Inter-decadal Change of Tibetan Plateau Vortices during the past four Decades and its Possible Mechanism

<p>The Tibetan Plateau vortices (TPVs) are mesoscale weather systems active at the near-surface of the Tibetan Plateau (TP), which are one of the major precipitation-producing systems over the TP and its surrounding areas. TPVs mainly occur in the warm season from May to September. In this paper, we investigate the inter-decadal change of TPVs in the warm seasons of 1979–2017 by analyzing five widely used reanalysis datasets. A significant change of the TPVs’ frequency appears around the mid-1990s, associated with less TPVs during 1979–1996 and more TPVs during 1997–2017. The abrupt change is caused by a transition of the Atlantic Multi-decadal Oscillation (AMO) from a cold phase to a warm phase in the mid-1990s. The shift of AMO leads to a silk-road pattern wave train and a spatially asymmetric change of tropospheric temperature. It modifies the intensity of the subtropical westerly jet and the TP heating, leading to the inter-decadal change of TPV activities.</p>

Two Types of Mid-High-Latitude Low-Frequency Intraseasonal Oscillations Near the Ural Mountains During Boreal Summer

Two types of mid-high-latitude low-frequency intraseasonal oscillations (LF ISOs), featuring eastward and westward propagation, have been identified over the Eurasian continent in the past 37 summers (1982–2018). The eastward and westward propagating modes commonly have a dominant periodicity of 30–50 days near the Ural Mountains (UM) but have different origins and evolutions. The eastward propagating LF-ISO initiates over the eastern North America, migrates northeastward across the northeastern North America-Western North Atlantic, central North Atlantic, Western Europe and the UM, then propagates southeastward to northwestern and eastern China, which is the Atlantic-Eurasian continental mode. In contrast, the westward propagating mode is quasi-circumpolar, initiating over the East Siberian Sea and moving southwestward across the UM, northern Europe and eventually reaching Greenland and Canadian Arctic Archipelago. These two mid-high-latitude LF-ISOs are accompanied by significant tropical intraseasonal variations with evident tropical-extratropical interactions. Meanwhile, these two LF-ISOs have different decadal preferences before and after 2000, which are ascribed to the decadal change of both intraseasonal efficient kinetic energy obtained from the mean flow over their genesis region and their background flow associated with the North Atlantic Oscillation/Arctic Oscillation decadal change. This study deepens the understanding of subseasonal variations for mid-high-latitudes and subseasonal prediction sources for low-latitude regions.