Abstract

The El Niño–related anomalous western North Pacific anticyclone (WNPAC) shows different latitudinal extensions during the El Niño decaying summer, which determines the moisture transport to different regions and leads to distinct climate impacts over East Asia. It is known that both the north Indian Ocean (NIO) sea surface temperature (SST) and the tropical North Atlantic (TNA) SST can generate a WNPAC in summer. However, the difference between the NIO SST-forced WNPAC and the TNA SST-forced WNPAC has hardly been noted before now. This study shows that the NIO SST warming makes the WNPAC contract southward, whereas the TNA SST warming makes the WNPAC extend northward. The NIO SST warming generates the WNPAC via a Kelvin wave response. Owing to the limited domain of Kelvin wave activity, the Kelvin wave–induced suppressed convection over the western Pacific is confined south of 20°N, resulting in the WNPAC being concentrated in the low latitudes. In contrast, the TNA SST warming generates the WNPAC via a Rossby wave–induced divergence/convergence chain response over the Pacific. The Rossby wave–induced suppressed convection over the central-eastern Pacific north of the equator leads to enhanced convection on its southwest side, which further generates the low-level anomalous divergent winds over the western North Pacific and suppresses convection there. In this process, the suppressed convection over the western North Pacific is pushed more northward, thus producing a WNPAC extending northward. Further study finds that there are good precursors for predicting the WNPAC latitudinal extension based on the El Niño spatial pattern and the NIO/TNA SST intensity in the previous winter and spring.

Abstract

The subseasonal variation of the anomalous western North Pacific anticyclone (WNPAC) has important implications for East Asian summer monsoon variability. How the WNPAC evolves on the subseasonal time scale under the different configurations of tropical North Atlantic (TNA) Ocean and north Indian Ocean (NIO) SST warming is elucidated in this study. The WNPAC forced by individual TNA SST warming shows an obvious subseasonal variation with a stepwise northward movement. In contrast, the WNPAC forced by individual NIO SST warming shows a weak subseasonal variation, being nearly stabilized at around 20°N from June to August and thereby causing long-lasting and intense positive mei-yu–baiu–changma rainfall anomalies. The physical mechanism for the different subseasonal variation of WNPAC is further investigated. The TNA SST warming generates a WNPAC via a Rossby wave–induced divergence/convergence chain response. In this process, the TNA SST warming-induced suppressed convection over the western Pacific moves northward with the northward movement of climatological intertropical convergence zone and summer monsoon region, which generates a northward shift of the WNPAC. However, the NIO SST warming produces a WNPAC via a Kelvin wave–induced suppressed convection over the western Pacific Ocean. This suppressed convection is stabilized at around 20°N because of the Kelvin wave activity scope being limited within 20°N, which finally produces a nearly stationary WNPAC from June to August. In addition, under the simultaneous occurrence of the TNA and NIO SST warming, the subseasonal variation of WNPAC bears a resemblance to that for the individual NIO SST warming condition, where the TNA SST warming fails to exert its impact.

Abstract

Although many studies have linked complex social processes with climate change, few have examined the connections between changes in environmental factors, resources, or energy and the evolution of civilizations on the Tibetan Plateau. The Chiefdom of Lijiang was a powerful chiefdom located on the eastern Tibetan Plateau during the Ming Dynasty; it began expanding after the 1460s. Although many studies have analyzed the political and economic motivations responsible for this expansion, no high-resolution climate records representing this period of the Chiefdom of Lijiang were available until now. Here, we obtain a 621-yr reconstruction of the April–July normalized difference vegetation index (NDVI) values derived from moisture-sensitive tree rings from the eastern Tibetan Plateau. Our NDVI reconstruction accounts for 40.4% of the variability in instrumentally measured NDVI values and can effectively represent the historical changes in regional vegetation productivity that occurred on the eastern Tibetan Plateau. In combination with a reconstruction of summer temperatures on the eastern Tibetan Plateau, these results reveal that the regional climate was relatively warm and persistently wet during the period 1466–1630. This period was characterized by long periods of above-mean vegetation productivity on the eastern Tibetan Plateau that coincided with the expansion of the Chiefdom of Lijiang. We therefore propose that the NDVI anomaly and associated favorable political environment may have affected the expansion of the Chiefdom of Lijiang. Instrumental climate data and tree rings also reveal that the early twenty-first-century drought on the eastern Tibetan Plateau was the hottest drought recorded over the past six centuries, in accordance with projections of warming over the Tibetan Plateau. Future climate warming may lead to the occurrence of similar droughts, with potentially severe consequences for modern Asia.

