Intraseasonal Oscillation in the Tropical Indian Ocean

1. Introduction The intraseasonal oscillation (ISO or Madden- Julian Oscillation, MJO) in the tropical atmosphere has been studied extensively, including its existence, structure, evolution and propagation (Madden and Ju- lian, 1971; Murakami, et al., 198…     

Relationships between the Global General Circulation and the Indian Summer Monsoon

The relationships between the global general circulation and the Indian monsoon during active and break phases are investigated with the help of FGGE IIIb data.It was found that the ultralong wave component positive and negative height anomalies over Tibet are associated with active and break monsoon phases respectively. This ultralong wave component has significant effect even upto 22°N over the Indian region which is the monsoon trough region. During a monsoon break, the general circulation was found to be more turbulent in the sense that more waves become energised.It was observed that during a break, blocking prevails over the Siberian region and cold air advection takes place toward Indian region from Siberian region depressing the temperatures over the Indian region by about 1℃. During the break, the Indian region gets connected with higher latitudes by the south winds blowing from polar Soviet regions to the Indian region. From active to break phase the zonal component weakens by about 25% from     

The Positive Indian Ocean Dipole–like Response in the Tropical Indian Ocean to Global Warming

Climate models project a positive Indian Ocean Dipole(p IOD)–like SST response in the tropical Indian Ocean to global warming. By employing the Community Earth System Model and applying an overriding technique to its ocean component(version 2 of the Parallel Ocean Program), this study investigates the similarities and differences of the formation mechanisms for the changes in the tropical Indian Ocean during the p IOD versus global warming. Results show that their formation processes and related seasonality are quite similar; in particular, wind–thermocline–SST feedback is the leading mechanism in producing the anomalous cooling over the eastern tropics in both cases. Some differences are also found, including the fact that the cooling effect of the vertical advection over the eastern tropical Indian Ocean is dominated by the anomalous vertical velocity during the p IOD but by the anomalous upper-ocean stratification under global warming. These findings are further examined through an analysis of the mixed layer heat budget.     

The Variability of the Interannual Oscillations of the Indian Summer Monsoon Rainfall

A new method of analysis namely, Singular Spectrum Analysis (SSA) is applied to the Indian Summer Monsoon (June-September) Rainfall (ISMR) series. The method is efficient in extracting the statistically significant oscillations with periods 2.8 and 2.3 year from the white noise of the ISMR series. The study shows that 2.8 / 2.3 year cycle captures the variability of the ISMR related to Southern Oscillation / Quasi Biennial Oscillation. The temporal structure of these oscillations show that these are in phase in extreme (excess and drought) monsoon conditions as well as in El Nino Southern Oscillation (ENSO) years. Both these oscillations show minimum variability during the period 1920-1940 and there is an increasing trend in the variability of these oscillations in the recent decades. The study enables to obtain pure signal consisting of reconstructed time series using these two Oscillations, from the original white noise series.     

How is the Indian Ocean Subtropical Dipole excited?

Based on experiments using a coupled general circulation model which resolves tropical ocean–atmosphere coupled phenomena such as El Niño/Southern Oscillation (ENSO) and the Indian Ocean Dipole, forcing mechanisms of the Indian Ocean subtropical dipole (IOSD) are investigated. In the control experiment, as in the observation, several types of the IOSD are generated by the variations in the Mascarene High during austral summer and characterized by a dipole pattern of sea surface temperature (SST) anomalies in the northeastern and southwestern parts of the southern Indian Ocean. In another experiment, where the SST outside the southern Indian Ocean is nudged toward the monthly climatology of the simulated SST, one type of the IOSD occurs, but it is less frequent and associated with the zonal wavenumber four pattern of equivalently barotropic geopotential height anomalies in high latitudes, suggesting an interesting link with the Antarctic Circumpolar Wave. This indicates that, even without the atmospheric teleconnection from tropical coupled climate modes, the IOSD may develop in association with the atmospheric variability in high latitudes of the Southern Hemisphere. In the other experiment, where only the southern Indian Ocean and the tropical Pacific are freely interactive with the atmosphere, two types of both positive and negative IOSD occur. Since the occurrence frequency of the IOSD significantly increases as compared to the second experiment, this result confirms that the atmospheric teleconnection from ocean-atmosphere coupled modes in the tropical Pacific such as ENSO may also induce the variations in the Mascarene High that generate the IOSD. The present research, even within the realm of model studies, shows clearly that the predictability of the IOSD in mid-latitudes is related to both low and high-latitudes climate variations.     

