Temporal variability of current‐driven upwelling at Jarvis Island

The temporal variability of upwelling on the western side of Jarvis Island, caused by the island blocking of free stream currents, is investigated using in situ and shipboard observations. Variations in upwelling are related to changes in the strength and depth of the Equatorial Undercurrent (EUC) caused by wind‐forcing at different timescales. Seasonally, the strongest upwelling at Jarvis occurs in boreal spring when a weakening of the trade winds in the eastern and central Pacific coincides with a locally shallow thermocline and shallow and strong EUC. Year to year differences in springtime upwelling are associated with the strength of the trade winds in the western Pacific; intensified trade winds associated with La Niña conditions favor the shoaling of the thermocline and the EUC at Jarvis. On intraseasonal timescales, upwelling events at Jarvis are linked to local trade wind bursts, which cause a regional upwelling that also enhances EUC‐driven upwelling. Strong (>1.00 m s −1 ) near‐surface eastward currents measured at Jarvis, associated with trade wind relaxations in the central Pacific, are downwelling favorable. During periods of EUC‐driven upwelling, interactions of the semidiurnal tide with Jarvis result in abrupt cold spikes (1–4°C) that tend to occur at ∼12 hour period. Tidal currents and temperature fluctuate 180° out of phase, such that minimum temperatures coincide with maximum eastward current. The high‐frequency upwelling appears to be most consistent with the presence of internal tides.