  We perform two-dimensional resistive magnetohydrodynamic simulations of
coronal jets driven by flux emergence along the lower boundary. The
reconnection layers are susceptible the formation of blobs that are ejected in
the jet. Our simulation with low plasma $\beta$ (Case I) shows that magnetic
islands form easily and propagate upwards in the jet. These islands are
multithermal and thus are predicted to show up in hot channels (335 \AA\, and
211 \AA) and the cool channel (304 \AA) in observations by the Atmospheric
Imaging Assembly (AIA) on the \emph{Solar Dynamics Observatory}. The islands
have maximum temperatures of 8 MK, lifetimes of 120 s, diameters of 6 Mm, and
velocities of 200 km s$^{-1}$. These parameters are similar to the properties
of blobs observed in EUV jets by<EMAIL_ADDRESS>The Kelvin-Helmholtz instability
develops in our simulation with moderately high plasma $\beta$ (Case II), and
leads to the formation of bright vortex-like blobs above the multiple high
magneto-sonic Mach number regions that appear along the jet. These vortex-like
blobs can also be identified in the AIA channels. However, they eventually move
downward and disappear after the high magneto-sonic Mach number regions
disappear. In the lower plasma $\beta$ case, the lifetime for the jet is
shorter, the jet and magnetic islands are formed with higher velocities and
temperatures, the current sheet fragments are more chaotic, and more magnetic
islands are generated. Our results show that the plasmoid instability and
Kelvin-Helmholtz instability along the jet are both possible causes of the
formation of blobs observed at extreme ultraviolet (EUV) wavelengths.
