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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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Xiaofan Li and Bin Wang

Abstract

The movement of a symmetric vortex embedded in a resting environment with a constant planetary vorticity gradient (the beta drift) is investigated with a shallow-water model. The authors demonstrate that, depending on initial vortex structure, the vortex may follow a variety of tracks ranging from a quasi-steady displacement to a wobbling or a cycloidal track due to the evolution of a secondary asymmetric circulation. The principal part of the asymmetric circulation is a pair of counterrotating gyres (referred to as beta gyres), which determine the steering flow at the vortex center. The evolution of the beta gyres is characterized by development/decay, gyration, and radial movement.

The beta gyres develop by extracting kinetic energy from the symmetric circulation of the vortex. This energy conversion is associated with momentum advection and meridional advection of planetary vorticity. The latter (referred to as “beta conversion”) is a principal process for the generation of asymmetric circulation. A necessary condition for the development of the beta gyres is that the anticyclonic gyre must be located to the east of a cyclonic vortex center. The rate of asymmetric kinetic energy generation increases with increasing relative angular momentum of the symmetric circulation.

The counterclockwise rotation of inner beta gyres (the gyres located near the radius of maximum wind) is caused by the advection of asymmetric vorticity by symmetric cyclonic flows. On the other hand, the clockwise rotation of outer beta gyres (the gyres near the periphery of the cyclonic azimuthal wind) is determined by concurrent intensification in mutual advection of the beta gyres and symmetric circulation and weakening in the meridional advection of planetary vorticity by symmetric circulation. The outward shift of the outer beta gyres is initiated by advection of symmetric vorticity by beta gyres relative to the drifting velocity of the vortex.

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Guosen Chen and Bin Wang

Abstract

Well-organized eastward propagation of the Madden–Julian oscillation (MJO) is found to be accompanied by the leading suppressed convection (LSC) over the Maritime Continent (MC) and the western Pacific (WP) when the MJO convection is in the Indian Ocean (IO). However, it remains unclear how the LSC influences the MJO and what causes the LSC. The present study shows that the LSC is a prevailing precursor for eastward propagation of the MJO across the MC. The LSC enhances the coupling of IO convection and the Walker cell to its east [front Walker cell (FWC)] by increasing the zonal heating gradient. The enhanced FWC strengthens the low-level easterly, which increases boundary layer (BL) convergence and promotes congestus convection to the east of the deep convection; the enhanced congestus convection preconditions the lower to middle atmosphere, which further promotes the transition from congestus to deep convection and leads to eastward propagation of the MJO. The MJO ceases eastward propagation once the FWC decouples from it. Further analysis reveals that LSC has two major origins: one comes from the eastward propagation of the preceding IO dry phase associated with the MJO, and the other develops concurrently with the IO convection. In the latter case, the development of the LSC is brought about by a two-way interaction between the MJO’s tropical heating and the associated tropical–extratropical teleconnection: the preceding IO suppressed convection induces a tropical–extratropical teleconnection, which evolves and forms an anomalous western North Pacific cyclone that generates upper-level convergence and induces significant LSC.

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Renguang Wu and Bin Wang

Abstract

Using station rainfall data and the NCEP–NCAR reanalysis, the authors investigate changes in the interannual relationship between the east Asian summer monsoon (EASM) and El Niño–Southern Oscillation (ENSO) in the late 1970s, concurrent with the Pacific climate shift. The present study focuses on decaying phases of ENSO because changes in developing phases of ENSO are less significant. Remarkable changes are found in the summer rainfall anomaly in northern China and Japan. From pre- to postshift period, the summer rainfall anomaly in eastern north China during decaying phases of El Niño changed from above to below normal, whereas that in central Japan changed from negative to normal. Consistent with this, the barotropic anticyclonic anomaly over the Japan Sea changed to cyclonic; the associated anomalous winds changed from southerly to northerly over the Yellow Sea–northeastern China and from northeasterly to northwesterly over central Japan.

The change in the ENSO–related east Asian summer circulation anomaly is attributed to changes in the location and intensity of anomalous convection over the western North Pacific (WNP) and India. After the late 1970s, the WNP convection anomaly is enhanced and shifted to higher latitudes due to increased summer mean SST in the Philippine Sea. This induces an eastward shift of an anomalous low pressure from east Asia to the North Pacific along 30°–45°N during decaying phases of El Niño. Thus, anomalous winds over northeastern China and Korea switch from southeasterly to northeasterly. Before the late 1970s, an anomalous barotropic anticyclone develops over east Asia and anomalous southerlies prevail over northeastern China during decaying phases of El Niño. This may relate to anomalous Indian convection through a zonal wave pattern along 30°–50°N. After the late 1970s, anomalous Indian convection weakens, which reduces the impact of the Indian convection on the EASM.

