Evolution of Warm Season Intense Rainfall in Yaan Against a Cold-Anomaly Background

This study investigates the rainfall characteristics during intense rainfall over Yaan against a cold-anomaly background, aiming to refine the understanding of different kinds of rainfall events across complex terrain. Hourly rain gauge records, ERA5 reanalysis data and the black body temperature of cloud tops derived from FY-2E were used. The results show that against a cold-anomaly background, the regional rainfall events (RREs) in Yaan exhibit west-to-east propagation, which is different from the north-to-south evolution of warm RREs. The middle and upper troposphere is dominated by a negative geopotential height anomaly corresponding to the cold anomaly. The cyclonic circulation at the higher level associated with the negative geopotential height anomaly bends the high-level jet to the south, forming a divergent zone over the Tibetan Plateau (TP) and guiding mid-level systems to move eastward. The cyclonic circulation at the mid-level produces a wind shear zone over the TP, generating anomalous vorticity that continuously moves eastward and develops to influence the rainfall over Yaan. The cold Yaan RREs are closely related to the TP low-pressure systems (both vortex and shearline). The anomalous vorticity over the TP can influence the local vortex over the eastern periphery of the TP at a distance mainly by the horizontal advection of anomalous vorticity by the mean flow and then enhance the local vortex mainly by anomalous convergence when it moves near Yaan.

The Quasi-Biweekly Oscillation of Summer Rainfall in Southern China and Its Relationship With the Geopotential Height Anomaly Over the North Atlantic Ocean

Summer precipitation in East Asia has significant quasi-biweekly (10–30-day) oscillation characteristics. By using gauge-based precipitation and ERA-Interim reanalysis data, the basic mode of the quasi-biweekly oscillation of summer precipitation in East Asia and the related circulation from 1979 to 2012 were analyzed. It was found that the middle and lower reaches of the Yangtze River and its south in China were among the key areas for the 10- to 30-day oscillation of summer precipitation. After selecting typical summer precipitation events with 10- to 30-day oscillation characteristics in key areas and conducting composite analysis, it is found that in the dry (wet) phase of quasi-biweekly precipitation in southern China, it is controlled by quasi-biweekly anticyclone (cyclone) at 500 hPa above the key area. During the evolution of quasi-biweekly precipitation, the ridge of the Northwest Pacific Subtropical High is located between 20 and 22°N latitude, and there is no significant variability in the large-scale background circulation. Furthermore, composite analysis of the precursory signal at 500 hPa during quasi-biweekly precipitation in southern China found that there was an obvious quasi-biweekly geopotential height anomaly over the North Atlantic Ocean almost 30 days before the peak day of quasi-biweekly precipitation. While the quasi-biweekly geopotential height anomaly at 500 hPa in the North Atlantic propagates eastward, it also leads the cold air to transport southward. Cold air from high latitudes and warm air from low latitudes converge in southern China, which affects the quasi-biweekly oscillation of precipitation. Hysteresis synthesis of precipitation based on 500 hPa geopotential height’s quasi-biweekly oscillation events over the North Atlantic Ocean comes to almost the same conclusion. Therefore, the 500 hPa geopotential height quasi-biweekly anomaly in the North Atlantic may have important prediction significance for an extended-range forecast of summer rainfall in China.

The Combined Effects of ENSO and Solar Activity on Mid-Winter Precipitation Anomalies Over Southern China

Both the El Niño-Southern Oscillation (ENSO) and the 11-years solar cycle had been identified as important factors that may influence the wintertime southern China precipitation (SCP). However, the interactions between these two factors remain less noticed. In this study, the combined effects of the ENSO and the solar activity on mid-winter (January) SCP are investigated using observational and reanalysis data. Results suggest that both the ENSO and the solar activity are positively correlated with the SCP, although exhibiting distinct spatial patterns. Under different combinations of the ENSO and solar phases, the SCP anomalies show superposition of these two factors to some extent. Generally, the ENSO-related SCP anomalies tend to be enhanced (disturbed) when the ENSO and the solar activity are in-phase (out-of-phase). But this solar modulation effect appears more clear and significant under cold ENSO (cENSO) phase rather than under warm ENSO (wENSO) phase. Further analysis suggests, during the wENSO phase, solar influences on the Northern Hemisphere circulation are generally weak with little significance. In contrast, during the cENSO phase, the solar effect resembles the positive phase of the Arctic Oscillation but with an evident zonal asymmetric component. Its manifestation over the Asia-Pacific domain features by negative geopotential height anomaly over the West Asia and positive geopotential height anomaly over the East Asian coast, a pattern that is favorable for the SCP, thus causing a significant solar modulation on the cENSO-related precipitation anomalies. Further, the potential physical causes of solar effects on circulation are also discussed. Our results highlight the importance of considering solar cycle phase when ENSO is used to predict the East Asian winter climate.

