Turbulent entrainment fluxes within the eastern Pacific warm pool

Abstract

Mechanisms controlling turbulent entrainment fluxes, or vertical turbulent fluxes at the mixed layer base (z = -h), and the specific influence of heat entrainment on SST within the eastern Pacific warm pool (EPWP) are investigated using a 19-day timeseries of upper-ocean and atmospheric measurements collected in September 2001 at 10°N, 95°W, co-located buoy measurements, and a semi-empirical entrainment model. Buoyancy entrainment scaled with the cube of the friction velocity, u*, and the inverse finescale (8-m) gradient Richardson number at z = -h, Ri-1(-h), with the variance of the latter largely due to both near-inertial and sub-inertial shear. These two parameters explained more than half of buoyancy entrainment variance over the 19-day timeseries. Surface buoyancy flux also modulated entrainment with heavy rainfall and intense solar radiation suppressing and buoyant convection generating entrainment. Variability of vertical gradients of temperature and salinity at z = -h due to coincident large surface heat and freshwater (rainfall) fluxes and the comparable roles of temperature and salinity in determining the stratification at h were also found to modulate entrainment heat and salt fluxes produced by elevated turbulence at h.The 19-days of entrainment observations allowed the evaluation and modification of the Ni-iler and Kraus (1977) (N-K) entrainment parameterization, which was used with mooring buoy measurements to obtain entrainment estimates over 2001. The three empirically-motivated modifi cations reduced the base N-K model root-mean-square error by 35% and mean bias from +40% to less than +2%. Mixed layer temperature budget inferences and entrainment model results indicate that changes in h associated with Ekman pumping and Rossby waves strongly controls entrainment and, thus, SST. Large entrainment heat fluxes and large resultant drops in SST occur when Ekman pumping decreases h, exposing the concentrated temperature gradients at z = -h to stronger, surface-forced turbulence. With changes in depth-averaged mixed layer temperature due to entrainment inversely proportional to h, shoaling h also amplifies the effect of these fluxes on SST changes. Entrainment may account for most of the cooling needed to offset the net annual ∼23°C of warming that would result from the divergence of the surface and penetrative heat fluxes acting alone, and, thus, strongly controls the region's SST.