TR-70 TECHNICAL REPORT ASWEPS REPORT NO. 7 SEA SURFACE TEMPERATURE SYNOPTIC ANALYSIS BLAIR W. GIBSON Forecasting Branch Oceanographic Prediction Division APRIL 1962 yy; U.S. NAVY HYDROGRAPHIC OFFICE WASHINGTON 25, D. C. ‘ T43, Price 55 cents ACB Sih RAI ii A technique is described for preparing detailed sea surface temperature analyses for large ocean areas. These analyses utilize injection temperature observations taken by commercial ships. The inadequacies of analyses based on averaged data and some difficulties inherent in contouring scalar fields are discussed. Sea surface temperatures are interpreted according to some concepts derived from cross-sectional profiles and surface current data. Isotach analyses of mean current drift are considered as flow patterns to aid temperature analysis in areas where data are sparse. FOREWORD A practical presentation of the daily sea surface temperature field in the form of contoured charts is of wide general utility and has specific applications in naval operations, as well as com- mercial enterprises. In particular, knowledge of the sea surface temperature at a given locality is used in the AntiSubmarine War- fare Environmental Prediction System (ASWEPS) being developed by the Hydrographic Office Rear Admiral, Hydrographer MBL/WHO! TNT 0 0301 0041302 7 i Lii FOREWORD . . . CONTENTS e e e e 2 e e e e e e e e ° e e e e e @ e e e e e I e INTRODUCTI ON e e e e e e ° e e 7 e e se e e e e e e e e e e e aA TAMU VALUA LONI sMelieiel (eliieiile) ie) toll lel telteliiell ei ilelt clliciieiaiieiienite III. SOME CONSIDERATIONS OF THE NATURE OF THE SEA SURFACE . . « « IV. ISOTACH PATTERNS OF MEAN CURRENTS AND THEIR APPLICATION TO TEMPERATURE ANALYSES . « » « © « « © © © « « VY. TEN-DAY COMPOSITE TEMPERATURE CHARTS . « »« © oe 0 e we wo VI e CONCLUSIONS e e ° e ° e e e e e e e e es e e e e e e e e e e BIBLIOGRAPHY . ILLUSTRATIONS Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure ll. Figure 12. e e e e e e e e e e e e e e ® e e e@ e e e e e e e Possible Misrepresentation of a Stable Temperature Field by MeanwChartsiverellclise) ie) ollelte Two Interpretations of a Temperature Field ErOnmiGentd Calaia GAM silcmrsilenlenicnenicliieliiel elle lelleiie Surface Temperature (Wave) Patterns Deduced from Different Data Distribution in Alternate Parallel Currents in the Same Area on Successive Days . e Temperature Profile Along the 50th Meridian South of the Grand Banks eeev3vev3w#w#eeee ¢ e@ Schematic Diagram Indicating Balanced Flow Through the Gulf Stream and Adjacent Waters . e« e Isotachs for the Caribbean Sea and Gulf Of Mexteal— OCLODET ells) \eleliie) ie) 1 e ellic! elle) elletre Composite Surface Temperature Analysis for the Caribbean Sea and Gulf of Mexico Mean Sea Surface Temperature February 1961 Compared to Mean Current Drift February 1935-1945 Temperature and Salinity Profiles Across the Calliforniial Currents) « « «6 «) «© © «6 «le «6 Composite Sea Surface Temperature Chart AS apie o 5 Jape USL 6 5 4G oOo OO Composite Sea Surface Temperature Chart Gap wAUSUStELOO Lm eentciell ciicnisiiomellclroiourciiel’c Composite Sea Surface Temperature Chart T6=CopApril TOOL) io et eilerielie et suis de siete: 6, 6 « 15 17 No) oOo au We) 14 16 Te Th : je) a = 4 7 =» I. INTRODUCTION In connection with the development of an oceanographic prediction system (ASWEPS), the Hydrographic Office is currently preparing exper- imental sea surface temperature charts. These charts cover a small area of the western North Atlantic and are prepared for periods of 1, 10, and 30 days. Sea surface temperature analytic techniques and theo- ry are discussed in the following sections. The isotherms of long-term mean sea surface temperature charts are generally west-east oriented with values increasing toward lower lati- tudes. Im these respects, they resemble mean isobaric patterns derived from large-scale migratory atmospheric systems. Moreover, apparent agreement between orientation of mean isobars and resultant ocean cur- rent drift suggests that water temperatures may be related to wind-driv- en currents. Since the patterns of mean charts depend only on resultant values averaged over specified unit areas, they do not necessarily rep- resent the true nature of a given field. This would be especially true if the systems comprising a particular field were