Simulated and observed circulation in the Indonesian Seas: 1/12° global HYCOM and the INSTANT observations

A 1/12° global version of the HYbrid Coordinate Ocean Model (HYCOM) using 3-hourly atmospheric forcing is analyzed and directly compared against observations from the International Nusantara STratification ANd Transport (INSTANT) program that provides the first long-term (2004–2006) comprehensive view of the Indonesian Throughflow (ITF) inflow/outflow and establishes an important benchmark for inter-basin exchange, including the net throughflow transport. The simulated total ITF transport (−13.4 Sv) is similar to the observational estimate (−15.0 Sv) and correctly distributed among the three outflow passages (Lombok Strait, Ombai Strait and Timor Passage). Makassar Strait carries ∼75% of the observed total ITF inflow and while the temporal variability of the simulated transport has high correlation with the observations, the simulated mean volume transport is ∼37% too low. This points to an incorrect partitioning between the western and eastern inflow routes in the model and is the largest shortcoming of this simulation. HYCOM simulates the very deep (>1250 m) overflow at Lifamatola Passage (−2.0 Sv simulated vs. −2.5 Sv observed) and indicates overflow contributions originating from the North (South) Equatorial Current in boreal winter–spring (summer–autumn). A new finding of INSTANT is the mean eastward flow from the Indian Ocean toward the interior Indonesian Seas on the north side of Ombai Strait. This flow is not robustly simulated at 1/12° resolution, but is found in a 1/25° version of global HYCOM using climatological forcing, indicating the importance of horizontal resolution. However, the 1/25° model also indicates that the mean eastward flow retroflects, turning back into the main southwestward Ombai Strait outflow, and in the mean does not enter the interior seas to become part of the water mass transformation process. The 1/12° global HYCOM is also used to fill in the gaps not measured as part of the INSTANT observational network. It indicates the wide and shallow Java and Arafura Seas carry −0.8 Sv of inflow and that the three major outflow passages capture nearly all the total Pacific to Indian Ocean throughflow.     

Pathways of intraseasonal variability in the Indonesian Throughflow region

The recent INSTANT measurements in the Indonesian archipelago revealed a broad spectrum of time scales that influence Indonesian Throughflow (ITF) variability, from intraseasonal (20–90 days) to interannual. The different time scales are visible in all transport and property fluxes and are the result of remote forcing by both the Pacific and Indian Ocean winds, and local forcing generated within the regional Indonesian seas. This study focuses on the time-dependent three-dimensional intraseasonal variability (ISV) in the ITF region, in particular at the locations of the INSTANT moorings at the Straits of Lombok, Ombai and Timor. Observations from the INSTANT program in combination with output from the Bluelink ocean reanalysis provide a comprehensive picture about the propagation of ISV in the ITF region. The reanalysis assimilates remotely sensed and in situ ocean observations into an ocean general circulation model to create a hindcast of ocean conditions. Data from the reanalysis and observations from the INSTANT program reveal that deep-reaching subsurface ISV in the eastern Indian Ocean and ITF is closely linked with equatorial wind stress anomalies in the central Indian Ocean. Having traveled more than 5000 km in about 14 days, the associated Kelvin waves can be detected as far east as the Banda Sea. ISV near the Straits of Ombai and Timor is also significantly influenced by local wind forcing from within the ITF region. At the INSTANT mooring sites the ocean reanalysis agrees reasonably well with the observations. Intraseasonal amplitudes are about ±1.0 °C and ±0.5 m/s for potential temperature and velocity anomalies. Associated phases of ISV are very similar in observations and the reanalysis. Where differences exist they can be traced back to likely deficits in the reanalysis, namely the lack of tidal dissipation, insufficient spatial resolution of fine-scale bathymetry in the model in narrow straits or errors in surface forcing.     

