470 JOURNAL OF CLIMATE VOLUME 15 Effects of Penetrative Radiation on the Upper Tropical Ocean Circulation RAGHU MURTUGUDDE AND JAMES BEAUCHAMP Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland CHARLES R. MCCLAIN NASA Goddard Space Flight Center, SeaWiFS Project Of®ce, Greenbelt, Maryland MARLON LEWIS Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada ANTONIO J. BUSALACCHI Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland (Manuscript received 23 February 2001, in ®nal form 3 August 2001) ABSTRACT The effects of penetrative radiation on the upper tropical ocean circulation have been investigated with an ocean general circulation model (OGCM) with attenuation depths derived from remotely sensed ocean color data. The OGCM is a reduced gravity, primitive equation, sigma coordinate model coupled to an advective atmospheric mixed layer model. These simulations use a single exponential pro®le for radiation attenuation in the water column, which is quite accurate for OGCMs with fairly coarse vertical resolution. The control runs use an attenuation depth of 17 m while the simulations use spatially variable attenuation depths. When a variable depth oceanic mixed layer is explicitly represented with interactive surface heat ¯uxes, the results can be counterintuitive. In the eastern equatorial Paci®c, a tropical ocean region with one of the strongest biological activity, the realistic attenuation depths result in increased loss of radiation to the subsurface, but result in increased sea surface temperatures (SSTs) compared to the control run. Enhanced subsurface heating leads to weaker strati®cation, deeper mixed layers, reduced surface divergence, and hence less upwelling and entrainment. Thus, some of the systematic de®ciencies in the present-day climate models, such as the colder than observed cold tongue in the equatorial Paci®c may simply be related to inaccurate representation of the penetrative radiation and can be improved by the formulation presented here. The differences in ecosystems in each of the tropical oceans are clearly manifested in the manner in which biological heat trapping affects the upper ocean. While the tropical Atlantic has many similarities to the Paci®c, the Amazon, Congo, and Niger Rivers' discharges dominate the attenuation of radiation. In the Indian Ocean, elevated biological activity and heat trapping are away from the equator in the Arabian Sea and the southern Tropics. For climate models, in view of their sensitivity to the zonal distribution of SST, using a basin mean of the ocean color±derived attenuation depth reduces the SST errors signi®cantly in the Paci®c although they occur in regions of high mean SST and may have potential feedbacks in coupled climate models. On the other hand, the spatial variations of attenuation depths in the Atlantic are crucial since using the basin mean produces signi®cant errors. Thus the simplest and the most economic formulation is to simply employ the annual mean spatially variable attenuation depths derived from ocean color. 1. Introduction interactions. These variations are largely controlled by the concentration of light-absorbing pigments associated Variations in the attenuation of visible radiation in with phytoplankton that vary over a wide range of time- the upper ocean alters the vertical distribution of local and space scales. These variations have been investi- heating, and has potential implications for thermal and gated in detail, not only for their in¯uence on the upper- dynamical processes as well as for ocean±atmosphere ocean structure, but also for generating full spectral ra- diance pro®les over a range of oceanic conditions (Den- man 1973; Simpson and Dickey 1981b; Lewis et al. Corresponding author address: Dr. Raghu Murtugudde, ESSIC/ CSS Bldg., University of Maryland, College Park, College Park, MD 1990; Siegel et al. 1995; Ohlmann et al. 1996; Ohlmann 20742. et al. 2000). A potential for positive feedback between E-mail: [email protected] stabilization of the mixed layer due to biological heat q 2002 American Meteorological Society Unauthenticated | Downloaded 09/26/21 03:21 PM UTC 1MARCH 2002 MURTUGUDDE ET AL. 471 trapping, which can lead to enhanced strati®cation, and, ing depth of 15 m in a coupled model to demonstrate in turn, to favorable conditions for phytoplankton several improvements in their simulation compared to growth was suggested by Sathyendranath et al. (1991). when the penetrative radiation was completely ignored. While this hypothesis can only be tested in a coupled Schneider et al. (1996) analyzed a coupled GCM run ecosystem model, the in¯uence of light attenuation due to conclude that penetrative radiation was important in to phytoplankton growth on ocean circulation can be their model in maintaining the vertical structure of the studied in ocean general circulation models (OGCMs). western Paci®c warm pool. The OGCM study of Nak- Such modeling studies have either been sophisticated amoto et al. (2000) shows that the seasonal variability solar transmission