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KARAKTERISTIK MUSIMAN DAN VARIABILITAS ARUS WYRTKI PERIODE 2000 – 2014
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The observational record is too short to confidently differentiate the relative contributions of Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO) on the interannual variability of the equatorial current system in the Indian Ocean because of the strong tendency of these two modes to co-occur. In this study, we analyse a five-decade simulation from an ocean general circulation model forced to describe the main interannual variations of surface and subsurface equatorial zonal currents in the Indian Ocean. This simulation is first shown to accurately capture the surface and subsurface zonal current variations in the equatorial region derived from the available observations. Through an EOF analysis on the model outputs, our results further reveals two main modes of equatorial current interannual variability: a dominant mode with largest amplitude in fall largely describing the variability of the fall Wyrtki jet intensity followed a few months later by a secondary mode maximum in winter largely describing the interannual variability of the subsurface currents in that season. Our analysis further confirms that the IOD is largely responsible for the interannual modulation of fall Wyrtki jet intensity by modulating the equatorial wind intensity during that season. The IOD is also responsible for strong subsurface current variations until December, induced by the delayed effect of the IOD wind signal onto the equatorial thermocline tilt. The equatorial current system response to ENSO is weaker and delayed compared to that of the IOD. The remote and delayed impact of ENSO in the IO indeed induces equatorial wind variations in winter that modulate the winter surface current intensity and the spring equatorial undercurrent intensity through its delayed impact on the thermocline tilt.
In-situ measurement of the upper ocean velocity discloses significant abnormal behaviors of two Wyrtki
Jets (WJs) respectively in boreal spring and fall, over the tropical Indian Ocean in 2013. The two WJs
both occurred within upper 130 m depth and persisted more than one month. The exceptional spring
jet in May was unusually stronger than its counterpart in fall, which is clearly against the previous
understanding. Furthermore, the fall WJ in 2013 unexpectedly peaked in December, one month later
than its climatology. Data analysis and numerical experiments illustrate that the anomalous changes in
the equatorial zonal wind, associated with the strong intra-seasonal oscillation events, are most likely
the primary reason for such anomalous WJs activities.
To what extent the Asian summer monsoon (ASM) rainfall is predictable has been an important but long-standing issue in climate science. Here we introduce a predictable mode analysis (PMA) method to estimate predictability of the ASM rainfall. The PMA is an integral approach combining empirical analysis, physical interpretation and retrospective predictions. The empirical analysis detects most important modes of variability; the interpretation establishes the physical basis of prediction of the modes; and the retrospective predictions with dynamical models and physics-based empirical (P-E) model are used to identify the “predictable” modes. Potential predictability can then be estimated by the fractional variance accounted for by the “predictable” modes.
For the ASM rainfall during June-July-August, we identify four major modes of variability in the domain (20oS-40oN, 40oE-160oE) during 1979-2010: (1) El Niño-La Nina developing mode in central Pacific, (2) Indo-western Pacific monsoon-ocean coupled mode sustained by a positive thermodynamic feedback with the aid of background mean circulation, (3) Indian Ocean dipole mode, and (4) a warming trend mode. We show that these modes can be predicted reasonably well by a set of P-E prediction models as well as coupled models’ multi-model ensemble. The P-E and dynamical models have comparable skills and complementary strengths in predicting ASM rainfall. Thus, the four modes may be regarded as “predictable” modes, and about half of the ASM rainfall variability may be predictable. This work not only provides a useful approach for assessing seasonal predictability but also provides P-E prediction tools and a spatial-pattern-bias correction method to improve dynamical predictions. The proposed PMA method can be applied to a broad range of climate predictability and prediction problems.
