Northward Propagation Research Articles

Mixed Layer Temperature Budget for the Northward Propagating Summer Monsoon Intraseasonal Oscillation (MISO) in the Central Bay of Bengal

AbstractComposite analyses of mixed layer temperature (MLT) budget terms from near‐surface meteorological and oceanic observations in the central Bay of Bengal are utilized to evaluate the modulation of air‐sea interactions and MLT processes in response to the summer monsoon intraseasonal oscillation (MISO). For this purpose, we use moored buoy data at 15°N, 12°N, and 8°N along 90°E together with TropFlux meteorological parameters and the Ocean Surface Current Analyses Real‐time (OSCAR) current product. Our analysis shows a strong cooling tendency in MLT with maximum amplitude in the central and northern BoB during the northward propagation of enhanced convective activity associated with the active phase of the MISO; conversely, warming occurs during the suppressed phase of the MISO. The surface mixed layer is generally heated during convectively inactive phases of the MISO primarily due to increased net surface heat flux into the ocean. During convectively active MISO phases, the surface mixed layer is cooled by the combined influence of net surface heat loss to the atmosphere and entrainment cooling at the base of mixed layer. The variability of net surface heat flux is primarily due to modulation of latent heat flux and shortwave radiation. Shortwave is mostly controlled by an enhancement or reduction of cloudiness during the active and inactive MISO phases and latent heat flux is mostly controlled by variations in air‐sea humidity difference.

Progress Towards Achieving the Challenge of Indian Summer Monsoon Climate Simulation in a Coupled Ocean‐Atmosphere Model

AbstractSimulation of the spatial and temporal structure of the monsoon intraseasonal oscillations (MISOs), which have effects on the seasonal mean and annual cycle of Indian summer monsoon (ISM) rainfall, remains a grand challenge for the state‐of‐the‐art global coupled models. Biases in simulation of the amplitude and northward propagation of MISOs and related dry rainfall bias over ISM region in climate models are limiting the current skill of monsoon prediction. Recent observations indicate that the convective microphysics of clouds may be critical in simulating the observed MISOs. The hypothesis is strongly supported by high fidelity in simulation of the amplitude and space‐time spectra of MISO by a coupled climate model, when our physically based modified cloud microphysics scheme is implemented in conjunction with a modified new Simple Arakawa Schubert (nSAS) convective parameterization scheme. Improved simulation of MISOs appears to have been aided by much improved simulation of the observed high cloud fraction and convective to stratiform rain fractions and resulted into a much improved simulation of the ISM rainfall, monsoon onset, and the annual cycle.

Crustal strain-dependent serpentinisation in the Porcupine Basin, offshore Ireland

Mantle hydration (serpentinisation) at magma-poor rifted margins is thought to play a key role in controlling the kinematics of low-angle faults and thus, hyperextension and crustal breakup. However, because geophysical data principally provide observations of the final structure of a margin, little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension. Here we present new observational evidence on how crustal strain-dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the Porcupine Basin, offshore Ireland. We present three new P-wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the Moho across and along the basin axis. We use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching (βc) of ∼2.5 in the north at 52.5° N and >10 in the south at 51.7° N. These values suggest that no crustal embrittlement occurred in the northernmost region, and that rifting may have progressed to crustal breakup in the southern part of the study area. We observed a decrease in mantle velocities across the basin axis from east to west. These variations occur in a region where βc is within the range at which crustal embrittlement and serpentinisation are possible (βc 3–4). Across the basin axis, the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting, indicating fault-controlled mantle hydration. Mantle velocities also suggest that the degree of serpentinisation, together with the amount of crustal faulting, increases southwards along the basin axis. Seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is >15%. This observation is consistent with laboratory measurements that show that at this degree of serpentinisation, mantle rocks are sufficiently weak to allow low-angle normal faulting. Based on these results, we propose two alternative formation models for the Porcupine Basin. The first involves a northward propagation of the hyperextension processes, while the second model suggests higher extension rates in the centre of the basin than in the north. Both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation, which eventually controls the development of detachments faults with advanced stretching.

