Intraseasonal Variability Research Articles

Vapor Pressure Deficit Controls the Extent of Burned Area Over the Himalayas

AbstractGlobally, increasing occurrences of forest fires are major threats to the ecosystem. The rich forests in the Himalayas also suffer from high incidents of forest fires in the pre‐monsoon summer months, March to June. Research studies are limited in identifying the meteorological factors governing the spatiotemporal distribution of forest fires. Using three satellite‐based data sets of monthly burned area (BA) for March to June, we found higher BA in the Eastern Himalayas (EH) compared to the Central (CH) and Western Himalayas (WH). Using statistical methods, we found Vapor Pressure Deficit (VPD) to be the most dominating variable controlling BA in the Himalayas. Precipitation, soil moisture and temperature, with their relative variability, control VPD that governs the interannual and intraseasonal variations of BA. Our results imply that a good forecast of VPD will facilitate alert generation for the Himalayan forest fires.

Observed Responses of Gravity Wave Momentum Fluxes to the Madden‒Julian Oscillation Around the Extratropical Mesopause Using Mohe Meteor Radar Observations

AbstractThe 12‐year continuous observation of gravity wave momentum fluxes (GWMFs) estimated by the Mohe meteor radar (53.5°N, 122.3°E) revealed prominent intraseasonal variability around the extratropical mesopause (82–94 km) during boreal winters. Composite analysis of the December‒January‒February (DJF) season according to the Madden‒Julian Oscillation (MJO) phases revealed that the zonal GWMFs notably increased in MJO Phase 4 (P4) by ∼2–4 m2/s2, and a Monte Carlo test was designed to examine the statistical significance. The response in zonal winds lags behind the GWMF response by two MJO phases (i.e., 1/2π), indicating a “force‒response” interaction between them. Additionally, time‐lagged composites revealed that strengthened westward GWMFs occurred ∼25–35 days after MJO P4, coincident with the MJO impact on the zonal winds in the stratosphere. The analysis results also suggested that the mechanism of MJO by which the MJO influences the stratospheric circulation might involve poleward propagating effects of stationary planetary waves with zonal wavenumber one. This work emphasizes the importance of GW intraseasonal variability, which impacts tropical sources from the troposphere to the extratropical mesopause.

Intraseasonal relationship of winter temperatures in North America and warm sea surface temperatures in the Northeast Pacific

Winter temperatures in the North America (NA) exhibit evident intraseasonal variation, which has become more pronounced under global climate change. In particular, the wintertime relationship between NA temperatures and long-lasting positive sea surface temperature anomalies (SSTAs) in the Northeast Pacific (NEP), known as warm blobs, has been hotly debated in recent years. However, their relationship on intraseasonal timescales remains unclear. Using reanalysis data and an Atmospheric General Circulation Model, this study reveals that the intraseasonal variation of atmospheric circulation is the primary factor to induce intraseasonal temperature anomalies in NA after classifying warm SSTA events based on their different evolutions. Moreover, the atmospheric circulation anomalies over the mid-to-high-latitude regions are in part contributed by the feedback of NEP warm SSTAs to the atmosphere. Specifically, for early-winter warm SSTA events (peaking in November), there are two intraseasonal reversals in geopotential height anomalies over the NEP and NA, corresponding to a “cold-warm-cold” temperature change in NA. However, for mid-winter warm SSTA events (peaking in January), the atmospheric circulation pattern is relatively stable, mainly characterized by a “west high-east low” dipole, associated with persistent cold anomalies in NA. Then, the role of warm SSTAs over the NEP in the intraseasonal temperature variation is generally verified by the numerical simulations. Hence, although the NEP warm SSTAs may not be the dominant driver of the intraseasonal variations of winter NA temperatures, we emphasize the implication of their evolutionary differences in reflecting the NA temperature variations, which contributes to better predictability.

