Abstract

Using 15-yr data from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-15), the authors found that rapid southeastward expansion of the rainy area from the western Amazon to southeastern Brazil is a result of midlatitude cold air intrusions. During austral spring, as the large-scale thermodynamic structure over Amazonia becomes destabilized, the incursions of extratropical cold air can trigger intense rainfall along the leading edge of northwest–southeast-oriented cold fronts east of the Andes. As these fronts penetrate into Amazonia, the northerly or northwesterly wind transports warm, moist air from the western Amazon to southeast Brazil. Moisture convergence consequently intensifies, resulting in northwest–southeast-elongated rainy areas. The latter contribute to the observed rapid, southeastward expansion of rainy areas shown in rainfall climatology during austral spring.

The authors’ analysis suggests that cold air intrusions during austral spring collectively assist the transformation of large-scale thermodynamic and dynamic environments to those favorable for the wet season onsets. Each time the cold fronts pass by, they tend to increase the atmospheric humidity and the buoyancy of the lower troposphere, which destabilizes the atmosphere. In the upper troposphere, the cold air intrusions supply kinetic energy for the development of anticyclonic flow. Cold air intrusions in the transitional season are not different from those occurring immediately before the wet season onsets except that the latter occurs under a more humid and unstable atmospheric condition. Thus, cold air intrusions can trigger the wet season onsets only when atmospheric and land surface conditions are “ready” for the onset.

Comparisons among early, normal, and late onsets on an interannual scale further suggest that more frequent and stronger cold air intrusions trigger the early onsets of wet seasons given suitable large-scale thermodynamic conditions. Likewise, less frequent and weaker cold air intrusions could delay the wet season onset even though the large-scale thermodynamic conditions appear to be favorable. Occasionally, strong unstable atmospheric thermodynamic conditions and northerly reversal of cross-equatorial flow can lead to wet season onsets without cold air intrusions. In such cases, enhanced precipitation is centered over central and eastern Amazon, and rainfall increases more gradually compared to the onset with cold air intrusions.

Abstract

Spring rainfall is important for agriculture and economics in North China (NC). Thus, there is an imperative need for accurate seasonal prediction of the spring precipitation. This study implements a novel rainfall framework to improve understanding of NC spring rainfall. The framework is built based on a three-cluster normal mixture model. Distribution parameters are sampled using Bayesian inference and a Markov chain Monte Carlo algorithm. The probability behaviors of light, moderate, and heavy rainfall events can be reflected by the three rainfall clusters, respectively. Analysis of 61-yr data indicates that moderate rainfall makes the largest contribution (67%) to the total rainfall amount. The moderate rainfall intensity is mainly influenced by the sea surface temperature anomaly (SSTA) in the previous season over the equatorial eastern Pacific, and rainfall frequency is influenced by geopotential height anomaly in the mid- to high latitudes in spring. It is also found that more extreme precipitation events can be observed in the spring following an eastern Pacific El Niño event in the previous autumn and winter. Based on these relationships, we develop a multiple linear regression model. Hindcasts for spring precipitation using the model indicates that its anomaly correlation is 0.48, significant at the 99% confidence level. The result suggests that the newly developed model can well predict spring rainfall amount in NC.

Abstract

Using 15-yr instantaneous European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA) data, the authors have examined the large-scale atmospheric conditions and the local surface fluxes through the transition periods from the dry to wet seasons over the southern Amazon region (5°–15°S, 45°–75°W). The composite results suggest that the transition can be divided into three phases: initiating, developing, and mature. The initiating phase is dominated by the local buildup of the available potential energy. This begins about 90 days prior to the onset of the wet season by the increase of local land surface fluxes, especially latent heat flux, which increases the available potential energy of the lower troposphere. The cross-equatorial flow and upper-tropospheric circulation remain unchanged from those of the dry season. The developing phase is dominated by the seasonal transition of the large-scale circulation, which accelerates by dynamic feedbacks to an increase of locally thermal-driven rainfall, starting about 45 days before the onset of the wet season. During this stage, the reversal of the low-level, cross-equatorial flow in the western Amazon increases moisture transport from the tropical Atlantic Ocean and leads to net moisture convergence in the southern Amazon region. In the upper troposphere, the divergent kinetic energy begins to be converted into rotational kinetic energy, and geopotential height increases rapidly. These processes lead to the onset of the wet season and the increase of anticyclonic vorticity at the upper troposphere. After onset, the lower-tropospheric potential energy reaches equilibrium, but the conversion from divergent to rotational kinetic energy continues to spin up the upper-tropospheric anticyclonic circulation associated with the Bolivian high until it reaches its full strength.