narrow, elongated bands separated by sharp gradient zones (Figure la), and the dimensions of the unit areas on which mean charts are based were relatively large when com- pared to the width of the systems (Figure 1b). @ ASSUMED STABLE CURRENT PATTERN IN ADJOINING ONE-DEGREE I | QUADRANGLES | | b MEAN TEMPERATURES DEDUCED FROM DATA IN THE ABOVE QUADRANGLES FIGURE | POSSIBLE MISREPRESENTATION OF A STABLE TEMPERATURE FIELD BY MEAN CHARTS Consecutive daily sea surface temperature charts show frequent abrupt changes in their isotherm patterns. Experience indicates that the major- ity of such changes arise from data errors, data distribution, and the analyst's concepts of temperature variation in the sea. Indeed, identical data on successive charts can be variously interpreted (Figure 2). (A) (B) FIGURE 2 TWO INTERPRETATIONS OF A TEMPERATURE FIELD FROM INDENTICAL DATA If one assumes the surface system to be alternate parallel warm and cold water bands (Figure 3), data distribution on successive charts may Hy ry W yj= FIRST DAY SECOND DAY FIGURE 3 SURFACE TEMPERATURE (WAVE) PATTERNS DEDUCED FROM DIFFERENT DATA DISTRIBUTION IN ALTERNATE PARALLEL CURRENTS IN THE SAME AREA ON SUCCESSIVE DAYS give rise to an erroneous temperature wave, the period of which would be dependent on the chart interval. If the plotted values in Figure 3 had actually resulted from a meandering current, they could just as well be interpreted as having arisen from straight currents. II. DATA EVALUATION Quasi-synoptic sea surface temperature charts for extensive ocean areas are of necessity based on injection temperature observations re- ported at 6-hour intervals to various meteorological organizations by commercial ships. Injection intake depths normally range between 20 and 25 feet, whereas bucket readings are taken near the air-water boundary. Assum- ing the thermometers are accurate, injection readings will be relatively low when the vertical temperature gradient is negative, relatively high when the gradient is positive, and equal to bucket readings when the water is uniformly mixed. Choice of temperature recording instruments obviously must be com- patible with planned use cf the data. The depth of the heated layer in cold currents varies from zero to roughly 200 feet, depending on lati- tude and season. Therefore, again assuming the thermometers are accu- rate, bucket temperatures are less suitable for outlining ocean currents during summer than injection temperatures; the latter fail completely between 15 and 20 degrees of latitude either side of the Equator. Teme peratures recorded by bucket thermometers in the heated layers of cold eurrents at low latitudes may slightly exceed those of warm currents. At low latitudes the current systems are apparently well outlined by isotach analyses - surface speeds of water masses being transmitted from below the heated layer. About 20 percent of injection temperature data is estimated to con- tain gross and minor errors from all causes. Sample checks of ship weather logs indicate that approximately 13 percent of these errors arises through processing of water temperatures for transmission in terms of surface air temperature and the air temperature minus sea temperature difference factor, D. Gross errors caused by incorrect difference factor signs occur near coastal areas and in the vicinity of warm and cold cur- rent systems where D values may be large. These errors result in water temperatures 2 x D too high over cold waters or 2 x D too low over warm waters; therefore, they are an additional cause of abrupt changes in iso- therm patterns. Such data errors are difficult to detect with certainty, because sharp horizontal temperature gradients (possibly as much as 8° F per mile) can be associated with oceanic currents. IIT. SOME CONSIDERATIONS OF THE NATURE OF THE SEA SURFACE Analyses of weather charts are facilitated by employing wind vectors to show isobaric spacing and orientation. Conversely, both salinity and current information are seldom available to aid construction of sea tem- perature charts. Experience indicates, however, that meaningful charts can be realized by employing injection temperatures according to a pre- scribed technique. As will be presently more evident, a number of important current systems with numerous tongues are present in