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Risheng Wang and Bin Wang

Abstract

The variability of El Niño–La Niña events was analyzed in a low-dimensional phase space, a concept derived from dynamic system theory. The space–time extended EOFs derived from the observed monthly mean SST field over tropical Pacific were used as the basis of the phase space that describes the time evolution of ENSO signals. It was shown that the essential features of the ENSO variability, such as the irregular oscillation, the phase locking to the annual cycle, and the interdecadal changes in its propagation and onset, can be effectively represented by a three-dimensional phase space. The typical El Niño–La Niña life cycle is four years with its mature phases in boreal winter. The intensity of the ENSO signal within one life cycle is closely linked to the frequency of its occurrence (onset). The interdecadal variability of the ENSO signals is characterized by both the intensity and the frequency of occurrence, displaying an irregularity with the gross feature comparable to the regime behavior and intermittency of some low-dimensional chaotic systems.

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Bin Wang and Albert Barcilon

Abstract

Cold season atmospheric observations of vacillation point to a wave-mean flow interaction of baroclinic, planetary waves with their mean flow, and the observational data show that wave 2 is the largest contributor to the energetics and the heat flux. To verify this hypothesis we present a weakly nonlinear analysis of the evolution of a single, most unstable Green mode interacting with mean zonal flow in the presence of internal and Ekman layer dissipations, the former being larger than the latter.

The derived amplitude equations for the wave and the mean fields transform into a Lorenz set of equations that admits stable, finite amplitude wave gates. No stable limit cycle or aperiodic solutions were found in the realistic parameter ranges that typify atmospheric winter conditions. When the system is disturbed away from these gable states, there is a monotonic or vacillators approach to equilibrium. Damped vacillation occurs when the internal dissipative time scale is longer than the efolding time scale of the inviscid, Green mode, a condition realized in the winter atmosphere. During the vacillation, due to the presence of the internal dissipation the tilt of the constant phase line may remain westward, and the horizontal he-at flux may be poleward throughout most of the cycle.

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Bin Wang and Yan Xue

Abstract

The effects of nonlinear (positive only or conditional) heating on moist Kelvin waves are examined with a simple equatorial zonal-plane model describing the gravest baroclinic mode.

The unstable perturbation subject to nonlinear beating emerges as a wave packet. A typical amplifying, eastward-moving wave packet is characterized by an asymmetric structure: 1) the ascending branch (wet region) is much narrower than the two descending ones (dry regions); and 2) the circulation cell to the east of the wet region center is smaller and stronger than its counterpart to the west of the center. The wet-dry asymmetry is primarily caused by the nonlinear beating effect, while the east-west asymmetry is a result of the movement of the wave packet relative to mean flow. The existence of Newtonian cooling and Rayleigh friction enhances the structural asymmetries.

The unstable wave packet is characterized by two zonal length scales: the ascending branch length (ABL) and total circulation extent (TCE). For a given basic state, the growth rate of a wave packet increases with decreasing ABL or TCE. However, up to a moderate growth rate (order of day−1) the energy spectra of all wave packets are dominated by zonal wavenumber one regardless of ABL size. In particular, the slowly growing (low frequency) wave packets normally exhibit TCEs of planetary scale and ABLs of synoptic scale.

Observed equatorial intraseasonal disturbances often display a narrow convection region in between two much broader dry regions and a total circulation of planetary scale. These structure and scale characteristics are caused by the effects of nonlinear heating and the cyclic geometry of the equator. It is argued that the unstable disturbance found in numerical experiments (e.g., Lau and Peng; Hayashi and Sumi) is a manifestation of the nonlinear wave packet.

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Qinghua Ding and Bin Wang

Abstract

Analysis of the 56-yr NCEP–NCAR reanalysis data reveals a recurrent circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere. This pattern represents the second leading empirical orthogonal function of interannual variability of the upper-tropospheric circulation. The CGT, having a zonal wavenumber-5 structure, is primarily positioned within a waveguide that is associated with the westerly jet stream. The spatial phases of CGT tend to lock to preferred longitudes. The geographically phase-locked patterns bear close similarity during June, August, and September, but the pattern in July shows shorter wavelengths in the North Pacific–North America sector. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of western Europe, European Russia, India, east Asia, and North America. This implies that the CGT may be a source of climate variability and predictability in the above-mentioned midlatitude regions.

