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Air-Sea Interaction in the Eastern Tropical Pacific ITCZ/Cold Tongue Complex

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July 1, 2006 - June 30, 2007

Dr. Robert A. Weller
Woods Hole Oceanographic Institution
Woods Hole, MA 02543

Program Manager: Dr. Jin Huang, NOAA Climate Prediction Program for the Americas

Related NOAA Strategic Plan Goal: Goal 2- Understand Climate Variability and Change to Enhance Society’s Ability to Plan and Respond

Project Overview
As part of the NOAA funded Pan American Climate Study (PACS), two surface moorings were deployed on 125º W, one at 3° S (cold tongue) and one at 10° N near the northernmost climatological position of the Inter-Tropical Convergence Zone (ITCZ). Each surface buoy carried two complete sets of meteorological sensors (wind velocity, air and sea temperature, incoming shortwave and longwave radiation, humidity, barometric pressure, precipitation, surface currents), and the heat, mass; and momentum fluxes have been computed using state-of-the-art bulk formulae (Fairall et al., 1996). The mooring lines carried temperature, conductivity, and velocity sensors to observe upper ocean variability in the upper 200 m. The data from the northern mooring returned the first accurate and complete time series of the air-sea fluxes of heat, freshwater, and momentum in the eastern Pacific warm pool beneath the northernmost climatological position of the Inter-Tropical Convergence Zone (ITCZ). This data set is also unique because it spans the strong El Niño event of 1997-98 and the onset of the subsequent La Niña. Tom Farrar completed his Ph.D. thesis during the past year under Weller’s supervision and manuscripts are being prepared for publication. That effort used the PACS mooring data and satellite observations to study air-sea interaction and the processes that set SST in these two contrasting regions. Analysis of this data has had two principle foci: (1) Understanding of the balance of processes that set SST, and (2) Characterization of air-sea fluxes of heat, momentum, and freshwater in these two climactically and meteorologically important regions.

1. Processes affecting SST at two contrasting sites in the eastern tropical Pacific
The well-resolved time series of upper-ocean temperature and velocity, together with the accurate estimates of air-sea heat fluxes and satellite observations of SST, allow examination of the relative importance of surface heat fluxes and horizontal advection in setting the local SST. The residual of the temperature balance equation can be used to assess the role of vertical mixing and other unresolved processes. Analysis of the surface layer temperature balance (e.g., Cronin and McPhaden, 1997) has been carried out at both mooring sites, and results are being prepared for publication.

At the southern site, horizontal advection was important throughout most of the mooring deployment. In particular, southward advection from the equatorial cold tongue by wind-driven Ekman transport was important in bringing about the local establishment of the equatorial cold tongue during the transition from El Niño to La Niña states. At the northern site, the surface temperature balance was primarily between surface heating and vertical mixing during the 1997 ITCZ season (July-December), but the balance shifted to one of surface heating and horizontal advection during the trade-wind season (January-May; Figure 2).

2. Mesoscale variability in SST, velocity, and sea surface height in the east Pacific warm pool
Prominent meridional current fluctuations with a period of about 2 months (Figure 2) were observed in the mooring data at the northern site, and these current fluctuations exerted a strong influence on the local SST through horizontal advection (Figure 1), causing SST to fluctuate with about a 2 month period from January-June of 1998. The SST fluctuations associated with this signal were substantial, with peak-to-peak amplitudes ranging from 0.5-0.8ºC. The two month signal in meridional currents was linked to a previously recognized sea surface height signal that is strongest in the latitude band 9-13ºN east of 120ºW (Miller et al., 1985; Perigaud, 1990; Giese et al., 1994). To resolve discrepancies in prior studies of the signal (Perigaud, 1990; Giese et al., 1994), an effort was undertaken to characterize the signal observed at the mooring within its larger spatial and temporal context using satellite SST and sea surface height measurements (Figure 3). The signal was found to be associated with relatively short (5-15º wavelength) baroclinic Rossby waves. There is evidence that the intraseasonal velocity variability and its annual cycle are associated with instability of the westward flowing North Equatorial Current as it intensifies in the spring of each year. The mooring observations were instrumental in this study, because they allowed establishment and understanding of the link between the intraseasonal SSH and SST fluctuations. These findings were published in the Journal of Geophysical Research (Farrar and Weller, 2006).

