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Bin Wang

Abstract

The characteristics of the onset of the Pacific basin-wide warming have experienced notable changes since the late 1970s. The changes are caused by a concurrent change in the background state on which El Niño evolves.

For the most significant warm episodes before the late 1970s (1957, 1965, and 1972), the atmospheric anomalies in the onset phase (November to December of the year preceding the El Niño) were characterized by a giant anomalous cyclone over east Australia whose eastward movement brought anomalous westerlies into the western equatorial Pacific, causing development of the basin-wide warming. Meanwhile, the trades in the southeastern Pacific (20°S–0°, 125°–95°W) relaxed back to their weakest stage, resulting in a South American coastal warming, which led the central Pacific warming by about three seasons. Conversely, in the warm episodes after the late 1970s (1982, 1986–87, and 1991), the onset phase was characterized by an anomalous cyclone over the Philippine Sea whose intensification established anomalous westerlies in the western equatorial Pacific. Concurrently, the trades were enhanced in the southeastern Pacific, so that the coastal warming off Ecuador occurred after the central Pacific warming.

It is found that the atmospheric anomalies occurring in the onset phase are controlled by background SSTs that exhibit a significant secular variation. In the late 1970s, the tropical Pacific between 20°S and 20°N experienced an abrupt interdecadal warming, concurrent with a cooling in the extratropical North Pacific and South Pacific and a deepening of the Aleutian Low. The interdecadal change of the background state affected El Niño onset by altering the formation of the onset cyclone and equatorial westerly anomalies and through changing the trades in the southeast Pacific, which determine whether a South American coastal warming leads or follows the warming at the central equatorial Pacific.

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Bin Wang

Abstract

In the tropical eastern central Pacific Ocean, the annual cycle in sea surface temperature (SST), surface winds and pressure, and clouds are alternatively dominated by an antisymmetric (with respect to the equator) monsoonal mode in February and August and a quasi-symmetric equatorial-coastal mode in May and November, both having a period of one year. The monsoonal mode is forced by the differential insulation between the Northern and Southern Hemispheres. The surface wind variation of the monsoonal mode tends to lead SST variation in late spring/fall. The equatorial-coastal mode originates from atmosphere–ocean interaction. Its development is characterized by contemporaneous intensification and spatial expansion (westward and poleward phase propagation).

The interaction between the forced monsoonal mode and the coupled equatorial-coastal mode plays a critical role in the annual cycle. From October to February, the decline of the southern winter regime of the monsoonal mode initiates and sustains the amplification of the equatorial-coastal mode, causing annual weakening of the cold tongue. From April to June, the enhancement of the poleward SST gradient associated with the decay of the equatorial-coastal mode initiates the eastern North Pacific summer monsoon. Atmosphere-ocean interaction is directly responsible for the annual weakening and reestablishment of the cold tongue, whereas the annual cycle in insulation regulates the interaction indirectly through the forced monsoonal mode.

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Bin Wang

Abstract

Annual distribution and phase propagation of tropical convection are delineated using harmonic and amplitude-phase characteristics analysis of climatological pentad mean outgoing longwave radiation and monthly frequencies of highly reflective cloud.

An annual eastward propagation of peak rainy season along the equator from the central Indian Ocean (60°E) to Arafura Sea (130°E) is revealed. This indicates a transition from the withdrawal of the Indian summer monsoon to the onset of the Australian summer monsoon. Significant bimodal variations are found around major summer monsoon regions. These variations originate from the interference of two adjacent regimes.

The convergence zones over the eastern North Pacific, the South Pacific, and the southwest Indian Ocean are identified as a marine monsoon regime that is characterized by a unimodal variation with a concentrated summer rainfall associated with the development of surface westerlies equatorward of a monsoon trough. Conversely, the central North Pacific and North Atlantic convergence zones between persistent northeast and southeast trades are classified as trade-wind convergence zones, which differ from the marine monsoon regime by their persistent rainy season and characteristic bimodal variation with peak rainy seasons occurring in late spring and fall.

The roles of the annual march of sea surface temperature in the phase propagation and formation of various climatic regimes of tropical convection are also discussed.

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Bin Wang and Yuqing Wang

Abstract

A coupled atmosphere–ocean–coastline model driven by solar radiation is advanced to understand the essential physics determining the annual cycle of the intertropical convergence zone (ITCZ)–equatorial cold tongue (ECT) complex and associated latitudinal climate asymmetry. With a thermocline depth similar to that of the western Pacific, the aquaplanet climate is latitudinal symmetric and stable. The presence of an oceanic eastern boundary supports an east–west asymmetric climate and an ECT due to unstable air–sea interaction and counter stabilization provided by zonal differential surface buoyancy flux. Formation of latitudinal climate asymmetry requires the presence of the ECT.