Seasonal Phase-Locking of Peak Events in the Eastern Indian Ocean

The sea surface temperature （SST） anomaly of the eastern Indian Ocean （EIO） exhibits cold anomalies in the boreal summer or fall during E1 Nino development years and warm anomalies in winter or spring following the E1 Nino events. There also tend to be warm anomalies in the boreal summer or fall during La Nina development years and cold anomalies in winter or spring following the La Nina events. The seasonal phase-locking of SST change in the EIO associated with E1 Nino/Southern Oscillation is linked to the variability of convection over the maritime continent, which induces an atmospheric Rossby wave over the EIO. Local air-sea interaction exerts different effects on SST anomalies, depending on the relationship between the Rossby wave and the mean flow related to the seasonal migration of the buffer zone, which shifts across the equator between summer and winter. The summer cold events start with cooling in the Timor Sea, together with increasing easterly flow along the equator. Negative SST anomalies develop near Sumatra, through the interaction between the atmospheric Rossby wave and the underneath sea surface. These SST anomalies are also contributed to by the increased upwelling of the mixed layer and the equatorward temperature advection in the boreal fall. As the buffer zone shifts across the equator towards boreal winter, the anomalous easterly flow tends to weaken the mean flow near the equator, and the EIO SST increases due to the reduction of latent heat flux from the sea surface. As a result, wintertime SST anomalies appear with a uniform and nearly basin-wide pattern beneath the easterly anomalies. These SST anomalies are also caused by the increase in solar radiation associated with the anticyclonic atmospheric Rossby wave over the EIO. Similarly, the physical processes of the summer warm events, which are followed by wintertime cold SST anomalies, can be explained by the changes in atmospheric and oceanic fields with opposite signs to those anomalies described above.     

Differences in atmospheric heat source between the Tibetan Plateau–South Asia region and the southern Indian Ocean and their impacts on the Indian summer monsoon outbreak

In this paper, the NCEP–NCAR daily reanalysis data are used to investigate the characteristics of the atmospheric heat source/sink (AHSS) over South Asia (SA) and southern Indian Ocean (SIO). The thermal differences between these two regions and their influence on the outbreak of the Indian summer monsoon (ISM) are explored. Composite analysis and correlation analysis are applied. The results indicate that the intraseasonal variability of AHSS is significant in SA but insignificant in the SIO. Large inland areas in the Northern Hemisphere still behave as a heat sink in March, similar to the situation in winter. Significant differences are found in the distribution of AHSS between the ocean and land, with distinct land–ocean thermal contrast in April, and the pattern presents in the transitional period right before the ISM onset. In May, strong heat centers appear over the areas from the Indochina Peninsula to the Bay of Bengal and south of the Tibetan Plateau (TP), which is a typical pattern of AHSS distribution during the monsoon season. The timing of SA–SIO thermal difference turning positive is about 15 pentads in advance of the onset of the ISM. Then, after the thermal differences have turned positive, a pre-monsoon meridional circulation cell develops due to the near-surface heat center and the negative thermal contrast center, after which the meridional circulation of the ISM gradually establishes. In years of early (late) conversion of the SA–SIO thermal difference turning from negative to positive, the AHSS at all levels over the TP and SIO converts later (earlier) than normal and the establishment of the ascending and descending branches of the ISM’s meridional circulation is later (earlier) too. Meanwhile, the establishment of the South Asian high over the TP is later (earlier) than normal and the conversion of the Mascarene high from winter to summer mode occurs anomalously late (early). As a result, the onset of the ISM is later (earlier) than normal. However, the difference in vorticity between early and late conversion only shows in the changes of strong vorticity centers’ location in the upper and lower troposphere.     

