Observed World Ocean Seasonal Surface Currents on a 5 Grid

Transcription

1 NCAR-TN/IA-159+STR NCAR TECHNICAL NOTE 1_,, 1 -, i: -.-.L- ;,, I I I. w I.. '... I. '.,.,.._,.. October 1980 Observed World Ocean Seasonal Surface Currents on a 5 Grid Gerald A. Meehl IIIIII ATMOSPHERIC ANALYSIS AND PREDICTION DIVISION - --spl----r~-rael-rrsl l~ I-~ l- II I CI 1 ~---Li~s~ I - - L81--~--~ II- I NATIONAL CENTER FOR ATMOSPHERIC RESEARCH BOULDER, COLORADO ~w

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3 i i i PREFACE A digitized ocean current data set for use in ocean model or diagnostic computer studies is described. Various sources of observed ocean currents are examined, and surface current data from pilot charts prepared by the U.S. Naval Oceanographic Office (NAVOCEANO) are used in the data set. The pilot chart data are long-term means of over four million ship drift records compiled since 1900 by NAVOCEANO. The ocean current data set prepared from these data and presented here combines the following: 1) current speeds and directions are digitized on a global 50 latitude-longitude grid; 2) current speeds are in cm/sec; 3) current speeds and directions are given for each season; 4) u and v components as well as resultant current speeds are included; 5) the data are stored on magnetic tape and are accessible for computer use. A portion of this study is supported by the Department of Energy as part of its Carbon Dioxide Effects Research and Assessment Program. The author thanks Warren Washington, Harry van Loon and Albert Semtner for their helpful comments and suggestions, Lynda Verplank and David Knight for their assistance in digitizing the data, Ann Modahl for typing the manuscript and Scott Johnson for drafting the figures. Gerald A. Meehl October 1980

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5 v CONTENTS Page I. INTRODUCTION II. DATA... 1 III. DESCRIPTION IV. CONCLUSION... 6 Appendix A References.. e.e e e e Figures

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7 vii LIST OF FIGURES Fig. la,b Fig. 2a,b Digitized ocean current data set represented by vectors Fig. 4 Fig. 5a,b Fig. 6a,b Fig. 7 on a 5 grid, length proportional to speed in cm/sec. Ocean surface current streamlines analyzed from ocean current data digitized onto a 5 grid from pilot charts. Dashed streamlines south of 50 S indicate only current direction was available from pilot charts. Fig. 3a,b Ocean surface current speeds in cm/sec from data digitized onto a 5 grid. Peak current core velocities indicated by arrows are sub-grid scale maximum speeds from original pilot chart data. Velocity data south of about 50 S do not appear on the pilot charts. Stippled areas indicate no velocity data in present OCDS. Day-by-day position plot of drifting buoy 470 in South Atlantic between 31 and 35 south latitude and 358 and 5 east longitude (Harris, 1978). u component ocean surface currents in cm/sec; positive values are eastward. v component ocean surface currents in cm/sec; positive values are northward. Surface current reversals between January and July, April and October. A reversal is defined as change of direction between two seasons. Solid arrows indicate current direction for January, dashed arrows for April. Currents flow 150 to 210 in opposite direction in July and April, respectively. Fig. Al Geostrophic current at sea surface relative to the 1000 dbar level calculated from dynamic height derived from 1 latitude by 2 longitude grid point oceanographic data (Gordon et az., 1978).

8 1 I. INTRODUCTION In 1942, Gerhard Schott published a chart of world ocean currents based on ship drift data accumulated until that time. The chart shows world ocean surface currents for the northern hemisphere winter and an inset map for 30'N to 20 0 S depicting currents during the northern hemisphere summer. Current speeds and directions are represented by unequally distributed vectors; velocity units are in nautical miles per day. This one chart has been the basis for many global surface current charts produced since then (e.g., Defant, 1961; Stommel, 1964; Neumann and Pierson, 1966). The recent proliferation of numerical models of ocean circulation, however, has created the need for a source of observed ocean surface currents more suitable for comparison with gridded seasonal model output as well as for use in diagnostic studies. The ocean surface current speeds and directions from such an observed source must be: 1) on a 50 latitude-longitude global grid, 2) in metric units, 3) seasonal, 4) on computer tape, facilitating access for computer use, 5) representative of the long-term mean, and 6) produced economically. II. DATA The source of observed long-term mean ocean surface currents until now has been through analysis of ship drifts (Neumann, 1968). With satellites, it may become possible to retrieve ocean surface currents directly through remote sensing (Shuchman et az., 1980). At this time, however, long-term mean current flow is usually obtained through analysis of ship drift records. The basis for inferring ocean surface currents from ship drift is described by Sverdrup et al. (1942) and Stidd (1974). A ship drift is the difference between its true position and its predicted position calculated 24 hours earlier by dead reckoning. This difference in position can be accounted for by two factors--wind and ocean currents.

