Specific Scientific Objectives

Here we formulate a set of questions about the atmospheric flow over the eastern Pacific and the underlying ocean which are relevant to the coupled ocean-atmosphere problem. The specific scientific objectives of EPIC2001 are to answer these questions. These objectives correlate with the overall objectives of EPIC, which are reproduced in the introduction.

1.

How are the location, strength, and other characteristics of ITCZ convection determined? This breaks into a number of sub-questions:

(a)

What mechanism or set of mechanisms forces convection in the east Pacific ITCZ? A wide variety of mechanisms proposed over the years can be tested, including Ekman pumping, temperature and pressure gradient forcing, enhancement of surface fluxes, free tropospheric moisture enhancement, etc.

(b)

What factors are responsible for the fluctuations in strength and position of the east Pacific ITCZ on weekly time scales? Are these truly due to impinging easterly waves acting directly or indirectly on the east Pacific? What causes the ITCZ to move northward under these circumstances? Do coastal jets play a role in this evolution? What about barotropic instability?

(c)

How do the characteristics of ITCZ convection vary through the diurnal cycle?

2.

How does the lower branch of the east Pacific Hadley cell evolve as it flows across the equator and into the ITCZ? Sub-questions:

(a)

How do the thermodynamic characteristics of this convective inflow change, in response to surface and boundary layer top fluxes, as the air moves from the southern hemisphere stratus region toward higher SSTs? We already know that the ABL is stably stratified over the cold equatorial waters and that surface winds are weak relative to the low level jet maximum there. Further north over higher SSTs surface winds are stronger (Wallace, Mitchell, and Deser, 1989).

(b)

How does the strength and structure of this inflow vary as ITCZ convection waxes and wanes?

(c)

How do cloudiness and wind stress curl in this region affect the ocean mixed layer?

3.

What factors determine the temperature, salinity, and thickness of the oceanic mixed layer in the east Pacific warm pool? Sub-questions:

(a)

What are the distributions of longwave and shortwave radiative fluxes at the ocean surface, the latent and sensible heat fluxes out of the ocean, the precipitation rate, and the wind stress curl in various conditions?

(b)

What controls the entrainment of water from below into the oceanic mixed layer? In particular, how important are the effects of energetic but episodic atmospheric forcing events relative to time-mean forcing?

(c)

How important to ocean mixed layer characteristics is horizontal advection relative to upwelling/downwelling, entrainment from below, and in situ thermodynamic processes in these regions?

4.

How effective are marine stratiform clouds in cooling the underlying ocean as southern hemisphere ABL air moves across the cold tongue and into the ITCZ region? Sub-questions:

(a)

What is the meridional distribution of liquid water content of the stratus clouds and their drop size distribution? These parameters determine the albedo of the stratus.

(b)

What are the fractional coverage and radiative properties of low clouds?

(c)

Can these factors be used to explain the observed radiation balance at the ocean surface?