scholarly journals Systematic Errors in South Asian Monsoon Simulation: Importance of Equatorial Indian Ocean Processes

2017 ◽  
Vol 30 (20) ◽  
pp. 8159-8178 ◽  
Author(s):  
H. Annamalai ◽  
Bunmei Taguchi ◽  
Julian P. McCreary ◽  
Motoki Nagura ◽  
Toru Miyama
Keyword(s):  
Indian Ocean ◽  
South Asia ◽  
South Asian ◽  
Coupled Model ◽  
Equatorial Indian Ocean ◽  
Coupled Processes ◽  
Cmip5 Models ◽  
Model Errors ◽  
Bjerknes Feedback ◽  
Sst Bias

Abstract Forecasting monsoon rainfall using dynamical climate models has met with little success, partly due to models’ inability to represent the monsoon climatological state accurately. In this article the nature and dynamical causes of their biases are investigated. The approach is to analyze errors in multimodel-mean climatological fields determined from CMIP5, and to carry out sensitivity experiments using a coupled model [the Coupled Model for the Earth Simulator (CFES)] that does represent the monsoon realistically. Precipitation errors in the CMIP5 models persist throughout the annual cycle, with positive (negative) errors occurring over the near-equatorial western Indian Ocean (South Asia). Model errors indicate that an easterly wind stress bias Δτ along the equator begins during April–May and peaks during November; the severity of the Δτ is that the Wyrtki jets, eastward-flowing equatorial currents during the intermonsoon seasons (April–May and October–November), are almost eliminated. An erroneous east–west SST gradient (warm west and cold east) develops in June. The structure of the model errors indicates that they arise from Bjerknes feedback in the equatorial Indian Ocean (EIO). Vertically integrated moisture and moist static energy budgets confirm that warm SST bias in the western EIO anchors moist processes that cause the positive precipitation bias there. In CFES sensitivity experiments in which Δτ or warm SST bias over the western EIO is artificially introduced, errors in the EIO are similar to those in the CMIP5 models; moreover, precipitation over South Asia is reduced. An overall implication of these results is that South Asian rainfall errors in CMIP5 models are linked to errors of coupled processes in the western EIO, and in coupled models correct representation of EIO coupled processes (Bjerknes feedback) is a necessary condition for realistic monsoon simulation.

scholarly journals Moist Dynamics of Extended Monsoon Breaks over South Asia

2012 ◽  
Vol 25 (11) ◽  
pp. 3810-3831 ◽  
Author(s):  
V. Prasanna ◽  
H. Annamalai
Keyword(s):  
Indian Ocean ◽  
South Asia ◽  
Coupled Model ◽  
Equatorial Indian Ocean ◽  
Hadley Circulation ◽  
Central India ◽  
Radiative Heating ◽  
Moisture Distribution ◽  
Advection Equation ◽  
Circulation Anomalies

In the present research to identify moist processes that initiate and maintain extended monsoon breaks over South Asia moisture and moist static energy (MSE) budgets are performed on the newly available European Centre for Medium-Range Weather Forecasts Interim reanalysis (ERA-Interim) and ensemble integrations from a coupled model. The hypothesis that interaction between moist physics and regional circulation and the role of cloud–radiation feedbacks are important is tested. Budget diagnostics show that dry advection is the principal moist process to initiate extended breaks. Its sources are (i) regional anticyclonic circulation anomalies forced by equatorial Indian Ocean negative rainfall anomalies advect low MSE air from north to central India, and (ii) rainfall enhancement over tropical west Pacific forces cyclonic circulation anomalies to its northwest as a Rossby wave response, and the northerlies at the poleward flank of this circulation advect air of low MSE content from north. The dominance of anomalous wind acting on climatological moisture gradient is confirmed from an examination of the moisture advection equation. A partition of various flux terms indicates that over central India, due to an increase in upwelling shortwave and longwave fluxes, radiative cooling increases during extended breaks. Here, enhanced rainfall over the equatorial Indian Ocean promotes anomalous radiative warming due to trapping of upwelling fluxes. The differential radiative heating anchors a local Hadley circulation with descent over central India. A direct implication of this research is that observational efforts are necessary to monitor the three-dimensional moisture distribution and cloud–radiation interaction over the monsoon region that would aid in better understanding, modeling, and predicting extended monsoon breaks.

scholarly journals Mixing state of refractory black carbon aerosol in the South Asian outflow over the northern Indian Ocean during winter