The Relative Importance of Ocean Advection and Surface Heat Fluxes During the Madden–Julian Oscillation in a Coupled Ocean–Atmosphere Model

AbstractIntraseasonal variability of ocean surface mixed layer temperature (MLT) in the tropics can be linked to the Madden–Julian Oscillation (MJO), the main source of intraseasonal atmospheric variability in the tropics. Here, we conduct a boreal winter mixed layer heat budget using 10‐day forecast composites of a coupled ocean–atmosphere Numerical Weather Prediction model of the UK Met Office to reveal that ocean advection plays a major role in modulating intraseasonal anomalies of MLT over the tropical Indian Ocean and the Maritime Continent. Large scale ( 10) intraseasonal anomalies of MLT ( 0.1 C) are driven by net surface heat flux. Ocean advection dominates at smaller horizontal scales ( 1), contributing up to 0.5 C to the intraseasonal MLT anomaly. Prior to the development of the enhanced MJO convection in the western Indian Ocean (phases 8 and 1), ocean advection reinforces the net heat flux warming in this region. However, ocean advection opposes the net heat flux warming in the Maritime Continent prior to the development of suppressed MJO convection in this region. When the MJO convection develops over the central Indian Ocean (phase 3), ocean advection (net surface heat flux) drives the intraseasonal MLT anomalies in the western Indian Ocean (central Indian Ocean and the Maritime Continent). Our results demonstrate the importance of ocean dynamics during the initiation and growth of the MJO, and raise implications for models that do not resolve these processes.

Exceptionally Strong Equatorial Intermediate Current Events in the Indian Ocean Associated with Climate Modes

Abstract Knowledge of the effects of climate modes on the equatorial intermediate current (EIC) remains limited. This paper investigates exceptional events of the EIC in the Indian Ocean and their relationships with climate modes at various time scales by using observations, reanalysis outputs, and a continuously stratified linear ocean model (LOM). A mooring at 80°E from 2015 to 2019 revealed four exceptionally strong EIC events, occurring in 2015 July–August (JA), 2016 January–February (JF), 2016 JA, and 2019 JF. Component analysis revealed that these exceptional events are attributed to the co-occurrence of the seasonal components peaking during JF and JA, as well as the larger current anomalies associated with intraseasonal and interannual components. In the intraseasonal band, the Madden–Julian oscillation (MJO) generates a significant EIC anomaly through a 40–50-day process involving equatorial waves. The MJO exerts a substantial effect when the amplitude of the MJO index exceeds 1 and the oscillation is in phase 4. In the interannual band, El Niño–Southern Oscillation and the Indian Ocean dipole (IOD) can each independently contribute to the EIC anomaly. This contribution initiates during the mature phase and persists throughout the subsequent year. Concurrent occurrences of these effects can lead to larger interannual anomalies. Notably, El Niño–positive IOD (El Niño–pIOD) and La Niña–negative IOD (La Niña–nIOD) synergies are most pronounced in August and in January of the following year. Significance Statement Four exceptional equatorial intermediate current (EIC) events characterized by strong eastward velocities are observed in the Indian Ocean during 2015–19. This study explored their underlying dynamics and their relationships with climate modes. Climatologically, the EIC has seasonal cycles with peaks in January–February and July–August, which is helpful in producing exceptional EIC events. However, the occurrence of these events is attributed to the intensity of the intraseasonal and interannual variability that occurs around the seasonal cycle peaks. This study revealed that climate modes, including the Madden–Julian oscillation (MJO), El Niño–Southern Oscillation (ENSO), and Indian Ocean dipole (IOD), contribute to exceptional EIC events in the intraseasonal and interannual bands. Notably, their influence is comparable, albeit contingent upon specific conditions.

