Intraseasonal Scales Research Articles

Unravelling the scaling characteristics of daily streamflows of Brahmani river basin, India, using arbitrary-order Hilbert spectral and detrended fluctuation analyses

This paper presents arbitrary-order Hilbert spectral analysis (AOHSA) and multifractal detrended fluctuation analysis (MF-DFA) approaches to describe the multifractality of daily streamflows from four stations, namely Tilga, Jeraikela, Gomlai and Jenapur of Brahmani river basin in India. In the former method, the spectral slopes of Hilbert spectra for different moment orders depict the multifractality, and in this study, AOHSA method detected the scale invariance between synoptic to intra-seasonal scales (3 days–3 months approximately) in the daily streamflows of all the four stations. The MF-DFA method detected a crossover within 80–110 days (~ 3 months) in the four time series in addition to the crossover at annual scale. The robust estimates of Hurst exponents made by following the adaptive detrending method of pre-processing detrending operation ranged between 0.7 and 0.73 for the four time series which confirmed the universal multifractal properties within Brahmani river basin. The behaviour of spectral slope plot of AOHSA, the characteristics of scaling exponent plot, mass exponent plot and multifractal spectra confirmed that the highest multifractal degree is for the streamflow records of Tilga station which is having the smallest drainage area, and it may be attributed to the faster response of this sub-catchment to local precipitation events. The multifractality of all the four streamflow time series is found to be due to the dominant influence of correlation properties than due to the broadness of probability density function.

The Simulation of East Asian Summer Monsoon Precipitation With a Regional Ocean‐Atmosphere Coupled Model

AbstractA fully coupled regional ocean‐atmosphere model was used to simulate the East Asian summer monsoon (EASM) precipitation. This coupled regional climate modeling system consists of the Regional Spectral Model (RSM) for the atmosphere and the Regional Ocean Modeling System for the ocean. The ocean and atmosphere share the same horizontal grid resolution. The coupled model is forced by the National Centers for Environmental Prediction‐Department of Energy (R‐2) global atmospheric reanalysis and Simplified Ocean Data Assimilation global oceanic reanalysis through the lateral boundary. This study examines EASM surface oceanic state and precipitation variability from a 22‐year (1984–2005) integration with a horizontal resolution of 40 km. The coupled model captures the features of observed sea surface temperature (SST), sea surface height, and ocean surface currents. Compared with the control run of the uncoupled RSM forced with observed SSTs, the coupled model shows more realistic simulation of the EASM precipitation climatology. The coupled model also improves the simulation of precipitation variability at both interannual and intraseasonal scales. It is the coupled model, not the uncoupled RSM, represents the observed SST‐precipitation and SST‐evaporation relationships. This study indicates that the ocean‐atmosphere coupling is essential for model simulations of the EASM precipitation.

Intraseasonal variation of the summer rainfall over the Southeastern United States

This study characterizes the intraseasonal variability (ISV) in the Southeastern United States (SE US) rainfall in boreal summer and delineates the associated dynamical processes featuring three-way interactions among the SE US rainfall, the central US low-level jet (LLJ), and the North Atlantic subtropical high (NASH). The analysis reveals that the ISV of the SE summer rainfall peaks at the 10‒20-day timescales. The physical mechanisms for the three-way interactions on the 10‒20-day timescales are proposed. When the NASH attains a minimum strength, the reduced size of the NASH is accompanied with an eastward retreat of the western ridge of the NASH, leading to a decrease in the zonal pressure gradient and consequently a weakening of the LLJ 1 day after. The weakened LLJ and the eastward-shifted NASH western ridge induces anomalous cyclonic circulation over the SE US, moves preferred regions of moisture convergence from central US to the SE US, and 3 days later the SE US rainfall attains its maximum strength. The excessive latent heating associated with the enhanced SE US rainfall excites an anomalous anticyclone northeast of the rainfall region, resulting in an increase in the NASH intensity that peaks 2 days after the maximum SE US rainfall. The NASH subsequently expands with its western ridge moving westward, zonal pressure gradient restored, and LLJ strength recovered. An anomalous anticyclone then emerges over the SE US and suppresses rainfall, marking the shift from an intraseasonal wet phase to dry phase in this region. A more rigorous proof of these causalities demand carefully designed numerical experiments and further statistical analysis in future. Our results suggest that improved prediction of SE US summer rainfall across intraseasonal scales depends critically on the model representation of the three-way coupling among the NASH, the central US LLJ, and the SE US rainfall.

