Deep ocean jets drive Atlantic weather (Science News)
Regular weather patterns in the tropical Atlantic are driven by deep ocean currents, not the winds and other atmospheric factors that cause El Niño and similar cycles. An international research team now finds evidence that 4.5-year cycles in temperature, precipitation and winds are driven by deep jets in equatorial waters that carry energy upward. These fast-paced currents appear to originate “in the abyss (perhaps as deep as several thousand meters),” the researchers report online May 19 in Nature, and vary independently of the seasons.
Mysterious Deep-Sea Jets Alter Global Climate (Live Science)
Strange jets of water rising from deep in the ocean repeatedly trigger anomalies in wind, rainfall and sea surface temperature across the tropical Atlantic, scientists find.
Decades of research have revealed that the oceans impact climate in a multitude of ways, most notably with the ocean-atmosphere phenomena known as El Niño and La Niña events, where patterns of warmth and cold in the Pacific regularly wreak havoc worldwide.
Scientists wanted to understand how the oceans influenced the tropical Atlantic around the equator. Previously, researchers had thought the Pacific and North Atlantic Oceans were the main sources for climate fluctuations there. [The World's Weirdest Weather]
Now oceanographers have discovered regular climate fluctuations in the tropical Atlantic are apparently caused by hitherto unknown deep jets of water traveling from the abyss up about 9,800 feet (3,000 meters).
"To date, when trying to explain tropical climate variations, we have always looked upwards, specifically to the atmosphere," said researcher Peter Brandt, a physical oceanographer at the Leibniz Institute of Marine Sciences in Kiel, Germany. "Our new data, for the first time, direct our attention towards the depths of the ocean, thereby opening new perspectives."
These currents flow at speeds of 1,180 to 2,360 feet per hour (10 to 20 centimeters per second) along the equator, crossing the entire Atlantic. The kinetic energy they pack alters surface currents, leading to fluctuations in climate that regularly cycle about every 4.5 years.
Among the areas these fluctuations impact is West Africa. Rainfall linked with the West African Monsoon has a major effect on agriculture, water resources and health concerns in one of the more densely populated regions in Africa, Brandt explained.
The scientists investigated these jets from the abyss using deep-sea moorings located along the equator, consisting of several miles of wire held upright in the water column by floats and buoys. Instruments mounted along the wire continuously recorded water speed, direction, salt levels and temperature over the long term. The researchers also gathered data using freely drifting deep-sea buoys called Argo floats, as well as from satellite.
"We have studied sea surface temperature variability in the tropical Atlantic for a long time and we also study deep circulation," Brandt told LiveScience. "At one point, we saw that they are both closely connected."
It remains uncertain what might cause these deep jets. The researchers currently speculate that strong monthly fluctuations in movement of the ocean's surface propagate downward to generate conditions for the jets in the abyss, Brandt said. [Video – Beauty from the Bottom of the Ocean]
"How large the effect of these deep jets is, and how they are generated is still somewhat of a mystery," Brandt said. "We still have a lot of work ahead of us."
On a research cruise this month, "we are going to retrieve and redeploy our moorings along the equator," Brandt said. "We hope that the new data will provide new insight into the processes of the deep sea, thereby also contributing to an improved climate prediction for West Africa."
The scientists detailed their findings online May 18 in the journal Nature.
Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean
Peter Brandt,1 Andreas Funk,1 Verena Hormann,1, 3 Marcus Dengler,1 Richard J. Greatbatch1 & John M. Toole2
Nature 2011 Published online18 May 2011
Climate variability in the tropical Atlantic Ocean is determined by large-scale ocean–atmosphere interactions, which particularly affect deep atmospheric convection over the ocean and surrounding continents1. Apart from influences from the Pacific El Niño/Southern Oscillation2 and the North Atlantic Oscillation3, the tropical Atlantic variability is thought to be dominated by two distinct ocean–atmosphere coupled modes of variability that are characterized by meridional4, 5 and zonal6, 7 sea-surface-temperature gradients and are mainly active on decadal and interannual timescales, respectively8, 9. Here we report evidence that the intrinsic ocean dynamics of the deep equatorial Atlantic can also affect sea surface temperature, wind and rainfall in the tropical Atlantic region and constitutes a 4.5-yr climate cycle. Specifically, vertically alternating deep zonal jets of short vertical wavelength with a period of about 4.5 yr and amplitudes of more than 10 cm s−1 are observed, in the deep Atlantic, to propagate their energy upwards, towards the surface10, 11. They are linked, at the sea surface, to equatorial zonal current anomalies and eastern Atlantic temperature anomalies that have amplitudes of about 6 cm s−1 and 0.4 °C, respectively, and are associated with distinct wind and rainfall patterns. Although deep jets are also observed in the Pacific12 and Indian13 oceans, only the Atlantic deep jets seem to oscillate on interannual timescales. Our knowledge of the persistence and regularity of these jets is limited by the availability of high-quality data. Despite this caveat, the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical Atlantic. Deep-jet generation and upward energy transmission through the Equatorial Undercurrent warrant further theoretical study.
|a, Anomalies of SST (colour scale), surface wind (arrows) and rainfall (white contours: solid, positive; dashed, negative; every 0.15 mm d−1) as determined through regression on the harmonic fit of the SST anomalies (microwave optimally interpolated…|