-Resolution AGCM Experiment

Abstract Shortwave absorption is one of the most important, but the most uncertain, components of direct radiative effect by mineral dust. It has a broad range of estimates from different observational and modeling studies and there is no consensus on the strength of absorption. To elucidate the sensitivity of the Middle East–North African (MENA) tropical summer rainbelt to a plausible range of uncertainty in dust shortwave absorption, AMIP-style global high-resolution (25 km) simulations are conducted with and without dust, using the High-Resolution Atmospheric Model (HiRAM). Simulations with dust comprise three different cases by assuming dust as a very efficient, as a standard, and as an inefficient absorber. Intercomparison of these simulations shows that the response of the MENA tropical rainbelt is extremely sensitive to the strength of shortwave absorption. Further analyses reveal that the sensitivity of the rainbelt stems from the sensitivity of the multiscale circulations that define the rainbelt. The maximum response and sensitivity are predicted over the northern edge of the rainbelt, geographically over the Sahel. The sensitivity of the responses over the Sahel, especially that of precipitation, is comparable to the mean state. Locally, the response in precipitation reaches up to 50% of the mean, while dust is assumed to be a very efficient absorber. Taking into account that the Sahel has a very high climate variability and is extremely vulnerable to changes in precipitation, the present study suggests the importance of reducing uncertainty in dust shortwave absorption for a better simulation and interpretation of the Sahel climate.

Opportunities for hydrologic research in the Congo Basin

We review the published results on the Congo Basin hydrology and summarize the historic and ongoing research. Annual rainfall is ~1900 mm/yr along an east-west trend across the basin, decreasing northward and southward to ~1100 mm/yr. Historic studies using lysimeters, pans, and models suggest that the annual potential evapotranspiration varies little across the basin at 1100 to 1200 mm/yr. Over the past century, river discharge data have been collected at hundreds of stream gauges with historic and recent data at 96 locations now publicly available. Congo River discharge at Kinshasa-Brazzaville experienced an increase of 21% during the 1960–1970 decade in comparison to most other decades. Satellite altimetry measurements of high and low flows show that water levels in the “Cuvette Centrale” wetland are 0.5 m to 3.0 m higher in elevation than the immediately adjacent Congo River levels. Wetland water depths are shallow at about a meter and there does not appear to be many sizable channels across the “Cuvette”; thus, wetland flows are diffusive. Cuvette waters alone are estimated to emit about 0.5 Pg CH4 and CO2 equivalents/yr, an amount that is significant compared to global carbon evasions. Using these results, we suggest seven hypotheses that focus on the source of the Cuvette waters and how these leave the wetland, on the river discharge generated by historic rainfall, on the connection between climate change and the rainfall-runoff generated by the migrating “tropical rainbelt,” on deforestation and hydroelectric power generation, and on the amount of carbon emitted from Congo waters.

A 21st century northward tropical precipitation shift caused by future anthropogenic aerosol reductions

AbstractThe tropical rain belt is a narrow band of clouds near the equator, where the most intense rainfall on the planet occurs. On seasonal timescales, the rain moves across the equator following the Sun, resulting in wet and dry seasons in the tropics. The position of the tropical rain belt also varies on longer timescales. Through the latter half of the twentieth century, for example, shifts in tropical rainfall have been associated with severe droughts, including the African Sahel and Amazon droughts. Here I show that climate models project a northward migration of the tropical rain belt through the 21st century, with future anthropogenic aerosol reductions driving the bulk of the shift. Models that include both aerosol indirect effects yield significantly larger northward shifts than models that lack aerosol indirect effects. Moreover, the rate of the shift corresponds to the rate of the decrease of anthropogenic aerosol emissions across different time periods and future emission scenarios. This response is consistent with relative warming of the Northern Hemisphere, a decrease in northward cross‐equatorial moist static energy transport, and a northward shift of the Hadley circulation, including the tropical rain belt. The shift is relatively weak in the Atlantic sector, consistent with both a smaller decrease in aerosol emissions and a larger reduction in northward cross‐equatorial ocean heat flux. Although aerosol effects remain uncertain, I conclude that future reductions in anthropogenic aerosol emissions may be the dominant driver of a 21st century northward shift of the tropical rain belt.

