Tinghai Ou

Recent recovery of the Siberian High intensity

This study highlights the fast recovery of the wintertime Siberian High intensity (SHI) over the last two decades. The SHI showed a marked weakening trend from the 1970s to 1980s, leading to unprecedented low SHI in the early 1990s according to most observational data sets. This salient declining SHI trend, however, was sharply replaced by a fast recovery over the last two decades. Since the declining SHI trend has been considered as one of the plausible consequences of climate warming, the recent SHI recovery seemingly contradicts the continuous progression of climate warming in the Northern Hemisphere. We suggest that alleviated surface warming and decreased atmospheric stability in the central Siberia region, associated with an increase in Eurasian snow cover, in the recent two decades contributed to this rather unexpected SHI recovery. The prominent SHI change, however, is not reproduced by general circulation model (GCM) simulations used in the IPCC AR4. The GCMs indicate the steady weakening of the SHI for the entire 21st century, which is found to be associated with a decreasing Eurasian snow cover in the simulations. An improvement in predicting the future climate change in regional scale is desirable.

Interannual teleconnections between the summer North Atlantic Oscillation and the East Asian summer monsoon

Here we present a study of the relationship between July-August (JA) mean climate over China, which is strongly linked to the East Asian summer monsoon (EASM), and the summer (JA) North Atlantic Oscillation (SNAO). The variations of temperature, precipitation, and cloud cover related to the SNAO were analyzed for the period 1951-2002 using gridded data sets as well as instrumental data from 160 stations in China. It was shown that the major patterns of summer climate over China are highly connected with the interannual variation of the SNAO, supporting a teleconnection between the North Atlantic region and East Asia. Based on the analyses of the daily and monthly reanalysis data sets, we propose possible mechanisms of this teleconnection. Changes in the position of the North Atlantic storm tracks and transient eddy activity associated with the positive (negative) SNAO phase contribute downstream to negative (positive) sea level pressure anomalies in northeastern East Asia. In negative SNAO years, a stationary wave pattern is excited from the southern SNAO center over northwestern Europe to northeastern East Asia. However, during positive SNAO years, a stationary wave pattern is excited extending from the SNAO center across the central Eurasian continent at around 40 degrees N and downstream to the southeast. This may explain a connection between the positive SNAO and atmospheric circulation in middle and southeastern China. (Less)

Evaluation of global climate models in simulating extreme precipitation in China

Variations in extreme precipitation can be described by various indices. In order to evaluate a climate models ability to simulate extreme precipitation, gridded extreme precipitation indices from observations are needed. There are two ways to obtain gridded extreme precipitation indices from station-based observations: either through interpolation of station-based extreme indices (EISTA) or estimated from gridded precipitation datasets (EIGRID). In this work, we evaluated these two methods and compared observational extreme precipitation indices in China to those obtained from a set of widely used global climate models. Results show that the difference between the two methods is quite large; and in some cases it is even larger than the difference between model simulations and observed gridded EISTA. Based on the sensitivity of the indices to horizontal resolution, it was suggested that EIGRID is more appropriate for evaluating extreme indices simulated by models. Subsequently, historic simulations of extreme precipitation from 21 CMIP5 (Coupled Model Intercomparison Project Phase 5) global climate models were evaluated against two reanalysis datasets during 1961–2000. It was found that most models overestimate extreme precipitation in the mountain regions in western China and northern China and underestimate extreme precipitation in southern China. In eastern China, these models simulate mean extreme precipitation fairly well. Despite this bias, the temporal trend in extreme precipitation for western China is well captured by most models. However, in eastern China, the trend of extreme precipitation is poorly captured by most models, especially for the so-called southern flood and northern drought pattern. Overall, our results suggest that the dynamics of inter-decadal summer monsoon variability should be improved for better prediction of extreme precipitation by the global climate models.

Changes in winter cold surges over Southeast China: 1961 to 2012

The present study investigates the overall changes in occurrences of winter cold surges over Southeast China for the period 1961–2012, using instrumental observations, reanalysis and model simulation datasets. Based on objectively defined criteria, cold surges were classified into 3 types according to their dynamical origin as inferred from daily evolution patterns of surface pressure systems with a focus on the Siberian High (SH): type A with an amplification of a quasi-stationary SH associated with high-pressure anomalies over the Ural mountains, type B with a developing SH associated with fast traveling upper-level waves, and type C with a high-pressure originated in the Arctic. Examination of the long-term change in cold surge occurrences shows different interdecadal variations among the 3 types. During 1961–2012, type A events (37.8%) decreased, while type B events, accounting for the majority (52.5%) of total winter cold surges, increased slightly. The contribution by type C to the total occurrence of the cold surges was small (8.8%) compared to that of A and B, but it became more frequent in the latest decade, related to the tendency of the Arctic Oscillation (AO) being more in its negative phase. Overall, we found slightly increased occurrences of cold surges over Southeast China since the early 1980s, despite the weakened SH intensity and warmer mean temperature compared to previous decades. The climate model projections of the phase 5 of the Coupled Model Intercomparison Project (CMIP5) suggests similar trend in the late 21st century under warmer climate.

