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

Potential evapotranspiration (PET) is one of the most critical parameters in the research on agro-ecological systems. The computational methods for the estimation of PET vary in data demands from very simple (empirically based), requiring only information based on air temperatures, to complex ones (more physically based) that require data on radiation, relative humidity, wind speed, etc. The current research is focused on three study areas in Greece that face different climatic conditions due to their location. Twelve PET formulae were used, analyzed and inter-compared in terms of their sensitivity regarding their input coefficients for the Ardas River basin in north-eastern Greece, Sperchios River basin in Central Greece and Geropotamos River basin in South Greece. The aim was to compare all the methods and conclude to which empirical PET method(s) better represent the PET results in each area and thus should be adopted and used each time and which factors influence the results in each case. The results indicated that for the areas that face Mediterranean climatic conditions, the most appropriate method for the estimation of PET was the temperature-based, Hamon’s second version (PET_Ham2). Furthermore, the PET_Ham2 was able to estimate PET almost similarly to the average results of the 12 equations. For the Ardas River basin, the results indicated that both PET_Ham2 and PET_Ham1 can be used to estimate PET satisfactorily. Moreover, the temperature-based equations have proven to produce better results, followed by the radiation-based equations. Finally, PET_ASCE, which is the most commonly used PET equation, can also be applied occasionally in order to provide satisfactory results.     

Mechanisms of low frequency intraseasonal oscillations of the Indian summer monsoon

Summary This work deals with idealized modelling experiments designed to understand the dynamical evolution of low frequency intraseasonal monsoonal oscillations that result from interactions between the large scale monsoon Reverse Hadley Cell (RHC) and moist convective processes. The monsoon differential heating, which primarily determines the low-level convergence of the large-scale monsoon flow, is found to play a decisive role in affecting the northward progression of the monsoonal modes. A strong north-south differential heating leads to a robust generation and steady maintenance of northward propagating monsoonal oscillations. A weaker land-ocean thermal contrast leads to feeble low frequency monsoonal modes that have relatively longer periods in the 30–50 day band. This increase in the period of the monsoonal oscillations due to weak north-south thermal contrast is in good agreement with the observational findings of Yasunari (1980) and Kasture and Keshavamurty (1987). It is speculated that such an increase in the oscillatory period may be an outcome from an elongation in the meridional scale of the transient Hadley type cells which act as resonating cavities for the monsoonal modes.A Mobile Wave CISK (MWC) form of interaction between the large scale monsoon and the transient circulations associated with the Madden Julian Oscillation (MJO) is projected as a viable physical mechanism for the northward movement of low frequency modes. It is demonstrated that the effective low level convergence, following such an interaction, tends to shift northward relative to the site of interaction. This enables the heating perturbations to be displaced northward which in turn causes the secondary circulations and wind perturbations to follow. The essential criterion for the occurrence of a prolonged northward propagation of the low frequency modes is that the heating perturbations should phase lead the wind perturbations at all times.An examination of the - interactions on the 30–50 day time scale reveals that the conversion from the transient divergent motions to rotational motions is quite intense (feeble) in the strong (weak) monsoon differential heating experiments. Because of the closer proximity to the monsoon heat source and also due to the latitudinal variation of earth's rotational effects, the - interactions tend to be more pronounced to the north of 15°N while they are less robust in the near equatorial latitudes.The regularity of the monsoonal modes is found to depend on the strength of the monsoon differential heating and also on the periodic behaviour of the equatorial intraseasonal oscillations. The monsoonal modes are quite steady and exhibit extreme regularity in the presence of a weak north-south differential heating provided the equatorial forcing due to the MJO varies in a periodic manner. This result supports the findings of Mehta and Krishnamurti (1988) who found greater regularity of the 30–50 day modes during bad monsoon years.The low frequency monsoonal modes are found to be quite sensitive to the moisture availability factor (m) and the vertical profile of heating used in the MWC parameterization. A small increase in the value of (m) is found to significantly intensify the amplitude of the monsoonal oscillations while there is no considerable shift in the spectral frequency within the 30–50 day band as such. The 30–50 day motions show significant enhancement, with a relatively sharp spectral peak around 45 days, when the vertical profile of MWC heating has a maximum in the lower troposphere. However an upward displacement of the heating maximum tends to weaken the low frequency oscillations.With 19 Figures     

AGCM simulations of intraseasonal variability associated with the Asian summer monsoon

