Interdecadal variations of different types of summer heatwaves in Northeast China associated with AMO and PDO

The summer heatwaves (HWs) in Northeast China (NEC) during 1961-2016 can be classified into two types, namely wave-train HWs and blocking HWs based on the hierarchical clustering algorithm by using ERA-Interim daily reanalysis datasets. Wave-train HWs occurred accompanied by eastward-moving wave trains with a "-+-+" structure formed over Eurasia, while the blocking HWs occurred with blocking circulation anomalies over Eurasia. In general, the blocking HWs could cause the positive temperature anomalies in NEC to last longer than wave-train HWs. During the period from 1961 to 2016, the wave-train HWs experienced an interdecadal variation from less to more, while the blocking HWs experienced interdecadal variations of less-more-less. Regression analysis and information flow indicate that the interdecadal variation of the wave-train HWs is associated with Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), while the interdecadal variation of the blocking HWs is more likely associated with PDO. The positive phase of AMO (negative phase of PDO) could increase the wave-train (blocking) HWs by strengthening the zonal wave-train similar to the Silk Road pattern (the arched wave-train like the polar-Eurasian pattern). The observed results are in agreement with the numerical experiments with the NCAR Community Atmosphere Model version 5.3.

The Evolution Characteristics of Daily-Scale Silk Road Pattern and Its Relationship with Summer Temperature in the Yangtze River Valley

By employing multi-reanalysis daily datasets and station data, this study focuses on the evolution characteristics of the daily-scale Silk Road pattern (SRP) and its effect on summer temperatures in the Yangtze River Valley (YRV). The results manifest that the evolution characteristics of positive- and negative-phase SRP (referred to SRP+ and SRP−) exhibit marked distinctions. The anomaly centers of SRP+ over West Central Asia (WCA) and Mongolia emerge firstly, vanishing simultaneously one week after peak date; however, the Far East (FE) anomaly centers can persist for a longer period. The SRP− starts with the WCA and FE centers, with a rapid decline in the strength of the WCA center and preservation of other anomaly centers after its peak. In the vertical direction, daily-scale SRP mainly concentrates in the mid-to-upper troposphere. Baroclinicity accounts for its early development and barotropic instability process favors the maintenance. Moreover, the SRP+ (SRP−) is inextricably linked to heat wave (cool summer) processes in the YRV. Concretely, before the onset of SRP+ events, an anomalous anticyclone and significant negative vorticities over East Asia related to SRP+ favor the zonal advance between the South Asia high (SAH) and western Pacific subtropical high (WPSH), inducing local descents over YRV area. The sinking adiabatic warming and clear-sky radiation warming can be considered as the possible causes for the YRV heat waves. The adiabatic cooling with the local ascents leads to more total cloud cover (positive precipitation anomalies) and less solar radiation incident to surface of the YRV, inducing the cool summer process during SRP−.

Skillful prediction of summer rainfall in the Tibetan Plateau on multiyear time scales

Skillful near-term climate predictions of rainfall over the Tibetan Plateau (TP), the Asian water tower, benefit billions of people. On the basis of the state-of-the-art decadal prediction models, we showed evidence that although the raw model outputs show low predicted ability for the summer Inner TP (ITP) rainfall due to low signal-to-noise ratios in models, we can produce realistic predictions by extracting the predictable signal from large ensemble predictions along with a postprocessing procedure of variance adjustment. The results indicate that the summer ITP rainfall is highly predictable on multiyear time scales. The predictability of ITP rainfall originates from the Silk Road pattern driven by sea surface temperature over the subpolar gyre region in North Atlantic. Real-time forecasts suggest that the ITP will become wetter, with 12.8% increase in rainfall during 2020–2027 relative to 1986–2005. Our results will help the water resources management in the surrounding regions.

Combined Effects of the British–Baikal Corridor Pattern and the Silk Road Pattern on Eurasian Surface Air Temperatures in Summer

The summer British–Baikal Corridor pattern (BBC) and the Silk Road pattern (SRP) manifest as zonally oriented teleconnections in the high and middle latitudes, respectively, of the Eurasian continent. In this study, we investigate the combined effects of the BBC and SRP on surface air temperatures over the Eurasian continent. It is found that the combination of the BBC and SRP results in two kinds of well-organized, large-scale circulation anomalies: the zonal tripole pattern and the Ω-like pattern in the 200-hPa geopotential height anomalies. The zonal tripole pattern is characterized by opposite variations between western Siberia/western Asia and Europe/central Asia/central Siberia, and the Ω-like pattern manifests as consistent variations over midlatitude Europe, western Siberia, and central Asia. Correspondingly, the resultant large-scale surface air temperature anomalies feature the same zonal tripole pattern and Ω-like pattern, respectively. Further results indicate that these two patterns resemble the two leading modes of surface air temperature anomalies over the middle to high latitudes of Eurasia. This study indicates that the temperature variations in the middle and high latitudes of Eurasia can be coordinated and evidently explained by the combination of the BBC and SRP, and it contributes to a more comprehensive understanding of the large-scale Eurasian climate variability.

