Principal Oscillation Pattern Analysis Research Articles

Distinct propagating and standing modes of tropical intraseasonal variability as represented by the two principal oscillation patterns

Although the tropical intraseaonal variability (TISV), as the most important predictability sources for subseasonal-to-seasonal (S2S) prediction, is dominated by Madden-Julian oscillation (MJO), its significant fraction does not always share the canonical MJO features, especially when the convective activity arrives at Maritime Continent. In this study, using principal oscillation pattern (POP) analysis on the combined fields of daily equatorial convection and zonal wind, two distinct leading TISV modes with relatively slower e-folding decay rates are identified. One is an oscillatory mode with the period of 51 days and e-folding time of 19 days, capturing the eastward propagating (EP) feature of the canonical MJO. The other is a non-oscillatory damping mode with e-folding time of 13.6 days, capturing a standing dipole (SD) with convection anomalies centered over the Maritime Continent and tropical central Pacific, respectively. Compared to the EP mode, the leading moisture anomalies at low level to the east of convection center are diminish for the SD mode, and instead, the strong negative anomalies of moisture and subsidence motion emerge in the tropical central Pacific area, which may be responsible for the distinct propagation features. Without filtering methods used, timeseries of the two POPs could be applied to the real-time monitoring of EP and SD events in the phase-space diagram. The two modes can serve as the simple and objective approach for a better characterization for diverse natures of TISV beyond the canonical MJO description, which may further shed light on dynamics of the TISV and its predictability.

Linear Inverse Modeling of Large-Scale Atmospheric Flow Using Optimal Mode Decomposition

Abstract Linear inverse modeling or principal oscillation pattern (POP) analysis is a widely applied tool in climate science for extracting from data dominant spatial patterns together with their dynamics as approximated by a linear Markov model. The system is projected onto a principal linear subspace and the system matrix is estimated from data. The eigenmodes of the system matrix are the POPs, with the eigenvalues providing their decay time scales and oscillation frequencies. Usually, the subspace is spanned by the leading principal components (PCs) and empirical orthogonal functions (EOFs). Outside of climate science, this procedure is now more commonly referred to as dynamic mode decomposition (DMD). Here, we use optimal mode decomposition (OMD) to address the full linear inverse modeling problem of simultaneous optimization of the principal subspace and the linear operator. The method is illustrated on two pedagogical examples and then applied to a three-level quasigeostrophic atmospheric model with realistic mean state and variability. The OMD models significantly outperform the EOF/DMD models in predicting the time evolution of the large-scale flow modes. The advantage of the OMD models stems from finding more persistent modes as well as from better capturing the nonnormality of the linear operator and the associated nonmodal growth. The dynamics of the large-scale flow modes turn out to be markedly non-Markovian and the OMD modes are superior to the EOF/DMD modes also in a modeling setting with a higher-order vector autoregressive process. The OMD modes could also be used as basis functions for a nonlinear dynamical model although they are not optimized for that purpose. Potential applications of the OMD method in weather and climate science include ENSO or MJO prediction, reduced-rank data assimilation, and generation of initial perturbations for ensemble prediction.

Interdecadal wind stress variability over the tropical Pacific causes ENSO diversity in an intermediate coupled model