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Jing Yang, Bin Wang, Bin Wang, and Qing Bao

Abstract

The lower reach of the Yangtze River basin (LYRB) is located at the central region of the mei-yu and baiu front, which represents the subtropical East Asian (EA) summer monsoon. Based on the newly released daily rainfall data, two dominant intraseasonal variation (ISV) modes are identified over the LYRB during boreal summer (May–August), with spectral peaks occurring on day 15 (the biweekly mode) and day 24 (the 21–30-day mode). These two modes have comparable intensities, and together they account for above about 57% of the total intraseasonal variance. Both ISV modes exhibit baroclinic structures over the LYRB at their extreme phases.

However, the genesis and evolutions associated with the two modes are different. Considering the genesis of their extreme wet phases over the LYRB, the biweekly mode is initiated by a midlatitude jet stream vorticity anomaly moving southeastward, while the 21–30-day mode is primarily associated with a low-level westward propagation of an anticyclonic anomaly from 145° to 120°E, which reflects the westward extension of the western North Pacific subtropical high (WNPSH). The development of the biweekly mode at LYRB is enhanced by the northwestward movement of a low-level anticyclonic anomaly from the Philippine Sea to the south of Taiwan, which is a result of the enhancement of the WNPSH resulting from its merger with a transient midlatitude high. In contrast, the development of the 21–30-day mode is enhanced by an upper-level trough anomaly moving from Lake Baikal to far east Russia. These two ISV periodicities are also found to be embedded in their corresponding source regions.

The new knowledge on the sources and evolutions of the two major LYRB ISV modes provides empirical predictors for the intraseasonal variation in the subtropical EA summer monsoon.

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Bin Wang, Jing Yang, Tianjun Zhou, and Bin Wang

Abstract

The present paper develops an integral view of the year-to-year variability across the entire Asian–Australian monsoon (A–AM) system, which covers one-third of the global tropics between 40° and 160°E. Using season-reliant empirical orthogonal function (S-EOF) analysis, the authors identified two major modes of variability for the period 1956–2004. The first exhibits a prominent biennial tendency and concurs with the turnabout of El Niño–Southern Oscillation (ENSO), providing a new perspective of the seasonally evolving spatiotemporal structure for tropospheric biennial oscillation. The second mode leads ENSO by one year. The remote El Niño forcing, the monsoon–warm pool ocean interaction, and the influence of the annual cycle are three fundamental factors for understanding the behavior of the first mode. The monsoon–ocean interaction is characterized by a positive feedback between the off-equatorial convectively coupled Rossby waves and the underlying sea surface temperature (SST) “dipole” anomalies.

Since the late 1970s the overall coupling between the A–AM system and ENSO has become strengthened. The relationships between ENSO and the western North Pacific, East Asian, and Indonesian monsoons have all become enhanced during ENSO’s developing, mature, and decaying phases, overriding the weakening of the Indian monsoon–ENSO anticorrelation during the developing phase. Prior to the late 1970s (1956–79), the first mode shows a strong biennial tendency, and the second mode does not lead ENSO. After 1980, the first mode shows a weakening biennial tendency, and the second mode provides a strong precursory signal for ENSO. These interdecadal changes are attributed to increased magnitude and periodicity of ENSO and the strengthened monsoon–ocean interaction.

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Soon-Il An and Bin Wang

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Conditional maximum covariance analysis is applied to investigate the coherent patterns between the tropical and North Pacific SST and the North Pacific 500-hPa geopotential height anomalies. Two leading modes are identified. One is an intrinsic midlatitude mode, the North Pacific (NP) mode, for which SST anomalies are mainly confined to the extratropical North Pacific. The other is a tropical ocean–atmosphere coupled mode, the ENSO mode, in which an ENSO-like SST pattern dominates the Tropics but extratropical SST anomalies are relatively weak.

The NP and ENSO modes exhibit distinct spatial and temporal characteristics. For the NP mode, atmospheric variation leads to changes in SST, while for the ENSO mode the opposite is true. The NP mode displays a persistence barrier during August–September whereas the ENSO mode has a March–April persistence barrier. The upper-tropospheric jet stream associated with the NP and ENSO mode intensifies, respectively, over the central North Pacific and the subtropical northeastern Pacific; consequently, the transient activities maximize in their corresponding jet exit regions. The expansion coefficients of the 500-hPa geopotential height associated with the two modes appear to be significantly correlated. However, by reducing the high-frequency part (e.g., shorter than the interannual time scale) in expansion coefficients, the correlation becomes insignificant, indicating that the significant correlation results from high-frequency signals that are unrelated to the corresponding SST variation. The results presented here suggest that the intrinsic coupled mode in the midlatitude North Pacific may be distinguished from the forced mode by remote ENSO, especially on the interannual time scale.

