  As each solar cycle progresses, remnant magnetic flux from active regions
(ARs) migrates poleward to cancel the old-cycle polar field. We describe this
polarity reversal process during Cycle 24 using four years (2010.33--2014.33)
of line-of-sight magnetic field measurements from the Helioseismic and Magnetic
Imager. The total flux associated with ARs reached maximum in the north in
2011, more than two years earlier than the south; the maximum is significantly
weaker than Cycle 23. The process of polar field reversal is relatively slow,
north-south asymmetric, and episodic. We estimate that the global axial dipole
changed sign in October 2013; the northern and southern polar fields (mean
above 60$^\circ$ latitude) reversed in November 2012 and March 2014,
respectively, about 16 months apart. Notably, the poleward surges of flux in
each hemisphere alternated in polarity, giving rise to multiple reversals in
the north. We show that the surges of the trailing sunspot polarity tend to
correspond to normal mean AR tilt, higher total AR flux, or slower mid-latitude
near-surface meridional flow, while exceptions occur during low magnetic
activity. In particular, the AR flux and the mid-latitude poleward flow speed
exhibit a clear anti-correlation. We discuss how these features can be
explained in a surface flux transport process that includes a field-dependent
converging flow toward the ARs, a characteristic that may contribute to solar
cycle variability.
