Asymmetry of the Indian Ocean Dipole. Part I: Observational Analysis

The physical mechanism for the amplitude asymmetry of SST anomalies (SSTA) between the positive and negative phases of the Indian Ocean dipole (IOD) is investigated, using Simple Ocean Data Assimilation (SODA) and NCAR–NCEP data. It is found that a strong negative skewness appears in the IOD east pole (IODE) in the mature phase [September–November (SON)], while the skewness in the IOD west pole is insignificant. Thus, the IOD asymmetry is primarily caused by the negative skewness in IODE. A mixed-layer heat budget analysis indicates that the following two air–sea feedback processes are responsible for the negative skewness. The first is attributed to the asymmetry of the wind stress–ocean advection–SST feedback. During the IOD developing stage [June–September (JJAS)], the ocean linear advection tends to enhance the mixed-layer temperature tendency, while nonlinear advection tends to cool the ocean in both the positive and negative events, thus contributing to the negative skewness in IODE. The second process is attributed to the asymmetry of the SST–cloud–radiation (SCR) feedback. For a positive IODE, the negative SCR feedback continues with the increase of warm SSTA. For a negative IODE, the same negative SCR feedback works when the amplitude of SSTA is small. After reaching a critical value, the cold SSTA may completely suppress the mean convection and lead to cloud free conditions; a further drop of the cold SSTA does not lead to additional thermal damping so that the cold SSTA may grow faster. A wind–evaporation–SST feedback may further amplify the asymmetry induced by the aforementioned nonlinear advection and SCR feedback processes.

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Mechanisms of asymmetry in sea surface temperature anomalies associated with the Indian Ocean Dipole revealed by closed heat budget

Scientific Reports ◽

10.1038/s41598-021-01619-2 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Mai Nakazato ◽

Shoichiro Kido ◽

Tomoki Tozuka

Keyword(s):

Indian Ocean ◽

Surface Temperature ◽

Indian Ocean Dipole ◽

Heat Budget ◽

Vertical Mixing ◽

Sea Surface ◽

Subsurface Temperature ◽

Thermocline Feedback ◽

The Indian Ocean ◽

Sst Anomalies

AbstractThe Indian Ocean Dipole (IOD) is an interannual climate mode of the tropical Indian Ocean. Although it is known that negative sea surface temperature (SST) anomalies in the eastern pole during the positive IOD are stronger than positive SST anomalies during the negative IOD, no consensus has been reached on the relative importance of various mechanisms that contribute to this asymmetry. Based on a closed mixed layer heat budget analysis using a regional ocean model, here we show for the first time that the vertical mixing plays an important role in causing such asymmetry in SST anomalies in addition to the contributions from the nonlinear advection and the thermocline feedback proposed by previous studies. A decomposition of the vertical mixing term indicates that nonlinearity in the anomalous vertical temperature gradient associated with subsurface temperature anomalies and anomalous vertical mixing coefficients is the main driver of such asymmetry. Such variations in subsurface temperature are induced by the anomalous southeasterly trade winds along the Indonesian coast that modulate the thermocline depth through coastal upwelling/downwelling. Thus, the thermocline feedback contributes to the SST asymmetry not through the vertical advection as previously suggested, but via the vertical mixing.

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A heat budget study on the mechanism of SST variations in the Indian ocean dipole regions

Journal of Ocean University of China ◽

10.1007/s11802-005-0054-y ◽

2005 ◽

Vol 4 (4) ◽

pp. 334-342 ◽

Cited By ~ 1

Author(s):

Ruijin Hu ◽

Qinyu Liu ◽

Chongyin Li

Keyword(s):

Indian Ocean ◽

Indian Ocean Dipole ◽

Heat Budget ◽

The Indian Ocean

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Genesis of Indian Ocean Mixed Layer Temperature Anomalies: A Heat Budget Analysis

Journal of Climate ◽

10.1175/2010jcli3072.1 ◽

2010 ◽

Vol 23 (20) ◽

pp. 5375-5403 ◽

Cited By ~ 28

Author(s):

