Indian Ocean Dipole

This study aims to find a clear relation between ENSO-IOD events combination and seasonal rainfall variability. Monthly rainfall data from 21 meteorology stations over Indonesia during 1970 – 2013 was used in this study. Monthly rainfall data was historically analysed in respect to its long term average to find out local response to the ENSO and IOD. It was observed that regions with monsoonal rainfall type were affected clearly by El Niño combined IOD positive or neutral especially in the dry season which is shown by negative anomaly e.g. Lampung, Banjarbaru, Ketapang, Siantan, Cilacap, Semarang, Karangploso, Ngurah Rai, Lombok, Kayuwatu, Bau-bau, Majene and Merauke. Paloh in contrast, showed no response to El Niño-La Niña. The areas with equatorial rainfall type, on the other hand, were affected with lower signal. This study found that Kenten, Jambi, and Sampali more affected by IOD than El Niño-La Niña while Ternate more affected by El Niño-La Niña. Sicincin was not affected by both El Niño-La Niña and IOD. Otherwise Siantan was affected by both El Niño-La Niña and IOD. Considering the upcoming 2014 El Niño event, this study suggests that there will be decreasing in rainfall over regions with monsoonal rainfall type, Ternate, and Siantan by approximately, 20-50% of its normal.

Equatorial Kelvin waves generated by westerly wind anomalies over the central Indian Ocean propagate eastward to Indonesia, where they can enter the outflow passages of the Indonesian Throughflow (ITF) and affect transports of mass and heat. This has potential consequences for local thermohaline properties and may also be related to larger scale climate modes such as the Indian Ocean Dipole (IOD) and Madden-Julian Oscillation (MJO). Moorings were deployed in Lombok and Ombai Straits, two ITF exit passages, as part of the three-year International Nusantara STratification ANd Transport (INSTANT) program. These data provide the first comprehensive, full-depth, high-resolution measurements of velocity and temperature in the exit passages and have allowed us to thoroughly characterize the properties of Kelvin waves in the throughflow region. Here, we discuss the relationship between ITF Kelvin waves and the structure and timing of the wind forcing over the Indian Ocean. Specifically, we focus on the partitioning of Kelvin wave energy between the ITF outflow passages, mode-1 versus mode-2 vertical structure of the Kelvin waves, and the connection between wind forcing, MJO, IOD, and monsoon dynamics.

The relationship between atmospheric subseasonal variability and interannual variation of SST over the tropical Indian Ocean is examined using winds and humidity from the NCEP-NCAR reanalysis, outgoing longwave radiation (OLR), and the monthly SST analysis. The primary focus is on whether and how the subseasonal variability is related to the zonal dipole structure of SST, which peaks during boreal fall.

This paper examines an issue concerning the
simulation of anomalously wet Indian summer monsoons
like 1994 which co-occurred with strong positive Indian
Ocean Dipole (IOD) conditions in the tropical Indian
Ocean. Contrary to observations it has been noticed that
standalone atmospheric general circulation models
(AGCM) forced with observed SST boundary condition,
consistently depicted a decrease of the summer monsoon
rainfall during 1994 over the Indian region. Given the
ocean–atmosphere coupling during IOD events, we have
examined whether the failure of standalone AGCM simulations
in capturing wet Indian monsoons like 1994 can be
remedied by including a simple form of coupling that
allows the monsoon circulation to dynamically interact
with the IOD anomalies. With this view, we have performed
a suite of simulations by coupling an AGCM to a
slab-ocean model with spatially varying mixed-layer-depth
(MLD) specified from observations for the 1994 IOD; as
well as four other cases (1983, 1997, 2006, 2007). The
specification of spatially varying MLD from observations
allows us to constrain the model to observed IOD conditions. It is seen that the inclusion of coupling significantly
improves the large-scale circulation response by
strengthening the monsoon cross-equatorial flow; leading
to precipitation enhancement over the subcontinent and
rainfall decrease over south-eastern tropical Indian
Ocean—in a manner broadly consistent with observations.
A plausible physical mechanism is suggested to explain the
monsoonal response in the coupled frame-work. These
results warrant the need for improved monsoon simulations
with fully coupled models to be able to better capture the
observed monsoon interannual variability.

