Global Adjustment in the Oceanic Circulation

Paola Cessi,
Scripps Institution of Oceanography – UCSD

Summary

1. Introduction

It has been somewhat puzzling that climate records taken near the equator reflect many events recorded in the proxy temperatures of the Greenland icecap. For example, anoxia events in the Santa Barbara Basin, with low surface density, higher rainfall and warmer conditions along the coast of California, correspond in time to cold periods recorded in the Greenland icecap over the past 60,000 years. Thus, a cold North Atlantic is associated with a shift of rainfall towards the eastern tropical Pacific, consistent with more frequent El Niño activity, while warm North Atlantic conditions are correlated with dry, cooler eastern tropical Pacific.

A possible mechanism for low frequency changes in the Equatorial Pacific is the surface branch of the conveyor belt which presently takes up heat in the tropical Pacific and takes it to the North Atlantic through the Cape of Good Hope and the Indonesian Passage. A stronger overturning circulation implies greater upwelling and cooler surface temperatures in the Pacific, but advection of warm water in the North Atlantic. Therefore variations of conveyor-belt strength produce sea surface temperature anomalies of opposite sign in the North Atlantic and Tropical Pacific.

2. Global seiching of upper ocean water

We illustrate the global response to low-frequency changes in the North Atlantic branch of the thermohaline circulation with two models: a high-resolution shallow water model and a low-resolution global three-dimensional ocean general circulation model (OGCM) coupled to an energy balance model and a sea ice model.

Our ocean models show that the surface branch of the overturning circulation connecting the North Atlantic to the Equatorial Pacific adjusts by exchanging thermocline water between ocean basins in response to changes in deep water formation in the northern North Atlantic.

The global propagation of a disturbance from the North Atlantic is illustrated in figure 1, where the arrival times of the first maximum of sea-surface height (SSH) are shown. The top panel illustrates that propagation in the shallow water model is counterclockwise around ocean basins in the Northern Hemisphere and clockwise in the Southern Hemisphere . From the source region in the North Atlantic a wave propagates equatorward along the American coast. At the equator it moves eastward and then splits into two waves moving north and south along the eastern boundary of the Atlantic. At the southern tip of Africa the wave turns eastward and until it finds its way to the Indian Ocean. At the equator eastward propagation by equatorially trapped Kelvin waves is rapid in spite of the barrier imposed by Indonesia. In the Pacific the wave acts like the El Ni ñ o, traveling eastward along the equator except that its low frequency allows it to penetrate all the way poleward along the eastern boundary. From there, the disturbance is radiated westward in the form of slower Rossby waves that eventually fill the entire basin. The Southern Ocean region to the south and east of Cape Horn is the last to adjust, since very slow Rossby waves reach the Southern Ocean only by radiating energy westward from the eastern South Pacific.

The bottom panel of figure 1 shows a similar propagation pattern in the OGCM, with the exception that communication from the Atlantic to the Pacific occurs through the Indian Ocean and through Drake Passage. The latter route to the South Pacific implies transport by the Antarctic Circumpolar Current (ACC), a flow which is absent in the simplified model, which has no wind-forcing. In summary, many decades are required for forced changes in SSH in the Atlantic to reach the Pacific and even longer to reach the Southern Ocean.

3 . Discussion

A direct comparison of our model results with proxy climate data is not possible. Our models provide reliable information on global SSH variations, while the proxy data are correlated with temperature and rainfall. In the eastern equatorial Pacific, fluctuations in SSH due to changes in the thermocline depth, are tightly correlated with sea-surface temperature (SST) anomalies. No such relation between SSH and SST exists in the western equatorial Pacific. Thus an increase in the equatorial SSH induced by the reduction of deep-water formation in the North Atlantic, leads to an anomaly in the equatorial east-west temperature gradient that can initiate the weakening of the trade winds that might trigger an El Niño event.

Figure 1. The time of arrival of the first maximum in SSH in response to a change in the northern North Atlantic is contoured in years. White areas are either continents or values outside the contoured range. (Top) A solution of a reduced-gravity shallow water model of the World Ocean in simplified geometry, forced in the northernmost North Atlantic with a 'switch on' sinusoid with a period of 100 years. (Bottom) Corresponding results for the SSH anomalies in a three-dimensional OGCM forced by wind and buoyancy gradients. The SSH anomalies are the difference between a perturbed and a control experiment: in the former deep convection is interrupted in the Labrador Sea by adding an anomalous freshwater flux near the Greenland coast; in the latter, freshwater flux is calculated by the coupled model itself (similar to modern observation). The anomalous freshwater flux is a sinusoid in time with a period of 100 years.

For further information contact:
Dr. P. Cessi
Scripps Institution of Oceanography --UCSD
La Jolla, CA 92093-0213
Phone: 858-534-0622
E-mail: pcessi@ucsd.edu

URL http://www.scidac.gov/March2004/ber_5.html
Updated: Monday, 12-Dec-2005 11:06:32 EST