The CGT has significant correlations with the Indian summer monsoon (ISM) and El Niño–Southern Oscillation (ENSO). However, in normal ISM years the CGT–ENSO correlation disappears; on the other hand, in the absence of El Niño or La Niña, the CGT–ISM correlation remains significant. It is suggested that the ISM acts as a “conductor” connecting the CGT and ENSO. When the interaction between the ISM and ENSO is active, ENSO may influence northern China via the ISM and the CGT. Additionally, the variability of the CGT has no significant association with the Arctic Oscillation and the variability of the western North Pacific summer monsoon. The circulation of the wave train shows a barotropic structure everywhere except the cell located to the northwest of India, where a baroclinic circulation structure dominates. Two possible scenarios are proposed. The abnormal ISM may excite an anomalous west-central Asian high and downstream Rossby wave train extending to the North Pacific and North America. On the other hand, a wave train that is excited in the jet exit region of the North Atlantic may affect the west-central Asian high and, thus, the intensity of the ISM. It is hypothesized that the interaction between the global wave train and the ISM heat source may be instrumental in maintaining the boreal summer CGT.

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Bin Wang and Tianming Li

Abstract

The tropical atmosphere model presented here is suitable for modeling both the annual cycle and short-term (monthly to decadal time scale) climate fluctuations in sole response to the thermal forcing from the underlying surface, especially the ocean surface. The present model consists of a well-mixed planetary boundary layer and a free troposphere represented by the gravest baroclinic mode. The model dynamics involves active interactions between the boundary-layer flow driven by the momentum forcing associated with sea surface temperature (SST) gradient and the free tropospheric flow stimulated by diabatic heating that is controlled by the thermal effects of SST. This process is demonstrated to be essential for modeling Pacific basinwide low-level circulations. The convective heating is parameterized by a SST-dependent conditional heating scheme based upon the proposition that the potential convective instability increases with SST in a nonlinear fashion.

The present model integrates the virtue of a Gill-type model with that of a Lindzen–Nigam model and is capable of reproducing both the shallow intertropical convergence zone (ITCZ) in the boundary layer and the deep South Pacific convergence zone (SPCZ) and monsoon troughs in the lower troposphere. The precipitation pattern and intensity, the trade winds and associated subtropical highs, and the near-equatorial trough can also be simulated reasonably well.

The thermal contrast between oceans and continents is shown to have a profound influence on the circulation near landmasses. Changes in land surface temperature, however, do not exert significant influence on remote oceanic regions. Both the ITCZ and SPCZ primarily originate from the inhomogeneity of ocean surface thermal conditions. The continents of South and North America contribute to the formation of these oceanic convergence zones through indirect boundary effects that support coastal upwelling changing the SST distribution. The diagnosis of observed surface wind and pressure fields indicates that the nonlinear advection of momentum is generally negligible, even near the equator, in the boundary-layer momentum balance. The large SST gradients in the subtropics play an important role in forcing rotational and cross-isobaric winds.

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Tianming Li and Bin Wang

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The development and movement of the tropical intraseasonal system (TIS) exhibit remarkable annual variations. It was hypothesized that spatial and temporal variation in sea surface temperature (SST) is one of the primary climatic factors that are responsible for the annual variation of TISs. This paper examines possible influences of SST on the TIS through numerical experiments with a 2.5-layer atmospheric model on an equatorial β plane, in which SST affects atmospheric heating via control of the horizontal distribution of moist static energy and the degree of convective instability.

