  Long-lasting activity complexes (ACs), characterised as a series of closely
located, continuously emerging solar active regions (ARs), are considered
generating prominent poleward surges from observations. The surges lead to
significant variations of the polar field, which are important for the
modulation of solar cycles. We aim to study a prominent poleward surge during
solar cycle 24 on the southern hemisphere, and analyse its originating ACs and
the effect on the polar field evolution. We automatically identify and
characterize ARs based on synoptic magnetograms from the Solar Dynamic
Observatory. We assimilate these ARs with realistic magnetic configuration into
a surface flux transport model, and simulate the creation and migration of the
surge. Our simulations well reproduce the characteristics of the surge and show
that the prominent surge is mainly caused by the ARs belonging to two ACs
during Carrington Rotations 2145-2159 (December 2013-January 2015). The surge
has a strong influence on the polar field evolution of the southern hemisphere
during the latter half of cycle 24. Without the about one-year-long flux
emergence in the form of ACs, the polar field around the cycle minimum would
have remained at a low level and even reversed to the polarity at cycle 23
minimum. Our study also shows that the long-lived unipolar regions due to the
decay of the earlier emerging ARs cause an intrinsic difficulty of
automatically identifying and precisely quantifying later emerging ARs in ACs.