the sea along boundary zones of waters of different origin. Because of the limited width and complex configuration of these tongues, the waters associated with oce- anic currents are more difficult to delineate than large-scale systems encountered in weather analyses. Figure 4 presents a vertical temperature cross section and surface current data along the 50th meridian. These data and sections for other ocean areas (not shown) form the bases for the analytical approach de- scribed below. Symmetrical undulation of the isotherms indicates four major water masses. Upon crossing each mass the surface current changes direction in an orderly manner; that is, the circulation is cyclonic for cold waters and anticyclonic for warm waters. There is also general agreement be- tween the magnitudes of temperature gradients and current velocity. If V is the surface current velocity, K a vertical vector, positive out- wards, and VY T is grad T, the relation V= K crossVT holds in principle. This relation, analogous to that which applies for straight air flow, suggests that water bands can be treated as greatly elongated air masses. Vertical symmetry manifested by the isotherms in Figure 4 is typical of temperature cross sections taken over wide areas of the North Atlantic Ocean, and indicates that wave action and meteorological conditions do not disturb deep current systems which regulate distribution of temper- ature and related physical properties in the sea. Temperature structure (including surface temperature) at any given time in the upper layers apparently depends on the origin and history of the masses as indicated by the two northernmost water masses (Figure 1) which lie in practically the same climatic environment. The forces expected to be present at various points in interdepend- ent warm and cold current systems are schematically shown in Figure 5, along with an appropriate velocity profile which is placed on the sea surface for convenience. If air masses were substituted for water bands in this figure, a similar velocity profile would be expected. Elonga- tion of the body of uniform warm water can be attributed to frictional drag due to high velocities in the northern transition zone. Under these conditions, the potential width of the uniform water, determined by frictional drag from the stronger transition zone, would decrease with increasing slope along the opposite boundary of the warm water. Warm waters flowing zonally in the tropics are subjected to long periods of maximum insolation. This results in mixed-layer character- istics which tend to be conserved by convective mixing at higher lati- tudes. Cold waters from higher latitudes, however, undergo short-term h SYNV8 GNVYS 3SHL JO HLNOS NVIGIYSW y,OS SHL ONONW 31Id0Nd (Jo) SUNLVYESdIWSL % AYNDIS OS6/] ¥Y3G0190 1OHM WV3SULS JINS SIN3YYND ,WYVM,, yodvysv 1 HiNOoS <> H LYON S3TIW WOILLAYN SS) O€ fe) i=) m a°) 4 =x 2 = m 4 m D n ltl 0 ¢-|\ \ROS ee ei 46 LL +S 8€ VE Zp | Ze SE BE 62 EI | 93S/W ‘ALIQOT3A GNV NOILO3YIG IN3YUND 3dvVsuNS KY 9Ll¢ Liv Blip 6LIv o80r rit zelv cely vely Sele SUBEWAN NOllvis (Ss) WEST (-) Foor NS -vp a“ a -Vvp Sa (€+8) g sis --G Ay ee V/ Ae JET \ —vP N \ SS Ye WEST (-) as Ge SOUTH EAST (+) COLD WATER Transition Zone WARM WATER Transition Zone COOL WATER EAST (+) LEGEND V- VELOCITY € — ENTRAINMENT FORCE fo — CORIOLIS FORCE ~V |) — PRESSURE GRADIENT FORCE __--. SURFACE VELOCITY PROFILE FIGURE 5 SCHEMATIC DIAGRAM INDICATING BALANCED FLOW THROUGH THE GULF STREAM AND ADJACENT WATERS (seasonal) periods of warming and cooling near the surface and tend to reach extremes in their mixed-layer structure. Cold waters at low lati- tudes tend to lose their identity because of surface heating. During all seasons, however, the surface positions of warm and cold currents in the North Atlantic are ordinarily discernable from injection temperatures. Because strong negative vertical temperature gradients exist in cold wa- ters during the warm season, upwelling does not satisfactorily explain the continued presence of relatively cold surface water in the open ocean. It appears rather that relatively cool waters mark the surface position of cold currents where advection occurs below the heated layer. IV. ISOTACH PATTERNS OF MEAN CURRENTS AND THETR APPLICATION TO TEMPERATURE ANALYSES In areas where data are sparse