Northward-propagating Intraseasonal Oscillation Research Articles

Dynamic and Thermal Effects of the Tibetan and Iranian Plateaus on the Northward-Propagating Intraseasonal Oscillation during Boreal Summer

Abstract This study investigates the dynamic and thermal effects of the Tibetan and Iranian Plateaus (TIP) on the characteristics of the northward-propagating boreal summer intraseasonal oscillation (BSISO) and reveals associated physical mechanisms. The relative importance of the Tibetan Plateau (TP) and Iranian Plateau (IP) to the northward propagation of BSISO is also detected. The findings are based on comparative analyses of the control run and six sensitivity experiments using the atmospheric model component of CAS FGOALS-f3-L (the low-resolution version of the Chinese Academy of Sciences Flexible Global Ocean–Atmosphere–Land System model, finite-volume version 3). It is demonstrated that the absence of TIP surface sensible heat or terrain restricts the occurrence of upper-level easterly and lower-level westerly anomalies, as well as moisture convergence over the Asian summer monsoon (ASM) region, which leads to a weakened easterly vertical wind shear with a narrowed range and the generation of a negative meridional specific humidity gradient in the northern Bay of Bengal. These mean-state changes are not conducive to the formation of positive barotropic vorticity together with planetary boundary layer moisture convergence to the north of the convective center, thus impeding the northward propagation of the BSISO. Results indicate that the combined TIP area, via its dynamic and thermal effects, modulates the northward-propagating BSISO by directly affecting the vertical wind shear and meridional moisture gradient in the northern Indian Ocean. By contrast, the TP plays a decisive role in the BSISO northward propagation due to the limited dynamic and thermal effects of IP on the background circulation and water vapor convergence over the ASM region.

Mechanisms of Northward-Propagating Intraseasonal Oscillation over the South China Sea during the Pre-Monsoon Period

AbstractIn the present study, the spatiotemporal structures of the northward-propagating intraseasonal oscillation (ISO) over the South China Sea (SCS) in the premonsoon period are analyzed by using the TropFlux air–sea flux and the JRA-55 reanalysis datasets. It is found that the SCS ISO is significant in the premonsoon season with a strong component of the northward propagation and that the mean state is different from that of summertime. Moreover, there are similar structures to those of a boreal summer ISO event except for the perturbation vorticity with no obvious phase leading. An internal atmospheric dynamics mechanism is proposed to understand the cause of the northward propagation of the ISO during the premonsoon period based on the spatial and temporal structures of the ISOs. The key process associated with this mechanism is the barotropic vorticity advection by the mean barotropic southerly winds, and the main barotropic vorticity around the convection center can be induced by the vertical advection of the mean vorticity. Low-level moisture convergence caused by anomalous flow is a supplementary mechanism to drive the ISOs northward during the premonsoon period, particularly over the northern SCS. In this mechanism, the SST-induced wind anomalies play a more important role than the convection-induced wind anomalies. The summer monsoon circulation has not built up during the premonsoon period, and thus the vertical wind shear effect and the barotropic vorticity effect associated with the meridional advection of baroclinic vorticity are not essential to cause the northward propagation of the ISOs over the SCS.

Environmental conditions regulating the formation of super tropical cyclone during pre-monsoon transition period over Bay of Bengal

Deadliest tropical cyclones (TCs) with category 4 or 5 intensity, which are referred to super TCs (STCs), often take place in Bay of Bengal (BoB) during pre-monsoon transition period. These STCs are often synchronized with the first-branch northward-propagating intraseasonal oscillation (FNISO) over BoB, but not all TCs synchronizing with FNISO were able to develop into STCs. The study analyzes the distinctive environmental conditions for regular and super TC groups. From the intraseasonal background, the most important difference appears in the intensity and relative location of FNISO. A stronger and closer ISO would lead to a colder upper-level air temperature and thus a more unstable stratification which promotes a greater TC intensity. Similar to intraseasonal timescale, upper-level temperature on the interannual timescale is also colder in the STC cases. The combined intraseasonal and interannual environmental temperature difference between lower and upper troposphere are responsible for significant difference between the STC and TC cases.