Revisiting the intraseasonal, interannual and interdecadal variability of tropical cyclones in the western North Pacific

This paper reviews the recent progress and research on the variability of tropical cyclones (TCs) at different time scales. Specific focus is placed on how different types of external forcings or climatic oscillations contribute to TC variability in the western North Pacific (WNP). At the intraseasonal scale, recent advances on the distinctive impacts of the Madden–Julian Oscillation (MJO), the Quasi-biweekly Oscillation, and the asymmetric MJO modulation under different El Niño–Southern Oscillation (ENSO) states, as well as the influences of the Pacific–Japan teleconnection, are highlighted. Interannually, recent progress on the influences of the ENSO cycle, different flavors of ENSO, and impacts of Indian Ocean warming is presented. In addition, the uncertainty concerning interdecadal TC variations is discussed, along with the recently proposed modulation mechanisms related to the zonal sea surface temperature gradient, the North Pacific Gyre Oscillation, and the Pacific Decadal Oscillation (PDO). It is hoped that this study can deepen our understanding and provide information that the scientific community can use to improve the seasonal forecasting of TCs in the WNP.

CHARACTERISTICS AND VARIABILITY OF THE FLORES ITF AND ITS COHERENCE WITH THE SOUTH JAVA COASTAL CURRENT

Characteristics and transport variability of the Indonesian Throughflow (ITF) in the western Flores Sea (FS) and its coherency with the South Java Coastal Current (SJCC) fluctuation are investigated using validated ocean general circulation model output (2008-2014) from the INDESO configuration. The results show that near-surface circulation in the study area is characterized by two distinct regimes: strong southwestward ITF flow and quasi-transient anti-cyclonic eddies. Vertical dimension of ITF crossing 7.5°S is about 112 km width, 250 m depth, and high velocity core at thermocline >0.3 m/s. Transport volume estimates along this latitude is -4.95 Sv (southward). Bifurcation of ITF flow appears north offshore Lombok Island where -2.92 Sv flowing into Lombok Strait and the rest flowing eastward into FS. Meanwhile, vertical dimension of SJCC crossing 114°E is about 89 km width, 120 m depth, and high velocity core at sub-surface >0.35 m/s. Mean transport of SJCC is +2.65 Sv. Coherency between Flores ITF and SJCC transport variability on intra-seasonal scales is significantly high, e.g., on 30 day period (coher=0.92) and phase-lags of 0.6-day with SJCC leading to Flores ITF. This result confirmed previous studies, related to intrusion of coastally trapped Kelvin waves into Flores Sea via Lombok Strait. Keywords: Indonesian Throughflow, western Flores Sea, South Java Coastal Current

Improving fungal disease forecasts in winter wheat: A critical role of intra-day variations of meteorological conditions in the development of Septoria leaf blotch

Meteorological conditions are important factors in the development of fungal diseases in winter wheat and are the main inputs of the decision support systems used to forecast disease and thus determine timing for efficacious fungicide application. This study uses the Fourier transform method (FTM) to characterize temporal patterns of meteorological conditions between two neighbouring experimental sites used in a regional fungal disease monitoring and forecasting experiment in Luxembourg. Three meteorological variables (air temperature, relative humidity, and precipitation) were included, all conducive to infection of wheat by Zymoseptoria tritici cause of Septoria leaf blotch (STB) in winter wheat, from 2006 to 2009. The intraday, diurnal, dekadal and intra-seasonal variations of the meteorological variables were assessed using FTM, and the impact of existing contrasts between sites on the development of STB was analyzed. Although STB severities varied between sites and years (P≤0.0003), the results indicated that the two sites presented the same patterns of meteorological conditions when compared at larger temporal scales (diurnal to intra-seasonal scales, with time periods >11h). However, the intraday variations of all the variables were well discriminated between the sites and were highly correlated to STB severities. Our findings highlight and confirm the importance of intraday meteorological variation in the development of STB in winter wheat fields. Furthermore, the FTM approach has potential for identifying microclimatic conditions prevailing at given sites and could help in improving the prediction of disease forecast models used in regional warning systems.

