Tropical Land Research Articles

Slower changes in vegetation phenology than precipitation seasonality in the dry tropics.

The dry tropics occupy ~40% of the tropical land surface and play a dominant role in the trend and interannual variability of the global carbon cycle. Previous studies have reported considerable changes in the dry tropical precipitation seasonality due to climate change, however, the accompanied changes in the length of the vegetation growing season (LGS)-the key period of carbon sequestration-have not been examined. Here, we used long-term satellite observations along with in-situ flux measurements to investigate phenological changes in the dry tropics over the past 40 years. We found that only ~18% of the dry tropics show a significant (p ≤ .1) increasing trend in LGS, while ~13% show a significant decreasing trend. The direction of the LGS change depended not only on the direction of precipitation seasonality change but also on the vegetation water use strategy (i.e. isohydricity) as an adaptation to the long-term average precipitation seasonality (i.e. whether the most of LGS is in the wet season or dry season). Meanwhile, we found that the rate of LGS change was on average ~23% slower than that of precipitation seasonality, caused by a buffering effect from soil moisture. This study uncovers potential mechanisms driving phenological changes in the dry tropics, offering guidance for regional vegetation and carbon cycle studies.

A Parameterization for Cloud Organization and Propagation by Evaporation‐Driven Cold Pool Edges

AbstractWhen the negatively buoyant air in the cloud downdrafts reaches the surface, it spreads out horizontally, producing cold pools. A cold pool can trigger new convective cells. However, when combined with the ambient vertical wind shear, it can also connect and upscale them into large mesoscale convective systems (MCS). Given the broad spectrum of scales of the atmospheric phenomenon involving the interaction between cold pools and the MCS, a parameterization was designed here. Then, it is coupled with a classical convection parameterization to be applied in an atmospheric model with an insufficient spatial resolution to explicitly resolve convection and the sub‐cloud layer. A new scalar quantity related to the deficit of moist static energy detrained by the downdrafts mass flux is proposed. This quantity is subject to grid‐scale advection, mixing, and a sink term representing dissipation processes. The model is then applied to simulate moist convection development over a large portion of tropical land in the Amazon Basin in a wet and dry‐to‐wet 10‐days period. Our results show that the cold pool edge parameterization improves the organization, longevity, propagation, and severity of simulated MCS over the Amazon and other different continental areas.

TREE-SPECIFIC EQUATIONS FOR ACCURATE CARBON ESTIMATION OF MAJOR FRUIT TREES IN BANGLADESH

Tropical fruit trees play an important role in mitigating climate change by sequestering carbon from the atmosphere. However, estimating their carbon sequestration potential requires tree-specific equations. This study aimed to develop such equations for five major fruit tree species in Bangladesh i.e. Jackfruit (Artocarpus heterophyllus), Mango (Mangifera indica), Litchi (Litchi chinensis), Guava (Psidium guajava), and Jujube (Ziziphus jujuba). The study was conducted in four major ecosystems of Bangladesh namely, the Coastal ecosystem, Barind ecosystem, Terrace ecosystem, and Hill ecosystem. Vegetation data were randomly collected from each ecosystem. The equations were derived for Jackfruit W (lb) = 0.26 × D² × H (for diameter <11 inches) and W (lb) = 0.14 × D² × H (for diameter >11 inches); for Mango W (lb) = 0.26 × D² × H (for diameter <11 inches) and W (lb) = 0.13 × D² × H (for diameter >11 inches); for Litchi W (lb) = 0.23 × D² × H (for diameter <11 inches) and W (lb) = 0.13 × D² × H (for diameter >11 inches); for Guava W (lb) = 0.22 × D² × H (for diameter <11 inches), and for Jujube W (lb) = 0.22 × D² × H (for diameter <11 inches) and W (lb) = 0.12 × D² × H (for diameter >11 inches). The equations will help in developing effective carbon management and climate change mitigation strategies in tropical agroforestry systems, aiding researchers, land managers, and policymakers in understanding tree growth and carbon sequestration.