Warming in the northwestern Indian Ocean associated with the El Niño event

This paper investigates possible warming effects of an E1 Nifio event on the sea surface temperature anomaly （SSTA） in the northwestern Indian Ocean. Most pure positive Indian Ocean dipole （IOD） events （without an E1 Nifio event co-occurring） have a maximum positive SSTA mainly in the central Indian Ocean south of the equator, while most co-occurrences with an E1 Nifio event exhibit a northwest-southeast typical dipole mode. It is therefore inferred that warming in the northwestern Indian Ocean is closely related to the E1 Nifio event. Based on the atmospheric bridge theory, warming in the northwestern Indian Ocean during co-occurring cases may be primarily caused by relatively less latent heat loss from the ocean due to reduced wind speed. The deepened thermocline also contributes to the warming along the east coast of Africa through the suppressed upwelling of the cold water. Therefore, the E1 Nifio event is suggested to have a modulating effect on the structure of the dipole mode in the tropical Indian Ocean.     

The ENSO Events in the Tropical Pacific and Dipole Events in the Indian Ocean

A depth map (close to that of the thermocline as defined by 20℃) of climatically maximum seatemperature anomaly was created at the subsurface of the tropical Pacific and Indian Ocean, based on which the evolving sea-temperature anomaly at this depth map from 1960 to 2000 was statistically analyzed. It is noted that the evolving sea temperature anomaly at this depth map can be better analyzed than the evolving sea surface one. For example, during the ENSO event in the tropical Pacific, the seatemperature anomaly signals travel counter-clockwise within the range of 10°S-10°N, and while moving, the signals change in intensity or even type. If Dipole is used in the tropical Indian Ocean for analyzing the depth map of maximum sea-temperature anomaly, the sea-temperature anomalies of the eastern and western Indian Oceans would be negatively correlated in statistical sense (Dipole in real physical sense), which is unlike the sea surface temperature anomaly based analysis which demonstrates that the inter-annual positive and negative changes only occur on the gradients of the western and eastern temperature anomalies. Further analysis shows that the development of ENSO and Dipole has a time lag features statistically, with the sea-temperature anomaly in the eastern equatorial Pacific changing earlier (by three months or so). And the linkage between these two changes is a pair of coupled evolving Walker circulations that move reversely in the equatorial Pacific and Indian Oceans.     

Indian summer monsoon influence on the climate in the North Atlantic–European region

Previous studies have shown that climate anomalies over the North Atlantic–Europe (NAE) can influence the Indian summer monsoon (ISM) variability. It is, however, still an outstanding question whether the latter has a significant impact on the former. In this study, observational evidences indicate that the interannual variability of ISM is closely linked to the climate anomalies over NAE. A strong ISM is often associated with significant above normal precipitation over most of western Europe. Meanwhile, positive surface air temperature (SAT) anomalies are usually observed over the Mediterranean, accompanied by below normal SAT in Western Europe during a strong ISM summer. The situation is just opposite during a weak ISM summer. A global primitive equation model is utilized to assess the mechanism of the above observed connection.     

Simulation of the Indian monsoon using the RegCM3–ROMS regional coupled model

A regional coupled atmosphere–ocean model was developed to study the role of air–sea interactions in the simulation of the Indian summer monsoon. The coupled model includes the regional climate model (RegCM3) as atmospheric component and the regional ocean modeling system (ROMS) as oceanic component. The two-way coupled model system exchanges sea surface temperature (SST) from the ocean to the atmospheric model and surface wind stress and energy fluxes from the atmosphere to the ocean model. The coupled model is run for four years 1997, 1998, 2002 and 2003 and the results are compared with observations and atmosphere-only model runs employing Reynolds SSTs as lower boundary condition. It is found that the coupled model captures the main features of the Indian monsoon and simulates a substantially more realistic spatial and temporal distribution of monsoon rainfall compared to the uncoupled atmosphere-only model. The intraseasonal oscillations are also better simulated in the coupled model compared to the atmosphere-only model. These improvements are due to a better representation of the feedbacks between the SST and convection and highlight the importance of air–sea coupling in the simulation of the Indian monsoon.     

