Normal modes, the eigenfunctions of the ocean waveguide, are useful in underwater acoustics because the lowest modes provide an efficient description of the most energetic signals at long ranges. Understanding the structure of the mode arrivals at megameter ranges is crucial to tomographic and matched field processing applications, but there have been few opportunities for experimental observation of these signals. First, this thesis develops a short-time Fourier transform (STFT) framework for estimating broadband signals propagating in the lowest modes. Since previous research has focused on narrowband sources, this work concentrates on broadband processing issues. Specifically, it addresses the fundamental issue of frequency resolution required for mode estimation, analyzes the performance characteristics of two modal beamforming algorithms and explores the time/frequency trade-offs inherent in STFT mode processing. Second, this work presents a detailed analysis of the low-mode arrivals at megameter ranges using data from the Acoustic Thermometry of Ocean Climate experiment. Short-time Fourier processing of receptions at a range of 3,515 kilometers reveal a complicated multipath arrival structure. This thesis characterizes the temporal coherence of the arrivals, computes average dispersion curves, and examines trends in arrival time over five months of data.