Linkages of Subtropical Stratospheric Intraseasonal Intrusions with Indian Summer Monsoon Deficit Rainfall

The authors investigate the life cycle of a strong subtropical stratospheric intrusion event and propose a hypothesis through which it might reduce the intensity of the Indian summer monsoon (ISM) rainfall (ISMR) after the monsoon onset during June 2014. The diagnostic analysis of ERA-Interim data revealed that stratospheric intrusion occurs in the region of the subtropical westerly jet (SWJ) as a result of Rossby wave breaking (RWB). The RWB event is associated with eddy shedding. These eddies transport extratropical stratospheric mass and energy fluxes downward and southward to north India (NI). As a result, the intrusion spreads dry, cold, and ozone-rich air deep into the troposphere (~500 hPa) over the NI. It enhanced the static stability and weakens the north–south upper-tropospheric temperature gradient. The intrusion of cold and dry air persisted for the entire June, which might have inhibited northward propagation of ISM convection and could be responsible for prolonged hiatus in northward phase propagation of the ISM after onset. The relation between stratospheric intrusion events and ISMR from long-term data (1979–2007) is also investigated. The analysis shows that the stronger negative anomalies of rainfall are associated with stratospheric intrusions during break spells. Thus, the study reveals that stratospheric intrusion is an important factor that may influence ISMR deficit.

South Asian summer monsoon breaks: Process‐based diagnostics in HIRHAM5

AbstractThis study assesses the ability of a high‐resolution downscaling simulation with the regional climate model (RCM) HIRHAM5 in capturing the monsoon basic state and boreal summer intraseasonal variability (BSISV) over South Asia with focus on moist and radiative processes during 1979–2012. A process‐based vertically integrated moist static energy (MSE) budget is performed to understand the model's fidelity in representing leading processes that govern the monsoon breaks over continental India. In the climatology (June–September) HIRHAM5 simulates a dry bias over central India in association with descent throughout the free troposphere. Sources of dry bias are interpreted as (i) near‐equatorial Rossby wave response forced by excess rainfall over the southern Bay of Bengal promotes anomalous descent to its northwest and (ii) excessive rainfall over near‐equatorial Arabian Sea and Bay of Bengal anchor a “local Hadley‐type” circulation with descent anomalies over continental India. Compared with observations HIRHAM5 captures the leading processes that account for breaks, although with generally reduced amplitudes over central India. In the model too, anomalous dry advection and net radiative cooling are responsible for the initiation and maintenance of breaks, respectively. However, weaker contributions of all adiabatic MSE budget terms, and an inconsistent relationship between negative rainfall anomalies and radiative cooling reveals shortcomings in HIRHAM5's moisture‐radiation interaction. Our study directly implies that process‐based budget diagnostics are necessary, apart from just checking the northward propagation feature to examine RCM's fidelity to simulate BSISV.

The onset of the Dead Sea transform based on calcite age-strain analyses

The onset and evolution of the Dead Sea transform are re-evaluated based on new in situ U-Pb dating and strain analyses of mechanically twinned calcites. Direct dating of 30 syn-faulting calcites from 10 different inactive fault strands of the transform indicates that the oceanic-to-continental plate boundary initiated between 20.8 and 18.5 Ma within an ∼10-km-wide distributed deformation zone in southern Israel. Ages from the northern Dead Sea transform (17.1–12.7 Ma) suggest northward propagation and the establishment of a well-developed >500-km-long plate-bounding fault in 3 m.y. The dominant horizontal shortening direction recorded in the dated twinned calcites marks the onset of left-lateral motion along the evolving plate boundary. The observed changes in the strain field within individual fault strands cannot be simply explained by local weakening effects along strands of the Dead Sea transform or by gradual changes in the Euler pole through time.

