VAMOS [Variability of the American Monsoon System] Ocean Cloud Atmosphere Land Study (VOCALS)

Collaborators: J. Thomas Farrar (Woods Hole Oceanographic Institution)
Robert A. Weller (Woods Hole Oceanographic Institution)

The Vamos Ocean-Cloud-Atomosphere-Land Study - Regional Experiment (VOCALS-REx) is an international field experiment designed to better understand physical and chemical processes central to the climate system of the Southeast Pacific (SEP) region. VOCALS Pic 1 The climate of the SEP region is a tightly coupled system involving poorly understood interactions between the ocean, the atmosphere, and the land. VOCALS-REx will focus on interactions between clouds, aerosols, marine boundary layer (MBL) processes, upper ocean dynamics and thermodynamics, coastal currents and upwelling, large-scale subsidence, and regional diurnal circulations, to the west of the Andes mountian range. The field experiment is ultimately driven by a need for improved model simluations of the coupled climate system in both the SEP and over the wider tropics and subtropics.

VOCALS-REx will provide detailed and targeted observations of those processes that impact the SEP climate system and are amenable to study with a month long program. The intensive field observations are a vital component of the broader VOCALS program VOCALS Pic 2 and have been carefully designed to complement a suite of enhanced long-term observations. The long-term observations provide important context for the intensive observations. In addition, a major thrust of the VOCALS program is to provide coordination for modeling activities, which will benefit from the intensive observations in a poorly observed region where coupled ocean-atmosphere models exhibit strong biases in sea surface temperature. The coordination through VOCALS of observational and modeling efforts will lead to an improved pull-through for climate and regional forecasting agencies.

Multi-disciplinary intensive observational datasets will be obtained during VOCALS-REx from several platforms including aircraft, research vessels, and a surface land site. These datasets will be used to test a coordinated set of hypotheses that are organized into two broad themes: (1) improved understanding of aerosol-cloud-drizzle interactions in the marine boundary layer (MBL) and the physicochemical and spatiotemporal properties of aerosols; (2) VOCALS Pic 3improved understanding of the chemical and physical couplings between the upper ocean, the land, and the atmosphere. The intensive observational period will be a month long and will take place during October 2008, chosen because it is the month during which the coverage of stratocumulus over the SEP is at its greatest, the southeast trade winds are at their strongest, and the coupling between the upper ocean and the lower atmosphere is at its tightest.

One goal of the VOCALS (VAMOS Ocean Cloud Atmosphere Land Study; VAMOS is Variability of the American Monsoon System) Regional Experiment (REx) is to improve understanding of the processes controlling sea surface temperature (SST) in the Southeastern Pacific off the west coast of South America. A connected set of upper-ocean processes-- near-inertial internal waves, mesoscale eddies, and vertical mixing-- are hypothesized to be important influences on the regional SST field. The relationship of eddies and near-inertial waves to SST, surface forcing and upper-ocean dissipation are of broader interest, but coincident observations of these processes are rare and exist only over periods of a few weeks.

This is a project with J. T. Farrar and R. Weller of WHOI to study the relationship of upper-ocean dissipation, near-inertial internal waves, and mesoscale eddies to SST in the VOCALS-REx study region over a period of one year. VOCALS Instrument Pic 1The project involves enhancement of an existing, heavily-instrumented air-sea interaction mooring with instruments to measure turbulent kinetic energy dissipation at six depths in the upper ocean and analysis of these observations in the context provided by the mooring record. The primary focus of the work is on understanding how physical processes in the upper ocean impact SST in the VOCALS study region, but the results would be of broader scientific interest, as the observations would be unique and would allow insight into some outstanding scientific questions. Specifically, the following were examined:

  1. the relationship of velocity, hydrography, and turbulent dissipation within eddies in the VOCALS region;
  2. the temperature balance of the mixed layer;
  3. near-inertial kinetic energy balance in the mixed layer; and
  4. the influence of near-inertial oscillations on SST.

There are no time series of turbulent kinetic energy dissipation and its vertical profile in the surface mixed layer of the deep ocean spanning an annual cycle. Recent advances in instrumentation, battery longevity, and data storage capacity make such measurements possible now. The dissipation measurements, combined with the existing moored measurements of surface forcing VOCALS Instrument Pic 2and detailed profiles of stratification and velocity, will allow unprecedented study of the energy balance of mixed-layer near-inertial oscillations and the temperature balance of the mixed layer over a period of one year.

A subject of great interest to those interested in the global ocean energy balance is the relative amount of wind-forced mixed-layer near-inertial kinetic energy that is dissipated locally compared to the amount of near-inertial energy that propagates to the deep-ocean. The episodic nature of wind-forced inertial oscillations makes a long-term study of their energetics desirable and necessary, and this will be the first such study that can directly address the relative contributions of wave radiation and mixed-layer dissipation to the loss of mixed-layer near-inertial kinetic energy from the mixed layer. For more information on VOCALS, click on this site.