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Equatorial Ocean Currents

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Equatorial Ocean Currents i KNAUSSTRIBUTE An Observer's View of the Equatorial Ocean Currents Robert H. Weisbeq~ University qf South Florida • St. Petersbm X, Florida USA Introduction The equator is a curious place. Despite high solar eastward surface current that flows halfway around the radiation that is relatively steady year-round, oceanog- Earth counter to the prevailing winds. raphers know to pack a sweater when sailing there. Curiosity notwithstanding, we study these equatori- Compared with the 28°C surface temperatures of the al ocean circulation features because they are adjacent tropical waters, the eastern halves of the equa- fundamental to the Earth's climate system and its vari- torial Pacific or Atlantic Oceans have surface ations. For instance, the equatorial cold tongue in the temperatures up to 10°C colder, particularly in boreal Atlantic (a band of relatively cold water centered on the summer months. The currents are equally odd. equator), itself a circulation induced feature, accounts Prevailing easterly winds over most of the tropical for a factor of about 1.5 increase in heat flux from the Pacific and Atlantic Oceans suggest westward flowing southern hemisphere to the northern hemisphere. In the currents. Yet, eastward currents oftentimes outweigh Pacific, the inter-annual variations in the cold tongue those flowing toward the west. Motivation for the first temperature are the ocean's part of the E1 Nifio- quantitative theory of the large scale, wind driven Southern Oscillation (ENSO) which affects climate and ocean circulation, in fact, came from observations of the people worldwide. It is against this backdrop of in- Pacific North Equatorial Counter Current (NECC), an tellectual curiosity and climate relevance that the decades following the DRIFTER OBSERVED GLOBAL OCEAN CIRCULATION at 15m DEPTH: 1978-1998 International Geophysical Year (IGY, 1958) experi- 0.3 nVsec 2 ° Latitude x 6 ° Longitude Average o.6 m/sec enced explosive growth in 0 ° 60°E 120°E 180" 120°W 60°W 0 ~ descriptions and under- - - - -- standings of the equatorial ocean circulation, the inter- actions that occur in the 60°N tropics between the ocean and the atmosphere, and the ocean's connections between the tropics and the 30'N subtropics. John Knauss , . played a seminal role in 0° ~.- ~~~. , .. 0o equatorial oceanography, both as an individual scien- tist exploring these strange, 3°°s eastward currents and as a mentor, providing opportu- nity and encouragement for 60°s his graduate students and associates. The following pages describe a small por- 0 ° 60°E 120°E 180 ° 120°W 60°W 0 ° tion of John's work and the Peter Niiler - SIO Mark Swenson - AOML/NOAA questions that he raised, Fik~ure 1. The smface currrnts of the worht's oceans as observed with satellite tracked dr!tiers ensemble aver- offer a glimpse at some of axed over 2 ° latitude by 6 lo~lxitude boxes.for the period 1978-1998. IFrom WOCE Aecomplish,te~its, 1999] the progress made, and Oceanography • Vol. 14 • No. 2/2001 27 show that there is still much to be done. I apologize for sional. Figure 1 shows a recent surface current map the many omissions of important concepts and produced using satellite-tracked drifters. The tropics advances, and for the lack of references to the extensive stand out as a place of relatively large and strong published literature. The purpose here is to provide a currents with many twists and turns. Amidst this conceptual overview of how the field evolved and complexity are well-organized gyres. Proceeding from perhaps where it is heading. south to north in either the Pacific or the Atlantic Oceans are: 1) an anti-cyclonic southern hemisphere The Currents of the Tropics and some of sub-tropical gyre, 2) a clockwise equatorial gyre, 3) a Knauss" Observations cyclonic tropical gyre, and 4) an anti-cyclonic northern Maps of the ocean's surface currents are available in hemisphere sub-tropical gyre. The transition regions all oceanography textbooks. Compiled over time from between these gyres are the named, zonally oriented, ship's logs, they provide a serpentine image of liquid ocean currents: the South Equatorial Current (SEC) water movement on the Earth's surface. These ocean between the southern hemisphere sub-tropical gyre and currents and their complementary atmospheric winds equatorial gyre, the North Equatorial Counter Current are nature's way of distributing internal energy around (NECC) between the equatorial gyre and the tropical the Earth in adjustment for the non-uniform radiative gyre, and the North Equatorial Current (NEC) between exchanges with the solar system. The routes are the tropical gyre and the northern hemisphere subtrop- complex because of the Earth's rotation and variations ical gyre. Sverdrup theory accounts for the gyres and in water density, and they are also fully three-dimen- currents in terms of the overlying wind field, the Earth's rotation, and its spherical shape. The description is incomplete, however, because the physics of a 28 the western and equatorial 24 boundary layers are more 20 involved. 