Impact of the Himalayas on the Meiyu–Baiu migration

The subseasonal migrations of the East Asian summer monsoon are nearly identical to that of the South Asian summer monsoon. In mid-May to mid-June (Phase 1), the South Asian westerly strengthens, the center of the South Asian high moves northwestward, and the East Asian frontal system coupled to and located north of the Western North Pacific (WNP) high moves northward, with all of these actions occurring rapidly. In mid-June to late July (Phase 2), the strength of the South Asian westerly reaches a maximum, the center of the South Asian high remains at approximately 30°N, and the northward propagation of the WNP high becomes stagnant. The speed of the northward movement of the WNP high in Phase 2 is only half the speed of that in Phase 1. By late June, the South Asian monsoon has reached northern India and been blocked by the Himalayas. This indicates that the Himalayas have an effect of constraining the speed of the northward movement of the South Asian high and, in turn, the WNP high. Climate model experiments further reveal that the near-stationary nature of the East Asian frontal system in Phase 2 is related to the blocking of the South Asian summer monsoon by the Himalayas. We suggest that the evolution of the WNP high is constrained by the evolution of the South Asian high, and the Meiyu–Baiu is connected to the South Asian summer monsoon through the impact of the South Asian monsoon heating on the upper tropospheric circulation.

Propagation and mechanisms of the quasi-biweekly oscillation over the Asian summer monsoon region

The propagation and underlying mechanisms of the boreal summer quasi-biweekly oscillation (QBWO) over the entire Asian monsoon region are investigated, based on ECMWF Interim reanalysis (ERA-Interim) data, GPCP precipitation data, and an atmospheric general circulation model (AGCM). Statistical analyses indicate that the QBWO over the Asian monsoon region derives its main origin from the equatorial western Pacific and moves northwestward to the Bay of Bengal and northern India, and then northward to the Tibetan Plateau (TP) area, with a baroclinic vertical structure. Northward propagation of the QBWO is promoted by three main mechanisms: barotropic vorticity, boundary moisture advection, and surface sensible heating (SSH). It is dominated by the barotropic vorticity effect when the QBWO signals are situated to the south of 20°N. During the propagation taking place farther north toward the TP, the boundary moisture advection and SSH are the leading mechanisms. We use an AGCM to verify the importance of SSH on the northward propagation of the QBWO. Numerical simulations confirm the diagnostic conclusion that the equatorial western Pacific is the source of the QBWO. Importantly, the model can accurately simulate the propagation pathway of the QBWO signals over the Asian monsoon region. Simultaneously, sensitivity experiments demonstrate that the SSH over northern India and the southern slope of the TP greatly contributes to the northward propagation of the QBWO as far as the TP area.

Origins and interrelationship of Intraseasonal rainfall variations around the Maritime Continent during boreal winter

Large intraseasonal rainfall variations are identified over the southern South China Sea (SSCS), tropical southeastern Indian Ocean (SEIO), and east coast of the Philippines (EPHI) in boreal winter. The present study contrasts origins and propagations and investigates interrelations of intraseasonal rainfall variations on the 10–20- and 30–60-day time scales in these regions. Different origins are identified for intraseasonal rainfall anomalies over the SSCS, SEIO, and EPHI on both time scales. On the 10–20-day time scale, strong northerly or northeasterly wind anomalies related to the East Asian winter monsoon (EAWM) play a major role in intraseasonal rainfall variations over the SSCS and EPHI. On the 30–60-day time scale, both the intraseasonal signal from the tropical Indian Ocean and the EAWM-related wind anomalies contribute to intraseasonal rainfall variations over the SSCS, whereas the EAWM-related wind anomalies have a major contribution to the intraseasonal rainfall variations over the EPHI. No relation is detected between the intraseasonal rainfall variations over the SEIO and the EAWM on both the 10–20-day and 30–60-day time scales. The anomalies associated with intraseasonal rainfall variations over the SSCS and EPHI propagate northwestward and northeastward, respectively, on the 10–20- and 30–60-day time scales. The intraseasonal rainfall anomalies display northwestward and northward propagation over the Bay of Bengal, respectively, on the 10–20- and 30–60-day time scales.

