Striking Deep Current Reversal In The Tropical Pacific Ocean

The ocean's immense heat storage capacity means that it has a dominant role in the regulation of heat exchange and of the Earth's climate. And it is the ocean's currents that drive thermal exchanges between ocean and atmosphere and contribute to climate balance. This they do in transporting warm- and cold-water masses from the Equator to the poles. The near-surface currents are generated essentially by the winds, whereas the deeper ones (known as thermohaline currents) result from water density variations induced by differences in temperature and salinity between the distinct masses. The prevailing winds in the tropical Pacific, the trade winds, blow from the American continent towards Asia, causing the warm surface waters to drift in a general East-West direction. As they approach the Asian continent, these waters accumulate, then change direction, part of them turning North and feeding the Kuroshio (the equivalent in the Pacific of the Gulf Stream), part going South to join up with the East Australian current, another portion flowing at depth and feeding the Equatorial Undercurrent (EUC), which runs at between 100 and 150 m below the surface. The EUC flows along the Equator, from Papua New Guinea to the Galapagos Islands, counter to the trade winds. That current extends over a width of nearly 300 km and transports a large mass of water eastwards (1), at a maximum velocity of around 2 knots (1 m/s or 3.6 km/h).

Scientists are currently seeking to describe ocean circulation and improve on data acquired, aiming to identify the physical mechanisms that regulate climate variability. The impact of the ENSO (El Niño-Southern Oscillation) event on the climatic situation in the southern Pacific Ocean is still not well known, for instance. In two oceanographic cruises run in October 1999 and April 2000 as part of the IRD's ECOP programme, the Institute's researchers were able to study this region and, in particular, the ENSO. The latter has a determinant effect on the distribution of ocean water masses, ocean/atmosphere exchanges in the tropical southern Pacific and many anomalies of climate that occur on the continents that border the Pacific. Physical determinations of currents and masses of water under transport were made from the surface down to 1 200 m over a large area, 1700 km in length, along the Equator (between the Equator and 10° S latitude, between 165° and 180° E longitude), using a Lowered Acoustic Doppler Current Profiler (L- ADCP) (2) installed aboard R/V Alis, the IRD oceanographic research vessel.

These series of measurements give a well-defined picture of the tropical circulation in this zone, for two specific dates. They show up in particular the horizontal alternation of bands of currents of opposing directions between the Equator and 10° S latitude, from the surface to 1200 m. Essentially, however, they reveal a surprising variability of intermediate equatorial currents (the equatorial intermediate current (EIC) and the lower equatorial intermediate current (LEIC)), which plunge at the Equator under the Equatorial Undercurrent and flow in the same direction, between about 300 and 1200 m (see Figure). Between October 1999 and April 2000, these equatorial intermediate currents changed direction, between 2° S latitude and the Equator, over the 1 700 km of the zone investigated. This reversal is already known, but its amplitude in this case is striking. The resulting variation in water mass transport is considerable, around 100 Sv (50 Sv towards the West in October 1999 and 50 Sv towards the East in April 2000).

The question is, what causes this change-about? One hypothesis put forward involves the passage of an oceanic instability wave, but no disturbance of the EUC was detected during the research cruises and the reversal remains unexplained. Further current measurement campaigns in the future should shed light on this event and bring clues for unravelling the dynamics of these currents. At present, such a change in ocean water mass transport must be taken into account in studies on the mass balance that exists in the equatorial Pacific Ocean. Mathematical models of ocean circulation are needed so that the variations in water transport can be reproduced, and thereby facilitate assessment of their impact on the climate variability, whether a seasonal, inter-annual or decennial temporal scales.

Notes:

(1) The average transport of this current is estimated at 30 Sv (1 Sverdrup, unit of volume transport = 1 000 000 m3/s, a value commensurable with the average discharge of the world's major rivers). For comparison, the average transport at the mouth of the Amazon, the most voluminous in the world, is about 0.3 Sv.

(2) Knowledge of ocean current circulation made great advances from the 1980s with the general use of current meters applying the Doppler effect (Acoustic Doppler Current Profiler, ADCP) which, fitted under the hull of the research vessels, make continuous measurements of currents (from ocean surface to an average 700 m depth). Since the 1990s investigation of deep ocean circulation has been using a new generation of ADCP, the L-ADCP (Lowered Acoustic Doppler Current Profiler), which are fixed on CTD (conductivity-temperature-depth) probes then lowered to a fixed point at depth. The principle of the Doppler effect (which is also employed in road and aviation radars) is founded on the instrument's emission of an acoustic signal. This signal is reflected by particles (zooplankton) transported by the ocean currents. These particles are in movement and therefore alter the frequency of the reflected sound wave. The difference in frequency between the emitted wave and the reflected wave is a function of the relative velocity between the instrument and the particles, and therefore of the current velocity.