9 2 The exact effects of wind on ship drift have not been satisfactorily determined, but Stidd (1974) estimates that the drift is roughly onequarter wind drag and three-quarters ocean current. Since these two media do not act in the same sense on the ship, this suggests that actual ocean currents would be somewhat different than those computed from ship drift. A pure wind drift surface current moves in a direction deviated 450 (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere) from the line of the wind due to Ekman drift (Neumann, 1968). However, even though the surface current and the wind are acting in different directions in the ship, both have components acting in the same direction which would affect the movement of the ship. Therefore, in the idealized case of a pure wind drift surface current, a current computed from the ship drift would be greater than the actual current due to the effect of the wind not accounted for in the surface current calculation. The degree to which this would actually affect surface current calculations from ship drifts would vary in space and time over the global oceans due to a myriad of other factors, but it is probable that surface currents derived from ship drifts are somewhat stronger due to wind effects than actual currents. Several options are available to formulate an observed seasonal source of global ocean surface currents on a 50 latitude-longitude grid. The first is the original U.S. Naval Oceanographic Office (NAVOCEANO) file of over four million ship drift observations compiled from a number of countries. NAVOCEANO has also converted these original ship drift data into a set of 42 surface current atlases. Each atlas lists seven parameters of current flow for each 1 latitudelongitude square, but the amount and type of data in these atlases make them difficult to use for the application of interest here. The National Oceanographic Data Center (NODC) has incorporated the original NAVOCEANO ship drift data into their Surface Current Data System (SCUDS). The SCUDS file is recorded on eleven computer tapes, but global analysis of the millions of original observations is both costly and time-consuming.

10 3 A second option is to seek sources of ship drift that have already been translated into seasonal surface currents. Stidd (1974) analyzed the NAVOCEANO ship drift file now incorporated in SCUDS and compiled an atlas of ship drift components by season on a global 50 latitude by 100 longitude grid, with a separate 50 by 50 grid for the North Pacific. Several problems exist with this analysis: the quality of the analyzed data is questionable near the coasts of continents, the global data are on a 50 by 100 grid instead of a 50 by 50 grid, and the tapes on which the data were recorded are inaccessible, thereby necessitating fresh digitization of the data in the atlas for computer use. A third possibility is non-gridded ocean current data from existing atlases. The Department of Commerce (1959, 1961) published two volumes which include ocean currents based on the NAVOCEANO ship drift file, but only the North Atlantic and North Pacific are covered. A better alternative with global coverage is the set of pilot charts prepared from NAVOCEANO data (U.S. Naval Oceanographic Office, 1955, 1966, 1979a, 1979b, 1979c). Among numerous other quantities, arrows on these charts show surface current directions labeled with current speeds in knots or nautical miles per day. Data for the pilot charts are based on long-term mean current speeds and directions already derived from the ship drift file at NAVOCEANO. Extracting a gridded ocean surface current data set from this source does not require great amounts of time~or expense for that reason, but involves hand analysis and digitization of the data. Considering the various requirements, we chose the ocean current data from the pilot charts. This data set provides the flexibility needed to produce a new source of observed surface currents for use in global ocean model and diagnostic computer studies. III. DESCRIPTION The pilot charts from which the ocean current data set (OCDS) the study was extracted are separated into five sets of ocean area in