Seasonality and mechanisms of tropical intraseasonal oscillations

This study has compared the monsoon intraseasonal oscillation (MISO) during the boreal summer and Madden Julian Oscillation (MJO) during the boreal winter. Based on MISO and MJO in high-resolution three-dimensional diabatic heating, the possible mechanisms are discussed through observational analyses of dynamical and thermodynamical variables. The MISO and MJO are extracted as nonlinear oscillations during boreal summer and winter, respectively, by applying multi-channel singular spectrum analysis on daily anomalies of diabatic heating over the Indo-Pacific region. Lead and lag relations among moisture, temperature and surface fields relative to diabatic heating are analyzed to compare the mechanisms of MISO and MJO. While both the oscillations show eastward propagation, MISO has a strong northward propagation and MJO has a weak southward propagation as well. The analysis shows that MJO and MISO are essentially driven by the same mechanisms but with some difference in the meridional propagation. The westerly shear leads the diabatic heating, while the vorticity has weak correlation. Large-scale circulation creates positive moisture preconditioning before convection and negative moisture preconditioning before suppressed conditions. A positive lower level horizontal advection of temperature and upper level temperature tendencies lead the convective state while a negative lower level horizontal advection of temperature and upper level temperature tendencies lead the suppressed state. There is positive feedback from the SST to atmosphere. The difference in the meridional propagation of MISO and MJO is hypothesized to be because of the different differential heating meridionally during the two seasons.

Evaluation of intra‐seasonal oscillation simulations in IPCC AR5 coupled GCMs associated with the Asian summer monsoon

ABSTRACTIn this study, we systematically evaluate the intra‐seasonal oscillation (ISO) associated with the Asian summer monsoon region in coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). Results show that most of the models simulate reasonable climatological circulation patterns in boreal summer, but insufficient precipitation over the Bay of Bengal (BOB) and western North Pacific summer monsoon region. Most models underestimate the variance of 12–90‐day ISO mode but overestimate the percent variance of 12–90‐day ISO accounting for raw precipitation anomalies. There remains a wide gap between model simulations and observations in the simulations of eastward‐propagating ISO (30–80‐day), while the models perform better in the northward propagation of the 30–80‐day mode and the westward propagation of the 12–24‐day mode.In addition, mean state circulation, atmospheric internal dynamic process, air–sea interactions and model resolution have varying degrees of impacts on the ISO simulation. The local Hadley and Walker cell biases are investigated in this study, which might relate well with the precipitation bias in models. Within the similar background of vertical easterly shear, few models have the capacity to simulate the equivalent barotropic vorticity to the north of the convection centre. Furthermore, many models have difficulty in reproducing the strong air–sea interactions over the Indian Ocean and the BOB. Our results indicate that simply improving model resolution is not an effective method to obtain more reasonable propagation characteristics of the ISO, especially for eastward propagation. Therefore, further improving the capacity of the ISO simulation remains a great challenge for future development of climate system models.

Atmospheric dynamics and internal processes during organization and intensification of Boreal Summer Intraseasonal Oscillation (BSISO) based on TRMM and reanalyses data

ABSTRACTBoreal summer intraseasonal oscillations (BSISOs) show a meridional propagation from the Indian Ocean (IO) region to the continental India. The BSISOs are the main source of rainfall variability over the Indian Summer Monsoon (ISM) region. The recent research has revealed a basic understanding of the scale selection and northward propagation of the BSISOs. But the complete understanding about the organization and intensification of BSISOs remains elusive. This study uses observation and reanalysis data that covers summers (June–September) of 1998–2013 to investigate the dynamical and atmospheric internal processes during the organization and intensification of BSISOs. Based on rainfall analyses, two types of BSISOs are defined, one with weaker and the other with stronger intensity. Stronger BSISOs show persisting lower level moisture convergence to the north of the convection centre (CC) and a strong vertical velocity collocated with CC. The analysis of mean kinetic energy (MKE) budget suggests that MKE significantly increases during the organization of the strong BSISOs but the strength is relatively weaker during the evolution of weak BSISOs. The enhanced MKE during the strong events is supported by the increase of baroclinic energy conversion from mean available potential energy (MAPE) to MKE but the MAPE to MKE conversion is significantly weak during the evolution of weak BSISOs. There is conversion from MKE to eddy kinetic energy (EKE) during organization of strong BSISOs and the conversion increases as the events approach towards the organized and intense phase. The eddy momentum‐vertical wind shear interaction is the significant contributor for conversion of MKE to EKE during the organization of strong BSISOs. The eddy momentum‐mean flow divergence interaction and eddy momentum–vorticity interaction at the lower level support to enhance EKE as the strong BSISOs approach the organized and intense phase. But the above interactions are weak during evolutions of the weak BSISOs.

