Indian Summer Monsoon Intraseasonal Oscillations Research Articles

Intraseasonal Variability of Surface Circulation in the Indo‐Pacific Warm Pool Induced by Summer Monsoon Intraseasonal Oscillations

AbstractThe Indian summer monsoon intraseasonal oscillation (MISO) is the most influential mode of tropical atmospheric intraseasonal variability in boreal summer (May–October). This study investigates how the MISO drives the intraseasonal variability (ISV) of zonal currents (U) in the Indo‐Pacific warm pool (IPWP) region. An analysis of satellite observations confirms a close association between U ISV and the MISO, with MISO‐induced surface U anomalies reaching 16–19 cm s−1 near the equator and 4–5 cm s−1 in the Northern Hemisphere portion of the IPWP. These ISVs account for 15%–35% of the intensity changes in the major zonal currents. The spatial distribution of U ISV shows coherent enhancement and suppression of MISO convections with a lag of approximately 7 days as a delayed response to the tropical wind forcing. ISV anomalies show northward emanation following propagation of the MISO. Ocean model experiments confirm that large‐scale ISV in surface U is primarily caused by the wind forcing of the MISO. Owing to the convergence/divergence of MISO‐induced U ISV, significant warm water volume anomalies occur in the western‐to‐central equatorial Pacific.

Role of North Indian Ocean Air–Sea Interaction in Summer Monsoon Intraseasonal Oscillation

Air–sea coupling processes over the north Indian Ocean associated with the Indian summer monsoon intraseasonal oscillation (MISO) are investigated. Observations show that MISO convection anomalies affect underlying sea surface temperature (SST) through changes in surface shortwave radiation and surface latent heat flux. In turn, SST anomalies may also affect the MISO precipitation tendency ( dP/ dt). In particular, warm (cold) SST anomalies can contribute to increasing (decreasing) precipitation rate through enhanced (suppressed) surface convergence associated with boundary layer pressure gradients. These air–sea interaction processes are manifest in a quadrature relation between MISO precipitation and SST anomalies. A local air–sea coupling model (LACM) is formulated based on these observed physical processes. The period of the LACM is proportional to the square root of seasonal mixed layer depth H, assuming other physical parameters remain unchanged. Hence, LACM predicts a relatively short (long) MISO period over the north Indian Ocean during the May–June monsoon developing (July–August monsoon mature) phase when H is shallow (deep). This result is consistent with observed MISO characteristics. A 30-day-period oscillating external forcing is also added to the LACM, representing intraseasonal oscillations propagating from the equatorial Indian Ocean to the north Indian Ocean. It is found that resonance will occur when H is close to 25 m, which significantly enhances the MISO amplitude. This process may contribute to the higher MISO amplitude during the monsoon developing phase compared to the mature phase, which is associated with the seasonal cycle of H.

The Indian Summer Monsoon Intraseasonal Oscillations in CFSv2 Forecasts: Biases and Importance of Improving Air–Sea Interaction Processes

Northward-propagating Indian summer monsoon intraseasonal oscillations (MISOs) are a major origin of the active–break spells of the monsoon rainfall. Forecast results for 28 active and 27 break spells from the National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2 (CFSv2), during 1999–2010 are analyzed. CFSv2 forecasts are able to represent the propagation of MISOs from the equator to central India, showing improvements in many aspects compared to its previous version. Systematic biases for MISOs, however, still exist, exhibiting apparently weaker amplitude and slower northward propagation compared to observations. The eastern Arabian Sea (EAS)–western Bay of Bengal (WBB) region (EAS–WBB region; 12°–20°N, 65°–85°E) is found to be critical for the MISO prediction. In that region, the forecast and observed MISO trajectories begin to bifurcate from each other, and forecast errors grow rapidly. Further diagnosis reveals that local air–sea interaction in that region is severely underrepresented in CFSv2. Sea surface temperature (SST) response to surface heat flux forcing and convection response to SST forcing are both too weak, leading to the underestimated MISO amplitude. The relationship between precipitation and SST in CFSv2 is much more chaotic than in observation. The misrepresentation of air–sea coupling results in longer MISO periods in the EAS–WBB region, manifesting as slower propagation and delayed arrival of MISOs in central India. Refining the air–sea coupling processes is crucial for improving the CFSv2 forecast. This includes taking into account the ocean skin layer, better resolving the diurnal cycle, and improving the ocean model physics.

