Abstract
Projections of future climate are sensitive to the representation of upper-ocean diurnal variability, including the diurnal cycle of winds. Two different methods suitable for time series with missing data are used here to characterize how observed diurnal winds vary over the year. One is based on diurnal composites of mooring data, and the other is based on harmonic analysis via a least squares fit and is able to isolate annual (i.e., 1 cycle per year) modulation of diurnal variability. Results show that the diurnal amplitude in meridional winds is larger than in zonal winds and peaks in the tropical Pacific, where diurnal variability in zonal winds is overall weaker compared to other basins. Furthermore, the amplitude and phasing of diurnal winds in the tropical oceans are not uniform in time, with overall larger differences through the year in the meridional component of tropical winds. Estimating the annual modulation of the diurnal signal implies resolving both the diurnal energy peak and also the modulation of this peak by the annual cycle. This leads to a recommendation for sampling at least 6 times per day and for a duration of at least 3 years.
Highlights
Diurnal variability is one of the most persistent features of the Earth’s climate system
This study describes how the diurnal cycle in surface winds changes in different months of the year over the tropical ocean
Both methods show that the largest amplitude in the annual mean diurnal cycle of surface winds is in meridional winds, consistent with previous studies
Summary
Diurnal variability is one of the most persistent features of the Earth’s climate system. Because large-scale vertical motion in the lower atmosphere and upper ocean is associated with mean convergence/divergence of surface winds, diurnal variations in surface winds can modulate oceanic mixing [9] as well as atmospheric convection in the deep tropics [10,11]. The diurnal cycle in winds and sea surface temperature is modulated on intraseasonal timescales (e.g., by the Madden Julian Oscillation as described in [12,13]), as well as seasonal and interannual timescales (see, e.g., in [4,14,15]). The sun crosses the equator twice a year, there is a strong annual cycle in the Tropics, and changes in temperatures and pressure gradients with seasons can drive changes in surface winds on diurnal and longer timescales [4]. Previous research [4] shows that the amplitudes of diurnal meridional wind variations in the tropical Pacific are larger during the cold season (June–November) than the warm season (December–May), with similar spatial patterns in the two seasons
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