U‐Net Segmentation for the Detection of Convective Cold Pools From Cloud and Rainfall Fields

AbstractConvective cold pools (CPs) mediate interactions between convective rain cells and help organize thunderstorm clusters, in particular mesoscale convective systems and extreme rainfall events. Unfortunately, the observational detection of CPs on a large scale has been hampered by the lack of relevant near‐surface data. Unlike numerical studies, where fields, such as virtual temperature or wind, are available at high resolution and frequently used to detect CPs, observational studies mainly identify CPs based on surface time series, for example, from weather stations or research vessels—thus limiting studies to a regional scope. To expand to a global scope, we here develop and evaluate a methodology for CP detection that relies exclusively on data with (a) global availability and (b) high spatiotemporal resolution. We trained convolutional neural networks to segment CPs in high‐resolution cloud‐resolving simulation output by deliberately restricting ourselves to only cloud top temperature and rainfall fields. Apart from simulations, such data are readily available from geostationary satellites that fulfill both (a) and (b). The networks employ a U‐Net architecture, popular with image segmentation, where spatial correlations at various scales must be learned. Despite the restriction imposed, the trained networks systematically identify CP pixels. Looking ahead, our methodology aims to reliably detect CPs over tropical land from space‐borne sensors on a global scale. As it also provides information on the spatial extent and the relative positioning of CPs over time, our method may unveil the role of CPs in convective organization.

Determining the Growing Potential of Jatropha curcas and Pongamia pinnata under Various Water Regimes

Pakistan is the among the most populated countries of the world with predominantly arid climate facing different challenges like food insecurity, energy crises, poor health system, poverty, small forest area cover, and significant effects of global warming and climate change. However, there is still a large area with scanty vegetation. Research on devising cultivation techniques for growing more economically profitable plants in these areas will not only improve the ecological environment but also the people’s living standards. Jatropha curcas (JC) and Pongamia pinnata (PP) are two economically important species (bioenergy plants) with potential to grow on dry tropical arid land. Therefore, this study was carried out to check the hypothesis “JC and PP are the suitable species to grow in the dry tropical arid land of Pakistan due to their drought tolerance ability”. The experiment was carried out at the nursery of Bahauddin Zakariya University, Multan. One year old seedlings of both plants were used in the experiment. The design involves two plants of the same species in each row, four treatments, and three replications. The levels of drought were T1 (100% water), T2 (75% water), T3 (50% water), and T4 (25% water). A generalized linear model was applied to check the variation among species and within the treatments of the same species. There is a significant difference (p ≥ 0.05) in growth among species and within the treatments of the same species. JC showed better growth as compared to PP in water stress conditions but PP grow as well. The growth parameters of JC showed a positive response in T2 (75% water) and T3 (50% water) drought levels while the growth parameters of PP showed a positive response in treatments T1 (100% water) and T2 (75% water)as compared to other drought levels. In the case of the fresh and dry weight of JC and PP, significant difference (p ≥ 0.05) was also found among species and within the treatments. Both JC and PP are recommended to grow in the dry tropical regions of Pakistan but JC is declared as the prime specie as compared to PP.

Comparison of UKESM1 and CESM2 simulations using the same multi-target stratospheric aerosol injection strategy

Abstract. Solar climate intervention using stratospheric aerosol injection (SAI) has been proposed as a method which could offset some of the adverse effects of global warming. The Assessing Responses and Impacts of Solar climate intervention on the Earth system with Stratospheric Aerosol Injection (ARISE-SAI) set of simulations is based on a moderate-greenhouse-gas-emission scenario and employs injection of sulfur dioxide at four off-equatorial locations using a control algorithm which maintains the global-mean surface temperature at 1.5 K above pre-industrial conditions (ARISE-SAI-1.5), as well as the latitudinal gradient and inter-hemispheric difference in surface temperature. This is the first comparison between two models (CESM2 and UKESM1) applying the same multi-target SAI strategy. CESM2 is successful in reaching its temperature targets, but UKESM1 has considerable residual Arctic warming. This occurs because the pattern of temperature change in a climate with SAI is determined by both the structure of the climate forcing (mainly greenhouse gases and stratospheric aerosols) and the climate models' feedbacks, the latter of which favour a strong Arctic amplification of warming in UKESM1. Therefore, research constraining the level of future Arctic warming would also inform any hypothetical SAI deployment strategy which aims to maintain the inter-hemispheric and Equator-to-pole near-surface temperature differences. Furthermore, despite broad agreement in the precipitation response in the extratropics, precipitation changes over tropical land show important inter-model differences, even under greenhouse gas forcing only. In general, this ensemble comparison is the first step in comparing policy-relevant scenarios of SAI and will help in the design of an experimental protocol which both reduces some known negative side effects of SAI and is simple enough to encourage more climate models to participate.