2020 ◽  
Author(s):  
Sobhan Kumar Kompalli ◽  
Surendran Nair Suresh Babu ◽  
Krishnaswamy Krishnamoorthy ◽  
Sreedharan Krishnakumari Satheesh ◽  
Mukunda M. Gogoi ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Black Carbon ◽  
South Asian ◽  
Arabian Sea ◽  
Equatorial Indian Ocean ◽  
Mixing State ◽  
Air Masses ◽  
Continental Outflow ◽  
Mass Concentrations

Abstract. Regional climatic implications of aerosol black carbon (BC) are well recognized over South Asia, which has a wide variety of anthropogenic sources in a large abundance. Significant uncertainties remain in its quantification due to lack of sufficient information on the microphysical properties (its concentration, size, and mixing state with other aerosol components), which determine the absorption potential of BC. Especially the information on mixing state of BC is extremely sparse over this region. In this study, first-ever observations of the size distribution and mixing state of individual refractory black carbon (rBC) particles in the south Asian outflow to Southeastern Arabian Sea, northern and equatorial Indian Ocean regions are presented based on measurements using a single particle soot photometer (SP2) aboard the ship cruise of the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB-2018) during winter-2018 (16 January to 13 February). The results revealed significant spatial heterogeneity of BC characteristics. Highest rBC mass concentrations (~ 938 ± 293 ng m−3) with the highest relative coating thickness (RCT; the ratio of BC core to its coating diameters) of ~ 2.16 ± 0.19 are found over the Southeast Arabian Sea (SEAS) region, which is in the proximity of the continental outflow. As we move to farther oceanic regions, though the mass concentrations decreased by nearly half (~ 546 ± 80 ng m−3), BC still remained thickly coated (RCT ~ 2.05 ± 0.07). The air over the remote equatorial Indian Ocean, which received considerable marine air masses compared to the other regions, showed the lowest rBC mass concentrations (~ 206 ± 114 ng m−3), with a moderately thick coating (RCT ~ 1.73 ± 0.16). Even over oceanic regions far from the landmass, regions which received the outflow from more industrialized east coast/the Bay of Bengal had thicker coating (~ 104 nm) compared to regions that received outflow from the west coast/peninsular India (~ 86 nm). Although different regions of the ocean depicted contrasting concentrations and mixing state parameters due to varying extent and nature of the continental outflow as well as the atmospheric lifetime of air masses, the modal parameters of rBC mass-size distributions were similar over all the regions. The observed mono-modal distribution with mean mass median diameters (MMD) in the range of 0.19–0.20 μm suggested mixed sources of BC. The mean fraction of BC containing particles (FBC) varied in the range 0.20–0.28 (suggesting significant amounts of non-BC particles), whereas the bulk mixing ratio of coating mass to rBC mass was highest (8.77 ± 2.77) over the outflow regions compared to the remote ocean (4.29 ± 1.54) highlighting the role of outflow in providing condensable material for coating on rBC. These parameters, along with the information on size-resolved mixing state of BC cores, throw light on the role of sources and secondary processing of their complex mixtures for coating on BC under highly polluted conditions. Examination of the non-refractory sub-micrometre aerosol chemical composition obtained using the aerosol chemical speciation monitor (ACSM) suggested that the overall aerosol system was sulfate dominated over the far-oceanic regions. In contrast, organics were equally prominent adjacent to the coastal landmass. Association between the BC mixing state and aerosol chemical composition suggested that sulfate was the probable dominant coating material on rBC cores.

scholarly journals Moist Dynamics of Severe Monsoons over South Asia: Role of the Tropical SST*

2012 ◽  
Vol 69 (1) ◽  
pp. 97-115 ◽  
Author(s):  
Prasanth A. Pillai ◽  
H. Annamalai
Keyword(s):  
Indian Ocean ◽  
South Asia ◽  
El Niño ◽  
El Nino ◽  
Coupled Model ◽  
Moisture Distribution ◽  
Northern Indian Ocean ◽  
Easterly Wind ◽  
Rainfall Anomalies ◽  
Sst Anomalies

