  The combined Greenwich and Solar Optical Observing Network (SOON) sunspot
group data during 1874-2013 are analyzed and studied the relatively long-term
variations in the annual sums of the areas of sunspot groups in 0-10 deg, 10-20
deg, and 20-30 deg latitude intervals of the Sun's northern and southern
hemispheres. The variations in the corresponding north-south differences are
also studied. Long periodicities in these parameters are determined from the
fast Fourier transform (FFT), maximum entropy method (MEM), and Morlet wavelet
analysis. It is found that in the difference between the sums of the areas of
the sunspot groups in 0-10 deg latitude intervals of northern and southern
hemispheres, there exist ~9-year periodicity during the high activity period
1940-1980 and ~12-year periodicity during the low activity period 1890-1939. It
is also found that there exists a high correlation (85% from 128 data points)
between the sum of the areas of the sunspot groups in 0-10 deg latitude
interval of the southern hemisphere during a Qth year (middle year of 3-year
smoothed time series) and the annual mean International Sunspot Number (Rz) of
(Q+9)th year. Implication of these results is discussed in the context of solar
activity prediction and predicted 50 + or - 10 for the amplitude of solar cycle
25, which is about 31% lower than the amplitude of cycle 24.