Region and cloud regime dependence of parametric sensitivity in E3SM atmosphere model

The Department of Energy (DOE)’s Energy Exascale Earth System Model (E3SM), including its atmosphere model (EAM), has many relatively new features. In a previous study we conducted a systematic parametric sensitivity analysis for EAM based on short, perturbed parameter ensemble (PPE) simulations, mainly focusing on global mean climate features and metrics. While parameter values in global climate models are generally invariant in space and time, model response to parameters perturbation may vary by regions and climate regimes, which motivates the need to better understand the EAM model behaviors and physics at regional scale and process level. In this study, using the same set of PPE simulations and a similar sensitivity analysis framework, we identify parameters that cause largest sensitivities over different regions and compare model responses in fast atmospheric processes to the parameters across different cloud regimes for several important cloud-related fidelity metrics. We find that cloud forcing has opposite response to some parameters over mid-latitude vs. tropical land. We also analyze how the parametric sensitivity varies as stratocumulus transitions to shallow convection and to deep convection over ocean. Low cloud forcing and shortwave cloud forcing in the subtropical eastern Pacific are most sensitive to the parameters controlling the width of the probability density function (PDF) of the subgrid vertical velocity (w’) (gamma) and the damping of the w’ skewness (c8) near the coast but become more sensitive to the parameter affecting the damping of the w’ variance (c1) further offshore. Detailed interpretation of the spatial dependence of parametric sensitivity is provided. We also investigate how the parametric sensitivity evolves with prediction duration. This study improves our process-level understanding of cloud physics and parameterization and provides insights for developing more advanced regime-aware parameterization schemes in global climate model.

Isolating the effect of biomass burning aerosol emissions on 20th century hydroclimate in South America and Southeast Asia

Biomass burning is a significant source of aerosol emissions in some regions and has a considerable impact on regional climate. Earth system model simulations indicate that increased biomass burning aerosol emissions contributed to statistically significant decreases in tropical precipitation over the 20th century. In this study, we use the Community Earth System Model version 1 Large Ensemble (CESM1-LENS) experiment to evaluate the mechanisms by which biomass burning aerosol contributed to decreased tropical precipitation, with a focus on South America and Southeast Asia. We analyze the all-but-one forcing simulations in which biomass burning aerosol emissions are held constant while other forcings (e.g., greenhouse gas concentrations) vary throughout the 20th century. This allows us to isolate the influence of biomass burning aerosol on processes that contribute to decreasing precipitation, including cloud microphysics, the radiative effects of absorbing aerosol particles, and alterations in regional circulation. We also show that the 20th century reduction in precipitation identified in the CESM1-LENS historical and biomass burning experiments is consistent across Coupled Model Intercomparison Project Phase 5 models with interactive aerosol schemes and the CESM2 single-forcing experiment. Our results demonstrate that higher concentrations of biomass burning aerosol increases the quantity of cloud condensation nuclei and cloud droplets, limiting cloud droplet size and precipitation formation. Additionally, absorbing aerosols (e.g., black carbon) contribute to a warmer cloud layer, which promotes cloud evaporation, increases atmospheric stability, and alters regional circulation patterns. Corresponding convectively coupled circulation responses, particularly over the tropical Andes, contribute to further reducing the flow of moisture and moisture convergence over tropical land. These results elucidate the processes that affect the water cycle in regions prone to biomass burning and inform our understanding of how future changes in aerosol emissions may impact tropical precipitation over the 21st century.

Same process, different patterns: pervasive effect of evolutionary time on species richness in freshwater fishes.

Tropical lands harbour the highest number of species, resulting in the ubiquitous latitudinal diversity gradient (LDG). However, exceptions to this pattern have been observed in some taxa, explained by the interaction between the evolutionary histories and environmental factors that constrain species' physiological and ecological requirements. Here, we applied a deconstruction approach to map the detailed species richness patterns of Actinopterygian freshwater fishes at the class and order levels and to disentangle their drivers using geographical ranges and a phylogeny, comprising 77% (12 557) of all described species. We jointly evaluated seven evolutionary and ecological hypotheses posited to explain the LDG: diversification rate, time for speciation, species-area relationship, environmental heterogeneity, energy, temperature seasonality and past temperature stability. We found distinct diversity gradients across orders, including expected, bimodal and inverse LDGs. Despite these differences, the positive effect of evolutionary time explained patterns for all orders, where species-rich regions are inhabited by older species compared to species-poor regions. Overall, the LDG of each order has been shaped by a unique combination of factors, highlighting the importance of performing a joint evaluation of evolutionary, historical and ecological factors at different taxonomic levels to reach a comprehensive understanding on the causes driving global species richness patterns.

