Equatorial Undercurrent

The Equatorial Undercurrent (EU) is a strong subsurface equatorial eastward jet, which is situated under the surface 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. The EU width is about 260 mi. (~420 km, plus or minus 2 degrees to the north and south from the equator). The thickness of the EU varies from 650 to 1,300 ft. (~200 to 400 m). Typical maximum velocity within the EU core is about 3.3–5 ft. (1 to 1.5 m) per second.

There are two principal reasons for EU, namely the southeast trade wind blowing over the equatorial zone and the convergence of equatorial eastward nonlinear jet. Long-term southeast trade wind forcing, as shown by George Philander, leads to a pressure gradient directed from western boundaries of the oceans to the east. Convergence of eastward nonlinear current at the equator (because of a change in the sign of Coriolis force between the Northern and Southern Hemispheres) causes the generation of a strong narrow undercurrent in the vicinity of the equator.

EU intensity is at a maximum in the Pacific Ocean (where it is called the Cromwell current, in memory of an American oceanographer). Maximum velocity of EU exceeds 60 in. (~150 cm) per second there. In the Atlantic Ocean (where it is sometimes called the Lomonosov current, in memory of Russian vessel Michael Lomonosov, which discovered the Atlantic EU), its velocity is about half that of the Cromwell current. In the Indian Ocean, the 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 the southwest monsoon action.

The EU is situated deeper on the western side of the Atlantic and Pacific oceans because thermocline deepens, only there, as a result of the longterm effect of the southeast trade wind. The depth of the EU core is up to 820 ft. (~250 m) in the western equatorial Pacific. To the east, the EU becomes shallower and more intense. Maximum EU velocity occurs in the mid-equatorial oceans. Further to the east, the EU shallows as well, but its intensity decreases. EU weakening in the western and eastern sides of the equatorial basins is because of intensified horizontal mixing there, restricting its velocity. Often, there is a secondary (deep) core of EU that is mostly because of a barotropic eastward pressure gradient alone the equator. However, the eastward velocity within this core does not usually exceed 8–12 in. (20–30 cm) per second.

Total transport of EU is about 50 Sverdrups in the central equatorial Pacific, while it is about 30 Sverdrups in the central equatorial Atlantic (1 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 of seasonal cycle between two oceans is because of 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 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 time.

Abrupt forcing of the equatorial ocean (e.g., before El Niño, 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 modified by EU. Low-frequency (decadal-to-decade) variability of southeast trade winds generates quasi-equilibrium EU variations. More or less intense southeast trade winds lead to more or less intense EU. As was shown by Albert Semtner and William Holland, the EU becomes unstable if its velocity exceeds 40 in. (~100 cm) per second. As a result of instability, long planetary equatorial waves are generated. Their periodicity is about 30 days, while wave length is about 500 mi. (~800 km).