The role of interdecadal wind stress variability in the genesis of ENSO diversity is examined by using an intermediate coupled model (ICM) in the tropical Pacific; two types of experiments are performed, one with the original ICM, and the other with interdecadal wind stress ( $${\uptau }_{interde}$$ ) effect being explicitly represented. The $${\uptau }_{interde}$$ component is derived from NCEP/NCAR reanalysis dataset as follows. First, the ensemble empirical mode decomposition (EEMD) is used to extract the interdecadal component of wind stress anomalies on about a 10–40 yr timescale. Next, an idealized interdecadal cycle of $${\uptau }_{interde}$$ is reconstructed by a principal oscillation pattern (POP) analysis based on the EEMD-extracted interdecadal wind component. A 110-yr model integration is then performed by explicitly incorporating the reconstructed $${\uptau }_{interde}$$ cycle into the ICM. Compared with the regular interannual oscillation in the original ICM, the simulated ENSO events become highly irregular with interdecadal variations in the amplitude and asymmetry when the $${\uptau }_{interde}$$ effect is included. Especially, the model reproduces two types of El Niño with different spatial distribution and temporal evolution of SST anomalies, namely Eastern-Pacific (EP) and Central-Pacific (CP) types. Further attribution analyses are performed to understand the modulating effects of $${\uptau }_{interde}$$ in the tropical Pacific using the ocean component of the ICM, forced by the added $${\uptau }_{interde}$$ effect. Two different roles of the Interdecadal Pacific Oscillation (IPO) in modulating different types of El Niño are illustrated. On the one hand, the warm phase of IPO favors for the emergence of EP-El Niño events, in association with the initial warming signals occurring in the eastern equatorial Pacific, which are absent in the original ICM. On the other hand, the cold phase of IPO tends to shift the El Niño (i.e., the single type of El Niño in the original ICM) to an CP type, with which the SST anomalies propagate eastward along the equator but cannot extend into the eastern boundary. This simple modeling study highlights the significant contributions of interdecadal wind variability to the genesis of ENSO irregularity and diversity theoretically.

What Contributes to the Inter‐Annual Variability in Tropical Lower Stratospheric Temperatures?

AbstractThe inter‐annual variability in mid and lower stratospheric temperatures for the period 1984–2019 is decomposed into dynamical and radiative contributions using a radiative calculation perturbed with changes in dynamical heating, trace gases and aerosol optical depth. The temperature timeseries obtained is highly correlated with the de‐seasonalized ERA5 temperature (r2 > 0.6 in the region 15 to 70 hPa, 1992 to 2019–after the Pinatubo volcanic eruption). Ozone and dynamical heating contributions are found to be equally important, with water vapor, stratospheric aerosols, and carbon dioxide playing smaller roles. Prominent aspects of the temperature timeseries are closely reproduced, including the 1991 Pinatubo volcanic eruption, the year‐2000 water vapor drop, and the 2016 Quasi‐biennial oscillation (QBO) disruption. Below 20 hPa, ozone is primarily controlled by transport and is positively correlated to the upwelling. This ozone‐transport feedback acts to increase the temperature response to a change in upwelling by providing an additional ozone‐induced radiative temperature change. This can be quantified as an enhancement of the dynamical heating of about 20% at 70 hPa. A Principal Oscillation Pattern (POP) analysis is used to estimate the contribution of the ozone QBO (±1 K at 70 hPa). The non‐QBO ozone variability is also shown to be significant. Using the QBO leading POP timeseries as representative of the regular QBO signal, the QBO 2016 disruption is shown to have an anomalously large radiative impact on temperature due to the ozone change ( at 70 hPa).

A Principal Oscillation Pattern Analysis of the Circulation Variability in the California Current System Associated With the El Niño Southern Oscillation

AbstractA linear inverse model of the circulation of the California Current System was constructed from a 31‐year sequence of circulation estimates based on a data assimilative configuration of the Regional Ocean Modeling System. Principal oscillation pattern (POP) analysis of the linear inverse model has identified several distinct dynamical modes, one with an oscillation period of 3.6 years that describes much of the observed circulation variability of the California Current System associated with the El Niño Southern Oscillation (ENSO). This POP captures not only the circulation changes due to large amplitude ENSO events but also that associated with many of the weak‐to‐moderate events as well. The POP describes both nearshore and offshore variability and captures many of the previously identified phase relations between different circulation variables, unifying them as part of a coherent three‐dimensional time‐evolving mode of the circulation. The connection between the POP and the ocean surface forcing was also explored revealing their spatiotemporal connections to the ENSO circulation variability. Analysis of the dynamical balances of POP circulation anomalies reveals fundamentally different dynamical regimes in nearshore and offshore regions and sheds light on the possible role of wind‐induced baroclinic instability in modulating eddy kinetic energy during large ENSO events.

Detecting, anticipating, and predicting critical transitions in spatially extended systems.