The intraseasonal variability associated with the Asian summer monsoon as simulated by a number of atmospheric general circulation models (AGCMs) are analyzed and assessed against observations. The model data comes from the Monsoon GCM Intercomparison project initiated by the CLIVAR/Asian–Australian Monsoon Panel. Ten GCM groups, i.e., the Center for Ocean–Land–Atmosphere Studies (COLA), Institute of Numerical Mathematics (DNM), Goddard Space Flight Center (GSFC), Geophysical Fluid Dynamics Laboratory (GFDL), Institute of Atmospheric Physics (IAP), Indian Institute of Tropical Meteorology (IITM), Meteorological Research Institute (MRI), National Center for Atmospheric Research (NCAR), Seoul National University (SNU), and the State University of New York (SUNY), participated in the intraseasonal component of the project. Each performed a set of 10 ensemble simulations for 1 September 1996–31 August 1998 using the same observed weekly SST values but with different initial conditions. The focus is on the spatial and seasonal variations associated with intraseasonal variability (ISV) of rainfall, the structure of each model's principal mode of spatial-temporal variation of rainfall [i.e. their depiction of the Intraseasonal Oscillation (ISO)], the teleconnection patterns associated with each model's ISO, and the implications of the models' ISV on seasonal monsoon predictability. The results show that several of the models exhibit ISV levels at or above that found in observations with spatial patterns of ISV that resemble the observed pattern. This includes a number of rather detailed features, including the relative distribution of variability between ocean and land regions. In terms of the area-averaged variance, it is found that the fidelity of a model to represent NH summer versus winter ISV appears to be strongly linked. In addition, most models' ISO patterns do exhibit some form of northeastward propagation. However, the model ISO patterns are typically less coherent, lack sufficient eastward propagation, and have smaller zonal and meridional spatial scales than the observed patterns, and are often limited to one side or the other of the maritime continent. The most pervasive and problematic feature of the models' depiction of ISV and/or their ISO patterns is the overall lack of variability in the equatorial Indian Ocean. In some cases, this characteristic appears to result from some models forming double convergence zones about the equator rather than one region of strong convergence on the equator. This shortcoming results in a poor representation of the local rainfall pattern and also significantly influences the models' representations of the global-scale teleconnection patterns associated with the ISO. Finally, analysis of the model ensemble shows a positive relationship between the strength of a model's ISV of rainfall and its intra-ensemble variability of seasonal monsoon rainfall. The implications of this latter relation are discussed in the context of seasonal monsoon predictability.     

Evaluation of the ENSEMBLES multi-model seasonal forecasts of Indian summer monsoon variability

The performance of the new multi-model seasonal prediction system developed in the frame work of the ENSEMBLES EU project for the seasonal forecasts of India summer monsoon variability is compared with the results from the previous EU project, DEMETER. We have considered the results of six participating ocean-atmosphere coupled models with 9 ensemble members each for the common period of 1960–2005 with May initial conditions. The ENSEMBLES multi-model ensemble (MME) results show systematic biases in the representation of mean monsoon seasonal rainfall over the Indian region, which are similar to that of DEMETER. The ENSEMBLES coupled models are characterized by an excessive oceanic forcing on the atmosphere over the equatorial Indian Ocean. The skill of the seasonal forecasts of Indian summer monsoon rainfall by the ENSEMBLES MME has however improved significantly compared to the DEMETER MME. Its performance in the drought years like 1972, 1974, 1982 and the excess year of 1961 was in particular better than the DEMETER MME. The ENSEMBLES MME could not capture the recent weakening of the ENSO-Indian monsoon relationship resulting in a decrease in the prediction skill compared to the “perfect model” skill during the recent years. The ENSEMBLES MME however correctly captures the north Atlantic-Indian monsoon teleconnections, which are independent of ENSO.     

