  Effects of an external magnetic field on the fluctuations of quark number,
fluctuations and correlations of conserved charges, including baryon number,
electric charge and strangeness, are studied in the 2+1 flavor
Polyakov--Nambu--Jona-Lasinio model. We find that magnetic field increases
fluctuations and correlations in the regime of chiral crossover. It makes the
transition of quadratic fluctuations more abrupt, and the peak structure of
quartic fluctuations more pronounced. Our calculations indicate that
$\chi_{4}^{B}/\chi_{2}^{B}$, $\chi_{4}^{Q}/\chi_{2}^{Q}$, and $\chi_{11}^{BQ}$
are very sensitive to the external magnetic field and maybe can be used for
probes for the strong magnetic field produced in the early stage of noncentral
collisions.