ABSTRAK Madden Julian Oscillation adalah sirkulasi skala besar di ekuator dan berpusat di Samudra Hindia dicirikan dengan pertumbuhan Super Cloud Cluster (SCC). MJO membutuhkan kondisi iklim yang sesuai untuk berpropagasi ke arah timur. Kondisi iklim di Benua Maritim dipengaruhi oleh fenomena Indian Ocean Dipole (IOD). Terdapat dua jenis fase IOD, yakni Dipole Mode positif dan Dipole Mode negatif. Menurut Marchant dkk. (2006) kedua fase IOD ini memberikan pengaruh yang berbeda terhadap variasi iklim di daerah sekitar Samudra Hindia Timur dan Benua Maritim. Dalam penelitian ini, dilakukan analisis anomali iklim di Benua Maritim saat IOD menggunakan data ERA Interim yang dikomposit berdasarkan fase IOD. Kemudian dilakukan analisis lebih lanjut mengenai dampaknya terhadap MJO menggunakan data angin zonal dan Outgoing Longwave Radiation (OLR) yang difilter menggunakan metode bandpass filter pada periode 20-100 hari. Analisis propagasi MJO dilakukan menggunakan diagram Hovmöller dan untuk melihat kekuatan sinyal MJO di Benua Maritim pada setiap fase IOD dilakukan analisis spektral. Dari hasil analisis didapatkan bahwa selama Dipole Mode negatif, MJO yang berpropagasi ke timur terlihat menguat. Hal ini kemungkinan terjadi dikarenakan adanya penguatan secara in situ akibat kondisi kelembapan yang lebih besar dan juga angin baratan menguat sehingga MJO dapat berpropagasi di Benua Maritim. Sedangkan selama Dipole Mode positif, MJO mengalami hambatan saat akan berpropagasi. Hal ini dikarenakan kondisi kelembapan yang lebih rendah dan angin timuran yang menguat sehingga propagasi MJO menuju timur tertahan saat akan memasuki Benua Maritim kemudian terdisipasi. Hal ini ditunjukkan dengan semakin kecilnya area anomali OLR negatif saat memasuki Benua Maritim dan sinyal MJO yang lebih lemah di Benua Maritim berdasarkan hasil analisis spektral.

Recent gridded and historical data are used in order to assess the relationships between interannual variability of the Indian summer monsoon (ISM) and sea surface temperature (SST) anomaly patterns over the Indian and Pacific oceans. Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoon season (June–September) are strongly influenced by rainfall and circulation anomalies observed during August and September, or the late Indian summer monsoon (LISM). Anomalous monsoons are linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-west Walker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISM circulation is weakened and shifted eastward during weak ISM years. Therefore, we focus on the predictability of the LISM. Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in the southeastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomalies are mainly linked to south Indian Ocean dipole events, studied by Besera and Yamagata (2001) and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies are highly persistent and affect the northwestward translation of the Mascarene High from austral to boreal summer. The southeastward (northwestward) shift of this subtropical high associated with cold (warm) SST anomalies off Australia causes a weakening (strengthening) of the whole monsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, it is suggested that the Mascarene High interacts with the underlying SST anomalies through a positive dynamical feedback mechanism, maintaining its anomalous position during the LISM. Our results also explain why a strong ISM is preceded by a transition in boreal spring from an El Niño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warm SST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in the development of a strong ISM by strengthening the local Hadley circulation during the LISM. On the other hand, a developing La Niña event in boreal spring and summer may also enhance the east–west Walker circulation and the monsoon as demonstrated in many previous studies.

The interannual variability in the tropical Indian Ocean, and in particular the Indian Ocean dipole mode (IODM), is investigated using both observations and a multi-decadal simulations performed by the coupled atmosphere–ocean general circulation model SINTEX. Overall, the characteristics of the simulated IODM are close to the features of the observed mode. Evidence of significant correlations between sea level pressure anomalies in the southeastern Indian Ocean and sea surface temperature anomalies in the tropical Indian and Pacific Oceans have been found both in observations and a multi-decadal simulation. In particular, a positive SLP anomaly in the southeastern part of the basin seems to produce favorable conditions for the development of an IODM event. The role played by the ocean dynamics both in the developing and closing phases of the IODM events is also investigated. Our results suggest that, during the developing phase, the heat content and SST variability associated with the IODM are influenced by a local response of the ocean to the winds, and a remote response with the excitation of Kelvin and Rossby waves. Ocean wave dynamics appear to be important also during the dying phase of the IODM, when equatorial downwelling Kelvin waves transport positive heat content anomalies from the western to the eastern part of the basin, suppressing the zonal heat content anomaly gradient. The results obtained from the model suggest a mechanism for the IODM. This mechanism is generally consistent with the characteristics of the observed IODM. Furthermore, it might give some clue in understanding the correlation between IODM and ENSO activity found both in the model and in the observations.

The variability in the southwest Indian Ocean is connected to the basin-scale and global-scale ocean circulation. Two bands of enhanced variability stretch across the Southern Indian Ocean east of Madagascar around 12 degrees S and 25 degrees S, respectively. They mark the preferred routes along which anomalies, generated by varying forcing over the central basin, near the eastern boundary or in the equatorial region, propagate westward as baroclinic Rossby waves. Sea-surface height anomalies pass along the northern tip of Madagascar and are observed by satellite altimetry to propagate into the central Mozambique Channel. There, eddies are subsequently formed that propagate southward into the Agulhas retroflection region. The anomalies along the southern band trigger the formation of large dipolar vortex pairs in the separation region of the East Madagascar Current at the southern tip of the island. South of Africa these eddies and dipoles trigger the shedding of Agulhas Rings that ...