Abstract: Used monthly datasets of temperature, salinity dissolved oxygen and pH from May 1997 to November
1981 and monthly mean atmospheric carbon dioxide concentration data in the Yellow Sea, the spatio-temporal 
modes on the seasonal cycle of pH field in the Yellow Sea and its mechanism are studied with Rotated Empirical Or-
thogonal Function ( REOF), harmonic analysis and the delay correlation analysis methods. The results show that 
the seasonal variation of pH field in Yellow Sea is divided into two kinds of the spatio-temporal modes. For the up-
per layer (surface io 20 m) . the spatial component of the first and second mode demonstrates a north-south distribution pattern, with main influencing factois of the air sea exchange CO2, photosynthesis respiration of biological
 active component and the downward transmission of the phase of pH modes from the surface to the deep layer. For 
the lower layer (30 m to bottom) , the spatial component of the first and second mode piesents an east-west distribution type. The main influencing factors are attributed to the sediment oxygen demand and the decomposition oxy
gen consumption of biological debris. The spatial component of the pH modes shows a vertically dual-layer structure. The periodic component of the pH modes is not the dominant factor and the stability of seasonal periodic cycle 
is relatively weak, which is susceptible to impacts from the inteiannual variation component and hence to produce
tfie seasonal cycle mutation. The pH modes in tfie surface, middle and bottom vary with seasonal disturbance sources.Air-sea exchange effect of carbon dioxide and physical-biological effects of apparent oxygen utilization with
 the nonlinear effect act as the firet and secondary influencing factors is in tfie surface, respectively. As to the middle
 layer, the physical-biological effects of apparent oxygen utilization of tfie middle and the incoming pH modes dis
turbance from the adjacent upper and lower layers play important roles. And for the bottom layer, the physical-bio-
logical effects of apparent oxygen utilization is believed to be important influence factor, while sediment oxygen de-
mand, bottom temperature, salinity effects seem to play the secondary roles. On Ae seasonal mean timescale, sea-
sonal disturbance of pH field induced by the surface air-aea exchange of carbon dioxide is the dominant factor with 
seasonal chaining phase propagating from surface to bottom less than one month. However, the stratification efiecls of 
the pH disturbances are more prominent on inteiannual timescale than that on seasonal timescale in tfie Yellow Sea.