Thermocline warming induced extreme Indian Ocean Dipole in 2019

2021 ◽  
Author(s):  
Yan Du ◽  
Yuhong Zhang ◽  
Lian-Yi Zhang ◽  
Tomoki Tozuka ◽  
Wenju Cai
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Rossby Waves ◽  
Deep Convection ◽  
Tropical Indian Ocean ◽  
Triggering Mechanism ◽  
Bjerknes Feedback ◽  
Easterly Wind ◽  
Positive Indian Ocean Dipole ◽  
The 1960S

<p>The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March-May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June-August, the easterly wind anomalies continued to evolve through ocean-atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September-November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity.</p>

Inter-basin and Multi-time Scale Interactions in generating the 2019 Extreme Indian Ocean Dipole

2021 ◽  
pp. 1-39
Author(s):  
Lei Zhang ◽  
Weiqing Han ◽  
Zeng-Zhen Hu
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Intraseasonal Oscillation ◽  
Tropical Indian Ocean ◽  
Forecast Model ◽  
Tropical Pacific ◽  
Boreal Summer ◽  
Indian Ocean Dipole Event ◽  
Easterly Wind ◽  
Western Tropical Pacific

AbstractAn unprecedented extreme positive Indian Ocean Dipole event (pIOD) occurred in 2019, which has caused widespread disastrous impacts on countries bordering the Indian Ocean, including the East African floods and vast bushfires in Australia. Here we investigate the causes for the 2019 pIOD by analyzing multiple observational datasets and performing numerical model experiments. We find that the 2019 pIOD is triggered in May by easterly wind bursts over the tropical Indian Ocean associated with the dry phase of the boreal summer intraseasonal oscillation, and sustained by the local atmosphere-ocean interaction thereafter. During September-November, warm sea surface temperature anomalies (SSTA) in the central-western tropical Pacific further enhance the Indian Ocean’s easterly winds, bringing the pIOD to an extreme magnitude. The central-western tropical Pacific warm SSTA is strengthened by two consecutive Madden Julian Oscillation (MJO) events that originate from the tropical Indian Ocean. Our results highlight the important roles of cross-basin and cross-timescale interactions in generating extreme IOD events. The lack of accurate representation of these interactions may be the root for a short lead time in predicting this extreme pIOD with a state-of-the-art climate forecast model.

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.

scholarly journals Extreme IOD induced tropical Indian Ocean warming in 2020

2021 ◽  
Vol 8 (1) ◽  
Author(s):  
Ying Zhang ◽  
Yan Du
Keyword(s):  
Indian Ocean ◽  
Surface Wind ◽  
Tropical Indian Ocean ◽  
First Record ◽  
Early Summer ◽  
Wind Anomaly ◽  
Surface Warming ◽  
Positive Indian Ocean Dipole ◽  
Indian Ocean Warming ◽  
The 1960S

AbstractThe tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer. This study reveals the important role of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode, which allows us to rethink the dynamic connections between the Indo-Pacific climate modes.

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.

scholarly journals Positive Indian Ocean Dipole events prevent anoxia along coast of India

2016 ◽  
Author(s):  
V. Parvathi ◽  
I. Suresh ◽  
Matthieu Lengaigne ◽  
Christian Ethé ◽  
Jérôme Vialard ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Indian Ocean Dipole ◽  
Boreal Summer ◽  
Low Oxygen ◽  
Easterly Wind ◽  
Positive Indian Ocean Dipole ◽  
Subsurface Waters ◽  
Anoxic Events ◽  
The Indian Ocean

Abstract. The seasonal upwelling along the west coast of India (WCI) brings nutrient-rich, oxygen-poor subsurface waters to the continental shelf, leading to very low oxygen concentrations at shallow depths during late boreal summer and fall. This yearly-recurring coastal hypoxia is sometimes more severe, 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 oxycline interannual variability off the WCI. Our analysis indicates a tight relationship between the oxycline and thermocline variations along the WCI at both seasonal and interannual timescales, thereby revealing a strong physical control of the WCI oxycline variability. As in observations, our model exhibits a shallow oxycline/thermocline along the WCI during fall that combines with interannual variability to create a window of opportunity for coastal anoxic events at this time of the year. We further demonstrate that boreal fall WCI oxycline fluctuations are strongly related to the Indian Ocean Dipole (IOD), with an asymmetric influence of positive and negative IOD 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 IOD events are associated with WCI thermocline and oxycline anomalies of opposite sign, but of smaller amplitude, and are hence a necessary, but not sufficient condition for coastal anoxia. As the IODs generally start developing in summer, these findings suggest some predictability to the occurrence of WCI coastal anoxia a couple of months ahead.

scholarly journals A Triggering Mechanism for the Indian Ocean Dipoles Independent of ENSO

2015 ◽  
Vol 28 (13) ◽  
pp. 5063-5076 ◽  
Author(s):  
Shuangwen Sun ◽  
Jian Lan ◽  
Yue Fang ◽  
Tana ◽  
Xiaoqian Gao
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Tropical Indian Ocean ◽  
Early Summer ◽  
Wind Anomaly ◽  
Triggering Mechanism ◽  
Easterly Wind ◽  
Summer Monsoon Onset ◽  
The Indian Ocean

Although the Indian Ocean dipole (IOD) and ENSO are significantly correlated, there are indeed some IODs independent of ENSO. In this research, the characteristics of independent IOD are investigated and a new triggering mechanism is proposed based on case study and statistical analysis. Results show that the independent IODs peak in an earlier season and have a weaker intensity compared with the IODs associated with ENSO. The wind anomaly associated with the independent IOD is very unique and shows a monsoonlike pattern, in addition to the equatorial easterly wind anomaly (EEWA) common to all IODs. The evolution of the EEWA associated with the independent IOD is well captured by the second EOF mode of the equatorial zonal wind interannual variability, suggesting that the independent IOD is an important climate mode inherent to the tropical Indian Ocean. The EEWA associated with the independent IOD is tightly linked to Indian summer monsoon activities in spring, and the convection anomalies associated with early summer monsoon onset in the Bay of Bengal plays a key role in inducing the EEWA. The EEWA can persist through spring and summer and causes a series of processes similar to those related to the IODs associated with ENSO. The correlation between the independent IOD and Indian summer monsoon activities increases dramatically after the 1980s, which is probably due to the mean state change in the tropical Indian Ocean climate system.

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