Seasonal and Intraseasonal Variability of Mesoscale Convective Systems over the South Asian Monsoon Region

Abstract Seasonal and intraseasonal differences in mesoscale convective systems (MCSs) over South Asia are examined using A-Train satellites, a ground-based lightning network, and reanalysis fields. Premonsoon (April–May) MCSs occur primarily over Bangladesh and the eastern Bay of Bengal. During the monsoon (June–September), small MCSs occur over the Meghalaya Plateau and northeast Himalayan notch, while large and connected MCSs are most widespread over the Bay of Bengal. Monsoon MCSs produce less lightning and exhibit more extensive stratiform and anvil reflectivity structures in CloudSat observations than do premonsoon MCSs. During the monsoon, Bay of Bengal and Meghalaya Plateau MCSs vary with the 30–60-day northward-propagating intraseasonal oscillation, while northeast Himalayan notch MCSs are associated with weak large-scale anomalies but locally enhanced CAPE. During intraseasonal active periods, a zone of enhanced large and connected MCSs, precipitation, and lightning extends from the northeastern Arabian Sea southeastward over India and the Bay of Bengal, flanked by suppressed anomalies. Spatial variability is observed within this enhancement zone: lightning is most enhanced where MCSs are less enhanced, and vice versa. Reanalysis composites indicate that Bay of Bengal MCSs are associated with monsoon depressions, which are frequent during active monsoon periods, while Meghalaya Plateau MCSs are most frequent at the end of break periods, as anomalous southwesterly winds strengthen moist advection toward the terrain. Over both regions, MCSs exhibit more extensive stratiform and anvil regions and less lightning when the large-scale environment is moister, and vice versa.

Strong modulations on the Bay of Bengal monsoon onset vortex by the first northward-propagating intra-seasonal oscillation

Monsoon onset vortex (OV) in the form of tropical cyclone is often observed in the pre-monsoon period and contributes to the subsequent abrupt establishment of summer monsoon over the Bay of Bengal (BoB). It is identified here that all historical OVs occurred during the convection-enhanced phase of the first northward-propagating intra-seasonal oscillation (FNISO). The individual contributions from the four large scale environmental fields associated with the intra-seasonal variations to the cyclone genesis are diagnosed with the aid of the genesis potential index. The significant moistening of mid-level atmosphere, which is embedded in the FNISO convection-enhanced phase, is shown to be the primary factor leading to the cyclone genesis. The water vapor budget analysis is further done to understand the governing process for the mid-level humidity increase. It is clearly seen that the vertical advection process, dominated by the anomalous vertical advection of the mean vertical water vapor gradient, plays the critical role. Hence the OVs are shown to be strongly modulated by FNISOs, both of which are important elements of the complex story of the BoB monsoon onset.

Partial lateral forcing experiments reveal how multi-scale processes induce devastating rainfall: a new application of regional modeling