Projecting Future Local Precipitation and Its Extremes for Sweden

Abstract A procedure to obtain future local precipitation characteristics focused on extreme conditions has been developed based on a weather generator. The method involves six major steps: (1) the weather generator was calibrated using observed daily precipitation at 220 wedish stations during 1961–2004; (2) present and future daily precipitation characteristics for the wedish stations from two global climate models, namely and HadCM3, were used to calculate weather generator parameters for the present and future climates at global climate model spatial scales; (3) the ratio of the weather generator parameters for the present climate simulated by the global climate models to those calculated for each station falling into the global climate model grid box were computed for all the stations; (4) these ratios were also assumed to be valid in the future climate, that way the future parameters for each station for the global climate model projected future climate could be calculated; (5) using the estimated future parameters of the weather generator, the future daily precipitation at each station could be simulated by the weather generator; (6) the simulated daily precipitation was used to compute eight indices describing mean and extreme precipitation climates. The future mean and extreme precipitation characteristics at the stations under the Second Report on Emission Scenarios A2 scenario were obtained and presented. An overall increasing trend for frequency and intensity of the indices are identified for the majority of the stations studied. The developed downscaling methodology is relatively simple but useful in deriving local precipitation changes, including changes in the precipitation extremes.

Satellite measurements reveal strong anisotropy in spatial coherence of climate variations over the Tibet Plateau.

This study uses high-resolution, long-term satellite observations to evaluate the spatial scales of the climate variations across the Tibet Plateau (TP). Both land surface temperature and precipitation observations of more than 10 years were analysed with a special attention to eight existing ice-core sites in the TP. The temporal correlation for the monthly or annual anomalies between any two points decreases exponentially with their spatial distance, and we used the e-folding decay constant to quantify the spatial scales. We found that the spatial scales are strongly direction-dependent, with distinctive patterns in the west-east and south-north orientations, for example. Meanwhile, in the same directions the scales are largely symmetric backward and forward. Focusing on the west-east and south-north directions, we found the spatial coherence in the first is generally stronger than in the second. The annual surface temperature had typical spatial scales of 302–480 km, while the annual precipitation showed smaller scales of 111–182 km. The majority of the eight ice-core sites exhibit scales much smaller than the typical scales over the TP as a whole. These results provide important observational basis for the selection of appropriate downscaling strategies, deployment of climate-data collection networks, and interpreting paleoclimate reconstructions.

Impact of model resolution on simulating the water vapor transport through the central Himalayas: implication for models’ wet bias over the Tibetan Plateau

Current climate models commonly overestimate precipitation over the Tibetan Plateau (TP), which limits our understanding of past and future water balance in the region. Identifying sources of such models’ wet bias is therefore crucial. The Himalayas is considered a major pathway of water vapor transport (WVT) towards the TP. Their steep terrain, together with associated small-scale processes, cannot be resolved by coarse-resolution models, which may result in excessive WVT towards the TP. This paper, therefore, investigated the resolution dependency of simulated WVT through the central Himalayas and its further impact on precipitation bias over the TP. According to a summer monsoon season of simulations conducted using the weather research forecasting (WRF) model with resolutions of 30, 10, and 2 km, the study found that finer resolutions (especially 2 km) diminish the positive precipitation bias over the TP. The higher-resolution simulations produce more precipitation over the southern Himalayan slopes and weaker WVT towards the TP, explaining the reduced wet bias. The decreased WVT is reflected mostly in the weakened wind speed, which is due to the fact that the high resolution can improve resolving orographic drag over a complex terrain and other processes associated with heterogeneous surface forcing. A significant difference was particularly found when the model resolution is changed from 30 to 10 km, suggesting that a resolution of approximately 10 km represents a good compromise between a more spatially detailed simulation of WVT and computational cost for a domain covering the whole TP.

Earlier occurrence and increased explanatory power of climate for the first incidence of potato late blight caused by Phytophthora infestans in Fennoscandia

Background Late blight (caused by Phytophthora infestans) is a devastating potato disease that has been found to occur earlier in the season over the last decades in Fennoscandia. Up until now the reasons for this change have not been investigated. Possible explanations for this change are climate alterations, changes in potato production or changes in pathogen biology, such as increased fitness or changes in gene flow within P. infestans populations. The first incidence of late blight is of high economic importance since fungicidal applications should be typically applied two weeks before the first signs of late blight and are repeated on average once a week. Methods We use field observations of first incidence of late blight in experimental potato fields from five sites in Sweden and Finland covering a total of 30 years and investigate whether the earlier incidence of late blight can be related to the climate. Results We linked the field data to meteorological data and found that the previous assumption, used in common late blight models, that the disease only develops at relative humidity levels above 90% had to be rejected. Rather than the typically assumed threshold relationship between late blight disease development and relative humidity we found a linear relationship. Our model furthermore showed two distinct responses of late blight to climate. At the beginning of the observation time (in Sweden until the early 90s and in Finland until the 2000s) the link between climate and first incidence was very weak. However, for the remainder of the time period the link was highly significant, indicating a change in the biological properties of the pathogen which could for example be a change in the dominating reproduction mode or a physiological change in the response of the pathogen to climate. Conclusions The study shows that models used in decision support systems need to be checked and re-parametrized regularly to be able to capture changes in pathogen biology. While this study was performed with data from Fennoscandia this new pathogen biology and late blight might spread to (or already be present at) other parts of the world as well. The strong link between climate and first incidence together with the presented model offers a tool to assess late blight incidence in future climates.