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Abstract Intraseasonal oceanic variations in the Indian Ocean are prominent mode of variability affecting evolution of the Indian Ocean Dipole, the semiannualWyrtki jets, and the Indonesian throughflow. This study describes the structure and propagation of wind-forced intraseasonal Kelvin waves along the equator in the Indian Ocean and their penetration into the Indonesian seas. For this purpose, we use an unprecedented multiyear (2004–2016) moored velocity time series data set from the equatorial central and eastern Indian Ocean, moored time series from Lombok, Makassar, and Ombai straits; satellite altimetry, and other oceanic and atmospheric data products. The waves are generated by zonal wind fluctuations with periods of approximately 20–90 days. They propagate eastward along the equator at speeds close to that of a second baroclinic mode (with estimates ranging between 1.3 and 1.7 ms−1 depending on method) and have an exponential latitudinal decay scale away from the equator of ∼250 km. Phase also propagates upward, implying downward energy propagation, consistent with wind energy source at the surface. As the waves encounter the coast of Sumatra, they partially reflect into Rossby waves that radiate energy westward back into the ocean interior. Some wave energy also propagates southward along the coasts of Sumatra and Java and can be traced into Lombok, Ombai, and Makassar straits at lags of about 2–3 weeks. The implications of these results for understanding variability in the Indian Ocean and its connectivity with the Indonesian seas are discussed. Plain Language Summary We investigate Indian Ocean equatorial Kelvin waves on weekly to monthly timescales and their propagation into the Indonesian seas. For this purpose, we use ocean current velocity data from an unprecedented number of moorings during 2004–2016, plus satellite data and other data sets. Consistent with theoretical expectations, the wave structures decay exponentially away from the equator to the north and south, with a decay scale of approximately 250 km. The phase of the zonal current variations propagates eastward and upward consistent with eastward and downward energy propagation expected for Kelvin waves forced by surface winds. When the waves encounter the west coast of Sumatra, they partially reflect into Rossby waves that propagate westward back into the interior of the ocean. Some energy also continues poleward as coastal waves along Sumatra and Java. These waves eventually make their way into Lombok, Ombai, and Makassar straits in the Indonesian seas, affecting the velocity there. We discuss the implications of these results for understanding broader regional circulation patterns in the Indian Ocean. [ABSTRACT FROM AUTHOR]

The onset of Madden–Julian oscillation (MJO) deep convection often occurs over the western Indian Ocean and has upper-tropospheric circulation precursors that consist of eastward-circumnavigating tropical easterlies and subtropical cyclonic Rossby gyres near eastern Africa. Moreover, the evolution of the large-scale circulation and its ability to reduce subsidence may be necessary for the initial development of organized deep convection. To better understand the evolution of the circulation precursors and their interaction with convective onset, this paper analyzes the upper-tropospheric zonal momentum budget using a regional index based on the temporal progression of the meridional structure of intraseasonal outgoing longwave radiation anomalies over eastern Africa and the western Indian Ocean. The circumnavigating intraseasonal easterly acceleration produces upper-level divergence when it reaches the western extent of a region of intraseasonal westerlies and may provide a forcing for the in-phase midtropospheric upward vertical motion. For about three-quarters of the identified cases, the easterly acceleration over the western Indian Ocean is a response to the zonal pressure gradient over the region. In the composite, the negative pressure gradient force may be initially induced by the injection of negative geopotential height anomalies from the extratropics of both hemispheres to the tropics over eastern Africa, though tropically circumnavigating and local signals may also contribute to the easterly acceleration, especially in the days following convective onset. [ABSTRACT FROM AUTHOR]

The central Indian Ocean (CIO) mode, an intrinsic coupled mode, plays an important role in the intraseasonal variabilities over the Indian monsoon region. Besides the intraseasonal variabilities, the CIO mode also has pronounced seasonal and interannual variabilities. The CIO mode is active during boreal summer but suppressed during boreal winter. The seasonality is mainly attributable to the barotropic instability, which is caused by the large meridional shear of zonal winds. By decomposing the temporal tendency of the meridional gradient of zonal winds, it is found that the zonal wind shear mainly follows the variation of the horizontal eddy flux, which indicates the importance of the multiscale interaction in tropical dynamics. The interannual variability of the CIO mode also depends on the energy transfer associated with the barotropic instability. The influences of El Niño or La Niña and Indian Ocean dipole-zonal mode (IODZM) on the CIO mode are analyzed. El Niño and La Niña have moderate impacts on the CIO mode. El Niño weakens the CIO mode and La Niña strengthens it via the changes in the low-level zonal wind shear. IODZM does not significantly change the amplitude of the CIO mode but can shift its latitudinal position by modifying the meridional shear of the zonal winds. The low-frequency variabilities of the CIO mode at seasonal and interannual time scales unveil the impacts of the background circulations at the intraseasonal variabilities during the Indian summer monsoon in a multiscale framework. While the low-frequency variabilities of this mode will clearly have an implication for monsoon variability and prediction, further studies are needed to quantify the impacts. [ABSTRACT FROM AUTHOR]