The 1998 extremely heavy rainfall events over East Asia are investigated through partial lateral forcing (PLF) experiments with the Weather Research and Forecasting model to determine the impacts of the synoptic (SY), intra-seasonal (IS), and inter-annual (IA) forcing across the lateral boundary on the extreme climate anomalies. The large-scale lateral boundary forcing was derived from an ensemble reanalysis dataset and decomposed into climatological, SY, IS, and IA components. The PLF experiments show that the IS forcing not only triggers the monsoon onset and drives two northward propagation events of the subtropical front but also has dominant contributions to the two heaviest rainfall events over the Yangtze River Basin (YRB) and South China, suggesting the critical role of the intra-seasonal variability in the devastating 1998 floods. Previous studies perceived that the northward propagating IS oscillation from the tropics regulates the extreme heavy rainfall of East Asia summer monsoon in 1998. However, we find that the IS forcing from the mid-latitude plays a more important role than the forcing from the tropics in generating the two extreme rainfall events in 1998. During the first extreme event in June, the IS forcing across the western boundary is the major cause of the northward advance of the subtropical front and the heavy rainfall over the YRB and South China, with the IS forcing across the northern boundary providing the second largest contribution. During the second extreme event (July 15–August 5), the IS forcing from the eastern boundary plays a dominant role in driving the southward retreat and northward advance of the subtropical front, causing another heavy rainfall over the YRB and South China. The western and northern IS forcing also has large contributions to the second extreme event. We have estimated the contributions to the seasonal anomalous rainfall by the three types of forcing. The SY forcing tends to have a moderate effect on the YRB rainfall but significant reduction of the rainfall in South China. The IS forcing has dominant contributions to the seasonal-mean rainfall anomalies over all three sub-regions of China (North China, the YRB, and South China). The IA forcing mainly enhances the rainfall in South China but reduces the precipitation in the YRB slightly. This study portends a promising application of regional climate models to identify key factors causing extreme climate events. The PLF methodology can be used to study a broad range of climate phenomena and to understand the effects of variety of dynamic and physical processes in climate variability and predictability.

Structures and mechanisms of the first-branch northward-propagating intraseasonal oscillation over the tropical Indian Ocean

The first-branch northward-propagating intra- seasonal oscillation (FNISO) over the tropical Indian Ocean (IO) often triggers the onset of the Asian summer monsoon. In this study we investigate the structures and mechanisms associated with FNISO through the diagnosis of ERA-Interim reanalysis data for the period of 1990-2009. A composite analysis is conducted to reveal the structure and evolution characteristics of the FNISO and associated background circulation changes. It is found that the FNISO convection originates from the southwest- ern IO and propagates eastward. After reaching the eastern IO, the major convective branch moves northward toward the northern Bay of Bengal (BoB). Two possible mecha- nisms may contribute to the northward propagation of the FNISO. One is the meridional asymmetry of the back- ground convective instability. A greater background con- vective instability over the northern BoB may destabilize Rossby waves and cause convection to shift northward. The other is the meridional phase leading of perturbation humidity in the planetary boundary layer (PBL). Maximum PBL moisture appears to the north of the convection center, which promotes a convectively unstable stratification ahead of the convection and leads to the northward prop- agation of the FNISO. A PBL moisture budget analysis reveals that anomalous zonal advection is a dominant process in contributing to the moisture asymmetry.

Variability in the East Asian Monsoon: a review

AbstractThis study presents reviews of recent research on the structure and the multiscale variability in the East Asian monsoon. The boreal summer and winter seasons in the East Asian monsoon region exhibit significant intraseasonal, interannual and interdecadal variabilities. The interannual intensity of the East Asian summer monsoon (EASM) is mainly associated with the position of the centre of the Bonin High, which may be distinguished from the North Pacific anticyclone. The frequencies of heavy rainfall events and associated rainfall amounts increase, and extreme heavy rainfall is higher in August than in July, due to changes that occurred in the August rainfall‐El Niño‐Southern Oscillation (ENSO) relationship around the mid‐1970s. This intraseasonal variability in EASM plays a more important role in the explanations of the interannual variability and climate change than does the annual mean. The interannual variability in the East Asian winter monsoon (EAWM) depends on the behaviour of the Siberian High (SH), Aleutian Low and the subtropical westerly jet stream. An EAWM index that takes into account the meridional shear of a 300 hPa zonal wind is a good indicator to represent the intensity of the EAWM. The Arctic Oscillation has a close relationship with the EAWM intensity on the decadal time scale. Distinct sub‐seasonal variability is characterized with northward propagation and is observed in the interdecadal change in the monsoonal intraseasonal oscillation (ISO)‐ENSO relationship. The preceding winter ENSO influenced the early summer northward propagating ISO (NPISO) activity before the late 1970s, whereas a strong NPISO–ENSO relationship appeared during the later summer after the late 1970s. The NPISO–ENSO relationship is robust owing to a tropical atmospheric bridge process involving the Walker Circulation and Rossby Wave propagation. Copyright © 2012 Royal Meteorological Society

