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A Theory of the Cromwell Current (The Equatorial Undercurrent) and of the Equatorial Upwelling an Interpretation in a Similarity to a Costal Circulation

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A Theory of the Cromwell Current (The Equatorial Undercurrent) and of the Equatorial Upwelling an Interpretation in a Similarity to a Costal Circulation A Theory of the Cromwell Current (the Equatorial Undercurrent) and of the Equatorial Upwelling An Interpretation in a Similarity to •\ a Costal Circulation*•\ Kozo YOSHIDA** Abstract: A theory is given to account for the CROMWELL Current-the Equatorial Undercurrent. The equatorial upwelling is closely related to the Current, and a remark- able similarity between the equatorial circulation and the circulation in the coastal upwell- ing regions. The widths of the equatorial upwelling as well as of the CROMWELL Current the maximum speeds of the upwelling and of the CROMWELL Current, the depth of the core and the thickness of the CROMWELL Current, are all found to be explained quantita- tively. The rate of change in the CORIOLIS parameter with latitude and the mean vertical stability of the waters are essential to determine those length scales of the phenomena. The easterly wind stress over the equatorial region is responsible for the processes. 1. Introduction In 1952, Townsend CROMWELLet al. (CRom- WELL et al., 1954) discovered an excitingly strong current flowing eastward just below the west-going surface equatorial current. This was the current which had not been known to us before and has been established by later investigation. Particularly, recent direct measurements of the current by John KNAUSS (1958) have revealed striking features of the current. This subsurface current which had hitherto been entirely unknown and has now become convinced to exist is named the CROMWELLCurrent or the Equatorial Undercurrent. Fig. 1. Sigma-t. Northbound section along We are so curious about this current not 140•‹W longitude, 7•‹S to 8•‹N, Stations 17- only because it is new to us, having entirely 31, June 3-8, 1952 (Hugh M. SMITH been overlooked by the world oceano- cruise 15). Sigma-t, in grams per liter, isopleth inter- graphers, but also because the discovery val 0.2 gr/1, (according to T. S. AUSTIN, seems to provide us with an extremely Mid-Pacific oceanography V, U.S. Fish and interesting phenomenon in the ocean. As Wildlife Serv., Spec. Sci. Rep.: Fish. No. KNAUSS has reported. the current is narrow, 136, Fig. 9) thin, deep and strong, the core of the cur- The temperature and density sections in rent sticking to the equator, with a strik- the meridional plane across the equator ing symmetry about the equator. All other reveal the outstanding features of "vertical ocean currents as far as we know do reveal divergence" of isotherms and isopycnals certain variations in their axes, mostly in responses to those in large-scale wind (Fig. 1), between 50 and 500 meter depths. Most of those sections to the east of about system. 180° longitude reveal this feature but not *Received Dec . 19, 1959 usually to the west. This feature looks **Geophysical Institute , Tokyo University, Tokyo very similar, at least apparently, to that ( 1 ) 160 Journ. Oceanogr. Soc. Japan, Vol.15, No.4 (1959) encountered in the regions where pronounced sponds to that of characteristic density- coastal upwelling take places, such as off divergent features as described before. California. Two different mechanism will be considered The CROMWELL Current appears to be for maintaining such subsurface conver- present right in the layer of such charac- gence: one is through the equatorward flow teristic density feature or at least to be with longitudinally uniform eastward flow, closely related to this density structure. and the other is through the eastward cur- In fact, it is apparent from the density rent decreasing its speed toward eastern distributions that the Undercurrent is nearly boundary. The former model would apply in geostrophic balance with the pressure to the regions in the central portion of the field. The question is then how such pres- equatorial oceans where the undercurrent sure field can be produced and maintained. exists with relatively uniform velocity longi- It will be attemped in this paper to give a tudinally. Although observations seem to possible dynamical explanation for the oc- show that the core velocity of the Current curence of the CROMWELL Current as so far does not vary appreciably between the observed. Galapagos Island and 140•‹W and disappears Of course, foregoing observations of the rather abruptly near the Island, the depth Current have been somewhat limited in the of the core seems to decrease toward the conditions. We have very little or no in- east, so that the latter model may be sug- formations as to the pressure of similar gested to apply for eastern regions. equatorial undercurrents in the Atlantic, It will be found from the following analy- in the Indian Ocean and even in the western sis that the principal features as observed portion of the Pacific. Weak