Renhe Zhang

Role of intraseasonal oscillation in asymmetric impacts of El Niño and La Niña on the rainfall over southern China in boreal winter

The impacts of El Niño and La Niña on the rainfall over southern China in winter half year (November–April) are investigated diagnostically. It is found that positive rainfall anomalies with statistical significance appear over southern China in El Niño winter but no reversed significant rainfall anomalies in La Niña winter. The asymmetry of the rainfall anomalies are related with the asymmetry in anomalous circulation over western North Pacific (WNP) in lower troposphere, with a strong anomalous anticyclone in El Niño winter but a weak anomalous cyclone in La Niña winter. It is revealed that the asymmetric responses in anomalous circulation to El Nino and La Niña are affected by the intraseasonal oscillation (ISO). In El Niño winter the interannual variation is dominant and the ISO is weak, and the strong interannual variation leads to a strong response of the anomalous anticyclone over WNP in winter half year. However, in La Niña winter the ISO is dominant and the interannual variation is weak, and the weak interannual variation results in a weak response of the anomalous cyclone over WNP.

Modulation of the atmospheric quasi-biweekly oscillation on the diurnal variation of the occurrence frequency of the Tibetan Plateau vortices

In this study, modulation of the atmospheric quasi-biweekly oscillation (QBWO) on diurnal variation of the occurrence frequency of Tibetan Plateau vortices (TPVs) during May–August of 2000–2009 was investigated. The diurnal variations of the occurrence frequency of the TPVs (OFTPVs) and the related dynamic and thermodynamic features in the positive and negative phases of QBWO were compared. In both the positive and negative phases, the OFTPVs reaches the maximum from evening to midnight (18–00 LT, LT indicates the local time), and minimum from early morning to noon (06–12 LT). At 18 LT, there is strongest convergence at 500xa0hPa and ascending motion, as well as the most abundant net water vapor budget over the Tibetan Plateau, which is in favor of the precipitation and the related condensation latent heat release, corresponding to the maximum of OFTPVs in 18–00 LT. On the contrary, in the early morning at 06 LT, the conditions are most unfavorable for genesis of TPVs in 06–12 LT. QBWO leads to stronger convergence at 500xa0hPa, ascending motion as well as more massive water vapor in the positive phases than those in the negative phases, resulting in larger numbers of TPVs occur in all of the four periods of a day (00–06 LT, 06–12 LT, 12–18 LT, and 18–00 LT) in the former. The TPVs generating from the early morning to noon (06–12 LT) are weaker and more sensitive and fragile to the disadvantageous background, while the TPVs occurring from evening to midnight (18–00 LT) are stronger and seem to be well tolerated, leading to more remarkable contrast between the OFTPVs in the negative and positive phases in 06–12 LT than in 18–00 LT.

Effect of the atmospheric quasi-biweekly oscillation on the vortices moving off the Tibetan Plateau

In the present study, the relationship between the atmospheric quasi-biweekly oscillation (QBWO) and Tibetan Plateau vortices (TPVs) moving off the Tibetan Plateau was investigated based on the radiosonde and reanalysis data. It is found that the number of TPVs moving off the Tibetan Plateau (moving-off TPVs) has the distinct feature of the 10–20-day QBWO. 77% of the moving-off TPVs occur in the positive phases of the 10–20-day filtered 500xa0hPa vorticity over eastern Tibetan Plateau. Besides, distributions of the zonal and meridional components of E-vectors coincide well with the trajectories of TPVs, indicating the moving-off TPVs are well related with the propagation of the QBWO energy. The atmospheric circulations and related thermodynamic fields are discussed to reveal the mechanism of the effect of 10–20-day QBWO on the moving-off TPVs. It is found that the atmospheric circulations and heating fields of 10–20-day QBWO have major impact on the moving-off TPVs. In positive QBWO phases, at 500xa0hPa over eastern Tibetan Plateau, there appear negative geopotential height anomalies and anomalous cyclonic wind shear; the anomalous jet stream and positive geopotential heights at 200xa0hPa lying over the northeast of the Tibetan Plateau stretch eastward gradually, benefiting for the upper level divergence and ascending motion. The condensation latent heat is released and shifts eastward with the heating centers located at 400xa0hPa, which depresses the isobaric surface at 500xa0hPa. All these conditions are in favor of the maintenance and eastward movement of TPVs in the positive QBWO phases.