Influence of the Madden-Julian Oscillation on the diurnal cycles of convection and precipitation over the Congo Basin

The Congolese rainforest is a hotspot for convection where thunderstorms and rainfall exhibit a strong diurnal cycle. Previous studies have shown that various modes of variability such as the Madden-Julian Oscillation (MJO), the leading mode of intraseasonal variability in the tropics, can impact the diurnal cycles of precipitation and convection over tropical land areas. Thus, this study analyzes the influence of the MJO on diurnal variations of convection and precipitation over the Congo and explores possible mechanisms leading to the observed changes, using Gridded Satellite (GridSat-B1), Tropical Rainfall Measuring Mission (TRMM), and ERA5 reanalysis data. Results show that convection and precipitation are increased during the MJO enhanced convective phase (RMM phases 1 and 2) and decreased during the suppressed convective phase (RMM phases 5 and 6), where the differences are found to be largest during the morning hours when convection is weakest. Convection is generally deeper during the enhanced phase and shallower during the suppressed phase, accompanied by a slower decay of convective clouds and anvils during the enhanced phase. Furthermore, the influence of the MJO was found to be stronger on stratiform precipitation compared to convective precipitation. While the diurnal cycles of convective precipitation fractions are similar during both MJO phases, the fraction of stratiform precipitation is higher during the enhanced phase, mainly in the morning hours. Suggested atmospheric drivers contributing to the observed differences between the two MJO phases include enhanced upward air motion in the mid- and upper levels and strong divergence in the upper levels during the enhanced phase, and strong mid-level divergence and upper- to mid-level subsidence during the suppressed phase, mainly during the nighttime and morning hours. Furthermore, decreased mid-level wind speed, increased upper-level wind speed, and enhanced relative humidity may be contributing factors to the increased formation of stratiform precipitation during the enhanced phase. These results enhance our understanding of the MJO's impacts on precipitation and convection variability, ultimately improving predictions of MJO modulated rainfall over the Congo.

Multiple Equilibria and Soil Moisture‐Precipitation Feedbacks in Idealized Convection‐Permitting Simulations With an Open Hydrological Cycle

AbstractSoil moisture‐precipitation feedbacks are influenced by both small‐scale land‐atmosphere coupling and large‐scale atmospheric circulations, and their sign has important implications for the stability of regional hydroclimate. However, the importance of both local and non‐local processes makes it difficult to model soil moisture‐precipitation feedbacks with high fidelity, limiting our ability to use models to understand controls on their sign. Here, we address this challenge by exploring a promising but seldom‐used approach to studying soil moisture‐precipitation feedbacks over tropical land: coupling small‐domain convection‐permitting simulations to a land‐like surface and a parameterization of large‐scale dynamics. The large‐scale dynamics parameterization, based on the weak temperature gradient (WTG) approximation, is a key component that produces an open hydrological cycle with interactive moisture convergence. We first show that WTG‐constrained simulations coupled to a freely‐evaporating land surface support both precipitating and non‐precipitating equilibria across a wide range of insolation. We then leverage this bistability to probe the influence of soil moisture feedbacks on dry spells by asking whether non‐precipitating equilibria remain stable as the underlying surface dries out. We find that surface drying can trigger transitions from dry equilibria back to precipitating equilibria—a negative soil moisture‐precipitation feedback—and attribute this transition to increasingly inefficient boundary layer ventilation by the parameterized large‐scale circulation. In sensitivity experiments, alternative versions of the WTG scheme modify the parameter space where the negative feedback occurs, but none eliminate it entirely. Our results provide a foundation for leveraging the rich behavior of WTG‐constrained simulations to probe controls on soil moisture‐precipitation feedbacks over tropical land.