The seasonal variation of the upper layers of the South China Sea (SCS) circulation and the Indonesian through flow (ITF): An ocean model study

The upper layer, wind-driven circulation of the South China Sea (SCS), its through-flow (SCSTF) and the Indonesian through flow (ITF) are simulated using a high resolution model, FVCOM (finite volume coastal ocean model) in a regional domain comprising the Maritime Continent. The regional model is embedded in the MIT global ocean general circulation model (ogcm) which provides surface forcing and boundary conditions of all the oceanographic variables at the lateral open boundaries in the Pacific and Indian oceans. A five decade long simulation is available from the MITgcm and we choose to investigate and compare the climatologies of two decades, 1960–1969 and 1990–1999.The seasonal variability of the wind-driven circulation produced by the monsoon system is realistically simulated. In the SCS the dominant driving force is the monsoon wind and the surface circulation reverses accordingly, with a net cyclonic tendency in winter and anticyclonic in summer. The SCS circulation in the 90s is weaker than in the 60s because of the weaker monsoon system in the 90s. In the upper 50 m the interaction between the SCSTF and ITF is very important. The southward ITF can be blocked by the SCSTF at the Makassar Strait during winter. In summer, part of the ITF feeds the SCSTF flowing into the SCS through the Karimata Strait. Differently from the SCS, the ITF is primarily controlled by the sea level difference between the western Pacific and eastern Indian Ocean. The ITF flow, consistently southwestward below the surface layer, is stronger in the 90s.The volume transports for winter, summer and yearly are estimated from the simulation through all the interocean straits. On the annual average, there is a ∼5.6 Sv of western Pacific water entering the SCS through the Luzon Strait and ∼1.4 Sv exiting through the Karimata Strait into the Java Sea. Also, ∼2 Sv of SCS water enters the Sulu Sea through the Mindoro Strait, while ∼2.9 Sv flow southwards through the Sibutu Strait merging into the ITF. The ITF inflow occurs through the Makassar Strait (up to ∼62%) and the Lifamatola Strait (∼38%). The annual average volume transport of the ITF inflow from the simulation is ∼15 Sv in the 60s and ∼16.6 Sv in the 90s, very close to the long term observations. The ITF outflow through the Lombok, Ombai and Timor straits is ∼16.8 Sv in the 60s and 18.9 Sv in the 90s, with the outflow greater by 1.7 Sv and 2.3 Sv respectively. The transport estimates of the simulation at all the straits are in rather good agreement with the observational estimates.We analyze the thermal structure of the domain in the 60s and 90s and assess the simulated temperature patterns against the SODA reanalysis product, with special focus on the shallow region of the SCS. The SODA dataset clearly shows that the yearly averaged temperatures of the 90s are overall warmer than those of the 60s in the surface, intermediate and some of the deep layers and the decadal differences (90s − 60s) indicate that the overall warming of the SCS interior is a local effect. In the simulation the warm trend from the 60s to the 90s in well reproduced in the surface layer. In particular, the simulated temperature profiles at two shallow sites at midway in the SCSTF agree rather well with the SODA profiles. However, the warming trend in the intermediate (deep) layers is not reproduced in the simulation. We find that this deficiency is mostly due to a deficiency in the initial temperature fields provide by the MITgcm.     

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

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

Annual variation of the southern boundary current in the Banda Sea

ADCP measurements in the southern Banda Sea, obtained with the bulk carrier “MS First Jupiter” from 1997 until 2000, have been analysed. The observations reveal the presence of an eastward flowing southern boundary current, bringing water from the Indonesian throughflow towards the connections with the Indian Ocean in Ombai Strait and the Timor Sea. The mean transport in the upper 300 m is estimated to be about 5 Sv, over 50% of the outflow towards the Indian Ocean in this layer through the eastern passages near Timor. The velocity in the boundary current shows a clear annual variation, more or less in phase with the annually varying inflow through Makassar Strait and the outflow near Timor. The phase of the annual variation cannot be explained by the monsoonal variation of the local winds. Therefore this annual variation of the throughflow is probably generated by large-scale forcing. A considerable reduction of the strength of the boundary current was observed in 1998, following the 1997–1998 El Niño with a delay of about half a year. On shorter time scales, Kelvin waves, entering the Banda Sea from the Indian Ocean, cause flow reversals of the boundary current.     