The Role of Oceanic Processes in the Initiation of Indian Summer Monsoon Intraseasonal Oscillations Over the Indian Ocean

AbstractObservational analyses and a hierarchy of ocean general circulation model (OGCM) experiments were performed to understand the influence of oceanic processes on the warm sea surface temperature anomalies (SSTAs) prior to the convection initiation of Indian summer monsoon intraseasonal oscillations (MISOs) in the equatorial Indian Ocean. Satellite observations revealed 41 strong MISO events that initiated over the central equatorial Indian Ocean and propagated northward to the Indian subcontinent and/or eastward to the Maritime Continent during the May–October season of the 2001–2012 period. Eight of those events were preceded by SSTAs that were strongly influenced by wind stress‐driven oceanic processes. Composite analyses for the eight events showed that more than one‐third of the preconvection warming in the initiation region resulted from oceanic processes, including advection and entrainment. Case studies of two strong MISO events associated with ocean‐dynamically induced SSTAs showed unmistakable evidence for the preconvection SSTAs and sea surface height anomalies (SSHAs) associated with eastern boundary‐reflected (in June/July 2004) and directly forced (in June 2006) intraseasonal oceanic Rossby waves. In those two case studies, a mixed‐layer heat budget analysis of the OGCM results showed that advection played a much larger role than entrainment in elevating SSTAs. Further analysis showed that intraseasonal currents associated with the Rossby waves advected high seasonal mean SSTs into the initiation region. As current forecasting skills for the onset of the MISO are low, these results underscore the importance of thoroughly understanding the ocean's role in affecting SSTAs, which contribute to MISO initiation.

Regional simulation of Indian summer monsoon intraseasonal oscillations at gray-zone resolution

Abstract. Simulations of the Indian summer monsoon by the cloud-permitting Weather Research and Forecasting (WRF) model at gray-zone resolution are described in this study, with a particular emphasis on the model ability to capture the monsoon intraseasonal oscillations (MISOs). Five boreal summers are simulated from 2007 to 2011 using the ERA-Interim reanalysis as the lateral boundary forcing data. Our experimental setup relies on a horizontal grid spacing of 9 km to explicitly simulate deep convection without the use of cumulus parameterizations. When compared to simulations with coarser grid spacing (27 km) and using a cumulus scheme, the 9 km simulations reduce the biases in mean precipitation and produce more realistic low-frequency variability associated with MISOs. Results show that the model at the 9 km gray-zone resolution captures the salient features of the summer monsoon. The spatial distributions and temporal evolutions of monsoon rainfall in the WRF simulations verify qualitatively well against observations from the Tropical Rainfall Measurement Mission (TRMM), with regional maxima located over Western Ghats, central India, Himalaya foothills, and the west coast of Myanmar. The onset, breaks, and withdrawal of the summer monsoon in each year are also realistically captured by the model. The MISO-phase composites of monsoon rainfall, low-level wind, and precipitable water anomalies in the simulations also agree qualitatively with the observations. Both the simulations and observations show a northeastward propagation of the MISOs, with the intensification and weakening of the Somali Jet over the Arabian Sea during the active and break phases of the Indian summer monsoon.

