Abstract

The characteristics and possible energy sources of the South Asian jet wave train in winter are analyzed, with the intraseasonal signal emphasized. The wave train is equivalently barotropic and strongest in the upper troposphere, with its daily evolution dominated by the intraseasonal (10–30 day) time scale. Along the wave train, the propagation of disturbances from the North Atlantic to the western North Pacific takes around 8 days, which is much faster than the eastward migration of activity centers. The energy sources of the intraseasonal wave train are complicated and can be separated into three categories depending on the role of the North Atlantic Oscillation (NAO). When NAO− precedes the wave train, it is northwest–southeast oriented. The energy is rooted in the lower troposphere over the high-latitude North Atlantic, and excites the Rossby wave source (RWS) over the western Mediterranean Sea via vortex stretching by abnormal divergence. When NAO+ precedes the wave train, it is southwest–northeast oriented. The energy rooted in the northeastern activity center excites RWS over the eastern Mediterranean Sea. Additionally, disturbances from the western North Atlantic and southwestern activity center of NAO+ excite the RWS over the western Mediterranean Sea. Hence, both NAO− and NAO+ can excite the same wave train, but with different orientation and via different paths. Without the NAO, the wave train can also be stimulated by enhanced disturbances over the midlatitude central North Atlantic. The signal lies mainly in the middle-upper troposphere, which might be related to atmospheric internal dynamic processes, such as kinetic energy conversion from synoptic disturbances.

Abstract

This study investigated the increased impact of the spring (March–May) snow-cover extent (SCE) over the western Tibetan Plateau (TP) (SSTP) to the mei-yu rainfall [June–July (JJ)] over the Yangtze River valley (YRV) (MRYRV) after the 1990s. The correlation between the MRYRV and SSTP is significantly increased from the period of 1970–92 (P1) to 1993–2015 (P2). In P1, the MRYRV-related SSTP anomalies are located over the southwest TP, which causes a perturbation near the subtropical westerly jet (SWJ) core and favors an eastward propagation in the form of a wave train. The wave train results in a southward shift of the SWJ over the ocean south of Japan in JJ and exerts a limited effect on the MRYRV. Differently, in P2, the MRYRV-related anomalous SSTP causes an anomalous cooling temperature and upper-level cyclonic system centered over the northwestern TP. The cyclonic system develops and extends eastward to the downstream region with time and reaches coastal East Asia in JJ. The anomalous westerly winds along its south flank cause an enhanced SWJ, which is accompanied by an anomalous lower-level air convergence and ascent motion near the YRV region, favoring enhanced MRYRV. In addition, the forecast experiments performed with empirical regression models illustrate that the prediction skill of the MRYRV variation is clearly increased in P2 with the additional forecast factor of the SSTP.

Abstract

This study revisits the long-term variabilities of the East Asian summer monsoon (EASM) in 1958–2017 through examination of diurnal cycles. We group monsoon days into four dynamic quadrants (Q1 to Q4), with emphasis on the strong daily southerlies coupled with a large (Q1) or small (Q4) diurnal amplitude over Southeast China. The occurrence day of Q1 increases in June–July with the seasonal progress of the EASM. It is most pronounced in the 1960s to the 1970s and declines to the lowest in the 1980s to the 1990s, whereas the Q4 occurrence increases notably from the 1970s to the 1990s; both groups return to normal in recent years. The interdecadal decrease (increase) of Q1 (Q4) occurrence corresponds well to the known weakening of EASM in the twentieth century, and it also coincides with the rainfall anomalies over China shifting from a “north flooding and south drought” to a “north drought and south flooding” mode. The rainfall under Q1 (Q4) can account for ~60% of the interannual variance of summer rainfall in northern (southern) China. The contrasting effects of Q1 and Q4 on rainfall are due to their remarkably different regulation on water vapor transport and convergence. The interannual/interdecadal variations of Q1 (Q4) occurrence determine the anomalous water vapor transports to northern (southern) China, in association with the various expansion of the western Pacific subtropical high. In particular, Q1 conditions can greatly intensify nighttime moisture convergence, which is responsible for the long-term variations of rainfall in northern China. The results highlight that the diurnal cycles in monsoon flow act as a key regional process working with large-scale circulation to regulate the spatial distributions and long-term variabilities of EASM rainfall.