Multiscale Atmosphere–Ocean Interactions and the Low-Frequency Variability in the Equatorial Region

Abstract In the present study a simplified multiscale atmosphere–ocean coupled model for the tropical interactions among synoptic, intraseasonal, and interannual scales is developed. Two nonlinear equatorial β-plane shallow-water equations are considered: one for the ocean and the other for the atmosphere. The nonlinear terms are the intrinsic advective nonlinearity and the air–sea coupling fluxes. To mimic the main differences between the fast atmosphere and the slow ocean, suitable anisotropic multispace/multitime scalings are applied, yielding a balanced synoptic–intraseasonal–interannual–El Niño (SInEN) regime. In this distinguished balanced regime, the synoptic scale is the fastest atmospheric time scale, the intraseasonal scale is the intermediate air–sea coupling time scale (common to both fluid flows), and El Niño refers to the slowest interannual ocean time scale. The asymptotic SInEN equations reveal that the slow wave amplitude evolution depends on both types of nonlinearities. Analytic solutions of the reduced SInEN equations for a single atmosphere–ocean resonant triad illustrate the potential of the model to understand slow-frequency variability in the tropics. The resonant nonlinear wind stress allows a mechanism for the synoptic-scale atmospheric waves to force intraseasonal variability in the ocean. The intraseasonal ocean temperature anomaly coupled with the atmosphere through evaporation forces synoptic and intraseasonal atmospheric variability. The wave–convection coupling provides another source for higher-order atmospheric variability. Nonlinear interactions of intraseasonal ocean perturbations may also force interannual oceanic variability. The constrains that determine the establishment of the atmosphere–ocean resonant coupling can be viewed as selection rules for the excitation of intraseasonal variability (MJO) or even slower interannual variability (El Niño).

Implementation and calibration of a stochastic multicloud convective parameterization in the NCEP Climate Forecast System (CFSv2)

AbstractA comparative analysis of fourteen 5 year long climate simulations produced by the National Centers for Environmental Predictions (NCEP) Climate Forecast System version 2 (CFSv2), in which a stochastic multicloud (SMCM) cumulus parameterization is implemented, is presented here. These 5 year runs are made with different sets of parameters in order to figure out the best model configuration based on a suite of state‐of‐the‐art metrics. This analysis is also a systematic attempt to understand the model sensitivity to the SMCM parameters. The model is found to be resilient to minor changes in the parameters used implying robustness of the SMCM formulation. The model is found to be most sensitive to the midtropospheric dryness parameter (MTD) and to the stratiform cloud decay timescale (τ30). MTD is more effective in controlling the global mean precipitation and its distribution while τ30 has more effect on the organization of convection as noticed in the simulation of the Madden‐Julian oscillation (MJO). This is consistent with the fact that in the SMCM formulation, midtropospheric humidity controls the deepening of convection and stratiform clouds control the backward tilt of tropospheric heating and the strength of unsaturated downdrafts which cool and dry the boundary layer and trigger the propagation of organized convection. Many other studies have also found midtropospheric humidity to be a key factor in the capacity of a global climate model to simulate organized convection on the synoptic and intraseasonal scales.

Analysis of spatial patterns of daily precipitation and wet spell extremes in Southeast Asia

ABSTRACTThe study investigates the spatial distribution of extreme daily rainfall and wet spells in Southeast Asia (SEA) at annual, seasonal, and intraseasonal scales using 16 years of data from the Tropical Rainfall Measurement Mission. A detailed analysis is conducted using various indices that characterize exceedance probabilities and relative contribution of extremes to the total rainfall. The selected indices include the Gini index, which is a measure of overall inequality, and is employed for the first time to examine SEA rainfall characteristics. The favoured regions (hotspots) of rainfall extremes during each season are identified, and their linkages with terrain and seasonal winds discussed. The intraseasonal variability in rainfall extremes is investigated by focusing on different phases of the Madden–Julian Oscillation (MJO). At the annual scale, four main hotspots are observed, which are located off the coast of Myanmar, over eastern Indian Ocean, northwestern Borneo and over Philippines. A majority of indices shows considerable land–sea contrast, with lower values over land. Lower Gini index as obtained for land regions indicate that the rainfall distribution, and the relative contribution of each class to the total rainfall are more uniform over land compared to sea. The land–sea contrast in indices is stronger during boreal summer and winter, and weaker during boreal fall. During boreal spring, the Myanmar coast has high probability of extremes, which further increases during boreal summer. The most evident hotspot regions during boreal fall and winter are along the coast of Vietnam, and the eastern coast of Malay Peninsula, respectively. The wind‐terrain interactions play a crucial role in determining the location of most hotspots. In general, the impact of active (suppressed) MJO phases is to increase (decrease) the probability of extremes in the study domain. However, local scale departures from this general pattern are observed, which are discussed in this work.