Dominant Characteristics of early autumn Arctic sea ice variability and its impact on Winter Eurasian climate

The present study investigated dominant characteristics of autumn Arctic sea ice concentration (SIC) interannual variations and impacts of September-October (SO) mean SIC anomalies in the East Siberian-Chukchi-Beaufort (EsCB) Seas on winter Eurasian climate variability. Results showed that the decreased SO EsCB sea ice is favorable for tropospheric warming and positive geopotential height anomaly over the Arctic region one month later through transporting much more heat fluxes to the atmosphere from the open water. When entering the early winter (ND(0)J(1)), enhanced upward propagation of quasi-stationary planetary waves in the mid-high latitudes generates anomalous Eliassen-Palm flux convergence in the upper troposphere, which decelerates the westerly winds and maintains the positive geopotential height anomaly in the Arctic region. This anticyclonic anomaly extends southward into the central-western Eurasia and leads to evident surface cooling there. Two months later, it further develops toward downstream accompanied by a deepened trough, making the northeastern China experience a colder late winter (JFM(1)). Meanwhile, an anticyclonic anomaly over the eastern North Pacific excites a horizontal eastward wave train and contributes to positive (negative) geopotential height anomaly around the Greenland (Europe), favoring negative surface temperature anomaly over western Europe. In addition, the stratospheric polar vortex is also significantly weakened in the wintertime, which is attributed to decreased meridional temperature gradient and decelerated westerly winds provides a favorable condition for much more quasi-stationary planetary waves propagating into the stratosphere. Some major features of atmospheric responses to EsCB sea ice loss are well reproduced in the CAM4 sensitivity experiments.

What Kinds of Atmospheric Anomalies Drive Wintertime North Pacific Basin-Scale Subtropical Oceanic Front Intensity Variation?

ABSTRACTThe basin-scale subtropical oceanic front zone (STFZ) is a key region for midlatitude air–sea interaction in the North Pacific. However, previous studies considered midlatitude sea surface temperature (SST) variabilities as a response to atmospheric stochastic forcing. With reanalysis and observational data, this study investigates what kinds of atmospheric anomalies drive the wintertime North Pacific STFZ intensity variation. Lead correlations show that prior to the STFZ’s enhancement, there exist persistent atmospheric anomalies characterized by a negative-phase Arctic Oscillation (AO) and a positive-phase Pacific–North American (PNA) pattern, lasting for up to 80 and 50 days and peaking at 20- and 8-day leads, respectively. It is further found that the long-lasting negative-phase AO is conducive to stronger low-tropospheric baroclinicity at around 40°N over North Pacific where there is a climatological baroclinic region. The stronger baroclinicity leads to more synoptic transient eddy activities, promoting an equivalent barotropic low geopotential height anomaly north of STFZ via transient eddy vorticity forcing. The geopotential height anomaly propagates downstream, triggering a PNA-like pattern. With such an AO-promoted atmospheric internal wave–flow feedback, the regional PNA pattern is intensified and embedded in the annular AO mode, accompanied with an intensified Aleutian low and surface westerly wind that peak at an 8-day lead, preconditioning a persistent (nonstochastic) atmospheric forcing on the STFZ. The intensified surface westerly predominantly tends to drive a southward Ekman transport and increase upward surface turbulent heat fluxes into the atmosphere through increasing surface wind speed and sea–air temperature difference, amplifying the underlying negative SST anomaly and cross-frontal meridional SST gradient, ultimately intensifying the STFZ.