Zonal propagation of zonal velocity along the equator in the Indian Ocean and its relationship with wind forcing are investigated with a focus on seasonal time scales using in situ observations from four acoustic Doppler current profilers (ADCPs) and an ocean reanalysis dataset. The results show that the zonal phase speed of zonal currents varies depending on season and depth in a very complicated way in relation to surface wind forcing. Surface layer zonal velocity propagates to the west in northern spring but to the east in fall in response to zonally propagating surface zonal winds, while in the pycnocline zonal phase speed is related to wind-forced ocean wave dynamics. In the western half of the analysis domain (78°-83°E), zonal phase speed in the pycnocline is eastward all year, which is attributed to the radiation of Kelvin waves forced in the western basin. In the eastern half of the domain (80°-90°E), zonal phase speed is westward at 50- to 100-m depths in northern fall, but eastward above and below, most likely due to Rossby waves generated at the eastern boundary. [ABSTRACT FROM AUTHOR]

Previous studies indicate that equatorial zonal winds in the Indian Ocean can significantly influence the Indonesian Throughflow (ITF). During the Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign, two strong MJO events were observed within a month without a clear suppressed phase between them, and these events generated exceptionally strong ocean responses. Strong eastward currents along the equator in the Indian Ocean lasted more than one month from late November 2011 to early January 2012. The influence of these unique MJO events during the field campaign on ITF variability is investigated using a high-resolution (1/25°) global ocean general circulation model, the Hybrid Coordinate Ocean Model (HYCOM). The strong westerlies associated with these MJO events, which exceed 10 m s−1, generate strong equatorial eastward jets and downwelling near the eastern boundary. The equatorial jets are realistically simulated by the global HYCOM based on the comparison with the data collected during the field campaign. The analysis demonstrates that sea surface height (SSH) and alongshore velocity anomalies at the eastern boundary propagate along the coast of Sumatra and Java as coastal Kelvin waves, significantly reducing the ITF transport at the Makassar Strait during January-early February. The alongshore velocity anomalies associated with the Kelvin wave significantly leads SSH anomalies. The magnitude of the anomalous currents at the Makassar Strait is exceptionally large because of the unique feature of the MJO events, and thus the typical seasonal cycle of ITF could be significantly altered by strong MJO events such as those observed during the CINDY/DYNAMO field campaign. [ABSTRACT FROM AUTHOR]

The Indian Ocean Dipole (IOD) is known as a climate mode in the tropical Indian Ocean accompanied by negative (positive) sea surface temperature (SST) anomalies over the eastern (western) pole during its positive phase. However, the western pole of the IOD is not always covered totally by positive SST anomalies. For this reason, the IOD is further classified into two types in this study based on SST anomalies in the western pole. The first type (hereafter 'canonical IOD') is associated with negative (positive) SST anomalies in the eastern (central to western) tropical Indian Ocean. The second type (hereafter 'IOD Modoki'), on the other hand, is associated with negative SST anomalies in the eastern and western tropical Indian Ocean and positive SST anomalies in the central tropical Indian Ocean. Based on composite analyses, it is found that easterly wind anomalies cover the whole equatorial Indian Ocean in the canonical IOD, and as a result, positive rainfall anomalies are observed over East Africa. Also, due to the basin-wide easterly wind anomalies, the canonical IOD is accompanied by strong sea surface height (SSH) anomalies. In contrast, zonal wind anomalies converge in the central tropical Indian Ocean in the IOD Modoki, and no significant precipitation anomalies are found over East Africa. Also, only weak SSH anomalies are seen, because equatorial downwelling anomalies induced by westerly wind anomalies in the west are counteracted by equatorial upwelling anomalies caused by easterly wind anomalies in the east. [ABSTRACT FROM AUTHOR]

The first part of this study examines the driving mechanisms of the equatorial intraseasonal relative atmospheric angular momentum ( AAM) and its dynamical relationship to the upper-tropospheric zonal wind over the Western Hemisphere ( WH) during the convective initiation of the Madden- Julian Oscillation ( MJO) over the Indian Ocean. The budget analysis shows that the main driver of the equatorial intraseasonal AAM anomaly is the meridional transport of momentum induced by the modulation of the background subtropical eddies by the intraseasonal eddies. While the subtropical eddies over the central Pacific basin partly drive the equatorial AAM by meridionally transporting the momentum, the equatorial zonal wind associated with the same subtropical eddies is zonally advected and locally amplified over the east Pacific and Atlantic basins. The common source phenomena that transport momentum result in simultaneous evolution of the WH upper-tropospheric zonal wind and the AAM on intraseasonal time-scales, but their main driving mechanisms are different. The second part of the study investigates the influence of the equatorial intraseasonal AAM state on the subsequent development of initiating MJO convection over the Indian Ocean. In the presence of the WH upper-tropospheric easterly wind, MJO convection tends to develop a stronger enhanced convective envelope when the initiation occurs during the negative intraseasonal AAM state, which strengthens and extends the upper-tropospheric easterly wind in the WH. When the AAM anomaly is positive, it tends to induce stronger mid-tropospheric convergence above the region of convective initiation, thereby suppressing the lower-tropospheric updraught and suppressing the further growth of convection. The results show that the combined effects of the WH circumnavigating circulation and the AAM can influence the subsequent development of MJO convection over the Indian Ocean. [ABSTRACT FROM AUTHOR]

