Interannual Influences of the Surface Potential Vorticity Forcing over the Tibetan Plateau on East Asian Summer Rainfall

The influences of interannual surface potential vorticity forcing over the Tibetan Plateau (TP) on East Asian summer rainfall (EASR) and upper-level circulation are explored in this study. The results show that the interannual EASR and associated circulations are closely related to the surface potential vorticity negative uniform leading mode (PVNUM) over the TP. When the PVNUM is in the positive phase, more rainfall occurs in the Yangtze River valley, South Korea, Japan, and part of northern China, less rainfall occurs in southern China, and vice versa. A possible mechanism by which PVNUM affects EASR is proposed. Unstable air induced by the positive phase of PVNUM could stimulate significant upward motion and a lower-level anomalous cyclone over the TP. As a result, a dipole heating mode with anomalous cooling over the southwestern TP and anomalous heating over the southeastern TP is generated. Sensitivity experiment results regarding this dipole heating mode indicate that anomalous cooling over the southwestern TP leads to local and northeastern Asian negative height anomalies, while anomalous heating over the southeastern TP leads to local positive height anomalies. These results greatly resemble the realistic circulation pattern associated with EASR. Further analysis indicates that the anomalous water vapor transport associated with this anomalous circulation pattern is responsible for the anomalous EASR. Consequently, changes in surface potential vorticity forcing over the TP can induce changes in EASR.

Large-Scale Background and the Role of Quasi-Biweekly Moisture Transport in the Extreme Yangtze River Rainfall in Summer 2020

A severe flooding hit southern China along the Yangtze River in summer 2020. The floods were induced by heavy rains, and the associated dynamic and thermodynamic conditions are investigated using daily gridded rainfall data of China and NCEP-NCAR reanalysis. It is found that the summer rainfall over the Yangtze River Basin (YRB) experienced pronounced subseasonal variation in 2020, dominated by a quasi-biweekly oscillation (QBWO) mode. The southwestward-moving anomalous QBWO circulation was essentially the fluctuation of cold air mass related to the tropospheric polar vortex or trough-ridge activities over the mid-high latitude Eurasian in boreal summer. The large-scale southwestward-transport of cold air mass from mid-high latitudes and the northeastward-transport of warm and moist air by the strong anomalous anticyclone over the western North Pacific provided important circulation support for the heavy rainfall in the YRB. The quasi-biweekly anomalies of potential and divergent component of vertically integrated water vapor flux played a major role in maintaining the moisture during summer 2020. The diagnosis of moisture budget shows that the enhanced moisture associated with the quasi-biweekly fluctuation rainfall was primarily attributed to the moisture convergence. The convergence of QBWO specific humidity by the background mean flow and convergence of mean specific humidity by QBWO flow played dominant roles in contributing to the positive moisture tendency. In combination with an adiabatic ascent induced by the warm temperature advection, the boundary layer moisture convergence strengthens the upward transport of moisture from lower troposphere. The vertical moisture transport associated with boundary layer convergence was of critical importance in causing low-level tropospheric moistening, whereas the horizontal advection of moisture showed a negative effect during the anomalous quasi-biweekly summer rainfall in 2020.

Impact of Solar Activity on Snow Cover Variation Over the Tibetan Plateau and Linkage to the Summer Precipitation in China

Solar activity is one of the main external forcing factors driving the Earth’s climate system to change. The snow cover over the Tibetan Plateau is an important physical factor affecting the East Asian climate. At present, insufficient research on the connection between solar activity and snow cover over the Tibetan Plateau has been carried out. Using Solar Radio Flux (SRF), Solar Sunspot Number (SSN), and Total Solar Irradiance (TSI) data, this paper calculated the correlation coefficients with snow indices over the Tibetan Plateau, such as winter and spring snow depth (WSD/SSD) and snow day number (WSDN/SSDN). These snow indices are obtained from the daily gauge snow data in the Tibetan Plateau. Through correlation analyses, it is found that there are significant synchronous or lag correlations between snow indices and solar parameters on multi-time scales. In particular, the Spring Snow Day Number (SSDN) is of significant synchronous or lag correlation with SRF, SSN, and TSI on multi-time scales. It is further found that SSDN over the Tibetan Plateau has more stable positive correlations with SRF by using the 21-year running mean and cross spectrum analyses. Therefore, SSDN can be ascertained to be the most sensitive snow index to the solar activity compared with other snow indices. Moreover, its influence on summer precipitation of China is strongly regulated by solar activity. In high solar activity years (HSAY), the significant correlated area of summer precipitation in China to SSDN is located further north than that in low solar activity years (LSAY). Such impact by solar activity is also remarkable after excluding the impact of ENSO (i.e., El Niño–Southern Oscillation) events. These results provide support for the application of snow indices in summer rainfall prediction in China.