Analysis of temporal and spatial characteristics of waves in the Indian Ocean based on ERA-40 wave reanalysis

Based on the 45-year (09/1957-08/2008) European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-40) wave reanalysis dataset, this study analyzes interannual and interdecadal variabilities and intraseasonal oscillations of sea surface wind speed (WS), wind sea wave height (Hw), swell wave height (Hs) and significant wave height (Hs) in the Roaring Forties and tropical waters of the Indian Ocean, to determine swell propagation characteristics. The results show: (1) monthly variabilities of Hs in the Roaring Forties are in good agreement with those in tropical waters of the Indian Ocean; swell plays a dominant role in mixed waves throughout most of the Indian Ocean; and WS, Hw, Hs, and Hs exhibit a significant increasing trend over the 45-year study period. (2) Hs in the Roaring Forties and tropical waters of the Indian Ocean share a common period of 9.8–10.4 years on an interdecadal scale; and WS and Hs in the Roaring Forties and Hs in the tropical waters of the Indian Ocean share a common period of approximately 8days (weekly oscillation) on an intraseasonal scale. (3) Swell of the Roaring Forties needs approximately 30h to fully respond to the wind in this region. Approximately 84h are required for Hs to propagate from the Roaring Forties to the tropical waters of the south Indian Ocean, while it takes approximately 132–138h for Hs to propagate from the Roaring Forties to the tropical waters of the north Indian Ocean.

Synchrony, compensatory dynamics, and the functional trait basis of phenological diversity in a tropical dry forest tree community: effects of rainfall seasonality

Interspecific variation in phenology is a key axis of functional diversity, potentially mediating how communities respond to climate change. The diverse drivers of phenology act across multiple temporal scales. For example, abiotic constraints favor synchronous reproduction (positive covariance among species), while biotic interactions can favor synchrony or compensatory dynamics (negative covariance). We used wavelet analyses to examine phenology of community flower and seed production for 45 tree species across multiple temporal scales in a tropical dry forest in Puerto Rico with marked rainfall seasonality. We asked three questions: (1) do species exhibit synchronous or compensatory temporal dynamics in reproduction, (2) do interspecific differences in phenology reflect variable responses to rainfall, and (3) is interspecific variation in phenology and response to a major drought associated with functional traits that mediate responses to moisture? Community-level flowering was synchronized at seasonal scales (∼5–6 mo) and at short scales (∼1 mo, following rainfall). However, seed rain exhibited significant compensatory dynamics at intraseasonal scales (∼3 mo), suggesting interspecific variation in temporal niches. Species with large leaves (associated with sensitivity to water deficit) peaked in reproduction synchronously with the peak of seasonal rainfall (∼5 mo scale). By contrast, species with high wood specific gravity (associated with drought resistance) tended to flower in drier periods. Flowering of tall species and those with large leaves was most tightly linked to intraseasonal (∼2 mo scale) rainfall fluctuations. Although the 2015 drought dramatically reduced community-wide reproduction, functional traits were not associated with the magnitude of species-specific declines. Our results suggest opposing drivers of synchronous versus compensatory dynamics at different temporal scales. Phenology associations with functional traits indicated that distinct strategies for coping with seasonality underlie phenological diversity. Observed drought responses highlight the importance of non-linear community responses to climate. Community phenology exhibits scale-specific patterns highlighting the need for multi-scale approaches to community dynamics.

A multi-scale spatiotemporal modeling approach to explore vegetation dynamics patterns under global climate change