The driving processes of concurrent hot and dry extreme events in China

<p>Sub-seasonal heatwave-driven concurrent hot and dry extreme events (HDEs) can cause substantial damage to crops, and hence to lives and livelihoods. However, the physical processes that lead to these devastating events are not well-understood.</p><p>Based on observations and reanalysis data for 1979-2016 over China, we show that HDEs occur preferentially over central and eastern China (CEC) and southern China (SC), with a maximum of 3 events year<sup>-1</sup> along the Yangtze Valley. The probability of longer-lived and potentially more damaging HDEs is larger in SC than in CEC. Over SC the key factors of HDEs—positive anomalies of surface air temperature and evapotranspiration, and negative anomalies of soil moisture—begin two pentads before maximising at the peak of the HDEs. These anomalies occur south of a positive height anomaly at 200 hPa, associated with a large-scale subsidence anomaly. The processes over CEC are similar to SC, but the anomalies begin one pentad before the peak. HDE frequency is strongly related to the Silk Road Pattern and the Boreal Summer Intraseasonal Oscillation. Positive phases of the Silk Road Pattern and suppressed phases of the Boreal Summer Intraseasonal Oscillation are associated with positive height anomalies over CEC and SC, increasing HDE frequency by about 35-54% relative to the climatological mean.  Understanding the effects of sub-seasonal and seasonal atmospheric circulation variability, such as the Silk Road Pattern and Boreal Summer Intraseasonal Oscillation, on HDEs is important to improve HDE predictions over China.</p>

Effects of upper-level background wind bias on Rossby waveguides, Rossby wave source hotspots, and predictability of the Silk Road pattern

<p>The Silk Road pattern (SRP) is a leading mode of atmospheric circulation over mid-latitude Eurasia during boreal summer. Its temporal phase is known to be unpredictable in many climate models. Previous studies have not reached a clear consensus on the role of sea surface temperature (SST) associated with SRP. To investigate role of SST on SRP formation, we begin by comparing reanalysis with seasonal hindcast experiments of the Pusan National University coupled climate model.<span> </span></p><p>Although SRP cannot be predicted temporally, the ensemble runs show potential predictability in SRP related to tropical Pacific SST. While reanalysis SRP is associated with North Atlantic SST anomalies, hindcast SRP is associated with tropical Pacific SST anomalies similar to El-Nino Southern Oscillation (ENSO). To explain the different SST associations, we propose two jet biases in the climate model which may affect Rossby wave propagation. Bias in North Atlantic jet exit results in a discontinuous waveguide from North Atlantic to Asia, which may hinder propagation of waves associated with North Atlantic SST to trigger SRP. In addition, bias in subtropical western Pacific westerlies reduces the evanescent region between subtropical western Pacific and Asian jet, which may favour westward dispersion of zonally elongated waves associated with ENSO SST to trigger SRP. Therefore, we propose that the role of SST on SRP can be substantially changed depending on fidelity of model upper-level background winds.<span> </span></p><p>To investigate more quantitatively the roles of waveguides and the Rossby wave sources (RWS), we perform wave-making experiments using an idealised barotropic model prescribed with two different upper-level background winds, namely from reanalysis and from climate model. By comparing with result using reanalysis background winds, the preferred forcing locations - RWS hotspots - of SRP are identified from all the RWS associated with SRP in reanalysis. In addition to previously identified hotspots from the literature, a new hotspot in central North Pacific is discovered which can force SRP by westward dispersion of zonally elongated Rossby wave.<span> </span></p><p>Wave-making result using climate model background winds reveals that the upper-level wind bias changes the RWS hotspots locations of SRP. Experimental result is consistent with theoretical analysis of waveguide bias, and support our conclusion that the relationship between SRP and SST can be substantially changed depending on model background winds bias. The impact of our study is that this sensitivity of SRP hotspots to background winds may reduce seasonal forecast skill of SRP in models with background winds bias.<span> </span></p>

The driving processes of concurrent hot and dry extreme events in China

Sub-seasonal heatwave-driven concurrent hot and dry extreme events (HDEs) can cause substantial damage to crops, and hence to lives and livelihoods. However, the physical processes that lead to these devastating events are not well-understood. Based on observations and reanalysis data for 1979-2016 over China, we show that HDEs occur preferentially over central and eastern China (CEC) and southern China (SC), with a maximum of 3 events year-1 along the Yangtze Valley. The probability of longer-lived and potentially more damaging HDEs is larger in SC than in CEC. Over SC the key factors of HDEs—positive anomalies of surface air temperature and evapotranspiration, and negative anomalies of soil moisture—begin two pentads before maximising at the peak of the HDEs. These anomalies occur south of a positive height anomaly at 200 hPa, associated with a large-scale subsidence anomaly. The processes over CEC are similar to SC, but the anomalies begin one pentad before the peak. HDE frequency is strongly related to the Silk Road Pattern and the Boreal Summer Intraseasonal Oscillation. Positive phases of the Silk Road Pattern and suppressed phases of the Boreal Summer Intraseasonal Oscillation are associated with positive height anomalies over CEC and SC, increasing HDE frequency by about 35-54% relative to the climatological mean. Understanding the effects of sub-seasonal and seasonal atmospheric circulation variability, such as the Silk Road Pattern and Boreal Summer Intraseasonal Oscillation, on HDEs is important to improve HDE predictions over China.