Simulation of Intra-seasonal Climatic Variability, Potential Yield and Yield Gap of Sugarcane for the South Gujarat Region, India

The validated CANEGRO (DSSAT v4.7) model was used to simulate the intra-seasonal climatic variation viz. temperature (-3oC to +3oC) and rainfall (-25% to +25%) variability in sugarcane yield and quantifying the yield gap of sugarcane at Navsari, Bharuch, Surat and Valsad districts of south Gujarat region under the various management options included dates of planting, irrigations and fertilizers application and compared with the reported yield of the districts. The cane yield increased with an increase in rainfall and a decrease in temperature. In contrast, the cane yield decreased with a decrease in rainfall and an increase in temperature. The extent of effect was found to vary with crop growth stages. The potential yield of sugarcane simulated by the model in different districts was found to vary between 107.0 to 114.7 t ha-1, while the reported yield in these districts varied between 68.1 to 72.0 t ha-1. This resulted in a total yield gap between 38.9 to 45.5 t ha-1 in different districts. The highest yield gap was in Navsari district while the lowest yield gap was found in Bharuch district.

Tropical Intraseasonal Variability as a Leading Moisture Dynamic Mode of the Warm-Pool Background State

Abstract Tropical intraseasonal variability (ISV) is dominated by the Madden–Julian oscillation (MJO), and its spatiotemporal characteristics vary with the Indo-Pacific warm-pool background on seasonal and longer timescales. Previous works have suggested ISV dynamics in various frameworks, whereas a unifying view remains challenging. Motivated by the recent advance in moisture mode theory, we revisit the ISV as a leading moisture mode modulated by varying background states derived from a reanalysis, using a moist linear baroclinic model (mLBM) improved with a simple convective scheme relating convective precipitation to tropospheric and boundary-layer moisture anomalies and simple cloud–radiative feedback representations. Under a boreal winter background state, this mLBM yielded a large-scale but local eastward-propagating mode with a phase speed of 3–5 m/s over the warm-pool region, resembling the MJO. Background lower-tropospheric winds and thermodynamic fields are important in determining the growth rate and periodicity of the leading mode, whose stability depends on cloud–radiative feedback and background state variations. We further demonstrate why the MJO is locally contained in the Indo-Pacific warm-pool region. The local thermal/moisture condition and Walker circulation greatly enhance its instability, but outside this region, this mode is heavily damped. Thus, the expansion/contraction of this warm-pool condition may enhance/reduce its instability and expand/reduce its domain of activity. Prescribing El Niño background causes eastward displacement of the wintertime ISV activity, reminiscent of the observed MJO modulations by El Niño. Under a summer background state, the eastward-propagating leading mode resembles the boreal summer ISV but biased, requiring further model improvements.

Indonesian Throughflow promoted eastward propagation of the Madden-Julian Oscillation

Understanding the impacts of the Indonesian Throughflow (ITF) on the eastward propagation of the Madden-Julian Oscillation (MJO) is crucial for accurately simulating the MJO and achieving high-skill sub-seasonal predictions. Our analyses demonstrate a significant enhancement of MJO eastward propagation due to the strong ITF. Blocking the ITF decreases the eastward sea surface temperature (SST) gradient over the tropical Indian Ocean, hindering MJO propagation across the Maritime Continent (MC). Removing the MJO circulation-induced intraseasonal variability of the ITF transport also weakens the eastward propagation of the MJO, as the MJO easterly winds enhance the ITF transport and warm the eastern tropical Indian Ocean. These experiments reveal that mean and intraseasonal variability of the ITF transport contribute to 73% and 42% of the eastward propagation of the MJO over the MC, respectively. The findings presented in this study highlight the significant role of the ITF in shaping the propagation of the MJO.