A data-driven linear framework for detecting, anticipating, and predicting incipient bifurcations in spatially extended systems based on principal oscillation pattern (POP) analysis is discussed. The dynamics are assumed to be governed by a system of linear stochastic differential equations which is estimated from the data. The principal modes of the system together with corresponding decay or growth rates and oscillation frequencies are extracted as the eigenvectors and eigenvalues of the system matrix. The method can be applied to stationary datasets to identify the least stable modes and assess the proximity to instability; it can also be applied to nonstationary datasets using a sliding window approach to track the changing eigenvalues and eigenvectors of the system. As a further step, a genuinely nonstationary POP analysis is introduced. Here, the system matrix of the linear stochastic model is time-dependent, allowing for extrapolation and prediction of instabilities beyond the learning data window. The methods are demonstrated and explored using the one-dimensional Swift-Hohenberg equation as an example, focusing on the dynamics of stochastic fluctuations around the homogeneous stable state prior to the first bifurcation. The POP-based techniques are able to extract and track the least stable eigenvalues and eigenvectors of the system; the nonstationary POP analysis successfully predicts the timing of the first instability and the unstable mode well beyond the learning data window.

Impacts of the global sea surface temperature anomaly on the evolution of circulation and precipitation in East Asia on a quasi-quadrennial cycle

Based on multi-source observational data from 1979 to 2013, we investigate the evolution of the global sea surface temperature anomaly (SSTA) on a quasi-quadrennial cycle and its impact on the seasonal evolution of circulation and precipitation in East Asia. We do this by using principal oscillation pattern analysis and associated correlation pattern analysis, together with numerical experiments from an atmospheric general circulation model. The results indicate that the global SSTA exerts profound impacts on the East Asian climate anomaly, while distinct responses exist between the SSTA developing and decaying stages. From winter to autumn in the developing year, southern China is typically wetter than normal. In the decaying year, however, the precipitation over most parts of East Asia is above normal from winter to spring, and the main rainfall band meandering from the Yangtze River to South Japan is enhanced in summer. During the developing and decaying years, an upper-level anomalous cyclone moves from Northeast Asia to the Tibetan Plateau, and then retreats back. A low-level anomalous cyclone occurs over Northeast Asia in spring, but weakens in summer and then strengthens gradually from autumn to the following summer. An anomalous Philippine Sea anticyclone occurs in winter and spring, but is replaced by an anomalous cyclone in summer before reappearing and persisting from autumn to the decaying year. The abrupt change in circulation patterns in summer of the developing year might be related to the anomalous weakening of the Tibetan Plateau heating. These anomalies can be attributed to the combined effects of the global large-scale heating, regional-scale oceanic forcing, and thermal feedback of the land.

Shifts in the spatiotemporal dynamics of schistosomiasis: a case study in Anhui Province, China.

BackgroundThe Chinese national surveillance system showed that the risk of Schistosoma japonicum infection fluctuated temporally. This dynamical change might indicate periodicity of the disease, and its understanding could significantly improve targeted interventions to reduce the burden of schistosomiasis. The goal of this study was to investigate how the schistosomiasis risk varied temporally and spatially in recent years.Methodology/Principal FindingsParasitological data were obtained through repeated cross-sectional surveys that were carried out during 1997-2010 in Anhui Province, East China. A multivariate autoregressive model, combined with principal oscillation pattern (POP) analysis, was used to evaluate the spatio-temporal variation of schistosomiasis risk. Results showed that the temporal changes of schistosomiasis risk in the study area could be decomposed into two sustained damped oscillatory modes with estimated period of approximately 2.5 years. The POPs associated with these oscillatory components showed that the pattern near the Yangtze River varied markedly and that the disease risk appeared to evolve in a Southwest/Northeast orientation. The POP coefficients showed decreasing tendency until 2001, then increasing during 2002-2005 and decaying afterwards.ConclusionThe POP analysis characterized the variations of schistosomiasis risk over space and time and demonstrated that the disease mainly varied temporally along the Yangtze River. The schistosomiasis risk declined periodically with a temporal fluctuation. Whether it resulted from previous national control strategies on schistosomiasis needs further investigations.

