PHYSICAL PROPERTIES OF SEA WATER - TEMPERATURE AND SALINITY

Transcription

1 NAME PHYSICAL PROPERTIES OF SEA WATER - TEMPERATURE AND SALINITY I. Sea water temperatures The distributin f surface temperatures fr the majr ceans is shwn in Figure 1. The istherms (lines f equal temperature) te t deflect twards the equatr n the eastern sides f the ceans a twards the ples n the western sides f the cean. This is due t the majr surface circulatin pattern f the ceans. Warm water frm lw latitudes is transprted twards the ples n the western sides f cean basins a cld water frm high latitudes is transprted twards the equatr n the eastern sides f cean basins. Figure 1. Distributin f surface cean temperatures ( C) fr the mnth f August. Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p. 73. Vertically the ceans can be divided int tw reservirs, a warm surface reservir a a cld deep water reservir. These tw reservirs are separated by a zne f rapidly changing temperature which is referred t as the permanent (r main) thermcline (Figure 2). The temperature f the tp (mixed) layer varies seasnally in respnse t variatins in slar heating. The slid curve shws the winter citins in the mixed layer a the dtted curve shws the seasnal thermcline that exists during spring warming. Nte that this seasnal thermcline is cnfined t the mixed layer. The dashed curve is the seasnal thermcline in extreme summer citins. The permanent thermcline is nt fu at high latitudes where the surface waters are very cld a there is little variatin in water temperature with depth. It is in these high latitude regins that cld surface waters can sink t depth. This prcess is respnsible fr the vertical circulatin f cean waters, called thermhaline Figure 2. The main thermcline. Slid line shws the winter thermcline a the dashed a dtted lines shw the summer thermcline. Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p

2 circulatin because density differences due t temperature a salinity differences are respnsible fr the circulatin. Tw ther terms, parallel t the term themcline, are used in ceangraphy. The halcline is the zne f rapidly changing salinity between relatively lw salinity surface waters a deeper mre saline waters. The pycncline marks a zne f rapidly changing density. Upwelling, the mvement f deeper water twards the surface a dwnwelling, mvement f surface waters t greater depth, are imprtant prcesses in the cean. Upwelling a dwnwelling can ccur lcally in castal waters r n a larger scale invlving significant vlumes f cean water. During upwelling deeper cld water is brught twards the surface a istherms deflect upwards. During dwnwelling warm surface water is mved t greater depths a istherms are deflected dwnwards. There are five basic types f upwelling (Figure 3). Nte that dwnwelling is simply the reverse f these prcesses. Fr example, if the wi is blwing nshre, rather than ffshre (Figure 3b), dwnwelling will ccur. Figure 3. Diagrams f the varius types f upwelling. Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p Ekman-transprt upwelling. Wi blwing acrss the surface water sets the water int mtin. Because f the Crilis effect water in the Nrthern Hemisphere is deflected t the right f the wi directin a water in the Suthern Hemisphere is deflected t the left. Sme f the mmentum f the mving surface waters is transferred t the deeper waters setting them in mtin. As we g dwn in the water clumn the mmentum transfer decreases a at sme depth the water mtin is zer. At all the depths abve this the water is deflected t the right (NH) r left (SH), the slwer the water mvement the greater the amunt f deflectin. This variatin in directin f water mvement with depth is referred t as the Ekman spiral (Figure 4). If we sum up the ttal water mvement thrughut the Ekman spiral the net mvement is at 90 t the surface wi. Referring t Figure 3a, yu are in the Nrthern Hemisphere a the wi is blwing t the nrth (a sutherly wi since wis are named fr the directin frm which they cme). The net mvement f water is t the right away frm -2-

3 the shre. The ffshre mvement f the surface water is ffset by the mvement f deeper waters t the surface, i. e., upwelling. 2. Wi-driven upwelling. In this case an ffshre wi causes water t mve away frm the castline. This water is replaced by deeper water mving t the surface (Figure 3b). 3. Open-cean Crilis-effect upwelling. The divergence f surface currents causes deeper waters t mve t the surface replacing surface waters that mve away frm the zne f divergence (Figure 3c). 4. Obstructin upwelling. A current mving past a headla r ther bstructin will draw water away frm the bstacle a upwelling will ccur (Figure 3d). 5. Density-driven upwelling. Cld denser surface waters sinking t depth frce less dense waters t the surface (Figure 3e). 1. Figure 5 shws a number f surface temperatures taken as part f a prgram knwn as the Organizatin f Persistent Upwelling Structures (OPUS). The investigatin centers n a regin f intensified upwelling between Pint Cnceptin a Pint Arguell, Califrnia. Figure 4. Diagram illustrating the Ekman spiral. (a) A bdy f water can be thught f as a set f slabs, the tp ne driven by the wi a each ne belw set in mtin by mmentum transfer. With depth each layer mves at a slwer speed. (b) Vectrs shwing water mtin as a functin f depth. Average waver mvement is at 90 t the surface wi directin. The example is set in the Nrthern Hemisphere. Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p. 73. a. Cntur the surface temperatures at a 0.5 C cntur interval. It is suggested that yu cntur whle degrees t start, then fill in half-degree intervals by interplatin. b. Explain the temperature pattern fu ff Pint Cnceptin. -3-

