In this study, The long-term trend of global sea-to-air dimethyl sulfide (DMS) flux was analyzed for 2000-2015, based on satellite observation and modeling data. In addition, DMS-related aerosol (e.g. sulfate) have been estimated using the DMS flux calculated in the east Asian ocean for the recent three years (2013-2015). The calculation of the AOD derived from DMS flux (AOD (DMS)) and direct radiative forcing was carried out using the OPAC and SBDART.
The DMS flux in the global ocean exhibited a gradual decreasing pattern from 2000-2015. For the latitude band (10° interval between 0° and 60°), the DMS flux at the low latitude (0°-30°) in both Northern (NH) and Southern Hemispheres (SH) was significantly higher than that at the middle latitude (30°-60°). From long-term analysis with the Mann-Kendall (MK) statistical test, the trend of DMS flux mostly showed a gradual decreasing pattern (NH: except for 10°N-20°N, 20°N-30°N, in winter, SH: except for 10°S-20°S in summer).
Yearly variations in DMS flux were 1.45±2.38 μmol/m^2/day in 2013, 0.75±1.03 μmol/m^2/day in 2014 and 0.57±0.68 μmol/m^2/day in 2015. The spatial distribution of annual mean DMS flux was highest in the coastal sea near Shanghai followed by Bohai sea, whereas its lowest value occurred in the East Sea. The DMS flux was higher in winter and/or fall and lower in spring during most years. For the latitude bands (4° interval between 29° and 45°), DMS flux was highest in the latitude band of 29°N-33°N and lowest at 41°N-45°N. The spatio-temporal characteristics of DMS flux were likely to be primarily affected by the SSW. However, at some latitudes, such DMS flux characteristics might be due to the combined effect of the DMS concentration and SST.
In the eastern Asian ocean during 2013-2015, the annual mean AOD derived from DMS flux (AOD (DMS)) was highest in the coastal sea near Shanghai (belong to 29°N-33°N), while its lowest value occurred in the East Sea followed by the western coastal sea near Japanese. The spatial distributions of the high AOD (DMS) (especially near Shanghai) were very or slightly similar to those of DMS flux. In addition, AOD (DMS) was highest at 29°N-33°N and lowest at 41°N-45°N, and this pattern for the latitude band was similar to that of DMS flux. To compare the trends between AOD (DMS) and total AOD (anthropogenic and natural), we analysed the trends in both AOD (τ550) derived from DMS flux (AOD (DMS)) and MODIS AOD (τ550) during 2003-2015. The contribution of AOD (DMS) to MODIS AOD in the study area was estimated to be 16.2% on average (range: 4.6%-45.9%). Seasonal contribution was highest in winter (24.1%) and lowest in spring
(2.4%).
The annual mean direct radiative forcing (SFC, TOA, and ATM) due to DMS-related aerosol (e.g. sulfate) were highest in the coastal sea near Shanghai, while their lowest values appeared in the East Sea followed by the western coastal sea near Japanese. during the study period (2013-2015). The negative direct radiative forcing (SFC, TOA, and ATM) showed a peak in December 2014 (a mean of -46.06±31.40 (SFC), -39.96±26.24 (TOA), 6.10±5.26 (ATM) W/m^2) and a minimum in March 2013 (-8.68±1.92 (TOA), 1.19±0.28 (ATM), -9.87±2.19 (SFC) W/m^2), with a highest value observed at 29°N-33°N and a lowest value at 41°N-45°N. In addition, the negative forcing was found to be higher in winter (highest) and fall than in summer and spring (lowest) during the study period. The negative radiative forcing thus suggests that the DMS-related aerosol in the target area can directly affect the change (especially, the negative impact) in the regional climate and radiative balance.