Efforts have been made to appreciate the extent to which we can predict the dominant modes of December–January–February (DJF) 2 m air temperature (TS) variability over the Asian winter monsoon region with dynamical models and a physically based statistical model. Dynamical prediction was made on the basis of multi-model ensemble (MME) of 13 coupled models with the November 1 initial condition for 21 boreal winters of 1981/1982–2001/2002. Statistical prediction was performed for 21 winters of 1981/1982–2001/2002 in a cross-validated way and for 11 winters of 1999/2000–2009/2010 in an independent verification. The first four observed modes of empirical orthogonal function analysis of DJF TS variability explain 69 % of the total variability and are statistically separated from other higher modes. We identify these as predictable modes, because they have clear physical meaning and the MME reproduces them with acceptable criteria. The MME skill basically originates from the models’ ability to capture the predictable modes. The MME shows better skill for the first mode, represented by a basin-wide warming trend, and for second mode related to the Arctic Oscillation. However, the statistical model better captures the third and fourth modes, which are strongly related to El Niño and Southern Oscillation (ENSO) variability on interannual and interdecadal timescales, respectively. Independent statistical forecasting for the recent 11-year period further reveals that the first and fourth modes are highly predictable. The second and third modes are less predictable due to lower persistence of boundary forcing and reduced potential predictability during the recent years. In particular, the notable decadal change in the monsoon–ENSO relationship makes the statistical forecast difficult.
Analyses of up-to-date data from satellite-tracked surface drifters indicate that the Wyrtki Jets (WJ) of the equatorial Indian Ocean (EIO) are developed firstly in the central EIO between 75°E and 80°E and then propagate westward along the equator at speeds of about 0.7 m s−1. Climatologically, the fall jet is both stronger and wider than its spring counterpart. This westward propagation phenomenon is supported by altimetry observation. It is suggested that the westward propagation of the jets in the western EIO (55°–75°E) is primarily forced directly by the westward propagating zonal winds. Whereas in the eastern EIO (east to 80°E), propagation of the jet signals is ambiguous although the zonal wind pattern is observed moving east. It is also evident that the WJs are subject to strong interannual variability, which may associate with El Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD).
During 2006-2008, the Indian Ocean (IO) experienced a rare realization of three consecutive positive IO Dipoles (pIODs), including an unusual occurrence with a La Niña in 2007. Common to all three pIODs is an early excitation of equatorial easterly anomalies. Argo profiles reveal that for the 2008 and 2006 pIODs the wind anomalies are generated by the following process: upwelling Rossby waves propagating into the western IO and their subsequent reflection as equatorial upwelling Kelvin waves enhance the seasonal upwelling, changing sea surface temperature (SST) gradients. For the 2007 pIOD, coastal upwelling Kelvin waves off the Sumatra-Java coast associated with the 2006 pIOD/El Niño, radiate into the IO as upwelling Rossby waves. They curve sharply equatorward to arrive at the central equatorial IO, inducing easterly anomalies, upwelling Kelvin waves, and the unusual pIOD. Our results suggest that real-time Argo observations, when assimilated into predictive systems, will enhance IOD forecasting skills.
Association between weakening/strengthening of the eastward equatorial jet (EEJ) in both seasons and the Indian Ocean dipole (IOD) was investigated using two independent observational datasets (October 1992 to September 2007): (a) the dipole mode index I(t) and (b) the 5-day Ocean Surface Current Analyses-Realtime (OSCAR) obtained from satellite altimetry and scatterometer data, which has strong seasonal variability, with the EEJ occurrence in spring and fall, shown from the time-longitude cross-section of equatorial zonal velocity (1°S-1°N). The association is detected in two ways. First, time series of averaged zonal velocity over (1°S-1°N, 42°E-100°E) U(t) shows a close association to the dipole mode index: positive IOD events (1994, 1997, 2006) correspond to negative U (westward equatorial current), and negative IOD events (1994, 1995, 1999, 2005) correspond to positive U (eastward equatorial current). Second, the EEJ weakening/strengthening is represented by the streamfunction anomaly relative to its climatological monthly mean fields. The streamfunction anomaly is further analyzed using the empirical orthogonal function (EOF) method. The first EOF mode accounts for 55% of the variance with corresponding principal component A
(1)(t) showing evident pattern of EEJ strengthening and weakening. The correlation coefficient between I(t) and A
(1)(t) is around 0.49. This may confirm the linkage in some sense (only EOF-1 considered) between the positive (negative) IOD events and the weakening (strengthening) of the EEJ. The dipole pattern of lag-correlation between the sea surface temperature anomaly and U confirms the connection between the EEJ weakening/strengthening and the IOD events.