The antisymmetric solar forcing due to annual variation of the solar declination angle can convert a stable latitudinal symmetric climate into a bistable-state latitudinal asymmetric climate by changing trade winds, which in turn control annual variations of the ECT. The ECT then interacts with ITCZ, providing a self-maintenance mechanism for ITCZ to linger in one hemisphere, either the northern or southern, depending on initial conditions. The establishment of the bistable-state asymmetry requires a delicate balance between counter effects of the antisymmetric solar forcing and self-maintenance. Two factors are critical for the latter: (i) The annual variation of ECT follows the SST of the ITCZ-free hemisphere and the meridional SST gradients between the ECT and ITCZ sustain moisture convergence, which prolongs residence of the ITCZ in summer hemisphere. (ii) The latent heat released in the ITCZ produces remarkable asymmetry in Hadley circulation and trades between the two hemispheres, and the stronger evaporation cooling in the ITCZ-free hemisphere delays and weakens the warming and convection development in that hemisphere.

The annual cycle of insolation due to the earth–sun distance variation may convert the bistable-state asymmetry into a preferred latitudinal asymmetric climate. The earth’s present orbit (with a minimum distance in December solstices) favors ITCZ staying north of the equator by compelling the ECT into a delayed in-phase variation with the Southern Hemisphere SST. With annual-mean solar forcing a tilted eastern boundary can support a weak preferred latitudinal asymmetry. Inclusion of the annual variation of insolation can dramatically amplify the asymmetry in the mean climate through the self-maintenance mechanism.

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Bin Wang and Yan Wang

Abstract

Wavelet transforms (WLT) and waveform transforms (WFT) are effective tools that reveal temporal structure of nonstationary time series. The authors discuss principles and practical aspects of their geophysical applications. The WLT can display variance as a continuous function of time and frequency, but the frequency (time) locality reduces at the high (low) frequency bands. The WFT, on the other hand, provides a sharp view of the locality in both time and frequency, but presents variance by discrete base functions. The two techniques are complementary. The authors use both Morlet WLT and Gabor WFT to analyze temporal structure of the Southern Oscillation (50).

The principal period of the SO has experienced two rapid changes since 1872, one in the early 1910s and the other in the mid-1960s. The dominant period was 3–4 years in the earliest four decades (1872–1910), 5–7 years in the ensuing five decades (1911–1960. except the 1920s), and about 5 years in the last two decades (1970–1992). Ale SO also exhibits noticeable amplitude changes. It was most energetic during two periods. 1872–1892 and 1970–1992, but powerless during the 1920s, 1930s. and 1960s. The powerless period is dominated by quasi-biennial oscillation. Excessively strong cold phases of the El Niño-Southern Oscillation cycle enhance annual variation of SST in the Equatorial eastern and central Pacific. The enhancement, however, appears to be modulated by an interdecadal variation.

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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 LinHo

Abstract

To date, the monsoon-research community has not yet reached a consensus on a unified definition of monsoon rainy season or on the linkage between the onsets over the Asian continent and the adjacent oceans. A single rainfall parameter is proposed, and a suite of universal criteria for defining the domain, onset, peak, and withdrawal of the rainy season are developed. These results reveal a cohesive spatial–temporal structure of the Asian–Pacific monsoon rainy season characteristics, which will facilitate validation of monsoon hydrological cycles simulated by climate system models and improve our understanding of monsoon dynamics.

The large-scale onset of the Asian monsoon rainy season consists of two phases. The first phase begins with the rainfall surges over the South China Sea (SCS) in mid-May, which establishes a planetary-scale monsoon rainband extending from the south Asian marginal seas (the Arabian Sea, the Bay of Bengal, and the SCS) to the subtropical western North Pacific (WNP). The rainband then advances northwestward, initiating the continental Indian rainy season, the Chinese mei-yu, and the Japanese baiu in early to mid-June (the second phase). The heights of the rainy seasons occur primarily in three stepwise phases: in late June over the mei-yu/baiu regions, the northern Bay of Bengal, and the vicinity of the Philippines, in late July over India and northern China; and in mid-August over the tropical WNP. The rainy season retreats northward over east Asia, yet it moves southward over India and the WNP.

Clear distinctions in the characteristics of the rainy season exist among the Indian, east Asian, and WNP summer monsoon regions. Nevertheless, the rainy seasons of the three subsystems also show close linkage. The causes of the regional distinctions and linkages are discussed. Also discussed are the atypical monsoon rainy seasons, such as the skewed and bimodal seasonal distributions found in various places of Asian monsoon domain.