Bay of Bengal salinity stratification and Indian summer monsoon intraseasonal oscillation: 2. Impact on SST and convection

AbstractThe Indian summer monsoon intraseasonal oscillations (MISOs) induce pronounced intraseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), which has important feedbacks to atmospheric convection. An ocean general circulation model (OGCM) is employed to investigate the upper‐ocean processes affecting intraseasonal SST variability and its feedback to the MISO convection. In the BoB, the MISO induces intraseasonal SST variability predominantly through surface heat flux forcing with comparable contributions from shortwave radiation and turbulent heat flux, and to a much smaller extent through wind‐driven ocean mixed layer entrainment. The ocean salinity stratification, represented by mixed layer depth (MLD) and barrier layer thickness (BLT), has a strong control on SST but weak impact on convection of the MISO. The MLD is critical for the amplitude of SST response to various forcing processes, while the BLT mainly affects entrainment by determining the temperature difference between the mixed layer and the water below. From May to mid‐June, the shallow MLD and thin barrier layer greatly enhance intraseasonal SST anomalies, which can amplify convection fluctuations of the MISO through air‐sea interaction and leads to intense but short‐duration postconvection break spells. When either the MLD or the BLT is large, intraseasonal SSTs tend to be weak. Further investigation reveals that freshwater flux of the monsoon gives rise to the shallow MLD and thick barrier layer, and its overall effect on intraseasonal SSTs is a 20% enhancement. These results provide implications for improving the simulation and forecast of the MISO in climate models.

Evaluation of NCEP TIGGE short-range forecast for Indian summer monsoon intraseasonal oscillation

This study focuses on the short-range prediction of Monsoon Intraseasonal Oscillations (MISOs) using the National Centers for Environmental Prediction(NCEP) Ensemble Prediction System (EPS) data from The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) archive. The Indian Summer Monsoon Rainfall (ISMR), which plays an important role in the socio-economic growth of the country, is highly variable and is mostly governed by the MISOs. In addition to this, deterministic forecasts of ISMR are not very reliable. Hence, a probabilistic approach at daily scale is required. Keeping this in mind, the present analysis is done by using daily forecast data for up to 7-day lead time and compared with observations. The analysis shows that the ensemble forecast well captures the variability as compared to observations even up to 7 days. The spatial characteristics and the northward propagation of MISO are observed thoroughly in the EPS. The evolution of dynamical and thermodynamical parameters such as specific humidity, moist static energy, moisture divergence, and vorticity is also captured well but show deviation from the observation from 96 h lead time onwards. The tropospheric temperature forecast captures the observed gradient but with certain bias in magnitude whereas the wind shear is simulated quite well both in pattern and magnitude. These analyses bring out the biases in TIGGE EPS forecast and also point out the possible moist processes which needs to be improved.

Indian summer monsoon intra-seasonal oscillation associated with the developing and decaying phase of El Niño

The present study investigates the characteristics of active and break cycles associated with the Indian summer monsoon (ISM) during developing and decaying phase of El Nino and in the years in which Indian Ocean Dipole (IOD) co-occurred with El Nino (co-occurred years). Observations show that break days are more in number (∼2–3 times) and long lasting (∼15–20 days) than active days in El Nino developing summers and vice versa for decay years. During El Nino developing years, northward propagation is well organized with significant anomalies in both active and break phases. The increased convection associated with active phase persists longer over Indian Ocean, than over the monsoon region, while the reduced convection in break phase propagates faster from ocean to land and persists there for a longer time. Compared to break events, active events have slower (faster) propagation over the monsoon (oceanic) region during El Nino decay years. The present study put forward the argument that the contrasting persistent circulations over Indo-western Pacific regions favour particular phase of intra-seasonal oscillation (ISO) in developing and decay phase of El Nino. These long-lasting circulations advect anomalous dry (moist) air to ISM region for longer period, resulting in long-lasting break (active) events in El Nino developing (decay) years. During co-occurrence years, the number of break (active) days is reduced by two to three times compared to the developing (decaying) phase of El Nino. It is found that 30- to 60-day scale ISO is strongly modulated, than 10 - to 20-day scale, by the changes in seasonal mean state associated with El Nino. Thus, this study demonstrates that the ISO characteristics such as its variance, northward propagation, spatial distribution and duration of active and break days are strongly modulated by seasonal background anomalies over the Indo-Pacific region.