The Asian Monsoon in the Superparameterized CCSM and Its Relationship to Tropical Wave Activity

Three general circulation models (GCMs) are used to analyze the impacts of air–sea coupling and superparameterized (SP) convection on the Asian summer monsoon: Community Climate System Model (CCSM) (coupled, conventional convection), SP Community Atmosphere Model (SP-CAM) (uncoupled, SP convection), and SP-CCSM (coupled, SP). In SP-CCSM, coupling improves the basic-state climate relative to SP-CAM and reduces excessive tropical variability in SP-CAM. Adding SP improves tropical variability, the simulation of easterly zonal shear over the Indian and western Pacific Oceans, and increases negative sea surface temperature (SST) biases in that region. SP-CCSM is the only model to reasonably simulate the eastward-, westward-, and northward-propagating components of the Asian monsoon. CCSM and SP-CCSM mimic the observed phasing of northward-propagating intraseasonal oscillation (NPISO), SST, precipitation, and surface stress anomalies, while SP-CAM is limited in this regard. SP-CCSM produces a variety of tropical waves with spectral characteristics similar to those in observations. Simulated equatorial Rossby (ER) and mixed Rossby–gravity (MRG) waves may lead to different simulations of the NPISO in each model. Each model exhibits some northward propagation for ER waves but only SP-CCSM produces northward-propagating MRG waves, as in observations. The combination of ER and MRG waves over the Indian Ocean influences the spatiotemporal structure of the NPISO and contributes to the differences seen in each model. The role of ocean coupling must be considered in terms of the time scale of the SST response compared to the time scale of tropical variability. High-frequency disturbances experience coupling via its changes to the basic state, while lower-frequency disturbances may respond directly to SST fluctuations.

Mechanisms of Northward-Propagating Intraseasonal Oscillation—A Comparison between the Indian Ocean and the Western North Pacific

Abstract Mechanisms of northward-propagating intraseasonal oscillations (ISOs) over the Indian Ocean (IO) and the western North Pacific (WNP) are examined for the possibility of their existence in observations. They include the following: 1) the vorticity advection effect, which is associated with the advection of anomalous baroclinic vorticity by mean baroclinic meridional winds; 2) the vertical wind shear effect, which is the vertical advection associated with the meridional gradient of baroclinic divergence and mean easterly vertical wind shear; 3) the moisture advection effect induced by mean flow; and 4) the air–sea interaction via surface latent heat flux. Because of differences in mean state, the influence of each mechanism on the northward-propagating ISOs is different between the IO and the WNP. The vorticity advection effect is consistently found over both the IO and the WNP, while the air–sea interaction has different impacts on the northward-propagating ISOs over the IO and the WNP. The vertical wind shear effect and the moisture advection effect are relatively important over the IO but not over the WNP. Processes to determine changes in SST are also different between the IO and the WNP. Over the IO, SST is mainly associated with surface solar radiation. Wind-stirring effects, surface latent heat flux, and subsurface water entrainment are secondary. Over the WNP, wind-stirring effects become important, but surface solar radiation is secondary.