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Bin Wang and Xihua Xu

Abstract

Using climatological pentad mean outgoing longwave radiation (OLR) and European Centre for Medium-Range Weather Forecasts analysis winds, the authors show that the Northern Hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations (CISOs). The extreme phases of CISO characterize monsoon singularities—monsoon events that occur on a fixed pentad with usual regularity, whereas the transitional phases of CISO represent the largest year-to-year monsoon variations.

The CISO results from a phase-locking of transient intraseasonal oscillation to annual cycle. It exhibits a dynamically coherent structure between enhanced convection and low-level convergent (upper-level divergent) cyclonic (anticyclonic) circulation. Its phase propagates primarily northward from the equator to the northern Philippines during early summer (May–July), and westward along 15°N from 170°E to the Bay of Bengal during August and September.

The propagation of CISO links monsoon singularities occurring in different regions. Four CISO cycles are identified from May to October. The first cycle has a peak wet phase in mid-May that starts the monsoon over the South China Sea and Philippines. Its dry phase in late May and early June brings the premonsoon dry weather over the regions of western North Pacific summer monsoon (WNPSM), Meiyu/Baiu, and Indian summer monsoon (ISM). The wet phase of Cycle II peaking in mid-June marks the onsets of WNPSM, continental ISM, and Meiyu, whereas the dry phase in early to mid-July corresponds to the first major breaks in WNPSM and ISM, and the end of Meiyu. The wet phase of Cycle III peaking in mid-August benchmarks the height of WNPSM, which was followed by a conspicuous dry phase propagating westward and causing the second breaks of WNPSM (in early September) and ISM (in mid-September). The wet phase of Cycle IV represents the last active WNPSM and withdrawal of ISM in mid-October.

The relationships among ISM, WNPSM, and East Asian Subtropical Monsoon (EASM) are season dependent. During Cycle II, convective activities in the three monsoon regions are nearly in phase. During Cycle III, however, the convective activities are out of phase between ISM and WNPSM; meanwhile, little linkage exists between WNPSM and EASM. The causes of unstable relationships and the phase propagation of CISO are discussed.

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Xiouhua Fu and Bin Wang

Abstract

A coupled tropical ocean–atmosphere model that fills the gap between anomalous coupled models and fully coupled general circulation models is described. Both the atmosphere and ocean are represented by two and one-half layer primitive equation models, which accentuate the physical processes in the oceanic mixed layer and atmospheric boundary layer. The two media are coupled through both momentum and heat flux exchanges without explicit flux correction. The coupled model, driven by solar radiation, reproduces realistic seasonal cycles of the mixed layer temperature, currents, and depth, and the surface winds and rainfall in the tropical Pacific.

The model results indicate that the equatorial westward phase propagation of the annual warming is primarily caused by zonal temperature advection and downward solar radiation modified by clouds, whereas the wind-evaporation-SST feedback plays a minor role. The meridional wind component appears to have a stronger impact than the zonal wind component on the seasonal cycle of the eastern Pacific cold tongue, because the cross equatorial winds have stronger annual variation, which is more effective in regulation of SST through changing surface evaporation and mixed layer entrainment. The annual variation of the solar forcing is shown to have a significant impact on the long-term mean state. Without the seasonal cycle forcing, the western Pacific warm pool would shift eastward and the latitudinal climate asymmetry in the eastern Pacific would be stronger.

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Guosen Chen and Bin Wang

Abstract

The skeleton model is one of the theoretical models for understanding the essence of the Madden–Julian oscillation (MJO). The heating parameterization scheme in the skeleton model assumes that precipitation tendency is in phase and proportional to the low-level moisture anomaly. The authors show that the observed MJO precipitation tendency is not in phase with the low-level moisture anomaly. The consequence of the wave activity envelope (WAE) scheme is reexamined by using a general MJO theoretical framework in which trio-interaction among convective heating, moisture, and wave–boundary layer (BL) dynamics are included and various simplified convective schemes can be accommodated. Without the BL dynamics, the general model framework can be reduced to the original skeleton model. The authors show that the original skeleton model yields a neutral mode that exhibits a “quadrupole” horizontal structure and a quadrature relationship between precipitation and low-level moisture; both are inconsistent with observations. With the BL dynamics and damping included, the model can produce a growing mode with improved horizontal structure and precipitation–moisture relationship, but deficiencies remain because of the WAE scheme. The authors further demonstrate that the general model with the simplified Betts–Miller scheme and BL dynamics can produce a realistic horizontal structure (coupled Kelvin–Rossby wave structure) and precipitation–moisture relationship (i.e., the BL moisture convergence leads precipitation, and column-integrated moisture coincides with precipitation).

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