Vertical Structure of Upper-Ocean Seasonality: Extratropical Spiral versus Tropical Phase Lock

Abstract

Abstract Seasonality is a fundamental feature of the coupled ocean–atmosphere system. The hemispherically opposite general pattern at the air–sea interface is well known, but its penetrative behavior below the sea surface is poorly understood. Over 10 years of Argo data reveal for the first time the spiral-like structure of vertical seasonality in the upper extratropical ocean: the amplitude (strength of annual sea temperature variability) decreases rapidly with depth while the phase (peaking time) rotates from August (February) at the sea surface to December (June) at the bottom of the mixed layer in the Northern (Southern) Hemisphere under a clockwise representation. It is found that, analogous to the Ekman current spiral, the oceanic seasonality is almost reversed at approximately 500-m depth with about 5% of the surface intensity left. In contrast, the seasonality of subtropical oceans below the thermocline exhibits a phase-lock pattern around May and October to the north and south of the equator, respectively. Meanwhile, a systematic westward progression of annual phase corresponding to the warmest month is observed in the equatorial regions between 10°S and 10°N. It is suggested that the seasonality spiral of extratropical oceans occurs as a result of the vertical decay of solar penetration in tandem with delayed annual maximum mixing in the context of buoyancy and turbulent induced convections, while the month of full summer appears to be “constant” May (October) in the subtropical oceans of the Northern (Southern) Hemisphere under the stratified subsurface layer due to a 9-month trapping of penetrating solar energy from May (October) to next January (June). The vertically locked westward annual phase progression in the equatorial regions is likely to be a consequence of the first baroclinic mode of β-refracted annual Rossby waves. The spiral and phase-lock behaviors of the upper oceans are of critical significance to the understanding of mixed layer and thermocline dynamics.