Warming in the Northwestern Indian Ocean Associated with the El Nio Event

This paper investigates possible warming effects of an El Nino event on the sea surface temperature anomaly (SSTA) in the northwestern Indian Ocean. Most pure positive Indian Ocean dipole (IOD) events (without an El Nino event co-occurring) have a maximum positive SSTA mainly in the central Indian Ocean south of the equator, while most co-occurrences with an El Nino event exhibit a northwest-southeast typical dipole mode. It is therefore inferred that warming in the northwestern Indian Ocean is closely related to the El Nino event. Based on the atmospheric bridge theory, warming in the northwestern Indian Ocean during co-occurring cases may be primarily caused by relatively less latent heat loss from the ocean due to reduced wind speed. The deepened thermocline also contributes to the warming along the east coast of Africa through the suppressed upwelling of the cold water. Therefore, the El Nino event is suggested to have a modulating effect on the structure of the dipole mode in the tropical Indian Ocean.     

Ocean–atmosphere coupled processes in the tropical Indian Ocean region prior to Indian summer monsoon onset over Kerala

Climate Dynamics - The present study examines atmosphere–ocean interaction before MOK using various observational data sets during the last 35 years (1982–2016). The analyses...     

Is the Indian Ocean SST variability a homogeneous diffusion process?

Whereas the predominance of El Niño Southern Oscillation (ENSO) mode in the tropical Pacific sea surface temperature (SST) variability is well established, no such consensus seems to have been reached by climate scientists regarding the Indian Ocean. While a number of researchers think that the Indian Ocean SST variability is dominated by an active dipolar-type mode of variability, similar to ENSO, others suggest that the variability is mostly passive and behaves like an autocorrelated noise. For example, it is suggested recently that the Indian Ocean SST variability is consistent with the null hypothesis of a homogeneous diffusion process. However, the existence of the basin-wide warming trend represents a deviation from a homogeneous diffusion process, which needs to be considered. An efficient way of detrending, based on differencing, is introduced and applied to the Hadley Centre ice and SST. The filtered SST anomalies over the basin (23.5N–29.5S, 30.5E–119.5E) are then analysed and found to be inconsistent with the null hypothesis on intraseasonal and interannual timescales. The same differencing method is then applied to the smaller tropical Indian Ocean domain. This smaller domain is also inconsistent with the null hypothesis on intraseasonal and interannual timescales. In particular, it is found that the leading mode of variability yields the Indian Ocean dipole, and departs significantly from the null hypothesis only in the autumn season.     

Influences of Madden–Julian Oscillations on the eastern Indian Ocean and the maritime continent

Oceanic response to Madden–Julian Oscillations (MJOs) is studied with satellite data, mooring observations, and reanalysis products to demonstrate that oceanic intraseasonal variabilities are a direct response to the atmospheric intraseasonal forcing. They propagate eastward to the Sumatran coast and southward along the coast to the southeastern Indian Ocean (SEIO) and the maritime continent, as coastal Kelvin waves. MJOs contribute to about 20% of the intraseasonal variabilities in the SEIO and the maritime continent. In addition, MJOs reduce the annual mean Indonesian Throughflow (ITF) and the associated westward temperature advection. However, MJOs only have slight influences on the peak ITF in boreal summer. The importance of INSTANT data is obvious not only for understanding of ITF but also for improving ocean reanalysis and should eventually lead to improved predictive understanding of phenomena such as MJOs.     