Given the complexity of vegetation dynamic patterns under global climate change, multi-scale spatiotemporal explicit models are necessary in order to account for environmental heterogeneity. However, there is no efficient time-series tool to extract, reconstruct and analyze the multi-scale vegetation dynamic patterns under global climate change. To fill this gap, a Multi-Scale Spatio-Temporal Modeling (MSSTM) framework which can incorporate the pixel, scale, and time-specific heterogeneity was proposed. The MSSTM method was defined on proper time-series models for multi-temporal components through wavelet transforms. The proposed MSSTM approach was applied to a subtropical mountainous and hilly agro-forestry ecosystem in southeast China using the moderate resolution imaging spectroradiometer enhanced vegetation index (EVI) time-series data sets from 2001 to 2011. The MSSTM approach was proved to be efficient in characterizing and forecasting the complex vegetation dynamic patterns. It provided good estimates of the peaks and valleys of the observed EVI and its average percentages of relative absolute errors of reconstruction was low (6.65). The complexity of the relationship between vegetation dynamics and meteorological parameters was also revealed through the MSSTM method: (1) at seasonal level, vegetation dynamic patterns are strongly associated with climatic variables, primarily the temperature and then precipitation, with correlations slight decreasing (EVI–temperature)/increasing (EVI–precipitation) with altitudinal gradients. (2) At inter-annual scale, obvious positive correlations were primarily observed between EVI and temperature. (3) Despite very low-correlation coefficients observed at intra-seasonal scales, considerable proportions of EVI anomalies are associated with climatic variables, principally the precipitation and sunshine durations.

Intra-annual variability of wood formation and δ13C in tree-rings at Hyytiälä, Finland

Investigation of the relationship between tree-ring stable carbon isotope composition (δ13C) and environmental variables at the intra-seasonal scale can inform on the understanding of the environmental forcing affecting trees during the active period of radial growth. Recent progress in the measurement techniques for assessing the δ13C signature of tree rings at high spatial resolution provides an opportunity to derive tree physiological information at fine temporal scale, within a given year. Three δ13C time series of resin-extracted wholewood from tree-rings of Scots pine trees (Pinus sylvestris L.) sampled at Hyytiälä (Finland) have been produced by using laser-ablation stable isotope mass spectrometry. These intra-seasonal stable isotope series exhibit a strong common signal (EPS=0.96) demonstrating the capacity of trees within a stand to preserve a common intra-seasonal response to external controls in a similar manner as found with annual measurements. To estimate when wood cells are active and responsive to environmental information, a Gompertz approach, assessed against microcore data, was adopted to model the timing of wood formation. The addition of a cell lifetime function into environment-growth models may evaluate more completely, the environmental effect on intra-annual tree-ring δ13C values and that during the growing season. Statistical analysis of the resulting tree-ring δ13C intra-annual signal implies a shift in importance from the influence of the environmental variables through out the growing season.

The influence of tropical forcing on extreme winter precipitation in the western Himalaya

Within the Karakoram and western Himalaya (KH), snowfall from winter westerly disturbances (WD) maintains the region’s snowpack and glaciers, which melt seasonally to sustain water resources for downstream populations. WD activity and subsequent precipitation are influenced by global atmospheric variability and tropical-extratropical interactions. On interannual time-scales, El Nino related changes in tropical diabatic heating induce a Rossby wave response over southwest Asia that is linked with enhanced dynamical forcing of WD and available moisture. Consequently, extreme orographic precipitation events are more frequent during El Nino than La Nina or neutral conditions. A similar spatial pattern of tropical diabatic heating is produced by the MJO at intraseasonal scales. In comparison to El Nino, the Rossby wave response to MJO activity is less spatially uniform over southwest Asia and varies on shorter time-scales. This study finds that the MJO’s relationship with WD and KH precipitation is more complex than that of ENSO. Phases of the MJO propagation cycle that favor the dynamical enhancement of WD simultaneously suppress available moisture over southwest Asia, and vice versa. As a result, extreme precipitation events in the KH occur with similar frequency in most phases of the MJO, however, there is a transition in the relative importance of dynamical forcing and moisture in WD to orographic precipitation in the KH as the MJO evolves. These findings give insight into the dynamics and predictability of extreme precipitation events in the KH through their relationship with global atmospheric variability, and are an important consideration in evaluating Asia’s water resources.

