Amplitude modulation of relative humidity by wind in Northeast China: the formation of variance annual cycle in relative humidity

Abstract

Relative humidity has an important impact not only on climate change and ecosystems but also on human life. The intensity of high-frequency fluctuations in relative humidity over Northeast China shows a predominant seasonally dependent structure, which may be closely related to regional monsoon activities. However, the factors responsible for this phenomenon remain unknown. This study defines the Variance Annual Cycle (VAC) to describe this seasonally dependent intensity structure of high-frequency relative humidity fluctuations. Relative humidity VAC shows a high correlation with low-frequency oscillations of wind speed. We examine the instantaneous amplitude-phase correlation map and amplitude modulation (AM) index between relative humidity and wind speed. We find that the wind speed with a period around 140-420 days has a significant amplitude modulation effect on the relative humidity with a period around 2-90 days over most regions in Northeast China, which reveals that the low-frequency oscillations of wind speed amplitudemodulate on the high-frequency fluctuations of relative humidity. To explore the physical mechanism behind this modulation, we examine the monthly mean patterns of the atmospheric fields. The patterns indicate that this amplitude modulation is induced by the evolution and transition of East Asian winter monsoon and summer monsoon.