Seasonal variability of sea level and sea-surface temperature on the north-east coast of New Zealand

Low-frequency, seasonal variations in sea level and sea-surface temperature (SST) for the north-east coast of the North Island, New Zealand were investigated using mainly multivariate analyses in the frequency domain over a 20-year period (1973-92). The dominant influence on the annual cycle of sea level (mean amplitude=378 mm) is associated with thermo-steric sea-level adjustments, which explains 50-80% of the variance in the annual frequency band. Sea levels generally peak at the end of April (austral autumn), lagging the SST cycle by around 2 months. The inclusion of secondary forcing variables (barometric pressure and alongshore wind stress) in the multivariate analysis increases the proportion of the variance explained to 70-90+%. Thermo-steric adjustments in sea level almost completely mask the inverted-barometer effect at annual and longer timescales. The response of sea level to wind stress (alongshore) is also anticorrelated to its response to barometric pressure, thereby appearing to reduce the magnitude of the barometric factor below 10 mm hPa(-1). These latter factors, along with changes in oceanic current patterns and seasonal coupling of El Nino-Southern Oscillation effects, cause secondary effects on seasonal sea-level variability. (C) 1998 Academic Press Limited.