Southern Subtropical Indian Ocean Research Articles

Separation of quasi-semiannual Rossby waves from the eastern boundary of the Indian Ocean

Westward propagating features, identified as semiannual Rossby waves, have been observed in the southern subtropical Indian Ocean between 20S and 35S. A previous study based on ERS satellite scatterometer wind stress data in this region has shown that the Rossby waves were not forced by local Ekman pumping in the ocean interior nor by local variations in wind stress near the Australian coast. In the present study, we investigate a third possible source, in the form of remotely forced semiannual coastal waves propagating along the Australian coast. Altimetric and tide gauge data show consistent semiannual anomalies along the northwestern and western coast, with a phase lag between the northernmost station and about 25S, producing a poleward phase speed of ∼0.4 m s -1 . South of the critical latitude for semiannual Rossby wave radiation at 27S, we still observe a significant sea level signal along the coast at the semiannual frequency that appears nearly in phase between the stations. Semiannual sea level anomalies lead the semiannual wind forcing by about one month, confirming that the signal is not locally forced. These semiannual coastal variations are in phase with the offshore radiation of Rossby waves observed between 20S and 35S. We suggest that, south of the critical latitude of 27S, the observed coastal wave signal could interact with the poleward coastal Leeuwin Current and that this wave-current interaction could stimulate nonlinear instabilities of this density-driven current and may be a key mechanism for the Rossby wave radiation in the subtropical Indian Ocean.

A 350,000 year summer-monsoon multi-proxy stack from the Owen Ridge, Northern Arabian Sea

Five summer-monsoon proxies from the Northern Arabian Sea are combined using stacking and principal components analysis (PCA) to create two very similar multi-proxy records of summer-monsoon variability. The five individual proxies all respond to monsoon variability but are largely independent in terms of the processes that complicate their interpretation as summer-monsoon indicators (e.g. preservation, dissolution, diagenesis, sediment reworking). As such, stacking and PCA average out non-monsoon variance, yielding a more pure monsoon signal. These stacked and PCA records (hereafter summer-monsoon stack and summer-monsoon factor) allow evaluation of relative monsoon strength through time as well as the relative concentration of variance within orbital bands; these two parameters are less reliable when estimated from individual proxy records. In fact, the summer-monsoon factor (SMF) accounts for only 33% of the total variance in the five records, suggesting that relative amplitude variations in each individual proxy time series are influenced by non-monsoon processes. The summer-monsoon stack (SMS) and SMF are spectrally very similar, dominated by variance in the 41-k.y. (obliquity) and 23-k.y. (precession) bands; there is very little variance at the 100-k.y. (eccentricity) band associated with large-scale changes in global ice-volume. Indeed, equally strong monsoons occur in both glacial and interglacial intervals. Within the 23-k.y. precession cycle, monsoon maxima fall at −121° relative to precession minima (June 21 perihelion, maximum Northern Hemisphere (NH) summer insolation). This phase falls midway between δ 18O minima (−78°) and December 21 perihelion (−180°) indicating that two mechanisms exert equal influence in determining the timing of strong summer monsoons within the precession band: (1) sensible heating of the Asian Plateau which is maximized at times of ice-volume minima (−78°), and (2) latent heat export from the southern subtropical Indian Ocean which is maximized at times of December 21 perihelion (−180°). The seasonal cycle at December 21 perihelion is characterized by warm Southern Hemisphere (SH) summers followed by cold SH winters, a combination that preconditions the ocean to export latent heat during the boreal summer-monsoon season. Summer-monsoon winds transport this latent heat into Asia where it is released during precipitation, enhancing the Asian monsoon low. Within the 41-k.y. obliquity cycle, monsoon maxima are in phase with obliquity maxima. This indicates that two mechanisms, quite similar to those in the precession band, influence the timing of strong summer-monsoons in the obliquity band: (1) sensible heating of the Asian Plateau but with no ice-volume delay, and (2) latent heat export from the southern subtropical Indian Ocean which is maximized at times of obliquity maxima. Again, the seasonal cycle at obliquity maximum is characterized by warm SH summers followed by cold SH winters, ideal for maximizing latent heat export during the boreal summer monsoon.

