Mixed layer depth (MLD) affects phytoplankton dynamics through controlling the availability of nutrients and light in the upper ocean. This study investigates variability of MLD and its relation with chlorophyll (CHL) in the North Pacific Ocean on seasonal to year-to-year timescales. The Pacific MLD field was calculated from SODA (Simple Ocean Data Assimilation) version 2.1.6 covering 51 years from January 1958 to December 2008. MLD was defined as the depth at which the potential density takes the reference depth (10 m) salinity value and the temperature value which is 0.2 degree centigrade lower than the reference value. The variability of maximum MLD is well correlated with CHL variability in some regions in the North Pacific Ocean including the Kuroshio Extension (KE). For example, increased maximum MLDs in the years of 2003-2007 compared with values in the years of 1998-2002 correspond to enhanced CHLs in the KE. The good correspondence between MLD and CHL suggests that increased maximum MLD helps to entrain deep nutrient into the upper ocean and thus to maintain high CHL in the KE. Changes in primary production based on nutrient entrainment by wind mixing at selected locations in the North Pacific also will be given and compared with observation.