Understanding the Eastward Shift and Intensification of the ENSO Teleconnection Over South Pacific and Antarctica Under Greenhouse Warming

The Pacific–South America (PSA) teleconnection pattern triggered by the El Niño/Southern Oscillation (ENSO) is suggested to be moving eastward and intensifying under global warming. However, the underlying mechanism is not completely understood. Previous studies have proposed that the movement of the PSA teleconnection pattern is attributable to the eastward shift of the tropical Pacific ENSO-driven rainfall anomalies in response to the projected El Niño-like sea surface temperature (SST) warming pattern. In this study, we found that with uniform warming, models will also simulate an eastward movement of the PSA teleconnection pattern, without the impact of the uneven SST warming pattern. Further investigation reveals that future changes in the climatology of the atmospheric circulation, particularly the movement of the exit region of the subtropical jet stream, can also contribute to the eastward shift of the PSA teleconnection pattern by modifying the conversion of mean kinetic energy to eddy kinetic energy

Multidecadal Changes in the Influence of the Arctic Oscillation on the East Asian Surface Air Temperature in Boreal Winter

The time-varying influences of the wintertime Arctic Oscillation (AO) on the concurrent East Asian surface air temperature (EAT) are investigated based on JRA-55 reanalysis data. Results reveal that there are multidecadal variations in the influences of wintertime AO on the EAT during 1958–2018. Before the mid-1980s, the impact of winter AO on the simultaneous EAT is weak and confined northward of 40° N over East Asia. After the mid-1980s, by contrast, the winter AO’s influence is stronger and can extend southward of 25° N over East Asia. The multidecadal variations of the winter AO–EAT relationship are mainly modulated by the magnitudes of the North Pacific center of the winter AO. During the periods with strong North Pacific center of the winter AO, in association with the positive phase of the winter AO, the low-level southeasterly anomalies on the west side of the anticyclone over North Pacific bring warm air from the ocean to East Asia and lead to a significant winter AO–EAT relationship. In contrast, the southerly anomalies are weak and even reversed to northerly over the coast of East Asia during the periods with weak North Pacific center of winter AO, which confines the influence of winter AO on northern East Asia and lead to an insignificant winter AO–EAT relationship. Our finding provides new insight into the understanding of the decadal changes of AO’s impacts on the regional climate

Recently Strengthened Influence of ENSO on the Wintertime East Asian Surface Air Temperature

Previous studies have indicated that El Niño–Southern Oscillation (ENSO) exerts a significant influence on the East Asian winter climate. This study reveals an interdecadal strengthening of the connection between ENSO and the East Asian surface air temperature (EAT) connection around the late 1990s and investigates the reason for this change. Before the late 1990s, the influence of ENSO on the EAT was weak, and the ENSO-related southerly wind anomalies were confined to the south of 30° N of East Asia. After the late 1990s, by contrast, ENSO’s influence became stronger and capable of extending northward to 50° N of East Asia. The decadal strengthening of the link between ENSO and EAT is primarily modulated by the magnitudes of the ENSO-related Kuroshio anticyclone. The intensity of the Kuroshio anticyclone contributes more than 50% of the variance of the oscillational ENSO–EAT variability. Further investigation indicates that the recovered magnitude of the Kuroshio anticyclone after the late 1990s has been closely tied to the eastward shrinking of the Aleutian Low (AL) pattern, which has weakened the link of atmospheric circulation between the AL and Kuroshio Extension region. Therefore, the offset effect of the AL-induced negative (positive) sea level pressure (SLP) anomalies on the El Niño (La Niña)-induced positive (negative) SLP anomalies over the Kuroshio Extension has also been weakened, which has facilitated the recovery of the significant influence of ENSO on the EAT

Biases of the wintertime Arctic Oscillation in CMIP5 models

Distinct biases are found in the pattern and teleconnections of the Arctic Oscillation (AO) in 32 climate models that participate the Coupled Model Intercomparison Project Phase 5 (CMIP5). Compared with observations, the Pacific (Atlantic) center of AO is excessively strong (weak) in most of the 32 CMIP5 models, and the AO-related surface air temperature anomalies are generally weak over the Eurasian continent and North America. These biases are closely tied to the excessively strong linkage, which is marginal in observations, between AO and the North Pacific mode (NPM)—the leading variability of the North Pacific sea level pressure. It implies that the AO in CMIP5 models may be compounded with some regional mode over the North Pacific. Accordingly, a bias-correction method was proposed via correcting the AO index (AOI) to improve the diagnostic estimates of the AO teleconnections. The results suggest that the biases in the pattern and teleconnections of AO can be significantly reduced when the NPM variability is linearly removed from the AOI