11 4 maps with slightly different characteristics. The North Atlantic, North Pacific and Indian Ocean are shown on 12 monthly charts. January, April, July and October represent the four seasons in the OCDS from these charts. The South Atlantic and South Pacific have four seasonal maps, and each season is represented in the OCDS. The North Atlantic, South Atlantic and Indian Ocean charts give current speeds in knots; the others are in miles per day. All current speeds, therefore, were converted to cm/sec for the OCDS. The South Pacific charts depict different ranges of current speeds, and an average speed for each range was used in the OCDS. The digitized ocean current data are shown in Fig. 1. Surface currents for the four mid-season months are represented by vectors on a 50 by 50 latitude-longitude grid. The length of each vector is proportional to the speed of the current in cm/sec. South of 45%S in the Atlantic and Indian Oceans and 50S in the Pacific in the Antarctic Circumpolar Current (ACC) region, there are no data because the pilot charts contained direction but no velocity information there. However, current directions from the pilot charts from the far southern oceans were used to represent schematically the streamline current flow and are represented by dashed streamlines in Fig. 2. Elsewhere, the digitized data in Fig. 1 were used to produce the solid streamlines. Similar maps have been drawn for the northern hemisphere winter and summer (e.g., Stommel, 1964; Bowditch, 1966). What is of interest here is that the streamline maps in Fig. 2 were produced from a global, uniformly gridded seasonal ocean current data set. It should be emphasized that the streamlines in Fig. 2 depict long-term mean ocean current flow. At any particular instant, the flow pattern would undoubtedly look much different from the mean, and current velocities would also vary (Knauss, 1978). Instantaneous flow would probably show a series of eddies superimposed on the general mean flow to the extent that the mean flow would possibly become unrecognizable in certain areas (Wooster and Reid, 1963; Wyrtki et az., 1976). For this reason, mean surface current charts such as those presented here should be used for oceanographic or climatological

12 5 studies where long-term averages of ocean or atmospheric variables are of interest. Surface current velocities in cm/sec shown in Fig. 3 are contoured from the resultant surface current magnitudes of the digitized data in Fig. 1. The long-term mean seasonal fluctuations of the various current systems can be seen clearly. Once again it should be emphasized that the current speeds shown here are long-term large-scale seasonal means. The pilot charts provide no measure of the variability of these currents, but studies of dynamic height in the Pacific Ocean, from which the sense of ocean surface current can be inferred, have shown that variability is largest in western boundary currents and equatorial currents where interannual variability sometimes exceeds seasonal variability (Wyrtki, 1975). Peak core current velocities from the pilot charts are noted in this figure even though the digitization process tended to smooth these current maxima slightly. This is unavoidable because the coarse resolution of the 50 grid does not allow narrow intense currents to be well represented. However, greatest velocities generally occur in the western boundary currents and equatorial currents as expected. An eastward current maximum not part of the ACC appears in all four seasons in the southern Atlantic between about 350 and 40'S in Fig. 3. The track of a drifting buoy launched in February 1976 (Harris, 1978) does not indicate the existence of a current maximum in this area (Fig. 4). However, -an eastward directed current maximum near 35%S in the southern Atlantic appears on a map of geostrophic surface currents computed from relative dynamic topography.' The geostrophic current maximum at 35%S on the map is distinct from the ACC maximum at as it is in the OCDS. (See Appendix A for further discussion of the strength and position of the ACC.) The fact that the drifting buoy did not reveal this feature seen in both the ship 'Gordon, 1980, personal communication. The map is part of a Southern Ocean Atlas being prepared by Arnold Gordon and Theodore Baker at the Lamont-Doherty Geological Observatory, Palisades, New York.

