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El Niño and It's Currents

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El Niño and It's Currents vii Abstract __________________________________ The central Pacific Ocean equatorial circulation system is comprised of surface currents, mainly the South Equatorial Current, the Equatorial Undercurrent, the North Equatorial Countercurrent, and the North Equatorial Current and underlying water masses- Surface Layer Water and Intermediate Water, specifically North Pacific Intermediate Water and Antarctic Intermediate Water. Annual variation of these waters is highly influenced by short-term climactic changes in atmospheric circulation, especially the El Nino Southern Oscillation cycle. Using data from CTD deployments, two stages in the decline of the 2002-2003 El Niño were geostrophically calculated. These observations showed that by April of 2003 the effects of this El Niño were minimal with only slight temperature and salinity anomalies remaining. The second cruise, from late March through early May, 54 Appendix a __________________________________ Hydrostatic equilibrium In order for an equilibrium to be maintained it is necessary to have a balancing of forces. It is often stated that the Coriolis force and the pressure force must balance due to this, but why is this the case? The vector form of the equation of motion is, d V = −∇α pg −Ω×+2V + F, dt where V is the total vector velocity, and the RHS reads from left to right, the pressure term, the Coriolis term, gravity and all other forces (per unit mass). The x-component (horizontal) of this is (Pond and Pickard, 1983), du∂ p = −+Ω−Ω+αφφ2sinvwF 2cos . dt∂ x x There is no gravity term in the pure horizontal equation, but rather there are two Coriolis terms, one involving the y-component of velocity (v) and another using the z-component of velocity (w). 55 When frictional forces and eddy viscosities are then taken into account, this equation becomes (Pond and Pickard, 1983), ∂∂∂uuu ∂ u ∂ p ∂222 u ∂ u ∂ u +++uvw =−+−Ω+αφ fvwAA2cos + + A. ∂∂∂txyz ∂ ∂ zzx ∂ x222 y ∂ y z ∂ z However, when the magnitudes of these terms are investigated, most of them are negligible. The magnitude of the pressure term is unknown. All other terms are at least 10-2 times smaller than the first Coriolis term (Pond and Pickard, 1983). Therefore, to an order of accuracy of 1% ∂p 0 = −+α fv . ∂x This holds true for all three directions. In the z-direction the pressure term is balanced by gravity. This means that for any column of water, the weight must be equal, even if the volume is not. 56 Bibliography __________________________________ Bjerknes J. (1969) Atmospheric Teleconnections from the Equatorial Pacific. Monthly Weather Review. 97(3): 163-172. Bradshaw AL and KE Schleicher. (1980) Electrical Conductivity of Seawater. IEEE Journal of Oceanographic Engineering. OE-5(1): 50-62. Chelliah, M. (1990) The Global Climate for June-August, 1989: A season of near normal conditions in the tropical Pacific. Journal of Climate. 3:138-160. D’Aleo Joseph S. with Pamela Grube. The Oryx Resource Guide to El Nino and La Nina. Oryx Press: Westport, CT, 2002. Dauphinee TM, J Ancsin, HP Klein and MJ Phillips. (1980) The Effect of Concentration and Temperature on the Conductivity Ratio of Potassium Chloride Solutions to Standard Seawater of Salinity 35‰ (Cl 19.3740‰). IEEE Journal of Oceanographic Research. OE-5(1): 17- 28. Dauphinee TM, J Ancsin, HP Klein and MJ Phillips. (1980) The Electrical Conductivity of Weight Diluted and Concentrated Standard Seawater as a Function of Salinity and Temperature. IEEE Journal of Oceanographic Engineering. OE-5(1): 28-50. 57 Donguy J and Meyers G. (1996) Mean annual variation of transport of major currents in the tropical Pacific Ocean. Deep Sea Research I, 41(7): 1105-1122. Eldin, G. (1983) Eastward flows of the south equatorial central pacific. Journal of Physical Oceanography. 13(): 1461-1467. Emery WJ and Pickard GL. Descriptive Physical Oceanography: An Introduction. Pergamon Press: Oxford, 1990. Fofonoff, NP. (1985) Physical Properties of Seawater: A New Salinity Scale and Equation of State for Seawater. Journal of Geophysical Research. 90(C2): 3332-3342. Johnson GC, Sloyan BM, Kessler WS, and McTaggart CE. (2002) Direct Measurements of upper ocean currents and water properties across the tropical Pacific during the 1990’s. Progress in Oceanography. 52(1): 31-61. Katz Richard W. (2002) Sir Gilbert Walker and a Connection between El Niño and Statistics. Statistical Science. 17(1): 97-112. Kling J and Rosenberg K. (2002) Pacific Equatorial Current Structure with respect to early onset of an El Nino Southern Oscillation Event. SSV Robert C. Seamans, Sea Education Association: Woods Hole, MA, Class 179. Lewis, EL. (1980) The Practical Salinity Scale 1978 and Its Antecedents. IEEE Journal of Oceanographic Engineering. OE-5(1): 3-8 Lu P, McCreary JP, and Klinger BA. (1998) Meridional Circulation Cells and the Source Waters of the Pacific Equatorial Undercurrent. Journal of Physical Oceanography 28(1): 62-84. McPhaden MJ, AJ Busalacchi, R Cheney, J Donguy, K Gage, D Halpern, M Ji, P Julian, G Meyers, G Mitchum, PP Niiler, J Picaut, RW Reynolds, N Smith, K Takeuchi. (1998) The Tropical Ocean- Global Atmosphere observing system: A decade of progress. Journal of Geophysical Research. 