Jiangyu Mao

Intraseasonal Variability of the South China Sea Summer Monsoon

The objective of this study is to explore, based on the National Centers for Environmental Prediction– National Center for Atmospheric Research (NCEP–NCAR) reanalysis data, the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM) in terms of its structure and propagation, as well as interannual variations. A possible mechanism that is responsible for the origin of the 10–20-day oscillation of the SCS SM is also proposed. The 30–60-day (hereafter the 3/6 mode) and 10–20-day (hereafter the 1/2 mode) oscillations are found to be the two intraseasonal modes that control the behavior of the SCSSM activities for most of the years. Both the 3/6 and 1/2 modes are distinct, but may not always exist simultaneously in a particular year, and their contributions to the overall variations differ among different years. Thus, the interannual variability in the intraseasonal oscillation activity of the SCS SM may be categorized as follows: the 3/6 category, in which the 3/6 mode is more significant (in terms of the percentage of variance explained) than the 1/2 mode; the 1/2 category, in which the 1/2 mode is dominant; and the dual category, in which both the 3/6 and 1/2 modes are pronounced. Composite analyses of the 3/6 category cases indicate that the 30–60-day oscillation of the SCS SM exhibits a trough–ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the SCS, with anomalous cyclones (anticyclones), along with enhanced (suppressed) convection, migrating northward from the equator to the midlatitudes. The northward-migrating 3/6-mode monsoon trough–ridge in the lower troposphere is coupled with the eastward-propagating 3/6-mode divergence–convergence in the upper troposphere. It is also found that, for the years in the dual category, the SCS SM activities are basically controlled by the 3/6 mode, but modified by the 1/2 mode. Composite results of the 1/2-mode category cases show that the 10–20-day oscillation is manifest as an anticyclone–cyclone system over the western tropical Pacific, propagating northwestward into the SCS. A close coupling also exists between the upper-level convergence (divergence) and the low-level anticyclone (cyclone). It is found that the 1/2 mode of the SCS SM mainly originates from the equatorial central Pacific, although a disturbance from the northeast of the SCS also contributes to this mode. The flow patterns from an inactive to an active period resemble those associated with a mixed Rossby–gravity wave observed in previous studies.

The Role of Bay of Bengal Convection in the Onset of the 1998 South China Sea Summer Monsoon

Abstract Assimilated analysis fields from the South China Sea Monsoon Experiment and the outgoing longwave radiation data from the National Center for Atmospheric Research (NCAR) have been employed to describe the large-scale and synoptic features of the subtropical circulation during the Bay of Bengal (BOB; 6°–20°N, 80°–100°E) and South China Sea (SCS; 7°–20°N, 110°–120°E) monsoon onsets in 1998. The results show that the Asian monsoon onset during May 1998 exhibited a typical eastward development from the BOB region to the SCS domain. The weakening and retreat of the subtropical anticyclone from the SCS were preceded by the intrusion of westerlies and the development of convective activities over the northern part of the SCS (NSCS; 15°–20°N, 110°–120°E). As the vertical shear of zonal wind changes in sign, the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the SCS. Based on these observational results, version 4 of the NCAR climate model (CCM3) ...

Genesis of the South Asian High and Its Impact on the Asian Summer Monsoon Onset

AbstractThe formation of the South Asian high (SAH) in spring and its impacts on the Asian summer monsoon onset are studied using daily 40-yr ECMWF Re-Analysis (ERA-40) data together with a climate-mean composite technique and potential vorticity–diabatic heating (PV–Q) analysis. Results demonstrate that, about 2 weeks before the Asian summer monsoon onset, a burst of convection over the southern Philippines produces a negative vorticity source to its north. The SAH in the upper troposphere over the South China Sea is then generated as an atmospheric response to this negative vorticity forcing with the streamline field manifesting a Gill-type pattern. Afterward, the persistent rainfall over the northern Indochinese peninsula causes the SAH to move westward toward the peninsula. Consequently, a trumpet-shaped flow field is formed to its southwest, resulting in divergence pumping and atmospheric ascent just over the southeastern Bay of Bengal (BOB).Near the surface, as a surface anticyclone is formed over t...