Abstract Diagnostics from observations and multicentury integrations of a coupled model [Geophysical Fluid Dynamics Laboratory (GFDL) coupled model version 2.1 (CM2.1)] indicate that about 65% of the severe monsoons (rainfall > 1.5 standard deviations of its long-term mean) over South Asia are associated with sea surface temperature (SST) anomalies over the equatorial Pacific during the developing phase of ENSO, and another 30% are associated with SST variations over the tropical Indo-Pacific warm pool. The present research aims to identify the moist processes that initiate the dryness (wetness) and provide a precursor for rainfall anomalies over South Asia in spring during El Niño (La Niña). The hypothesis in this paper, based on CM2.1 composites, is that at low levels El Niño–forced equatorial easterly wind anomalies over the Indian Ocean, resulting from Ekman pumping, promote anticyclonic vorticity over the northern Indian Ocean, whose poleward flank advects dry air from northern latitudes to South Asia. This is tested by performing ensemble simulations with the atmospheric component of CM2.1 (AM2.1) and applying moisture and moist static energy budgets. During El Niño, AM2.1 solutions capture the anticyclonic vorticity formation over the northern Indian Ocean 20–25 days earlier than organized negative rainfall anomalies over South Asia, and the advection of climatological air of lower moisture content by these anomalous winds initiates the dryness over South Asia from April onward. This long lead time embodied in this precursor signal can be exploited for predicting severe monsoons. During ENSO neutral conditions, the amplitude of regional SST anomalies during spring is insufficient to produce such a precursor signal. The dominance of the term warrants monitoring the three-dimensional moisture distribution for better understanding, modeling, and predicting of severe monsoons.

Association between mean and interannual equatorial Indian Ocean subsurface temperature bias in a coupled model

2017 ◽  
Vol 50 (5-6) ◽  
pp. 1659-1673 ◽  
Author(s):  
G. Srinivas ◽  
Jasti S. Chowdary ◽  
C. Gnanaseelan ◽  
K. V. S. R. Prasad ◽  
Ananya Karmakar ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Coupled Model ◽  
Equatorial Indian Ocean ◽  
Subsurface Temperature ◽  
Temperature Bias

scholarly journals On The Simulation of Northeast Monsoon Rainfall Over Southern Peninsular India in CMIP5 Models

2021 ◽  
Author(s):  
P.P. Sree ◽  
C. A. Babu ◽  
S. Vijaya Bhaskara Rao
Keyword(s):  
Pacific Ocean ◽  
Indian Ocean ◽  
South China Sea ◽  
South China ◽  
Cmip5 Models ◽  
Northeast Monsoon ◽  
Peninsular India ◽  
China Sea ◽  
Sst Bias ◽  
The Mean

Abstract The skill of 34 CMIP5 models to simulate the mean state and interannual variability of Northeast Monsoon Rainfall (NEMR) is studied here. The mean (1979-2005) NEMR over southern Peninsular India (SPIRF), Indian Ocean and Maritime continents (10°S-30°N,40°E- 120°E) is simulated reasonably well by CMIP5 models with pattern correlation ranges from 0.6 to 0.93. Diverse behaviour in the simulation of Indian and Pacific Ocean SST is observed in the CMIP5 models. A set of models (high skill models: HSM), which shows a Negative Indian Ocean Dipole (NIOD) like mean (1979-2005) SST bias in Indian Ocean and strong La Nina like mean SST bias in the Pacific Ocean, are able to simulate the mean NEMR more realistically. Another set of models (low skill models: LSM) which shows a Positive IOD (PIOD) like mean SST bias in the Indian Ocean and weak La Nina like mean SST bias in the Pacific Ocean are not able to simulate the observed equatorial Indian Ocean westerlies, which leads to an abnormal ascending motion and unrealistic wet bias over the western Indian Ocean and dry bias over the southern Peninsular India, southeast Asia and southeast Indian Ocean. The observation analysis reveals that the establishment of South China Sea anticyclone and Bay of Bengal anticyclone during El Nino and PIOD are strongly related with the ascending motion over south peninsular India and enhances the south Peninsular Indian rainfall during NEM season. Around 70% of the CMIP5 models were not able to capture the observed positive correlation that exist between SPIRF and Nino3.4 SST as well as SPIRF and DMI. Unrealistic westward extension of South China Sea anticyclone and Bay of Bengal anticyclone (up to 70°E) in the low skill models (LSM-IAV) manifested as the abnormal descending anomalies and unrealistic dry bias over the southern Peninsular India. This leads to a negative Correlation coefficient (CC) between SPIRF and Nino 3.4 SST as well as SPIRF and DMI in the low skill models. The descending anomalies over South China Sea and ascending anomalies over the western Indian Ocean and southern Peninsular India (50°E-80°E) is well captured but with lower intensity in the high skill models (HSM-IAV) and hence it captures the observed positive CC between SPIRF and Nino3.4 SST as well as SPIRF and DMI.

