# **Global Adjustment in the Oceanic Circulation**

## Paola Cessi, Scripps Institution of Oceanography -- UCSD

***Summary***

***The global adjustment of the upper ocean in response to localized
anomalies is affected by planetary waves which propagate from one basin
to another in several decades. Planetary ocean waves give rise to a
global seiche, such that the volume of thermocline water decreases in
the Pacific-Indian Ocean while increasing in the Atlantic Ocean.***

**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 Niño, 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.

![](media/image1.png){width="3.002083333333333in"
height="2.782638888888889in"}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](mailto:pcessi@ucsd.edu)**\@ucsd.edu**