dynamical response of equatorial indian ocean to intraseasonal winds zonal flow nonlinear and linear 4 layer ocean models are used to explore the dynamics of intraseasonal 20 90 day periods zonal flow in the equatorial indian ocean the model simulations suggest that the observed 40 60 day zonal surface current is forced primarily by wind associated with the madden julian oscillation mjo which peaks at 40 60 days the strongest spectral peak of zonal flow however occurs at 90 day period in the model and a corresponding 90 day peak appears in the observed sea level data the 90 day current results from the preferential excitation of kelvin and rossby waves by the lower frequency component of intraseasonal wind and from the enhancement by rossby waves reflected from the eastern ocean boundary

origins and dynamics of the 90 day and 30 60 day variations in the equatorial indian ocean abstract sea level observations in the equatorial indian ocean show a dominant spectral peak at 90 days and secondary peaks at 30 60 days over an intraseasonal period 20 90 days a detailed investigation of the origins and dynamics of these variations is carried out using an ocean general circulation model namely the hybrid coordinate ocean model hycom two parallel experiments are performed in the tropical indian ocean basin for the period 1988 2001 one is forced by ncep 3 day mean forcing fields together with the climate prediction center cpc merged analysis of precipitation cmap pentad precipitation and the other is forced by monthly mean fields to help to understand the role played by the wind driven equatorial wave dynamics a linear continuously stratified ocean model is also used both the observed and modeled 90 day sea level anomaly fields and hycom surface current clearly show equatorial kelvin and first meridional mode rossby wave structures that are forced by the 90 day winds the

intraseasonal variability in the upper layer currents observed in the eastern equatorial indian ocean 1 direct observations of the upper ocean velocity in the eastern equatorial indian ocean by an acoustic doppler current profiler from november 2000 to october 2001 on the equator at 90 e demonstrate that the dominant periods of variability in the upper layer zonal and meridional currents are in intraseasonal frequency bands with periods of 30 to 50 days and 10 to 20 days respectively the strong intraseasonal variability in the zonal current obscures the semiannual wyrtki jets which can be seen clearly in the monthly averaged field in addition a zone of strong vertical shear of the zonal current and a distinct equatorial undercurrent with semiannual period are observed the results provide us with a new perspective on importance of the energetic intraseasonal variability in the eastern equatorial indian ocean which indicates strong correlation with the wind variability near the mooring location

understanding the origins of interannual thermocline variations in the tropical indian ocean 1 based on the data analysis of the 1000 hpa wind sst and ssh anomalies it is revealed that the atmospheric variations associated with indian ocean dipole iod or referred as indian ocean zonal dipole mode iozdm consist of a pair of anticyclones closely north and south of the equator with accompanying intense equatorial easterly anomalies while the atmospheric variations related to el nino southern oscillation enso include a strong anticyclone in the southeastern tropical indian ocean tio at higher latitudes with strong along shore wind anomalies near java sumatra coast the different atmospheric forcing patterns lead to the fact that oceanic thermocline variations associated with iod iozdm are more closely confined to the region north of 10 s while enso induced thermocline variations are dominant south of 10 s

subsurface equatorial zonal current in the eastern indian ocean 1 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

impacts of the iod associated temperature and salinity anomalies on the intermittent equatorial undercurrent anomalies the study of equatorial undercurrent euc has attracted a broad attention in recent years due to its strong response and feedback to the indian ocean dipole in this paper we first produce a high quality simulation of three dimensional temperature salinity and zonal current simulation from 1982 to 2014 using a high resolution ocean general circulation model on this basis with two sensitivity experiments we investigate the role of temperature and salinity anomalies in driving and enhancing the euc during the positive iod events by examining the variation of the euc seasonal cycle and diagnosing the zonal momentum budget along the equatorial indian ocean our results show that during january march the euc can appear along the entire equatorial indian ocean in all years but during august november the euc can appear and reach the eastern indian ocean only during the positive iod events the zonal momentum budget analysis indicates that the pressure gradient force contributes most to the variation of the eastward acceleration of zonal currents in the subsurface during the positive iod events strong negative subsurface temperature anomalies exist in the eastern indian ocean with negative surface salinity anomalies in the central and eastern indian ocean resulting in a large pressure gradient force to drive euc during the august november further the results of two sensitivity experiments indicate that the temperature anomalies significantly impact the pressure gradient force playing a leading role in driving the euc while the surface salinity anomalies can secondarily help to intensify the eastward euc through increasing the zonal density gradient in the eastern indian ocean and impacting the vertical momentum advection in the subsurface

