A liquid-electrode discharge system excited by an alternating-current sinusoidal voltage is employed to investigate the discharge modes with varying liquid conductivity (σ). The results indicate that with σ increasing, the discharge transitions from the uniform mode to the pattern mode, which undergoes various self-organized patterns such as gear, circular saw, discrete spots, single-arm spiral, and concentric rings on the liquid surface. The voltage and current waveforms reveal that the discharge occurs only in the negative half-cycle of applied voltage (when the liquid acts as the instantaneous anode). After gas breakdown, the discharge current rises rapidly to a peak, and then slowly decreases. For the uniform mode, the current decreases monotonically. However, during the current decreasing in the pattern mode, there appears a plateau in which the current keeps almost invariant with time. As σ increases, the values of the peak current and the plateau increase, and the breakdown moment advances. In addition, fast photography achieved through an intensified charge-coupled device (ICCD) shows that regardless of the discharge mode, a uniform disk is initially generated on the liquid surface, and various non-uniform patterns are formed during the plateau stage. Based on the reaction-diffusion model, numerical simulations are carried out through changing ion strength and current strength, which are related to the variables m and l. The simulated discharge modes are well in line with those obtained in the experiments. Moreover, spectral line intensity ratios related to electron temperature and electron density are determined through the spectra emitted from the discharge near the liquid surface. By fitting the spectra, gas temperature and molecular vibration temperature are obtained, which show an increasing trend with σ increasing.