13 6 drift and geostrophic current data could be accounted for by interannual variation of the current's strength and position. Figs. 5 and 6 show seasonal u and v component current speeds. Stippled areas indicate positive eastward and northward components. In these figures, one can follow the seasonal evolution of features such as the equatorial counter-currents in all oceans and monsoonal currents (e.g., the Somali Current in the western Indian Ocean). The directional variability of surface currents, as indicated by changes in sign in Figs. 5 and 6, is shown in Fig. 7. Two sets of two seasons each (January and July, April and October) were chosen to outline areas of current reversals, defined as change of direction between respective seasons. Three different reversal criteria were tried: , and The size of the affected areas changed slightly, but locations remained consistent. The directions of currents in January and April are represented in Fig. 7 by solid and dashed lines, respectively. Those current directions change in the opposite seasons of July and October according to the reversal criteria. The most obvious feature of Fig. 7 is the great directional variability of equatorial currents in all oceans. Current directions also change with slight seasonal shifts of mid-ocean gyre convergence areas. IV. CONCLUSION An ocean surface current data set suitable for ocean model comparisons or diagnostic studies has been described. The long-term mean ocean current data are based on over four million ship drift records kept since 1900 and recorded on pilot charts published by the U.S. Naval Oceanographic Office. The data set derived from those observations and presented here contains current speeds (in cm/sec) and directions digitized on a global 50 latitude-longitude grid for each season and stored on tape for computer use.

14 7 APPENDIX A. OCEAN SURFACE CURRENTS SOUTH OF 45 0S The pilot chart data give only some directional and no magnitudinal ocean current data south of 50 S in the Pacific and south of 45 S in the Atlantic and Indian Oceans. Fig. Al gives more details of ocean surface current speed and direction in the far southern oceans. This chart depicts geostrophic surface currents derived from values of dynamic topography calculated from oceanographic grid point data (Gordon et al., 1978). Surface currents calculated in this manner are usually somewhat less than half the total actual current velocity (Harris, 1978). Therefore, maximum actual current velocities of the ACC, seen as the current maximum girdling Antarctica between 40 and 60 S, are probably in the neighborhood of 30 to 40 cm/sec. The current maximum of the ACC in Fig. Al generally lies south of 45 S except in the region to the south of Madagascar in the southwestern Indian Ocean. Here the ACC makes its most northward excursion. Maximum velocities for this part of the ACC from Fig. Al are roughly 20 cm/sec. The complete pilot chart ocean current data end north of the ACC current maximum except for this one area of northward excursion which can be seen clearly in Fig. 3, the current magnitudes, and in Fig. 5, the u component current speeds. A more complete chart of geostrophic currents to be published as part of a Southern Ocean Atlas mentioned previously shows that this northward excursion of the ACC extends from about 10 E to 60 E longitude at about 35 S to 40 S latitude. This corresponds to the current maxima seen in Fig. 3. Even though the OCDS cannot depict the ACC maximum elsewhere, an indication of the presence of the ACC south of the OCDS data can be seen in Fig. 5 as an increase of u component current velocities with decreasing southern latitude.

15 8 REFERENCES Bowditch, N., 1966: American Practical Navigator. U.S. Naval Hydrographic Office, 1966 corrected print, H. 0. Pub. No. 9, Washington, DC, 1524 pp. Defant, A., 1961: Physical Oceanography. Pergamon Press, London, 729 pp. Department of Commerce, 1959: Climatological and Oceanographic Atlas for Mariners, Vol. l, North Atlantic Ocean. Washington, DC, 182 pp., 1961: Climatological and Oceanographic Atlas for Mariners, Vol. 2, North Pacific Ocean. Washington, DC, 159 pp. Gordon, A. L., E. Molinelli, and T. Baker, 1978: Large-scale relative dynamic topography of the southern ocean. Journal of Geophysical Research, 83, Harris, T. F. W., 1978: Satellite-tracked drifters between Africa and Antarctica. Bulletin of the American Meteorological Society, 59, 1, Knauss, J. A., 1978: Introduction to Physical Oceanography. Prentice Hall, New Jersey, 338 pp. Neumann, G., 1968: Ocean Currents. Elsevier, New York, 352 pp., and W. J. Pierson, Jr., 1966: Principles of Physical Oceanography. Prentice Hall, New Jersey, 545 pp. Schott, G., 1942: Die Grundlagen einer Weltkarte der Meeresstromungen. Annalen der Hydrographie und Maritimen Meteorologie, Shuchman, R. H., C. L. Rufenach, F. I. Gonzalez, and A. Klooster, 1980: The feasibility of measurement of ocean surface currents using synthetic aperture radar. Environmental Research Institute of Michigan, Ann Arbor, Michigan, 10 pp. Stidd, C. K., 1974: Ship drift components: Means and standard deviations. SIO Reference Series, 74-33, Scripps Institution of Oceanography, 57 pp. Stommel, H., 1964: Summary charts of the mean dynamic topography and current field at the surface of the ocean and related functions of the mean wind stress. In Studies on Oceanography, K. Yoshida, ed., University of Tokyo Press, Japan, Sverdrup, H. V., M. W. Johnson, and R. H. Fleming, 1942: The Oceans, Their Physics, Chemistry, and General Biology. Prentice Hall, New Jersey, 1087 pp.