Boreal summer intraseasonal oscillations and seasonal Indian monsoon prediction in DEMETER coupled models

Even though multi-model prediction systems may have better skill in predicting the interannual variability (IAV) of Indian summer monsoon (ISM), the overall performance of the system is limited by the skill of individual models (single model ensembles). The DEMETER project aimed at seasonal-to-interannual prediction is not an exception to this case. The reasons for the poor skill of the DEMETER individual models in predicting the IAV of monsoon is examined in the context of the influence of external and internal components and the interaction between intraseasonal variability (ISV) and IAV. Recently it has been shown that the ISV influences the IAV through very long breaks (VLBs; breaks with duration of more than 10 days) by generating droughts. Further, all VLBs are associated with an eastward propagating Madden–Julian Oscillation (MJO) in the equatorial region, facilitated by air–sea interaction on intraseasonal timescales. This VLB-drought–MJO relationship is analyzed here in detail in the DEMETER models. Analyses indicate that the VLB-drought relationship is poorly captured by almost all the models. VLBs in observations are generated through air–sea interaction on intraseasonal time scale and the models’ inability to simulate VLB-drought relationship is shown to be linked to the models’ inability to represent the air–sea interaction on intraseasonal time scale. Identification of this particular deficiency of the models provides a direction for improvement of the model for monsoon prediction.     

Indian summer monsoon rainfall variability in global coupled ocean-atmospheric models

Performance of seven fully coupled models in simulating Indian summer monsoon climatology as well as the inter-annual variability was assessed using multi member 1 month lead hindcasts made by several European climate groups as part of the program called Development of a European multi-model ensemble system for seasonal-to-inter-annual prediction (DEMETER). Dependency of the model simulated Indian summer monsoon rainfall and global sea surface temperatures on model formulation and initial conditions have been studied in detail using the nine ensemble member simulations of the seven different coupled ocean–atmosphere models participated in the DEMETER program. It was found that the skills of the monsoon predictions in these hindcasts are generally positive though they are very modest. Model simulations of India summer monsoon rainfall for the earlier period (1959–1979) are closer to the ‘perfect model’ (attainable) score but, large differences are observed between ‘actual’ skill and ‘perfect model’ skill in the recent period (1980–2001). Spread among the ensemble members are found to be large in simulations of India summer monsoon rainfall (ISMR) and Indian ocean dipole mode (IODM), indicating strong dependency of model simulated Indian summer monsoon on initial conditions. Multi-model ensemble performs better than the individual models in simulating ENSO indices, but does not perform better than the individual models in simulating ISMR and IODM. Decreased skill of multi-model ensemble over the region indicates amplification of errors due to existence of similar errors in the individual models. It appears that large biases in predicted SSTs over Indian Ocean region and the not so perfect ENSO-monsoon (IODM-monsoon) tele-connections are some of the possible reasons for such lower than expected skills in the recent period. The low skill of multi-model ensemble, large spread among the ensemble members of individual models and the not so perfect monsoon tele-connection with global SSTs points towards the importance of improving individual models for better simulation of the Indian monsoon.     

Simulation of monsoon intraseasonal variability in NCEP CFSv2 and its role on systematic bias

We have evaluated the simulation of Indian summer monsoon and its intraseasonal oscillations in the National Centers for Environmental Prediction climate forecast system model version 2 (CFSv2). The dry bias over the Indian landmass in the mean monsoon rainfall is one of the major concerns. In spite of this dry bias, CFSv2 shows a reasonable northward propagation of convection at intraseasonal (30–60 day) time scale. In order to document and understand this dry bias over the Indian landmass in CFSv2 simulations, a two pronged investigation is carried out on the two major facets of Indian summer monsoon: one, the air–sea interactions and two, the large scale vertical heating structure in the model. Our analysis shows a possible bias in the co-evolution of convection and sea surface temperature in CFSv2 over the equatorial Indian Ocean. It is also found that the simulated large scale vertical heat source (Q1) and moisture sink (Q2) over the Indian region are biased relative to observational estimates. Finally, this study provides a possible explanation for the dry precipitation bias over the Indian landmass in the simulated mean monsoon on the basis of the biases associated with the simulated ocean–atmospheric processes and the vertical heating structure. This study also throws some light on the puzzle of CFSv2 exhibiting a reasonable northward propagation at the intraseasonal time scale (30–60 day) despite a drier monsoon over the Indian land mass.     