Asymmetry in space–time characteristics of Indian summer monsoon intraseasonal oscillations during extreme years: Role of seasonal mean state

ABSTRACTA diagnostic study of the space–time characteristics of the summer monsoon intraseasonal oscillations (MISO) during strong monsoon (SM) and weak monsoon (WM) years is carried out to gain insight on the role of the seasonal mean states on the MISOs using long observational and reanalysis datasets. Prominent asymmetry is noted in the duration and magnitude of active‐break spells associated with MISO during SM and WM years. Such difference is also reflected in the northward propagation of MISOs over the Indian subcontinent that show fast (slow) propagation during active phase of WM (SM), while the situation is just opposite during breaks. We hypothesize that considerable differences in the seasonal mean states during these extreme monsoon years may be responsible for these asymmetries. In support of this hypothesis, it is found that during WM (SM) years, the seasonal mean zonal winds associated with the convective region have anomalous barotropic (baroclinic) vertical structure, while a baroclinic (barotropic) structure dominates over region of suppressed convection. Evolution of meridional–vertical structure of the MISO anomalies during SM and WM also highlights the role of barotropic zonal wind anomalies in the northward propagation. From the barotropic zonal momentum budget analysis, it is demonstrated that meridional and vertical advection of the seasonal mean zonal wind anomaly may be largely responsible for such asymmetry in intensity, life span and propagation characteristics of the MISOs during SM and WM years.

Role of ocean–atmosphere interaction on northward propagation of Indian summer monsoon intra-seasonal oscillations (MISO)

Atmospheric dynamical mechanisms have been prevalently used to explain the characteristics of the sum- mer monsoon intraseasonal oscillation (MISO), which dictates the wet and dry spells of the monsoon rainfall. Recent studies show that ocean-atmosphere coupling has a vital role in simulating the observed amplitude and rela- tionship between precipitation and sea surface temperature (SST) at the intraseasonal scale. However it is not clear whether this role is simply 'passive' response to the atmospheric forcing alone, or 'active' in modulating the northward propagation of MISO, and also whether the extent to which it modulates is considerably noteworthy. Using coupled NCEP-Climate Forecast System (CFSv2) model and its atmospheric component the Global Forecast System (GFS), we investigate the relative role of the atmospheric dynamics and the ocean-atmosphere coupling in the initiation, maintenance, and northward propagation of MISO. Three numerical simulations are performed including (1) CFSv2 coupled with high frequency inter- active SST, the GFS forced with both (2) observed monthly SST (interpolated to daily) and (3) daily SST obtained from the CFSv2 simulations. Both CFSv2 and GFS simulate MISO of slightly higher period (*60 days) than observations (*45 days) and have reasonable seasonal rainfall over India. While MISO simulated by CFSv2 has realistic northward propagation, both the GFS model experiments show standing mode of MISO over India with no northward propagation of convection from the equator. The improvement in northward propagation in CFSv2, therefore, may not be due to improvement of the model physics in the atmospheric component alone. Our analysis indicates that even with the presence of conducive vertical wind shear, the absence of meridional humidity gradient and moistening of the atmosphere column north of con- vection hinders the northward movement of convection in GFS. This moistening mechanism works only in the pres- ence of an 'active' ocean. In CFSv2, the lead-lag rela- tionship between the atmospheric fluxes, SST and convection are maintained, while such lead-lag is unreal- istic in the uncoupled simulations. This leads to the con- clusion that high frequent and interactive ocean- atmosphere coupling is a necessary and crucial condition for reproducing the realistic northward propagation of MISO in this particular model.

Simulation of Indian summer monsoon intraseasonal oscillations in a superparameterized coupled climate model: need to improve the embedded cloud resolving model

The characteristic features of Indian summer monsoon (ISM) and monsoon intraseasonal oscillations (MISO) are analyzed in the 25 year simulation by the superparameterized Community Climate System Model (SP-CCSM). The observations indicate the low frequency oscillation with a period of 30–60 day to have the highest power with a dominant northward propagation, while the faster mode of MISO with a period of 10–20 day shows a stationary pattern with no northward propagation. SP-CCSM simulates two dominant quasi-periodic oscillations with periods 15–30 day and 40–70 day indicating a systematic low frequency bias in simulating the observed modes. Further, contrary to the observation, the SP-CCSM 15–30 day mode has a significant northward propagation; while the 40–70 day mode does not show prominent northward propagation. The inability of the SP-CCSM to reproduce the observed modes correctly is shown to be linked with inability of the cloud resolving model (CRM) to reproduce the characteristic heating associated with the barotropic and baroclinic vertical structures of the high-frequency and the low-frequency modes. It appears that the superparameterization in the General Circulation Model (GCM) certainly improves seasonal mean model bias significantly. There is a need to improve the CRM through which the barotropic and baroclinic modes are simulated with proper space and time distribution.