In recent years, the Indian Ocean (IO) has been discovered to have a
much larger impact on climate variability than previously thought. This
paper reviews climate phenomena and processes in which the IO is, or
appears to be, actively involved. We begin with an update of the IO mean
circulation and monsoon system. It is followed by reviews of
ocean/atmosphere phenomenon at intraseasonal, interannual, and longer
time scales. Much of our review addresses the two important types of
interannual variability in the IO, El Niño-Southern Oscillation
(ENSO) and the recently identified Indian Ocean Dipole (IOD). IOD events
are often triggered by ENSO but can also occur independently, subject to
eastern tropical preconditioning. Over the past decades, IO sea surface
temperatures and heat content have been increasing, and model studies
suggest significant roles of decadal trends in both the Walker
circulation and the Southern Annular Mode. Prediction of IO climate
variability is still at the experimental stage, with varied success.
Essential requirements for better predictions are improved models and
enhanced observations.
[1] The zonal wind in the equatorial Indian Ocean (EqIO) is westerly almost throughout the year. It has a strong semiannual cycle and drives the spring and fall Wyrtki jets. In addition, high resolution daily satellite winds show “westerly wind bursts” lasting 10–40 days, associated with atmospheric convection in the eastern EqIO. These bursts have the potential to produce intraseasonal eastward equatorial jets in the ocean. Using an ocean model driven by QuikSCAT scatterometer winds, we show that strong westerly bursts associated with summer monsoon intraseasonal oscillations can drive “monsoon jets” in the eastern EqIO, which have been observed recently. Although there are distinct equatorial wind bursts associated with Madden-Julian oscillations in January–March, they do not produce equatorial jets in the ocean. The role of ocean dynamics in producing the selective response of the ocean is discussed.
The Tibetan plateau, like any landmass, emits energy into the atmosphere in the form of dry heat and water vapour, but its mean surface elevation is more than 5 km above sea level. This elevation is widely held to cause the plateau to serve as a heat source that drives the South Asian summer monsoon, potentially coupling uplift of the plateau to climate changes on geologic timescales. Observations of the present climate, however, do not clearly establish the Tibetan plateau as the dominant thermal forcing in the region: peak upper-tropospheric temperatures during boreal summer are located over continental India, south of the plateau. Here we show that, although Tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model, the large-scale South Asian summer monsoon circulation is otherwise unaffected by removal of the plateau, provided that the narrow orography of the Himalayas and adjacent mountain ranges is preserved. Additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm, moist air over continental India from the cold and dry extratropics. These results call for both a reinterpretation of how South Asian climate may have responded to orographic uplift, and a re-evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades.