Irregularity and decadal variation in ENSO: a simplified model based on Principal Oscillation Patterns

A new method of estimating the decay time, mean period and forcing statistics of El Nino-Southern Oscillation (ENSO) has been found. It uses a two-dimensional stochastically forced damped linear oscillator model with the model parameters estimated from a Principal Oscillation Pattern (POP) analysis and associated observed power spectra. It makes use of extended observational time series of 150 years of sea surface temperature (SST) and sea level pressure (SLP) as well as climate model output. This approach is motivated by clear physical relationships that SST and SLP POP patterns have to the ENSO cycle, as well as to each other, indicating that they represent actual physical modes of the climate system. Moreover, the leading POP mode accounts for 20–50 % of the variance on interannual time scales. The POP real part is highly correlated with several Nino indices near zero lag while the imaginary part exhibits a 6–9 month lead time and thus is a precursor. The observed POP power spectra show markedly different behavior for the peak and precursor, the former having more power at ENSO frequencies and the latter dominating at low frequencies. The results realistically suggest a period of oscillation of 4–6 years and a decay time of 8 months, which corresponds to the practical ENSO prediction limit. A fundamental finding of this approach is that the difference between the observed peak and precursor spectra at low frequencies can be related to the forcing statistics using the simple model, as well as to the difference between patterns of decadal and interannual variability in the Pacific.

Pacific Decadal Variability: Paced by Rossby Basin Modes?

Abstract A systematic study is presented of decadal climate variability in the North Pacific. In particular, the hypothesis is addressed that oceanic Rossby basin modes are responsible for enhanced energy at decadal and bidecadal time scales. To this end, a series of statistical analyses are performed on a 500-yr control integration of the Community Climate System Model, version 3 (CCSM3). In particular, a principal oscillation pattern (POP) analysis is performed to identify modal behavior in the subsurface pressure field. It is found that the dominant energy of sea surface temperature (SST) variability at 25 yr (the model equivalent of the Pacific decadal oscillation) cannot be explained by the resonant excitation of an oceanic basin mode. However, significant energy in the subsurface pressure field at time scales of 17 and 10 yr appears to be related to internal ocean oscillations. However, these oscillations lack the characteristics of the classical basin modes, and must either be deformed beyond recognition by the background circulation and inhomogeneous stratification or have another dynamical origin altogether. The 17-yr oscillation projects onto the Pacific decadal oscillation and, if present in the real ocean, has the potential to enhance the predictability of low-frequency climate variability in the North Pacific.

Multivariate principal oscillation pattern analysis of ICA sources during seizure.

Mapping the dynamics of neural source processes critically involved in initiating and propagating seizure activity is important for effective epilepsy diagnosis, intervention, and treatment. Tracking time-varying shifts in the oscillation modes of an evolving seizure may be useful for both seizure onset detection as well as for improved non-surgical interventions such as microstimulation. In this report we apply a multivariate eigendecomposition method to analyze the time-varying principal oscillation patterns (POPs, or eigenmodes) of maximally-independent (ICA) sources of intracranial EEG data recorded from subdural electrodes implanted in a human patient for evaluation of surgery for epilepsy. Our analysis of a subset of the most dynamically important eigenmodes reveals distinct shifts in characteristic frequency and damping time before, throughout, and following seizures providing insight into the dynamical structure of the system throughout seizure evolution.

Principal-Oscillation-Pattern Analysis of Gene Expression

Principal-oscillation-pattern (POP) analysis is a multivariate and systematic technique for identifying the dynamic characteristics of a system from time-series data. In this study, we demonstrate the first application of POP analysis to genome-wide time-series gene-expression data. We use POP analysis to infer oscillation patterns in gene expression. Typically, a genomic system matrix cannot be directly estimated because the number of genes is usually much larger than the number of time points in a genomic study. Thus, we first identify the POPs of the eigen-genomic system that consists of the first few significant eigengenes obtained by singular value decomposition. By using the linear relationship between eigengenes and genes, we then infer the POPs of the genes. Both simulation data and real-world data are used in this study to demonstrate the applicability of POP analysis to genomic data. We show that POP analysis not only compares favorably with experiments and existing computational methods, but that it also provides complementary information relative to other approaches.