The gradient of the antisymmetric (with respect to the equator) component of SST causes a southward propagation of the model TIS toward northern Australia in boreal winter and a northward propagation over the Indian and western Pacific Oceans in boreal summer. The phase speed of the meridional propagation increases with the magnitude, of antisymmetric SST gradients. The poleward propagation of the equatorial disturbance takes the form of moist antisymmetric Rossby modes and influences the summer monsoon.

During May when SST is most symmetric in the western Pacific, a disturbance approaching the date line may evolve into westward-moving, double cyclonelike, symmetric Rossby modes due to the suppression of the moist Kelvin mode by the cold ocean surface cast of the date line. The disturbance over the equatorial Indian Ocean, however, may evolve into an eastward-moving, moist Kelvin–Rossby wave packet; meanwhile, a cyclonic circulation may be induced over the Gulf of Thailand and Malaysia, drifting slowly westward into the Indian subcontinent.

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Bin Wang and Hualan Rui

Abstract

A simple theoretical analysis on the stability of a resting tropical atmosphere to semigeostrophic perturbations is given using a free atmosphere–boundary layer coupled model on an equatorial β-plane.

An unstable mode emerges when sea surface temperature is higher than a critical value. The growing mode is a moist Kelvin wave modified through coupling with a Rossby wave of the lowest meridional index. The modified Rossby modes, however, remain damped even for high SST. The unstable mode selection can be explained in terms of wave energy generation due to the latent heating induced by frictional moisture convergence.

The horizontal mode-coupling has profound impacts on wave instability. It favors the amplification of long planetary-scale waves, slows down eastward propagation, and suppresses unrealistically fast growth of the uncoupled moist Kelvin mode by creating substantial meridional flows. These effects make the coupled unstable mode more resemble observed equatorial intraseasonal disturbances.

The results also demonstrate that when maximum SST moves from the equator to 7.5°N, the growth rate of the unstable wave is significantly reduced, suggesting that the annual march of the “thermal equator” and associated convective heating is likely responsible for annual variations of the equatorial 40–50 day wave activity.

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Hualan Rui and Bin Wang

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The development and dynamical structure of intraseasonal low-frequency convection anomalies in the equatorial region are investigated using 10 years (1975–85) of outgoing longwave radiation (OLR) and 7 years (1979–85) of 200 and 850 mb wind data.

The composite OLR anomalies for 36 cases show a four-stage development process: initiation over equatorial Africa, rapid intensification when passing through the Indian Ocean, mature evolution characterized by a weakening in the maritime continent and redevelopment over the western Pacific, and dissipation near the date line in moderate events or emanation from the equator toward North America and southeastern Pacific in strong events.

A noticeable feature in vertical structure is that the 850 mb convergence leads convection and midtropospheric upward motion by about 30 degrees longitude in both developing and mature phases. Equatorial upper- (lower-) level easterly (westerly) anomalies and associated twin anomalous anticyclonic (cyclonic) circulation anomalies couple with equatorial convection anomalies. The wind anomalies, however, generally lag convection anomalies in development and early mature phases, but nearly overlap in late mature phase and slightly lead the convection anomalies in dissipation phase. The upper-level twin cyclonic cells associated with the westerly anomalies in front of the convection travel across eastern Pacific after the convection ceases in the central Pacific, while the low-level wind anomalies die out east of the date line.

The implications of the findings in relation to theoretical hypotheses on low-frequency motion are discussed.

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Hisayuki Kubota and Bin Wang

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The authors investigate the effects of tropical cyclones (TCs) on seasonal and interannual rainfall variability over the western North Pacific (WNP) by using rainfall data at 22 stations. The TC-induced rainfall at each station is estimated by using station data when a TC is located within the influential radius (1000 km) from the station. The spatial–temporal variability of the proportion of TC rainfall is examined primarily along the east–west island chain near 10°N (between 7° and 13°N) and the north–south island chain near 125°E (between 120° and 130°E).