Agus Santoso ◽

Alexander Sen Gupta ◽

Matthew H. England

Keyword(s):

Indian Ocean ◽

Time Scales ◽

Mixed Layer ◽

Heat Budget ◽

Heat Fluxes ◽

Temperature Anomalies ◽

Mixed Layer Temperature ◽

The Indian Ocean ◽

The Tropics

Abstract The genesis of mixed layer temperature anomalies across the Indian Ocean are analyzed in terms of the underlying heat budget components. Observational data, for which a seasonal budget can be computed, and a climate model output, which provides improved spatial and temporal coverage for longer time scales, are examined. The seasonal climatology of the model heat budget is broadly consistent with the observational reconstruction, thus providing certain confidence in extending the model analysis to interannual time scales. To identify the dominant heat budget components, covariance analysis is applied based on the heat budget equation. In addition, the role of the heat budget terms on the generation and decay of temperature anomalies is revealed via a novel temperature variance budget approach. The seasonal evolution of the mixed layer temperature is found to be largely controlled by air–sea heat fluxes, except in the tropics where advection and entrainment are important. A distinct shift in the importance and role of certain heat budget components is shown to be apparent in moving from seasonal to interannual time scales. On these longer time scales, advection gains importance in generating and sustaining anomalies over extensive regions, including the trade wind and midlatitude wind regimes. On the other hand, air–sea heat fluxes tend to drive the evolution of thermal anomalies over subtropical regions including off northwestern Australia. In the tropics, however, they limit the growth of anomalies. Entrainment plays a role in the generation and maintenance of interannual anomalies over localized regions, particularly off Sumatra and over the Seychelles–Chagos Thermocline Ridge. It is further shown that the spatial distribution of the role and importance of these terms is related to oceanographic features of the Indian Ocean. Mixed layer depth effects and the influence of model biases are discussed.

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Decadal Modulations of the Indian Ocean Dipole in the SINTEX-F1 Coupled GCM

Journal of Climate ◽

10.1175/jcli4168.1 ◽

2007 ◽

Vol 20 (13) ◽

pp. 2881-2894 ◽

Cited By ~ 65

Author(s):

Tomoki Tozuka ◽

Jing-Jia Luo ◽

Sebastien Masson ◽

Toshio Yamagata

Keyword(s):

Indian Ocean ◽

Heat Transport ◽

Indian Ocean Dipole ◽

Decadal Variability ◽

Heat Budget ◽

Coupled Model ◽

Tropical Indian Ocean ◽

Decadal Modulation ◽

Mascarene High ◽

Budget Analysis

Abstract The decadal variation in the tropical Indian Ocean is investigated using outputs from a 200-yr integration of the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F1) ocean–atmosphere coupled model. The first EOF mode of the decadal bandpass- (9–35 yr) filtered sea surface temperature anomaly (SSTA) represents a basinwide mode and is closely related with the Pacific ENSO-like decadal variability. The second EOF mode shows a clear east–west SSTA dipole pattern similar to that of the interannual Indian Ocean dipole (IOD) and may be termed the decadal IOD. However, it is demonstrated that the decadal air–sea interaction in the Tropics can be a statistical artifact; it should be interpreted more correctly as decadal modulation of interannual IOD events (i.e., asymmetric or skewed occurrence of positive and negative events). Heat budget analysis has revealed that the occurrence of IOD events is governed by variations in the southward Ekman heat transport across 15°S and variations in the Indonesian Throughflow associated with the ENSO. The variations in the southward Ekman heat transport are related to the Mascarene high activities.