structure of intraseasonal kelvin waves in the equatorial pacific ocean abstract previous studies have shown that intraseasonal kelvin waves are a prominent mode of variability in the eastern and central equatorial pacific these waves appear to be remotely forced by wind variations in the western pacific and propagate eastward at approximately first baroclinic mode phase speeds in this study moored temperature and velocity time series data from 1984 to 1987 between 110 and 140 w are studied to isolate the kelvin wave structures and to document their wave mean flow interactions with the south equatorial current equatorial undercurrent ruc system observed structures at periods of 59 125 days have rms amplitudes of about 10 cm s 1 for zonal velocity and about 1 c for temperature in the thermocline these structures exhibit significant departures from linear no mean flow theory zonal velocity is intensified below the euc core and temperature variability in the thermocline is weaker on the equator than 1 5 off the equator additionally the wave temperature signal at the

volume transports of the wyrtki jets and their relationship to the indian ocean dipole 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

short term upper ocean variability in the central equatorial indian ocean during 2006 indian ocean dipole event 1 intensive observations using an array of surface and subsurface moored buoys are conducted around 80 5 e in the equatorial indian ocean during october november 2006 an intriguing data set of atmospheric and oceanic variables during a peak phase of a positive indian ocean dipole is obtained the ocean observation data shows relatively shallow thermocline which intensifies with time during the one month period and eastward subsurface zonal flow under westward flowing surface current generating unusually strong vertical shear above the thermocline intraseasonal meridional current variability is also observed upper ocean volume budget analysis indicates that a strong upwelling event larger than 10 m day and associated upward movement of the isotherms below the thermocline occur for a few days in early november these observed data demonstrates unusual conditions of the upper ocean during boreal autumn in 2006

seasonal cycle of cross equatorial flow in the central indian ocean this study investigates the seasonal cycle of meridional currents in the upper layers of central equatorial indian ocean using acoustic doppler current profiler adcp and other data over the period 2004 2013 the adcp data set collected along 80 5 e is the most comprehensive collection of direct velocity measurements in the central indian ocean to date providing new insights into the meridional circulation in this region we find that mean volume transport is southward across the equator in the central indian ocean in approximate sverdrup balance with the wind stress curl in addition mean westerly wind stress near the equator drives convergent ekman flow in the surface layer and subsurface divergent geostrophic flow in the thermocline at 50 150 m depths in response to a mean northward component of the surface wind stress the maximum surface layer convergence is shifted off the equator to between 0 5 and 1 n evidence is also presented for the existence of a shallow equatorial roll consisting of a northward wind driven surface drift overlaying the southward directed subsurface sverdrup transport seasonal variations are characterized by cross equatorial transports flowing from the summer to the winter hemisphere in quasi steady sverdrup balance with the wind stress curl in addition semiannually varying westerly monsoon transition winds lead to semiannual enhancements of surface layer ekman convergence and geostrophic divergence in the thermocline these results quantify expectations from ocean circulation theories for equatorial indian ocean meridional circulation patterns with a high degree of confidence given the length of the data records

indian ocean response to anomalous conditions in 2006 1 the equatorial indian ocean eio exhibited anomalous conditions characteristic of an indian ocean dipole iod during 2006 the eastern eio had cold sea surface temperature anomalies ssta lower sea level shallow thermocline and higher chlorophyll than normal the anomalies in the east restricted to the south of the equator were highest off sumatra the western pole of the iod was marked by warm ssta and deeper thermocline with maxima on either side of the equator an ocean general circulation model of the indian ocean forced by quikscat winds reproduces the iod of 2006 remarkably well the switch over to cooling in the east and warming in the west happened during may and july respectively in the east airsea heat flux initiated cold ssta in the model which were sustained later by oceanic processes in the west surface heat fluxes and horizontal advection caused warm ssta and contribution by the latter decreased after august citation vinayachandran p n j kurian and c p neema 2007 indian ocean response to anomalous conditions in 2006 geophys res lett 34 l15602 doi 10 1029 2007gl030194

behavior of the wyrtki jet observed with surface drifting buoys and satellite altimeter 1 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 nino southern oscillation enso and indian ocean dipole iod

impact of indian ocean dipole and el nino southern oscillation wind forcing on the wyrtki jets 1 interannual variability of the wyrtki jets is studied in the context of indian ocean dipole iod and el nino and southern oscillation enso wind forcing using a three dimensional numerical ocean model and observations the boreal fall october november wyrtki jet is more significantly affected than the boreal spring april may wyrtki jet since both the iod and enso tend to peak toward the end of the calendar year various statistical methods are used in an attempt to separate the impacts of the iod and enso on these jets with emphasis on the fall jet the first two modes of an empirical orthogonal function eof decomposition account for about 90 and 85 of variability in zonal currents and wind stress respectively along the equator in the indian ocean but eof analysis does not cleanly separate out iod and enso forcing and response partial correlation analysis reveals that iod wind forcing and zonal equatorial current response are stronger on average than for enso and extend further west across the basin composite analysis of iod only enso only and combined iod and enso years provides a complementary definition of the relative contributions of these two phenomena on wyrtki jet variability and in general is consistent with the results of the partial correlation analysis

