Improved Modeling of SST in the Pacific Cold Tongue: Implications for the NCEP GODAS and CFS

Kristopher B. Karnauskas

Earth System Science Interdisciplinary Center

University of Maryland, College Park

It would be difficult to overemphasize the importance of the Pacific cold tongue (CT) in global hydrological and biogeochemical cycles, as it plays a key role in the formation of tropical cloud and precipitation patterns, the supply of nutrients for surface ocean biological productivity, and carbon cycling. The east-central tropical Pacific is the largest oceanic source of CO2 to the atmosphere. Furthermore, large SST anomalies associated with El Niño and La Niña events are manifested as variations about the mean state of the cold tongue, which have long been known to influence weather patterns globally.  

In spite of the importance of the equatorial Pacific cold tongue in global hydrological and biogeochemical cycles, most ocean general circulation models and coupled atmosphere-ocean general circulation models produce a cold bias in the east-central equatorial Pacific ocean, including an exaggerated westward extent of the cold tongue. The core ocean model of the present National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS), the Modular Ocean Model version 3 (MOM v.3), also exhibits such a cold bias in the cold tongue region, up to 1°C on the annual mean (Fig.1).

The existence of the Galápagos Archipelago on the equator near 90°W, made famous after the nineteenth century expeditions of British naturalist Charles Darwin, presents the potential for topographic interaction with the equatorial current system and other processes related to the cold tongue. Currently, the Global Ocean Data Assimilation System (GODAS), the oceanic component of the CFS, does not include the Galápagos Islands.

A reduced gravity ocean general circulation model of the tropical Pacific Ocean (Gent and Cane, 1989) was recently used to determine the improvements to the simulated Pacific cold tongue region arising from increased horizontal resolution and the inclusion of the Galápagos Islands (Fig.2).

It is found that a more realistic treatment of the Galápagos Islands results in the obstruction of the equatorial undercurrent (EUC) (Fig. 3), which leads to improvements in the simulated spatial structure of the cold tongue, including a basin-wide warming of up to 2°C in the east-central Pacific (Figs. 4, 5, and 6a).

The obstruction of the EUC is related to the improvements east of the Galápagos Islands, and for the basin-wide reduction of the tropical cold bias through an equatorial dynamical adjustment leading to much reduced entrainment-mixing (Fig. 6b), as well as reduced meridional import/export circulation.

In many respects, simply increasing the resolution without including the Galápagos Islands did not result in improvements, but instead exacerbated the cold bias and produced an EUC that is too strong east of where the islands should be. On the other hand, differences due to the Galápagos Islands without increasing the horizontal resolution were negligible. Only when the Galápagos Islands were given a proper treatment with sufficient horizontal resolution did a more realistic depiction of the CT and a reduction in the tropical cold bias problem emerge. In other words, the horizontal resolution must be fine enough to produce an EUC that extends far enough eastward, but the Galápagos Islands must be there to obstruct it.

The pattern of SST warming due to the inclusion of the Galápagos Islands is similar to that of the known cold biases in ocean models and the current NOAA CFS. It is thought that such an improvement would have a considerable impact on the ability of coupled ocean-atmosphere and ocean-ecosystem models to produce realistic clouds, precipitation, biological activity, and carbon cycling in

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

the tropical Pacific Ocean. The next logical step is to exploit these effects on interannual variability and the implications for the predictability of the couple ocean-atmosphere system. In particular, the potential benefits to the predictive skill of, for example, future iterations of the NCEP GODAS and CFS by implementing the Galápagos Islands at sufficient resolution should be given serious consideration. Preliminary sensitivity experiments are planned for the MOM v.4 within the NCEP Environmental Modeling Center.

Thanks to Drs. Tony Busalacchi and Raghu Murtugudde, and the NOAA PACS grant # NA17EC1483.  Also, a special  thanks to Drs. David Behringer, Jiande Wang, and Sudhir Nadiga (EMC) for enlightening discussions about the NCEP GODAS.  For more information, please see:

Karnauskas, K.B., R. Murtugudde, and A.J. Busalacchi, 2006: The impact of the Galápagos Islands on the equatorial Pacific cold tongue.  In revisions at the Journal of Physical Oceanography.

Other references:

Gent, P.R. and M.A. Cane, A reduced gravity, primitive equation model of the upper equatorial ocean. J. Computational Phys., 81, 444-480, 1989.

Vecchi, G., M. Harrison, A. Wittenberg, T. Rosati, 2005: Biases in Ocean and Coupled General Circulation Models. Workshop on the Application of EPIC2001 Data for Improving and Testing Coupled Atmosphere-Ocean Models, Seattle, WA., 11-13 May 2005.

 

(Contact Kristopher B. Karnauskas)