Pacific and Atlantic multidecadal variability in the Kiel Climate Model

Pacific decadal variability (PDV) and Atlantic multidecadal variability (AMV), the two leading decadal modes of observed Northern Hemisphere sea surface temperature (SST) variability, are investigated in a multi‐millennial control integration of the Kiel Climate Model (KCM). It is shown that the two phenomena are independent modes in the model and can be easily separated by Principal Oscillation Pattern (POP) analysis of model SST. PDV‐related variability covers the whole North Pacific with strong signals in both the mid‐latitude North Pacific and the western Tropical Pacific. Strong signals are also simulated in the eastern Indian Ocean Sector. PDV's memory, however, resides in the North Pacific and is linked to the subtropical gyre. The AMV mechanism is related to the Atlantic meridional overturning circulation (AMOC). A stochastic mechanism applies to both PDV and AMV.

Equatorial Atlantic interannual variability: Role of heat content

The dynamics of the equatorial Atlantic zonal mode are studied using observed sea surface height (SSH), sea surface temperature (SST), and heat flux and reanalysis wind stress and upper ocean temperature. Principal oscillation pattern (POP) analysis shows that the zonal mode is an oscillatory normal mode of the observed coupled system, obeying the delayed action/recharge oscillator paradigm for El Niño Southern Oscillation. Variations in equatorial averaged SSH, a proxy for upper ocean heat content, precede SST anomalies in the cold tongue by 4–5 months, about a quarter of the POP period. Positive subsurface temperature anomalies appear in the west, as a delayed response to the preceding cold event. These propagate eastward, where due to the shallow thermocline they can influence SST, leading to the next warm event. Although SST variations exhibit weak westward propagation during some zonal mode events, POP analysis indicates that to first order there is no zonal propagation in SST. Net surface heat flux anomalies generally act to damp SST anomalies. The zonal mode explains a large amount (70%) of SST variability in the east and a significant fraction (19%) of equatorial variability. Thus, the predictability potential in the equatorial Atlantic on seasonal time scales may be considerably higher than currently thought.

The 20–30-day oscillation of the global circulation and heavy precipitation over the lower reaches of the Yangtze River valley

Based on the observational data in summer, the variations of intraseasonal oscillation (ISO) of the daily rainfall over the lower reaches of the Yangtze River valley (LYRV) were studied by using the non-integer spectrum analysis. The NCEP/NCAR reanalysis data for the period of 1979–2005 were analyzed by principal oscillation pattern analysis (POP) to investigate the spatial and temporal characteristics of principal ISO patterns of the global circulation. The relationships of these ISO patterns to the rainfall ISO and the heavy precipitation process over LYRV were also discussed. It is found that the rainfall over LYRV in May–August is mainly of periodic oscillations of 10–20, 20–30 and 60–70 days, and the interannual variation of the intensity of its 20–30-day oscillation has a strongly positive correlation with the number of the heavy precipitation process. Two modes (POP1, POP2) are revealed by POP for the 20–30-day oscillation of the global 850 hPa geopotential height. One is a circumglobal teleconnection wave train in the middle latitude of the Southern Hemisphere (SCGT) with an eastward propagation, and the other is the southward propagation pattern in the tropical western Pacific (TWP). The POP modes explain 7.72% and 7.66% of the variance, respectively. These two principal ISO patterns are closely linked to the low frequency rainfall and heavy precipitation process over LYRV, in which the probability for the heavy precipitation process over LYRV is 54.9% and 60.4% for the positive phase of the imaginary part of POP1 and real part of POP2, respectively. Furthermore, the models of the global atmospheric circulation for the 20–30-day oscillation in association with or without the heavy precipitation process over LYRV during the Northern Hemisphere summer are set up by means of the composite analysis method. Most of the heavy precipitation processes over LYRV appear in Phase 4 of SCGT or Phase 6 of TWP. When the positive phases of 20–30-day oscillations for the rainfall over LYRV are associated with (without) the heavy precipitation process, a strong westerly stream appears (disappears) from the Arabian Sea via India and Bay of Bengal (BOB) to southern China and LYRV for the global 850 hPa filtered wind field during Phase 4 of SCGT. This situation is favorable (unfavorable) for the forming of the heavy precipitation process over LYRV. Similarly, a strong (weak) western wind belt forms from India through BOB to southern China and LYRV and the subtropical northwestern Pacific and central and eastern equatorial Pacific during Phase 6 of TWP for the cases with (without) the heavy precipitation process. The evolutions of these ISO patterns related to the 20–30-day oscillation are excited by either the interaction of extratropical circulation in both hemispheres or the heat source forcing in Asia monsoon domain and internal interaction of circulation in East Asia. These two global circulation models might therefore provide valuable information for the extended-range forecastof the heavy precipitation process over LYRV during the 10–30 days.

