Ocean Systems

Ocean Systems in Baja

Ocean Currents

Two main ocean currents occur off the west coast of Baja California. The first is the California current, driven by a wind called the Westerlies. The California current runs down the eastern coast of North America from Alaska, United States to Baja California, Mexico. This current is slow moving, cool, shallow, and broad (Feldman, 2001). The coolness of the waters encourages a phenomenon called upwelling, which typically occurs on western boundaries of continents. This phenomenon is a collaboration of wind and currents. The colder water rises to replace warm surface water blown out to sea by strong offshore winds. (See fig 1)

Figure 1: Upwelling (USC, http://earth.usc.edu/~stott/Catalina/Oceans.html)

Upwelling supports about half of the world's fisheries, although these cool waters account for only 10 percent of the surface area of the global ocean (Gross, 1993; Feldman, Website 2001).
The second current is the equatorial counter-current.

This current is a warm water current moving in a northward direction up the coast of Mexico. When the two currents meet they turn and move out in a westerly direction toward Asia (see Fig 2). The point where these two currents meet is at Punta Eugenia, Baja California, Mexico. Hence, the waters north of this area are typically cooler than the waters found south of this point. This in turn effects the number of species found in the tidal zone. One thought that I would like to see explored is does the warmer waters/ temperatures have the ability to support a greater number of species than cooler waters similar to the latitudinal effect of fewer species and larger organism as you move northward? (See the species account section for more thoughts of this topic.)

Figure 2: Major oceanic surface currents of the world (Advanced Computing Lab / Los Alamos Nat. Lab., http://www.acl.lanl.gov/GrandChal/GCM/currents.html)

Surface current waters make up about 10% of all the water in the ocean. These waters are the upper 400 meters of the ocean (USC, Website). Typical surface currents run in a clockwise direction in the northern hemisphere and counter clockwise in the southern hemisphere. The California Current and the Peru Current are strengthened by the Westerlies and the Equatorial Currents by the Tradewinds. Not only are surface currents moving in a circular fashion, but the subsurface currents also rotate moving the warmer surface waters to deeper depths unreached by heat diffusion. These currents push away from each other at the equator creating two distinct systems of water movements, hence the different rotations of the northern and southern hemispheres (see Fig. 3).

Figure 3: A schematic diagram that shows the complex surface and subsurface currents in the tropical Pacific Ocean (Philander, 1999)

The above illustrates the normal conditions in the Pacific Ocean, but the southern Pacific Ocean exhibits a rotation called the Southern Oscillation where the main current in the southern hemisphere oscillates between a westward flow (normal) and an eastward flow (El Niño conditions)

Southern Oscillation

The Southern Oscillation is a seesaw of air pressures on the eastern and western halves of the Pacific. Normally, the atmosphere above the eastern South Pacific is dominated by a persistent high-pressure zone, while a low pressure zone dominates the west. These two systems are coupled: when the pressure rises in the east, it falls in the west and vice versa. This difference in pressures drives the trade winds from east to west along the equator under normal conditions. At the same time, high above the ocean surface, this wind circulation is completed, as it continues to blow around from west to east. This convection of air is called the Walker Circulation (named after identifier Sir Gilbert Walker). (Amaral, Website)

Every 4-7 years a random disturbance disrupts the normal flow causing the air pressure in the east to decrease and the west end to increase, thus affecting the winds and current to slow down and even reverse causing an anomaly called El Niño (Amaral, Website; Philander, 1999). This event is typically followed by another anomaly called La Niña with each event lasting about 14-22 months (Amaral, Website). The events are caused by a coupling between the ocean and atmosphere by the winds and sea surface temperatures (Amaral, Website). La Niña is characterized by unusually cold ocean temperatures in the equatorial Pacific, as compared to El Niño, which is inversely characterized by unusually warm ocean temperatures in the equatorial Pacific (NOAA & USDOC, 2001a) (See Fig. 4).

Figure 4: Current and atmospheric conditions that occurs during the El Niño (bottom) and La Niña (top) events (Philander, 1999).

The root cause of (El Niño) could be buried in the lengthy time it takes for that information to propagate across the Pacific. The ocean attempts to respond on a yearly cycle with the sun, but due to its width it is unable to comply, which could be a cause of El Niño (Amaral, Website).
During an El Niño event, the trade winds and ocean currents reverse causing the warm waters of the south pacific to extend across the ocean toward South America. A nearly flat thermocline extends across the ocean preventing the normal upwelling along the west coast of South America, unlike the inclined thermocline shallow on the South America side during La Niña years (see Fig. 4) (Philander, 1999).
There are three events or conditions that prevail, but are not always present during each Southern Oscillation. The three include the warm El Niño, the cool La Niña, and an in-between normal state (see Fig. 5). The effect of these conditions on Baja California could bring warm Pacific water entirely up to the west side during El Niño, bring warm Pacific water up to Punta Eugenia in the normal conditions, and bring a cooling off the Pacific side during La Niña events. The Sea of Cortez typically stays warm during each event having a shallow basin, but may become warmer than normal during El Niño years.

Figure 5: The three prevailing weather patterns of the Southern Oscillation. La Niña (top), Normal (middle), and El Niño (bottom). SST= Sea Surface Temperature. (NOAA & USDOC, 2001c).

El Niño is a disruption of the ocean-atmosphere system in the Tropical Pacific having important consequences for weather and climate around the globe (NOAA and USDOC, 2001b). El Niño events tend to dry out Australia and Indonesia, while heavy rains causing mud slides and flooding become the forecast on the west coast of South and Central America (Amaral, Website; Philander, 1999). It also moves the jet stream off course, supplying California with a more-than-healthy supply of rain and suppressing Atlantic hurricanes (Amaral, Website; Philander, 1999). The effects of El Niño have even been felt as far south as Antarctica more than 6,000 kilometers away. James W. Testa noticed that the number of births in Weddell seals declines every four to six years which coincides with El Niño events (Amaral, Website). This suggests that the seal declines may result from changes in the fish populations, possibly caused by shifts in ocean currents (Amaral, Website).

Much can be gained then, if El Niño could be predicted accurately. Peru, for instance, could anticipate a rainy season, and plant crops--like rice--adapted for the weather. So far, several scientists have been successful developing models to predict another El Niño onslaught. Mark Cane and Stephen Zebiak of Columbia University's Lamont-Doherty Geological Survey have developed a model they think can predict an El Niño event at least six months--and perhaps even a year--ahead of time. They use essential elements in the model, such as, sea-surface temperatures, wind data, and estimate thermocline depth using information about surface winds. Their model correctly predicted El Niño events for three-fourths of the time over 15 years. (Amaral, Website)