Opportunities and barriers for skillful sub-seasonal prediction of East Asian summer precipitation

Abstract Accurate sub-seasonal (2-8 weeks) prediction of monsoon precipitation is crucial for mitigating flood and heatwave disasters caused by intra-seasonal variability (ISV). However, current state-of-the-art sub-seasonal-to-seasonal (S2S) models have limited prediction skills beyond one week when predicting weekly precipitation. Our findings suggest that predictability primarily arises from strong ISV events, and the prediction skills for ISV events depend on the propagation stability of preceding signals, regardless of models. This allows us to identify opportunities and barriers (OBs) within S2S models, clarifying what the models can and cannot achieve in ISV event prediction. Focusing on the complex East Asian summer monsoon (EASM), we discover that stable propagation of Eurasian and tropical atmospheric wave trains towards East Asia serves as an opportunity. This opportunity offers a one-week leading prediction skill of up to 0.85 and skillful prediction up to 13 days ahead for 43% of all ISV events. However, the Tibetan Plateau barrier highlights the limitation of EASM predictability. Identifying these OBs will help us gain confidence in making more accurate sub-seasonal prediction.

Manifold increase in the spatial extent of heatwaves in the terrestrial Arctic

It is widely acknowledged that the intensity, frequency and duration of heatwaves are increasing worldwide, including the Arctic. However, less attention has been paid to the land area affected by heatwaves. Here, using atmospheric reanalysis and global climate models, we show that the area covered by heatwaves is substantially expanding in the terrestrial Arctic. Compared to the mid-20th century, the total land area affected by severe heatwaves in the Arctic has doubled, the area of extreme heatwaves has tripled, and the area of very extreme heatwaves has quadrupled. Furthermore, climate model projections suggest that the extent of heatwaves will continue to increase in the 21st century, but with large regional differences in heatwave magnitudes due to summer intraseasonal temperature variability. Our findings underscore the growing vulnerability of the Arctic region to extreme heat, potentially leading to severe impacts on both ecosystems and societies.

Diversity of 10–20-day propagation of genesis potential index and its roles in tropical cyclogenesis over the South China Sea

The genesis potential index (GPI) is used to quantify the major large-scale environmental parameters associated with tropical cyclogenesis (TCG). It is demonstrated that the 10–20-day GPI dominates the intraseasonal variability of the GPI over the South China Sea (SCS) from June to September during 1979–2021. The spatiotemporal evolutions of the 10–20-day GPI in the TCG processes show distinct diverse characteristics. Through extended empirical orthogonal function (EEOF) analysis and spatial correlation coefficients, the 10–20-day GPI propagation characteristics of the TCG processes are mainly classified into 3 categories: WN-Pattern, N-Pattern, and W-Pattern. The WN-Pattern originates from the tropical western North Pacific (WNP), then gradually intensifies and propagates northwestward to the northern SCS. The N-Pattern moves northward from Indonesia to the central-northern SCS and distinctly strengthens from 2 days before the cyclogenesis. The W-Pattern features westward propagation from the subtropical WNP to the northern SCS. These three propagating patterns correspond to the respective locations of SCS TCG. In the GPI terms of these three patterns, the low-level absolute vorticity and mid-level relative humidity are considered as the dominant environmental factors. Furthermore, the significant cyclonic circulation anomalies and water vapor flux anomalies are well coordinated with the 10–20-day GPI in all the three patterns, constituting favorable dynamical and thermodynamic conditions for the SCS TCG. This study may be beneficial for deeper comprehension of the subseasonal large-scale environmental factors about SCS TCG.

Large Scale Oscillations in the Martian Tropical Cloud Belt

AbstractThe Tropical Cloud Oscillation (TCO) in the Martian atmosphere is a shift of clouds in the northern spring and summer tropical cloud belt between the eastern and western hemispheres on an intra‐seasonal timescale of about 10–40 sols. The TCO is a significant intraseasonal variation and may strongly affect the Martian general circulation, water cycle, and dust cycle. We examine TCOs using multiple data sets with a focus on the clouds observed in Mars Daily Global Maps during Mars Year (MY) 29–35. One or more TCO cycles are observed in each MY and the phenomenon is most prominent during Ls = 135°–185°. Space‐time spectral analysis shows a variety of waves which appear to follow the theoretical dispersion relationships of equatorial waves, such as Kelvin waves, Rossby waves, and Mixed Rossby Gravity waves. The TCO appears to be controlled by zonal wavenumber one traveling waves with Kelvin and Rossby wave characteristics and exhibits a fine‐scale latitudinal structure that requires modeling with sufficient resolution. Issues with current data assimilation products for use in studies of Martian equatorial waves due to this fine‐scale structure are discussed.

