THE FORTNIGHTLY AND MONTHLY TIDES - RESONANT ROSSBY WAVES OR NEARLY EQUILIBRIUM GRAVITY-WAVES

The fortnightly and monthly tides are discussed in the light of recent sea level observations and numerical modeling results. Within the tid e gauge network of the low-latitude Pacific, the fortnightly tide is s hown to possess a large-scale phase lag of roughly 10-40 degrees. Alth ough the nonequilibrium part of the fortnightly tide is traditionally thought to be dominated by Rossby wave dynamics, it is shown, via glob al shallow-water modeling studies, that this large-scale phase lag is explicable in terms of remotely forced gravity waves whose origin is m ainly in the Arctic Ocean. Although future observations outside the lo w-latitude region of the Pacific may eventually reveal Rossby wave exc itation, the fortnightly tidal signal in the tide gauge network at han d appears to reveal at most only weak excitation of Rossby waves compa red to the phase lag due to remotely forced gravity waves. The observe d monthly tide appears to be only slightly closer to equilibrium than the fortnightly tide. The reason for this remains unclear since the mo nthly tide is less affected by the remotely forced gravity waves than the fortnightly tide.