Three inflow pathways of the Indonesian throughflow as seen from the simple ocean data assimilation

The SODA product is used to investigate three Indonesian throughflow (ITF) branches: the flow through the Makassar Strait; through the South China Sea; and through the eastern Indonesian basins. The results reveal strong interannual variation in the Makassar Strait and the eastern Indonesian basins throughflow. Inspection of vertically integrated dynamic height (0–1000 db), a proxy of transport function, suggests that this interannual variation can be traced to the New Guinea Coastal Current, indicative of a strong influence of the South Pacific. The vertically integrated dynamic height along the south Java coast is related to variation in the North Pacific and in particular near the east coast of Mindanao Island, whereas the vertically integrated dynamic height along the coast of West Australia is related to variation in the South Pacific, and in particular near the coast of New Guinea. The integrated dynamic height difference between the Java and New Guinea coast appears to be a good proxy of ITF transport on the interannual time scale. Regression analysis shows a phase dependence of the three ITF pathways on the Nino3.4 index. Decoupling of current anomalies between the surface and subsurface layers is identified in the developing and mature phase of El Nino, reflecting different effects of local and remote forcing through oceanic pathways at the Makassar Strait and eastern Indonesian basins.     

Wave dynamics of super cloud clusters over the tropical region

A wave dynamics for the super cloud clusters associated with the tropical intraseasonal oscillation is constructed. Under the present framework, the basic state of super cloud clusters is considered to be a large-scale convergence of the zonal wind. This convergence is created by a nonlinear diabatic Kelvin wave front, which moves eastward slowly without change of shape. When interacting with free equatorial waves with n=−1, 0, 1 and 2, this basic flow will suppress those waves which move eastward except for free Kelvin waves, and permit westward propagating modes such as the mixed Rossby-gravity wave, the inertio-gravity waves with n=1 and 2 to appear in the large-scale convergent region. Among these waves, only fast modes, that is, inertio-gravity waves with n=1 and 2 are regarded to be responsible for the presence of cloud clusters which move westward within a super cloud cluster, while the multiplicity of the occurrences of super cloud clusters, which move eastward within the large-scale convergent region, is due to the superposition of free Kelvin wave upon these inertio-gravity waves.     

The role of equatorial waves in the onset of the South China Sea summer monsoon and the demise of El Niño during 1998

The observed sequence of events leading to the onset of the summer monsoon in the South China Sea (SCS) is described, with a particular focus on conditions during the South China Sea monsoon experiment (SCSMEX) in May–June 1998. During SCSMEX, SCS monsoon onset occurred within the context of a multitude of scale interactions within the ocean-atmosphere system on intraseasonal time scales. Results from the 1998 SCSMEX case study illustrate that SCS monsoon onset is preceded by the development of an eastward-propagating Madden-Julian Oscillation (MJO) in the Indian Ocean, as suggested by previous authors, and the subsequent emanation of a convectively coupled Kelvin wave into the Pacific. Remarkably similar results are obtained in an independent composite of 25 years of data. Since both the MJO and Kelvin waves generate westerly surface winds in their wake, it is suggested that these waves may accelerate or trigger the monsoon onset process in the southern SCS. A detailed analysis of the Kelvin wave that propagated through the SCS during SCSMEX shows that it was responsible for a large portion of the surface wind shift leading to monsoon onset in 1998. Finally, easterly wind anomalies in the eastern Pacific associated with the Indian Ocean MJO event during the SCSMEX period are shown to result in the sudden demise of the 1997–1998 El Niño event.     