Pacific decadal oscillation and variability of the Indian summer monsoon rainfall

Recent studies have furnished evidence for interdecadal variability in the tropical Pacific Ocean. The importance of this phenomenon in causing persistent anomalies over different regions of the globe has drawn considerable attention in view of its relevance in climate assessment. Here, we examine multi-source climate records in order to identify possible signatures of this longer time scale variability on the Indian summer monsoon. The findings indicate a coherent inverse relationship between the inter-decadal fluctuations of Pacific Ocean sea surface temperature (SST) and the Indian monsoon rainfall during the last century. A warm (cold) phase of the Pacific interdecadal variability is characterized by a decrease (increase) in the monsoon rainfall and a corresponding increase (decrease) in the surface air temperature over the Indian subcontinent. This interdecadal relationship can also be confirmed from the teleconnection patterns evident from long-period sea level pressure (SLP) dataset. The SLP anomalies over South and Southeast Asia and the equatorial west Pacific are dynamically consistent in showing an out-of-phase pattern with the SLP anomalies over the tropical central-eastern Pacific. The remote influence of the Pacific interdecadal variability on the monsoon is shown to be associated with prominent signals in the tropical and southern Indian Ocean indicative of coherent inter-basin variability on decadal time scales. If indeed, the atmosphere–ocean coupling associated with the Pacific interdecadal variability is independent from that of the interannual El Niño-Southern Oscillation (ENSO), then the climate response should depend on the evolutionary characteristics of both the time scales. It is seen from our analysis that the Indian monsoon is more vulnerable to drought situations, when El Niño events occur during warm phases of the Pacific interdecadal variability. Conversely, wet monsoons are more likely to prevail, when La Niña events coincide during cold phases of the Pacific interdecadal variability.     

Role of the cloud adjustment time scale in simulation of the interannual variability of Indian summer monsoon

The simulation of precipitation in a general circulation model relying on relaxed mass flux cumulus parameterization scheme is sensitive to cloud adjustment time scale (CATS). In this study, the frequency of the dominant intra-seasonal mode and interannual variability of Indian summer monsoon rainfall (ISMR) simulated by an atmospheric general circulation model is shown to be sensitive to the CATS. It has been shown that a longer CATS of about 5 h simulates the spatial distribution of the ISMR better. El Niño Southern Oscillation–ISMR relationship is also sensitive to CATS. The equatorial Indian Ocean rainfall and ISMR coupling is sensitive to CATS. Our study suggests that a careful choice of CATS is necessary for adequate simulation of spatial pattern as well as interannual variation of Indian summer monsoon precipitation.     

Temperature variability over the Indian Ocean and its relationship with Indian summer monsoon rainfall

Summary The present study examines the long term trend in sea surface temperatures (SSTs) of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean in the context of global warming for the period 1901–2002 and for a subset period 1971–2002. An attempt has also been made to identify the relationship between SST variations over three different ocean areas, and All-India and homogeneous region summer monsoon rainfall variability, including the role of El-Ni?o/Southern Oscillation (ENSO). Annual sea surface temperatures of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean show a significant warming trend of 0.7 °C, 0.6 °C and 0.5 °C per hundred years, respectively, and a relatively accelerated warming of 0.16 °C, 0.14 °C and 0.14 °C per decade during the 1971–2002 period. There is a positive and statistically significant relationship between SSTs over the Arabian Sea from the preceding November to the current February, and Indian monsoon rainfall during the period 1901–2002. The correlation coefficient increases from October and peaks in December, decreasing from February to September. This significant relationship is also found in the recent period 1971–2002, whereas, during 1901–70, the relationship is not significant. On the seasonal scale, Arabian Sea winter SSTs are positively and significantly correlated with Indian monsoon rainfall, while spring SSTs have no significant positive relationship. Nino3 spring SSTs have a negative significant relationship with Indian monsoon rainfall and it is postulated that there is a combined effect of Nino3 and Arabian Sea SSTs on Indian monsoon. If the Nino3 SST effect is removed, the spring SSTs over the Arabian Sea also have a significant relationship with monsoon rainfall. Similarly, the Bay of Bengal and Equatorial South Indian Ocean spring SSTs are significantly and positively correlated with Indian monsoon rainfall after removing the Nino3 effect, and correlation values are more pronounced than for the Arabian Sea. Authors’ address: Dr. D. R. Kothawale, A. A. Munot, H. P. Borgaonkar, Climatology and Hydrometeorology divisions, Indian Institute of Tropical Meteorology, Pune 411008, India.     