Sea Surface Temperature (SST) anomalies that develop in spring in the central Pacific are crucial to the El Niño Southern Oscillation (ENSO) development. Here we use a linear, continuously stratified, ocean model, and its impulse response to a typical ENSO wind pattern, to derive a simple equation that relates those SST anomalies to the low frequency evolution of zonal wind stress anomalies τ over the preceding months. We show that SST anomalies can be approximated as a 'causal' filter of τ, τ( t − t) − c τ( t − t), where t is ~1-2 months, t − t is ~6 months and c ranges between 0 and 1 depending on τ location (i.e. SST anomalies are approximately proportional to the wind stress anomalies 1-2 months earlier minus a fraction of the wind stress anomalies 7-8 months earlier). The first term represents the fast oceanic response, while the second one represents the delayed negative feedback associated with wave reflection at both boundaries. This simple approach is then applied to assess the relative influence of the Indian Ocean Dipole (IOD) and of the Indian Ocean Basin-wide warming/cooling (IOB) in favouring the phase transition of ENSO. In agreement with previous studies, Atmospheric General Circulation Model experiments indicate that the equatorial Pacific wind responses to the IOD eastern and (IOB-related) western poles tend to cancel out during autumn. The abrupt demise of the IOD eastern pole thus favours an abrupt development of the IOB-cooling-forced westerly wind anomalies in the western Pacific in winter-spring (vice versa for an IOB warming). As expected from the simple SST equation above, the faster wind change fostered by the IOD enhances the central Pacific SST response as compared to the sole IOB influence. The IOD thereby enhances the IOB tendency to favour ENSO phase transition. As the IOD is more independent of ENSO than the IOB, this external influence could contribute to enhanced ENSO predictability. [ABSTRACT FROM AUTHOR]

The superparameterized Community Atmosphere Model (SPCAM) is used to investigate the impact and geographic sensitivity of positive Indian Ocean Dipole (+IOD) sea-surface temperatures (SSTs) on Madden-Julian oscillation (MJO) propagation. The goal is to clarify potentially appreciable +IOD effects on MJO dynamics detected in prior studies by using a global model with explicit convection representation. Prescribed climatological October SSTs and variants of the SST distribution from October 2006, a +IOD event, force the model. Modest MJO convection weakening over the Maritime Continent occurs when either climatological SSTs, or +IOD SST anomalies restricted to the Indian Ocean, are applied. However, severe MJO weakening occurs when either +IOD SST anomalies are applied globally or restricted to the equatorial Pacific. MJO disruption is associated with time-mean changes in the zonal wind profile and lower moist static energy (MSE) in subsiding air masses imported from the Subtropics by Rossby-like gyres. On intraseasonal scales, MJO disruption arises from significantly smaller MSE accumulation, weaker meridional advective moistening, and overactive submonthly eddies that mix drier subtropical air into the path of MJO convection. These results (1) demonstrate that SPCAM reproduces observed time-mean and intraseasonal changes during +IOD episodes, (2) reaffirm the role that submonthly eddies play in MJO propagation and show that such multiscale interactions are sensitive to interannual SST states, and (3) suggest that boreal fall +IOD SSTs local to the Indian Ocean have a significantly smaller impact on Maritime Continent MJO propagation compared to contemporaneous Pacific SST anomalies which, for October 2006, resemble El Niño-like conditions. [ABSTRACT FROM AUTHOR]

Hindcast simulations of MJO events during the dynamics of the MJO (DYNAMO) field campaign are conducted with two models, one with conventional parameterization (CAM5) and a comparable model that utilizes superparameterization (SP-CAM). SP-CAM is shown to produce a qualitatively better reproduction of the fluctuations of precipitation and low-level zonal wind associated with the first two DYNAMO MJO events compared to CAM5. Interestingly, skill metrics using the real-time multivariate MJO index (RMM) suggest the opposite conclusion that CAM5 has more skill than SP-CAM. This inconsistency can be explained by a systematic increase of RMM amplitude with lead time, which results from a drift of the large-scale wind field in SP-CAM that projects strongly onto the RMM index. CAM5 hindcasts exhibit a contraction of the moisture distribution, in which extreme wet and dry conditions become less frequent with lead time. SP-CAM hindcasts better reproduce the observed moisture distribution, but also have stronger drift patterns of moisture budget terms, such as an increase in drying by meridional advection in SP-CAM. This advection tendency in SP-CAM appears to be associated with enhanced off-equatorial synoptic eddy activity with lead time. Systematic drift moisture tendencies in SP-CAM are of similar magnitude to intraseasonal moisture tendencies, and therefore are important for understanding MJO prediction skill. [ABSTRACT FROM AUTHOR]

Although the Indian Ocean dipole (IOD) and ENSO are significantly correlated, there are indeed some IODs independent of ENSO. In this research, the characteristics of independent IOD are investigated and a new triggering mechanism is proposed based on case study and statistical analysis. Results show that the independent IODs peak in an earlier season and have a weaker intensity compared with the IODs associated with ENSO. The wind anomaly associated with the independent IOD is very unique and shows a monsoonlike pattern, in addition to the equatorial easterly wind anomaly (EEWA) common to all IODs. The evolution of the EEWA associated with the independent IOD is well captured by the second EOF mode of the equatorial zonal wind interannual variability, suggesting that the independent IOD is an important climate mode inherent to the tropical Indian Ocean. The EEWA associated with the independent IOD is tightly linked to Indian summer monsoon activities in spring, and the convection anomalies associated with early summer monsoon onset in the Bay of Bengal plays a key role in inducing the EEWA. The EEWA can persist through spring and summer and causes a series of processes similar to those related to the IODs associated with ENSO. The correlation between the independent IOD and Indian summer monsoon activities increases dramatically after the 1980s, which is probably due to the mean state change in the tropical Indian Ocean climate system. [ABSTRACT FROM AUTHOR]