The Indonesian throughflow during 2004–2006 as observed by the INSTANT program

The Indonesian seas provide a sea link between the tropical Pacific and Indian Oceans. The connection is not simple, not a single gap in a ‘wall’, but rather composed of the intricate patterns of passages and seas of varied dimensions. The velocity and temperature/salinity profiles Indonesian throughflow (ITF) are altered en route from the Pacific into the Indian Ocean by sea–air buoyancy and momentum fluxes, as well as diapycnal mixing due to topographic boundary effects and dissipation of tidal energy. The INSTANT program measured the ITF in key channels from 2004 to 2006, providing the first simultaneous view of the main ITF pathways. The along-channel speeds vary markedly with passage; the Makassar and Timor flow is relatively steady in comparison to the seasonal and intraseasonal fluctuations observed in Lombok and Ombai Straits. The flow through Lifamatola Passage is strongly bottom intensified, defining the overflow into the deep Indonesian basins to the south. The 3-year mean ITF transport recorded by INSTANT into the Indian Ocean is 15 × 10⁶ m³/s, about 30% greater than the values of non-simultaneous measurements made prior to 2000. The INSTANT 3-year mean inflow transport is nearly 13 × 10⁶ m³/s. The 2 × 10⁶ m³/s difference between INSTANT measured inflow and outflow is attributed to unresolved surface layer transport in Lifamatola Passage and other channels, such as Karimata Strait. Introducing inflow within the upper 200 m to zero the water column net convergence still requires upwelling within the intervening seas, notably the Banda Sea. A layer of minimum upwelling near 600 m separates upwelling within the thermocline from a deep water upwelling pattern driven by the deep overflow in Lifamatola Passage. For a steady state condition upwelling thermocline water is off-set by a 3-year mean sea to air heat flux of 80 W/m² (after taking into account the shoaling of thermocline isotherms between the inflow and outflow portals), which agrees with the climatic value based on bulk formulae sea–air flux calculations, as well as transport weighted temperature of the inflow and outflow water. The INSTANT data reveals interannual fluctuations, with greater upwelling and sea to air heat flux in 2006.     

Generation and termination of Indian Ocean dipole events in 2003, 2006 and 2007

Evolution of Indian Ocean Dipole (IOD) events in 2003, 2006 and 2007 is investigated using observational and re-analysis data products. Efforts are made to understand various processes involved in three phases of IOD events; activation, maturation and termination. Three different triggers are found to activate the IOD events. In preceding months leading to the IOD evolution, the thermocline in southeastern Indian Ocean shoals by reflection of near equatorial upwelling Rossby waves at the East African coast into anomalous upwelling equatorial Kelvin waves. Strengthening (weakening) of northern (southern) portion of ITCZ in March/April and May/June of IOD years, leads to strengthening of alongshore winds along Sumatra/Java coasts. With the combined shallow thermocline and increased latent heat flux due to enhanced wind speeds, the SST in the southeastern Indian Ocean cools in following months. On intraseasonal time scales convection-suppressing phase of Madden-Julian oscillation (MJO) propagates from west to east in May/June of IOD year, and easterlies associated with this phase of MJO causes further shoaling of thermocline in southeastern Indian Ocean, through anomalous upwelling Kelvin wave. All these three mechanisms appear to be involved in initiating IOD event in 2006. On the other hand, except the strengthening/weakening of ITCZ, all other mechanisms are involved in activation of 2003 IOD event. Activation of 2007 IOD event was due to propagation of convection-suppressing MJO in May/June and strengthening of mean winds along Sumatra/Java coast from March to June through changes in convection. The IOD events matured into full-fledged events in the following months after activation, by surface heat fluxes, vertical and horizontal advection of cool waters supported by local along-shore upwelling favorable winds and remote equatorial easterly wind anomalies through excitation of upwelling Kelvin waves. Propagating MJO signals in the tropical Indian Ocean brings significant changes in evolution of IOD events on MJO time scales. Termination of 2003 and 2007 IOD events is achieved by strong convection-enhancing MJOs propagating from west to east in the tropical Indian Ocean which deepen the thermocline in the southeastern equatorial Indian Ocean. IOD event in 2006 was terminated by seasonal reversal of monsoon winds along Sumatra/Java coasts which stops the local coastal upwelling.     

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

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

Influence of monsoon over the warm pool on interannual variation on tropical cyclone activity over the western North Pacific

The relationship between the interannual variation in tropical cyclone （TC） activity over the western North Pacific （WNP） and the thermal state over the warm pool （WP） is examined in this paper. The results show that the subsurface temperature in the WP is well correlated with TC geographical distribution and track type. Their relation is linked by the East Asian monsoon trough. During the warm years, the westward-retreating monsoon trough creates convergence and vorticity fields that are favorable for tropical cyclogenesis in the northwest of the WNP, whereas more TCs concentrating in the southeast result from eastward penetration of the monsoon trough during the cold years. The steering flows at 500 hPa lead to a westward displacement track in the warm years and recurving prevailing track in the cold years.
The two types of distinct processes in the monsoon environment triggering tropical cyclogenesis are hypothesized by composites centered for TC genesis location corresponding to two kinds of thermal states of the WP. During the warm years, low-frequency intraseasonal oscillation is active in the west of the WNP such that eastward-propagating westerlies cluster TC genesis in that region. In contrast, during the cold years, the increased cyclogenesis in the southeast of the WNP is mainly associated with tropical depression type disturbances transiting from equatorially trapped mixed Rossby gravity waves. Both of the processes may be fundamental mechanisms for the inherent interannual variation in TC activity over the WNP.     