Partial least regression approach to forecast the East Asian winter monsoon using Eurasian snow cover and sea surface temperature

Seasonal prediction of the East Asian (EA) winter monsoon (EAWM) is of great significance yet a challenging issue. In this study, three statistical seasonal prediction models for the EAWM are established using three leading modes of the Eurasian snow cover (ESC), the first leading mode of sea surface temperature (SST) and the four leading modes of the combination of the ESC and SST in preceding autumn, respectively. These leading modes are identified by the partial-least square (PLS) regression. The first PLS (PLS1) mode for the ESC features significantly anomalous snow cover in Siberia and Tibetan Plateau regions. The ESC second PLS (PLS2) mode corresponds to large areas of snow cover anomalies in the central Siberia, whereas the third PLS (PLS3) mode a meridional seesaw pattern of ESC. The SST PLS1 mode basically exhibits an El Niño-Southern Oscillation developing phase in equatorial eastern Pacific and significant SST anomalies in North Atlantic. A strong EAWM tends to emerge in a La Niña year concurrent with cold SST anomalies in the North Atlantic, and vice versa. After a 35-year training period (1967–2001), three PLS seasonal prediction models are constructed and the 11-year hindcast is performed for the period of 2002–2012, respectively. The PLS model based on combination of the autumn ESC and SST exhibits the best hindcast skill among the three models, its correlation coefficient between the observation and the hindcast reaching 0.86. This indicates that this physical-based PLS model may provide another practical tool for the EAWM. In addition, the relative contribution of the ESC and SST is also examined by assessing the hindcast skills of the other two PLS models constructed solely by the ESC or SST. Possible physical mechanisms are also discussed.

Seasonal prediction and predictability of Eurasian spring snow water equivalent in NCEP Climate Forecast System version 2 reforecasts

The spring snow water equivalent (SWE) over Eurasia plays an important role in East Asian and Indian monsoon rainfall. This study evaluates the seasonal prediction capability of NCEP Climate Forecast System version 2 (CFSv2) retrospective forecasts (1983–2010) for the Eurasian spring SWE. The results demonstrate that CFSv2 is able to represent the climatological distribution of the observed Eurasian spring SWE with a lead time of 1–3xa0months, with the maximum SWE occurring over western Siberia and Northeastern Europe. For a longer lead time, the SWE is exaggerated in CFSv2 because the start of snow ablation in CFSv2 lags behind that of the observation, and the simulated snowmelt rate is less than that in the observation. Generally, CFSv2 can simulate the interannual variations of the Eurasian spring SWE 1–5xa0months ahead of time but with an exaggerated magnitude. Additionally, the downtrend in CFSv2 is also overestimated. Because the initial conditions (ICs) are related to the corresponding simulation results significantly, the robust interannual variability and the downtrend in the ICs are most likely the causes for these biases. Moreover, CFSv2 exhibits a high potential predictability for the Eurasian spring SWE, especially the spring SWE over Siberia, with a lead time of 1–5xa0months. For forecasts with lead times longer than 5xa0months, the model predictability gradually decreases mainly due to the rapid decrease in the model signal.

Interannual variability and dynamics of intraseasonal wind rectification in the equatorial Pacific Ocean

The rectification of intraseasonal wind forcing on interannual sea surface temperature anomalies (SSTA) and sea level anomalies (SLA) associated with El Niño–Southern Oscillation (ENSO) during 1993–2016 are investigated using the LICOM ocean general circulation model forced with daily winds. The comparisons of the experiments with and without the intraseasonal wind forcing have shown that the rectified interannual SSTA and SLA by the intraseasonal winds are much weaker than the total interannual SSTA and SLA in the cold tongue, due to the much weaker rectified than the total interannual Kelvin and Rossby waves in the equatorial Pacific Ocean. The dynamics of the rectification are through the nonlinear zonal and vertical advection by the background currents, which produces downwelling equatorial Kelvin waves during El Niño. The meridional advection is much smaller than the zonal and vertical advection, suggesting that the rectification is not induced by the Ekman dynamics or the thermocline rectification. The rectified interannual Kelvin waves are found to be much smaller than reflected at the Pacific western boundary and those forced by the interannual winds, suggesting that the latter two play a much more important role in ENSO dynamics than the intraseasonal winds. The results of this study suggest an unlikely significant role of oceanic nonlinear rectification by intraseasonal winds during the onset and cycling of El Niño.