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Impact of Equatorial Wind Stress on Ekman Transport During the Mature Phase of the Indian Ocean Dipole

10.21203/rs.3.rs-390257/v1 ◽

2021 ◽

Author(s):

Linfang Zhang ◽

Yaokun Li ◽

Jianping Li

Keyword(s):

Indian Ocean ◽

Wind Stress ◽

Zonal Wind ◽

Indian Ocean Dipole ◽

Meridional Overturning Circulation ◽

Ekman Transport ◽

Zonal Wind Stress ◽

Mature Phase ◽

The Indian Ocean ◽

The Impact

Abstract This paper investigates the impact of equatorial wind stress on the equatorial Ekman transport during the Indian Ocean dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper levels. In normal years, the zonal wind stress south of the equator is easterly and that north of the equator is westerly, which contributes to southward Ekman transport at the upper levels to form the climatological Indian Ocean shallow meridional overturning circulation. During the years of positive IOD events, abnormal easterly winds near the equator bring southward Ekman transport south of the equator while they bring northward Ekman transport north of the equator. This causes the seawater to move away from the equator and hence induces upwelling near the equator, which forms a pair of small circulation cells that are symmetric about the equator at the upper levels (approximately 100 m deep). The abnormal circulation cell south (north) of the equator strengthens (weakens) the southward (southward) motion south (north) of the equator. During years with negative IOD events, the opposite occurs. In addition, during the mature period of IOD, the remote sea surface temperature anomaly (SSTA) such as El Niño–Southern Oscillation (ENSO) may exert some influence on equatorial wind stress and Ekman transport anomaly but the influence is weak.

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Asymmetry of the Indian Ocean Dipole. Part II: Model Diagnosis*

Journal of Climate ◽

10.1175/2008jcli2223.1 ◽

2008 ◽

Vol 21 (18) ◽

pp. 4849-4858 ◽

Cited By ~ 27

Author(s):

Chi-Cherng Hong ◽

Tim Li ◽

Jing-Jia Luo

Keyword(s):

Heat Flux ◽

Indian Ocean ◽

Latent Heat ◽

Latent Heat Flux ◽

Indian Ocean Dipole ◽

Heat Budget ◽

Coupled Model ◽

Sensitivity Experiment ◽

Negative Skewness ◽

Model Diagnosis

Abstract In this second part of a two-part paper, the mechanism for the amplitude asymmetry of SST anomalies (SSTA) between positive and negative Indian Ocean dipole (IOD) events is investigated through the diagnosis of coupled model simulations. Same as the observed in Part I, a significant negative skewness appears in the IOD east pole (IODE) in September–November (SON), whereas there is no significant skewness in the IOD west pole (IODW). A sensitivity experiment shows that the negative skewness in IODE appears even in the case when the ENSO is absent. The diagnosis of the model mixed layer heat budget reveals that the negative skewness is primarily induced by the nonlinear ocean temperature advection and the asymmetry of the cloud–radiation–SST feedback, consistent with the observation (Part I). However, the simulated latent heat flux anomaly is greatly underestimated in IODE during the IOD developing stage [June–September (JJAS)]. As a result, the net surface heat flux acts as strong thermal damping. The underestimation of the latent heat flux anomaly in the IODE is probably caused by the westward shift of along-coast wind anomalies off Sumatra.

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Asymmetry of the Indian Ocean Basinwide SST Anomalies: Roles of ENSO and IOD

Journal of Climate ◽

10.1175/2010jcli3320.1 ◽

2010 ◽

Vol 23 (13) ◽

pp. 3563-3576 ◽

Cited By ~ 35

Author(s):

Chi-Cherng Hong ◽

Tim Li ◽

LinHo ◽

Yin-Chen Chen

Keyword(s):