Air–sea interaction and formation of the Asian summer monsoon onset vortex over the Bay of Bengal

In spring over the southern Bay of Bengal (BOB), a vortex commonly develops, followed by the Asian summer monsoon onset. An analysis of relevant data and a case study reveals that the BOB monsoon onset vortex is formed as a consequence of air–sea interaction over BOB, which is modulated by Tibetan Plateau forcing and the land–sea thermal contrast over the South Asian area during the spring season. Tibetan Plateau forcing in spring generates a prevailing cold northwesterly over India in the lower troposphere. Strong surface sensible heating is then released, forming a prominent surface cyclone with a strong southwesterly along the coastal ocean in northwestern BOB. This southwesterly induces a local offshore current and upwelling, resulting in cold sea surface temperatures (SSTs). The southwesterly, together with the near-equatorial westerly, also results in a surface anticyclone with descending air over most of BOB and a cyclone with ascending air over the southern part of BOB. In the eastern part of central BOB, where sky is clear, surface wind is weak, and ocean mixed layer is shallow, intense solar radiation and low energy loss due to weak surface latent and sensible heat fluxes act onto a thin ocean layer, resulting in the development of a unique BOB warm pool in spring. Near the surface, water vapor is transferred from northern BOB and other regions to southeastern BOB, where surface sensible heating is relatively high. The atmospheric available potential energy is generated and converted to kinetic energy, thereby resulting in vortex formation. The vortex then intensifies and moves northward, where SST is higher and surface sensible heating is stronger. Meanwhile, the zonal-mean kinetic energy is converted to eddy kinetic energy in the area east of the vortex, and the vortex turns eastward. Eventually, southwesterly sweeps over eastern BOB and merges with the subtropical westerly, leading to the onset of the Asian summer monsoon.

Diurnal variations of summer precipitation over the Asian monsoon region as revealed by TRMM satellite data

Climatological characteristics of diurnal variations in summer precipitation over the Asian monsoon region are comprehensively investigated based on the Tropical Rainfall Measuring Mission (TRMM) satellite data during 1998–2008. The topographic influence on the diurnal variations and phase propagations of maximum precipitation are identified according to spatiotemporal distributions of the amplitude and peak time of the diurnal precipitation. The amplitude and phase of diurnal precipitation show a distinct geographical pattern. Significant diurnal variations occur over most of continental and coastal areas including the Maritime Continent, with the relative amplitude exceeding 40%, indicating that the precipitation peak is 1.4 times the 24-h mean. Over the landside coasts such as southeastern China and Indochina Peninsula, the relative amplitude is even greater than 100%. Although the diurnal variations of summer precipitation over the continental areas are characterized by an afternoon peak (1500–1800 Local Solar Time (LST)), over the central Indochina Peninsula and central and southern Indian Peninsula the diurnal phase is delayed to after 2100 LST, suggesting the diurnal behaviors over these areas different from the general continental areas. The weak diurnal variations with relative amplitudes less than 40% exist mainly over oceanic areas in the western Pacific and most of Indian Ocean, with the rainfall peak mainly occurring from midnight to early morning (0000–0600 LST), indicating a typical oceanic regime characterized by an early morning peak. However, apparent exceptions occur over the South China Sea (SCS), Bay of Bengal (BOB), and eastern Arabian Sea, with the rainfall peak occurring in daytime (0900–1500 LST). Prominent meridional propagations of the diurnal phase exist in South Asia and East Asia. Along the eastern Indian Peninsula, there is not only the southward phase propagation with the peak occurring around 25°N but also the northward phase propagation with the peak beginning with the southernmost continent, and both reach the central Indian continent to finish. Along the same longitudes where southern China and Kalimantan are located, the diurnal phase of the former propagates from the oceanic area (northern SCS) toward the inland continent, while the phase of the latter propagates from the land area toward the outside sea, showing a landward or seaward coastal diurnal regime. A distinct zonal propagation of the diurnal phase is observed over the BOB oceanic area. The maximum precipitation zone originates from the land-sea boundary of the eastern coast of the Indian peninsula at around 0300 LST, and then propagates eastward with increasing time to reach the eastern coast of the BOB on 1800 LST, finally migrates into the Indochina continent on about 2100 LST.

Characteristics and Mechanism of the 10–20-Day Oscillation of Spring Rainfall over Southern China

AbstractThe intraseasonal oscillations (ISOs) of southern China spring rainfall (SCSR) are examined based on daily rain gauge rainfall data and NCEP/Department of Energy Reanalysis 2 (NCEP-2) products for the period 1980–2008. The objective of this study is to reveal the structure and propagation of the dominant ISO of SCSR as well as its driving mechanisms, thereby gaining an understanding of the causes of extreme wet and dry SCSR.The EOF analysis and power spectrum analysis show that the 10–20-day oscillation is a predominant ISO of SCSR in most years. Composite analyses and wave-activity propagation diagnosis demonstrate that the 10–20-day oscillation of SCSR is characterized by an alternate occurrence of a huge anomalous anticyclone (cyclone) encircling the Tibetan Plateau in the lower troposphere, with anomalous low-level northeasterly (southwesterly) winds prevailing over southern China, producing lower-tropospheric divergence (convergence). In the middle and upper troposphere, the oscillation appea...