meridional may be explained quantitatively by those components of flow have not been measured. dynamical models which are highly simpli- It is uncertain whether the current might fied. The similarity of the equatorial circu- reveal certain fluctuation from season to lation to the coastal circulation will also season. Such a situation may still allow be considered to be real, and the narrow for various different possibilities of theore- coastal undercurrent in certain regions of tical accounts for what have so far been coastal upwelling may be suspected. observed. Therefore, the present attempt may only provide one of those possible 2. The observed facts to be accounted for models. will be those as follows: (1) The CROMWELL The model considered in this paper is Current exists at and very near the equator, that within the framework of a linear between 2•‹S and 2•‹N approximately and is system. The CORIOLIS change with latitudes symmetrical about the equator: (2) The is the most important factor, and the den- core of the current is at about 100 meters; sity stratification is essential to the process (3) The thickness of the current is about as well. The equatorial upwelling which is 400 meters between the depth of 50 and 500 confined near the equator is considered to meters; (4) The maximum velocities of the be closely related to the Undercurrent. Current (at the core) are about 100- The easterly winds over the equatorial re- 500 cm-sec-1; (5) The core of the current gions will be responsible for the upwelling is likely to be fixed at the equator and to and consequently for the Undercurrent. be quite steady; (6) The temperature and The Undercurrent will be present in the density profiles on the meridional section layer below the surface layer within which across equator indicate striking features the wind-induced horizontal divergence is of the vertical stretching of the layer near assumed to be limited. This lower layer is the thermocline which appear to be charac- thus a layer of convergence compensating teristic to the CROMWELL Current. the surface divergence layer and corre- Of course, it would be quite probable that ( 2 ) Kozo Yoshida: A Theory of the Cromwell Current and of the Equatorial Upwelling 161 nonlinear terms are comparable with other intensity at the longitudes nearly in the half terms in the dynamic balances* very near way between the 180th meridiam and the the equator, especially around the core of eastern coast, that is, around 130° to 135° W the undercurrent. The significances of the in all seasons (HIDAKA, 1958) . The surface use of vertical mixing without these non- current charts (such as CROMWELL et al., liner effects might be doubtful, so that it 1959; the Oceans) seem to show that the would not be justified to go much in details westward surface drift has the maximum of the Undercurrent at the very equator, speed at some longitudes between 110° and such as the depth of the core and the 140°W, nearly in consistence with the wind thickness of the Current witnin the frame- distributions. work of the linear system considered. Such distributionsin the westward surface Highly concentrated activities within the drifts will tend to give the surface divergence narrow equatorial zone are not the direct in the east and convergence in the west. consequence of the external forces such as However, the equatorial upwelling may seem winds having similar concentration. Instead, to be indicated to occur at least in the the winds have rather much more uniform eastern half of the equator, to the east of distribution over larger areas. The physical 180•‹ meridian. Hence the principal factor factors which may cause such a concentrated which may be responsible for the equato- pattern in the ocean must therefore be rial upwelling in wide range of the equator seeked for in dynamical mechanism within will be considered to be the horizontal the interior of the ocean system. divergence due to the distribution of the As far as the surface winds over the meridional component of flow. equatorial regions are relatively uniform, The mechanism may then be similar to the distributions of the upwelling centered for the coastal upwelling and may be close at the equator will be nearly symmetrical to that described qualitatively by CROMWELL about the equator. The width of the upwell- ing zone may not essentially depend upon (1952). The following analysis will give the quantitative explanation for the latitudinal wind conditions but only on characteristics width of the equatorial upwelling, the speed of density stratification and on a rate of of the upwelling, and so forth in terms of change in the CORIOLIS parameter. It will the measurable quantities. be shown that if the horizontal divergence Taking x eastward, y northward (from the associated with the wind-induced upwelling equator), and z downward (from the sea sur- is limited within the surface layer, the con- face), the equations for the layer above are vergence to maintain the surface divergence would require the eastward corrent in the layer just below the surface layer.
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