103(C7): 14,169-14,240. 58 Perkin RG and EL Lewis. (1980) The Practical Salinity Scale 1978: Fitting the Data. IEEE Journal of Oceanographic Engineering. OE-5(1): 9-16. Picaut J and R Tournier. (1991) Monitoring the 1979-1985 Equatorial Pacific current transports with expendable bathythermography data. Journal of Geophysical Research. 96(supplement): 3263-3277. Pond, Stephen and George L Pickard. Introduction to Dynamical Oceanography; 2nd Edition. Butterworth-Heinemann Ltd: Woburn, MA, 1983. Schmitz WJ. On the World Ocean Circulation (Volume II): The Pacific and Indian Oceans/A Global Update. Woods Hole Oceanographic Institution: Woods Hole, Massachusetts, 1996. Taylor AC, Richardson P (Peter the Exalted), Hunter M, Crotty EA, Gross SH and Scherr MS. (2003) Central Pacific Ocean Equatorial Water Circulation. SSV Robert C. Seamans, Sea Education Association: Woods Hole, MA, Class 186. Tomczak M and JS Godfrey. (2003) Regional Oceanography: an Introduction 2nd edn Walker GW. (1928) World Weather. Monthly Weather Review. 56(5): 167-170. Wyrtki K and Kilonsky B. (1984) Mean Water and Current Structure during the Hawaii-to-Tahiti Shuttle Experiment. Journal of Physical Oceanography. (14): 242-254. 53 Conclusion __________________________________ This work has shown that the methods used to calculate geostrophic currents do show the large scale shifts in current structure as seen in the phases of the El Niño Southern Oscillation. In particular, the weakening of the 2002-2003 El Niño is observed. However, the error incurred by this method of approximation is extreme. In order to even decrease the error by half, the increased number of measurements necessary would be logistically prohibitive for a vessel with multiple research objectives. 36 discussion __________________________________ El niño and the currents The same mechanisms that cause the weakening of westerly flowing currents during the onset of El Niño also result in restoring those currents during El Niño's decline. The primary mechanism is the change in the strength of easterly trade winds. As these easterly trade winds increase in strength, the surface waters should be blown into areas of greater potential height difference. With the increase in the difference in geostrophic height, stronger currents will theoretically return. This can be seen in the change of maximum velocities of the South and North Equatorial Currents between cruises 185 and 186 (Figure 5.1). The maximum velocity of the SEC changed from 0.363 ms-1 to 0.875 ms-1 in the six weeks between passes. The NEC's velocity changed from 0.195 ms-1 to 0.315 ms-1. But how much of this is due to where CTDs were taken rather than the current structure? For both 37 Core Current Velocities 1 0.9 0.8 0.7 0.6 Cruise 185 0.5 0.4 Cruise 186 0.3 Velocity (m/s) 0.2 0.1 0 SEC(SS) SEC(S) SEC(N) SSCC EUC NEC NECC NSCC Current Figure 5.1. Comparison of maximum current velocities on cruise 185 (blue) and cruise 186 (purple). cruises, there were between three and four CTD drops in the region of the NEC, but those for cruise 185 were more evenly distributed from 10°N to 17°N while those for cruise 186 were more densely distributed, primarily from 9°N to 12°N. Due to this closer distribution on cruise 186, the area of greatest velocities may have been observed more thoroughly than the rest of the current, leading to a greater average velocity. The geopotentials are within the same range, and could possibly include the slope seen in cruise 186, but this is unknown. For the SEC, there are much better data for cruise 185 than for 186. Cruise 185 has seven data points fairly evenly spaced, with six falling between 14.5°S and 2.9°S, whereas there is a large gap in data from cruise 186 between 12.3°S and 5.2°S. Despite this lack of data, it appears highly 38 Geopotential Sea Surface 21 SEC NECC NEC 19 17 15 Geopotential Distance (Nm) 13 -20 -10 0 10 20 Latitude 185 surface 186 surface Figure 5.2. Geopotential sea surface of cruise 185 and cruise 186 measured as a geopotential distance and graphed as a function of latitude. The currents which flow at the surface are labeled along the top of the graph. unlikely that the sea surface of 186, even with more points added, would have a different slope more similar to that of cruise 185 (Figure 5.2). When considering the EUC in particular, there are two ways to conceptualize the effects of El Niño. One would be that the westerly flowing currents are weaker than normal, creating less friction on the EUC, so it can therefore flow faster back towards the west.
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