scholarly journals Simulation of Indian summer monsoon rainfall, interannual variability and teleconnections: Evaluation of CMIP6 models

2021 ◽  
Author(s):  
Kavirajan Rajendran ◽  
Sajani Surendran ◽  
Stella Jes Varghese ◽  
Anjali Sathyanath
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Summer Monsoon ◽  
Inverse Relationship ◽  
Monsoon Rainfall ◽  
Coupled Model ◽  
Equatorial Indian Ocean ◽  
Summer Monsoon Rainfall ◽  
Boreal Summer ◽  
Model Intercomparison

Abstract We analyse the performance of global climate models of 6th generation of Coupled Model Intercomparison Project (CMIP6) in simulating climatological summer monsoon rainfall over India, interannual variability (IAV) of all-India summer monsoon rainfall (ISMR) and its teleconnections with rainfall variability over equatorial Pacific and Indian Oceans. The multimodel ensemble mean (MME) of 61 CMIP6 models shows the best skill in simulating mean monsoon rainfall over India compared to the MMEs of 6th generation atmosphere-only models (AMIP6) and the previous generations of Atmospheric and Coupled Model Intercomparison Projects (AMIPs and CMIPs). Systematic improvement and reduction in bias are evident from lower to higher AMIPs/CMIPs. Still, there exists dry bias over a narrow region of the monsoon zone of central India besides wet and cold bias over the surrounding oceans. The persistence of errors in atmosphere-only models hints that the source of errors could be with atmosphere models. Fifteen CMIP6 models selected through objective criteria, perform the best in simulating mean monsoon, IAV of ISMR, the strong inverse relationship between ISMR and Boreal summer El Nino-Southern Oscillation (ENSO), and the inverse relationship between all-India rainfall and north-west tropical Pacific rainfall in June. Several models reproduce the dipole structure of Equatorial Indian Ocean Oscillation (EQUINOO) with the centres over western and eastern equatorial Indian Ocean. But, ISMR-EQUINOO relationship in many of them is opposite to the observed. Our analysis implies the need for capturing ISMR-EQUINOO link to improve the simulation of IAV of ISMR which is crucial for reliable monsoon prediction and projection.

scholarly journals On the Weakening of Northward Propagation of Intraseasonal Oscillations During Positive Indian Ocean Dipole Events.

2021 ◽  
Author(s):  
Aditya Kottapalli ◽  
Vinayachandran P N
Keyword(s):  
Indian Ocean ◽  
Zonal Wind ◽  
Indian Ocean Dipole ◽  
Monsoon Rainfall ◽  
Equatorial Indian Ocean ◽  
Summer Monsoon Rainfall ◽  
Boreal Summer ◽  
Positive Indian Ocean Dipole ◽  
Northward Propagation ◽  
Intraseasonal Oscillations

Abstract The northward propagation of intraseasonal oscillations (ISO) is one of the major modes of variability in the tropics during boreal summer, associated with active and break spells of monsoon rainfall over the Indian region, and modulate the Indian summer monsoon rainfall (ISMR). The northward march starts close to the equator over warm waters of the Indian Ocean and continues till the foothills of the Himalayas. The northward propagations tend to be weaker during positive Indian Ocean Dipole (pIOD) years. We have used the "moisture mode" framework to understand the processes responsible for the weakening of northward propagations during IOD years. Our analyses show that moistening caused by the horizontal advection was the major contributor for the northward propagations during negative IOD (nIOD) years, and its amplitude is much smaller during pIOD years. The reduction in the zonal advection during pIOD is responsible for the weakening of northward propagations. Also, the mean structure of entropy between 925hpa – 500hpa levels remained similar over most of the monsoon region across the contrasting IOD years. The reason for weaker northward propagations can be attributed to the weaker zonal wind perturbations at intraseasonal timescales. The weaker zonal wind perturbations during ISO events in pIOD years owing to cooler sea surface temperatures (SST) in the South-East Equatorial Indian Ocean (SEIO) and warmer West Equatorial Indian Ocean (WEIO) and South-East Arabian Sea (SEAS) is proposed to be the possible reason for the weakening of northward propagations during pIOD years.

Dependence of Indian summer monsoon rainfall forecast skill of CFSv2 on initial conditions and the role of bias in SST boundary forcing

2021 ◽  
Author(s):  
Stella Jes Varghese ◽  
Kavirajan Rajendran ◽  
Sajani Surendran ◽  
Arindam Chakraborty
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Lead Time ◽  
Monsoon Rainfall ◽  
Indian Summer Monsoon Rainfall ◽  
Initial Conditions ◽  
Summer Monsoon Rainfall ◽  
Forecast Skill ◽  
Boreal Summer ◽  
Central Pacific