Dominant Characteristics of early autumn Arctic sea ice variability and its impact on Winter Eurasian climate

<p>The present study investigated dominant characteristics of autumn Arctic sea ice concentration (SIC) interannual variations, and examined impacts of SIC anomalies in the East Siberian-Chukchi-Beaufort (EsCB) Seas on winter Eurasian climate variability and the associated possible physical mechanism. Results showed that the Arctic SIC variations in both September and October display a certain continuity to some extent, thus, we chose the September-October (SO) mean SIC as a factor to explore its delayed impacts on winter atmosphere. Dominant features of Arctic SIC variability in SO is characterized by sea ice loss in the EsCB Seas, with more evident interannual variability since the late 1990s. Such a change can be attributed to the central Arctic pattern of atmospheric variability. Along with the global warming, the interannual variation of sea ice in the EsCB Seas seemingly exerts an increasingly role in the Northern Hemispheric climate variability. When the EsCB sea ice decreases in the early autumn (SO), a barotropic response of wave number 2 structure with significant warming and positive geopotential height anomaly dominates the Arctic region a month later. Then, in the early winter (ND(0)J(1)), the Arctic anticyclonic anomaly extends southward into the central-western Eurasia and leads to evident surface cooling there. Two month later, it further develops toward downstream accompanied by a deepened trough, making the East Asia experience a colder late winter (JFM(1)), especially in the northeastern China. Meanwhile, enhanced North Pacific anticyclonic perturbation excites an eastward wave train and contributes to positive geopotential height anomaly around the Greenland. Combined with a cyclonic anomaly to its southeast, a dipole structure forms and favors negative surface temperature anomaly covering the western Europe. In addition, a weakened polar vortex in the lower stratosphere can be observed during the boreal winter. Similar atmospheric responses to EsCB sea ice loss are well reproduced in the simulation experiments, not only supporting the conclusions from observational analyses, but also illustrating the possible physical mechanism to some extent.</p>

Frequency of Persistent Blocking and Ridge Events Related to Precipitation over Eastern China during August and Its Preceding Atmospheric Signals

Based on a recently developed approach that can recognize both persistent blocking and ridge events effectively, the contributions of the frequency of these persistent events (FOPE) over different regions in Eurasia to precipitation over eastern China were investigated. The results reveal that, the FOPE over the longitudinal range of 110°–130°E, near the Stanovoy Mountains and the Okhotsk Sea, is significantly correlated with precipitation over the middle and lower reaches of the Yangtze River (MLRYR) during summer, particularly in August. The preceding full July (or 1–20 July) mean Balkhash Lake–Caucasus geopotential height index, which measures the combined effect of the Balkhash Lake and Caucasus geopotential height anomalies, is closely related to the August geopotential height anomaly around the Stanovoy Mountains and the Okhotsk Sea, and can therefore reflect the August 110°–130°E FOPE. The predictability based on this preceding atmospheric signal seems to be attributable to slow-varying atmospheric processes on a subseasonal (20-day mean) time scale. On this time scale, the Balkhash Lake and Caucasus geopotential height anomalies occur prior to, and seem to modulate, the geopotential height anomaly around the Stanovoy Mountains and the associated 110°–130°E FOPE through an eastward extension and through exciting a positive–negative–positive pattern in 500-hPa geopotential heights, respectively. As a result of the slow-varying atmospheric processes, this preceding atmospheric signal performs well in predicting the August 110°–130°E FOPE, which also facilitates the prediction of the MLRYR precipitation.

Observing System Experiments in a 3DVAR Data Assimilation System at CPTEC/INPE

The Center for Weather Forecast and Climate Studies [Centro de Previsão e Tempo e Estudos Climáticos (CPTEC)] at the Brazilian National Institute for Space Research [Instituto Nacional de Pesquisas Espaciais (INPE)] has recently operationally implemented a three-dimensional variational data assimilation (3DVAR) scheme based on the Gridpoint Statistical Interpolation analysis system (GSI). Implementation of the GSI system within the atmospheric global circulation model from CPTEC/INPE (AGCM-CPTEC/INPE) is hereafter referred to as the Global 3DVAR (G3DVAR) system. The results of an observing system experiment (OSE) measuring the impacts of radiosonde, satellite radiance, and GPS radio occultation (RO) data on the new G3DVAR system are presented here. The observational impact of each of these platforms was evaluated by measuring the degradation of the geopotential height anomaly correlation and the amplification of the RMSE of the wind. Losing the radiosonde, GPS RO, and satellite radiance data in the OSE resulted in negative impacts on the geopotential height anomaly correlations globally. Nevertheless, the strongest impacts were found over the Southern Hemisphere and South America when satellite radiance data were withheld from the data assimilation system.