Indian summer monsoon prediction and simulation in CFSv2 coupled model

Using carefully designed coupled model experiments, we have demonstrated that the prediction skill of the all India summer monsoon rainfall (AISMR) in Climate Forecast System version 2 (CFSv2) model basically comes from the El-Niňo Southern Oscillation-Monsoon teleconnection. On the other hand, contrary to observations, the Indian Ocean coupled dynamics do not have a crucial role in controlling the prediction skill of the AISMR in CFSv2. We show that the inadequate representation of the Indian Ocean coupled dynamics in CFSv2 is responsible for this dichotomy. Hence, the improvement of the Indian Ocean coupled dynamics is essential for further improvement of the AISMR prediction skill in CFSv2

The resolution sensitivity of the Asian summer monsoon and its inter-model comparison between MRI-AGCM and MetUM

In this study, we compare the resolution sensitivity of the Asian Summer Monsoon (ASM) in two Atmospheric General Circulation Models (AGCMs): the MRI-AGCM and the MetUM. We analyze the MetUM at three different resolutions, N96 (approximately 200-km mesh on the equator), N216 (90-km mesh) and N512 (40-km mesh), and the MRI-AGCM at TL95 (approximately 180-km mesh on the equator), TL319 (60-km mesh), and TL959 (20-km mesh). The MRI-AGCM and the MetUM both show decreasing precipitation over the western Pacific with increasing resolution, but their precipitation responses differ over the Indian Ocean. In MRI-AGCM, a large precipitation increase appears off the equator (5–20°N). In MetUM, this off-equatorial precipitation increase is less significant and precipitation decreases over the equator. Moisture budget analysis demonstrates that a changing in moisture flux convergence at higher resolution is related to the precipitation response. Orographic effects, intra-seasonal variability and the representation of the meridional thermal gradient are explored as possible causes of the resolution sensitivity. Both high-resolution AGCMs (TL959 and N512) can represent steep topography, which anchors the rainfall pattern over south Asia and the Maritime Continent. In MRI-AGCM, representation of low pressure systems in TL959 also contributes to the rainfall pattern. Furthermore, the seasonal evolution of the meridional thermal gradient appears to be more accurate at higher resolution, particularly in the MRI-AGCM. These findings emphasize that the impact of resolution is only robust across the two AGCMs for some features of the ASM, and highlights the importance of multi-model studies of GCM resolution sensitivity

The resolution sensitivity of the South Asian monsoon and Indo-Pacific in a global 0.35◦ AGCM

The South Asian monsoon is one of the most significant manifestations of the seasonal cycle. It directly impacts nearly one third of the world’s population and also has substantial global influence. Using 27-year integrations of a high-resolution atmospheric general circulation model (Met Office Unified Model), we study changes in South Asian monsoon precipitation and circulation when horizontal resolution is increased from approximately 200 to 40 km at the equator (N96 to N512, 1.9 to 0.35◦). The high resolution, integration length and ensemble size of the dataset make this the most extensive dataset used to evaluate the resolution sensitivity of the South Asian monsoon to date. We find a consistent pattern of JJAS precipitation and circulation changes as resolution increases, which include a slight increase in precipitation over peninsular India, changes in Indian and Indochinese orographic rain bands, increasing wind speeds in the Somali Jet, increasing precipitation over the Maritime Continent islands and decreasing precipitation over the northern Maritime Continent seas. To diagnose which resolution related processes cause these changes we compare them to published sensitivity experiments that change regional orography and coastlines. Our analysis indicates that improved resolution of the East African Highlands results in the improved representation of the Somali Jet and further suggests that improved resolution of orography over Indochina and the Maritime Continent results in more precipitation over the Maritime Continent islands at the expense of reduced precipitation further north. We also evaluate the resolution sensitivity of monsoon depressions and lows, which contribute more precipitation over northeast India at higher resolution. We conclude that while increasing resolution at these scales does not solve the many monsoon biases that exist in GCMs, it has a number of small, beneficial impacts

Projected changes of rainfall seasonality and dry spells in a high greenhouse gas emissions scenario