or unevenly distributed or in unfamil- iar areas where flow patterns are unknown, surface temperatures covering extended periods should be compared with mean isotach patterns. This pro- cedure is especially helpful where currents are apparently constrained by topography; for example, in inland seas and near coastal areas. Studies indicate that high temperatures are related to high isotach values and deep water. An isotach analysis for the Caribbean Sea and the Gulf of Mexico is shown in Figure 6. Isotach configurations were employed as warm and cold flow patterns for interpretation of the temperatures on the composite chart (Figure 7). A series of temperature charts based on the configurations in Figure 6 show only minor pattern changes over a period of several months. Figure 7 indicates an overall surface temperature range of about 9° F even in these latitudes; temperature contrasts be- tween adjacent waters are expected to be appreciably greater below the depth of wave mixing. Although significant details of the circulation are lost by averag- ing temperatures and drift values over one-degree quadrangles, Figure 8, in which February drift values (1935-45) are compared with mean isotherms for February 1961, shows that low temperatures correspond to minimum drift and vice versa. Of particular interest is evidence of counter cur- rents south of the 32nd parallel. In connection with the temperature pro- file and currents off the California coast (Figure 9), it will be noted that, except at the extreme western end of the section where the warm axis is slightly displaced to the right of a marked salinity minimum, currents change direction near the maximal and minimal portions of the trace in agreement with the relation discussed on page }. Since mean drift values are ordinarily computed for one-degree quad- rangles over long tine intervals, only the more gross and permanent fea- tures of the circulation are indicated by them. This may be especially true in the deep ocean where current systems are not topographically con- strained. Effectiveness of isotach analyses should be greatly enhanced when data density permits averaging of drift values over much smaller unit areas than the present one-degree quadrangle. For example, com- putations of mean drift for a portion of a permanent warm tongue (Figure la) are low, because drift values, although high along its boundary, are oppositely directed. pg 7 VA a oe \ o y Ga fas wr 9 MEXICO Y (b ie \P Q, UNITED 25 $e X Se a STATES LEGEND 23 NUMBER OF OBSERVATIONS PER ONE-DEGREE QUADRANGLE ~<1_ RESULTANT SET 86 RESULTANT DRIFT “S (MILES PER DAY) FIGURE 6 ISOTACHS FOR THE CARIBBEAN SEA AND GULF OF MEXICO (OCTOBER) O,, UNITE D STATES LEGEND © 27,20, 29 OCT A 30,31 ‘OCT x< 2,5 NOV FIGURE 7 COMPOSITE SURFACE TEMPERATURE ANALYSIS FOR THE CARIBBEAN SEA AND GULF OF MEXICO BASED ON FIGURE 6 2 LEGEND 69.2 MEAN SEA SURFACE TEMPERATURE (° F) 7 CURRENT SET 6 CURRENT SPEED (TENTHS OF KNOT) 1935—1945 74.5 73.0 WP USD TOS ee Nt PA == 7.5 76.8 eee ie FIGURE 8 MEAN SEA SURFACE TEMPERATURE FEBRUARY 1961 COMPARED TO MEAN CURRENT SPEED FEBRUARY 1935-45 10 SLN3SYYND VINYOSINVO SHL SSOYDV S3ANSOUd ALININVS ONY 3YdNVeadWal 6 3YyNndls (9¢61 SHON WOU) OE oZI1 ,00 o€€ ,00 o8! Of o8 ll 00 o6l! O€ 06! 00 002! PS i ‘| SLNAWS 19 AES | / INAH ND Y¥3.1NN09 eat OF oce OF off IN3YYND VINYOSINV9 LN3YYND VINYOSIIVO! 2 imei meee FX N / eis Meza aie Aare | i] a / iV WNIIWLV9 | 1 SYTIOOIN nush\ (SANIVA HOIH OL MOT) LA —. LA\LA LA LAL LAj1A LIN3SIGVYS dWal —— | —<— <= => > x1 > > |< 14334 NI Hidad hal V. TEN-DAY COMPOSITE TEMPERATURE CHARTS Ten-day composite charts should be sufficient for determination of general temperature patterns in areas where the axes of currents are fairly constant. In practice, 10-day composite charts are drawn every 5 days on an overlapping basis. This procedure allows for a relatively large number of reports over small time intervals. Daily sea surface temperature charts utilize the latest composite chart or daily pattern as a guide. The prime object of composite charts is provision of means for achieving continuity between successive iso- therm patterns. In order to conserve space and provide visual data con- trol, the following symbols and color code are used on composite charts. Ten-Day Composite Chart Data Code Day Symbol Color