The Problematic psi1 Tide

The geophysical importance of the ψ1 tidal constituent is out of all proportion to its tiny size. Throughout the global ocean its amplitude is rarely more than a few mm. That such a tiny tide can be important owes solely to a geophysical coincidence: the ψ1 frequency is close to that of the Free Core Nutation (FCN), a resonant free mode of the earth's rotation. Studies of the ψ1 earth tide inform us about that resonance and set constraints on the coupling of core and mantle and the shape of the core-mantle boundary. To do this, the effects of the ψ1 ocean tide must be removed, yet ψ1 is too poorly known for the task. Determining the ψ1 ocean tide is problematic because of its small size, making it difficult to measure directly, and also because of the multitude of effects that combine to generate it. The tide has multiple components, each one problematic in its own way: (1) The gravitational component of ψ1 is the easiest to handle since it can be inferred from K1, but only if the FCN resonance is accurately known since the FCN severely perturbs the Love numbers of both ψ1 and K1. (2) The radiational component of ψ1 is almost as large. It is forced by the ψ1 atmospheric tide, which itself represents a semiannual variation of the dominant S1 air tide. The ψ1 air tide is affected by climatic changes in S1. (3) A compound tide, RP1 coincides with ψ1. (4) Climate-induced seasonal changes in K1 show up at ψ1. This last mechanism appears to be responsible for the largest observed amplitude of ψ1: over 5 cm near Vancouver. We have accounted for these various effects and have arrived at a new model of the ψ1 ocean tide which can be used in other investigations, but we are by no means convinced that it is realistic, nor even that it is stable, given the time-varying nature of some of the above forcing. It is, however, more accurate than models previously used to study the FCN.