Interaksi laut dan atmosfer baik secara lokal, regional maupun global sangat mempengaruhi variasi temporal arus Wyrtki yang terjadi pada arus permukaan ekuator Samudera Hindia yang bergerak ke arah timur. Kajian ini difokuskan pada variasi musiman dan variasi antar-tahunan (interannual) yang dihubungkan dengan fenomena Indian Ocean Dipole (IOD). Analisis dilakukan dengan menggunakan data Ocean Surface Current Analysis-Real time (OSCAR) Project. Hasil penelitian menunjukkan bahwa arus Wyrtki musim peralihan II (Oktober – November) lebih kuat dan berlangsung lebih lama jika dibandingkan dengan arus Wyrtki musim peralihan I. Arus Wyrtki musim peralihan II membentang di sepanjang ekuator dari bujur 50oBT hingga sisi timur Samudera Hindia. Sementara itu, arus Wyrtki musim peralihan I terkonsentrasi di sisi timur Samudera Hindia. Dalam skala antar-tahunan, arus Wyrtki musim peralihan II termodulasi oleh fenomena IOD. Pada kejadian IOD positif, arus Wyrtki musim peralihan II mengalami pelemahan atau bahkan berbalik arah, sementara pada kejadian IOD negatif arus Wyrtki musim peralihan II mengalami peningkatan intensitas. Pola dan amplitudo arus Wyrtki sangat dipengaruhi oleh pola dan amplitudo angin baratan di atas ekuator Samudera Hindia. Angin baratan pada musim peralihan II lebih kuat dan berlangsung lebih lama dibandingkan dengan angin baratan musim peralihan I. Lebih lanjut lagi, ketika terjadi IOD positif di ekuator Samudera Hindia terdapat angin timuran selama musim peralihan II, sedangkan pada saat IOD negatif angin baratan mengalami peningkatan intensitas
The equatorial Indian Ocean is characterized by strong eastward flows in the upper 80-100 m during boreal spring and fall referred to as the Wyrtki jets. These jets are driven by westerly winds during the transition seasons between the southwest and northeast monsoons and represent a major conduit for mass and heat transfer between the eastern and western sides of the basin. Since their discovery over 40 years ago, there have been very few estimates from direct observations of the volume transports associated with these currents. In this paper we describe seasonal-to-interannual time scale variations in volume transports based on 5 years of unique measurements from an array of acoustic Doppler current profilers in the central equatorial Indian Ocean. The array was centered at 0°, 80.5°E and spanned latitudes between 2.5°N and 4°S from August 2008 to December 2013. Analysis of these data indicates that the spring jet peaks in May at 14.9±2.9 Sv and the fall jet peaks in November at 19.7±2.4 Sv, around which there are year-to-year transport variations of 5-10 Sv. The relationship of the interannual transport variations to zonal wind stress forcing, sea surface temperature, sea surface height, and surface current variations associated with the Indian Ocean Dipole (IOD) are further highlighted. We also illustrate the role of wind-forced equatorial waves in affecting transport variations of the fall Wyrtki jet during the peak season of the IOD. This article is protected by copyright. All rights reserved.
Interannual variability of zonal currents in the eastern equatorial Indian Ocean thermocline is significantly correlated with sea surface temperature (SST) in the eastern pole of the Indian Ocean Dipole (IOD). This relationship is more significant than for zonal currents and the Dipole Mode Index, the latter of which measures zonal SST anomaly gradient. Variability of zonal currents in the thermocline is consistent with variations of eastward pressure gradient force in the thermocline, and the equatorward geostrophic thermocline flows associated with the shallow meridional overturning circulations in the eastern tropical Indian Ocean. Our analysis suggests a positive feedback between zonal currents in the thermocline and SST in the eastern pole of IOD. The combination of equatorial wind and off-equatorial wind stress curls associated with SST anomalies in the eastern pole of the IOD drives interannual variability in equatorial zonal thermocline flows and its covariability with the shallow meridional overturning cells.
The relative importance of local versus remote forcing on intraseasonal-to-interannual sea level and thermocline variability of the tropical south Indian Ocean (SIO) is systematically examined by performing a suite of controlled experiments using an ocean general circulation model and a linear ocean model. Particular emphasis is placed on the thermocline ridge of the Indian Ocean (TRIO; 5°-12°S, 50°-80°E). On interannual and seasonal time scales, sea level and thermocline variability within the TRIO region is primarily forced by winds over the Indian Ocean. Interannual variability is largely caused by westward propagating Rossby waves forced by Ekman pumping velocities east of the region. Seasonally, thermocline variability over the TRIO region is induced by a combination of local Ekman pumping and Rossby waves generated by winds from the east. Adjustment of the tropical SIO at both time scales generally follows linear theory and is captured by the first two baroclinic modes. Remote forcing from the Pacific via the oceanic bridge has significant influence on seasonal and interannual thermocline variability in the east basin of the SIO and weak impact on the TRIO region. On intraseasonal time scales, strong sea level and thermocline variability is found in the southeast tropical Indian Ocean, and it primarily arises from oceanic instabilities. In the TRIO region, intraseasonal sea level is relatively weak and results from Indian Ocean wind forcing. Forcing over the Pacific is the major cause for interannual variability of the Indonesian Throughflow (ITF) transport, whereas forcing over the Indian Ocean plays a larger role in determining seasonal and intraseasonal ITF variability.