Long range forecasting parameters vis-a-vis sub-divisional summer rainfall over India

Interannual variability of Empirical Orthogonal Functions (EOF) based upon regional/global parameters, associated with the summer monsoon rainfall over different meteorological sub-divisions of the country have been discussed, based upon the data during the years 1958 to 1990 enabling us to identify three broad sub-divisions of the country. It was interesting to note that the first empirical orthogonal function did not show significant correlation with monsoon rainfall over most SUB-DIVISIONS of the NE and SE parts of the country. However, this EOF was found to be significantly correlated with the rainfall over the remaining meteorological sub-divisions of the country.

Modulation of coupled modes of Tibetan Plateau heating and Indian Summer Monsoon on summer rainfall over Central Asia

It has been suggested that summer rainfall over Central Asia (CA) is significantly correlated with the summer thermal distribution of the Tibetan Plateau (TP) and the Indian summer monsoon (ISM). However, relatively few studies have investigated their synergistic effects of different distribution. This study documents the significant correlations between precipitation in CA and the diabatic heating of TP and the ISM based on the results of statistical analysis and numerical simulation. Precipitation in CA is is dominated by two water vapor transport branches from the south which are related to the two primary modes of anomalous diabatic heating distribution related to the TP and ISM precipitation, that is, the “+-” dipole mode in the southeastern TP and the Indian subcontinent (IS), and the “+-+” tripole mode in the southeastern TP, the IS, and southern India. Both modes exhibit obvious mid-latitude Silk Road pattern (SRP) wave trains with cyclone anomalies over CA, but with different transient and stationary eddies over south Asia. The different locations of anomalous anticyclones over India govern two water vapor transport branches to CA, which are from the Arabian Sea and the Bay of Bengal. The water vapor flux climbs while being transported northward and can be transported to CA with the cooperation of cyclonic circulation. The convergent water vapor and ascending motion caused by cyclonic anomalies favor the precipitation in CA. Further analysis corroborates the negative South Indian Ocean Dipole (NSIOD) in February could affect the tripole mode distribution of TP heating and ISM via the atmospheric circulation, water vapor transport and an anomalous Hadley cell circulation. The results indicate a reliable prediction reference for precipitation in CA.

Mechanism of The Summer Rainfall Variation in Transitional Climate Zone in East Asia From the Perspective of Moisture Supply During 1979-2010 Based On The Lagrangian Method

Transitional Climate Zone (TCZ) over East Asia, characterized by semi-arid climate, is ecologically fragile environment with limited water resources, making atmospheric moisture supply being the key influential factor. This study investigates the moisture sources of summer (JJA) rainfall in the TCZ over East Asia during 1979-2010 with the Lagrangian particle dispersion model. Seven moisture source regions and associated contribution are quantified: Eurasia continent to northwest of the TCZ (EC, 18.01%), central-eastern China (CEC, 17.14%), western Pacific Ocean (WPO, 7.46%), South China Sea and Indonesia (SCSI, 3.56%), Bay of Bengal (BOB, 2.55%), Arabian Sea (AS, 2.13%) and local evaporation (TCZ, 19.96%). The moisture contribution from ocean (16.06%) is less than those from the continent (55.11%), due to the great loss en-route. In particular, the local evaporation not only contributes the most moisture among 7 selected source regions, but also has the greatest influence in summer precipitation variability in TCZ. Furthermore, westerlies precipitation and monsoon precipitation are discriminated according to the dominant system of water vapor source regions. It is found that summer monsoon contributes most of water vapor (33.2%) to summer rainfall in TCZ, while only 18.01% comes from the mid-latitude westerlies dominant area. Finally, further analysis of dry and wet years shows that summer monsoon system also takes more responsibility for a drier or wetter summer in TCZ from the perspective of moisture supply, followed by local evaporation and mid-latitude westerlies.

Changement climatique dans le bassin versant de l'Ogooué : évolution récente et impact sur les écoulements