Correlation between Indian Ocean summer monsoon and North Atlantic climate during the Holocene

There has been a number of investigations for the correlation between the Asia monsoon and the North Atlantic climate for the last glacial; however, little research has been done for the present interglacial, the Holocene. Here we present for the first time a high-resolution composite proxy record for the Indian Ocean summer monsoon spanning around 12 000 years based on the δ 13C time series of both a single plant species ( Carex mulieensis) remains cellulose and the total plant assemblage cellulose in the Hongyuan peat bog from the Tibet Plateau. The records show that the strength of the Indian Ocean summer monsoon had abrupt variations during the last 12 000 years. The weakest monsoon occurred in the Younger Dryas period. Following rapid strengthening from around 11 200 to 10 800 a BP the monsoon kept a generally strong level for around 5300 years. From around 5500 a BP onwards the monsoon strength tended to gradual decrease. In addition, there are a series of abrupt variation events of the monsoon strength on centennial to millennial time scales, which superimpose the general tendency of the monsoon variation. In every case when the ice-rafted debris events in the North Atlantic occurred, the summer monsoon strength decreased correspondingly. These evidences show that teleconnection between the Indian Ocean summer monsoon and the North Atlantic climate is present not only in the last glacial but also in the Holocene, which may be linked to abrupt reorganizations of the ocean thermohaline circulation, leading to redistribution of energy, changing temperature and moisture gradient over the southern subtropical Indian Ocean, and eventually controlling the variability of the Indian Ocean summer monsoon.

Ocean heat transport and its relationship to ocean circulation in the CMIP coupled models

The Coupled Model Intercomparison Project was set up to examine climate variability and predictability as simulated by coupled atmosphere-ocean general circulation models, and to evaluate the model results against available observations. This study considers the results from a subproject that concerns the ocean heat transport and its relationship to ocean circulation in these coupled models. In particular, the relative roles of the meridional overturning circulation and horizontal circulation in determining the ocean heat transport are assessed and contrasted between the major ocean basins. Many consistent patterns are found among the models and with observations. A number of discrepancies and uncertainties are also identified. Some of the discrepancies have already been seen in ocean only models, and many of the uncertainties are also present in observational studies. In the Atlantic, the meridional overturning circulation that consists of the southward transport of the North Atlantic Deep Water and the northward transport of the upper layer water masses is captured by all the models, and so is the associated northward heat transport, although quantitatively there are large differences. In all the models, the North Atlantic Deep Water is too warm compared with that derived from direct oceanographic measurements, an indication that high-latitude processes responsible for the production and transport of the North Atlantic Deep Water are not well represented. In the Pacific, most of the models produced the upper layer wind-driven circulation. However, none of the models produced a deep circulation pattern close to that deduced from direct oceanographic measurements and consequently the heat transport exhibits large differences between the models. In the Indian Ocean, all models agree on a southward heat transport resulting from a meridional overturning circulation of warm southward flow of upper layer water and cold northward flow of deep water, but disagree on the magnitude of the heat transport and the strength of the overturning. Observational studies agree and disagree in a similar way. All the models produced poleward heat transport by the horizontal circulation in the subtropical regions of the South Atlantic and the Pacific Ocean basins. In the southern subtropical Indian Ocean a number of the models produced equatorward heat transport by the horizontal circulation, contrary to the common belief of a warm western boundary current. In the North Atlantic, a negligible contribution from the horizontal circulation is expected but several models produced significant equatorward heat transport.

Signals of interannual and interdecadal variability of air‐sea interaction in the basin‐wide Indian Ocean

Many climatolo gists are concerned with the variability of air‐sea interactions. In this paper, signals of interannual and interdecadal variability of air‐sea interactions in the southern subtropical Indian Ocean (SSIO) have been extracted from both monthly sea surface temperatures (SST) and monthly surface winds and compared to air‐sea interactions of the tropical Indian Ocean (TIO). Empirical orthogonal function (EOF) expansion has been used in the analysis of observed sea surface temperature anomalies (SSTA) over the basin‐wide Indian Ocean (35°S to 15°N, 30°E to120°E)for 1950–1998. The results show that another dipole structure, with centres in the south‐western subtropics of the ocean and near Australia, is different from the TIO dipole mode in several aspects. The SSIO dipole mode reflects internal variations of the Indian Ocean, exhibiting obvious interannual and interdecadal changes. The variability of this independent SSTA mode, which is represented by a dipole mode index (DMI), is strongly connected with the air‐sea coupling through monsoon precipitation, wind field, and heat flux changes. Examining the wind fields correlated with the SSIO SSTA DMI, an oscillation in the meridional surface wind or the wind DMI is also defined and analyzed. Composite results of wind fields from extreme years and anomalous periods of the SSIO SSTA DMI show that interannual and interdecadal air‐sea interactions are evident in the Indian Ocean and are connected with the activity of the east Asian, the south Asian, the African, and the Australian monsoons.