3. Impact of mesoscale SST variability on atmospheric convection and clouds at intraseasonal timescales
The PACS buoy observations from the 10°N site further indicate that there is variability in surface solar radiation coupled to the sea surface temperature (SST) signal of the Rossby wave, which suggests that oceanic Rossby waves may affect atmospheric convection by modulating SST. This hypothesis was investigated using satellite measurements of SST, columnar cloud liquid water (CLW), cloud reflectivity, and surface solar radiation. A statistically significant relationship between SST and these cloud properties was identified within the wavenumber-frequency band of oceanic Rossby waves (e.g., Figure 4a). For example, analysis of seven years of data indicates that 10-20% of the variance in the logarithm of CLW at intraseasonal periods and zonal scales on the order of 10° longitude can be ascribed to SST signals driven by oceanic Rossby waves (Figure 4b). The robust relationship identified in multiple data sets suggests that the oceanic mesoscale SST variability in the region modulates the likelihood and/or intensity of atmospheric convection.

Farrar, J.T. and Weller, R.A. 2006. Intraseasonal variability near 10ºN in the eastern tropical Pacific Ocean. J. Geophys. Res., 111, C05015, doi:10.1029/2005JC002989.

Plueddemann, A.J. and Farrar, J.T. 2006. Observations and models of the energy flux from the wind to mixed layer inertial currents. Deep Sea Research II, 53, 5-30, doi:10.1016/j.dsr2.2005.10.017.

Farrar, J.T. 2007. Air-sea interaction at contrasting sites in the eastern tropical Pacific: Oceanic mesoscale variability and atmospheric convection at 10°N. Massachusetts Institute of Technology- Woods Hole Oceanographic Institution, Ph.D. thesis.

Farrar, J.T. and Weller, R.A. Oceanic mesoscale variability and atmospheric convection on 10°N in the eastern Pacific. NOAA Climate Prediction Program for the Americas PI Meeting, August 2006, Tucson, AZ.

Farrar, J.T. Oceanic mesoscale variability and atmospheric convection on 10°N in the eastern Pacific. Oceanography and Climate Sack Lunch Seminar, Massachusetts Institute of Technology, September 2006.

Farrar, J.T. and Weller, R.A. The relationship between oceanic mesoscale motions and atmospheric convection on 10°N in the eastern tropical Pacific Ocean. EOS Trans. AGU, 87(52), Fall Meet. Suppl., Abstract OS51E-06. 2006.

Farrar, J.T. Oceanic mesoscale variability and atmospheric convection on 10°N in the eastern Pacific. Ocean and Climate Physics Seminar, Lamont-Doherty Earth Observatory, April 2007.

Summary of Education and Outreach Activity
Under Weller’s supervision, graduate student Tom Farrar completed his Ph. D. thesis work in February 2007 using the high quality PACS observations to study the role of air-sea exchange and upper-ocean processes in setting SST in these two contrasting regions. Farrar continues to work under Weller’s supervision as a post-doc, and manuscripts are being prepared for publication.

Works cited:
Cronin, M.F. and McPhaden, M.J. 1997. The upper ocean heat balance in the western equatorial Pacific warm pool during September-December 1992. J. Geophys. Res., 102:8533-8553.
Fairall, C.W., Bradley, C.W., Rogers, D.P., Edson, J.B. and Young, G.S. 1996. Bulk parameterization of air-sea fluxes during TOGA COARE. J. Geophys. Res., 101:3747-3764.
Giese, B.S., Carton, J.A., and Holl, L.J. 1994. Sea level variability in the eastern tropical Pacific as observed by TOPEX and the Tropical Ocean-Global Atmosphere Tropical Atmosphere-Ocean Experiment. J. Geophys. Res., 99:24,739-24,748.
Miller, L., Watts, D.R., and Wimbush, M. 1985. Oscillations in dynamic topography in the eastern Pacific. J. Phys. Oceanogr., 15:1759-1770.
Perigaud, C. 1990. Sea level oscillations observed with Geosat along the two shear fronts of the North Equatorial Counter Current. J. Geophys. Res., 95:7239-7248.

Last updated: August 19, 2008

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