Winter‐time near‐surface currents in the strait of Juan de Fuca

Abstract

During November 1976 to February 1977 near‐surface wind, current and temperature measurements were made at three sites along the Strait of Juan de Fuca. Strong tidal currents and major intrusions of warmer, fresher offshore coastal water were superimposed upon the estuarine circulation of near‐surface seaward flow. The r.m.s. amplitudes of the diurnal and semidiurnal tidal currents were ～30 cms^‐1 and 30–47 cm s^‐1, respectively. The vector‐mean flow at 4 m‐depth was seaward and decreased in speed from 28 cm s^‐1 at 74 km from the entrance to 9 cm s^‐1 at 11 km from the entrance. On five occasions intrusions of 1–3 C warmer northeast Pacific coastal water occurred for durations of 1–10 days. The 25 cm s^‐1 up‐strait speed of the intrusive lens agreed to within 20% of the gravity current speed computed from Benjamin's (1968) hydraulic model. The near‐surface currents associated with the intrusions and the southerly coastal winds were significantly correlated, indicating that the intrusions were initiated when shoreward Ekman currents advected Pacific coastal water into the Strait. The reversals were not significantly coherent with the along‐strait sea surface slope measured along the north side of the Strait nor were they strongly related to local wind forcing.     

涡分辨率全球海洋环流模式LICOM模拟的吕宋海峡流场的季节变化

分析了一个1/10°的涡分辨率全球环流模式LICOM(LASG/IAP Climate system Ocean Model)对吕宋海峡附近海洋环流的模拟能力。结果表明,模拟的吕宋海峡附近上层环流及输运具有明显的季节变化特征,除6月是东向净流出外,其余月份均为西向流入,冬季流量最大。年平均流量在-3.76 Sv(1 Sv=106 m3/s),其中上层(600 m以上)流量起主要贡献,为-3.60 Sv,与目前已有的研究结果基本一致。南海通过6个海峡完成与外界的水交换,其中吕宋海峡和巴拉巴克海峡是大洋水进入南海的主要通道,其余海峡均以流出为主,流出量最大的是台湾海峡(1.99 Sv),其次是卡里玛塔海峡(1.03 Sv)。进一步分析表明,由季风引起的埃克曼输送量约占吕宋海峡流量的11%,而由季风引起的吕宋海峡压力梯度形成的西向的地转流对吕宋海峡的输运起支配作用。作为黑潮源头的太平洋北赤道流流量对吕宋海峡输运的季节变化也有一定影响。     

Interannual spring Wyrtki jet variability and its regional impacts

The role of spring Wyrtki jets in modulating the equatorial Indian Ocean and the regional climate is an unexplored problem. The source of interannual variability in the spring Wyrtki jets is explored in this study. The relationship between intraseasonal and interannual variability from 1958 to 2008 and its relation with Indian Summer Monsoon is further addressed. Analysis reveals that the interannual variability in spring Wyrtki jets is controlled significantly by their intraseasonal variations. These are mostly defined by a single intraseasonal event of duration 20 days or more which either strengthens or weakens the seasonal mean jet depending on its phase. The strong spring jets are driven by such intraseasonal westerly wind bursts lasting for 20-days or more, whereas the weak jets are driven by weaker intraseasonal westerlies. During the years of strong jets, the conventional westward phase propagation of Wyrtki jets is absent and instead there is an eastward phase propagation indicating the possible role of Madden Julian Oscillation (MJO) in strengthening the spring Wyrtki jets. These strong intraseasonal westerly wind bursts with eastward phase propagation during strong years are observed mainly in late spring and have implications on June precipitation over the Indian and adjoining land mass. Anomalously strong eastward jets accumulate warm water in the eastern equatorial Indian Ocean (EIO), leading to anomalous positive upper ocean heat content and supporting more local convection in the east. This induces subsidence over the Indian landmass and alters monsoon rainfall by modulating monsoon Hadley circulation. In case of weak current years such warm anomalies are absent over the eastern EIO. Variations in the jet strength are found to have strong impact on sea level anomalies, heat content, salinity and sea surface temperature over the equatorial and north Indian Ocean making it a potentially important player in the north Indian Ocean climate variability.     