Along 10°N the seasonality of total rainfall is mainly determined by non-TC rainfall that is influenced by the WNP monsoon trough. The proportion of the TC rain is relatively low. During the high TC season from July to December, TC rainfall accounts for 30% of the total rainfall in Guam, 15%–23% in Koror and Yap, and less than 10% at other stations. In contrast, along 125°E where the WNP subtropical high is located, the TC rainfall accounts for 50%–60% of the total rainfall between 18° and 26°N during the peak TC season from July to October. In Hualien of Taiwan, TC rainfall exceeds 60% of the total rainfall.

The interannual variability of the TC rainfall and total rainfall is primarily modulated by El Niño–Southern Oscillation (ENSO). Along 10°N, the ratio of TC rainfall versus total rainfall is higher than the climatology during developing and mature phases of El Niño (from March to the following January), whereas the ratio is below the climatology during the decaying phase of El Niño. The opposite is true for La Niña, except that the impact of La Niña is shorter in duration. Furthermore, in summer of El Niño developing years, the total seasonal rainfall increases primarily because of the increase of TC rainfall. In the ensuing autumn, an anticyclonic anomaly develops over the Philippine Sea and TC rainfall shifts eastward; as a result, the total rainfall over the Philippines and Taiwan decreases. The total rainfall to the east of 140°E, however, changes little, because the westward passage of TCs enhances TC rainfall, which offsets the decrease of non-TC rainfall. Along the meridional island chain between 120° and 130°E, the total rainfall anomaly is affected by ENSO starting from the autumn to the following spring, and the variation in TC rainfall dominates the total rainfall variation only in autumn (August–November) of ENSO years.

The results from this study suggest that in the tropical WNP and subtropical East Asian monsoon regions (east of 120°E), the seasonal and interannual variations of rainfall are controlled by changes in nonlocal circulations. These changes outside the monsoon domain may substantially affect summer monsoon rainfall by changing TC genesis and tracks.

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Kazuyoshi Kikuchi and Bin Wang

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Diurnal variations of the global tropical precipitation are documented by using two complementary Tropical Rainfall Measuring Mission (TRMM) datasets (3B42 and 3G68) for 1998–2006 in an attempt to provide a unified view of the diurnal cycle and a metric for evaluating numerical model performance. The 3B42 data have better spatial coverage; the 3G68 data offer more accurate diurnal phase information. The first and second empirical orthogonal function (EOF) modes represent the diurnal cycle and account for 89% of the total variance in 3B42. The third and fourth EOF modes, which account for 10% of the total variance, represent the semidiurnal cycle. Both datasets yield consistent spatial structures and temporal evolution, but they have different advantages: the patterns derived from 3B42 exhibit less noise, while 3G68 yields an arguably more accurate diurnal phase. The diurnal phase derived from 3G68 systematically leads 3B42 by about 3 h.

Three tropical diurnal cycle regimes (oceanic, continental, and coastal) are identified according to the amplitude, peak time, and phase propagation characteristics of the diurnal precipitation. The oceanic regime is characterized by moderate amplitude and an early morning peak [0600–0900 Local Solar Time (LST)], found primarily in the oceanic convergence zones in the Pacific, Atlantic, and Indian Oceans. In contrast, the continental regime features a large amplitude and an afternoon peak (1500–1800 LST), which is particularly pronounced in South America and equatorial Africa near Lake Victoria. Both the oceanic and continental regimes show little spatial phase propagation. The coastal regime, however, shows not only large amplitude but also prominent phase propagation. Two subregimes can also be recognized, often concurring along the same land–sea boundary. The seaside coastal regime is characterized by offshore phase propagation, with peaks occurring from late evening to noon of the next day (2100–1200 LST), whereas the landside coastal regime has landward phase propagation with peaks occurring from noon to evening (1200–2100 LST). The coastal regime is prominent along the land–sea boundaries of the Maritime Continent, the Indian subcontinent, northern Australia, the west coast of America extending from Mexico to Ecuador, the west coast of equatorial Africa, and Northeast Brazil. Note that the amplitude of the diurnal cycle is dependent on season, but the diurnal phase characteristics are not. The underlying mechanism suggested by this analysis, especially over the coastal areas, is also discussed.

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