On the Spatio-Temporal Variations of the Tropopause Height over India and Indian Summer Monsoon Activity

The spatio-temporal variation of the tropopause height (TH) over the Indian region (5°N-35°N, 70°E-95°E) has been studied using monthly mean TH data, for 22-year period, 1965 to 1986. The study revealed that the stations south of 20° showed maximum TH in April / May and minimum in September. This variation in TH has been attributed to the corresponding variation of average sea surface temperature (SST) over ± 20° latitudinal belt over Indian Ocean, Arabian Sea and Bay of Bengal. Further the stations north of 20°N showed maximum in June and minimum in October/ November. This maximum in TH has primarily been attributed to the increased insolation and convection. Furthermore it is noticed that the anomaly of TH moved northwards during the period April to July.The interannual variability of the Indian Summer Monsoon Activity (ISMA) has been studied in relation to all India mean TH (at 12 GMT) for six months April through September. The composites of mean TH for good and bad monsoon years showed that     

Processes controlling the surface temperature signature of the Madden�CJulian Oscillation in the thermocline ridge of the Indian Ocean

During boreal winter, there is a prominent maximum of intraseasonal sea-surface temperature (SST) variability associated with the Madden?CJulian Oscillation (MJO) along a Thermocline Ridge located in the southwestern Indian Ocean (5°S?C10°S, 60°E?C90°E; TRIO region). There is an ongoing debate about the relative importance of air-sea heat fluxes and oceanic processes in driving this intraseasonal SST variability. Furthermore, various studies have suggested that interannual variability of the oceanic structure in the TRIO region could modulate the amplitude of the MJO-driven SST response. In this study, we use observations and ocean general circulation model (OGCM) experiments to quantify these two effects over the 1997?C2006 period. Observational analysis indicates that Ekman pumping does not contribute significantly (on average) to intraseasonal SST variability. It is, however, difficult to quantify the relative contribution of net heat fluxes and entrainment to SST intraseasonal variability from observations alone. We therefore use a suite of OGCM experiments to isolate the impacts of each process. During 1997?C2006, wind stress contributed on average only about 20% of the intraseasonal SST variability (averaged over the TRIO region), while heat fluxes contributed about 70%, with forcing by shortwave radiation (75%) dominating the other flux components (25%). This estimate is consistent with an independent air-sea flux product, which indicates that shortwave radiation contributes 68% of intraseasonal heat flux variability. The time scale of the heat-flux perturbation, in addition to its amplitude, is also important in controlling the intraseasonal SST signature, with longer periods favouring a larger response. There are also strong year-to-year variations in the respective role of heat fluxes and wind stress. Of the five strong cooling events identified in both observations and the model (two in 1999 and one in 2000, 2001 and 2002), intraseasonal-wind stress dominates the SST signature during 2001 and contributes significantly during 2000. Interannual variations of the subsurface thermal structure associated with the Indian Ocean Dipole or El Ni?o/La Ni?a events modulate the MJO-driven SST signature only moderately (by up to 30%), mainly by changing the temperature of water entrained into the mixed layer. The primary factor that controls year-to-year changes in the amplitude of TRIO, intraseasonal SST anomalies is hence the characteristics of intraseasonal surface flux perturbations, rather than changes in the underlying oceanic state.     

The Kelvin Wave Processes in the Equatorial Indian Ocean during the 2006–2008 IOD Events

The present study investigates the role of Kelvin wave propagations along the equatorial Indian Ocean during the 2006–2008 Indian Ocean Dipole (IOD). The 2006 IOD lasted for seven months, developing in May and reaching its peak in December, while the 2007 and 2008 IODs were short-lived events, beginning in early May and ending abruptly in September, with much weaker amplitudes. Associated with the above IODs, the impulses of the sea surface height (SSH) anomalies reflect the forcing from an intraseasonal time scale, which was important to the evolution of IODs in 2007 and 2008. At the thermocline depth, dominated by the propagation of Kelvin waves, the warming/cooling temperature signals could reach the surface at a particular time. When the force is strong and the local thermocline condition is favorable, the incoming Kelvin waves dramatically impact the sea surface temperature (SST) in the eastern equatorial Indian Ocean. In July 2007 and late July 2008, the downwelling Kelvin waves, triggered by the Madden-Julian Oscillation (MJO) in the eastern and central equatorial Indian Ocean, suppressed the thermocline in the Sumatra and the Java coast and terminated the IOD, which made those events short-lived and no longer persist into the boreal fall season as the canonical IOD does.