Temp. (° F) lst through 5th ee 3 Purple oe e@ Oo < 30 6th ©) lemme) ese. -osves 0 O Light Blue eo e 30 = 39 Green «2 © © o ho = kg Black . « « e 50 - 59 re ee Pi (5) (5) @ ct roa Ko ee <) Purple . -« -« « 70 = 79 GMS 6 5 9 60 YO) = ES) Green «2 e« o eo >90 Examples: 55-4° F for the kth day is coded as °5.4 in black 74° © for the 7th day is coded as A in purple Date indicators serve to show daily variations of water temperatures; the color code minimizes data congestion, aids in scanning large quantities of data, and helps to screen out gross data errors. The last 5 days of data on a 10-day composite chart are carried forward to the next composite chart using dot symbols; the remaining 5 days of data, accumulated from daily charts, are added by using individual symbols as shown in the preced- ing table. (The last 5 days of data on an 11-20 August composite chart would be transferred to a 16-25 August chart using dot symbols, the re- maining days would carry individual symbols. ) Portions of two composite charts for 26 July-5 August and 16-25 August 1961 are shown in Figures 10 and 11. General agreement of the patterns in these charts suggests that oceanic current systems are more stable than ale) 1961 LSNONY S—AINC 9% (40) LYVHD SYNLVYSdW3L JOVSYNS VSS 3LISOdWOD O} SYNDIS - (21. a9vd NO NOllwInav. 33s Wes SIOGWAS 40 NOILWL3YdY3LNI HOS) ee ° - FFONS .@ 13 IS961 LSNONY S2—-91 (se) LYVWHD SYNIVYSdWSL JOVIYNS VSS SLISOdWOD I! 3YNDI4 3) = b Ko. 7-5 Un five (‘21 39Vd NO NOILVINE@VL 33S ¢ “SIOSWAS JO NOILVLSYdYSINI YO4) é 14 ° 7 SS sf previously believed. Indeed, many of the variations may be due to data errors. However, the fact that data from a large number of reporting ships form definite patterns indicates that injection temperatures are of operational use. Figures 10 and 11 show numerous warm and cold water tongues origi- nating from four different current systems (excluding coastal waters). It is noted that a tongue is surrounded by water of approximately the same temperature; whereas a current is flanked by waters of different temperature. The southerly flow of cold water along the eastern edges of warm tongues would result in increased gradients at intervals along the main body of warm water and apparently contributes to the wide dis- tribution of warm and cold waters. A succession of many charts is usually required to establish a reason- ably stable temperature pattern. Winter conditions are most favorable for reslization of this end, because temperature contrasts and mixed layers are greatest during this season. Patterns indicated by composite charts are considered to represent only approximate envelopes of the major current systems responsible for distribution of surface temperature in the sea. Figure 12 presents an enlarged portion of a composite chart for 16- 25 April 1961. The data, plotted in the usual manner, include a rela- tively large proportion of bathythermograph surface temperatures in whole degrees and tenths (Fahrenheit) which may be compared with injec- tion values. This figure shows the remarkable symmetry of data of both sources over a 10-day period in an area of complex, small-scale current features. VI. CONCLUSIONS Although present analytical techniques permit preparation of repre- sentative sea surface temperature charts, much improvement is required in the quantity and quality of reports from the synoptic observational net. Continuation of the present analysis program and expansion of the coverage to the entire North Atlantic and North Pacific are planned. 15 75° W LEGEND | ISOTHERMS IN DEGREES F | SURFACE OBSERVATIONS FROM | BATHY THERMOGRAPH UNDERLINED 75° W 74° 73° Ue 7I° 70° W FIGURE 12 COMPOSITE SEA SURFACE TEMPERATURE CHART (° F) I6—25 APRIL 196I 16 BIBLIOGRAPHY COMMITTEE ON UNDERSEA WARFARE OF THE NATIONAL RESEARCH COUNCIL. ‘The Application of Oceanography to Subsurface Warfare, National Defense Research Council Summary Technical Report. 106 p. Reprinted March 1951. FUGLISTER, F. C., and WORTHINGTON, L. V. Some Results of a Multiple Ship Survey of the Gulf Stream, Woods Hole Oceanographic Institution Technical Report No. 18. 19 p. 1951. 17 ‘ Ese i sts nul sh ai iat " Bins Re ies ae ay, a aripealnng ore OL-aL °O °H aosqty) °M IfeTd -sou ny stsfTeuy oTz.douks emyeredmay, SoBJMS Bes :oT7T. 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