El Ni?o发展年和La Ni?a年东亚夏季风季节内变化的比较

基于1979~2013年多种再分析资料,合成分析了El Ni?o发展年和La Ni?a年东亚夏季风的季节内变化。结果表明,东亚夏季风在两种情况下呈现出不同的季节内变化特征。在El Ni?o发展年,初夏期间高纬度地区出现偏北风异常,造成东亚地区位势高度场偏低,西太平洋副热带高压偏东,但均不显著。盛夏期间,El Ni?o强迫造成中太平洋对流增强,副热带西太平洋出现气旋异常,位势高度显著降低,副热带高压明显偏东。与此不同的是,La Ni?a年春季暖池海温偏高,造成夏季对流偏强,西太平洋地区位势高度场偏低,副热带高压减弱东退。此外,La Ni?a年东亚夏季风的季节内变化较为复杂,6月异常较弱,7月达到最强,8月又开始减弱。因此,虽然El Ni?o发展年和La Ni?a年夏季平均副高异常有一定的相似性,但季节内变化则有很大差异,其成因也完全不同。     

Coupling between the El-Niño and planetary-scale waves and their linkage with the Indian monsoon rainfall

Summary The relationships between the El-Niño phenomenon and the planetary-scale waves, and the interannual variations in the Indian monsoon (June–September) rainfall have been analysed in order to investigate how the sea surface temperature (SST) in the equatorial eastern Pacific associated with the El-Niño can produce reduced monsoon rainfall over India by teleconnections.The longitude of ridge location over the Indian region of the integrated planetary waves (numbers 1–3) along 15° N latitude circle in the height field of 200 mb pressure level in May is significantly (r=0.93, significant at 98% CL) related to the May SST anomaly at Puerto Chicama. This implies that warmer (colder) SST anomalies are associated with eastward (westward) longitude of the ridge location. The variations of the ridge location in May appear to be significantly inversely (r=–0.95, significant at 99% CL) related to the Indian monsoon rainfall, with rainfall tending to be less (more) than normal during eastward (westward) longitude of the ridge location suggesting some predictive value for the Indian monsoon rainfall. The Indian monsoon rainfall and May SST anomaly at Puerto Chicama are inversely related (r=–0.90, significant at 96% CL).In terms of the observed relationships, a plausible mechanism for linking El-Niño with the reduced Indian monsoon rainfall is discussed. The relationships noted suggest that excessive warm SST anomalies associated with El-Niño induce an eastward shift in the planetary waves which in turn reduce the Indian monsoon rainfall.With 8 Figures     

Evidence for zonally-trapped propagating waves in the eastern atlantic from satellite sea surface temperature observations

It has been hypothesized (Moore et al., 1978; O'Brien et al., 1978), that equatorial upwelling and subsequent coastal upwelling on the eastern boundary of the Atlantic Ocean are the result of eastward propagating equatorially trapped Kelvin waves in the Atlantic. Concurrent satellite and ship sea surface temperature observations taken during the GATE experiment permit validation of the satellite data as well as relating sea surface temperature (SST) variability to the local current dynamics. A method based on cross-correlations and cross-spectra of the SST field at various locations is utilized to test the Kelvin wave hypothesis. Significant periodic variation of time lags in the SST variability in the eastern Atlantic is observed by the spectral techniques. Satellite data for the 1974 summer show periodic variability which fits either eastward or westward propagating waves with 1 m s^-1 phase speed, i.e., SST supports the quasi-continuous presence of Kelvin or Yanai waves. We find no evidence for a seasonally solitary eastward propagating signal in the eastern Atlantic from SST.     