Diurnal to interannual variability of low‐level cloud cover over western equatorial Africa in May–October

AbstractThis study examines the diurnal to interannual variations of the stratiform cloud cover in May–October (1971–2019) from a 3‐hourly station database and from ERA5 reanalyses over western equatorial Africa (WEA). The main diurnal variations of the local‐scale fraction and genus of stratiform clouds are synthesized into three canonical diurnal types (i.e., “clear,” “clear afternoon,” “cloudy” days). The interannual variations of frequencies of the three diurnal types during the cloudiest months (JJAS) are mostly associated with two main mechanisms: a meridional shallow overturning cell associating more “cloudy” and less “clear” and “clear afternoon” days to anomalous southerlies below 900 hPa over and around WEA, anomalous ascent around 5°–7°N, anomalous northerlies between 875 and 700 hPa, and anomalous subsidence over the equatorial Atlantic. This circulation is strongly related to interannual variations of the equatorial Atlantic upwelling (i.e., more clouds when the upwelling is strong) associated with a meridional shift of the Intertropical Convergence Zone over the Tropical Atlantic and adjacent continents. The second mechanism operates mostly in the zonal direction and involves again the coupled ocean–atmosphere system over the equatorial Atlantic, but also the remote El Niño–Southern Oscillation (ENSO). An anomalously cold equatorial Atlantic drives increased low‐level westerlies toward the Congo Basin. Warm ENSO events promote broad warm and easterly anomalies in the middle and upper troposphere, which increase the local static stability, and thus the local stratiform cloud cover over WEA. The present study suggests new mechanisms responsible for interannual variations of stratiform clouds in WEA, thus providing avenues of future research regarding the stability of the stratiform cloud deck under the ongoing differential warming of tropical ocean and land masses.

Response of high-altitude clouds to the galactic cosmic ray cycles in tropical regions

Galactic cosmic rays are one of the possible mediators of the solar influence on climate. However, the impacts of GCR on clouds and climate systems are not fully understood. In this paper, we show that the high-altitude clouds associated with deep convective activities are responding to the decadal-scale cycles of GCRs and that the susceptible areas are seasonally variable. Most notable responses were found in August over tropical land areas, suggesting that the susceptivity of clouds to GCRs depends on the depth of convective activities and the abundance of aerosol precursor materials. Furthermore, following the activation of high-altitude cloud formation, an increase in sea surface temperature (SST) gradient was observed over the Pacific. Although the response of sea surface temperature to solar activity has mostly been discussed as mediated by solar radiations, we propose that another mechanism is possible: through the impact of GCRs on clouds and the resultant changes in atmospheric circulations.

Combining local model calibration with the emergent constraint approach to reduce uncertainty in the tropical land carbon cycle feedback

Abstract. The role of the land carbon cycle in climate change remains highly uncertain. A key source of the projection spread is related to the assumed response of photosynthesis to warming, especially in the tropics. The optimum temperature for photosynthesis determines whether warming positively or negatively impacts photosynthesis, thereby amplifying or suppressing CO2 fertilisation of photosynthesis under CO2-induced global warming. Land carbon cycle models have been extensively calibrated against local eddy flux measurements, but this has not previously been clearly translated into a reduced uncertainty in terms of how the tropical land carbon sink will respond to warming. Using a previous parameter perturbation ensemble carried out with version 3 of the Hadley Centre coupled climate–carbon cycle model (HadCM3C), we identify an emergent relationship between the optimal temperature for photosynthesis, which is especially relevant in tropical forests, and the projected amount of atmospheric CO2 at the end of the century. We combine this with a constraint on the optimum temperature for photosynthesis, derived from eddy covariance measurements using the adjoint of the Joint UK Land Environment Simulator (JULES) land surface model. Taken together, the emergent relationship from the coupled model and the constraint on the optimum temperature for photosynthesis define an emergent constraint on future atmospheric CO2 in the HadCM3C coupled climate–carbon cycle under a common emissions scenario (A1B). The emergent constraint sharpens the probability density of simulated CO2 change (2100–1900) and moves its peak to a lower value of 497 ± 91 compared to 607 ± 128 ppmv (parts per million by volume) when using the equal-weight prior. Although this result is likely to be model and scenario dependent, it demonstrates the potential of combining the large-scale emergent constraint approach with a parameter estimation using detailed local measurements.

