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 surface 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 surface wind speed. We examine the instantaneous amplitude-phase correlation map and amplitude modulation (AM) index between surface relative humidity and surface wind speed. We find that the surface wind speed with a period around 140–420 days has a significant amplitude modulation effect on the surface relative humidity with a period of around 2–90 days over most regions in Northeast China, which reveals that the low-frequency oscillations of surface wind speed amplitude modulate on the high-frequency fluctuations of surface relative humidity. To explore the physical mechanism behind this modulation, we examine the monthly mean patterns of the surface wind and sea level pressure fields. The patterns indicate that this amplitude modulation is induced by the evolution and transition of East Asian winter monsoon and summer monsoon.
Highlights
Relative humidity plays a crucial role in climate change, vegetation growth and human living and health (Byrne and O'Gorman, 2018; Sherwood and Fu, 2014; Sun et al, 2016)
Relative humidity Variance Annual Cycle (VAC) shows the deviation of raw data from climatology each day in a calendar year (Figure 1b), which corresponds to the multi-year average of the intensity changes of relative humidity anomalies in a year
To explore the formation of relative humidity VAC, we compare the climatology annual cycle (CAC) of wind speed measured at the same meteorological station with the VAC of relative humidity
Summary
Relative humidity plays a crucial role in climate change, vegetation growth and human living and health (Byrne and O'Gorman, 2018; Sherwood and Fu, 2014; Sun et al, 2016). As a direct observation of atmospheric moisture content, relative humidity is an important atmospheric variable that can reflect the combined effect of temperature and moisture budget (Wang and Gaffen, 2001). In which the temperature and local water vapor budget change drastically during the beginning, development, and ending process, relative humidity is a physical quantity with great potential to describe its evolution process. Previous studies have found that the change of relative humidity in West Africa is controlled by the West African monsoon system, and it can be well predicted with the occurrence and disappearance of the monsoon (Broman et al, 2014).
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