Impact of tropical cyclones on flood risk in southeastern China: Spatial patterns, causes and implications

Using flood peaks data from 62 hydrological stations covering a period of 1951–2014 and tropical cyclone (TC) tracks, floods due to TCs were differentiated and relations between occurrences of floods and TCs were investigated. Then, the impact of TCs on the flood magnitude and frequency was evaluated. Results indicated that there was more than one external influencing factor for flood occurrences, i.e. convective precipitation and TC-related heavy precipitation. In general, the landed TC events have a crucial influence on the occurrence of floods in eastern and western parts of Guangdong province. The mountainous topography limits the northward propagation of TC events and hence limits their impact on the occurrence of floods in the northern part. More than 60% of the largest 10 hazardous floods in the northern and eastern parts were attributed to TCs and hence they significantly modified the probability distributions of flood peaks, increasing the location and scale parameters of the generalized extreme value (GEV) probability distribution model. Therefore, development of measures for mitigation of coastal flooding would need to consider the occurrences of TCs.

The role of shallow convection in promoting the northward propagation of boreal summer intraseasonal oscillation

By conducting idealized experiments in a general circulation model (GCM) and in a toy theoretical model, we test the hypothesis that shallow convection (SC) is responsible for explaining why the boreal summer intraseasonal oscillation (BSISO) prefers propagating northward. Two simulations are performed using ECHAM4, with the control run using a standard detrainment rate of SC and the sensitivity run turning off the detrainment rate of SC. These two simulations display dramatically different BSISO characteristics. The control run simulates the realistic northward propagation (NP) of the BSISO, while the sensitivity run with little SC only simulates stationary signals. In the sensitivity run, the meridional asymmetries of vorticity and humidity fields are simulated under the monsoon vertical wind shear (VWS); thus, the frictional convergence can be excited to the north of the BSISO. However, the lack of SC makes the lower and middle troposphere very dry, which prohibits further development of deeper convection. A theoretical BSISO model is also constructed, and the result shows that SC is a key to convey the asymmetric vorticity effect to induce the BSISO to move northward. Thus, both the GCM and theoretical model results demonstrate the importance of SC in promoting the NP of the BSISO.

Space–Time Evolution of the Low- and High-Frequency Intraseasonal Modes of the Indian Summer Monsoon

AbstractIn this study, rainfall estimates by the Tropical Rainfall Measuring Mission are used to understand the spatiotemporal structures of convection in the intraseasonal time scale and their intensity during the boreal summer over South Asia. A quantitative analysis on how these intraseasonal modes modulate the central Indian rainfall is also provided.Two dominant modes of variability with periodicities of 10–20 and 20–60 days are found, with the latter strongly modulated by sea surface temperature. The 20–60-day mode shows northward propagation from the equatorial Indian Ocean linked with eastward-propagating modes of convective systems over the tropics. The 10–20-day mode shows a complex space–time structure with a northwestward-propagating anomalous pattern emanating from the Indonesian coast. This pattern is found to be interacting with a structure emerging from higher latitudes propagating southeastward, the development of which is attributed to the vertical shear of the zonal wind. The two modes exhibit profound variability in their intensity on the interannual time scale and they contribute a comparable amount to the daily rainfall variability in a season. The intensity of the 20–60 and 10–20-day modes shows a significantly strong inverse and direct relationship with the all-India June–September rainfall, respectively.This study establishes that the probability of the occurrence of substantial rainfall over central India increases significantly if the two intraseasonal modes simultaneously exhibit positive anomalies over the region. The results presented in this paper will provide a pathway to understand, using observations and numerical model simulations, intraseasonal variability and its relative contribution to the Indian summer monsoon. It can also be used for model evaluation.