POST-EL-NIÑO WEAKENING OF COASTAL UPWELLING AND ITS ECOLOGICAL IMPACTS IN THE SOUTH-CENTRAL VIET NAM REGION

Upwelling that occurs in the summer offshore of South-Central Viet Nam in the South China Sea (SCS) also known as the East Viet Nam Sea (EVS), is one of the region’s key dynamic processes. The weakening of upwelling phenomena during post-El-Niño events and their ecological responses were studied based on a reanalysis dataset derived from the HYCOM/NCODA system, coupled with a local Finite Element Model (FEM) and observed data. Long-term warming, along with orographic and wind factors, play significant roles in the formation and weakening of upwelling phenomena during normal and post-El-Niño episodes, respectively. The weakening of upwelling during post-El-Niño periods is reflected by: Extreme weakening of wind forcing and Ekman pumping; a northward shift of the cold-saline tongue and current dipole, with a limited eastward extent; dominance of northward circulation in the surface layer and westward circulation in deeper layers; and the formation of a homogeneous surface thermohaline layer of about 50 meters thick, with a thermo-halocline layer at 50-60 meters depth. These abrupt changes strongly influenced the ecological characteristics of the upwelling area. Coral bleaching during the summers of 2010 and 2016, as well as anomalous distributions of chlorophyll-a (Chl-a) in the surface layer in 1998, 2003, 2010, and 2016, were concrete indicators of the ecological responses of Viet Nam's coastal upwelling waters in post-El-Niño years. Detailed intra-seasonal variations of temperature during the summer showed a warming of the waters, leading to coral bleaching and abnormal Chl-a levels during post-El-Niño years. Organisms in this region struggled to adapt to these rapid environmental changes, leading to a potential reduction in living resources.

Correction: A new insight into monsoon intraseasonal variability as revealed from distinct wind-precipitation regimes over the southwest coast of India

Correction: A new insight into monsoon intraseasonal variability as revealed from distinct wind-precipitation regimes over the southwest coast of India

A New Paradigm for Active–Break Cycle in the Indian Summer Monsoon

Abstract A multichannel singular spectrum analysis of 121 years of daily rainfall over India and 43 years of outgoing longwave radiation and horizontal wind has revealed two dominant intraseasonal oscillations (ISOs) that describe the space–time variability of monsoon rainfall. The two nonlinear oscillations are not perfectly periodic but exhibit broadband spectra centered at 45 and 28 days. These modes have coherent and near-regular spatial structure and temporal variations. This study posits that the two nonlinear oscillations provide a consistent and comprehensive framework to study the variability and predictability of intraseasonal rainfall variations. The active and break cycles of monsoon rainfall over India have been traditionally defined by using an arbitrarily chosen duration of persistence of arbitrarily chosen amount of rainfall averaged over an arbitrarily chosen area. This study shows that the phases of the two objectively derived modes of variability provide an objective method for defining the active and break cycles. The two ISOs are further shown to make negligible contribution to the seasonal mean rainfall and its interannual variability. This study proposes that the investigations of the origins of these nonlinear modes, and modulations of their amplitudes and phases, provide a new paradigm for future research on validation and predictability of intraseasonal variations in climate models.