direct measurements of upper ocean currents and water properties across the tropical pacific during the 1990s abstract meridional sections of upper ocean zonal currents potential temperature and salinity are estimated at ten longitudes from 143 e to 95 w using conductivity temperature depth and acoustic doppler current profiler data from 172 synoptic sections taken in the tropical pacific between 138 e and 86 w mostly in the 1990s data reduction is carried out in a potential isopycnal and mixed layer framework to preserve a sharp pycnocline a mixed layer water property extrema and velocity extrema mean zonal currents potential temperatures and salinities are produced at each longitude the seasonal cycles of these fields are also estimated as well as a simple el nino southern oscillation enso cycle zonal sections along the equator are also presented properties of the near equatorial zonal currents including transports temperatures and salinities are estimated separately from the synoptic sections the seasonal cycles of these quantities and their correlations with the southern oscillation index are investigated the work is distinguished from most existing literature in that direct estimates of zonal velocity are combined with contemporaneous temperature and salinity data allowing trans pacific estimates of near equatorial current transports and properties including those of the northern branch of the south equatorial current the new guinea coastal undercurrent and the equatorial undercurrent

the dynamics of zonal current variations in the central equatorial indian ocean 1 this paper examines the depth integrated zonal momentum balance in the central equatorial indian ocean 0 80 5 e for two periods in 1993 1994 and 2004 2006 using a combination of in situ and satellite observations one to two year record length mean balances are approximately linear and steady state monthly to seasonal time scale variations are also governed by linear dynamics with imbalances between the pressure gradient force and surface wind stress leading to zonal mass transport variations along the equator interannual variations in zonal transports along the equator associated with indian ocean dipole events in 1994 and 2006 are likewise consistent with linear dynamics these results provide a valuable benchmark for evaluating models of equatorial indian ocean circulation on seasonal time scales

dynamics of wind forced intraseasonal zonal current variations in the equatorial indian ocean 1 this study examines the structure and dynamics of wind forced intraseasonal zonal current variability in the equatorial indian ocean we take advantage of a variety of satellite and in situ data sets including unprecedented 4 8 year long velocity time series records from the research moored array for african asian australian monsoon analysis and prediction rama program spectral analysis reveals prominent intraseasonal zonal currents variations along the equator with periods of 30 70 days these oscillations are vertically in phase above the thermocline and propagate eastward with the local zonal winds in the thermocline intraseasonal zonal velocity variations also propagate eastward across a broad range of phase speeds expected for low baroclinic equatorial kelvin waves amplitudes decrease with depth with deeper levels leading those near surface collectively these results suggest that the near surface layer responds directly to intraseasonal zonal wind stress forcing and that subsequently energy radiates downward and eastward in the thermocline in the form of wind forced equatorial kelvin waves in addition intraseasonal zonal current variability on the equator is coherent with off equatorial sea surface height fluctuations in the eastern and central of the basin this coherence is primarily due to the fact that equatorial zonal wind variations are associated with off equatorial wind stress curls that can generate local ekman pumping and westward propagating rossby waves

orbital scale variation of primary productivity in the central equatorial indian ocean maldives during the early pliocene abstract we investigate the response of the central equatorial indian ocean odp site 716 maldives upwelling system to orbital forcing between 3 64 and 4 19 ma this time interval in the early pliocene was characterized by global average temperatures considerably warmer than present and reduced zonal and meridional sea surface temperature gradients across the major oceans chlorin a degradation product of chlorophyll and a potential indicator of primary productivity was previously shown to record orbital scale variations at this site to confirm that chlorin records primary productivity and to understand the mechanism controlling the orbital variations we compare changes in the planktic foraminiferal assemblages with the chlorin content of the sediment cross wavelet analysis demonstrates the in phase relationship between the abundance of thermocline species and chlorin content indicating that upwelling and a shallower thermocline drive the productivity changes the aragonite content in periplatform carbonates is known to reflect variations in sea level and or climate with high values generally indicating sea level highstands the aragonite content in the studied interval shows precession and eccentricity astronomical cycles whereas the primary productivity proxies show only precessional cycles the negative correlation between aragonite content and the relative abundance of thermocline species on the precessional band implies that the primary productivity increased during sea level lowstands this phase coupling is consistent with quaternary productivity records from the central indian ocean that were proposed to be controlled by orbital induced variations of the el nino southern oscillation enso we therefore propose that a similar mechanism was active in the early pliocene despite major changes in the global boundary conditions