4 Figure 5. Surface temperature pattern ff Pint Cnceptin, Califrnia, taken n April 14-15, Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p Figure 6 is a temperature crss sectin ff Pint Cnceptin. Cntur the prfile dwn t the 8 C istherm using a 0.5 C cntur interval. It is suggested that yu begin cnturing at the bttm (8 C istherm) a wrk upward. a. What prcess is iicated by the shape f the cnturs at shallwer depths? b. Based n the appearance f the cnturs, what seems t be the mtin f the deeper water? c. Cmpare the temperature map a crss sectin t the upwelling prcesses shwn n Figure 3. What type f upwelling best describes what is taking place at Pint Cnceptin? Why? -4-

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6 Figure 6. Vertical temperature prfile ff Pint Cnceptin. Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p

7 3. Figure 1 shws the distributin f surface temperatures in August in the majr cean basins. With reference t this figure a. Where is water f the greatest density frmed? Explain yur answer. b. Hw wuld a depth-temperature (BT) curve frm the Arctic regins differ frm ne at mid-latitude r ne frm the equatrial regins? Explain yur answer. c. Why is water s much cler alng the west cast f Nrth America than at equivalent latitudes n the ppsite side f the Pacific Ocean? II. Sea water salinity Salinity is defined as the amunt f disslved material in grams per kilgram f sea water when all the idine a brmine have been replaced by chlrine, the carbnates have been cnverted t xides a all the rganic matter has been xidized. Salinity varies with latitude (Figure 7). The majr prcesses respnsible fr these variatins are evapratin, precipitatin a mixing. Where evapratin exceeds precipitatin salinity values are high. Where precipitatin exceeds evapratin salinity values are lw. Ishalines are lines f equal salinity. -7-

8 Figure 7. Salinity distributin in the surface waters f the ceans in August. Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p Study the ishalines n Figure 7 a answer the fllwing questins. a. Hw des salinity vary frm the equatr t plar regins in the Pacific Ocean? Give exact values a general latitudinal znes. b. Which f the tw ceans shwn is saltier a by what amunt? Hw might yu explain this? (Hint: Relate t the majr wi belts.) c. Explain the tngue f lw-salinity water exteing frm Baffin Bay west f Greenla t the eastern cast f Canada. (Remember that it is summertime in the Nrthern Hemisphere.) -8-

9 An imprtant cncept in ceangraphy a marine gechemistry is that f residence time. Residence time, R, is equal t the ttal quantity f a substance, C, in a particular reservir divided by the rate f additin, A, f the substance: R = C/A. Hence residence time is the mean length f time an atm, mlecule, r particle f a given substance spes in a reservir. Highly reactive substances have shrt residence times, a vice versa. There are several primary assumptins in the cncept: (1) that the substance is thrughly mixed in the reservir, a (2) that the rates f supply a remval are cnstant ver at least several residence times. 5. Using the data in Table 1, calculate the residence times fr water, disslved salt a sdium in the ceans. Table 1. Data fr residence times calculatins Water Disslved salt Sdium Oceans (C) 1.4 x 10 kg 5 x 10 kg 14 x 10 kg Rivers (A) 26.4 x 10 kg yr 2.7 x 10 kg yr 15 x 10 kg yr a. Residence time fr water in years. b. Residence time fr disslved salt in years. c. Residence time fr sdium in years. -9-

10 6. A series f statins ff the Califrnia cast have been pltted n Figure 8. Cntur the values f the O salinity at intervals f 0.5 / (that is, at 32, 32.5, 33, etc.) Shade in the areas where the salinity is greater O O than /. Briefly explain the high-salinity statin (34.00 /) shwn near the entrance t San Francisc Bay. Figure 8. Surface salinity bservatins (in parts per thusa) at 37 statins ff the cast f suthern Califrnia a Baja Califrnia, summer Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p

11 III. Temperature-salinity diagrams a the identificatin f water masses A water mass is a large vlume f water that can be identified as having a cmmn rigin r surce area. Water masses are frmed thrugh interactins with the atmsphere r by the mixing f tw r mre bdies f water. Because mixing between water masses a their surruings is slw, water masses te t retain their riginal temperature a salinity. These distinctive temperatures a salinities can be used t identify the water masses. This identificatin is imprtant because it gives us infrmatin abut their place f rigin, deep circulatin, a the rates at which waters f different densities mix. Table 2 lists the characteristics f the water masses fu in the Nrth Atlantic Ocean. Table 2. Water masses f the Nrth Atlantic Ocean Water Mass Surce Identifying characteristics Antarctic bttm water Weddell Sea Bttm temperature minimum (AABW) Nrth Atlantic deep water Area near Greenla Intermediate salinity maximum, may shw (NADW) intermediate temperature maximum Surface-water masses Reginal Variable, generally warm Mediterranean intermediate Mediterranean Sea High salinity a temperature layer at water (MIW) ff Turkey intermediate depths Table 3. Oceangraphic data frm a Nrth Atlantic statin at 20 N Depth (m) T ( C) O S ( /)

12 7. Oceangraphic data frm a typical statin in the Nrth Atlantic lcated at abut 20 N latitude are listed in Table 3. Plt these data n Figure 9 a draw a line cnnecting all the pints in the rder f depth. Using the diagnstic parameters in Table 2, name the majr water masses represented in the diagram. Figure 9. Blank T-S diagram fr questin 7. Plt the data frm Table 3 n this diagram a draw a line cnnecting all f the pints in rder f increasing depth. Frm Pipkin et al., Labratry Exercises in Oceangraphy, 2 Ed. New Yrk: Freeman, p