The U.S. contribution to

Climate Variability and Predictability

Pacific Upwelling and Mixing Physics

PUMP is a process study designed to improve our understanding of the complex of mechanisms that connect the thermocline to the surface in the equatorial Pacific cold tongue. Its goal is to observe and understand the interaction of upwelling and mixing with each other and with the larger-scale equatorial current system. Its premises are, first, that the least understood contributions to the modulation of equatorial SST are upwelling and mixing, and second, that climate-scale ocean models are now ready to exploit realistic vertical exchange processes, but need adequate observational guidance.

Overall science objectives: To observe and understand

1) The evolution of the equatorial cell under varying winds;
2) The mixing mechanisms that determine:

• the depth of wind-input momentum
• the transmission of surface heat fluxes in the thermocline

3) The processes that allow and control exchange across the sharp SST front north of the cold tongue

Science and Implementation plans and other PUMP documents can be found at: www.pmel.noaa.gov/~kessler/clivar/pump.html

• Reanalysis of historical data
• Multi-scale modeling effort
• 2-3 year moored array along 140W, to establish the scales of variability of equatorial upwelling
• Two IOPs (intense observing periods), both on and just north of the equator at 140W, to quantify the relative effects of upwelling and mixing

• Improve the forcing fields
• Provide benchmark data sets to compare model circulations across the upwelling cell
• Improve mixing parameterizations
• Learn to use sparse sustained observations (ENSO OS), assimilated into models to infer equatorial mixing

Expected Outcomes

• The processes of mixing and upwelling that control equatorial SST are poorly understood and modeled.
• Present-generation OGCM representations of the upwelling cell are not adequately constrained by observed reality and differ widely among models.
• This deficiency contributes to the fundamental problems of coupled models of the tropical climate.
• The tools both to observe these phenomena and to improve the models are at hand.