Abstract
The East Asian Summer Monsoon driven by temperature and moisture gradients between the Asian continent and the Pacific Ocean, leads to approximately 50% of the annual rainfall in the region across 20–40°N. Due to its increasing scientific and social importance, there have been several previous studies on identification of moisture sources for summer monsoon rainfall over East Asia mainly using Lagrangian or Eulerian atmospheric water vapor models. The major source regions for EASM previously proposed include the North Indian Ocean, South China Sea and North western Pacific. Based on high-precision and high-frequency 6-year measurement records of hydrofluorocarbons (HFCs), here we report a direct evidence of rapid intrusion of warm and moist tropical air mass from the Southern Hemisphere (SH) reaching within a couple of days up to 33°N into East Asia. We further suggest that the combination of direct chemical tracer record and a back-trajectory model with physical meteorological variables helps pave the way to identify moisture sources for monsoon rainfall. A case study for Gosan station (33.25°N, 126.19°E) indicates that the meridional transport of precipitable water from the SH accompanying the southerly/southwesterly flow contributes most significantly to its summer rainfall.
Highlights
The East Asian summer monsoon (EASM) is characterized by the abrupt transition from the dry season with dominant northerly winds to the rainy season with prevailing southwesterly/southerly winds bringing substantial moisture[1,2]
A chemical compound that is conservative on a time scale of the EASM seasonality, but has detectable regional gradients in its atmospheric mixing ratio, can be used as a tracer (see Supplementary Information (SI) text for more details)) to identify meridional transports of air masses
Vertical dilution by free tropospheric air masses cannot be an explanation for the drawdowns given that no vertical gradients in HFCs mixing ratios were observed during the fourth and fifth HIAPER Pole-to-Pole Observations (HIPPO-4 and HIPPO-5) campaigns traversing the entire Pacific in June-early September 201126
Summary
High-precision and high-frequency measurements of halogenated compounds introduced in this study, were made continuously every two hours from 2008 to 2013 using a gas chromatograph-mass spectrometer coupled with an online cryogenic pre-concentration system (“Medusa”)[34] within the AGAGE program. Analytical uncertainties of the measurements are 0.2% (RSD). Flask measurements of HFC-152a and HFC-134a at MLO in Figs 2(b) and S2, respectively, have been taken since 2008. The data are provided from the Global Monitoring Division of the Nationals Oceanic and Atmospheric Administration’s Earth System Research Laboratory (NOAA/ ESRL/GMD) (ftp://ftp.cmdl.noaa.gov/hats/hfcs/)
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