Heat Flux ◽

Indian Ocean ◽

Mixed Layer ◽

Surface Heat Flux ◽

Surface Heat ◽

Ocean Temperature ◽

Positive Skewness ◽

Cold Events ◽

The Indian Ocean ◽

The Asymmetry

A basinwide warming (cooling) in the Indian Ocean is observed following the El Niño (La Niña) mature phase, with the amplitude of the warming being significantly larger than the cooling. A composite analysis reveals that the amplitude asymmetry (positive skewness) between the warm and cold Indian Ocean basinwide sea surface temperature anomaly pattern (IOB) appears only when ENSO is concurrent with the Indian Ocean dipole (IOD). The amplitude asymmetry becomes insignificant during the ENSO-only and the IOD-only events. The physical mechanism for the amplitude asymmetry is investigated by analyzing the mixed layer heat budget based on the Simple Ocean Data Assimilation (SODA) 2.0.2 data. It is found that the positive skewness in the IOD west pole (IODW) is mainly caused by the asymmetry of ocean temperature advection, whereas the positive skewness in the IOD east pole (IODE) is caused by the asymmetry of the surface heat flux anomaly (primarily shortwave radiation) in response to the ENSO remote forcing. The asymmetry of the mixed layer depth (MLD) between warm and cold events is another factor contributing to the IOB positive skewness. The MLD in IODE during the warm events (27 m) is shallower than that of the cold events (45 m), resulting a larger (smaller) temperature tendency during the warm (cold) events. In contrast, the MLD in IODW during the warm events (44 m) is deeper than that of the cold events (37 m). Because the positive skewness in IODW is caused by the ocean temperature advection and the surface heat flux plays a damping role, a larger (smaller) MLD leads to a weaker (stronger) thermodynamic damping. Thus the asymmetry of MLD in both IODE and IODW favors a greater basinwide warming than cooling.

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A Contest of Status and Legitimacy in the Indian Ocean

10.1093/oso/9780199479337.003.0002 ◽

2018 ◽

Author(s):

David Brewster

Keyword(s):

Indian Ocean ◽

International System ◽

Special Role ◽

Indian Ocean Region ◽

Major Powers ◽

Threat Perceptions ◽

Ocean Region ◽

The Status ◽

The Indian Ocean ◽

The Asymmetry

This chapter examines Indian and Chinese perspectives of each other as major powers and their respective roles in the Indian Ocean. It focuses on the following elements: (a) China’s strategic imperatives in the Indian Ocean Region, (b) India’s views on its special role in the Indian Ocean and the legitimacy of the presence of other powers, (c) China’s strategic vulnerabilities in the Indian Ocean and India’s wish to leverage those vulnerabilities, (d) the asymmetry in Indian and Chinese threat perceptions, and (d) Chinese perspectives of the status of India in the international system and India’s claims to a special role in the Indian Ocean. The chapter concludes that even if China were to take a more transparent approach to its activities, significant differences in perceptions of threat and over status and legitimacy will produce a highly competitive dynamic between them in the maritime domain.

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Analysis of Drought-Sensitive Areas and Evolution Patterns through Statistical Simulations of the Indian Ocean Dipole Mode

Water ◽

10.3390/w11061302 ◽

2019 ◽

Vol 11 (6) ◽

pp. 1302 ◽

Cited By ~ 2

Author(s):

Qing-Gang Gao ◽

Vonevilay Sombutmounvong ◽

Lihua Xiong ◽

Joo-Heon Lee ◽

Jong-Suk Kim

Keyword(s):

Indian Ocean ◽

Indian Ocean Dipole ◽

Diagnostic Techniques ◽

Eastern Coast ◽

Indochina Peninsula ◽

Indian Ocean Dipole Mode ◽

Sensitive Areas ◽

The Indian Ocean ◽

Long Term Trend

In this study, we investigated extreme droughts in the Indochina peninsula and their relationship with the Indian Ocean Dipole (IOD) mode. Areas most vulnerable to drought were analyzed via statistical simulations of the IOD based on historical observations. Results of the long-term trend analysis indicate that areas with increasing spring (March–May) rainfall are mainly distributed along the eastern coast (Vietnam) and the northwestern portions of the Indochina Peninsula (ICP), while Central and Northern Laos and Northern Cambodia have witnessed a reduction in spring rainfall over the past few decades. This trend is similar to that of extreme drought. During positive IOD years, the frequency of extreme droughts was reduced throughout Vietnam and in the southwestern parts of China, while increased drought was observed in Cambodia, Central Laos, and along the coastline adjacent to the Myanmar Sea. Results for negative IOD years were similar to changes observed for positive IOD years; however, the eastern and northern parts of the ICP experienced reduced droughts. In addition, the results of the statistical simulations proposed in this study successfully simulate drought-sensitive areas and evolution patterns of various IOD changes. The results of this study can help improve diagnostic techniques for extreme droughts in the ICP.

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