Controls on the depth penetration of diurnal surfaced-forced heat and momentum in the near-surface tropical Pacific

Abstract:

The TPOS enhanced moorings project deployed instrumentation on TAO moorings to observe the near-surface ocean boundary layer at nine separate locations across the tropical Pacific in near real-time. At each site, approximately one year of velocity, temperature, salinity and meteorological observations have shed light on the transfer of heat and momentum from the atmosphere to the ocean. In particular, the diurnal cycle of surface heating and cooling can introduce a daytime warm layer that traps wind momentum in the near-surface ocean at each site. At some locations, this heat and momentum is trapped at the surface and mixes away at night. In regions and times of high near-surface shear, this surface forcing can reach at least as deep as 60 meters, extending below the nighttime convective layer and into the ocean interior.

We detail the surface and subsurface conditions that give rise to these deep-reaching daytime layers at each enhanced mooring site. We calculate a diurnal cycle of Richardson number to help detangle the effects of stratification and shear that set the depth of the diurnal layer. In the eastern equatorial Pacific, where the shallow Equatorial Undercurrent (EUC) creates a great deal of near-surface background shear, we observe the temperature and velocity signatures of the persistent deep cycle turbulence that has been identified by many previous experiments in the region. Year-long records at the three enhanced moorings in this region allow us to characterize the modulation of the deep cycle by the passage of tropical instability waves and other factors. We also identify a few episodic events of deep-reaching diurnal surface-forced heat and momentum at sites away from the EUC as well.

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