Use of Wavelet Transform to Analyse Observed Precipitation at Bukoba, Tanzania to Recognize Impacts to Hydrological Response in Lake Victoria Basin

This research exploited seventy years of daily precipitation data from Bukoba, Tanzania to understand precipitation variability and change important for water resource management. Morlet Wavelet Transform was applied to discriminate the distinct time-frequency rainfall variability in the 92 days long MAM ((March and May)) and OND (October to December) primary rainfall seasons for intraseasonal characteristics over the periods 1931-1960, 1961-1990 and 1971-2000. The time-frequency analysis yielded wavelets outlining the intraseasonal nature of sporadic wet and dry spells in each epoch. The characters of the spells designate changing distribution, intensity and frequency of occurrence across the three epochs. The profile of the erratic wet and dry spells speculates shift change and fading of high frequency, quasi biweekly and low frequency oscillations. The oscillations which act together across atmosphere, oceans and land surfaces through convective processes are likely to influence seasonal precipitation anomalies at intraseasonal scale. The variability of the observed daily precipitation is thus hypothesized to be linked to the fading oscillations in the later two epochs particularly during the main MAM season and thus the declining precipitation in the study domain. Global increased atmospheric carbon dioxide concentration could catalyze this process.

West African monsoon intraseasonal activity and its daily precipitation indices in regional climate models: diagnostics and challenges

The West African monsoon intraseasonal variability has huge socio-economic impacts on local populations but understanding and predicting it still remains a challenge for the weather prediction and climate scientific community. This paper analyses an ensemble of simulations from six regional climate models (RCMs) taking part in the coordinated regional downscaling experiment, the ECMWF ERA-Interim reanalysis (ERAI) and three satellite-based and observationally-constrained daily precipitation datasets, to assess the performance of the RCMs with regard to the intraseasonal variability. A joint analysis of seasonal-mean precipitation and the total column water vapor (also called precipitable water—PW) suggests the existence of important links at different timescales between these two variables over the Sahel and highlights the relevance of using PW to follow the monsoon seasonal cycle. RCMs that fail to represent the seasonal-mean position and amplitude of the meridional gradient of PW show the largest discrepancies with respect to seasonal-mean observed precipitation. For both ERAI and RCMs, spectral decompositions of daily PW as well as rainfall show an overestimation of low-frequency activity (at timescales longer than 10 days) at the expense of the synoptic (timescales shorter than 10 days) activity. Consequently, the effects of the African Easterly Waves and the associated mesoscale convective systems are substantially underestimated, especially over continental regions. Finally, the study investigates the skill of the models with respect to hydro-climatic indices related to the occurrence, intensity and frequency of precipitation events at the intraseasonal scale. Although most of these indices are generally better reproduced with RCMs than reanalysis products, this study indicates that RCMs still need to be improved (especially with respect to their subgrid-scale parameterization schemes) to be able to reproduce the intraseasonal variance spectrum adequately.

What caused the spring intensification and winter demise of the 2011 drought over Texas?

The 2011 Texas drought, the worst 1-year drought on record, was characterized by spring intensification of rainfall deficit and surface dryness. Such spring intensification was led by an unusually strong increase of convective inhibition (CIN), which suppressed convection at the time critical for the onset of the April–June rainfall season. The CIN increase appeared to be caused by strong sub-seasonal anomalously westerly winds at 850 hPa (U850) in April, in addition to surface dryness due to cumulative rainfall deficit since fall of 2010. The anomalous U850 advected warm dry air from the Mexican Plateau to Texas, enhancing cap inversion, and exacerbating static stability initially elevated by an anomalously high surface Bowen ratio due to rainfall deficits from winter through spring over Texas. Strengthened westerly U850 in April, in addition to the persistent rainfall deficits from winter through spring, are common characteristics in other strong drought events experienced over Texas. Atmospheric Model Intercomparison Project-type simulations with prescribed La Nina SSTAs in the tropical Pacific do not show a strengthening of westerly U850 in April, suggesting that internal atmospheric variability at intraseasonal scale, instead of La Nina, may initiate the spring drought intensification over Texas. Soil moisture deficits in late spring are significantly correlated with positive 500 hPa geopotential height anomalies over the south central U.S. 2–3 weeks later, suggesting that intensified surface dryness in late-spring could reinforce the drought-inducing anomalous mid-tropospheric high. The drought diminished in the winter of 2011/2012 despite a second La Nina event. Our analysis suggests an important role for strong westerly wind anomalies, the resultant increase of CIN in spring, and subsequent positive feedback between dry surface anomalies and the anomalous large-scale circulation pattern in drought intensification. Clarification of the mechanisms behind the strong increase of CIN and land–atmosphere feedbacks may provide a key for improving our understanding of drought predictability in spring and summer, and a scientific basis for the early warning of strong summer drought. The demise of the 2011 drought appears to have resulted from internal atmospheric circulation variability, thus intrinsically unpredictable.