Monsoon source shifts during the drying mid-Holocene: Biomarker isotope based evidence from the core ‘monsoon zone’ (CMZ) of India

A better understanding of past variations of the Indian Summer Monsoon (ISM), that plays a vital role for the still largely agro-based economy in India, can lead to a better assessment of its potential impact under global climate change scenarios. However, our knowledge of spatiotemporal patterns of ISM strength is limited due to the lack of high-resolution, continental paleohydrological records. Here, we reconstruct centennial-scale hydrological variability during the Holocene associated to changes in the intensity of the ISM based on a record of lipid biomarker abundances and compound-specific stable isotopic composition of a 10 m long sediment core from saline–alkaline Lonar Lake, situated in the core ‘monsoon zone’ of central India.We identified three main periods of distinct hydrology during the Holocene in central India. The period between 10.1 and 6 cal ka BP was likely the wettest during the Holocene. Lower average chain length (ACL) index values (29.4–28.6) and negative δ13Cwax values (−34.8‰ to −27.8‰) of leaf wax n-alkanes indicate the dominance of woody C3 vegetation in the catchment, and negative δDwax values (concentration weighted average) (−171‰ to −147‰) suggest a wet period due to an intensified monsoon. After 6 cal ka BP, a gradual shift to less negative δ13Cwax values (particularly for the grass derived n-C31) and appearance of the triterpene lipid tetrahymanol, generally considered as a marker for salinity and water column stratification, mark the onset of drier conditions. At 5.1 cal ka BP an increasing flux of leaf wax n-alkanes along with the highest flux of tetrahymanol indicate a major lowering of the lake level. Between 4.8 and 4 cal ka BP, we find evidence for a transition to arid conditions, indicated by high and strongly variable tetrahymanol flux. In addition, a pronounced shift to less negative δ13Cwax values, in particular for n-C31 (−25.2‰ to −22.8‰), during this period indicates a change of dominant vegetation to C4 grasses. In agreement with other proxy data, such as deposition of evaporite minerals, we interpret this period to reflect the driest conditions in the region during the last 10.1 ka. This transition led to protracted late Holocene arid conditions after 4 ka with the presence of a permanent saline lake, supported by the sustained presence of tetrahymanol and more positive average δDwax values (−122‰ to −141‰). A late Holocene peak of cyanobacterial biomarker input at 1.3 cal ka BP might represent an event of lake eutrophication, possibly due to human impact and the onset of cattle/livestock farming in the catchment.A unique feature of our record is the presence of a distinct transitional period between 4.8 and 4 cal ka BP, which was characterized by some of the most negative δDwax values during the Holocene (up to −180‰), when all other proxy data indicate the driest conditions during the Holocene. These negative δDwax values can as such most reasonably be explained by a shift in moisture source area and/or pathways or rainfall seasonality during this transitional period. We hypothesize that orbital induced weakening of the summer solar insolation and associated reorganization of the general atmospheric circulation, as a possible southward displacement of the tropical rainbelt, led to an unstable hydroclimate in central India between 4.8 and 4 ka.Our findings shed light onto the sequence of changes during mean state changes of the monsoonal system, once an insolation driven threshold has been passed, and show that small changes in solar insolation can be associated with major hydroclimate changes on the continents, a scenario that may be relevant with respect to future changes in the ISM system.

Role of dust direct radiative effect on the tropical rain belt over Middle East and North Africa: A high‐resolution AGCM study

AbstractTo investigate the influence of direct radiative effect of dust on the tropical summer rain belt across the Middle East and North Africa (MENA), the present study utilizes the high‐resolution capability of an Atmospheric General Circulation Model, the High‐Resolution Atmospheric Model. Ensembles of Atmospheric Model Intercomparison Project style simulations have been conducted with and without dust radiative impacts, to differentiate the influence of dust on the tropical rain belt. The analysis focuses on summer season. The results highlight the role of dust‐induced responses in global‐ and regional‐scale circulations in determining the strength and the latitudinal extent of the tropical rain belt. A significant response in the strength and position of the local Hadley circulation is predicted in response to meridionally asymmetric distribution of dust and the corresponding radiative effects. Significant responses are also found in regional circulation features such as African Easterly Jet and West African Monsoon circulation. Consistent with these dynamic responses at various scales, the tropical rain belt across MENA strengthens and shifts northward. Importantly, the summer precipitation over the semiarid strip south of Sahara, including Sahel, increases up to 20%. As this region is characterized by the “Sahel drought,” the predicted precipitation sensitivity to the dust loading over this region has a wide range of socioeconomic implications. Overall, the study demonstrates the extreme importance of incorporating dust radiative effects and the corresponding circulation responses at various scales, in the simulations and future projections of this region's climate.

