No abstract

The East Asian summer monsoon (EASM) may exhibit rather large variability between years characterized by the same ENSO phase. Such inconsistency reduces the EASM predictability based on ENSO. Results in this study show that the Tibetan Plateau snow cover (TPSC) exerts a modulating effect on ENSO teleconnections and ENSO significantly correlates with the EASM only during the reduced TPSC summers. Three-dimensional circulation structures are examined to manifest that the typical ENSO signals in reduced TPSC summers tend to be stronger than in excessive TPSC summers. Numerical and theoretical evidences indicate that the anomalously reduced TPSC can force positive geopotential height anomalies at the upper troposphere and weaken the jet streams across eastern Asia and northwestern Pacific. Governed by such basic state zonal flows, the extratropical Rossby wave response to the ENSO forcing usually has a larger amplitude and pronounced westward development. In such case, ENSO extends its influences to eastern Asia and enhances its connection with the EASM.

[1] Seasonal prediction of heat wave variability is a scientific challenge and of practical importance. This study investigates the heat wave frequency (HWF) variability over North America (NA) during the past 53 summers . It is found that the NA HWF is dominated by two distinct modes: the interdecadal (ID) mode and the interannual (IA) mode. The ID mode primarily depicts a HWF increasing pattern over most of the NA continent except some western coastal areas. The IA mode resembles a tripole HWF anomaly pattern with three centers over the northwestern, central, and southern NA. The two leading modes have different dynamic structures and predictability sources. The ID mode is closely associated with the prior spring sea surface temperature anomaly (SSTA) in the tropical Atlantic and tropical western Pacific that can persist throughout the summer, whereas the IA mode is linked to the development of El Niño-Southern Oscillation. A simplified general circulation model is utilized to examine the possible physical mechanism. For the ID mode the tropical Atlantic SSTA can induce a Gill-type response which extends to NA, while the northwestern Pacific SSTA excites a Rossby wave train propagating eastward toward NA. These two flow patterns jointly contribute to the formation of the large-scale circulation anomalies associated with the ID mode. For the IA mode the corresponding circulation anomalies are basically similar to a Pacific-North America pattern. The subsidence associated with high-pressure anomalies warms and dries the boundary layer, inhibiting cloud formation. The resulting surface radiative heating further warms the surface. For the low-pressure anomalies the situation is just opposite. Through such processes these SSTAs can exert profound influences on the HWF variability over NA.

No abstract

This study aims to clarify the relationship between chemical weathering of rocks and the carbon budget of rivers and to better understand the weathering mechanisms of plateau watersheds. We chose to study the Jinsha River, which originates from the Tibetan Plateau and also is in the upper reaches of the Changjiang River. Analysis of hydrochemistry, radiogenic strontium isotope and stable calcium isotopes were conducted of the Jinsha River water samples, which were collected along its mainstream and main tributaries in the summer. The results show that the water chemistry of the mainstream is dominated by evaporite weathering, which has a low 87 Sr/ 86 Sr (0.7098 to 0.7108) and wide range of Sr contents (2.70 to 9.35 μmol/L). In contrast, tributaries of the Jinsha River have higher 87 Sr/ 86 Sr (0.7090 to 0.7157) and lower Sr contents (~1μmol/L). Moreover, the Ca isotopic compositions in the mainstream (0.87-1.11‰) are heavier than the tributaries (0.68-0.88 ‰) and could not attribute to the conventional mixing of different sources. We suggest that secondary carbonate precipitation fractionates Ca isotopes in the Jinsha River, and fractionation factors are between 0.99935 and 0.99963. At least 66% of dissolved Ca is removed in the mainstream of the Jinsha River through secondary mineral precipitation, and the average value is ~35% in the tributaries. The results highlight that evaporite weathering results in more carbonate precipitation influencing Ca transportation and cycling in the riverine system constrained by stable Ca isotopic compositions and water chemistry.

Homochiral and racemic polymorphs show different spin-crossover behaviours due to different intermolecular interactions, and reversible LIESST effects can be realized on homochiral complexes.

No abstract

This study reviews the development of the omega equation and the relation between omega and potential vorticity (PV) advection and considers the application of PV theory to investigate the impact of large‐scale mountains on downstream weather development. A diabatic quasi‐geostrophic omega equation is introduced to reveal the feedback of diabatic heating (Q) on PV advection and vertical velocity. A challenge therein concerning the use of the diagnostic omega equation to interpret weather system development is considered from the PV‐Q perspective based on a severe weather event that occurred downstream of the Tibetan Plateau (TP) on 17–21 January 2008. Results demonstrate that owing to PV restructuring, positive PV was generated over the eastern flank of the TP, and its eastward advection triggered development of isentropic displacement vertical velocity and cyclogenesis in the lower troposphere. A converging southeasterly wind accompanied with ascending isentropic gliding vertical velocity in the lower troposphere was induced to the east of the cyclone center, which transported not only warm moist air but also negative PV from the south, contributing to developments of both local diabatic ascent and precipitation and eastward migration of the cyclone. During the cyclone life cycle, three omega components induced by different processes interacted with each other, and diabatic heating associated with precipitation exerted considerable feedback to vertical motion as well as PV advection. It is the vertical differential PV advection and feedback from diabatic heating that control the evolution of the circulation and the associated precipitation downstream of the TP.