This analysis suggests that a weaker (stronger) increase of land surface latent (sensible) heat flux during the dry season and the initiating phase tends to delay the large-scale circulation transition over the Amazon. The influence of land surface heat fluxes becomes secondary during the developing and mature phases after the transition of the large-scale circulation begins. A later northerly reversal and/or weaker cross-equatorial flow, a later southerly withdrawal of the upper-tropospheric westerly wind, and a stronger subsidence could delay and prolong the developing phase of the transition and consequently delay the onset of the Amazon wet season.

Abstract

The Pacific–North America–North Atlantic sector in general experienced a dryer and warmer climate in summer during the past 40 years. These changes are partly associated with declining midlatitude synoptic variability in boreal summer, especially over the two ocean basins. A nonmodal instability analysis of the boreal summer background flow is conducted for two periods, 1979–94 and 2000–15, to understand dynamical processes potentially responsible for the observed decline of synoptic variability. The synoptic variability associated with fast, nonmodal growth of atmospheric disturbances shows a decline over northern midlatitudes in the later period, in both a barotropic model and a two-level quasigeostrophic model. These results highlight the importance of the changing summer background flow in contributing to the observed changes in synoptic variability. Also discussed are factors likely associated with background flow changes including sea surface temperature and sea ice change.

Abstract

The variability of summer precipitation in the southeastern United States is examined in this study using 60-yr (1948–2007) rainfall data. The Southeast summer rainfalls exhibited higher interannual variability with more intense summer droughts and anomalous wetness in the recent 30 years (1978–2007) than in the prior 30 years (1948–77). Such intensification of summer rainfall variability was consistent with a decrease of light (0.1–1 mm day⁻¹) and medium (1–10 mm day⁻¹) rainfall events during extremely dry summers and an increase of heavy (>10 mm day⁻¹) rainfall events in extremely wet summers. Changes in rainfall variability were also accompanied by a southward shift of the region of maximum zonal wind variability at the jet stream level in the latter period. The covariability between the Southeast summer precipitation and sea surface temperatures (SSTs) is also analyzed using the singular value decomposition (SVD) method. It is shown that the increase of Southeast summer precipitation variability is primarily associated with a higher SST variability across the equatorial Atlantic and also SST warming in the Atlantic.

Abstract

This study investigates the changes of the North Atlantic subtropical high (NASH) and its impact on summer precipitation over the southeastern (SE) United States using the 850-hPa geopotential height field in the National Centers for Environmental Prediction (NCEP) reanalysis, the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), long-term rainfall data, and Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) model simulations during the past six decades (1948–2007). The results show that the NASH in the last 30 yr has become more intense, and its western ridge has displaced westward with an enhanced meridional movement compared to the previous 30 yr. When the NASH moved closer to the continental United States in the three most recent decades, the effect of the NASH on the interannual variation of SE U.S. precipitation is enhanced through the ridge’s north–south movement. The study’s attribution analysis suggested that the changes of the NASH are mainly due to anthropogenic warming. In the twenty-first century with an increase of the atmospheric CO₂ concentration, the center of the NASH would be intensified and the western ridge of the NASH would shift farther westward. These changes would increase the likelihood of both strong anomalously wet and dry summers over the SE United States in the future, as suggested by the IPCC AR4 models.

Abstract

Recently Diem questioned the western ridge movement of the North Atlantic subtropical high (NASH) reported in a 2011 paper of Li et al. This reply shows more analysis that further strengthens the conclusions originally put forth by Li et al. Diem’’s analysis of the trend in the western ridge of the NASH was based on the data over a 30-yr period (1978–2007), whereas the main conclusions in Li et al. were drawn according to the data over a 60-yr period (1948–2007). Over the last 60 years, the NASH has shown a significant trend of westward movement, the meridional movement of the western ridge of the NASH has enhanced in the recent three decades, and the potential impact of global warming cannot be ruled out in an attempt to explain these changes of the NASH.