Red Sea circulation during marine isotope stage 5e

AbstractWe have employed a regional Massachusetts Institute of Technology oceanic general circulation model of the Red Sea to investigate its circulation during marine isotope stage (MIS) 5e, the peak of the last interglacial, approximately 125 ka before present. Compared to present‐day conditions, MIS 5e was characterized by higher Northern Hemisphere summer insolation, accompanied by increases in air temperature of more than 2°C and global sea level approximately 8 m higher than today. As a consequence of the increased seasonality, intensified monsoonal conditions with increased winds, rainfall, and humidity in the Red Sea region are evident in speleothem records and supported by model simulations. To assess the dominant factors responsible for the observed changes, we conducted several sensitivity experiments in which the MIS 5 boundary conditions or forcing parameters were used individually. Overall, our model simulation for the last interglacial maximum reconstructs a Red Sea that is colder, less ventilated and probably more oligotrophic than at present day. The largest alteration in Red Sea circulation and properties was found for the simulation of the northward displacement and intensification of the African tropical rain belt during MIS 5e, leading to a notable increase in the fresh water flux into the Red Sea. Such an increase significantly reduced the Red Sea salinity and exchange volume of the Red Sea with the Gulf of Aden. The Red Sea reacted to the MIS 5e insolation forcing by the expected increase in seasonal sea surface temperature amplitude and overall cooling caused by lower temperatures during deep water formation in winter.

Resolution dependence in simulating the African hydroclimate with the HadGEM3-RA regional climate model

This study documents the effect of horizontal resolution on the ability of the Met Office third-generation Global Atmosphere Regional Climate Model (HadGEM3-RA), a regional atmospheric configuration of the HadGEM3 model, to simulate rainfall variability over Africa. It is based on six 20-year long RCM simulations driven by ERA-Interim reanalysis and performed at 12, 25, 50, 70, 90, and 150 km over the CORDEX-Africa domain. To gain further insight into model errors, we also compared the HadGEM3-RA’s performance to that of the parent General Circulation Model using three different spatial resolutions (70, 100, and 150 km), and to HadRM3P—the current Met Office regional climate model. It is found that the 50 km resolution RCM reproduces reasonably the spatial and temporal features of rainfall variability across regions. These include the seasonal progression of the tropical rainbelt, its extent and location, the annual cycle and interannual variability. Although model biases vary across seasons and locations, a prominent feature is the over-prediction of rainfall totals over Central Africa, and underestimation of rainfall in coastal areas of the Guinea Gulf during boreal spring and autumn. HadGEM3-RA improves with increased horizontal resolution, but some model errors persist. Comparison with the parent global model simulations demonstrates generally a realistic and consistent behaviour over large scales—suggesting that the physical formulation is able to capture the key driving processes, but also confirms the benefit of increasing the model horizontal resolution. Despite the model errors, HadGEM3-RA rainfall shows superiority over that from HadRM3P, ERA-Interim and MERRA datasets—indicating that the associated dynamical features of HadGEM3-RA can complement the physical understanding gained from reanalyses. This article also highlights the challenges for evaluating climate models in data sparse regions where satellite derived rainfall and gridded observational datasets often diverge.

Abrupt glacial climate shifts controlled by ice sheet changes.

During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard-Oeschger (DO) events. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record, and has drawn broad attention within the science and policy-making communities alike. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified. Here we show, by using a comprehensive fully coupled model, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere-ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses--including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw--are generally consistent with empirical evidence. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere-ocean-sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere-ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials.

Ten year observations of gravity waves from thunderstorms in western Africa

AbstractA new study of gravity waves produced by thunderstorms was performed using continuous recordings at the IS17 (Ivory Coast) infrasound station of the International Monitoring System developed for the verification of the Comprehensive Nuclear Test‐Ban Treaty. A typical case study is presented for a large thunderstorm on 10–11 April 2006 lasting near 14 h. Comparison with cloud temperature measured by the Meteosat 6 satellite shows that wave activity is large when the cloud temperature is low inside convection cells located over the station. Statistics based on 10 year data show that the wave activity is intense throughout the year with peak periods in May and October and less intense activity in January, in good agreement with the local keraunic level. The seasonal variations of the wave azimuth highlight clear trends from northward direction from February to August to southward direction from August to December. Lightning flashes, observed from space, show a similar motion confirming that thunderstorms are the main sources of the gravity wave activity. The gravity wave azimuth follows the seasonal motion of the tropical rain belt partly related to the Intertropical Convergence Zone of the winds. The contribution of other possible sources, such as wind over relief, is weak because surface winds are weak in this region and only oceans are present south of the station. We conclude that the large observed wave activity is mainly produced by convection associated to thunderstorms.