Abstract:
The contribution of tropical instability waves to the momentum and
energy balances of the Pacific Equatorial Undercurrent is investigated using
velocity and temperature time series from the three-dimensional Equatorial
Pacific Ocean Climate Study mooring array at 110°W. Tropical instability
waves are an energetic band of variability typically with periods between 14
and 36 days which are thought to be generated by instability of the equatorial
currents. They are frequently observed as meanders of the equatorial front in
satellite sea surface temperature maps. Here, they are observed as large
oscillations in the meridional velocity records at l10°W with an energy peak
at 21 days. Westward phase propagation is observed in this band with a phase
speed of -0.9 (±0.3) m s-1 and a wavelength of 1660 km. Upward phase
propagation is observed which is consistent with downward energy
propagation. The observed propagation characteristics are compared with
those of the mixed Rossby-gravity wave.
The variability in this band produces large northward fluxes of
eastward momentum and southward fluxes of temperature which affect the
dynamics of the mean Undercurrent through the Reynolds stress divergence,
and the Eliassen-Palm flux divergence. The waves produce a northward flux
of eastward momentum, uv, which is largest at the northern mooring in the
upper part of the array. The meridional divergence of eastward momentum,
-δ(uv)/δy, decelerates the Undercurrent core down to 150 m. This implies a
coupling between the Undercurrent and the South Equatorial Current with
the eastward momentum of the Undercurrent transferred to the westward
flowing South Equatorial Current. To estimate the vertical momentum flux
divergence, the vertical eddy flux of eastward momentum, uw, is inferred
using the eddy temperature equation. The vertical eddy momentum flux is
positive and largest at the core of the Undercurrent, implying an acceleration
of the eastward flow above the core and a deceleration below. The Eliassen-Palm
flux divergence is small above the core of the Undercurrent at 75 m, but
below the core, is sufficient to balance the deeply penetrating eastward
pressure gradient force. The instability waves are important to the energetics of the mean
Undercurrent. An exchange of kinetic energy from the mean Undercurrent
to the waves through shear production is estimated. A local exchange is
suggested since the rate at which the mean Undercurrent loses kinetic energy
through instability is comparable to the rate at which the waves gain energy
through shear production. The conversion from mean to eddy potential
energy is an order of magnitude smaller with the waves gaining potential
energy through conversion of mean available potential energy. The
observations of upward phase propagation and downward Eliassen-Palm flux
suggest that the waves propagate energy downward into the deep ocean.
The energetics and momentum balance of the mean Undercurrent is
investigated further by analyzing the downstream change in the Bernoulli
function on the equator along isentropes or potential density surfaces using
mean hydrographic sections at 150°W and 110°W. A downstream decrease in
the Bernoulli function is observed which is due to a decrease in the
Acceleration Potential since the mean kinetic energy of the Undercurrent
changes little from 150°W to 110°W. The lateral divergence of eddy
momentum fluxes calculated on isotherms is sufficient to balance the
observed decrease in the Acceleration Potential. The downstream decrease in
the Acceleration potential has further implications for the mean energetics
since this "downhill" flow releases mean available potential energy stored in
the east-west sloping thermocline. The rate at which the Undercurrent
releases available potential energy, is shown to be comparable to the rate at
which the mean flow loses kinetic energy by interaction with the waves, with
the waves gaining kinetic energy in the process. Thus, it is hypothesized that
in the eastern Pacific this downstream release of available potential energy is
ultimately converted into a downstream increase in the kinetic energy of the
waves rather than the kinetic energy of the mean flow as occurs in the
western Pacific. To maintain an equilibrium, the waves radiate energy into
the deep ocean as is suggested by the upward phase propagation and the
downward Eliassen-Palm flux.