numerical simulation of the observed near surface east india coastal current on the continental slope we simulate the east india coastal current eicc using two numerical models resolution 0 1 circ times 0 1 circ an oceanic general circulation model ogcm called modular ocean model and a simpler linear continuously stratified lcs model and compare the simulated current with observations from moorings equipped with acoustic doppler current profilers deployed on the continental slope in the western bay of bengal bob we also carry out numerical experiments to analyse the processes both models simulate well the annual cycle of the eicc but the performance degrades for the intra annual and intraseasonal components in a model resolution experiment both models run at a coarser resolution of 0 25 circ times 0 25 circ simulate well the currents in the equatorial indian ocean eio but the performance of the high resolution lcs model as well as the coarse resolution ogcm which is good in the eicc regime degrades in the eastern and northern bob an experiment on forcing mechanisms shows that the annual eicc is largely forced by the local alongshore winds in the western bob and remote forcing due to ekman pumping over the bob but forcing from the eio has a strong impact on the intra annual eicc at intraseasonal periods local equatorial forcing dominates in the south north because the kelvin wave propagates equatorward in the western bob a stratification experiment with the lcs model shows that changing the background stratification from eio to bob leads to a stronger surface eicc owing to strong coupling of higher order vertical modes with wind forcing for the bob profiles these high order modes which lead to energy propagating down into the ocean in the form of beams are important only for the current and do not contribute significantly to the sea level

intraseasonal variations in the surface layer heat balance of the central equatorial indian ocean the importance of zonal advection and vertical mixing 1 we examine the ocean mixed layer response to intraseasonal atmospheric forcing using moored time series data in the central equatorial indian ocean for october 2004 to march 2005 a period coincident with two active phases of the madden julian oscillation mjo both mjo events were accompanied by a sea surface temperature decrease that was partially the consequence of reduced net surface heat flux in addition during the first event in october november 2004 advection by an enhanced wyrtki jet contributed substantial cooling while during the second event in december 2004 to january 2005 vertical processes most likely related to entrainment mixing were pronounced heavy rainfall at the mooring location during the first event may have contributed to the formation of a 30 40 m thick barrier layer that limited turbulent vertical transfers between the mixed layer and the thermocline there was no barrier layer present during the second event which presumably allowed for much freer vertical turbulent exchanges

seasonal to interannual time scale dynamics of the equatorial undercurrent in the indian ocean abstractthis paper investigates the structure and dynamics of the equatorial undercurrent euc of the indian ocean by analyzing in situ observations and reanalysis data and performing ocean model experiments using an ocean general circulation model and a linear continuously stratified ocean model the results show that the euc regularly occurs in each boreal winter and spring particularly during february and april consistent with existing studies the euc generally has a core depth near the 20 c isotherm and can be present across the equatorial basin the euc reappears during summer fall of most years with core depth located at different longitudes and depths in the western basin the euc results primarily from equatorial kelvin and rossby waves directly forced by equatorial easterly winds in the central and eastern basin however reflected rossby waves from the eastern boundary play a crucial role while the first two baroclinic modes make the largest contribution intermediate modes 3 8 are also

variability of zonal currents in the eastern equatorial indian ocean on seasonal to interannual time scales this study examines equatorial zonal current variations in the upper layers of eastern indian ocean in relation to variations in the indian ocean dipole iod the analysis utilizes data from the research moored array for african asian australian monsoon analysis and prediction rama and the european centre for medium range weather forecasts ocean reanalysis system 4 ecmwf oras4 surface currents are characterized by semiannual eastward flowing wyrtki jets along the equator in boreal spring and fall forced by westerly monsoon transition winds the fall jet intensifies during negative iod niod events when westerlies are anomalously strong but significantly weakens during positive iod piod events when westerlies are anomalously weak as zonal wind stress weakens during piod events sea surface height becomes unusually low in the eastern basin and high in the west setting up an anomalous pressure force that drives increased eastward transport in the thermocline opposite tendencies are evident during niod events in response to intensified equatorial westerlies current transport adjustments to anomalous zonal wind forcing during iod events extend into the following year consistent with the cycling of equatorial wave energy around the basin a surface layer mass budget calculation for the eastern sea surface temperature sst pole of the iod indicates upwelling of 2 9 0 7 sv during normal periods increasing by 40 50 during piod events and reducing effectively to zero during niod events iod related variations in wyrtki jet and thermocline transports are major influences on these upwelling rates and associated water mass transformations which vary consistently with sst changes