The tropical Indian Ocean has experienced a faster warming rate in the west than in the east over the twentieth century. The warming pattern resembles a positive Indian Ocean dipole (IOD) that is well captured by climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), forced with the two main anthropogenic forcings, long-lived greenhouse gases (GHGs), and aerosols. However, much less is known about how GHGs and aerosols influence the IOD asymmetry, including the negative sea surface temperature (SST) skewness in the east IOD pole (IODE). Here, it is shown that the IODE SST negative skewness is more enhanced by aerosols than by GHGs using single-factor forcing experiments from 10 CMIP5 models. Aerosols induce a greater mean zonal thermocline gradient along the tropical Indian Ocean than that forced by GHGs, whereby the thermocline is deeper in the east relative to the west. This generates strong asymmetry in the SST response to thermocline anomalies between warm and cool IODE phases in the aerosol-only experiments, enhancing the negative IODE SST skewness. Other feedback processes involving zonal wind, precipitation, and evaporation cannot solely explain the enhanced SST skewness by aerosols. An interexperiment comparison in one model with strong skewness confirms that the mean zonal thermocline gradient across the Indian Ocean determines the magnitude of the SST-thermocline asymmetry, which in turn controls the SST skewness strength. The findings suggest that as aerosol emissions decline and GHGs increase, this will likely contribute to a future weakening of the IODE SST skewness. [ABSTRACT FROM AUTHOR]

The Tropical Meteorology Project at Colorado State University currently issues seasonal forecasts for Atlantic basin hurricane activity in early April, June, and August. This paper examines the potential for issuing an additional seasonal forecast on 1 July, using a two-predictor forecast model. The two predictors are selected from the ECMWF Interim Re-Analysis (ERA-Interim) and explain over 60% of the cross-validated variance in post-30 June accumulated cyclone energy over the hindcast period from 1979 to 2012. The two predictors selected are May-June-averaged 2-m temperatures in the eastern tropical and subtropical Atlantic along with May-June 200-mb zonal winds in the tropical Indian Ocean. The May-June-averaged 2-m temperatures are shown to strongly correlate with August-October 2-m temperatures in the main development region, while the 200-mb zonal wind flow over the tropical Indian Ocean is shown to strongly correlate with El Niño-Southern Oscillation. In addition, each predictor is shown to correlate significantly with accumulated cyclone energy, both during the hindcast period of 1979-2012 and with an independent period from 1948 to 1978. [ABSTRACT FROM AUTHOR]

The moist static energy (MSE) anomalies and MSE budget associated with the Madden-Julian oscillation (MJO) simulated in the Iowa State University General Circulation Model (ISUGCM) over the Indian and Pacific Oceans are compared with observations. Different phase relationships between MJO 850-hPa zonal wind, precipitation, and surface latent heat flux are simulated over the Indian Ocean and western Pacific, which are greatly influenced by the convection closure, trigger conditions, and convective momentum transport (CMT). The moist static energy builds up from the lower troposphere 15-20 days before the peak of MJO precipitation, and reaches the maximum in the middle troposphere (500-600 hPa) near the peak of MJO precipitation. The gradual lower-tropospheric heating and moistening and the upward transport of moist static energy are important aspects of MJO events, which are documented in observational studies but poorly simulated in most GCMs. The trigger conditions for deep convection, obtained from the year-long cloud-resolving model (CRM) simulations, contribute to the striking difference between ISUGCM simulations with the original and modified convection schemes and play the major role in the improved MJO simulation in ISUGCM. Additionally, the budget analysis with the ISUGCM simulations shows the increase in MJO MSE is in phase with the horizontal advection of MSE over the western Pacific, while out of phase with the horizontal advection of MSE over the Indian Ocean. However, the NCEP analysis shows that the tendency of MJO MSE is in phase with the horizontal advection of MSE over both oceans. [ABSTRACT FROM AUTHOR]

The tropical Indian Ocean dipole/zonal mode (IOD) is phase locked with the austral winter and spring seasons. This study describes three types of the IOD in terms of their peak time and duration. In particular, the authors focus on a new type that develops in May-June and matures in July-August, which is distinctively different from the canonical IOD, which may develop later and peak in September-November or persist from June to November. Such 'unseasonable' IOD events are only observed since the mid-1970s, a period after which the tropical Indian Ocean has a closer relationship with the Pacific Ocean. The unseasonable IOD is an intrinsic mode of the Indian Ocean and occurs without an ensuing El Niño. A change in winds along the equator is identified as a major forcing. The wind change is in turn related to a weakening Walker circulation in the Indian Ocean sector in austral winter, which is in part forced by the rapid Indian Ocean warming. Thus, although the occurrence of the unseasonable IOD may be partially influenced by oceanic variability, the authors' results suggest an influence from the Indian Ocean warming. This suggestion, however, awaits further investigation using fully coupled climate models. [ABSTRACT FROM AUTHOR]