<p>Indian summer monsoon seasonal reforecasts by CFSv2, initiated from January (4-month lead time, L4) through May (0-month lead time, L0) initial conditions (ICs), are analysed to investigate causes for the highest Indian summer monsoon rainfall (ISMR) forecast skill of CFSv2 with February (3-month lead time, L3) ICs. Although theory suggests forecast skill should degrade with increase in lead-time, CFSv2 shows highest skill with L3, due to its forecasting of ISMR excess of 1983 which other ICs failed to forecast. In contrast to observation, in CFSv2, ISMR extremes are largely decided by sea surface temperature (SST) variation over central Pacific (NINO3.4) associated with El Niño-Southern Oscillation (ENSO), where ISMR excess (deficit) is associated with La Niña (El Niño) or cooling (warming) over NINO3.4. In 1983, CFSv2 with L3 ICs forecasted strong La Niña during summer, which resulted in 1983 ISMR excess. In contrast, in observation, near normal SSTs prevailed over NINO3.4 and ISMR excess was due to variation of convection over equatorial Indian Ocean, which CFSv2 fails to capture with all ICs. CFSv2 reforecasts with late-April/early-May ICs are found to have highest deterministic ISMR forecast skill, if 1983 is excluded and Indian monsoon seasonal biases are also reduced. During the transitional ENSO in Boreal summer of 1983, faster and intense cooling of NINO3.4 SSTs in L3, could be due to larger dynamical drift with longer lead time of forecasting, compared to L0. Boreal summer ENSO forecast skill is also found to be lowest for L3 which gradually decreases from June to September. Rainfall occurrence with strong cold bias over NINO3.4, is because of the existence of stronger ocean-atmosphere coupling in CFSv2, but with a shift of the SST-rainfall relationship pattern to slightly colder SSTs than the observed. Our analysis suggests the need for a systematic approach to minimize bias in SST boundary forcing in CFSv2, to achieve improved ISMR forecasts.</p>

scholarly journals Weakening Trend of the Tropical Easterly Jet Stream of the Boreal Summer Monsoon Season 1950–2009

2013 ◽  
Vol 26 (23) ◽  
pp. 9408-9414 ◽  
Author(s):  
B. Abish ◽  
P. V. Joseph ◽  
Ola M. Johannessen
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Monsoon Season ◽  
Equatorial Indian Ocean ◽  
Boreal Summer ◽  
Jet Stream ◽  
Moist Convection ◽  
Summer Monsoon Season ◽  
Upper Troposphere ◽  
Tropical Easterly Jet

Recent research has reported that the tropical easterly jet stream (TEJ) of the boreal summer monsoon season is weakening. The analysis herein using 60 yr (1950–2009) of data reveals that this weakening of the TEJ is due to the decreasing trend in the upper tropospheric meridional temperature gradient over the area covered by the TEJ. During this period, the upper troposphere over the equatorial Indian Ocean has warmed due to enhanced deep moist convection associated with the rapid warming of the equatorial Indian Ocean. At the same time, a cooling of the upper troposphere has taken place over the Northern Hemisphere subtropics including the Tibetan anticyclone. The simultaneous cooling of the subtropics and the equatorial heating has caused a decrease in the upper tropospheric meridional thermal gradient. The consequent reduction in the strength of the easterly thermal wind has resulted in the weakening of the TEJ.

scholarly journals The Role of the Western Arabian Sea Upwelling in Indian Monsoon Rainfall Variability

2008 ◽  
Vol 21 (21) ◽  
pp. 5603-5623 ◽  
Author(s):  
Takeshi Izumo ◽  
Clémentde Boyer Montégut ◽  
Jing-Jia Luo ◽  
Swadhin K. Behera ◽  
Sébastien Masson ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Arabian Sea ◽  
Monsoon Rainfall ◽  
Indian Monsoon ◽  
Indian Summer Monsoon Rainfall ◽  
Summer Monsoon Rainfall ◽  
Indian Monsoon Rainfall ◽  
Precipitation Anomalies

Abstract The Indian summer monsoon rainfall has complex, regionally heterogeneous, interannual variations with huge socioeconomic impacts, but the underlying mechanisms remain uncertain. The upwelling along the Somalia and Oman coasts starts in late spring, peaks during the summer monsoon, and strongly cools the sea surface temperature (SST) in the western Arabian Sea. They restrict the westward extent of the Indian Ocean warm pool, which is the main moisture source for the monsoon rainfall. Thus, variations of the Somalia–Oman upwelling can have significant impacts on the moisture transport toward India. Here the authors use both observations and an advanced coupled atmosphere–ocean general circulation model to show that a decrease in upwelling strengthens monsoon rainfall along the west coast of India by increasing the SST along the Somalia–Oman coasts, and thus local evaporation and water vapor transport toward the Indian Western Ghats (mountains). Further observational analysis reveals that such decreases in upwelling are caused by anomalously weak southwesterly winds in late spring over the Arabian Sea that are due to warm SST/increased precipitation anomalies over the Seychelles–Chagos thermocline ridge of the southwestern Indian Ocean (and vice versa for years with strong upwelling/weak west Indian summer monsoon rainfall). The latter SST/precipitation anomalies are often related to El Niño conditions and the strength of the Indonesian–Australian monsoon during the previous winter. This sheds new light on the ability to forecast the poorly predicted Indian monsoon rainfall on a regional scale, helped by a proper ocean observing/forecasting system in the western tropical Indian Ocean.

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