热带对流层大气低频振荡机制的初步研究

本文从赤道β平面近似下的线性化扰动方程组出发,基于第二类条件不稳定(CISK)理论,研究了热带对流层大气准40天低频振荡的动力机制。研究发现,当对流层中、上层存在较大的对流凝结加热时可激发出纬向波数为1、周期为40天左右的不稳定Kelvin波,它以每天8到11个经度的相速缓慢向东移动。由此指出,观测到的热带对流层大气30—50天的低频振荡可能正是这种由对流凝结加热所驱动的缓慢东移的Kelvin波的具体表现。这可对热带对流层大气30—50天低频振荡现象的动力机制给以初步的物理解释。      

Modulation of low frequency intraseasonal oscillations of northern summer monsoon by El Nino and Southern Oscillation (ENSO)

Summary In order to improve our understanding of the interannual variability of the 30–50 day oscillations of the northern summer monsoon, we have performed numerical experiments using a 5-level global spectral model (GSM). By intercomparing the GSM simulations of a control summer experiment (E1) and a warm ENSO experiment (E2) we have examined the sensitivity of the low frequency intraseasonal monsoonal modes to changes in the planetary scale component of the monsoon induced by anomalous heating in the equatorial eastern Pacific during a warm ENSO phase.It is found that the anomalous heating in the equatorial eastern Pacific induces circulation changes which correspond to weakening of the time-mean divergent planetary scale circulation in the equatorial western Pacific, weakening of the east-west Walker cell over the western Pacific ocean, weakening of the time-mean Reverse Hadley circulation (RHC) over the summer monsoon region and strengthening of the time-mean divergent circulation and the subtropical jet stream over the eastern Pacific and Atlantic oceans. These changes in the large scale basic flow induced by the anomalous heat source are found to significantly affect the propagation characteristics of the 30–50 day oscillations. It is noticed that the reduction (increase) in the intensity of the time-mean divergent circulation in the equatorial western (eastern) Pacific sectors produces weaker (stronger) low-level convergence as a result of which the amplitude of the eastward propagating 30–50 day divergent wave decreases (increases) in the western (eastern) Pacific sectors in E2. One of the striking aspects is that the eastward propagating equatorial wave arrives over the Indian longitudes more regularly in the warm ENSO experiment (E2). The GSM simulations reveal several small scale east-west cells in the longitudinal belt between 0–130°E in the E1 experiment. On the other hand the intraseasonal oscillations in E2 show fewer east-west cells having longer zonal scales. The stronger suppression of small scale east-west cells in E2 probably accounts for the greater regularity of the 30–50 day oscillations over the Indian longitudes in this case.The interaction between the monsoon RHC and the equatorial 30–50 day waves leads to excitation of northward propagating modes over the Indian subcontinent in both cases. It is found that the zonal wind perturbations migrate northward at a rate of about 0.8° latitude per day in E1 while they have a slightly faster propagation speed of about 1° latitude per day in E2. The low frequency monsoonal modes have smaller amplitude but possess greater regularity in E2 relative to E1. As the wavelet trains of low latitude anomalies progress northward it is found that the giant meridional monsoonal circulation (RHC) undergoes well-defined intraseasonal oscillations. The amplitude of the monsoon RHC oscillations are significantly weaker in E2 as compared to E1. But what is more important is that the RHC is found to oscillate rapidly with a period of 40 days in E1 while it executes slower oscillations of 55 days period in E2. These results support the observational findings of Yasunari (1980) who showed that the cloudiness fluctuations on the 30–60 day time scale over the Indian summer monsoon region are associated with longer periods during El Nino years. The oscillations of the monsoon RHC show an enhancement of the larger scale meridional cells and also a stronger suppression of the smaller scale cells in E2 relative to E1 which seems to account for the slower fluctuations of the monsoon RHC in the warm ENSO experiment. It is also proposed that the periodic arrival of the eastward propagating equatorial wave over the Indian longitudes followed by a stronger inhibition of the smaller meridional scales happen to be the two primary mechanisms that favour steady and regular northward propagation of intraseasonal transients over the Indian subcontinent in the warm ENSO experiment (E2). This study clearly demonstrates that the presence of E1 Nino related summertime SST anomalies and associated convection anomalies in the tropical central and eastern Pacific are favourable criteria for the detection and prediction of low frequency monsoonal modes over India.With 11 Figures