Abstract

A third-order numerical model is developed for global advection transport computation. The multimoment constrained finite-volume scheme has been implemented to the hexagonal geodesic grid for spherical geometry. Two kinds of moments (i.e., point value and volume-integrated average) are used as the constraint conditions to derive the time evolution equations to update the computational variables, which are the values defined at the specified points over each mesh element in the present model. The numerical model has rigorous numerical conservation and third-order accuracy. One of the major merits of the present method is that it does not explicitly involve numerical quadrature, which leads to great convenience in accurately computing curved geometry and source terms. The present paper provides an accurate and practical formulation for advection calculation in the hexagonal-type geodesic grid.

Abstract

An adaptive global shallow-water model is proposed on cubed-sphere grid using the multimoment finite volume scheme and the Berger–Oliger adaptive mesh refinement (AMR) algorithm that was originally designed for a Cartesian grid. On each patch of the cubed-sphere grid, the curvilinear coordinates are constructed in a way that the Berger–Oliger algorithm can be applied directly. Moreover, an algorithm to transfer data across neighboring patches is proposed to establish a practical integrated framework for global AMR computation on the cubed-sphere grid.

The multimoment finite volume scheme is adopted as the fluid solver and is essentially beneficial to the implementation of AMR on the cubed-sphere grid. The multimoment interpolation based on both volume-integrated average (VIA) and point value (PV) provides the compact reconstruction that makes the present scheme very attractive not only in dealing with the artificial boundaries between different patches but also in the coarse–fine interpolations required in the AMR computations. The single-cell-based reconstruction avoids involving more than two nesting levels during interpolations. The reconstruction profile of constrained interpolation profile–conservative semi-Lagrangian scheme with third-order polynomial function (CIP-CSL3) is adopted where the slope parameter provides a flexible and convenient switching to get the desired numerical properties, such as high-order (fourth order) accuracy, monotonicity, and positive definiteness.

Numerical experiments with typical benchmark tests for both advection equation and shallow-water equations are carried out to evaluate the proposed model. The numerical errors and the corresponding CPU times of numerical experiments on uniform and adaptive meshes verify the performance of the proposed model. Compared to the uniformly refined grid, the AMR technique is able to achieve the similar numerical accuracy with less computational cost.

Abstract

An improved Doppler radar radial velocity assimilation observation operator is proposed based on the integrating velocity–azimuth process (IVAP) method. This improved operator can ingest both radial wind and its spatial distribution characteristics to deduce the two components of the mean wind within a given area. With this operator, the system can be used to assimilate information from tangential wind and radial wind. On the other hand, because the improved observation operator is defined within a given area, which can be uniformly chosen in both the observation and analysis coordinate systems, it has a thinning function. The traditional observation operator and the improved observation operator, along with their corresponding data processing modules, were implemented in the community Gridpoint Statistical Interpolation analysis system (GSI) to demonstrate the superiority of the improved operator. The results of single analysis unit experiments revealed that the two operators are comparable when the analysis unit is small. When the analysis unit becomes larger, the analysis results of the improved operator are better than those of the traditional operator because the former can ingest more wind information than the latter. The results of a typhoon case study indicated that both operators effectively ingested radial wind information and produced more reasonable typhoon structures than those in the background fields. The tangential velocity relative to the radar was retrieved by the improved operator through ingesting tangential wind information from the spatial distribution characteristics of radial wind. Because of the improved vortex intensity and structure, obvious improvements were seen in both track and intensity predictions when the improved operator was used.

Abstract

Tropical cyclones (TCs) of a particular track type move northward along the open oceans to the east of Taiwan and later pass over or near northern Taiwan. Their northward movement may be associated with intensified monsoon southwesterly flows from the northern South China Sea (SCS) toward Taiwan. Prolonged heavy rainfall then occurs in western Taiwan across the landfall and postlandfall periods, leading to severe floods. Characteristics of this TC–southwesterly flow association and related large-scale regulatory processes of intraseasonal oscillations (ISOs) are studied. For summers from 1958 to 2009, 16 out of 108 TCs affecting Taiwan exhibit the aforementioned northward-moving track. Among them, four TCs (25%) concur with enhanced southwesterly flows. Intensified moisture supplies from the SCS result in strong moisture convergence and prolonged heavy rainfall in western Taiwan. Both 30–60- and 10–24-day ISOs make positive contributions to the TC–southwesterly flow association. Both ISOs exhibit the northward progress of a meridional circulation pair from the tropics toward Taiwan. During landfall and the ensuing few days, Taiwan is surrounded by a cyclonic anomaly to the north and an anticyclonic anomaly to the south of these two ISOs. The appearance of anomalous southwesterly–westerly flows acts to prolong heavy rainfall in western Taiwan after the departure of a TC. The TC–southwesterly flow association tends to occur during the minimum phase of the 30–60-day ISO featuring a cyclonic anomaly in the vicinity of Taiwan but in various phases of the 10–24-day ISO. Rainfall in western Taiwan increases when these two ISOs simultaneously exhibit a cyclonic anomaly to the north of Taiwan.