Trade, Institutions, and Ethnic Tolerance: Evidence from South Asia

2013 ◽  
Vol 107 (4) ◽  
pp. 806-832 ◽  
Author(s):  
SAUMITRA JHA
Keyword(s):  
Indian Ocean ◽  
South Asia ◽  
South Asian ◽  
Colonial History ◽  
Local Institutions ◽  
Occupational Choices ◽  
Household Level ◽  
Political Incentives ◽  
Ocean Shipping ◽  
Level Data

I provide evidence that the degree to which medieval Hindus and Muslims could provide complementary, nonreplicable services and a mechanism to share the gains from exchange has resulted in a sustained legacy of ethnic tolerance in South Asian towns. Due to Muslim-specific advantages in Indian Ocean shipping, interethnic complementarities were strongest in medieval trading ports, leading to the development of institutional mechanisms that further supported interethnic exchange. Using novel town-level data spanning South Asia's medieval and colonial history, I find that medieval ports, despite being more ethnically mixed, were five times less prone to Hindu-Muslim riots between 1850 and 1950, two centuries after Europeans disrupted Muslim overseas trade dominance, and remained half as prone between 1950 and 1995. Household-level evidence suggests that these differences reflect local institutions that emerged to support interethnic medieval trade, continue to influence modern occupational choices and organizations, and substitute for State political incentives in supporting interethnic trust.

Indian Ocean Dipole Response to Global Warming in the CMIP5 Multimodel Ensemble*

2013 ◽  
Vol 26 (16) ◽  
pp. 6067-6080 ◽  
Author(s):  
Xiao-Tong Zheng ◽  
Shang-Ping Xie ◽  
Yan Du ◽  
Lin Liu ◽  
Gang Huang ◽  
...  
Keyword(s):  
Global Warming ◽  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Coupled Model ◽  
Equatorial Indian Ocean ◽  
Easterly Wind ◽  
Thermocline Feedback ◽  
Intercomparison Project ◽  
Eastern Equatorial Indian Ocean ◽  
Dipole Response

Abstract The response of the Indian Ocean dipole (IOD) mode to global warming is investigated based on simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). In response to increased greenhouse gases, an IOD-like warming pattern appears in the equatorial Indian Ocean, with reduced (enhanced) warming in the east (west), an easterly wind trend, and thermocline shoaling in the east. Despite a shoaling thermocline and strengthened thermocline feedback in the eastern equatorial Indian Ocean, the interannual variance of the IOD mode remains largely unchanged in sea surface temperature (SST) as atmospheric feedback and zonal wind variance weaken under global warming. The negative skewness in eastern Indian Ocean SST is reduced as a result of the shoaling thermocline. The change in interannual IOD variance exhibits some variability among models, and this intermodel variability is correlated with the change in thermocline feedback. The results herein illustrate that mean state changes modulate interannual modes, and suggest that recent changes in the IOD mode are likely due to natural variations.

The Response of the Indian Ocean Dipole Asymmetry to Anthropogenic Aerosols and Greenhouse Gases

2015 ◽  
Vol 28 (7) ◽  
pp. 2564-2583 ◽  
Author(s):  
Tim Cowan ◽  
Wenju Cai ◽  
Benjamin Ng ◽  
Matthew England
Keyword(s):  
Indian Ocean ◽  
Greenhouse Gases ◽  
Indian Ocean Dipole ◽  
Climate Models ◽  
Coupled Model ◽  
Tropical Indian Ocean ◽  
Cmip5 Models ◽  
Anthropogenic Aerosols ◽  
The West ◽  
The Indian Ocean