16 - L. 9 U.S. Naval Oceanographic Office, 1955: Atlas of Pilot Charts, Central American Waters and South Atlantic Ocean. Pub. 106, Defense Mapping Agency Hydrographic Center, Washington, DC, 16 pp : Atlas of Pilot Charts, South Pacific and Indian Oceans. Pub. 107, Defense Mapping Agency Hydrographic Center, Washington DC, 16 pp., 1979a: Atlas of Pilot Charts, Indian Ocean. Pub. 109, Defense Mapping Agency Hydrographic Center, Washington, DC, 12 pp. D 1979b: Pilot Charts of the North Pacific Ocean. 55 Series, Defense Mapping Agency Hydrographic Center, Washington, DC, 12 charts c: Pilot Charts of the North Atlantic Ocean. 16 Series, Defense Mapping Agency Hydrographic Center, Washington, DC, 12 charts. Wooster, W. S., and J. L. Reid, Jr., 1963: Eastern boundary currents. In The Sea, Vol. 2, M. N. Hill, ed., Interscience Publishers, New York, 554 pp. Wyrtki, K., 1975: Fluctuations of the dynamic topography in the Pacific Ocean. Journal of Physical Oceanography, 5, Magaard, and J. Hager, 1976: Eddy energy in the oceans. Journal of Geophysical Research, 81,

17 I OBSERVED OCEAN CURRENTS FOR JANUARY sow 0 90E IA( ~~~~~~- -S - I I I I I I I I I I I I I I I I I I I I I I I I I _ I -I I I I I I I I I I I I I 1, V _ r \ W<, ~~~~~~~~~~~~~~~4 4 l r S -> -, -- / / - -, it^a f K «- - ; *-'-' -A-- <,^. ^^^\ Ce^ C3 ' : : - ' I- - i I - I F o \ \ \ ^ _ 40N '- ' tu 15 >^ I15 ^ 4 4< N------^iC^ ox sj _\t S 3 gr33 / Z Z / / I /z Xt Z / 111, b ~~CM/SEC tf -\111 *- ' * i 1 J \ / ', -~ i \\ u ^ ^ -2 t,,, y^_ z ~~~~~~~~~~~~~vi _, <, 1\< ~z_-7-ee 40 s,_,' oW 0 90E W 90NL - I I I I I I -F-T 1 1 I 1 I 1 I I I r OBSERVED OCEAN CURRENTS FOR JULY 0 90E 180 I I._.. I I II-I -I-II-I-II -, I a I I I I I I III- I II -I 9ON -I I I I I I I I I I T F - I I I I I I I 14 40N -- -,. * ^ ' L^ < X :: -- v ;s L - ^^ r/ _\r y ^/ * N -A^ i 7io0t \ \ ;-*- / * ^ \ (Tf/ rt ' ^ i ^ CM/SEC _ t O r / > ,.- D \ _ N, 5 - _/ e, ^ ^», / 1 N / 1 i-nm / ^ 40N 0 4 t~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~ t 40S 9 0 S I I I I t I I I I I I I I I I -I -i I I I q O F I I I - I., W u uvr_ ]90S Ig0os 180 QnF~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~l I % Fig. la Digitized ocean current data set represented by vectors on a 5 grid, length proportional to speed in cm/sec.