Constrained tropical land temperature-precipitation sensitivity reveals decreasing evapotranspiration and faster vegetation greening in CMIP6 projections

Over the tropical land surface, accurate estimates of future changes in temperature, precipitation and evapotranspiration are crucial for ecological sustainability, but remain highly uncertain. Here we develop a series of emergent constraints (ECs) by using historical and future outputs from the Coupled Model Inter-comparison Project Phase 6 (CMIP6) Earth System Models under the four basic Shared Socio-economic Pathway scenarios (SSP126, SSP245, SSP370, and SSP585). Results show that the temperature sensitivity to precipitation during 2015–2100, which varies substantially in the original CMIP6 outputs, becomes systematically negative across SSPs after application of the EC, with absolute values between −1.10 °C mm−1 day and −3.52 °C mm−1 day, and with uncertainties reduced by 9.4% to 41.4%. The trend in tropical land-surface evapotranspiration, which was increasing by 0.292 mm yr−1 in the original CMIP6 model outputs, becomes significantly negative (−0.469 mm yr−1) after applying the constraint. Moreover, we find a significant increase of 58.7% in the leaf area index growth rate.

Changes in the mean and variability of temperature and precipitation over global land areas

The frequency and intensity of extreme weather and climate events may change in response to shifts in the mean and variability of climate, which pose high risks to societies and natural ecosystems. Gridded near-surface temperature, precipitation, and the number of wet days from the Climatic Research Unit dataset were analyzed for two 30 year periods to explore changes in the mean and variability of temperature and precipitation over global land areas in the recent period (1991–2020) compared to the reference period (1951–1980). Global land areas are characterized by warmer and slightly wetter conditions in the recent period, while the variability of temperature and precipitation has remained nearly unchanged. Changes in the mean and variability of both temperature and precipitation are also analyzed over tropical, subtropical, and midlatitude land areas. The annual mean temperature over all these three latitudinal regions has increased in the recent period compared to the reference period, with the highest increase in subtropical and midlatitude land areas (0.7 ∘C), followed by tropical land areas (0.5 ∘C), while temperature variability has remained nearly unchanged. The annual precipitation has decreased over tropical, subtropical, and midlatitude land areas in the recent period compared to the reference period. Precipitation variability has not changed considerably over subtropical land areas. However, it has substantially increased over tropical land areas, which indicates a higher risk of droughts and periods of excess water in the recent period. In contrast, precipitation variability has decreased over midlatitude land areas, indicating narrower swings between wet and dry conditions, which decrease the risk of droughts and periods of excess water in the recent period.

Determination of the chemical equator from GEOS-Chem model simulation: a focus on the tropical western Pacific region

Abstract. The tropical western Pacific (TWP) plays an important role in global stratosphere–troposphere exchange and is an active region of the interhemispheric transport (IHT). Common indicators for transport between the hemispheres like the tropical rain belt are too broad or lack precision in the TWP. In this paper, we provide a method to determine the atmospheric chemical equator (CE), which is a boundary for air mass transport between the two hemispheres in the tropics. This method used the model output from an artificial passive tracer simulated by the chemical transport model GEOS-Chem in the troposphere. We investigated the movement of the CE in the tropics, which indicates the migration of atmospheric circulation systems and air mass origins. Our results show the CE on different timescales, suggesting that the different features of the IHT in different regions are highly related to the variation in the circulation systems. We compared the CE with the tropical wind fields, indicating that the region of IHT does not coincide with the convergence of the 10 m wind fields in the tropical land sectors and the TWP region. We compared the CE with the atmospheric composition such as satellite data of CH4 and model simulation of sulfur hexafluoride (SF6). The results show that the CE and north–south gradient of CH4 in the Indian Ocean in January are well consistent with each other, which indicates the CE has good potential to estimate the IHT inferred by observations. We discussed the vertical extent and the meridional extent of the IHT. We find that the vertical structure above 2 km has a slight northern tilt in the Northern Hemisphere (NH) winter season and a southern tilt in the NH summer, meaning the seasonality of the migration of the CE at the lower altitude is larger than that at the higher altitude. The meridional extent of the CE indicates a narrow transition zone where IHT happens throughout the year. We find that the meridional extent above South America is larger compared to other regions. The distribution of the land–sea contrast plays an important role in the meridional extent of IHT. We focus on the TWP region and further compared the tropical rain belt with the CE. There is a broad region of high precipitation occurring in the TWP region, and it is difficult to determine the IHT by the rain belt. In the NH winter, the CE is not consistent with the tropical rain belt in the TWP but is confined to the southern branch of the peak of the rain belt. For the other seasons, both indicators of IHT agree.