Development and eastward movement mechanisms of the Tibetan Plateau vortices moving off the Tibetan Plateau

Based on the Final operational global analysis data from the Global Forecasting System of the National Centers for Environment Prediction and the radiosonde data, the development and eastward movement mechanisms of 15 Tibetan Plateau vortices (TPVs) after they move off the plateau are investigated. The results show that the convergence to the east of the TPVs at 500xa0hPa, the divergence associated with the westerly jet stream at 200xa0hPa, as well as the corresponding ascending motion provide favorable conditions for the development and eastward movement of the TPVs. The spatial structures of the atmospheric apparent heat source (Q1) and the apparent moisture sink (Q2) are studied, showing that the heating centers of Q1 at 400xa0hPa mainly sourced from the condensation latent heat are beneficial to the eastward movement of the TPVs, while the horizontal distribution of Q1 at 500xa0hPa goes against that. The development and eastward movement mechanisms of the TPVs after they move off the plateau are further discussed through diagnosing the potential vorticity (PV) tendency equation. It is revealed that the horizontal PV flux convergence to the east of the TPVs related to the convergence at 500xa0hPa plays as the dominant role, exerting positive contribution to the PV tendency. Meanwhile, the heating fields induce feeble PV tendency, indicating more important effect of the dynamic factor. The development and eastward movement mechanisms of the TPVs after they move off the plateau are different from those before the TPVs move off, and the dynamic effect is vital in the former stage while the effect of Q1 is revealed as the dominant influencing factor in the latter.

Interdecadal changes in the asymmetric impacts of ENSO on wintertime rainfall over China and atmospheric circulations over western North Pacific

Based on data diagnoses, in this study the interdecadal changes in asymmetric impacts of ENSO on wintertime East Asian climate are investigated. It is found that the Pacific Decadal Oscillation (PDO) can significantly modulate the asymmetry in the responses of the East Asian climate to El Niño and La Niña events. In positive PDO phase the response is asymmetric with a strong anomalous anticyclone over western North Pacific (WNP) and significant positive rainfall anomalies over southern China during El Niño winters, but a weak anomalous cyclone over WNP and insignificant negative rainfall anomalies over southern China during La Niña winters. However, such asymmetric responses do not appear in negative PDO phase, with comparable amplitudes of anomalous circulations over WNP and rainfall over southern China in El Niño and La Niña winters. Further analyses reveal that the warm background of the tropical Pacific in positive PDO phase causes significant difference in the amplitudes of convection anomalies over tropical western Pacific, resulting in asymmetric responses of the WNP circulation and rainfall over southern China to El Niño and La Niña events. Nevertheless, the cold background in negative PDO phase reduces the amplitude difference between El Niño and La Niña winters. A comparable convection anomalies appear in tropical western Pacific in El Niño and La Niña winters and the asymmetric responses do not occur.

Modulation of the Intensity of Nascent Tibetan Plateau Vortices by Atmospheric Quasi-Biweekly Oscillation

The modulation of the intensity of nascent Tibetan Plateau vortices (ITPV) by atmospheric quasi-biweekly oscillation (QBWO) is investigated based on final operational global analysis data from the National Centers for Environmental Prediction. The spatial and temporal distributions of the ITPV show distinct features of 10–20-day QBWO. The average ITPV is much higher in the positive phases than in the negative phases, and the number of strong TPVs is much larger in the former, with a peak that appears in phase 3. In addition, the maximum centers of the ITPV stretch eastward in the positive phases, indicating periodic variations in the locations where strong TPVs are generated. The large-scale circulations and related thermodynamic fields are discussed to investigate the mechanism by which the 10–20-day QBWO modulates the ITPV. The atmospheric circulations and heating fields of the 10–20-day QBWO have a major impact on the ITPV. In the positive QBWO phases, the anomalous convergence at 500 hPa and divergence at 200 hPa are conducive to ascending motion. In addition, the convergence centers of the water vapor and the atmospheric unstable stratification are found in the positive QBWO phases and move eastward. Correspondingly, condensational latent heat is released and shifts eastward with the heating centers located at 400 hPa, which favors a higher ITPV by depressing the isobaric surface at 500 hPa. All of the dynamic and thermodynamic conditions in the positive QBWO phases are conducive to the generation of stronger TPVs and their eastward expansion.摘要本研究基于NCEP的FNL资料对大气准双周振荡(QBWO)对青藏高原初生涡强度(ITPV)的调制进行了分析. 结果指出, ITPV的时空分布具有显著的10–20天的准双周振荡特征. QBWO正位相时, 平均IPTV比负位相时高, 强高原低涡的个数也明显高于负位相时, 强高原低涡数量的峰值出现在第三位相. 正位相时, ITPV的最大值中心向东延伸, 表示强高原的发生位置存在周期性变化. 10–20天滤波的大尺度环流场和相关加热场对ITPV有重要的调制作用. 在QBWO正位相期间, 从第1到第4位相, 500hPa的异常辐合, 200hPa的异常辐散, 以及相应的垂直上升运动逐渐东移; 水汽的辐合中心和大气不稳定层结也向东移动, 这些热力和动力条件有利于降水的发生. 因此, 与降水相关的凝结潜热加热逐渐东移, 其加热中心位于400hPa, 有利于500hPa等压面的降低, 进而有利于强高原低涡的发生.