  N131 is a typical infrared dust bubble showing an expanding ringlike shell.
We study what kinds of CO line ratios can be used to trace the interaction in
the expanding bubble. We carry out new $\rm CO\,(3-2)$ observations towards
bubble N131 using the JCMT 15-m telescope, and derive line ratios by combining
with our previous $\rm CO\,(2-1)$ and $\rm CO\,(1-0)$ data from the IRAM 30-m
telescope observations. To trace the interaction between the molecular gas and
the ionized gas in the HII region, we use RADEX to model the dependence of CO
line ratios on kinetic temperature and H$_2$ volume density, and examine the
abnormal line ratios based on other simulations. We present $\rm CO\,(3-2)$,
$\rm CO\,(2-1)$, and $\rm CO\,(1-0)$ integrated intensity maps convolved to the
same angular resolution (22.5$"$). The three different CO transition maps show
apparently similar morphology. The line ratios of $W_{\rm CO\,(3-2)}$/$W_{\rm
CO\,(2-1)}$ mostly range from 0.2 to 1.2 with a median of $0.54\pm0.12$, while
the line ratios of $W_{\rm CO\,(2-1)}$/$W_{\rm CO\,(1-0)}$ range from 0.5 to
1.6 with a median of $0.84\pm0.15$. The high CO line ratios $W_{\rm
CO\,(3-2)}$/$W_{\rm CO\,(2-1)}\gtrsim 0.8 $ and $W_{\rm CO\,(2-1)}$/$W_{\rm
CO\,(1-0)}\gtrsim 1.2$ are beyond the threshold predicted by numerical
simulations based on the assumed density-temperature structure for the inner
rims of ringlike shell, where are the compressed areas in bubble N131. These
high CO integrated intensity ratios, such as $W_{\rm CO\,(3-2)}$/$W_{\rm
CO\,(2-1)}\gtrsim0.8$ and $W_{\rm CO\,(2-1)}$/$W_{\rm CO\,(1-0)}\gtrsim1.2$,
can be used as a tracer of gas compressed regions with a relatively high
temperature and density. This further suggests that the non-Gaussian part of
the line-ratio distribution can be used to trace the interaction between the
molecular gas and the hot gas in the bubble.