An examination of the interannual variations of tropical cyclone (TC) activity over the North Indian Ocean during 1983-2008 has been carried out. The results suggest that instead of local sea surface temperatures, such variations, at least over the Bay of Bengal (BB) during October-November-December (OND), can be attributed to similar variations in the atmospheric flow patterns and moist static energy that are apparently forced largely by the El Niño/Southern Oscillation (ENSO). In an El Niño year, conditions for TC genesis and development, including 850-hPa relative vorticity, 200-850-hPa vertical shear of zonal wind, moist static energy, 500-hPa zonal wind, 500-hPa and 850-hPa geopotential height and 200-hPa divergence, are generally less favourable in BB and fewer intense cyclones are observed during OND. The reverse occurs during a La Niña event. However, causes of the variability of TC activity over BB during April-May-June and that over Arabian Sea have yet to be found, which may be due to the small sample size. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The semiannual cycle in zonal wind over the equatorial Indian Ocean is investigated by use of ocean-atmospheric reanalyses, and linear ocean-atmospheric models. In observations, the semiannual cycle in zonal wind is dominant on the equator and confined in the planetary boundary layer (PBL). Results from a momentum budget analysis show that momentum advection generated by the cross-equatorial monsoon circulation is important for the semiannual zonal-wind cycle in the equatorial Indian Ocean. In experiments with a linearized primitive model of the atmosphere, semiannual momentum forcing due to the meridional advection over the central equatorial Indian Ocean is important to simulate the observed maxima of the semiannual cycle in equatorial zonal wind. Off Somalia, diabatic heating and surface friction over land weaken the semiannual response to large momentum forcing there. Results from a linear ocean model suggest that the semiannual zonal wind stress over the central equatorial Indian Ocean generates large semiannual variability in zonal current through a basin-mode resonance. [ABSTRACT FROM AUTHOR]

The impact of mountains and ice sheets on the large-scale circulation of the world''s oceans is investigated in a series of simulations with a new coupled ocean--atmosphere model [Oregon State University--University of Victoria model (OSUVic)], in which the height of orography is scaled from 1.5 times the actual height (at T42 resolution) to 0 (no mountains). The results suggest that the effects of mountains and ice sheets on the buoyancy and momentum transfer from the atmosphere to the surface ocean determine the present pattern of deep ocean circulation. Higher mountains reduce water vapor transport from the Pacific and Indian Oceans into the Atlantic Ocean and contribute to increased (decreased) salinities and enhanced (reduced) deep-water formation and meridional overturning circulation in the Atlantic (Pacific). Orographic effects also lead to the observed interhemispheric asymmetry of midlatitude zonal wind stress. The presence of the Antarctic ice sheet cools winter air temperatures by more than 20°°C directly above the ice sheet and sets up a polar meridional overturning cell in the atmosphere. The resulting increased meridional temperature gradient strengthens midlatitude westerlies by ~25%% and shifts them poleward by ~10°°. This leads to enhanced and poleward-shifted upwelling of deep waters in the Southern Ocean, a stronger Antarctic Circumpolar Current, increased poleward atmospheric moisture transport, and more advection of high-salinity Indian Ocean water into the South Atlantic. Thus, it is the current configuration of mountains and ice sheets on earth that determines the difference in deep-water formation between the Atlantic and the Pacific. [ABSTRACT FROM AUTHOR]

The impact of mountains and ice sheets on the large-scale circulation of the world''s oceans is investigated in a series of simulations with a new coupled ocean--atmosphere model [Oregon State University--University of Victoria model (OSUVic)], in which the height of orography is scaled from 1.5 times the actual height (at T42 resolution) to 0 (no mountains). The results suggest that the effects of mountains and ice sheets on the buoyancy and momentum transfer from the atmosphere to the surface ocean determine the present pattern of deep ocean circulation. Higher mountains reduce water vapor transport from the Pacific and Indian Oceans into the Atlantic Ocean and contribute to increased (decreased) salinities and enhanced (reduced) deep-water formation and meridional overturning circulation in the Atlantic (Pacific). Orographic effects also lead to the observed interhemispheric asymmetry of midlatitude zonal wind stress. The presence of the Antarctic ice sheet cools winter air temperatures by more than 20°°C directly above the ice sheet and sets up a polar meridional overturning cell in the atmosphere. The resulting increased meridional temperature gradient strengthens midlatitude westerlies by ~25%% and shifts them poleward by ~10°°. This leads to enhanced and poleward-shifted upwelling of deep waters in the Southern Ocean, a stronger Antarctic Circumpolar Current, increased poleward atmospheric moisture transport, and more advection of high-salinity Indian Ocean water into the South Atlantic. Thus, it is the current configuration of mountains and ice sheets on earth that determines the difference in deep-water formation between the Atlantic and the Pacific. [ABSTRACT FROM AUTHOR]

The sea level pressure (SLP) variability in 30–60 day intraseasonal timescales is investigated using 25 years of reanalysis data addressing two issues. The first concerns the non-zero zonal mean component of SLP near the equator and its meridional connections, and the second concerns the fast eastward propagation (EP) speed of SLP compared to that of zonal wind. It is shown that the entire globe resonates with high amplitude wave activity during some periods which may last for few to several months, followed by lull periods of varying duration. SLP variations in the tropical belt are highly coherent from 25°S to 25°N, uncorrelated with variations in mid latitudes and again significantly correlated but with opposite phase around 60°S and 65°N. Near the equator (8°S–8°N), the zonal mean contributes significantly to the total variance in SLP, and after its removal, SLP shows a dominant zonal wavenumber one structure having a periodicity of 40 days and EP speeds comparable to that of zonal winds in the Indian Ocean. SLP from many of the atmospheric and coupled general circulation models show similar behaviour in the meridional direction although their propagation characteristics in the tropical belt differ widely. [ABSTRACT FROM AUTHOR]