Effect of Stratiform Heating on the Planetary-Scale Organization of Tropical Convection

Abstract It is widely recognized that stratiform heating contributes significantly to tropical rainfall and to the dynamics of tropical convective systems by inducing a front-to-rear tilt in the heating profile. Precipitating stratiform anvils that form from deep convection play a central role in the dynamics of tropical mesoscale convective systems. The wide spreading of downdrafts that are induced by the evaporation of stratiform rain and originate from in the lower troposphere strengthens the recirculation of subsiding air in the neighborhood of the convection center and triggers cold pools and gravity currents in the boundary layer, leading to further lifting. Here, aquaplanet simulations with a warm pool–like surface forcing, based on a coarse-resolution GCM of approximately 170-km grid mesh, coupled with a stochastic multicloud parameterization, are used to demonstrate the importance of stratiform heating for the organization of convection on planetary and intraseasonal scales. When the model parameters, which control the heating fraction and decay time scale of the stratiform clouds, are set to produce higher stratiform heating, the model produces low-frequency and planetary-scale MJO-like wave disturbances, while parameters associated with lower-to-moderate stratiform heating yield mainly synoptic-scale convectively coupled Kelvin-like waves. Furthermore, it is shown that, when the effect of stratiform downdrafts is reduced in the model, the MJO-scale organization is weakened, and a transition to synoptic-scale organization appears despite the use of larger stratiform heating parameters. Rooted in the stratiform instability, it is conjectured here that the strength and extent of stratiform downdrafts are key contributors to the scale selection of convective organizations, perhaps with mechanisms that are, in essence, similar to those of mesoscale convective systems.

Sensitivity of MJO propagation to a robust positive Indian Ocean dipole event in the superparameterized CAM

AbstractThe superparameterized Community Atmosphere Model (SPCAM) is used to investigate the impact and geographic sensitivity of positive Indian Ocean Dipole (+IOD) sea‐surface temperatures (SSTs) on Madden‐Julian oscillation (MJO) propagation. The goal is to clarify potentially appreciable +IOD effects on MJO dynamics detected in prior studies by using a global model with explicit convection representation. Prescribed climatological October SSTs and variants of the SST distribution from October 2006, a +IOD event, force the model. Modest MJO convection weakening over the Maritime Continent occurs when either climatological SSTs, or +IOD SST anomalies restricted to the Indian Ocean, are applied. However, severe MJO weakening occurs when either +IOD SST anomalies are applied globally or restricted to the equatorial Pacific. MJO disruption is associated with time‐mean changes in the zonal wind profile and lower moist static energy (MSE) in subsiding air masses imported from the Subtropics by Rossby‐like gyres. On intraseasonal scales, MJO disruption arises from significantly smaller MSE accumulation, weaker meridional advective moistening, and overactive submonthly eddies that mix drier subtropical air into the path of MJO convection. These results (1) demonstrate that SPCAM reproduces observed time‐mean and intraseasonal changes during +IOD episodes, (2) reaffirm the role that submonthly eddies play in MJO propagation and show that such multiscale interactions are sensitive to interannual SST states, and (3) suggest that boreal fall +IOD SSTs local to the Indian Ocean have a significantly smaller impact on Maritime Continent MJO propagation compared to contemporaneous Pacific SST anomalies which, for October 2006, resemble El Niño‐like conditions.

Variability in AIRS CO2 during active and break phases of Indian summer monsoon

Due to human activities, the atmospheric concentration of Carbon Dioxide (CO2) has been rising extensively since the Industrial Revolution. Indian summer monsoon (ISM) has a dominant westerly component from ocean to land with a strong tendency to ascend and hence may have role in CO2 distribution in lower and middle troposphere over Indian sub-continent. A substantial component of ISM variability arises from the fluctuations on the intra-seasonal scale between active and break phases which correspond to strong and weak monsoon circulation. In view of the above, an attempt is made in this study to examine the AIRS/AQUA satellite retrieved CO2 distribution in response to atmospheric circulation with focus on active and break phase. Correlation analysis indicates the increase in AIRS CO2 linked with strong monsoon circulation. Study also reveals that anomalous circulation pattern during active and break phase show resemblance with high and low values of AIRS CO2. Homogeneous monsoon regions of India show substantial increase in CO2 levels during active phase. Hilly regions of India show strong contrast in CO2 and vertical velocity during active and break phases.