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.

scholarly journals Influence of Indian Ocean Dipole on Poleward Propagation of Boreal Summer Intraseasonal Oscillations

2008 ◽  
Vol 21 (21) ◽  
pp. 5437-5454 ◽  
Author(s):  
R. S. Ajayamohan ◽  
Suryachandra A. Rao ◽  
Toshio Yamagata
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Critical Role ◽  
Equatorial Indian Ocean ◽  
Summer Monsoon Rainfall ◽  
Specific Humidity ◽  
Boreal Summer ◽  
Intraseasonal Oscillations ◽  
Precipitation Anomalies

Abstract The influence of the Indian Ocean dipole (IOD) on the poleward propagation of boreal summer intraseasonal oscillations (BSISOs) is examined using observed datasets. This study finds that coherent (incoherent) poleward propagation of precipitation anomalies from 5°S to 25°N are observed during negative (positive) IOD years. Disorganized poleward propagation of BSISO in the south equatorial Indian Ocean is observed during positive IOD years. The rationale behind such an anomaly in the poleward propagation of BSISO in contrasting IOD years is identified based on the theory of northward-propagating BSISO, which suggests the influential role of air–sea interaction on the genesis and propagation of BSISO. It is found that the mean structure of moisture convergence and meridional specific humidity distribution undergoes radical changes in contrasting IOD years, which in turn influences the meridional propagation of BSISO. This study assumes significance, considering the critical role of BSISO in modulating the seasonal mean summer monsoon rainfall.

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 A biological Indian Ocean Dipole event in 2019

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Wei Shi ◽  
Menghua Wang
Keyword(s):  
Biological Activity ◽  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Infrared Imaging ◽  
Equatorial Indian Ocean ◽  
The West ◽  
Indian Ocean Dipole Event ◽  
Chl A ◽  
Positive Indian Ocean Dipole ◽  
Dipole Response

AbstractThe 2019 positive Indian Ocean Dipole (IOD) event in the boreal autumn was the most serious IOD event of the century with reports of significant sea surface temperature (SST) changes in the east and west equatorial Indian Ocean. Observations of the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (SNPP) between 2012 and 2020 are used to study the significant biological dipole response that occurred in the equatorial Indian Ocean following the 2019 positive IOD event. For the first time, we propose, identify, characterize, and quantify the biological IOD. The 2019 positive IOD event led to anomalous biological activity in both the east IOD zone and west IOD zone. The average chlorophyll-a (Chl-a) concentration reached over ~ 0.5 mg m−3 in 2019 in comparison to the climatology Chl-a of ~ 0.3 mg m−3 in the east IOD zone. In the west IOD zone, the biological activity was significantly depressed. The depressed Chl-a lasted until May 2020. The anomalous ocean biological activity in the east IOD zone was attributed to the advection of the higher-nutrient surface water due to enhanced upwelling. On the other hand, the dampened ocean biological activity in the west IOD zone was attributed to the stronger convergence of the surface waters than that in a normal year.

scholarly journals Positive Indian Ocean Dipole events prevent anoxia off the west coast of India

2017 ◽  
Vol 14 (6) ◽  
pp. 1541-1559 ◽  
Author(s):  
Parvathi Vallivattathillam ◽  
Suresh Iyyappan ◽  
Matthieu Lengaigne ◽  
Christian Ethé ◽  
Jérôme Vialard ◽  
...  
Keyword(s):  
Indian Ocean ◽  
West Coast ◽  
Indian Ocean Dipole ◽  
Boreal Summer ◽  
Low Oxygen ◽  
The West ◽  
Easterly Wind ◽  
West Coast Of India ◽  
Positive Indian Ocean Dipole ◽  
Subsurface Waters

Abstract. The seasonal upwelling along the west coast of India (WCI) brings nutrient-rich, oxygen-poor subsurface waters to the continental shelf, favoring very low oxygen concentrations in the surface waters during late boreal summer and fall. This yearly-recurring coastal hypoxia is more severe during some years, leading to coastal anoxia that has strong impacts on the living resources. In the present study, we analyze a 1/4° resolution coupled physical–biogeochemical regional oceanic simulation over the 1960–2012 period to investigate the physical processes influencing the oxycline interannual variability off the WCI, that being a proxy for the variability on the shelf in our model. Our analysis indicates a tight relationship between the oxycline and thermocline variations in this region on both seasonal and interannual timescales, thereby revealing a strong physical control of the oxycline variability. As in observations, our model exhibits a shallow oxycline and thermocline during fall that combines with interannual variations to create a window of opportunity for coastal anoxic events. We further demonstrate that the boreal fall oxycline fluctuations off the WCI are strongly related to the Indian Ocean Dipole (IOD), with an asymmetric influence of its positive and negative phases. Positive IODs are associated with easterly wind anomalies near the southern tip of India. These winds force downwelling coastal Kelvin waves that propagate along the WCI and deepen the thermocline and oxycline there, thus preventing the occurrence of coastal anoxia. On the other hand, negative IODs are associated with WCI thermocline and oxycline anomalies of opposite sign but of smaller amplitude, so that the negative or neutral IOD phases are necessary but not the sufficient condition for coastal anoxia. As the IODs generally start developing in summer, these findings suggest some predictability to the occurrence of coastal anoxia off the WCI a couple of months ahead.

Dynamics of 2015 positive Indian Ocean Dipole

2019 ◽  
Vol 69 (1) ◽  
pp. 75
Author(s):  
Putri Adia Utari ◽  
Mokhamad Yusup Nur Khakim ◽  
Dedi Setiabudidaya ◽  
Iskhaq Iskandar
Keyword(s):  
Heat Flux ◽  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Tropical Indian Ocean ◽  
Boreal Summer ◽  
The South ◽  
South Eastern ◽  
Positive Indian Ocean Dipole ◽  
Sst Warming ◽  
Dipole Pattern

Evolution of typical positive Indian Ocean Dipole (pIOD) event was dominated by a significant sea-surface temperature (SST) cooling in the south-eastern tropical Indian Ocean. Interestingly, during the evolution of 2015 pIOD event, the SST in the south-eastern tropical Indian Ocean did not reveal significant cooling, instead anomalous strong SST warming took place in the western tropical Indian Ocean off the East African coast. This anomalous SST warming was associated with a weakening of the Asian summer monsoon. Furthermore, analysis on the mixed layer heat budget demonstrated that the evolution of the 2015 pIOD event could be attributed mainly to the air-sea heat flux. By decomposing the air-sea heat flux, it is found that reduced latent heat loss plays an important role on the SST warming in the western pole and keeping SST warm in the eastern pole. We note that a residual term also may play a role during the initial development of the event. In contrast to the SST pattern, the subsurface temperature revealed a clear positive dipole pattern. Shallow (deep) 20°C isothermal layer in the eastern (western) equatorial Indian Ocean was observed during boreal summer. This robust subsurface dipole pattern indicated that the subsurface ocean response was largely wind driven through the equatorial wave dynamics as previously suggested.

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