formulations in a one-dimensional in surface chlorophyll results in seasonally dependent (1D) setting (e.g., Ohlmann and Siegel 2000) or sim- absorption of solar irradiance and heating rate in the plistic representations of penetrative radiation in real- upper ocean. Our simulations are more comprehensive istic three-dimensional (3D) models (e.g., Chen et al. and address dynamical and thermodynamical effects of 1994a; Schneider and Zhu 1998). One recent study in heat trapping by surface chlorophyll in all three tropical an isopycnal OGCM addressed the modulation of SST oceans with realistic interactions between mixed layer by surface chlorophyll (Nakamoto et al. 2000), but with depth variations and surface heat ¯uxes, and also be- relaxation to climatological quantities, and only for the tween mixed layer depths and subsurface variabilities. Arabian Sea. Based on our simulations, we conclude that the spatially Simpson and Dickey (1981a) used a 1D, second-mo- variable annual mean of the attenuation depths derived ment turbulent closure scheme with modi®cation to ac- from satellite ocean color data are suf®cient for cap- count for solar ¯ux divergence. Their investigation of turing the largest part of the heat trapping. the role of downward irradiance in determining the up- The coastal zone color scanner (CZCS) data are only per-ocean structure showed that SST, mixed layer depths accurate enough for a composite annual mean over the (MLDs), eddy diffusivity of heat, and the mean hori- global Tropics and the newly available SeaWiFS ocean zontal velocities were dependent on the particular form color data are not suf®ciently long for computing cli- of solar irradiance divergence and the in¯uence of the matological monthly mean attenuation depths. Some de- vertical pro®le of radiation depended also on the wind tails of the CZCS and SeaWiFS data can be found in speeds. One-dimensional studies such as these have McClain et al. (1993) and McClain et al. (1998). In- been instrumental in driving home the point that proper terannual simulations of the tropical Paci®c with con- representation of solar irradiance is important for ac- stant and spatially variable annual mean attenuation curate computation of SSTs. The simulated pro®les of depths are analyzed to understand the effects of inter- full spectral radiance by Ohlmann and Siegel (2000) annual variability in ecosystems and attenuation depths. show that solar transmission at around 20 m of the ocean Access to routine space-based observations of ocean can exceed 40 W m22 for a surface irradiance of 200 color and optical property data related to attenuation Wm22 as predicted by Lewis et al. (1990) and con®rmed depth (McClain et al. 1998) make the present study by Siegel et al. (1995). While these numbers are sig- timely and feasible. ni®cant, state-of-the-art coupled climate prediction The paper is organized as follows. Section 2 contains models, or even stand-alone OGCMs used for process a brief description of the model and the formulation of studies, do not resolve the upper ocean to better than penetrative radiation. Section 3 contains detailed com- an order of ;10 m. Layer models such as the one em- parisons of climatological model simulations with var- ployed here only resolve a bulk mixed layer and level iable and constant attenuation depths. Discussion of in- models such as the one used by Schneider and Zhu terannual runs for the Paci®c Ocean and effects of time- (1998) have levels of order 10 m near the surface. Thus, varying attenuation depths are presented in section 4. in a practical sense, especially for climate prediction A summary of results is reported in section 5. models, we only need to worry about solar radiation that escapes the top layer or level of the model. As pointed out by Ohlmann et al. (1998), solar transmission 2. Model ocean in the visible band within the oceanic mixed layer below The ocean model is a reduced-gravity, primitive- 10 m, can be represented with a single exponential pro- equation OGCM con®gured separately for each of the ®le to within 10% accuracy. tropical oceans. It consists of 19 sigma-coordinate layers Our goal is mainly to address the potential for en- beneath a variable-thickness, surface mixed layer (Chen hancing the simulation of tropical SSTs and in turn, et al. 1994b). Surface heat ¯uxes are determined by ENSO prediction models via improved representation coupling a simple model of the atmospheric boundary of vertical pro®les of solar radiation without incurring layer to the OGCM and freshwater forcing is included excessive computational costs. We thus focus on the as a natural boundary condition. Model details are pre- Tropics and investigate a single exponential pro®le with sented in Murtugudde et al. (1996, 1998) and Murtu- constant attenuation depth and compare it to model sim- gudde and Busalacchi (1999). Model salinity, temper- ulations with spatially variable attenuation depths de- ature, and layer thicknesses are relaxed to Levitus rived from satellite ocean color data.