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Soon-Il An and Bin Wang

Abstract

The peaks of El Niño in the Cane–Zebiak (CZ) model tend to appear most frequently around November when the ocean Rossby waves, which were amplified during the previous unstable season (February–May), turn back to the eastern Pacific and when the local instability in the eastern Pacific is very weak. The peaks of La Niña in the CZ model occur most frequently in boreal summer, in contrast to the observed counterpart that usually occurs in boreal winter. Sensitivity experiments indicate that the phase locking of the La Niña to boreal summer is primarily caused by seasonal variations of the tropical convergence zone, which regulate convective heating through atmospheric convergence feedback. The observed thermocline and the wind anomalies in the western Pacific exhibit considerable seasonal variations. These were missed in the original CZ model. In a modified CZ model that includes the seasonal variations of the western Pacific wind anomalies and the basic-state thermocline depth, the peaks of La Niña preferably occur in boreal winter, suggesting that the seasonal variation of the western Pacific surface wind anomalies and the mean thermocline depth are critical factors for the phase locking of the mature La Niña to boreal winter. The mechanisms by which these factors affect ENSO phase locking are also discussed.

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Soon-Il An and Bin Wang

Abstract

In the late 1970s, the ENSO cycle exhibited frequency change. The oscillation period increased from 2–4 yr (high frequency) during 1962–75 to 4–6 yr (low frequency) during 1980–93. Observations suggest that this frequency change was accompanied by a significant change in the structure of the coupled ENSO mode. In comparison with the high-frequency regime, the structure of the coupled mode in the low-frequency regime shows three distinctive features during the warm phase of ENSO: the eastward shift of the westerly anomalies, the meridional expansion of the westerly anomalies, and the weaker intensity of the easterly anomalies in the eastern Pacific.

To test the robustness of the relationship between the oscillation period and the structure of the coupled mode, the authors designed empirical atmospheric models based on observations and coupled them with the ocean model of Zebiak and Cane. Numerical experiments demonstrate that the ENSO period is sensitive to changes in the wind anomaly pattern in a way much like the observed ENSO frequency–structure relation. The increase of the ENSO period after 1980 is mainly due to the eastward shift of the zonal wind stress with respect to the SST anomalies. Physical explanations of the dependence of ENSO frequency on the structure of the coupled mode are provided by diagnosing the relative contributions of the thermocline feedback and zonal advection feedback on ENSO evolution.

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Susan Kemball-Cook and Bin Wang

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A composite study of the life cycle of the boreal summer intraseasonal oscillation (BSISO) was performed using data from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis and National Oceanic and Atmospheric Administration polar-orbiting satellites. Because of pronounced differences in their climatologies, the boreal summer periods May–June (MJ) and August–October (AO) were composited separately. Characteristics of the BSISO life cycle common to MJ and AO were initiation and eastward propagation of the convective anomaly over the Indian Ocean, followed by poleward propagation, with the northward-moving branch having greater amplitude than the southward-moving branch. The transition of convection from the Indian Ocean to the western Pacific occurred next, followed by dissipation of the current cycle and initiation of the subsequent cycle. The MJ and AO life cycles were found to have several significant differences. The MJ shows strong eastward movement of convection along the equator in both the Indian and western Pacific Oceans. Convection in AO has a weaker eastward-propagating signal along the equator and displays a discontinuous jump from the Indian Ocean to the western Pacific. In marked contrast to MJ, AO shows strong northwestward propagation of convection in the western Pacific during the latter half of the BSISO life cycle. The change in the BSISO life cycle from MJ to AO reflects the seasonal shift in the distributions of vertical wind shear and low-level specific humidity from early to late summer. Rossby waves emitted by equatorial convection play a critical role in the BSISO in both the Indian and western Pacific Oceans. These waves are instrumental in the northward propagation of convection in MJ and AO. Both MJ and AO composites suggest that air–sea interactions are present in the BSISO, fostering both eastward and northward propagation of convective anomalies in the Indian Ocean and in the western Pacific. The complexity and pronounced seasonal dependence of the BSISO reflected in the composites suggest that its simulation is a rigorous test for general circulation models.

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