Subtropical SST dipole events in the southern Indian Ocean

Sea surface temperature (SST) anomalies in the subtropical southern Indian Ocean show interannual dipole events that are seasonally phase‐locked to the austral summer. A positive phase of the event is characterized by cold SST anomalies in the eastern part i.e. off Australia and warm SST anomalies in the southwestern part, south of Madagascar. Such an event is found to produce above normal rainfall over many regions in south‐central Africa. The cooling of SST in the eastern part is mainly caused by the enhanced evaporation. This is associated with stronger winds along the eastern edge of the subtropical high, which is strengthened and shifted slightly to the south during the event. On the other hand, relative decrease in the seasonal latent heat loss due to reduced evaporation dominates the warming in the southwestern part. Evolution of such subtropical dipole events shows quite a contrast to that of the tropical dipole events discovered recently in the Indian Ocean.

Interhemispheric moisture transport in the Indian Ocean summer monsoon: Data‐model and model‐model comparisons

Exchange of energy and mass between the ocean and the atmosphere is an integral component of Indian Ocean summer‐monsoon dynamics. Most previous general circulation model simulations of the monsoon have prescribed sea surface temperature (SST), thus severely constraining atmosphere‐ocean energy exchanges. Here we compare model physics, based on both prescribed and computed SST simulations, to observed climate links between northern hemisphere monsoon strength and the cross‐equatorial transport of moisture (latent heat) from the southern subtropical Indian Ocean. For the computed SST case, stronger southern hemisphere southeast trades and colder southern subtropical SSTs are associated with increased cross‐equatorial moisture flux. Increased moisture flux induces strengthened monsoon circulation via increased condensational heating and decreased sea level pressure over Asia. For the prescribed SST case, stronger southeast trades and colder southern subtropical SSTs are also associated with increased cross‐equatorial moisture flux and, hence, strengthened monsoon circulation. However, in this case, the increased moisture flux is induced by strengthened cross‐equatorial winds associated with subsidence over the cold southern hemisphere SSTs (i.e., increased sea level pressure in the southern subtropics). Thus the model physics associated with changes in monsoon strength are very different for the prescribed and computed SST simulations. Only the computed SST simulation replicates observed meteorological relationships.

Late Quaternary forcing of Indian Ocean summer‐monsoon winds: A comparison of Fourier model and general circulation model results

Past fluctuations in the strength of summer monsoon winds in the Arabian Sea can be inferred from the grain size of lithogenic material blown in from the peripheral deserts. We partition a 400 kiloyear (kyr) time series of inferred wind strength into its dominant frequency components, and develop a Fourier model to evaluate the relative influence of potential monsoon forcing mechanisms operating over these dominant frequencies. Our results indicate that the primary external and internal forcing mechanisms determining the timing and strength of Arabian Sea monsoon winds over the late Quaternary are (1) solar insolation and (2) latent heat collected from the southern subtropical Indian Ocean and released over the Asian Plateau. Our results further suggest that the extent of glacial conditions plays only a minor role in determining the timing and strength of the Arabian Sea paleomonsoon. In contrast, sensitivity experiments using the National Center for Atmospheric Research community climate model (NCAR CCM) indicate that glacial boundary conditions play a dominant role in forcing the strength of Arabian Sea paleomonsoon winds. Such inconsistencies are illustrated by comparing CCM wind strengths to those inferred from geological data. CCM monsoon winds in the Arabian Sea are uniformly strong during interglacial oxygen isotope stages 1 and 5 and weak during glacial stage 2. The geological record, in contrast, indicates strong and weak monsoons during both glacial and interglacial stages. Examining CCM monsoon strength in the context of latent heat flux versus regional sea surface temperatures (SST) suggests that these discrepancies may be attributed to incorrect ocean‐atmosphere heat transfer in the southern hemisphere, possibly associated with the effects of prescribed model SST fields.

Forcing mechanisms of the Indian Ocean monsoon

Sediments in the Arabian Sea provide biological, biogeochemical and lithogenic evidence of past changes in the Indian Ocean summer monsoon winds. For the past 350,000 years, this system has been externally forced by cyclical changes in solar radiation, and internally phase-locked to the transport of latent heat from the southern subtropical Indian Ocean to the Tibetan Plateau. In contrast to the results of general circulation models, these geological data suggest that the climate change associated with variability in global ice volume is not a primary factor in determining the strength and timing of the monsoon winds.