Abstract
The Northern Hemisphere summer climate isstrongly affected by a circumglobal stationary Rossby wave train, which can be manifested by the first EOF mode of the geopotential height at 200 hPa. Interannual variation of this Northern Hemisphere wave (NHW) pattern has a significant impact on remarkably warm surface temperature anomalies over the North Atlantic, Northeast Europe, East Asia to Central-North Pacific, and America, particularly in 2018 and 2010. The NHW pattern is likely generated by atmospheric diabatic heating and vorticity forcing: diabatic heating is mainly confined in the Indian summer monsoon (ISM) precipitation region, whereas the anti-cyclonic vorticity forcing is distributed in the globe. The ISM is a well-known diabatic heat source; however, the main source of vorticity forcing has not been established. In general, the tropical vorticity anomaly comes from diabatic heating-induced atmospheric waves and randomly generated inherent internal waves. The linear baroclinic model experiment reveals that the NHW pattern can be generated by the westward propagating tropical waves generated by the ISM diabatic heat forcing.
Highlights
More than 90% of the world’s population lives in the Northern Hemisphere, and the human life is significantly influenced by weather and climate variability
With dominant stationary wave pattern over the Northern Hemisphere in July, the NHW pattern is related to the anomalous ISM precipitation and sequential westward propagating tropical easterly waves
The tropical vorticity forcing comes from both the internal and forced atmospheric waves; here, the forced waves are primarily caused by the steady heat forcing, while atmospheric internal waves are regarded to exist randomly
Summary
More than 90% of the world’s population lives in the Northern Hemisphere, and the human life is significantly influenced by weather and climate variability. We note that the upper troposphere positive geopotential height anomalies and negative vorticity forcing over subtropical regions can be generated by westward propagating atmospheric waves. This means that the tropical–subtropical diabatic heat forcing generates the westward propagating Rossby waves, and again this anomalous vortex flow can be an additional vorticity forcing on the atmosphere. The potential diabatic heat forcing regions, represented by forcing in LBM experiments inducing significant value of pattern correlation coefficient between geopotential height anomalies of EOF1 and LBM simulations, are shown, where forcing lies along the subtropical regions from the Eastern Atlantic to South China. Note that most of North African vorticity perturbations are induced by random tropical forcing with internal tropical easterly waves, and these waves are typically regarded as an atmospheric noise; the rigorous signal of 5–20% has a grateful meaning to understand the origin of the NHW pattern
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