Monsoon intraseasonal oscillation and land–atmosphere interaction in an idealized model

A simple coupled model is used in a zonally-symmetric configuration to investigate the effect of land–atmosphere coupling on the Asian monsoon intraseasonal oscillation. The atmospheric model is a version of the Quasi-equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature. The land model is the simple one-layer model SLand. The complete nonlinear version and a linear version of the model are used to understand how land–atmosphere interaction influences the northward-propagating intraseasonal oscillation that has been documented in the atmospheric model (Bellon and Sobel in J Geophys Res 113, 2008a, J Atmos Sci 65:470–489, 2008b). Our results show that this interaction damps the intraseasonal variability in most cases. The small heat capacity of land surfaces is the main factor that intervenes directly in the dynamics of the intraseasonal oscillation and explains the damping of intraseasonal variability. But in a few peculiar cases, the small heat capacity of land can also cause a strong interaction between the intraseasonal oscillation and the mean state via the nonlinearity of precipitation, that enhances the monsoon intraseasonal variability. High land albedo indirectly influences the intraseasonal variability by setting the seasonal mean circulation to conditions unfavorable for the monsoon intraseasonal oscillation.

Interdecadal Change in the Relationship between ENSO and the Intraseasonal Oscillation in East Asia

AbstractThe northward-propagating intraseasonal oscillation (NPISO) during the boreal summer is closely linked to the onset/retreat and intensity of the East Asian summer monsoon (EASM). In this study, interdecadal variability in the relationships between the NPISO and El Niño–Southern Oscillation (ENSO) was investigated using long-term outgoing longwave radiation data obtained from the 40-yr ECMWF Re-Analysis (ERA-40) for a 44-yr period (1958 to 2001). It was found that before the late 1970s, the preceding winter ENSO influenced the early summer (i.e., May to June) NPISO activity, whereas after the late 1970s a strong relationship appeared during the later summertime (i.e., July to August). The May–June NPISO before the late 1970s was modulated by springtime Indian Ocean sea surface temperature warming and central North Pacific suppressed convection anomalies and was consequently related to the ENSO-induced west Pacific (WP) pattern, which shows a north–south dipole structure over the North Pacific from winter through spring. After the late 1970s, because of an anomalously strengthened Walker–Hadley circulation, Indian Ocean SST warming was significantly maintained until summer, which promoted a strong suppressed convection anomaly over the Philippine Sea during summer and consequently an enhanced western North Pacific subtropical high and Pacific–Japan (PJ) pattern.

Development of a Regional Climate Model (CREM) and Evaluation on Its Simulation of Summer Climate over Eastern China

This paper describes and evaluates the performance of a regional climate model developed at the State Key Laboratory of Numerical Modeling for Atmospherics Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics (LASG/IAP). The model has been developed based on the numerical forecast model, Advanced Regional Eta-coordinate Model (AREM), of LASG/IAP. An advanced radiation package and a common land surface scheme have been included into the AREM. The new model is regarded as the Climate version of AREM and named CREM. To evaluate its performance in reproducing the summer climate over eastern China, the CREM with 37 km horizontal resolution was integrated from 1 May to 1 September of 1995-2004. The lateral boundary forcing data is derived from the NCEP-DOE (National Centers for Environmental Prediction-Department of Energy) reanalysis data at 6-h intervals. Evaluations on the model performance indicate that the CREM can reasonably reproduce the spatial distributions of climatological monthly (June, July, and August) mean precipitation and circulation over eastern China. The simulated monthly mean precipitation has high spatial correlation with the observation over central-eastern China, except with larger variability. Compared with the precipitation, both the spatial distribution and the spatial variability of geopotential height at 500-hPa and the zonal wind at 200-hPa are better simulated. The interannual variation of precipitation anomalies over central-eastern China is well captured by the model. The variation of the Meiyu front is realistically depicted except that the simulated rainbelt shifts northward in July. A case study of the 1998 flooding year suggests that the model can reproduce not only the seasonal mean precipitation and circulation, but also the temporal evolution of the rainfall and monsoon system over eastern China. In particular, the two northward-propagating intra-seasonal oscillation events are successfully captured in the simulation. The main deficiency of CREM is the overestimation of rainfall amount and the northward shift of the rainbelt, which probably results from the high frequency of rainstorms and exaggerated thermal contrast between central-eastern China and southern China.