An hierarchy of ocean models is used to investigate the dynamics of the eastward surface jets that develop along the Indian Ocean equator during the spring and fall, the Wyrtki jets (WJs). The models vary in dynamical complexity from 2 1/2 -layer to 4 1/2 -layer systems, the latter including active thermodynamics, mixed layer physics, and salinity. To help identify processes, both linear and nonlinear solutions are obtained at each step in the hierarchy. Specific processes assessed are as follows: direct forcing by the wind, reflected Rossby waves, resonance, mixed layer shear, salinity effects, and the influence of the Maldive Islands. In addition, the sensitivity of solutions to forcing by different wind products is reported. Consistent with previous studies, the authors find that direct forcing by the wind is the dominant forcing mechanism of the WJs, accounting for 81% of their amplitude when there is a mixed layer. Reflected Rossby waves, resonance, and mixed layer shear are all necessary to produce jets with realistic strength and structure. Completely new results are that precipitation during the summer and fall considerably strengthens the fall WJ in the eastern ocean by thinning the mixed layer, and that the Maldive Islands help both jets to attain roughly equal strengths. In both the ship-drift data and the authors' 'best' solution (i.e., the solution to the highest model in the authors' hierarchy), the semiannual response is more than twice as large as the annual one, even though the corresponding wind components have comparable amplitudes. Causes of this difference are as follows: the complex zonal structure of the annual wind, which limits the directly forced response at the annual frequency; resonance with the semiannual wind; and mixed layer shear flow, which interferes constructively (destructively) with the rest of the response for the semiannual (annual) component. Even in the most realistic solution, however, the annual component still weakens the fall WJ and strengthens the spring one in the central ocean, in contrast to the ship-drift; this model/data discrepancy may result from model deficiencies, inaccurate driving winds, or from windage errors in the ship-drift data themselves.
A majority of positive Indian Ocean dipole (IOD) events in the last 50 years were accompanied by enhanced summer monsoon circulation and above-normal precipitation over central-north India. Given that IODs peak during boreal autumn following the summer monsoon season, this study examines the role of the summer monsoon flow on the Indian Ocean (IO) response using a suite of ocean model experiments and supplementary data diagnostics. The present results indicate that, if the summer monsoon Hadley-type circulation strengthens during positive IOD events, then the strong off-equatorial southeasterly winds over the northern flanks of the intensified Australian high can effectively promote upwelling in the southeastern tropical Indian Ocean and amplify the zonal gradient of the IO heat content response. While it is noted that a strong monsoon cross-equatorial flow by itself may not generate a dipolelike response, a strengthening (weakening) of monsoon easterlies to the south of the equator during positive IOD events tends to reinforce (hinder) the zonal gradient of the upper-ocean heat content response. The findings show that an intensification of monsoonal winds during positive IOD periods produces nonlinear amplification of easterly wind stress anomalies to the south of the equator because of the nonlinear dependence of wind stress on wind speed. It is noted that such an off-equatorial intensification of easterlies over the SH enhances upwelling in the eastern IO off Sumatra-Java, and the thermocline shoaling provides a zonal pressure gradient, which drives anomalous eastward equatorial undercurrents (EUC) in the subsurface. Furthermore, the combination of positive IOD and stronger-than-normal monsoonal flow favors intensification of shallow transient meridional overturning circulation in the eastern IO and enhances the feed of cold subsurface off-equatorial waters to the EUC.