Different uncertainty in tropical oceanic and land precipitation sensitivities under global warming

The change in precipitation (P) under global warming has far-reaching implications for society. However, the high degree of intermodel uncertainty in future projections undermines the reliability of the multi-model mean. The present study investigates the sources of intermodel uncertainty in tropical-mean oceanic and land P sensitivities under global warming, based on an analysis of 28 models from CMIP6. Results show that the uncertainty in tropical-mean P sensitivity is greater over ocean compared to that over land, and both are closely tied to uncertainties in tropical-mean circulation sensitivity. Nevertheless, the underlying drivers of the uncertainty in tropical-mean circulation sensitivity vary between ocean and land. For the tropical-mean oceanic part, the tropical sea surface temperature (SST) warming pattern, particularly the SST warming over the western equatorial Pacific, plays a crucial role by modifying the atmospheric stability over the tropical Pacific. On the other hand, for the tropical-mean land part, the combined impacts of changes in land sea level pressure, relative humidity and SST warming over the equatorial Pacific contribute to the intermodel uncertainty in circulation sensitivity. The results highlight the importance of improving the simulation of these factors to reduce the discrepancies in tropical-mean oceanic and land circulation sensitivities among climate models, ultimately leading to a more accurate depiction of P sensitivities under global warming.

Constraints on the Ratio between Tropical Land and Ocean Precipitation Derived from a Conceptual Water Balance Model

Abstract Which processes control the mean amounts of precipitation received by tropical land and ocean? Do large-scale constraints exist on the ratio between the two? We address these questions using a conceptual box model based on water balance equations. With empirical but physically motivated parameterizations of the water balance components, we construct a set of coupled differential equations that describe the dynamical behavior of the water vapor content over land and ocean as well as the land’s soil moisture content. For a closed model configuration with one ocean and one land box, we compute equilibrium solutions across the parameter space and analyze their sensitivity to parameter choices. The precipitation ratio χ, defined as the ratio between mean land and ocean precipitation rates, quantifies the land–sea precipitation contrast. We find that χ is bounded between zero and one as long as the presence of land does not affect the relationship between water vapor path and precipitation. However, for the tested parameter values, 95% of the obtained χ values are even larger than 0.75. The sensitivity analysis reveals that χ is primarily controlled by the efficiency of atmospheric moisture transport rather than by land surface parameters. We further investigate under which conditions precipitation enhancement over land (χ > 1) would be possible. An open model configuration with an island between two ocean boxes and nonzero external advection into the domain can yield χ values larger than one, but only for a small subset of parameter choices, characterized by small land fractions and a sufficiently large moisture influx through the windward boundary.

A Multimodel Real-Time System for Global Probabilistic Subseasonal Forecasts of Precipitation and Temperature

Abstract A global multimodel probabilistic subseasonal forecast system for precipitation and near-surface temperature is developed based on three NOAA ensemble prediction systems that make their forecasts available publicly in real time as part of the Subseasonal Experiment (SubX). The weekly and biweekly ensemble means of precipitation and temperature of each model are individually calibrated at each grid point using extended logistic regression, prior to forming equal-weighted multimodel ensemble (MME) probabilistic forecasts. Reforecast skill of week-3–4 precipitation and temperature is assessed in terms of the cross-validated ranked probability skill score (RPSS) and reliability diagram. The multimodel reforecasts are shown to be well calibrated for both variables. Precipitation is moderately skillful over many tropical land regions, including Latin America, sub-Saharan Africa and Southeast Asia, and over subtropical South America, Africa, and Australia. Near-surface temperature skill is considerably higher than for precipitation and extends into the extratropics as well. The multimodel RPSS skill of both precipitation and temperature is shown to exceed that of any of the constituent models over Indonesia, South Asia, South America, and East Africa in all seasons. An example real-time week-3–4 global forecast for 13–26 November 2021 is illustrated and shown to bear the hallmarks of the combined influences of a moderate Madden–Julian oscillation event as well as weak–moderate ongoing La Niña event. Significance Statement This paper develops a system for forecasting of precipitation and temperatures globally over land, several weeks in advance, with a focus on biweekly averaged conditions between three and four weeks ahead. The system provides the likelihood of biweekly and weekly conditions being below, near, or above their long-term averages, as well the probability of exceeding (or not exceeding) any threshold value. Using historical data, the precipitation forecasts are demonstrated to have skill in many tropical regions, and the temperature forecasts are more widely skillful. While weather and seasonal range forecasts have become quite generally available, this is one of the first examples of a publicly available, calibrated multimodel probabilistic real-time forecasting system for the subseasonal range.