Long‐term observations of transport, eddies, and Rossby waves in the Mozambique Channel

Data from an array of current meter moorings covering a period of two and a half years are used to estimate the varying transport through the Mozambique Channel. The total transport during this period is small (8.6 · 106 m3 s−1 or 8.6 Sv southward). Below 1200 m the transport is weak but a prominent deep western boundary undercurrent with cores at 1700 and 2200 m is found that transports 1.5 Sv to the north. The transport shows a large temporal variability, and neither a continuous upper layer western boundary current nor a continuous deep undercurrent is found. The variability in the upper layer is dominated by a period of 68 days and results mainly from eddies that migrate southward through the Mozambique Channel. In addition to this southward propagation, a westward‐propagating signal is evident from a space‐time diagram of the throughflow. The signal is interpreted as a Mozambique Channel Rossby normal mode. This interpretation is consistent with results from a Principal Oscillation Pattern Analysis (that estimates normal modes from the data) and a quasi‐geostrophic channel model. A detailed inspection of a single “eddy event” shows that a precursor of an anticyclone is a strong southward current along the Madagascar coast that propagates westward to the center of the Channel. During the westward propagation, the current becomes unstable inducing an anticyclone. This scenario connects the westward‐propagating mode with the eddy growth and explains the coincidence of the eddy and Rossby mode frequency. Still, the type of instability that leads to eddy growth could not be determined yet.

Multiple Oscillatory Modes of the Argentine Basin. Part I: Statistical Analysis

Abstract Observations of the sea surface height in the Argentine Basin indicate that strong variability occurs on a time scale of 20−30 days. The aim of this study is to determine the physical processes responsible for this variability. First, results are presented from two statistical techniques applied to a decade of altimetric data. A complex empirical orthogonal function (CEOF) analysis identifies the recently discovered dipole mode as the dominant mode of variability. A principal oscillation pattern (POP) analysis confirms the existence of this mode, which has a period of 25 days. The second CEOF displays a propagating pattern in the northern Argentine Basin, plus a rotating dipole in the southwest corner. The POP analysis identifies both patterns as individual modes, with periods of 30 and 20 days, respectively. Second, the barotropic normal modes of the Argentine Basin are studied, using a shallow-water model capturing the full bathymetry of the basin. Coherences between the spatial patterns of these modes and altimeter data suggest that several of the basin modes are involved in the observed variability. This analysis implies that the 20-day mode detected by recent bottom-pressure measurements is a true barotropic mode. However, the 25-day variability, as found in altimeter data, cannot be directly attributed to the excitation of a free Rossby basin mode. This study indicates that the results of several apparently conflicting observations of the flow variability in the Argentine Basin can be reconciled by assuming that multiple basin modes are involved.

Multivariate autoregressive modelling of sea level time series from TOPEX/Poseidon satellite altimetry

