  We present Gemini-NIFS, VLT-SINFONI and Keck-OSIRIS observations of
near-infrared [Fe II] emission associated with the well-studied jets from three
active T Tauri stars; RW Aur A, RY Tau and DG Tau taken from 2012-2021. We
primarily covered the redshifted jet from RW Aur A, and the blueshifted jets
from RY Tau and DG Tau, to investigate long-term time variabilities potentially
related to the activities of mass accretion and/or the stellar magnetic fields.
All of these jets consist of several moving knots with tangential velocities of
70-240 km s-1, ejected from the star with different velocities and at irregular
time intervals. Via comparison with literature, we identify significant
differences in tangential velocities for the DG Tau jet between 1985-2008 and
2008-2021. The sizes of the individual knots appear to increase with time, and
in turn, their peak brightnesses in the 1.644-micron emission decreased up to a
factor of ~30 during the epochs of our observations. A variety of the decay
timescales measured in the [Fe II] 1.644 micron emission can be attributed to
different pre-shock conditions if the moving knots are unresolved shocks.
However, our data do not exclude the possibility that these knots are due to
non-uniform density/temperature distributions with another heating mechanism,
or in some cases due to stationary shocks without proper motions. Spatially
resolved observations of these knots with significantly higher angular
resolutions are necessary to better understand their physical nature.