Abstract

In many countries, thunderstorms are the main contributor to hourly extreme precipitation (HEP). Prior studies have shown that the number of thunderstorms decreased steadily in whole country of China; however, HEP has increased significantly in several areas over the past half-century. The role of thunderstorms in changes in HEP occurrence remains largely unknown in China. In this study, for the first time, we used continuous 32-yr records of hourly precipitation and thunder, and the fifth-generation European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5), to analyze changes in thunderstorms under various vertical wind shear (VWS) environments, and their contribution to HEP occurrence. The number of HEP events associated with thunderstorms (TD-HEP) increased significantly in southern China (SC) but decreased significantly in northeastern China (NEC) and east of the Tibetan Plateau (ETP). Weak VWS thunderstorms accounted for 69.1% of TD-HEP in SC. Changes in the most unstable convective available potential energy and precipitable water (PW) in SC favored an increase in weak-VWS thunderstorms, which resulted in an increase of 2.35 h per warm season in overall “station-mean” TD-HEP events from 1980 to 2011. As the major contributor to HEP in NEC, moderate VWS thunderstorms decreased by 0.37 h per warm season, due mainly to a reduction in PW, leading to a negative trend in TD-HEP events. Similarly, the decreasing TD-HEP occurrence on the ETP was due to a decrease of 1.12 h per warm season of moderate VWS thunderstorms. Studying the VWS environments of thunderstorms, and changes therein under a warming climate, can improve understanding of the changes in HEP in China.

Abstract

The mass footprints associated with atmospheric blocks over the North Pacific are evaluated by constructing daily tendencies of total mass over the blocking domain from three-dimensional mass fluxes throughout the life cycle of a composite blocking event. The results highlight the major role of mass convergence driven by low-frequency (with periods >1 week) atmospheric disturbances during both the development and decay stage of a block. Specifically, low-frequency eddies are responsible for the accelerated mass buildup 4 days prior to the peak intensity of a block, and they also account for the rapid mass loss afterward. High-frequency, subweekly scale disturbances have statistically significant positive contributions to the mass loss during the decay stage, and also show weak negative contributions to the development of the blocking high prior to the peak of the high. The majority of the mass convergence (divergence) responsible for the intensification (decay) of the blocking high occurs in the middle-to-lower troposphere and is largely attributed to mass flux driven by low-frequency meridional (zonal) winds. Also discussed are the implications of this new mass perspective of atmospheric blocks for understanding dynamics of blocking highs and for model bias detection and attribution.

Abstract

Unforced global mean surface air temperature ( ) is stable in the long term primarily because warm anomalies are associated with enhanced outgoing longwave radiation ( ) to space and thus a negative net radiative energy flux ( , positive downward) at the top of the atmosphere (TOA). However, it is shown here that, with the exception of high latitudinal and specific continental regions, warm unforced surface air temperature anomalies at the local spatial scale [T(θ, ϕ), where (θ, ϕ) = (latitude, longitude)] tend to be associated with anomalously positive N(θ, ϕ). It is revealed that this occurs mainly because warm T(θ, ϕ) anomalies are accompanied by anomalously low surface albedo near sea ice margins and over high altitudes, low cloud albedo over much of the middle and low latitudes, and a large water vapor greenhouse effect over the deep Indo-Pacific.

It is shown here that the negative versus relationship arises because warm anomalies are associated with large divergence of atmospheric energy transport over the tropical Pacific [where the N(θ, ϕ) versus T(θ, ϕ) relationship tends to be positive] and convergence of atmospheric energy transport at high latitudes [where the N(θ, ϕ) versus T(θ, ϕ) relationship tends to be negative]. Additionally, the characteristic surface temperature pattern contains anomalously cool regions where a positive local N(θ, ϕ) versus T(θ, ϕ) relationship helps induce negative . Finally, large-scale atmospheric circulation changes play a critical role in the production of the negative versus relationship as they drive cloud reduction and atmospheric drying over large portions of the tropics and subtropics, which allows for greatly enhanced .