Ocean–Atmosphere Coupling in the Monsoon Intraseasonal Oscillation: A Simple Model Study

Abstract A simple coupled model is used in a zonally symmetric aquaplanet configuration to investigate the effect of ocean–atmosphere coupling on the Asian monsoon intraseasonal oscillation. The model consists of a linear atmospheric model of intermediate complexity based on quasi-equilibrium theory coupled to a simple, linear model of the upper ocean. This model has one unstable eigenmode with a period in the 30–60-day range and a structure similar to the observed northward-propagating intraseasonal oscillation in the Bay of Bengal/west Pacific sector. The ocean–atmosphere coupling is shown to have little impact on either the growth rate or latitudinal structure of the atmospheric oscillation, but it reduces the oscillation’s period by a quarter. At latitudes corresponding to the north of the Indian Ocean, the sea surface temperature (SST) anomalies lead the precipitation anomalies by a quarter of a period, similarly to what has been observed in the Bay of Bengal. The mixed layer depth is in phase opposition to the SST: a monsoon break corresponds to both a warming and a shoaling of the mixed layer. This behavior results from the similarity between the patterns of the predominant processes: wind-induced surface heat flux and wind stirring. The instability of the seasonal monsoon flow is sensitive to the seasonal mixed layer depth: the oscillation is damped when the oceanic mixed layer is thin (about 10 m deep or thinner), as in previous experiments with several models aimed at addressing the boreal winter Madden–Julian oscillation. This suggests that the weak thermal inertia of land might explain the minima of intraseasonal variance observed over the Asian continent.

Relationship between ENSO and northward propagating intraseasonal oscillation in the east Asian summer monsoon system

Observational studies are presented on the relationship between El Niño–Southern Oscillation (ENSO) and the northward propagating intraseasonal oscillation (NPISO) in the east Asian summer monsoon (EASM) system. The summer NPISO activity shows a significant correlation with the preceding winter extreme phase of ENSO cycles. A higher correlation appears during late summer, which is consistent with frequent heavy rainfall events at that time as revealed in some previous case studies. The westward expansion of broad anticyclonic circulation over the western North Pacific and the smaller cyclonic circulation around Korea and Japan are found to be associated with the NPISO activity. ENSO affects the late summer NPISO activity through an atmospheric bridge and wave propagation; the springtime Indian Ocean sea surface temperature warming induced by ENSO through the Walker circulation leads to the downward motion and suppressed convection over the Philippine Sea, and this generates the forced Rossby wave train, forming the above south‐to‐north low‐level circulation anomalies.

The impact of ocean–atmosphere coupling on the predictability of boreal summer intraseasonal oscillation

The impact of ocean–atmosphere coupling on the simulation and prediction of the boreal summer intraseasonal oscillation (ISO) has been investigated by diagnosing 22-year retrospective forecasts using the Seoul National University coupled general circulation model (CGCM) and its atmospheric GCM (AGCM) forced with SSTs derived from the CGCM. Numerous studies have shown that the ocean–atmosphere coupling has a significant effect on the improvement of ISO simulation and prediction. Contrary to previous studies, this study shows similar results between CGCM and AGCM, not only in regard to the ISO simulation characteristics but also the predictability. The similarities between CGCM and AGCM include (1) the ISO intensity over the entire Asian-monsoon region; (2) the spatiotemporal evolution of the northward propagating ISO (NPISO); and (3) the potential and practical predictability. A notable difference between CGCM and AGCM is the phase relationship between precipitation and SST anomalies. The CGCM and observation exhibits a near-quadrature relationship between precipitation and SST, with the former lagging about two pentads. The AGCM shows a less realistic phase relationship. The similar structure and propagation characteristics of ISO between the CGCM and AGCM suggest that the internal atmospheric dynamics could be more essential to the ISO than the ocean–atmosphere interaction over the Indian monsoon region.

Horizontal and Vertical Structures of the Northward-Propagating Intraseasonal Oscillation in the South Asian Monsoon Region Simulated by an Intermediate Model*

Abstract 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.

