As shown in Fig. 5c, monthly averaged MLD had a similar seasonal pattern of Chl a. It was shallow before October 2009, and increased gradually after that. The MLD level was much shallower than cruise observation values in winter, which could reach 90 m, but it was comparable to the field observation in remaining part of the year ( 20 m) (Tseng et al., 2005). The MLD value obtained from the ARGO observations in winter could be expected to be shallower than the in situ values, since it was a monthly averaged mean for entire season.