Quantifying predictability of Indian summer monsoon intraseasonal oscillations using nonlinear time series analysis

Quantifying predictability of Indian summer monsoon intraseasonal oscillations using nonlinear time series analysis

Forecasting the Indian summer monsoon intraseasonal oscillations using genetic algorithm and neural network

The correct and timely forecast of the Indian summer monsoon Intraseasonal Oscillations (ISOs) is very important. It has great impact on the agriculture and economy of the Indian subcontinent region. The applicability of Genetic Algorithm (GA) is demonstrated for nonlinear curve fitting of the inherently chaotic and noisy Lorenz time series and the ISO data. A robust method is developed for the very long‐range prediction of the ISO using a feed‐forward time delay backpropagation Artificial Neural Network (ANN). Using an iterative one‐step‐ahead prediction strategy, five years (120 pentads) of advanced prediction is made for the ISO data with good forecast skill. It is shown that a hybrid GA‐ANN model may be used as an early forecast model followed by ANN only model as a more reliable model.

Extension of potential predictability of Indian summer monsoon dry and wet spells in recent decades

AbstractAn understanding of the limit on potential predictability is crucial for developing appropriate tools for extended‐range prediction of active/break spells of the Indian summer monsoon (ISM). The global low‐frequency changes in climate modulate the annual cycle of the ISM and can influence the intrinsic predictability limit of the ISM intraseasonal oscillations (ISOs). Using 104‐year (1901–2004) long daily rainfall data, the change in potential predictability of active and break spells are estimated by an empirical method. It is found that the potential predictability of both active and break spells have undergone a rapid increase during the recent three decades. The potential predictability of active spells has shown an increase from one week to two weeks while that for break spells increased from two weeks to three weeks. This result is interesting and intriguing in the backdrop of recent finding that the potential predictability of monsoon weather has decreased substantially over the same period compared to earlier decades due to increased potential instability of the atmosphere. The possible role of internal dynamics and external forcing in producing this change has been explored. The changes in energy exchange between the synoptic and ISO scale and the different ISO modes as evidenced by energetics computations in frequency domain also support the increased potential predictability of ISO. Our finding provides optimism for improved and useful extended‐range prediction of monsoon active and break spells. Copyright © 2010 Royal Meteorological Society

Forecasting the duration of active and break spells in intrinsic mode functions of Indian monsoon intraseasonal oscillations

Several indices of the observed Indian summer monsoon intraseasonal oscillations (ISO) are decomposed into their respective intrinsic mode functions (IMFs). For each ISO index, it is shown that for most of the dominant IMFs, the peak in the active (break) regime may be used as a predictor for the duration of the subsequent break (active) regime. Similar results are also obtained for a Stochastic Forced Lorenz model proposed earlier as a paradigm model for ISO.

Clustering of synoptic activity by Indian summer monsoon intraseasonal oscillations

Active and break phases of the Indian summer monsoon are characterized by enhancement and decrease of precipitation over the monsoon trough region. Using genesis data of monsoon low pressure systems (LPS) and circulation data for the period 1954 to 1993, it is shown that the frequency of occurrence of LPS is nearly 3.5 times higher in the active phase of monsoon as compared to the break phase. In addition, the tracks of these synoptic systems are also strongly spatially clustered along the monsoon trough during the active phase of the monsoon. The enhanced (decreased) frequency of occurrence of LPS during active (break) phases is due to modulation of meridional shear of zonal winds and cyclonic vorticity along the monsoon trough by the intraseasonal oscillations (ISO).