  Advancing ocean science has a significant impact to the development of the
world, from operating a safe navigation for vessels to maintaining a healthy
and diverse ocean ecosystem. Various ocean software systems have been
extensively adopted for different purposes, for instance, predicting hourly sea
level elevation across shorelines, simulating large-scale ocean circulations,
as well as integrating into Earth system models for weather forecasts and
climate projections. Regardless of their significance, guaranteeing the
trustworthiness of ocean software and modelling systems is a long-standing
challenge. The testing of ocean software suffers a lot from the so-called
oracle problem, which refers to the absence of test oracles mainly due to the
nonlinear interactions of multiple physical variables and the high complexity
in computation. In the ocean, observed tidal signals are distorted by
non-deterministic physical variables, hindering us from knowing the "true"
astronomical tidal constituents existing in the timeseries. In this paper, we
present how to test tidal analysis and prediction (TAP) software based on
metamorphic testing (MT), a simple yet effective testing approach to the oracle
problem. In particular, we construct metamorphic relations from the periodic
property of astronomical tide, and then use them to successfully detect a
real-life defect in an open-source TAP software. We also conduct a series of
experiments to further demonstrate the applicability and effectiveness of MT in
the testing of TAP software. Our study not only justifies the potential of MT
in testing more complex ocean software and modelling systems, but also can be
expanded to assess and improve the quality of a broader range of scientific
simulation software systems.