Using a set of models, including one with a resolution of ¼°, several aspects of the simulated seasonal currents in the deep ocean are considered. It is shown that over vast areas of the deep interior, particularly in the Indian Ocean, annual-mean circulation represents a small residual of much stronger seasonal flows. In many places the seasonal horizontal velocities are of the order of 10-2 m s-1, reaching locally to 10-1 m s-1; the corresponding vertical velocities are of the order of 10-5 m s-1. An idealized geometry model is employed to confirm the notion that much of this seasonal variability in the deep-ocean circulation can be attributed to the annual cycle of wind stress, combined with the significant increase in the vertical trapping depth for basin-scale seasonal forcing. It is suggested that, at least on seasonal time scales, the so-called bottom pressure torque can be an important term in the depth-integrated vorticity balance. An interaction of these relatively strong flows (of nontidal origin) with bottom topography may contribute to diapycnal mixing in the deep ocean in a manner similar to that proposed recently for the Southern Ocean. In addition, it is found that under a plausible climate change scenario, the amplitude of the mean annual cycle of wind stress may change. Among the regions where such changes are most pronounced is that in the extratropical North Pacific. It is shown that the data on surface wind stress can be effectively used to identify the seasons with the largest changes in the deep-reaching overturning cells. Finally, unlike what might be expected from the earlier theories, the annual-mean circulation simulated by the model with ¼° resolution has the deep interior flows that tend to group into jetlike structures, often having a predominant equatorward rather than poleward direction. [ABSTRACT FROM AUTHOR]

While the canonical life cycle of the Madden–Julian oscillation (MJO) is relatively well studied, little attention has been given to the determination of its periodicity and the detailed low-level dynamic structure during the onset phase. Here we present observational evidence of significant dynamical linkage between MJO convection and life span of MJO events. It is revealed that over the Indian Ocean, the low-level moisture convergence (LLMC) induced by easterly zonal wind anomaly and equatorward converging meridional wind anomaly preconditions about 3–5 days prior to the development of onset convection. Further, it is explicitly shown that the stronger LLMC tends to induce more intense MJO convection in the Indian Ocean, indicating that the LLMC paradigm is effectively operative in the initiation of MJO convection. The results show that the life span of the MJO is determined by the strength of the convective coupling with the large-scale circulation. That is, stronger MJO convection exhibits a longer life span due to stronger coupling between the tropical convection and circulation, and hence the slower eastward propagation of low-level circulation. In contrast, the weak MJO events favor a short period of the cycle and bear a similarity to the convectively coupled equatorial Kelvin waves. Thus, the greater LLMC tends to induce the greater MJO convection, which in turn gives rise to the strong coupling with lower-level circulation and propagating slowly to the east. The linear relationship between the convection strength and life span is shown to be greater in boreal winter. [ABSTRACT FROM AUTHOR]

The structures and mechanism of the northward-propagating boreal summer intraseasonal oscillation (BSISO) in the southern Asian monsoon region are simulated and investigated in a three-dimensional intermediate model (3D model). The horizontal structure of the intraseasonal variability in the 3D model depicts the Kelvin–Rossby wave–type disturbance, which may or may not produce the northward-propagating disturbance in the Indian Ocean, depending on the seasonal-mean background winds. Two experiments are conducted in order to identify what characteristic of seasonal-mean background can induce the northwestward-tilted band in the Kelvin–Rossby wave, whose overall eastward movement gives the impression of the northward propagation at a given longitude. When the prescribed boreal summer mean winds are excluded in the first experiment, the phase difference between the barotropic divergence tendency and convection disappears. Consequently, the Rossby wave–type convection forms a zonally elongated band. As a result, the northward propagation of convection at a given longitude disappears. When the easterly vertical shear is introduced in the second experiment, the horizontal and the vertical structures of BSISO become similar to that of the northward-propagating one. The reoccurrence of the northwestward-directed convective band and the phase difference between the barotropic divergence tendency and the convection suggest that the summer mean zonal winds in the boreal Indian summer monsoon region are a critical condition that causes the horizontal and vertical structures of northward-propagating BSISO in the southern Asian monsoon region. [ABSTRACT FROM AUTHOR]

Using 200 yr of coupled general circulation model (CGCM) results, causes for the termination of Indian Ocean dipole (IOD) events are investigated. The CGCM used here is the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F1) model, which consists of a version of the European Community–Hamburg (ECHAM4.6) atmospheric model and a version of the Ocean Parallelise (OPA8.2) ocean general circulation model. This model reproduces reasonably well the present-day climatology and interannual signals of the Indian and Pacific Oceans. The main characteristics of the intraseasonal disturbances (ISDs)/oscillations are also fairly well captured by this model. However, the eastward propagation of ISDs in the model is relatively fast in the Indian Ocean and stationary in the Pacific compared to observations. A sudden reversal of equatorial zonal winds is observed, as a result of significant intraseasonal disturbances in the equatorial Indian Ocean in November–December of IOD events, which evolve independently of ENSO. A majority of these IOD events (15 out of 18) are terminated mainly because of the 20–40-day ISD activity in the equatorial zonal winds. Ocean heat budget analysis in the upper 50 m clearly shows that the initial warming after the peak of the IOD phenomenon is triggered by increased solar radiation owing to clear-sky conditions in the eastern Indian Ocean. Subsequently, the equatorial jets excited by the ISD deepen the thermocline in the southeastern equatorial Indian Ocean. This deepening of the thermocline inhibits the vertical entrainment of cool waters and therefore the IOD is terminated. IOD events that co-occur with ENSO are terminated owing to anomalous incoming solar radiation as a result of prevailing cloud-free skies. Further warming occurs seasonally through the vertical convergence of heat due to a monsoonal wind reversal along Sumatra–Java. On occasion, strong ISD activities in July–August terminated short-lived IOD events by triggering downwelling intraseasonal equatorial Kelvin waves. [ABSTRACT FROM AUTHOR]