Abstract The tropical Indian Ocean has experienced a faster warming rate in the west than in the east over the twentieth century. The warming pattern resembles a positive Indian Ocean dipole (IOD) that is well captured by climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), forced with the two main anthropogenic forcings, long-lived greenhouse gases (GHGs), and aerosols. However, much less is known about how GHGs and aerosols influence the IOD asymmetry, including the negative sea surface temperature (SST) skewness in the east IOD pole (IODE). Here, it is shown that the IODE SST negative skewness is more enhanced by aerosols than by GHGs using single-factor forcing experiments from 10 CMIP5 models. Aerosols induce a greater mean zonal thermocline gradient along the tropical Indian Ocean than that forced by GHGs, whereby the thermocline is deeper in the east relative to the west. This generates strong asymmetry in the SST response to thermocline anomalies between warm and cool IODE phases in the aerosol-only experiments, enhancing the negative IODE SST skewness. Other feedback processes involving zonal wind, precipitation, and evaporation cannot solely explain the enhanced SST skewness by aerosols. An interexperiment comparison in one model with strong skewness confirms that the mean zonal thermocline gradient across the Indian Ocean determines the magnitude of the SST–thermocline asymmetry, which in turn controls the SST skewness strength. The findings suggest that as aerosol emissions decline and GHGs increase, this will likely contribute to a future weakening of the IODE SST skewness.

scholarly journals Mixing state of refractory black carbon aerosol in the South Asian outflow over the northern Indian Ocean during winter

2021 ◽  
Vol 21 (11) ◽  
pp. 9173-9199
Author(s):  
Sobhan Kumar Kompalli ◽  
Surendran Nair Suresh Babu ◽  
Krishnaswamy Krishna Moorthy ◽  
Sreedharan Krishnakumari Satheesh ◽  
Mukunda Madhab Gogoi ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Black Carbon ◽  
South Asian ◽  
Equatorial Indian Ocean ◽  
Mixing State ◽  
The South ◽  
Air Masses ◽  
Continental Outflow ◽  
Mass Concentrations

Abstract. Regional climatic implications of aerosol black carbon (BC), which has a wide variety of anthropogenic sources in large abundance, are well recognized over South Asia. Significant uncertainties remain in its quantification due to a lack of sufficient information on the microphysical properties (its concentration, size, and mixing state with other aerosol components) that determine the absorption potential of BC. In particular, the information on the mixing state of BC is extremely sparse over this region. In this study, the first observations of the size distribution and mixing state of individual refractory black carbon (rBC) particles in the South Asian outflow to the south-eastern Arabian Sea and the northern and equatorial Indian Ocean regions are presented based on measurements using a single particle soot photometer (SP2) aboard the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB-2018) ship during winter 2018 (16 January to 13 February). The results revealed significant spatial heterogeneity of BC characteristics. The highest rBC mass concentrations (∼938±293 ng m−3) with the highest relative coating thickness (RCT; the ratio of BC core to its coating diameters) of ∼2.16±0.19 are found over the south-east Arabian Sea (SEAS) region, which is in the proximity of the continental outflow. As we move to farther oceanic regions, though the mass concentrations decreased by nearly half (∼546±80 ng m−3), BC still remained thickly coated (RCT∼2.05±0.07). The air over the remote equatorial Indian Ocean, which received considerable marine air masses compared to the other regions, showed the lowest rBC mass concentrations (∼206±114 ng m−3) with a moderately thick coating (RCT∼1.73±0.16). Even over oceanic regions far from the landmass, regions that received the outflow from the more industrialized east coast/the Bay of Bengal had a thicker coating (∼104 nm) compared to regions that received outflow from the west coast and/or peninsular India (∼86 nm). Although different regions of the ocean depicted contrasting concentrations and mixing state parameters due to the varied extent and nature of the continental outflow as well as the atmospheric lifetime of air masses, the modal parameters of rBC mass–size distributions (mean mass median diameters ∼ 0.19–0.20 µm) were similar over all regions. The mean fraction of BC-containing particles (FBC) varied in the range of 0.08–0.12 (suggesting significant amounts of non-BC particles), whereas the bulk mixing ratio of coating mass to rBC mass was highest (8.31±2.40) over the outflow regions compared to the remote ocean (4.24±1.45), highlighting the role of outflow in providing condensable material for coatings on rBC. These parameters, along with the information on the size-resolved mixing state of BC cores, throw light on the role of sources and secondary processing of their complex mixtures for coatings on BC under highly polluted conditions. Examination of the non-refractory sub-micrometre aerosol chemical composition obtained using the aerosol chemical speciation monitor (ACSM) suggested that the overall aerosol system was sulfate-dominated over the far-oceanic regions. In contrast, organics were equally prominent adjacent to the coastal landmass. An association between the BC mixing state and aerosol chemical composition suggested that sulfate was the probable dominant coating material on rBC cores.

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