16 oC In driving these surface cur- 12 rents the winds cause mass and 10 heat flux distributions that set 8 up the pressure field. Off the 6 equator the pressure gradient 4 force is balanced largely by the Coriolis effect of the currents, b o but within the equatorial 36.2 boundary layer the pressure 200 35,8 gradient itself tends to balance 35.4 the force of the wind stress over the upper ocean. When com- --~ 400 35.0 bined with other boundary 34.6 layer physics this results in 6O0 subsurface eastward currents. 34.2 An example of these, calculated 800 33.8 from eight years of repeated hydrographic sections in the 40 C central Pacific, is given in Figure ~0 2. The lower panel shows 0 predominantly eastward flow L cm/s occurring below a thin west- ward flowing surface veneer. 4 Situated within about two 1o degrees of the equator is 30 the subsurface Equatorial 70 Undercurrent (EUC), and just 10°S 0 ° IO°N 20°N outboard of the EUC, and a little L at/rude deeper, are the flanking subsurface Tsuchiya jets. These Figure 2. Meridional sections of temperature, salinity, and zonal velocity aver- subsurface eastward currents agedfrom 1987-1995 using data from quarterly cruises between Fiji and Hawaii. are consequences of the equato- [Plate l from Morris et al. ,1998.] rial boundary layer, and the 28 Oceanography • VoL 14 • No. 2/2001 EUC along with the surface NECC are among the many which he reports on obervational studies of the Pacific topics that John Knauss investigated. John's descriptions EUC. The EUC discovery is attributed to Cromwell, et and analyses motivated both theoretical and experimen- al. (1954) who followed up on the long line fishing tal work that eventually fed into a new era of numerical vessel observations in 1951 that tackle and ship on the ocean circulation modeling, a tectmique that was virtu- equator tend to drift in opposite directions (I wonder ally non-existent at the time. Many questions still remain how many equatorial oceanographers will admit to unanswered, particularly those regarding how the their wire being tugged under the hull). The Dolphin currents are linked and how they affect sea surface Expedition, a multi-ship survey in boreal spring of the temperature and hence the coupled ocean-atmosphere IGY set out to map the EUC from the central Pacific to system so vital to the Earth's climate. Let's take a look at where we are and how we got to this point. We now recognize that the serpentine nature of the I I J I I surface currents is also reflected at depth. A dynamical /z / / / • ~\ \\\ framework in which to describe the thermocline 111 I '/ currents and the linkages between the subtropics and tropics was advanced in the mid-1980s by Luyten, Pedlosky, and Stommel. Their layered thermocline ,~___100 2 ~.~..~'~'KA ,' theory (with subsequent boundary layer modifications) makes connections between the eastward flowing subsurface equatorial currents and the mid-latitude zones of subduction. Water properties are determined at mid-latitudes in winter when the depth of the mixed a - ^A // -12 layer and the density of its water are at their annual 200 maxima. Once overlain by less dense water the sub- ducted fluid moves slowly equatorward subject to potential vorticity conservation for individual water masses (geostrophic most of the way with relative vor- ticity added in the boundary layers) and the overall 300 / E Sverdrup (wind stress curl) constraint for the entire S 3 ° 2 ° m° O ° ~° 2 ° 3 ° N thermocline. Upon arrival at the equatorial boundary layer, these thermocline waters from mid-latitudes turn eastward, accelerate, and methodically upwell and mix to form the equatorial cold tongues of the Pacific and Atlantic Oceans. Since the cold tongues are the primary source of atmospheric pressure gradients in the tropics they largely determine the tropical winds and hence the 100 currents that respond to these winds. The combined ocean-atmosphere system therefore works through an E intimate coupling between the basin-scale ocean circu- v lation, the sea surface temperature, and the winds. T I-- Analogous to the way that zonal (i.e. East-West) EL LLI changes along the equator are related to the inter- D 200 amlual climate variations known as ENSO, meridional (i.e. North-South) changes in water mass movements produce decadal climate variations through modula- tions of the equatorial thermocline and ENSO. This all seems very straightforward in hindsight, but it was not so at the time of the IGY. 300 I will limit my attention to a few of Knauss' works, notably observations associated with the eastward flowing NECC and EUC, and I will then track the progress made through the present time.
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