New satellite and in situ observations show large intraseasonal (10–60 day) variability of surface winds and upper-ocean current in the equatorial Indian Ocean, particularly in the east. An ocean model forced by the Quick Scatterometer (QuikSCAT) wind stress is used to study the dynamics of the intraseasonal zonal current. The model has realistic upper-ocean currents and thermocline depth variabilities on intraseasonal to interannual scales. The quality of the simulation is directly attributed to the accuracy of the wind forcing. At the equator, moderate westerly winds are punctuated by strong 10–40-day westerly wind bursts. The wind bursts force swift, intraseasonal (20–50 day) eastward equatorial jets in spring, summer, and fall. The zonal momentum balance is between local acceleration, stress, and pressure, while nonlinearity deepens and strengthens the eastward current. The westward pressure force associated with the thermocline deepening toward the east rapidly arrests eastward jets and, subsequently, generates (weak) westward flow. Thus, in accord with direct observations in the east, the spring jet is a single intraseasonal event, there are intraseasonal jets in summer, and the fall jet is long lived but strongly modulated on an intraseasonal scale. The zonal pressure force is almost always westward in the upper 120 m, and changes sign twice a year in the 120–200-m layer. Transient eastward equatorial undercurrents in early spring and late summer are associated with semiannual Rossby waves generated at the eastern boundary following thermocline deepening by the spring and fall jets. An easterly wind stress is not necessary to generate the undercurrents. Experiments with a single westerly wind burst forcing show that apart from the intraseasonal response, the zonal pressure force and current in the east have an intrinsic 90-day time scale that arises purely from equatorial adjustment. [ABSTRACT FROM AUTHOR]

The article presents a study on Indian summer monsoon rainfall and its link with El Niño Southern Oscillation (ENSO) and Indian Ocean climate indices. It examines data from 1881 to 1998 on anomalies in zonal wind and sea surface temperature (SST) which were used as indices for the Indian Ocean. The study uses multiple regression with the NINO3 model to illustrate the relationship between Indian summer monsoon rainfall and zonal winds. It found that there is a negative correlation between Indian summer monsoon rainfall and zonal winds during El Niño. It also found that there is a weak relationship between Indian summer monsoon rainfall and the SST index.

Centered composite analysis is described and applied to gain a better understanding of the initial phases of the Madden–Julian oscillation (MJO). Centered composite analysis identifies the dates and central locations of key events. The elements of the composite means are centered on these central locations before averages are calculated. In this way much of the spatial fuzziness, which is inherent in traditional composite analysis, is removed. The results for the MJO, based on MJO-filtered outgoing longwave radiation for the reference data and 40-yr ECMWF Re-Analysis (ERA-40) and NCEP–NCAR reanalysis products for the composites, show highly significant composites of unfiltered data for not only zero lag, but also lags back to 20 days before the target events. These composites identify propagating patterns of surface pressure, upper- and lower-troposphere zonal winds, surface temperature, and 850-hPa specific humidity associated with MJO convective events in the Indian Ocean. The propagation characteristics of important features, especially surface pressure, differ substantially for MJO convective anomalies centered over the Indian or western Pacific Oceans. This suggests that distinctly different mechanisms may be dominant in these two regions, and that many earlier analyses may be mixing properties of the two. [ABSTRACT FROM AUTHOR]

An ocean general circulation model (OGCM) is used to study the roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial Indian Ocean. The western boundary reflection is defined as the total Kelvin waves leaving the western boundary, which include the reflection of the equatorial Rossby waves as well as the effects of alongshore winds, off-equatorial Rossby waves, and nonlinear processes near the western boundary. The evaluation of the reflection is based on a wave decomposition of the OGCM results and experiments with linear models. It is found that the alongshore winds along the east coast of Africa and the Rossby waves in the off-equatorial areas contribute significantly to the annual harmonics of the equatorial Kelvin waves at the western boundary. The semiannual harmonics of the Kelvin waves, on the other hand, originate primarily from a linear reflection of the equatorial Rossby waves. The dynamics of a dominant annual oscillation of sea level coexisting with the dominant semiannual oscillations of surface zonal currents in the central equatorial Indian Ocean are investigated. These sea level and zonal current patterns are found to be closely related to the linear reflections of the semiannual harmonics at the meridional boundaries. Because of the reflections, the second baroclinic mode resonates with the semiannual wind forcing; that is, the semiannual zonal currents carried by the reflected waves enhance the wind-forced currents at the central basin. Because of the different behavior of the zonal current and sea level during the reflections, the semiannual sea levels of the directly forced and reflected waves cancel each other significantly at the central basin. In the meantime, the annual harmonic of the sea level remains large, producing a dominant annual oscillation of sea level in the central equatorial Indian Ocean. The linear reflection causes the semiannual harmonics of the incoming and reflected sea levels to enhance each other at the meridional boundaries. In addition, the weak annual harmonics of sea level in the western basin, resulting from a combined effect of the western boundary reflection and the equatorial zonal wind forcing, facilitate the dominance by the semiannual harmonics near the western boundary despite the strong local wind forcing at the annual period. The Rossby waves are found to have a much larger contribution to the observed equatorial semiannual oscillations of surface zonal currents than the Kelvin waves. The westward progressive reversal of seasonal surface zonal currents along the equator in the observations is primarily due to the Rossby wave propagation. [ABSTRACT FROM AUTHOR]