West Pacific subtropical high double ridges and intraseasonal variability of the South China Sea summer monsoon

In this study, the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data from 1979 to 2005 is used to investigate the possible impact of IntraSeasonal Oscillation (ISO) of the South China Sea (SCS) monsoon on the West Pacific Subtropical High (WPSH) double ridges. Three WPSH modes are defined as the 3/6 mode, the 1/2 mode and the dual mode, for the amplitude of the 30–60-day oscillation of SCS summer monsoon which is larger, smaller and comparable to the 10–20-day oscillation, respectively. The results show that there are no double ridges of WPSH present during 3/6 mode, but a weak process can be found during 1/2 mode. However, a powerful double-ridge structure of WPSH is present in several phases of the dual mode during both the 10–20-day oscillation and the 30–60-day oscillation. Then two typical WPSH double ridge cases in 1999, a special year of the dual mode, are chosen to further discuss this interesting phenomenon. Case 1 (24 July–27 July) is much weaker, and in this case the southern ridge disappears after several days, while during case 2 (3 August–11 August), the southern ridge finally replaces the northern ridge. The double ridges are much stronger compared to case 1. The ISO evolution feature is different between case 1 and case 2. The anomalous circulation of 10–20-day oscillation is anticyclonic over the southern ridge during both case 1 and case 2. However, the anomalous circulation of 30–60-day oscillation is cyclonic during case 1 and anticyclonic during case 2. It is this difference that leads to the double ridges being more powerful in case 2 than in case 1. This indicates that the 10–20-day oscillation of the SCS summer monsoon plays a key role in the WPSH double-ridge formation, while the 30–60-day oscillation provides a favorable background for it.     

Influence of sea surface temperature on the intraseasonal variability of the South China Sea summer monsoon

The objective of this study is to examine, based on recently available high resolution satellite and observational data, the evolution and role of sea surface temperature (SST) in influencing the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM). The study focuses on the 30–60?day timescale when the northward propagating anomalies are dominant over the SCS. Composite analysis of the SST maximum events during SCS SM shows that increased SST anomalies over the SCS are significantly influenced by the downward shortwave radiation flux anomalies, with the suppressed surface latent heat flux anomalies supplementing to it. A thermal damping of the positive SST anomalies induces positive upward heat fluxes, which then destabilize the lower atmosphere between 1,000 and 700?hPa. The positive SST anomalies lead the positive precipitation anomalies over the SCS by 10?days, with a significant correlation (r?=?0.44) between the SST-precipitation anomalies. The new findings here indicate an ocean-to-atmosphere effect over the SCS, where underlying SST anomalies tend to form a favorable condition for convective activity and sustain enhanced precipitation during the SCS SM. It is also argued, based on our observations, that the negative sea level pressure anomalies induced by the positive SST anomalies play a role in enhancing the northward propagation of the intraseasonal anomalies over the SCS.     

The fidelity of a regional coupled model in capturing the relationship between intraseasonal variability and the onset/demise of the Indian summer monsoon

Climate Dynamics - Onset and demise of the Indian summer monsoon (ISM) and intraseasonal variability (ISV) embedded within the ISM are dominant climatological phenomena observed over the Indian...     

Influence of ENSO and of the Indian Ocean Dipole on the Indian summer monsoon variability

Indian summer monsoon (ISM) variability is forced from external factors (like the El Niño Southern Oscillation, ENSO) but it contains also an internal component that tends to reduce its potential for predictability. Large-scale and local monsoon indices based on precipitation and atmospheric circulation parameters are used as a measure of ISM variability. In a 9-members ensemble of AMIP-type experiments (with same boundary SST forcing and different initial conditions) their potential predictability is comparable using both local and large-scale monsoon indices. In the sample analyzed, about half of more predictable monsoon years coincide with El Niño and/or positive Indian Ocean Dipole (IOD) events. Summer monsoon characteristics during ENSO and IOD years are analyzed through composites computed over a three years period (i.e. one year before and one year after the event peak) to investigate the mutual relationship between the events lagged in time. The connection between ISM and IOD is mostly confined in the summer and autumn, while that with ENSO is stronger and extends more in time. In the coupled model results the IOD influence on the monsoon is large, even because in the model IOD events are intense and easily reproduced due to a strong air-sea feedback in the eastern side of the basin. Monsoon seasons preceding or following an El Niño or a La Niña event are not exactly symmetric, even in terms of their biennial character. In most of the cases, both in reanalysis and model, El Niño and positive IOD events tend to co-occur with larger anomalies either in the Indo-Pacific ocean sector or over India, while La Niña and negative IOD do not. From the observed record, the ENSO-IOD correlation is positive strong and significant since mid-60s and it may correspond with either strong or weak ENSO-monsoon relationship and with strong or weak IOD-monsoon relationship. A main difference between those periods is the relationship between Indian monsoon rainfall and SST in other ocean basins rather than the Indo-Pacific sector alone.