Predictability Studies of the Intraseasonal Oscillation with the ECHAM5 GCM

Abstract Our ability to predict active and break periods of the Asian summer monsoon is intimately tied to our ability to predict the intraseasonal oscillation (ISO). The present study analyzes the upper limit of potential predictability of the northern summer ISO, as it is simulated by the ECHAM5 atmospheric general circulation model forced with climatological SSTs. The leading extended empirical orthogonal functions of precipitation, computed from a 10-yr control simulation, are used to define four different phases of the ISO. Fourteen-member ensembles of 90-day hindcasts are run for each phase of the three strongest ISO events identified in the 10-yr control run. Initial conditions for each ensemble are created from the control simulation using a breeding method. The signal-to-noise ratio is analyzed over a region that covers the core of the Asian summer monsoon activity. Over Southeast Asia, the upper limit for predictability of precipitation and 200-hPa zonal wind is about 27 and 33 days, respectively. Over India, values of more than 15 days occur for both variables. A spatial analysis of the different phases of the ISO reveals that the predictability follows the eastward- and northward-propagating ISO during the active and break phases of the monsoon. Precipitation reveals increased predictability at the end of the convective phase. Analogous, 200-hPa zonal wind shows strongest predictability during low and easterly anomalies. This potential predictability is considerably higher than for numerical forecasts of typical weather variations, particularly for the Tropics, indicating that useful forecasts of monsoon active and break events may be possible with lead times of more than two weeks for precipitation and the dynamics. A closer look at the breeding method used here to initialize the hindcasts shows the importance of appropriate ensemble experiment designs.

Mechanism of the Northward-Propagating Intraseasonal Oscillation: Insights from a Zonally Symmetric Model*

Abstract The propagation and initiation mechanisms of the boreal summer intraseasonal oscillation (BSISO) in the south Asian summer monsoon are examined with a zonally symmetric atmospheric model. In the axially symmetric model the effects of zonally propagating atmospheric waves are intentionally excluded. The model specifies mean flows and depicts the lowest baroclinic mode and a barotropic mode in the free troposphere. The two vertical modes are coupled by the time-mean vertical wind shear. The model atmosphere produces a 15–20-day oscillation, which is characterized by northward propagation of convection from south of the equator to the Indian monsoon trough region and a reinitiation of convection in the region between 10°S and the equator. The northward propagation in the model is produced by the free troposphere barotropic divergence, which leads convection by about a quarter of a cycle. The vertical advection of summer-mean easterly vertical wind shear by perturbation vertical motion inside the convective region induces barotropic divergence (convergence) to the north (south) of convection. This barotropic divergence triggers the moisture convergence in the boundary layer to the north of convection, causing the northward propagation of precipitation. The development of convection in the Southern Hemisphere near the equator is also produced by the development of the barotropic divergence in the free troposphere. When the BSISO convection is located in the Indian monsoon trough region, it creates Hadley-type anomalous circulation. This Hadley-type circulation interacts with the monsoon flow through the meridional and vertical advections creating anomalous barotropic divergence and boundary layer convergence.

Contrasting Characteristics between the Northward and Eastward Propagation of the Intraseasonal Oscillation during the Boreal Summer

Abstract This study investigates the structural and evolutionary characteristics of the eastward- and northward-propagating intraseasonal oscillation (ISO) in the Indian Ocean and western Pacific during the boreal summer. Along the equator, the near-surface moisture convergence located to the east of the deep convection region appears to result in the eastward propagation of the ISO, consistent with the frictional wave–CISK (conditional instability of the second kind) mechanism proposed in previous studies. The eastward propagation is characterized by sequentially downstream development of deep convection occuring mainly in certain regions such as 60°, 95°, 120°, and 145°E, and the date line. The northward propagation of deep convection can be attributed to the low-level moisture convergence located to the north. This convergence is a deep structure extending from the surface to the middle troposphere. Near-surface convergence appears only after the systems approach the landmass in the north. It is sugges...