L'objectif de cet article est de faire une évaluation des tendances récentes des grandeurs hydroclimatiques dans le bassin de l'Ogooué en contexte de changement climatique. Pour cela, les données de pluies et de débits de ce bassin ont été analysées au moyen du test de Pettitt. Les résultats de cette étude révèlent une diminution statistiquement significative des modules annuels que le test de Pettitt situe en 1972–1973, mais rien de tel pour les pluies à ce même pas de temps. Les écarts des moyennes décennales à la moyenne interannuelle montrent cependant des liens plus nets entre ces variables, se traduisant essentiellement par une diminution synchrone au cours des décennies 1970 et 1980. Cette diminution est suivie d'une reprise au cours des deux décennies d'après (1990 et 2000), laquelle s'estompe à nouveau au cours de la décennie 2010. Les écoulements des saisons pluvieuses (printemps et automne) ont enregistré les modifications les plus importantes, suite aux variations des régimes pluviométriques des saisons sèches (hiver et été) qui les précèdent. La saison sèche d'hiver a connu entre les décennies 1970–1990 une diminution importante des précipitations qui a provoquée une chute des écoulements du printemps. Le rehaussement et la flexion respectivement notés au cours des décennies 2000 et 2010 s'accompagnent des mêmes tendances dans les écoulements du printemps. A l'inverse, entre les décennies 1980–1990, il a été noté une hausse conjointe des pluies d'été et des écoulements d'automne. La flexion des pluies d'été notée depuis la décennie 2000 est également perceptible dans les écoulements d'automne. Ces résultats pourraient servir à renforcer les capacités de gestion des ressources en eau dans le bassin versant concerné et dans la région. Ils apportent également de nouveaux éléments pour étudier et comprendre la variation saisonnière et la disponibilité de l'eau douce en aval, dans les estuaires et les zones côtières des rivières régionales. The objective of this article is to assess recent trends of hydroclimatic quantities in Ogooue basin in the context of climate change. For this, the rainfall and discharges data of this basin were analyzed using the Pettitt test. The results of this study reveal a statistically significant decrease in runoff that the Pettitt test situates in 1972–1973, but nothing like that for rainfall at this same time scale. The decadal deviations from the interannual average, however show links between these variables essentially marked by a synchronous decrease in the 1970s and 1980s. This decrease is followed by a recovery in the two decades following (1990 and 2000), which stops again during the 2010s. The flow of the rainy seasons (Spring and Autumn) recorded the most important modifications, following variations in the rainfall regimes of the dry seasons (Winter and Summer) which precede them. The dry Winter season experienced a significant decrease in precipitation between the 1970s and 1990s, which caused a decrease in Spring flows. The increase and flexion noted respectively during the 2000 and 2010 decades are accompanied by the same trends in Spring flows. Conversely, between the 1980s and the 1990s, there was a joint increase in Summer rainfall and Autumn flows. The decrease of Summer rainfall noted since the 2000s is also noticeable in the Autumn flows. These results could be used to strengthen water resources management capacities in the watershed concerned and the region. They also provide new insights to study and understand seasonal variation and availability of freshwater downstream, in estuaries and coastal areas of regional rivers.

Annual yield and botanical composition of four dryland grass species with or without nitrogen over six years

Nitrogen (N) and water availability affect pasture production and persistence. Yield and botanical composition of four monocultures of brome (BR), cocksfoot (CF), perennial ryegrass (RG) and tall fescue (TF) were evaluated with (+N) or without (-N) N at Ashley Dene farm, Canterbury, over six growth seasons from establishment in 2014/15 (Year 1) to 2019/20 (Year 6). Total annual yields ranged from 2.04 (RG-N; Year 1) to 12.7 t DM/ha/yr (CF+N; Year 3). Yields differed among species in Years 1, 3, 4 and 6 when TF pastures had the lowest production. There was no difference in DM production from BR, CF and RG pastures. Additionally, +N pastures produced ~55% more yield than –N pastures in Years 3 and 5 when spring/summer rainfall was adequate to maintain growth. Sown grasses accounted for >89% of total DM yield in Years 1 and 2 but the proportion of total annual DM production from sown species declined from Year 3. By Year 6, sown species accounted for 48±3.3 (TF) to 64±3.3% (BR, CF and RG) of total annual DM production. Generally, TF failed to perform in this dryland environment. In contrast, the production and persistence of the other three species were not different when subjected to water deficits alone.

Forecasting northern Australian summer rainfall bursts using a seasonal prediction system

Rainfall bursts are relatively short-lived events that typically occur over consecutive days, up to a week. Northern Australian industries like sugar farming and beef are highly sensitive to burst activity, yet little is known about the multi-week prediction of bursts. This study evaluates summer (December to March) bursts over northern Australia in observations and multi-week hindcasts from the Bureau of Meteorology’s multi-week to seasonal system, ACCESS-S1 (Australian Community Climate and Earth-System Simulator, Seasonal version 1). The main objective is to test ACCESS-S1’s skill to confidently predict tropical burst activity, defined as rainfall accumulation exceeding a threshold amount over three days, for the purpose of producing a practical, user-friendly burst forecast product. The ensemble hindcasts, made up of 11 members for the period 1990–2012, display good predictive skill out to lead week 2 in the far northern regions, despite overestimating the total number of summer burst days and the proportion of total summer rainfall from bursts. Coinciding with a predicted strong Madden-Julian Oscillation (MJO), the skill in burst event prediction can be extended out to four weeks over the far northern coast in December, however this improvement is not apparent in other months or over the far northeast, which shows generally better forecast skill with a predicted weak MJO. The ability of ACCESS-S1 to skillfully forecast bursts out to 2-3 weeks suggests the Bureau's recent prototype development of a Burst Potential forecast product would be of great interest to northern Australia’s livestock and crop producers, who rely on accurate multi-week rainfall forecasts for managing business decisions.