In this diagnostic study we analyze changes of rainfall seasonality and dry spells by the end of the twenty-first century under the most extreme IPCC5 emission scenario (RCP8.5) as projected by twenty-four coupled climate models contributing to Coupled Model Intercomparison Project 5 (CMIP5). We use estimates of the centroid of the monthly rainfall distribution as an index of the rainfall timing and a threshold-independent, information theory-based quantity such as relative entropy (RE) to quantify the concentration of annual rainfall and the number of dry months and to build a monsoon dimensionless seasonality index (DSI). The RE is projected to increase, with high inter-model agreement over Mediterranean-type regions---southern Europe, northern Africa and southern Australia---and areas of South and Central America, implying an increase in the number of dry days up to 1Â month by the end of the twenty-first century. Positive RE changes are also projected over the monsoon regions of southern Africa and North America, South America. These trends are consistent with a shortening of the wet season associated with a more prolonged pre-monsoonal dry period. The extent of the global monsoon region, characterized by large DSI, is projected to remain substantially unaltered. Centroid analysis shows that most of CMIP5 projections suggest that the monsoonal annual rainfall distribution is expected to change from early to late in the course of the hydrological year by the end of the twenty-first century and particularly after year 2050. This trend is particularly evident over northern Africa, southern Africa and western Mexico, where more than 90% of the models project a delay of the rainfall centroid from a few days up to 2Â weeks. Over the remaining monsoonal regions, there is little inter-model agreement in terms of centroid changes

On the relationship between Indian summer monsoon withdrawal and Indo-Pacific SST anomalies before and after 1976/1977 climate shift

A clear shift in the withdrawal dates of the Indian Summer Monsoon is observed in the long term time series of rainfall data. Prior (posterior) to the 1976/1977 climate shift most of the withdrawal dates are associated with a late (an early) withdrawal. As a result, the length of the rainy season (LRS) over the Indian land mass has also undergone similar changes (i. e., longer (shorter) LRS prior (posterior) to the climate shift). In this study, probable reasons for this significant shift in withdrawal dates and the LRS are investigated using reanalysis/observed datasets and also with the help of an atmospheric general circulation model. Reanalysis/observational datasets indicate that prior to the climate shift the sea surface temperature (SST) anomalies in the eastern equatorial Pacific Ocean and the Arabian Sea exerted a strong influence on both the withdrawal and the LRS. After the climate shift, the influence of the eastern equatorial Pacific Ocean SST has decreased and surprisingly, the influence of the Arabian Sea SST is almost non-existent. On the other hand, the influence of the southeastern equatorial Indian Ocean has increased significantly. It is observed that the upper tropospheric temperature gradient over the dominant monsoon region has decreased and the relative influence of the Indian Ocean SST variability on the withdrawal of the Indian Summer Monsoon has increased in the post climate shift period. Sensitivity experiments with the contrasting SST patterns on withdrawal dates and the LRS in the pre- and post- climate shift scenarios, confirm the observational evidences presented above

Why ensemble mean projection of south Asian monsoon rainfall by CMIP5 models is not reliable?

Future projections of the Indian summer monsoon rainfall (ISMR) and its large-scale thermodynamic driver are studied by using CMIP5 model outputs. While all models project an increasing precipitation in the future warming scenario, most of them project a weakening large-scale thermodynamic driver arising from a weakening of the upper tropospheric temperature (UTT) gradient over south Asian summer monsoon region. The weakening of the UTT gradient under global warming scenarios is related to the increase in sea surface temperature (SST) over the equatorial Indian Ocean (EIO) leading to a stronger increase of UTT over the EIO region relative to the northern Indian region, a hypothesis supported by a series of Atmospheric General Circulation Model (AGCM) experiments forced by projected SSTs. To diagnose the inconsistency between the model projections of precipitation and the large-scale thermodynamic driver, we have examined the rate of total precipitation explained by convective and stratiform precipitations in observations and in CMIP5 models. It is found that most models produce too much (little) convective (stratiform) precipitation compared to observations. In addition, we also find stronger precipitable water-precipitation relationship in most CMIP5 models as compared to observations. Hence, the atmospheric moisture content produced by the model immediately gets converted to precipitation even though the large-scale thermodynamics in models weaken. Therefore, under global warming scenarios, due to increased temperature and resultant increased atmospheric moisture supply, these models tend to produce unrealistic local convective precipitation often not in tune with other large-scale variables. Our results questions the reliability of the ISMR projections in CMIP5 models and highlight the need to improve the convective parameterization schemes in coupled models for the reliable projections of the ISMR. © 2014 Springer-Verlag Berlin Heidelberg

