1.1
Observational basis for air-sea interaction in the tropical Atlantic
James A. Carton, University of Maryland, College Park, MD; and S. Grodsky
In the past few years the observational network in the tropical Atlantic has expanded tremendously, now encompassing a mooring array (PIRATA), floats (ARGO), VOS XBT lines, near-surface drifters, as well as several focused scientific experiments and a variety of remote-sensed observations. In this talk the authors seek to apply this new wealth of information to evaluate models of air-sea interaction in the tropical Atlantic. We begin by considering the seasonal cycle because of its strength. We then explore lower frequency interactions.
We begin by considering surface fluxes, whose seasonal fluctuations can be evaluated at the PIRATA mooring locations (Foltz, et al., 2002). There the dominant terms in the radiative budget are latent heat loss and net downwelling surface solar radiation. Both are influenced by the latitudinal movement of the narrow band of clouds associated with the Intertropical Convergence Zone. Surface solar radiation is also influenced by semiannual variations changes in the solar declination. We find, for example that north of 5ºN surface solar radiation reaches a maximum in boreal spring, when the ITCZ is near its southernmost position and the solar zenith angle is high. Between the equator and 5ºN there is a strong semiannual component, with maxima in boreal spring and fall, while on the equator the annual harmonic is again significant with increasing amplitude toward the west and maximum in boreal fall. The reduced amplitude in the east is due to the appearance of reflective stratus clouds in boreal fall over the cool waters of the eastern basin (Klein and Hartmann, 1993; Philander et al., 1996). Two additional factors that need to be considered are seasonal variations in Saharan dust and seasonal changes in the vertical profile of solar absorption within the oceanic mixed layer.
Latent heat loss presents a similarly complex pattern. In the northern tropics latent heat loss is controlled by seasonal changes in wind speed as well as relative humidity (itself a result of wind direction and PBL dynamics). In the eastern equatorial zone latent heat loss has only weak seasonal variations, as low-level relative humidity and wind speed are fairly steady throughout the year. The importance of advection and entrainment in controlling SST is generally recognized close to the equator, but the details put forth by different authors differ substantially (Hastenrath 1977; Weingartner and Weisberg 1991a,b; Carton and Zhou 1997). We will discuss some of the differences in the scenarios put forward in these studies and their implications for air-sea interaction.
At longer interannual-decadal periods the importance of direct solar forcing is reduced and the potential for extra-basin influences grows. Here we examine the observational evidence for dynamic as well as flux-driven air-sea interactions, the former acting though wind stress driving of the mixed layer, while the latter appears to act primarily through interannual changes in surface heat flux.
Session 1, Tropical Atlantic coupled variability
Monday, 10 February 2003, 9:00 AM-10:15 AM
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