A case study of the impact of boreal summer intraseasonal oscillations on Yangtze rainfall

The impact on Yangtze rainfall of the boreal summer intraseasonal oscillations (BSISOs) over the entire Asian summer monsoon region during summer 1996 was investigated using the APHRODITE gridded rainfall and the NCEP–DOE reanalysis II products. Wavelet analyses suggest that the ISOs of Yangtze rainfall were regulated mainly by both 30–60- and 10–25-day oscillations, respectively linked to BSISO1 and BSISO2 activity. Phase locking of the wet phases of these two ISOs resulted in a prolonged wet episode from late June to mid-July. The circulation evolution of the BSISO1 mode showed that active convection accompanied by strong convergence of anomalous zonal winds first developed over the equatorial Indian Ocean, with suppressed convection over the South China Sea (SCS)–Philippine Sea and with active convection over the Yangtze Basin. The triple convection anomaly that aligned meridionally in the East Asian sector arose from a local meridional–vertical cell associated with a Rossby wave-like coupled circulation–convection system. The opposite flow patterns occurred during the dry phase of Yangtze rainfall. The composite BSISO2 cases demonstrated a weak convective anomaly initially appeared around the Maritime Continent, with a huge anomalous anticyclone accompanied by suppressed convection over the SCS–Philippine Sea. The low-level convergence of the anomalous southwesterlies on the northwestern side of the anticyclone and the consequent ascent led to positive rainfall anomalies over the Yangtze Basin. When the entire SCS was dominated by an anomalous cyclone resulting from interaction with upstream systems, convection over the Yangtze Basin was suppressed.

Interdecadal modulation of ENSO-related spring rainfall over South China by the Pacific Decadal Oscillation

The interdecadal modulation of the relationship between El Niño–Southern Oscillation (ENSO) and the South China spring rainfall (SCSR) by the Pacific Decadal Oscillation (PDO) is investigated using long-term observational datasets. When ENSO and PDO are in-phase [i.e., El Niño events during warm PDO (EN_WPDO) and La Niña events during cold PDO (LN_CPDO)], the positive correlations between ENSO and SCSR are enhanced significantly, with above-normal (below-normal) SCSR generally following EN_WPDO (LN_CPDO) events. In contrast, the ENSO–SCSR relationship becomes ambiguous when ENSO and PDO are out-of-phase [i.e., El Niño events during cold PDO (EN_CPDO) and La Niña events during warm PDO (LN_WPDO)]. The PDO modulates the ENSO–SCSR relationship through the impact of variations in the lower-tropospheric subtropical anticyclone over the western North Pacific (WNP) and upper-tropospheric westerly jets over East Asia and the midlatitude North Pacific. An EN_WPDO (LN_CPDO) event induces an enhanced subtropical anticyclone (cyclone) over the WNP and intensified (weakened) subtropical westerly jet around the southern Tibetan Plateau due to modification by the PDO-forced anomalous circulation. Thus, South China falls just under the influence of the anomalous lower-tropospheric southwesterlies (northeasterlies) and upper-tropospheric divergent (convergent) environment, leading to above-normal (below-normal) SCSR. In contrast, the SCSR anomalies exhibit no wet or dry preference following EN_CPDO (LN_CPDO) events, because ENSO-induced and PDO-forced circulation anomalies tend to cancel each other out. These modulating effects by the PDO on the ENSO–SCSR relationship and related physical processes are also examined with coupled model simulations.

Different multi-fractal behaviors of diurnal temperature range over the north and the south of China

Multi-fractal behaviors of diurnal temperature range (DTR for short) from 100 stations over China during 1956–2010 are analyzed by means of multi-fractal detrended fluctuation analysis. By making a Monte-Carlo simulation, we obtain two criterions which can be used to decide whether a DTR series is significantly multi-fractal or not. With these criterions, different multi-fractal behaviors are found over the north and the south of China, and Yangtze River is roughly the dividing line. Over the north region, nearly all the considered DTR series do not show multi-fractal behaviors, while the results are completely the opposite over the south. The findings are confirmed by the scaling behaviors of the corresponding DTR magnitude series and indicate that more scale-dependent structure differences may be hidden in DTR series over the north and the south of China. Therefore, an extensive analysis of the multi-fractal behaviors are essential for a better understanding of the complex structures of the climate changes.

Novel exact solutions to the short pulse equation

In this paper, the bridge connecting the short pulse equation (SPE for short) with the sine-Gordon equation is applied to construct the novel solutions to the short pulse equation. It is shown that the solutions of the sine-Gordon equation can be used to obtain many different kinds of solutions to the short pulse equation with the aid of symbolic computation and plot representation of Maple.