Intraseasonal Variations and Extreme Occurrence in the Local Sea Level Along the Western Coast of India Remotely Controlled by a Basin‐Scale Climate Variability

AbstractThe equatorial Kelvin waves, remotely excited by basin‐scale climate modes, and subsequent coastal trapped waves significantly influence the intraseasonal variations, their low‐frequency modulations, and the frequency of extreme sea level events along the western coast of India. This study demonstrates that the frequency of extreme events are linked to the phase of the Indian Ocean Dipole mode. The temporal changes in the occurrence frequency of extremes are simulated in an eddy‐resolving ocean model consistently with observations. However, a non‐eddying model significantly underestimate the occurrence frequency of extreme sea level events, suggesting the importance of coastal trapped wave propagations regulated by the horizontal scale with the Rossby radius of deformation. This result implies that many state‐of‐the‐art climate models with a one‐degree ocean horizontal resolution may underestimate future coastal sea level variability and the frequency of extreme events under global warming and potential modulations of major internal climate modes.

The Role of Cloud-Radiative Interaction in Tropical Circulation and the Madden–Julian Oscillation

Abstract Cloud-radiative interaction (CRI) is a fundamental process that modulates tropical circulation and intraseasonal variability, including the Madden–Julian oscillation (MJO). In this study, we investigate how the mean state of the tropical atmosphere and the MJO respond to CRI intensity changes and provide insights into the underlying mechanisms, using the aquaplanet configuration in the Community Earth System Model, version 2 (CESM2). By enhancing CRI through tuning the DCS parameter (an autoconversion threshold size in the Morrison and Gettelman cloud microphysics scheme), we demonstrate that DCS-induced CRI intensification is linked to a warmer troposphere, increased tropical moisture, strengthened Hadley cell (HC), stronger trade winds, and a stronger equatorward intertropical convergence zone (ITCZ) with more clouds and precipitation, reflecting stronger cloud–radiation–circulation feedback. The intensified CRI also leads to the intensification and slower propagation of the simulated MJO-like mode despite the MJO-like signals becoming less distinguishable from the background due to the influence of other waves. The MJO intensification is likely associated with the mean state changes that support the development of deep convection. Moreover, the CRI itself, especially the interaction with the longwave radiation, also directly influences the MJO’s maintenance and propagation, more contributing to the maintenance of column moist static energy (MSE) and deceleration of its eastward propagation on intraseasonal time scales.

Tropical and mid-latitude causal drivers of the eastern Mediterranean Etesians during boreal summer

During boreal summer, large scale subsidence and a persistent northerly flow, known as the Etesians, characterize the tropospheric circulation over the eastern Mediterranean. The Etesians bring clear skies and alleviate the impact of heat waves over the region. The intraseasonal variability of the Etesians and subsidence over the eastern Mediterranean has been thought to be influenced by the South Asian monsoon and atmospheric processes over the North Atlantic. Here, we employ causal effect networks and causal maps, obtained by applying the Peter and Clark Momentary Conditional Independence (PCMCI) causal discovery algorithm, to identify causal precursors of Etesians. We find that both wave train activity over the North Atlantic/North American region and convective activity over South Asia associated with the Indian summer monsoon (ISM) are causally related to the Etesians at 3-day time scale. Thus, intraseasonal ISM variability affects the eastern Mediterranean circulation, though its influence is conveyed via a Middle East ridge. On longer weekly time scale, the mid-latitude influence weakens, while the influence of the tropical convective activity via the Middle East ridge remains stable. Moreover, the heat low over the Arabian Peninsula, a feature strongly responsible for the development of the Etesians, is caused by a stronger Middle East ridge and not by North Atlantic wave activity. Finally, we discuss potential implication for circulation changes in the eastern Mediterranean due to anthropogenic global warming.

Role of ocean-atmosphere interaction in intraseasonal variability of summer rainfall over the Indo–Northwest Pacific

Role of ocean-atmosphere interaction in intraseasonal variability of summer rainfall over the Indo–Northwest Pacific

A new insight into monsoon intraseasonal variability as revealed from distinct wind-precipitation regimes over the southwest coast of India

A new insight into monsoon intraseasonal variability as revealed from distinct wind-precipitation regimes over the southwest coast of India