Monsoon intra-seasonal variability in a high-resolution version of Met Office Global Coupled model

Intra-seasonal oscillation (ISO) is a key ingredient of the East Asia and western North Pacific (EAWNP) summer monsoon and particularly important for seasonal forecast. This paper evaluates the seasonal means and ISOs of the EAWNP summer monsoon simulated by the latest version of the Met Office Global Coupled Model (HadGEM3-GC2) with two different atmospheric model resolutions at ~130 and ~25 km coupled to a same 0.25° × 0.25° resolution ocean model. Results show that the mean states of sea surface temperature (SST), low-level specific humidity and the western Pacific subtropical high are all improved in HadGEM3-GC2 with higher atmosphere resolution. Moreover, although ISO variance is overestimated over the western North Pacific, the model has good fidelity in characterising ISO basic features over the EAWNP including the dominant EOF structure, northward propagation and cycle evolution, as well as the zonal displacement of western Pacific Subtropical High and South Asian High associated with the northward propagating ISOs. Increasing atmosphere model resolution yields improvements in most aspects of the Monsoon ISO over the EAWNP, especially for its northward propagation. Further analysis indicates that this improvement is mainly due to the better description of ISO-related air–sea interaction in higher resolution experiment, as evidenced by the enhanced intra-seasonal SST variance and more coherent northward propagation of rainfall, SST, and the associated surface dynamic and thermodynamic variables in the higher resolution model.

Evolution of the broadly rifted zone in southern Ethiopia through gravitational collapse and extension of dynamic topography

The Broadly Rifted Zone (BRZ) is a ~315km wide zone of extension in southern Ethiopia. It is located between the Southern Main Ethiopian Rift and the Eastern Branch of the East African Rift System (EARS) represented by the Kenya-Turkana Rift. The BRZ is characterized by NE-trending ridges and valleys superimposed on regionally uplifted (~2km average elevation) terrain. Previous studies proposed that the BRZ is an overlap zone resulted from northward propagation of the Kenya-Turkana Rift and southward propagation of the Southern Main Ethiopian Rift. To understand the relationship between the BRZ's extensional style and its crustal and upper mantle structures, this work first estimated the Moho depth using the two-dimensional (2D) radially-averaged power spectral analysis of the World Gravity Map. Verification of these results was accomplished through lithospheric-scale 2D forward gravity models along E-W profiles. This work found that the Moho topography beneath the BRZ depicts a dome-like shape with a minimum depth of ~27km in the center of the dome. This work proposes that the Moho doming, crustal arching underlying the BRZ and associated topographic uplift are the result of asthenospheric mantle upwelling beneath the BRZ. This upwelling changed to a NE-directed lateral mantle flow at shallower depth. This is supported by seismic tomography imaging which shows slow S-wave velocity anomaly at lithospheric depth of 75km to 150km stretching in a NE-SW direction from beneath the BRZ to the Afar Depression. This work proposes that the asthenospheric upwelling created gravitationally unstable dynamic topography that triggered extensional gravitational collapse leading to the formation of the BRZ as a wide rift within the narrow rift segments of the EARS.

High‐resolution simulation of the boreal summer intraseasonal oscillation in Met Office Unified Model

The present study investigates the fidelity and resolution sensitivity of Met Office Unified Model Global Atmosphere 3.0 in simulating the intraseasonal oscillation (ISO) of the East Asia and western North Pacific summer monsoon. Two sets of model simulations at horizontal resolutions of N216 (60 km) and N96 (130 km) forced by observed daily high‐resolution sea‐surface temperature are examined and compared with the observations. Diagnostic results show that the model can well reproduce the gross spatio‐temporal features of observed summer ISO over East Asia and the western North Pacific in terms of period, dominant rotated empirical orthogonal function (REOF) mode, variance, northward propagation, cycle evolution and vertical structure. The intraseasonal change in intensity and position of the western North Pacific subtropical high and upper troposphere westerly jet associated with the northward propagating ISOs are also reasonably captured in the model. Moreover, increasing horizontal resolution improves most aspects of the ISO simulation, especially the intensity and northward propagation of the ISO‐related convection and circulation systems. Further analysis indicates that the improvement as resolution increases is related to the weakening in background state of the East Asian summer monsoon, which is due to the realistic simulation of land–sea thermal contrast in the higher resolution model. This study suggests that enhanced model resolution can have some beneficial impacts on the ISO simulation.

