Diurnal variability of upper ocean temperatures observed from buoys in the northwestern Pacific

The significant role of the diurnal temperature variability in the top few meters of the ocean on air-sea interactions is increasingly recognized. In present study, the diurnal cycle over the northwestern Pacific (NWP) is examined using in situ observations from two buoys simultaneously measuring oceanic and meteorological data for five months. Observations suggest the temperature variations in the upper few meters are strongly influenced by the insolation varying at a predominant diurnal period and a minor semidiurnal period, reflecting solar atmospheric tides S₁ and S₂, respectively. The diurnal heating can reach 7-12 m depth by warming the water ~0.7 ^oC on average in SST, but with a time lag of about 4 hours. The lagging time can be shortened during high winds. As a result, the upper thermal structure in our 5-month observation can be categorized as (1) the convectively stable regime (~20 %) during the warming stage, when the insolation is sufficient (>400 W m^-2) and the wind is weak (<8 m s^-1), (2) the convectively unstable regime (~5 %), when the temperature overturns due to the atmospheric cooling or rainfall and (3) the well-mixed regime (~75 %) occurring during stronger winds (>10 m s^-1). Previous satellite investigation suggested the most significant diurnal warming of SST (~1 ^oC) occurs during spring and summer in the zonal band of subtropical high in NWP (Tu et al., 2016). Our buoys, located at the western end of the band, show SST differences close to satellite observations. The thermal structure, range of diurnal warming, and time lag revealed in the observations are well consistent with the Price-Weller-Pinkel (PWP) mixed layer model results.