Abstract

South China experiences extreme heat (EH) most frequently in eastern China. This study specifically explores the large-scale circulation anomalies associated with long-lived EH events in south China. The results show that there is an anomalous cyclone (anticyclone) and active (inactive) convection over south China (the western Pacific) before the EH onset; then, an anticyclone develops and moves northwestward and dominates over south China on the onset day. The anomalous anticyclone maintains its strength over south China and then diminishes and is replaced by another cyclone migrating from the western Pacific after the final day of the EH event. Consequently, the temperature increases over south China around the onset day and is anomalously warm for approximately 10 days on average and then decreases shortly thereafter. The fluctuating anomalies over south China and the western Pacific are intimately related to two intraseasonal oscillation (ISO) modes, namely, the 5–25- and 30–90-day oscillations, which originate from the tropical western Pacific and propagate northwestward. The 5–25-day oscillation is vital to triggering and terminating EH, accounting for approximately half of the original temperature and circulation anomaly transitions. The 30–90-day oscillation favors the persistent warming during EH events, accounting for approximately one-third of the original prolonged warming and anticyclonic anomaly. This result suggests that different ISO modes play crucial roles at different stages of the events. Moreover, a higher annual frequency of long-lived EH days in south China is associated with the transition phase from El Niño to La Niña. It is suggested that both medium-range and interannual forecasting of long-lived EH in south China are possible.

Abstract

The present study investigates the possible linkage between the monsoon trough and the interannual variability of the activity of westward-propagating tropical waves (WTW) over the western North Pacific (WNP) during July–November for the period 1979–2007. It is shown that the interannual variability of WTW activity is closely related to the location of the monsoon trough. During the years when the enhanced (weakened) monsoon trough extends eastward (retreats westward), the lower-tropospheric WTW activity is above (below) normal within the southeastern quadrant of the WNP. Furthermore, this study evaluates different wave structures and dynamics of two types of WTWs, equatorial Rossby (ER) waves and mixed Rossby–gravity (MRG)–tropical depression (TD)-type waves, in strong monsoon trough (S-MT) and weak monsoon trough (W-MT) years over the WNP. There is a significant change in the three-dimensional structure as those waves propagate westward to the east of the monsoon trough. For the TD–MRG waves, an apparent transition from MRG waves to off-equatorial TD disturbances is identified in the region of the monsoon trough. For the ER waves, their amplitudes have a faster growth, but their structures and propagation characters have no marked change. Differences in the location of the monsoon trough may lead to an east–west contrast in the WTWs. In a companion study (Part II), diagnostics of energetics and numerical experiments are conducted to explain the observed results in the present study.

Abstract

Southern China, located in the tropical–subtropical East Asian monsoonal region, presents a unique anticyclonic–cyclonic circulation pattern during extreme heat (EH), obviously different from the typical anticyclone responsible for EH in many other regions. Associated with the evolution of EH in southern China, the anticyclonic–cyclonic anomalies propagate northwestward over the Philippines and southern China. Before the EH onsets, the anticyclonic anomaly dominates southern China, resulting in stronger subsidence over southern China and stronger southerly (southwesterly) flow over the western (northern) margins of southern China. The southerly (southwesterly) flow transports more water vapor to the north of southern China, thus, together with the local stronger subsidence, resulting in drier air condition and accordingly favoring the occurrence of EH. Conversely, after the EH onsets, the cyclonic component approaches southern China and offsets the high temperature.

The oscillations of temperature and circulation anomalies over southern China exhibit a periodicity of about 10 days and indicate the influence of a quasi-biweekly oscillation, which originates from the tropical western Pacific and propagates northwestward. Therefore, the 5–25-day-filtered data are extracted to further analyze the quasi-biweekly oscillation. It turns out that the evolution of the filtered circulation remarkably resembles the original anomalies with comparable amplitudes, indicating that the quasi-biweekly oscillation is critical for the occurrence of EH in southern China. The quasi-biweekly oscillation could explain more than 50% of the intraseasonal variance of daily maximum temperature T _max and vorticity over southern China and 80% of the warming amplitude of EH onsets. The close relationship between the circulation of the quasi-biweekly oscillation and the EH occurrence indicates the possibility of medium-range forecasting for high temperature in southern China.

Abstract

The upper-level tropical easterly jet (TEJ) is a crucial component of the summer monsoon system and tropical general circulation. The simulation and projection of the TEJ, however, have not been assessed. Here we evaluate models’ fidelity and assess the future change of the TEJ by utilizing 16 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). Most of the models can reproduce the TEJ reasonably well in terms of climatology, seasonal evolution, and interannual variability. Nevertheless, underestimation of the TEJ’s intensity and extent is identified, with the maximum bias occurring in the jet centers over the tropical Indian Ocean (IO) and the tropical eastern Pacific (EP). Under the shared socioeconomic pathway 5–8.5, the multimodel ensemble projects a remarkable reduction in the central TEJ intensity by about 18% over the IO and 77% over the EP toward the end of the twenty-first century. The mean intensity of TEJ will weaken by about 11%, and the extent will reduce by 6%, suggesting a significantly weakened upper-level monsoon circulation in the future climate. The projected El Niño–like warming pattern over the tropical Pacific may play a critical role in the future weakening of the TEJ via inducing suppressed rainfall over the tropical eastern IO and Central America. The model uncertainties in the projected TEJ changes may arise from the uncertainties in the models’ projected tropical EP warming. The sensitivity of future projections to model selection is also examined. Results show that the selection of models based on different physical considerations does not yield a significantly different projection.