Role of Indian Ocean SST variability on the recent global warming hiatus

Previous studies have shown a slowdown in the warming rate of the annual mean global surface temperature in the recent decade and it is referred to as the hiatus in global warming. Some recent studies have suggested that the hiatus in global warming is possibly due to strong cooling in the tropical Pacific. This study investigates the possible role of the Indian Ocean warming on the tropical Pacific cooling. Despite the continued rise in sea surface temperature (SST) over the tropical Indian Ocean, SST over the tropical Pacific has shown a cooling trend in the recent decade (2002 − 2012). It is well known fact that the Indian Ocean and the Pacific Ocean are strongly coupled to each other and the Indian Ocean basin wide warming is triggered by El Niño on interannual time scale. However, in the recent decade, this relationship is weakening. The recent Indian Ocean warming is triggering a Matsuno-Gill type response in the atmosphere by generating anomalous cyclonic circulations on either side of equator over the tropical Indian Ocean and anomalous easterlies along the tropical Pacific Ocean. These anomalous easterlies result in Ekman divergence in the equatorial Pacific and produce upwelling Kelvin waves, cools the tropical Pacific and therefore indirectly contributes to the hiatus in global warming

Large-scale teleconnection patterns of Indian summer monsoon as revealed by CFSv2 retrospective seasonal forecast runs

The seasonal prediction skill [defined as the linear correlation (cc) between the observed and forecasted rainfall] of the Indian Summer Monsoon Rainfall (ISMR) is evaluated in the Climate Forecast System version 2 (CFSv2) model, at different lead times on the basis of its representation of large scale tropical teleconnection. Surprisingly, the model exhibits reasonable skill at a longer lead time (e.g. forecasts initialized with February initial conditions, Feb IC run, cc > 0.5) that is reasonably better when compared with that with forecast initialized at shorter lead time [April/May IC (Apr/May IC) runs, cc < 0.5]. Although the prediction skill of ISMR improves as lead time increases, the ENSO forecast skill improves as lead time decreases. Probable reasons for these counter-intuitive findings are investigated in this study

Modulation of monsoon intraseasonal oscillations in the recent warming period

The Indian Ocean sea surface temperature (SST) during the boreal summer has shown a significant warming of 0.3°C in the recent decade (2001-2010) compared to a former decade (1979-1988), and it is most pronounced in the central tropical Indian Ocean. By using reanalysis and satellite-derived data sets, we investigated how the monsoon intraseasonal oscillation (MISO) over the South Asian summer monsoon (ASM) region has been influenced by the recent warming in the Indian Ocean. It is found that the MISO variance has increased over the ASM region in the recent period compared with the earlier decade. It is also noted that the characteristic northward propagation of the MISO has slowed over 2001-2010, resembling more of a standing oscillation near the equator. Mechanisms implicated in the observed MISO changes are explored by conducting several model sensitivity experiments with an atmospheric general circulation model. The model experiments suggest that the mean SST increase over the Indian Ocean, and the associated changes in the air-sea interaction, the increased mean moisture convergence, and changes in the large-scale circulation are responsible for the changes in the characteristics of the MISO. The influence of the recent Indian Ocean warming on the MISO characteristics must be understood fully since they determine the seasonal amount of rainfall over the Indian subcontinent. An examination of future projections of the MISO using the MPI-ESM-LR model from the Coupled Model Intercomparison Project phase 5 archive also gives consistent result