The zonal circulation south of Sri Lanka is an important link for the exchange of water between the Bay of Bengal and the Arabian Sea. Results from a first array of three moorings along 80° 30'E north of 4° 10'N from January 1991 to March 1992 were used to investigate the Monsoon Current regime [Schott et al., 1994]. Measurements from a second array of six current meter moorings are presented here. This array was deployed along 80° 30'E between 45'S and 5°N from July 1993 to September 1994 to investigate the annual cycle and interannual variability of the equatorial currents at this longitude. Both sets of moorings contribute to the Indian Ocean current meter array ICM8 of the World Ocean Circulation Experiment. The semiannual equatorial jet (EJ) was showing a large seasonal asymmetry, reaching a monthly mean eastward transport of 35 Sv (1Sv=1×106m3s-1) in November 1993, but just 5 Sv in May 1994. The Equatorial Undercurrent (EUC) had a maximum transport of 17 Sv in March to April 1994. Unexpectedly, compared to previous observations and model studies, the EUC was reappearing again in August 1994 at more than 10 Sv transport and was still flowing when the moorings were recovered. In addition, monthly mean ship drifts near the equator are evaluated to support the interpretation of the moored observations. Interannual variability of the EJ in our measurements and ship drift data appears to be related to the variability of the zonal winds and Southern Oscillation Index. The output of a global numerical model (Parallel Ocean Climate Model) driven by the winds for 1993/1994 is used to connect our observations to the larger scale. The model reproduces the EJ asymmetry and shows the existence of the EUC and its reappearance during summer 1994.
The influences of El Niño–Southern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 1950–95 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios. The essential characteristics of the model climatology for the Asian–Australian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding warm and cold ENSO events reveal that a majority of the warm episodes are accompanied by below-normal summer rainfall in India and northern Australia, and above-normal winter rainfall in southeast Asia. The polarity of these anomalies is reversed in the cold events. These relationships are particularly evident in the TOGA experiment. Composite charts of the simulated flow patterns at 850 and 200 mb indicate that the above-mentioned precipitation
Variations of subsurface zonal current in the eastern equatorial Indian Ocean are investigated by examining 6-year data (December 2000–November 2006) from acoustic Doppler current profiler (ADCP) mooring at 0°S, 90°E. The analysis indicates the presence of an eastward equatorial subsurface current between 90 and 170 m depths during both boreal winter and summer. During boreal winter, the generation of eastward pressure gradient, which drives an eastward flow in the thermocline, is caused primarily by upwelling equatorial Kelvin waves excited by prevailing easterly winds. On the other hand, the downwelling Rossby waves generated by the reflection of the spring downwelling Kelvin waves in the eastern boundary, as well as the upwelling equatorial Kelvin waves triggered by easterlies, create an oceanic state that favors the generation of the eastward pressure gradient during boreal summer. The subsurface current reveals a distinct seasonal asymmetry. The maximum eastward speed of 63 cm s−1 is observed in April, and secondary maximum of 49 cm s−1 is seen in October. The zonal transport per unit width within depth of the subsurface current exhibits similar variations: reaching maximum eastward transport of 35 m2 s−1 in April and secondary maximum of 29 m2 s−1 in October. Moreover, the subsurface current during boreal summer undergoes significant interannual variations; it was absent in 2003, but it was anomalously strong during 2006.
This study examines the dynamics of the Wyrtki jets, which are strong equatorial zonal flows that occur typically during boreal spring and fall in the Indian Ocean. Our diagnosis relies primarily on a continuously stratified linear longwave ocean model driven by QuikSCAT zonal winds. Model results, which compare well with satellite altimetry and in situ current observations, indicate that the zonal currents propagate westward along the equator at semiannual periods with an average speed of −1.5 m s−1. This propagation speed is three times faster than the propagation speed of the dominant wave mode in model zonal velocity, namely the first meridional, second baroclinic mode Rossby wave. We interpret this result in terms of a superposition of Rossby waves on a wind-forced jet, with the jet stronger than the waves by a factor of 2. Sea surface height (SSH), on the other hand, shows propagating features that vary in both speed and direction from region to region. This contrasting behavior between SSH and zonal velocity results from differing influences of Kelvin and Rossby wave dynamics on the variability. These results are in many respects analogous to the distinction between SSH and zonal current behavior found in previous studies of the equatorial Pacific and Atlantic oceans on seasonal time scales.