Abstract

The leading mode of boreal spring precipitation variability over the tropical Pacific experienced a pronounced interdecadal change around the late 1990s. The pattern before 1998 features positive precipitation anomalies over the equatorial eastern Pacific (EP) with positive principle component years. The counterpart after 1998 exhibits a westward shift of the positive center to the equatorial central Pacific (CP). Observational evidence shows that this interdecadal change in the leading mode of precipitation variability is closely associated with a distinctive sea surface temperature (SST) anomaly pattern. The westward shift of the anomalous precipitation center after 1998 is in tandem with a similar shift of maximum warming from the EP to CP. Diagnostic analyses based on a linear equation of the mixed layer temperature anomaly exhibit that an interdecadal enhancement of zonal advection (ZA) feedback process plays a vital role in the shift in the leading mode of both the tropical Pacific SST and the precipitation anomaly during spring. Moreover, the variability of the anomalous zonal current at the upper ocean dominates the ZA feedback change, while the mean zonal SST gradient associated with a La Niña–like pattern of the mean state only accounts for a relatively trivial proportion of the ZA feedback change. It was found that both the relatively rapid decaying of the SST anomalies in the EP and the La Niña–like mean state make it conceivable that the shift of the leading mode of the tropical precipitation anomaly only occurs in spring. In addition, the largest variance of the anomalous zonal current in spring might contribute to the unique interdecadal change in the tropical spring precipitation anomaly pattern.

Abstract

Part I of this study examined the modulation of the monsoon trough (MT) on tropical depression (TD)-type–mixed Rossby–gravity (MRG) and equatorial Rossby (ER) waves over the western North Pacific based on observations. This part investigates the interaction of these waves with the MT through a diagnostics of energy conversion that separates the effect of the MT on TD–MRG and ER waves. It is found that the barotropic conversion associated with the MT is the most important mechanism for the growth of eddy energy in both TD–MRG and ER waves. The large rotational flows help to maintain the rapid growth and tilted horizontal structure of the lower-tropospheric waves through a positive feedback between the wave growth and horizontal structure. The baroclinic conversion process associated with the MT contributes a smaller part for TD–MRG waves, but is of importance comparable to barotropic conversion for ER waves as it can produce the tilted vertical structure. The growth rates of the waves are much larger during strong MT years than during weak MT years.

Numerical experiments are conducted for an idealized MRG or ER wave using a linear shallow-water model. The results confirm that the monsoon background flow can lead to an MRG-to-TD transition and the ER wave amplifies along the axis of the MT and is more active in the strong MT state. Those results are consistent with the findings in Part I. This indicates that the mean flow of the MT provides a favorable background condition for the development of the waves and acts as a key energy source.

Abstract

This study investigates interannual aerosol variations over the Maritime Continent and the western North Pacific Ocean and aerosol–cloud–precipitation relationship during the period 2000–10 based on monthly-mean anomalies. The local aerosol–cloud–precipitation relationship displays strong regional characteristics. The aerosol variation is negatively correlated with cloud and precipitation variation over the Maritime Continent, but is positively correlated with cloud and precipitation variation over the region southeast of Japan. Over broad subtropical oceanic regions, the aerosol variation is positively correlated with cloud variation, but has a weak correlation with precipitation variation. Aerosol variations over the Maritime Continent and over the region southeast of Japan display a biennial feature with an obvious phase lag of about 8 months in the latter region during 2001–07. This biennial feature is attributed to the impacts of El Niño events on aerosol variations in these regions through large-scale circulation and precipitation changes. Around October of El Niño–developing years, the suppressed precipitation over the Maritime Continent favors an aerosol increase by reducing the wet deposition and setting up dry conditions favorable for fire burning. During early summer of El Niño–decaying years, suppressed heating around the Philippines as a delayed response to El Niño warming induces an anomalous lower-level cyclone over the region to the southeast of Japan through an atmospheric teleconnection, leading to an accumulation of aerosol and increase of precipitation. The aerosol–precipitation relationship shows an obvious change with time over eastern China, leading to an overall weak correlation.