Heat and Freshwater Budgets in the Eastern Pacific Warm Pool

Hemantha Wijesekera, Daniel Rudnick¹, Clayton Paulson, Stephen Pierce, and Scott Pegau

College of Oceanic and Atmospheric Sciences, 104 OC Admin Bldg, Oregon State University, Corvallis, OR 97331

¹ Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA

The study focuses on the upper ocean budgets of heat and freshwater which provide independent estimates of net surface heat flux and rainfall from a 19-day R/V New Horizon survey made as a part of EPIC2001. We collected underway hydrographic data from a SeaBird CTD mounted on an undulating platform, SeaSoar, and horizontal velocity data from ship-mounted ADCP, along a 146 x 146 km butterfly pattern centered near 10N, 95W. The cruise averaged CTD profiles showed weak stratification in the upper 20 m atop a strong pycnocline with buoyancy frequency of about 22 cph between 25 and 40 m depth. The average depth of active mixing layer was 6.5 m (with 2.0 m std). The depth of the active mixing layer was defined as the depth at which the insitu density increased by 0.01 kg m^-3 above a near-surface value. Both hydrographic and velocity data were used to evaluate time variability and advection of heat and freshwater. Heat and salt budgets were evaluated as a function of depth and it was found that budget estimates were unreliable for depths > ~ 25 m because uncertainty in estimated advection became large. The upper 15-m heat budget indicates that storage (42 W/m² of cooling), advective divergence (15-19 W/m² of cooling), and penetrative solar radiation (23 W/m²) are major components of surface heat balance. By estimating that turbulent heat flux was about 8 W/m² (Gregg et al., 2002; preliminary results), we evaluated the net surface heat flux as 27-31 W/m², which is consistent with R/V Ron Brown measurements. The salinity budget revels that cruise-averaged rain rate was about 14-18 mm/day, which is smaller than that of R/V Ron Brown (~26 mm/day) and R/V New Horizon (34 mm/day), but larger than climatelogical estimates (10 mm/day).