Chinese blue days: a novel index and spatio-temporal variations

As part of the Blue-Sky Protection Campaign, we develop the Chinese Blue Days Index based on meteorology data from 385 stations in China during 1980–2014. This index is defined as the days with no rain, low cloud cover ≤75th percentile, and visibility ≥15 km at 2 pm. The spatio-temporal variations and possible driving factors of Chinese Blue Days (CBD) are further investigated, revealing a steadily rising rate of 1.6 day (d)/10 year (y) for the nationally averaged CBD during 1980–2014. At regional scales, the CBD exhibit an increasing trend >4 d/10 y in western China and a decreasing trend <−2 d/10 y in southeastern China, northwestern Xinjiang, and Qinghai. The minimum/maximum trends (−7.5/9.5 d/10 y) appear in Yangtze–Huai River Valley (YHRV)/southwestern China (SWC). The interannual variations in CBD are highly related to wind speed and windless days in YHRV but are closely associated with wind speed, rainless days and relative humidity in SWC, suggesting that the two regions are governed by different meteorological factors. Moreover, a dynamic adjustment method called partial least squares is used to remove the atmospheric circulation-related CBD trend. The residual CBD contributions for the total trend in summer and winter are 43.62% and 35.84% in YHRV and are 14.25% and 60.38% in SWC. The result indicates that considerable parts of the CBD trend are due to the change of atmospheric circulation in the two regions

The Spatiotemporal Patterns of Human Settlement during the Longshan and Erlitou Periods in Relation to Extreme Floods and Subsistence Strategy in the Upper and Middle Qin River Reaches, Central China

Human settlement numbers have significantly changed before and after ~4000 cal. y BP in the upper and middle Qin River reaches, but the external and internal factors driving this change remain unclear. In this study, we examine changing spatial and temporal patterns of the Longshan and Erlitou settlements in relation to extreme flooding at ~4000 cal. y BP and a variety of subsistence strategies during the Longshan and Erlitou periods. The results indicate that settlement number, settlement distribution, and subsistence strategies exhibited obvious shifts between the Longshan and Erlitou periods, and the episode at ~4000 cal. y BP was an extreme-flood-rich interval within and around the Qin River Basin. During the Longshan and Erlitou periods, millet-based agriculture dominated local subsistence strategy, and ancient people would prefer to reside in the areas suitable for farming, causing the valley plains in the upper and middle Qin River reaches to contain most Longshan and Erlitou settlements. However, the frequent occurrence of extreme floods at ~4000 cal. y BP, in conjunction with intergroup conflicts due to a large amount of population immigration during the late Longshan period, is likely to have jointly decreased the settlement number and shrunk the spatial range of human settlement distribution. Subsequently, with the end of the extreme-flood-rich episode and the increasing proportion of higher-water-requirement foxtail millet in cropping structures of human subsistence strategy, more Erlitou settlements were distributed in the wetter valley plains of the middle Qin River reaches

Spatial Expansion of Human Settlement during the Longshan Period (~4.5–~3.9 ka BP) and Its Hydroclimatic Contexts in the Lower Yellow River Floodplain, Eastern China

Obvious spatial expansion of human settlement occurred in the lower Yellow River floodplain during the Longshan period, but the external factors driving this expansion remain unclear. In this study, we first delineated the hydroclimatic changes at both regional and local scales within and around the lower Yellow River floodplain and then examined the relationships of human settlements with hydroclimatic settings between the pre-Longshan and Longshan periods. The results indicate that the site distribution, site density and hydroclimatic conditions exhibited significant shifts during the pre-Longshan and Longshan periods. In the pre-Longshan period, the intense East Asian summer monsoon and abundant monsoon-related precipitation caused widespread development of lakes and marshes in the lower Yellow River floodplain. As a result, the circumjacent highlands of the lower Yellow River floodplain contained concentrated human settlements. However, the persistent weakening of the East Asian summer monsoon and consequent precipitation decline, in conjunction with accelerated soil erosion due to decreasing forest vegetation and strengthening of human activities on the upstream Loess Plateau in the Longshan period, are likely to have jointly caused both shrinking and faster filling of preexisting lakes and marshes. Subsequently, a large area of arable land had been created in the lower Yellow River floodplain and thus was occupied by locally rapid increasing population, resulting in the notable spatial expansion of human settlements during the Longshan period