Effects of atmospheric intraseasonal oscillations (ISOs) on the Indian Ocean zonal dipole mode (IOZDM) are investigated by analyzing available observations and a suite of solutions to an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Data and model solutions for the period 1991–2000 are analyzed, a period that includes two strong IOZDM events, during 1994 and 1997, and a weak one, in 1991. Both the data analysis and model results suggest that atmospheric ISOs play a significant role in causing irregularity of the two strong IOZDM events and the premature termination of the weak one. Of particular interest is a basinwide, wind-driven oceanic resonance with a period near 90 days, involving the propagation of equatorial Kelvin and first-meridional-mode Rossby waves across the basin. Before the onset of the strong 1997 dipole, wind variability had significant power near 90 days, and the resonance was strongly excited. Associated with the resonance was a deepened thermocline in the eastern basin during August and early September, which reduced the upwelling in the eastern antinode region of the IOZDM, thereby delaying the reversal of the equatorial zonal SST gradient—an important indicator of a strong IOZDM—by over a month. A similar deepened thermocline in the eastern basin also contributed to the premature termination of the weak 1991 dipole. During the 1994 IOZDM, the winds had little power near 90 days, and the resonant mode was not prominent. The ISOs influenced the IOZDM through both surface fluxes and thermocline variability. They enhanced warming in the western antinode region during October, the peak phase of the IOZDM, intensifying its strength. During November, strong winds significantly cooled the western and central basin through upwelling and surface fluxes, cooling SST there and contributing to the early and quick termination of the 1994 event. [ABSTRACT FROM AUTHOR]

An ocean general circulation model (OGCM) is used to investigate the low-frequency (period longer than 90 days) rectification of atmospheric intraseasonal variability (10–90-day periods) in zonal surface current and transport of the equatorial Indian Ocean. A hierarchy of OGCM solutions is found in an actual tropical Indian Ocean basin for the period of 1988–2001. To help to identify and isolate nonlinear processes, a linear continuously stratified model and a 4½-layer intermediate ocean model are also used. Results from the OGCM solutions suggest that intraseasonal atmospheric forcing acts to weaken the equatorial seasonal surface currents. Amplitudes of the spring and autumn eastward surface jets, the Wyrtki jets (WJ), and the westward surface current during January­March are reduced by as much as 15–25 cm s&sup-1; by intraseasonal atmospheric forcing, and strengths of the rectification exhibit a significant interannual variability. Important processes that cause the low-frequency rectification are asymmetric response of mixed layer depth to easterly and westerly winds, entrainment, and upwelling of momentum. During spring and autumn, the westerly (easterly) phase of an intraseasonal event enhances (weakens or even reverses) the seasonal westerly winds, increases (decreases) equatorial convergence and entrainment, and thus deepens (thins) the mixed layer. A net, westward current is generated over an event mean because easterly wind acts on a thinner surface mixed layer whereas westerly wind acts on a thicker one. In contrast, during January­March when the seasonal winds are equatorial easterlies, surface currents are westward and equatorial undercurrents (EUC) develop. The rectified surface currents are eastward, which reduces the westward surface flow. This eastward rectification results largely from the vertical advection and entrainment of the EUC. The seasonal-to-interannual variability of the rectified surface flow is determined primarily from the seasonal cycle and interannual variability of the background state. Seasonal-to-interannual variability of the intraseasonal wind forcing also contributes. The rectified low-frequency zonal volume (heat) transports integrated over the entire water column along the Indian Ocean equator are persistently eastward with an amplitude of 0–15 × 106 m³ s&sup-1; (0–1.2 pW). This is because westerly winds generate equatorial downwelling, advecting the surface eastward momentum downward and giving an eastward subsurface current. Easterly winds cause equatorial upwelling and produce an eastward pressure gradient force that drives an eastward subsurface current. This eastward subsurface current is advected upward by upwelling. The mean effect over an intraseasonal event at the equator is to increase the eastward transport in the water column. In the layers above the thermocline, the rectified zonal volume (heat) transports are in the same direction as the rectified surface currents. Results from this paper may have important implications for understanding climate variability because modification of WJ strength and transports can affect the SST and heat storage in the equatorial Indian Ocean warm pool. [ABSTRACT FROM AUTHOR]

Based on the ensemble empirical mode decomposition of low‐level winds and outgoing longwave radiation, this paper analyzes the influence of tropical intraseasonal oscillations (ISOs) on one extremely late case of South China Sea (SCS) summer monsoon withdrawal (SMW). Compared to the climatological monsoon withdrawal of September 25, the South China Sea summer monsoon withdrawal in 2010 was delayed for approximately 1 month (October 26) and is the latest in the past 60 years. The quasi‐biweekly oscillation (QBWO) and Madden‐Julian oscillation (MJO) originating from the equatorial Indian Ocean propagate northeastward to the SCS. The accompanying circulation first induced a westerly burst event and prolonged the lifespan of the summer monsoon. After this transitory recovery, the circulation anomalies associated with the current propagation of ISOs into the SCS contributed to the large‐scale circulation adjustment, leaded to the shift of zonal wind (from westerlies to easterlies) and triggered the SCSSM withdrawal. This case study shows that, in addition to the well‐studied impacts of ISOs on the monsoon onset and active/break cycle, the ISOs can also trigger the monsoon withdrawal. (a) The October rainfall at Haikou station (bar, units in mm) and the SCSSMW pentad (line). (b) OLR (shading, units in W/m2) and winds (vectors, units in m/s) at 850 hPa averaged in 110°–120°E during 2010. Westerlies are highlighted by the bold vectors, and wind speed less than 5 m/s are omitted. [ABSTRACT FROM AUTHOR]