Abstract

Climate change can lead to variations in the probability distribution of precipitation. In this study, quantile regression (QR) is undertaken to identify the quantile trends in precipitation over China and to examine the quantile effects of various climate oscillations on precipitation. The results show that the quantile trends show apparent seasonal variations, with a greater number of stations showing trends in winter (especially at quantile levels ≥ 0.5), and larger average magnitudes of trends at nearly all quantile levels in summer. The effects of El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Pacific decadal oscillation (PDO) exhibit evident variations with respect to the quantile level. Spatial clusters are subsequently identified based on the quantile trends, and the individual and combined effects from the teleconnection patterns are further investigated from the perspective of moisture budget. Seven spatial clusters with distinct seasonal quantile trends can be identified; three of them are located in southeastern China and are characterized by increasing trends in summer and winter precipitation. Summer precipitation over this region is positively influenced by ENSO and negatively influenced by NAO, with the former affecting both the dynamic and thermodynamic components of vertically integrated moisture divergence and the latter affecting only the dynamic component. The interaction effect of ENSO and NAO on summer precipitation anomalies in months that are extremely wetter than normal is statistically significant. In comparison, winter precipitation in this region is under the positive influence of ENSO and NAO and the negative influence of PDO; the effect of ENSO on moisture convergence can be mainly attributed to its dynamic component.

Abstract

The Tarim basin (TB) in northwestern China is one of the most arid regions in the middle latitudes, where water is scarce year-round. This study investigates the variations of summer precipitation in the TB and their association with water vapor fluxes and atmospheric circulation. The results suggest that the variations of summer precipitation in the TB are dominated by the water vapor fluxes from the south and east, although the long-term mean water vapor mostly comes from the west. The anomalous water vapor fluxes are closely associated with the meridional teleconnection pattern around 50°–80°E and the zonal teleconnection pattern along the Asian westerly jet in summer. The meridional teleconnection connects central Asia and the tropical Indian Ocean; the zonal teleconnection resembles the “Silk Road pattern.” The two teleconnections lead to negative height anomalies in central Asia and positive height anomalies in the Arabian Sea and India and in northern central China. The anomalous pressure gradient force, caused by these height anomalies, leads to anomalous ascending motion in the TB and brings low-level moisture along the eastern periphery of the Tibetan Plateau and water vapor from the Arabian Sea passing over the Tibetan Plateau to influence precipitation development in the study region.

Abstract

The data collected on 10 August 1990 from the Hawaiian Rainband Project were analyzed to study the evolution of katabatic flow. Results from this study show that for a relatively dry case thermal forcing is important to account for the onset, evolution, depth, and offshore extension of the katabatic flow on the windward side of the island of Hawaii where the mean winds are weak because of island blocking.

During the evening transition, the initiation of katabatic flow on the windward slopes was mainly driven by the diurnal heating cycle. At the windward coast, the wind shift was caused by the arrival of the drainage front from the windward lowlands. After the arrival of the drainage front at Hilo, slopewise instability was generated because of rain showers along the coast and diminishing orographic clouds and showers on the slopes. Along the coast, because of the reduced radiative cooling due to cloud cover and vertical mixing associated with rains, a warm period was observed at Hilo. On the slopes, the katabatic flow dissipated the orographic clouds and produced strong surface radiative cooling. As a result, the surface air on the lower slopes became potentially colder than the surface air along the coast and continued to move downward toward the coast. Furthermore, because of surface radiative cooling, the surface air at higher elevations moved to lower elevations along the isentropic surfaces but remained within the katabatic flow. With continued cold air advection down the slope under opposing trade winds, the nocturnal inversion was deeper along the coast than on the windward slopes.

Throughout the night, the katabatic flow extended gradually over the ocean. A narrow updraft of ∼0.5 m s⁻¹ was observed along the leading edge of the offshore flow, which resembled a density current. The location of the convergence zone between the offshore flow and the incoming trade winds is related to the offshore extension of katabatic flow and is not solely determined by the upstream Froude number.