Abstract. This work addresses the autoregressive modelling of sea level time series from TOPEX/Poseidon satellite altimetry mission. Datasets from remote sensing applications are typically very large and correlated both in time and space. Multivariate analysis methods are useful tools to summarise and extract information from such large space-time datasets. Multivariate autoregressive analysis is a generalisation of Principal Oscillation Pattern (POP) analysis, widely used in the geosciences for the extraction of dynamical modes by eigen-decomposition of a first order autoregressive model fitted to the multivariate dataset of observations. The extension of the POP methodology to autoregressions of higher order, although increasing the difficulties in estimation, allows one to model a larger class of complex systems. Here, sea level variability in the North Atlantic is modelled by a third order multivariate autoregressive model estimated by stepwise least squares. Eigen-decomposition of the fitted model yields physically-interpretable seasonal modes. The leading autoregressive mode is an annual oscillation and exhibits a very homogeneous spatial structure in terms of amplitude reflecting the large scale coherent behaviour of the annual pattern in the Northern hemisphere. The phase structure reflects the seesaw pattern between the western and eastern regions in the tropical North Atlantic associated with the trade winds regime. The second mode is close to a semi-annual oscillation. Multivariate autoregressive models provide a useful framework for the description of time-varying fields while enclosing a predictive potential.

Viscosity-Dependent Internal Variability in a Model of the North Pacific

Abstract A 2°-resolution isopycnal model of the North Pacific Ocean is shown to produce anomalies that propagate around the subtropical gyre on the decadal time scale that do not appear in a 1°-resolution version of the same model. A principal oscillation pattern (POP) analysis of the isopycnal interface anomaly is performed to examine the dynamics responsible for the anomaly generation. The POPs show a coherent oscillation around the entire subtropical gyre with two centers of action, one in the Central Mode Water (CMW) region, the other in the Subtropical Countercurrent (STCC). Lead–lag covariances between the subduction rate in the CMW and the layer thickness along the oscillation path indicate that anomalous subduction events are not the driving mechanism for the oscillation. A linearized quasigeostrophic mode analysis shows that the anomalies are generated by flow instability in the region of the STCC. The instability disappears in the 1° model because of changes in the horizontal viscosity, which is set in each model to the minimum value necessary to resolve the western boundary current and preserve numerical stability. A criterion for model resolution of an instability of a given length and time scale damped by biharmonic viscosity is derived. The enhancement of the large-scale instabilities in the low-resolution model emphasizes the importance of achieving mesoscale resolution in ocean models used for climate studies.

Active / break cycles: diagnosis of the intraseasonal variability of the Asian Summer Monsoon

In this study, various diagnostics have been applied to daily observed outgoing longwave radiation (OLR) and ECMWF ReAnalysis (ERA) products to provide a comprehensive description of the active/break cycles associated with the Asian Summer Monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability, 10–20 days and 30–60 days. Composite analysis of OLR based on filtered daily All-India rainfall (AIR) for the 40 day (30–60 days) intraseasonal mode indicates that during active phases, convection is significantly enhanced over the Indian continent, extending over the Bay of Bengal, Maritime continent and equatorial west Pacific, while convection is suppressed over the equatorial Indian Ocean and northwest tropical Pacific, resulting in a ‘quadrapole’ structure over the Asian monsoon domain. In response to this heating pattern, the large-scale Hadley (lateral) and the two east-west (transverse) tropical circulations are enhanced. There is also a significant impact on the extra-tropical circulation through excitation and propagation of Rossby waves. In contrast, the 15-day mode is more regional to the monsoon domain and has a prominent east-west orientation in convection. Only the local Hadley circulation over the monsoon region is modulated by this mode. The evolution of these two modes as revealed by POP (principal oscillation pattern) analysis, shows that the 40-day mode originates over the equatorial Indian Ocean. Once formed it has poleward propagation on either side of the equator, and eastward propagation into the equatorial west Pacific. From the equatorial west Pacific, northward propagation over the west Pacific and westward propagation into the Indian longitudes are prominent. The propagative features are complex and interactive and are responsible for the ‘quadrapole’ structure in convection seen from the composites. The interannual variability, assessed from the POP coefficient time series, indicates that the 40-day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic TCZ (tropical convergence zone). The POP analysis of the 15-day mode indicates that this event originates over the␣equatorial west Pacific, associated with westward propagating Rossby waves, amplifies over the northwest tropical Pacific and modulates both the continental and oceanic TCZs over Indian longitudes simultaneously. This mode is pronounced during the established phase of the monsoon. Due to the complexity in the propagational features of both the intraseasonal modes, it is concluded that understanding the subseasonal variability of one regional component of the Asian Summer Monsoon (ASM), requires understanding the entire ASM system.