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Using multiple proxy data of historical hydroclimate variations, this study first selects the five heaviest megadrought decades in North China during the last millennium and then depicts the spatial pattern of hydroclimate over the Northern Hemisphere through these five megadrought decades. The results shows that abnormally dry conditions existed in East Asia, North America, and Europe when megadroughts in North China during the last millennium. The intensity of dry conditions in North America was stronger than that in Europe. Then, through comparing the spatial patterns of hydroclimate from proxy data with ensemble modeling of nine CMIP5/PMIP3 models, we find that such megadroughts in North China, and the contemporaneously abnormal dry conditions over the Northern Hemisphere were very likely mainly induced by external forcing, i.e., large volcanic eruptions and weakened solar irradiation. These external forcing may lead to climate cooling and, hence, air motion downward at mid-lower layer of troposphere, inducing a much more stable troposphere; as well as, the land-ocean pressure differences were weakened, and, hence, there would be decreased water vapor flux from ocean to land. Both of intensified stability of troposphere and weakened vapor transportations to land would lead to precipitation decline and, as a result, would be favorable for drought. It is therefore necessary to consider these external forcing into future climate change predicting to improve our ability of predicting megadrought.

An extreme drought occurred from 1876 to 1878 in most regions of northern China, leading to a series of social impacts, including harvest failures, price inflation, and population migration. Most regions of the Northern Hemisphere concurrently experienced extreme drought. Here, we use reconstructed high-resolution hydroclimatic (Palmer Drought Severity Index/precipitation) datasets and investigate the seasonal–annual hydroclimatic spatial patterns and drought intensity in North China from 1876 to 1878. Furthermore, we select combined sea surface temperature (SST) modes with positive Indian Ocean Dipole (IOD)/Pacific Decadal Oscillation (PDO) and El Nino conditions from the 1200-year control run simulations of HadCM3 to determine the possible causes of this severe drought. The extent and intensity of the selected SST modes are similar to those in the Pacific and Indian oceans during the 1876–1878 period from the National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed Sea Surface Temperature (ERSST) dataset. The results show that the large-scale drought of 1876–1878 is mainly driven by El Nino and a positive PDO, while the effect of IOD is not significant. El Nino may trigger the circumglobal teleconnection of the Northern Hemisphere. And, the meridional disturbance of the eastward Rossby wave train at mid-latitudes may change the intensity of the troughs and ridges and further block water vapor transport from ocean to land.

The long-term variations in meteorological drought and its large-scale climate patterns in each season in Central Asia from 1901 to 2015 remain unclear. Here, this issue is addressed using meteorology measurements and reanalysis data through correlation and composite analyses. The drought intensity index (DII) and extent index (DEI) do not exhibit significant linear trends from 1901 to 2015 but do exhibit interannual to interdecadal variations. Both the DII and DEI are highly correlated with the tropical Nino 4 sea surface temperature (SST) and extratropical atmospheric teleconnections, including the East Atlantic (EA) pattern, the East Atlantic/West Russia (EAWR) pattern, and the Arctic oscillation (AO), but with seasonal discrepancies in terms of the degree of influence. The winter drought is strongly linked to both the negative EA and negative EAWR patterns, while spring and autumn drought are strongly linked to the negative EAWR and negative EA patterns, respectively. In the winter, spring, and autumn, drought is also closely linked to below normal Nino 4 SST. The links to EA and EAWR patterns are mainly derived from their impacts on precipitation in the central and northern sectors, while the link to Nino 4 SST is mainly derived from its impacts on precipitation in the southern sector. By considering both drought intensity and drought extent, the ten extreme drought years for each season are selected and, through composite analysis, their large-scale climate patterns are studied. The extreme drought generally occurs in the contexts of a negative EA pattern in winter, a negative EAWR pattern in spring, and a negative AO pattern in autumn. As an exception, summer drought is weakly correlated with Nino 4 SST and is not correlated with extratropical atmospheric teleconnections.