On the barotropic instability using wave ray theory

Based on the spherical Rossby wave ray theory, dynamic characteristics of the barotropic disturbances associated with the westerly jet are discussed, and the barotropic instability conditions are reconsidered. The well-known Rayleigh-Kuo instability criterion indicates that the gradient of potential vorticity must change sign in the unstable area. Kuo uses the method of eigenvalues by assuming disturbances to be simple harmonic waves; however, in this work we assume the atmosphere to be a slowly varying medium, and calculate the amplitude of the Rossby wave by ray tracing. We find a band where the quasi-geostrophic potential vorticity gradient ( β M) is negative, and the amplitude of the northward perturbation grows substantially in this area, making it an unstable region for northward perturbations. In addition the perturbations cannot cross the line of β M=0, and therefore, this is referred to as the “trapped line”. Characteristics of the nonstationary Rossby wave are first analyzed using an ideal zonal mean wind profile u = 30 sin 8 ( 2 φ ) + 0.2 , with the “trapped line” located at 58°N and 70°N, and β M l = 8, but with different zonal wave numbers k . The results suggest that the behavior of the disturbances depends on the initial wave numbers k and l . Northward disturbances with smaller k can propagate further, while disturbances with larger k cannot reach high latitudes and turn back towards the south at turning points β M / ( u M − σ k ) − k 2 = 0. As such, only long Rossby waves with small k values can reach the “trapped line”, while short Rossby waves with larger k values cannot reach it. Long Rossby waves with k = 1, l = 8, first propagate northwest, and then turn eastwards at the “trapped line”. Their energy increases while propagating northward, reaching its maximum in the northern part of the jet stream, and then decays rapidly to zero near the “trapped line”. The energy increases during northward propagation, which means the disturbance obtains energy from the base flow in the southern part of the jet stream, and gives it back to the base flow near the “trapped line”, which completes energy transport from south to north. Further, we also calculated the energy evolution with realistic January and June wind profiles using NCEP data. We conclude that long waves with k <3 are easily trapped by the “trapped line” in both winter and summer, while the propagation of short waves differs substantially between winter and summer. The non-stationary disturbances at low latitudes are easily blocked by easterly winds during winter; however, they can cross easterlies and then propagate to the southern hemisphere in summer, forming a westward wave in the easterlies. It is known that stationary Rossby waves cannot propagate in the easterlies, because only non-stationary waves that meet many conditions can propagate in the easterlies; therefore, this scenario will not be detailed in this paper.

Boreal summer sub-seasonal variability of the South Asian monsoon in the Met Office GloSea5 initialized coupled model

Boreal summer sub-seasonal variability in the Asian monsoon, otherwise known as the monsoon intra-seasonal oscillation (MISO), is one of the dominant modes of intraseasonal variability in the tropics, with large impacts on total monsoon rainfall and India’s agricultural production. However, our understanding of the mechanisms involved in MISO is incomplete and its simulation in various numerical models is often flawed. In this study, we focus on the objective evaluation of the fidelity of MISO simulation in the Met Office Global Seasonal forecast system version 5 (GloSea5), an initialized coupled model. We analyze a series of nine-member hindcasts from GloSea5 over 1996–2009 during the peak monsoon period (July–August) over the South-Asian monsoon domain focusing on aspects of the time-mean background state and air–sea interaction processes pertinent to MISO. Dominant modes during this period are evident in power spectrum analysis, but propagation and evolution characteristics of the MISO are not realistic. We find that simulated air–sea interactions in the central Indian Ocean are not supportive of MISO initiation in that region, likely a result of the low surface wind variance there. As a consequence, the expected near-quadrature phase relationship between SST and convection is not represented properly over the central equatorial Indian Ocean, and northward propagation from the equator is poorly simulated. This may reinforce the equatorial rainfall mean state bias in GloSea5.