THE EQUATORIAL UNDERCURRENT (EU) is a strong subsurface equatorial eastward jet, which is situated under the surface of the westward south equatorial current in the Atlantic, Indian, and Pacific oceans. The main EU core (in which velocity exceeds 20 in. [50 cm.] per second) occurs within an equatorial thermocline. EU wideness is about 428 mi. (400 km.) (plus or minus 2 degrees to the north and south from the Equator). Thickness of EU varies from 656-1,312 ft. (200-400 m.). Typical maximum velocity within the EU core is about 3.3-5 ft. (1-1.5 m.) per second.
Two primary factors
There are two principal reasons for EU, namely the southeast trade wind blowing over the equatorial zone, and the convergence of equatorial eastward nonlinear current.Convergence of eastward nonlinear current at the equator (due to changes in the sign of Coriolis force between the Northern and Southern Hemispheres) causes the generation of strong narrow undercurrents in the vicinity of the equator.
EU intensity is at a maximum in the Pacific Ocean (where it is called the Cromwell current). The maximum velocity of EU exceeds 59 in. (150 cm.) per second there. In the Atlantic Ocean (where it is sometimes called the Lomonosov current), its velocity is about half that of the Cromwell current. In the Indian Ocean, EU as a strong subsurface equatorial jet does not exist throughout the year. It occurs in boreal winter, when northeast monsoons are developing, and disappears in summer during southwest monsoon action.
EU is situated deeper in the western side of Atlantic and Pacific oceans because thermocline deepens just there (a long-term effect of southeast trade winds). The depth of EU core is up to 820 ft. (250 m.) in the western equatorial Pacific. To the east, EU becomes shallower and more intense. Maximum EU velocity occurs in the mid-equatorial oceans. Further to the east, EU is shallower, too. However, its intensity decreases. EU intensity decreases in the western and eastern sides of the equatorial basins is due to intensified horizontal mixing there, restricting its velocity. Often, there is the secondary (deep) core of EU that is mostly due to a barotropic eastward pressure gradient along the equator. However, the eastward velocity within this core does not usually exceed 8-12 in. (20-30 cm.) per sec.
The total transport of EU is about 50 Sverdrups in the central equatorial Pacific, while it is about 30 Sverdrups in the central equatorial Atlantic (one Sverdrup = 106 cu. m. per second). This transport is at a maximum in boreal spring (in the Pacific Ocean) and in fall (in the Atlantic). Such phase differences in seasonal cycles between two oceans is due to their different sizes. As was shown by Vitaly Bubnov in his comprehensive monograph, a seasonal EU cycle in the equatorial Atlantic is approximately in phase with the southeast trade wind forcing (in accuracy of a month), while in the equatorial Pacific they are approximately out of phase, as a result of different sizes and, hence, different equilibrium times.
Abrupt forcing of equatorial ocean (for instance, before El Nino, when trade winds weaken very quickly) generates different classes of equatorially trapped waves (such as, Kelvin, Rossby, inertia-gravitational, and mixed Rossby-gravitational or Yanai waves), most of them are modified by EU. Low-frequency (decade-to-decade) variability of southeast trade wind generates quasi-equilibrium EU variations. More/less intense southeast trade wind leads to more/less intense EU. As was shown by Albert Semtner and William Holland, EU becomes unstable if its velocity exceeds 39 in. (100 cm.) per second. As a result of instability, long planetary equatorial waves are generated. Their periodicity/wave length is about 30 days per 497 mi. (800 km.)