18 I 1 90E IE180 9ON ON CM/SEC I t ON N 0 4( OS -7 j j I N N - 0 PI S ";zl 9 one ( L - I - I - I..... Hill 9 Gow I I I I I I- I I I I I I I I I I I I I I I I I I I 1-1 I I I I I I I I I I I I I I I I I I -I-A- I I I I I I I I I I I I I I I I I I I I 90S 90E if180 OBSERVED OCEAN CURRENTS FOR OCTOBER 4ON 0 1% V 0 15 CM/SEC JR, 4ON Fig lbdigitized ocean current data set represented by vectors on

19 12 OBSERVED OCEAN SURFACE CURRENTS - JANUARY 90W 0 90E OBSERVED OCEAN SURFACE CURRENTS- JULY Fig. 2a Ocean surface current streamlines analyzed from ocean current data digitized onto a 5 grid from pilot charts. Dashed streamlines south of 50 S indicate only current direction was available from pilot charts.

20 1 3 OBSERVED OCEAN SURFACE CURRENTS - APRIL OBSERVED OCEAN SURFACE CURRENTS-OCTOBER Fig. 2b Ocean surface current streamlines analyzed from ocean current data digitized onto a 50 grid from pilot charts. Dashed streamlines south of 50 0 S indicate only current direction was available from pilot charts.

21 14 SURFACE CURRENT SPEEDS CM/SEC -JANUARY 90W 0 90E SURFACE CURRENT SPEEDS, CM-SEC-JULY Fig. 3a Ocean surface current speeds in cm/sec from data digitized onto a 5 grid. Peak current core velocities indicated by arrows are sub-grid scale maximum speeds from original pilot chart data. Velocity data south of about 50 S do not appear on the pilot charts. Stippled areas indicate no velocity data in present OCDS.

22 15 SURFACE CURRENT SPEEDS, CM/SEC -APRIL W 0 90E ON pr I I II III II I I II II III II II II I I III II II II II III I I II II 11 III II I I I.IIIIII I I I I... ~ s.-i... I I.... I : ,. -1 ""\ '**- ^^ ^ **^ :! C 20\:.,< ^ )...* ( /""-jr60'***.**.*..,..-***).).. ^-A--....'.. S. \**..: *....~., 40N -20N~~~~~~~~\y\ l<9 ~ ~~~...- *., "***\ *.,if..^^ A 40N l t I ' V - (5 ^.129.ll 144 M4 4 4 I ' I. ~.. ~I uvn 1 I I... M e k I I I II I I I I1 I 0 (20 IC /I ks40 7i...0.::: :,:p\ i < /^.. ^>'40^6 <5R?(B-^^ V.^~4 0 40S 40S -duv r I I II I IIIII[ L it '' I L II I 'L111' QnAC ' ' I ' II 11' I'l' I - I.. ' I... III III I 9OW U 9U0 l10 SURFACE CURRENT SPEEDS, CM/SEC -OCTOBER Fig. 3b Ocean surface current speeds in cm/sec from data digitized onto a 5 grid. Peak current core velocities indicated by arrows are sub-grid scale maximum speeds from original pilot chart data. Velocity data south of about 50 S do not appear on the pilot charts. Stippled areas indicate no velocity data in present OCDS.

23 Fig. 4 Day-by-day position plot of drifting buoy 470 in South Atlantic between 31 and 35 south latitude and 358 and 5 east longitude (Harris, 1978). 16

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25 18 u COMPONENT, JANUARY u COMPONENT, JULY Fig. 5a u component ocean surface currents in cm/sec; positive values are eastward.

26 19 u COMPONENT, APRIL 0 u COMPONENT, OCTOBER Fig. 5b u component ocean surface currents in cm/sec; positive values are eastward.

27 20 v COMPONENT, JANUARY v COMPONENT, JULY Fig. 6a v component ocean surface currents in cm/sec; positive values are northward.

28 21 v COMPONENT, APRIL v COMPONENT, OCTOBER Fig. 6b v component ocean surface currents in cm/sec; positive values are northward.

29 22 OCEAN CURRENT VARIABILITY Fig. 7 Surface current reversals between January and July, April and October. A reversal is defined as change of direction between two seasons. Solid arrows indicate current direction for January, dashed arrows for April. Currents flow 150 to 210 in opposite direction in July and April, respectively.

30 23 Fig. Al Geostrophic current at sea surface relative to the 1000 dbar level calculated from dynamic height derived from 1 latitude by 2 longitude grid point oceanographic data (Gordon et al., 1978).