The early 21st century experienced a transition in global ocean observing systems from the expendable bathythermograph (XBT) to the Argo. There has been a decrease in XBT observations, and a significant increase in Argo profiling floats in the global ocean. However, numerical XBT observation evaluations during this transition period have been under presented. This study investigates the XBT use effects on the global ocean observing systems using a coupled data assimilation model developed by the Geophysical Fluid Dynamics Laboratory (GFDL). Results show that the inclusion of XBT data significantly increases the accuracy of heat content and sea level change estimations during the pre-Argo period. During the Argo period, the amount of heat content correction by XBT assimilation is significantly weakened, especially in the upper ocean. However, it remains in the deeper oceans below 700 m depths, which is the residual effects of assimilating XBT data with the pre-Argo period. This study also confirms that although XBT only provides temperature observations mostly in the upper 700 m of the northern hemisphere, it can affect both the temperature and salinity fields of data assimilation systems, especially in the deep and southern oceans, which is also supported by the significant change in steric height.