This study examines interannual variability in the equatorial Indian Ocean using observations and a continuously stratified linear long-wave ocean model driven by European Centre for Medium-Range Weather Forecasts winds. Our focus is on the relationship between wind stress, zonal velocity, and sea surface height (SSH) in association with the Indian Ocean dipole (IOD). The model correctly simulates the dominant pattern of variability associated with the IOD in which SSH anomalies near the equator tend to tilt zonally in phase with zonal wind forcing. Both observations and the model also show that surface zonal velocity on the equator tends to lead zonal wind stress by about 1 month on interannual time scales. This phasing occurs because velocity anomalies reverse before the wind anomalies reverse during the decay of IOD events. The model simulations indicate that this reversal of velocity earlier than winds is caused by reflected Rossby waves radiating from the eastern boundary. These results have important implications for understanding the evolution of IOD events because of the role of zonal advection in determining interannual variations in equatorial Indian Ocean sea surface temperature anomalies.
AbstractPrevious studies have investigated how second-baroclinic-mode (n = 2) Kelvin and Rossby waves in the equatorial Indian Ocean (IO) interact to form basin resonances at the semiannual (180 day) and 90-day periods. This paper examines unresolved issues about these resonances, including the reason the 90-day resonance is concentrated in the eastern ocean, the time scale for their establishment, and the impact of complex basin geometry. A hierarchy of ocean models is used: an idealized one-dimensional (1D) model, a linear continuously stratified ocean model (LCSM), and an ocean general circulation model (OGCM) forced by Quick Scatterometer (QuikSCAT) wind during 2000–08. Results indicate that the eastern-basin concentration of the 90-day resonance happens because the westward-propagating Rossby wave is slower, and thus is damped more than the eastward-propagating Kelvin wave. Results also indicate that superposition with other baroclinic modes further enhances the eastern maximum and weakens sea level ...
The onset and interannual variability of the Asian summer monsoon in relation to land-sea thermal contrast and its contributing factors are studied using a 14-yr (1979-1992) dataset. The onset of the Asian summer monsoon is concurrent with the reversal of meridional temperature gradient in the upper troposphere south of the Tibetan Plateau. The reversal is the result of large temperature increases in May to June over Eurasia centered on the Plateau with no appreciable temperature change over the Indian Ocean. In spring the Tibetan Plateau is a heat source that is distinctly separate from the heat source associated with the rain belt in the equatorial Indian ocean. The Tibetan heat source is mainly contributed by sensible heat flux from the ground surface, while the oceanic heat source is due to the release of latent heat of condensation. It is the sensible heating over the Plateau region in spring that leads to the reversal of meridional temperature gradient. Despite its intensity the condensational heating over the Indian Ocean does not result in tropospheric warming because it is offset by the adiabatic cooling of ascending air.A monsoon intensity index, based on the magnitude of the summer mean vertical shear of zonal wind over the North Indian Ocean, is used to compare the years of strong and weak Asian summer monsoon circulation. The strong (weak) Asian summer monsoon years are associated with (a) positive (negative) tropospheric temperature anomalies over Eurasia, but negative (positive) temperature anomalies over the Indian Ocean and the eastern Pacific; (b) negative (positive) SST anomalies in the equatorial eastern Pacific, Arabian Sea, Bay of Bengal. and South China Sea, but positive (negative) SST anomalies in the equatorial western Pacific; and (c) strong (weak) heating and cumulus convection over the Asian monsoon region and the western Pacific, but weaker (stronger) heating and convection in the equatorial Pacific.
At the surface of the Indian Ocean along the equator a narrow, jet-like current flows eastward at high speed during both transition
periods between the two monsoons. The formation of the jet is accompanied by thermocline uplifting at the western origin of
the jet and by sinking at its eastern terminus. This demonstrates that a time-variable current can have profound effects in
changing the mass structure in the ocean.
Intraseasonal variability in the upper layer currents observed in the eastern equatorial Indian Ocean
Y, Masumoto, et al., "Intraseasonal variability
in the upper layer currents observed in the
eastern equatorial Indian Ocean," Journal of
Geophysical Research, vol 32, Issue 2, 2005.