Mixing in the Barents Sea Polar Front near Hopen in spring

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365 = 12.6 ± 4.5 as the mean and standard deviation over the 6 data points. 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 Of the stirring terms, wind dominates for x< 45 km where the ratio of the mean tide and wind stirring terms is R wind-stir = 0.1. Toward the bank the contribution due to tidal stirring increases, and on the bank for x>70 km they are of comparable magnitude with R wind-stir = 0.9. Melting increases the potential energy anomaly by 4.710 -5 W m -3 , approximately 4 times that due to heating. For a reduced melt rate of 0.5 m per month, the melting term still dominates with a contribution twice as much as the heating term. The stabilizing components due to heat and melting are about 5 times the mixing contributions from wind and tidal stirring (3.2 times for a reduced melt rate of 0.5 m per month). When tidal straining is included, assuming that it destabilizes the water column in the ebb half cycle, the positive and negative contributions nearly balance: the ratio of stabilizing and destabilizing terms are 1.5 and 0.9, respectively, for melt rates of 1 and 0.5 m per month. We conclude that the ice melting dominates over heating in increasing the potential density anomaly. Stirring by winds and tides, on the bank, is of comparable magnitude but not sufficient to completely mix the water column. Tidal straining, periodically acting to reduce the stratification, dominates over the wind and tide stirring, complements the stirring to overcome the positive induced by melting and heating. A tidal front is thus maintained. 382 383 384 385 386 387 6. Implications for nutrient fluxes Nitrate profiles collected at different times but at the same location show significant variability that can be explained by tidal stirring and straining. Figure 10 shows the occupation time, relative to the tide, of selected stations near the bank that were visited three times, twice for nutrient sampling and the third time for microstructure profiling. The tidal velocity was inferred at about x = 70 km, close to the station 222/249/259, and had a peak-to- 18
388 389 390 391 392 393 peak amplitude reaching 1 m s -1 during spring tides. The station locations on the section are shown in Figure 10c together with the isopycnals. The water column on the bank, for x>80 m was characterized by a strong horizontal density gradient, rather homogeneous throughout the water column. Stations 218 to 222 were collected on floods during spring tides. While most of the remaining profiles were collected also during flood periods later in transition to neap tides, stations 249 and 259 were taken during ebb periods. 394 395 396 397 398 399 400 401 402 403 404 Pairs of nutrient profiles for each repeat station are shown in Figure 11. There was no significant difference in the vertical structure of profiles at 220 and 247, occupied during floods. At 247, occupied about 4 days layer, the nitrate concentration was higher below the depleted zone, suggesting vertical transport of nutrients consistent with D mixing > D mixed episodes observed at this site. This, however, could have been a result of temporal variability, advection or internal wave heaving of density surfaces. The difference between the profiles of 218 and 245 was more significant. The location of this station is closer to the region where the mixing depth did not penetrate below the mixed layer depth (x < 23 km). We propose a mechanism whereby high nitrate concentrations at the base of the pycnocline are transported with the T and S anomalies along the isopycnals which deepen toward the PW side of the front, which are then effectively mixed upward and then depleted by the phytoplankton. 405 406 407 408 409 410 The most striking difference was the well-mixed profile at 222 in contrast to significantly stratified vertical structure of 249. Since station 249 was occupied during an ebb, tidal straining will act to enhance stratification and suppress vertical mixing. Recall that profile 222 was taken at spring tides during flood. Tidal stirring together with the vertical mixing favourable tidal straining will act to mix the nitrate profile. Nutrients brought on to the shelf at deeper layers of the water column will thus be differentially mixed at different phases of 19
Page 1 and 2: 1 Mixing in the Barents Sea Polar F
Page 3 and 4: 28 29 30 31 32 33 34 35 36 37 38 39
Page 5 and 6: 76 standard methods (Parsons et al.
Page 7 and 8: 122 diffusivity is then calculated
Page 9 and 10: 168 169 170 171 172 173 174 175 176
Page 11 and 12: 214 enhancement can not be attribut
Page 13 and 14: 260 261 262 263 264 265 266 267 268
Page 15 and 16: 307 308 309 310 311 of the front to
Page 17: 343 344 345 346 347 In the presence
Page 21 and 22: 435 436 437 438 439 440 441 442 443
Page 23 and 24: 482 483 484 485 486 487 488 489 490
Page 25 and 26: 502 REFERENCES 503 504 505 506 507
Page 27 and 28: 547 548 549 550 551 552 553 554 555
Page 29 and 30: 585 Figure captions 586 587 588 589
Page 31 and 32: 630 631 fluorescence (red) are show
Page 33 and 34: Greenland 77 o N Svalbard Barents S
Page 35 and 36: 683 0 231 280 278 276 274 272 270 2
Page 37 and 38: 697 0 213 215 217 219 221 223 225 N
Page 39 and 40: 0 27.94 θ 0.4 Depth [m] 50 100 27.
Page 41 and 42: 732 Mean Isopycnal Depth [m] Mean I
Page 43 and 44: 0 a b c 25 Depth [m] 50 75 100 218
Page 45: 763 Depth [m] 0 20 40 θ [°C] 2 1

Mixing in the Barents Sea Polar Front near Hopen in spring

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