Journal Title
Title of Journal: J Oceanogr
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Abbravation: Journal of Oceanography
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Authors: Motoyo Itoh Koji Shimada Takashi Kamoshida Fiona McLaughlin Eddy Carmack Shigeto Nishino
Publish Date: 2012/06/21
Volume: 68, Issue: 4, Pages: 575-592
Abstract
We examined the interannual variability of Pacific Winter Water PWW both upstream in the northeastern Chukchi Sea and Barrow Canyon using mooring observations from 2000 to 2006 and downstream in the Canada Basin using hydrographic data acquired in 2002–2006 The interannual variation of PWW salinity is governed by two factors 1 variability in the salinity of Pacific Water that flows northward through Bering Strait in winter and 2 the input of salt associated with sea ice formation during winter in an intermittent coastal polynya located along the Alaskan coast between Cape Lisburne and Point Barrow During the winters of 2000/2001 and 2001/2002 an increased transport of cold and saline PWW S 335 to the basin via Barrow Canyon was observed In 2000/2001 enhanced ice formation in the polynya contributed to the increased salinity of PWW whereas in 2001/2002 the salinity of water entering through the Bering Strait was higher and this resulted in more saline PWW being delivered to the basin In the following four winters 2002/2003 2003/2004 2004/2005 and 2005/2006 the transport of cold and saline PWW in winter to the basin was less than that in the two preceding winters In three of these four winters 2003/2004 being the exception the coastal polynya was less active thus reducing the input of salt due to brine enrichment In the winter of 2003/2004 however warmer water within the polynya region constrained ice formation and thus less cold and saline PWW was produced despite the fact that the coastal polynya was active and frequently openWe are greatly indebted to the officers and crew of the R/V Mirai CCGS Louis S StLaurent and CCGS Sir Wilfrid Laurier and the scientists and technicians who collected the data We sincerely thank Andrey Proshutinsky and acknowledge support from the US NSF Office of Polar Programs Grant OPP 0424864 and Fisheries and Oceans Canada We appreciate Takatoshi Takizawa Bon van Hardenberg Kiyoshi Hatakeyama Sarah Zimmermann Hirokatsu Uno and Akinari Murata for their dedicated support of our research program We thank Rebecca Woodgate and Knut Aagaard for providing mooring data in the Bering Strait Comments from the editor Hirohiko Nakamura and two anonymous reviewers were very helpful for improving the manuscriptTemperature salinity and velocity data measured in Barrow Canyon were linearly interpolated to every 2 m between the observational levels shown in Fig 4 The shallowest data were extended from the shallowest observational level to the surface and the deepest data were extended from the deepest observational level to the bottom Most of the mooring data were recorded at hourly intervals We calculated daily averaged data using a 25h running mean Some instruments failed Fig 4 thus we had to use different interpolation methods to create time series of temperature salinity and velocityAt Stn BCC00 the Aanderaa recording current meters RCMs at 195 and 235 m failed to record data from December 2000 to September 2001 and from January 2001 to September 2001 respectively We thus extended the velocity observed at 140 m to the bottom decreasing by 47 every 10 m This rate was estimated from the acoustic Doppler current profiler ADCP data observed at Stn BCC from October 2001 to June 2002 and from October 2005 to September 2006 these periods had good temporal records of temperature salinity and velocity From July 2002 to August 2002 at Stn BCC01 the upwardlooking ADCP at 150 m failed and we used velocity data recorded from the surface to 50 m at Stn BCE01 instead Between 50 and 150 m we linearly interpolated velocity data observed at 50 m at Stn BCE01 75 m at Stn BCC01 and 150 m at Stn BCC01To evaluate the error arising from insufficient velocity data at Stns BCC01 and BCC00 the PWW transport obtained from real velocity profiles ADCP data was compared to that calculated from estimated velocity profile data from October 2001 to June 2002 and from July to August 2006 There were good temporal records of temperature salinity and velocity during these periods Fig 4 The difference between the annual mean transport using ADCP data and that estimated using the interpolated data was only a small percentage of the total transport Therefore the error of PWW transport arising from these interpolation methods was negligibleAt Stn BCC02 the Seabird Microcat SBE instrument at 50 m failed from September 2002 to September 2003 and the SBE at 158 m failed from November 2002 to September 2003 In the upper layer temperature and salinity at Stn BCC were in between values measured at Stns BCE and BCW Fig 5 Therefore mean temperature and salinity values Stns BCE02 and BCW02 were calculated and used for Stn BCC02 above 50 m Temperature and salinity properties at Stn BCW02 at 152 m were used to determine the T–S curve at Stn BCC02 from 89 to 252 m However the vertical salinity profile at Stn BCC02 which is nearly the same as the density profile could not be estimated from BCW02 Fig 5 Thus the monthly averaged profiles of the salinity gradient from 89 to 252 m were calculated for the following three cases using the differences of velocity at 150 m at Stn BCC 1 southwestward or weak northward current current speed in northwestward direction v 20 cm/s 2 northeastward current v 20 cm/s and 3 strong northeastward current v 40 cm/s using data from October 2001 to August 2002 and from October 2005 to September 2006 Calculated monthly averaged salinity profiles for these three cases were used to estimate the salinity profile at Stn BCC02 from November 2002 to September 2003 From September 2002 to September 2003 no velocity data from the surface to 120 m were available at Stn BCC02 and thus the velocity measured at 50 m at Stn BCE02 was used from the surface to 120 m at Stn BCC02To evaluate the error at Stn BCC02 due to the missing data we applied the interpolation method used for Stn BCC02 to data from October 2001 to June 2002 and from July to August 2006 We then compared PWW transport values obtained from reliable temperature salinity and velocity data with those calculated from the interpolated data for these periods PWW transport 32 S 335 decreased by 0042 Sv for 2001–2002 and by 0047 Sv for 2005–2006 when compared to values calculated using real temperature salinity and velocity profile data In Fig 10 the error bar indicates the magnitude of the potential underestimation of PWW transport at Stn BCC02 in 2002–2003The SBEs at 180 and 250 m at Stn BCC03 failed from October 2003 to September 2005 thus no temperature and salinity data were collected below 120 m Warm Atlantic Water is generally observed near the bottom at Stn BCC Fig 5 thus the mean properties of Atlantic Water from Stn BCC at 250 m T = 03 S = 3464 from October 2001 to June 2002 and from October 2005 to September 2006 were used instead However the temperature and salinity recorded at 120 m at Stn BCC03 were extended to the bottom when cold and saline brine water T −15 °C was observed at the bottom of Stn BCW03 Temperature and salinity properties at Stn BCW03 at 158 m were used to determine the T–S curve at Stn BCC03 from 120 to 250 m The monthly averaged profile of the salinity gradient between 120 and 250 m at Stn BCC03 was calculated as we interpolated salinity data for Stn BCC02 between 89 and 252 m from November 2002 to September 2003 Using the T–S curve observed at Stn BCW03 and calculated monthly averaged salinity profiles temperature and salinity properties from 120 to 250 m at Stn BCC03 were estimated from September 2003 to September 2005 The upwardlooking ADCP at 240 m failed from August 2004 to the end of September 2005 and only RCM velocity data at 57 m were available for Stn BCC03 Thus the velocity measured at 57 m was applied to the velocity from the surface to the bottom at Stn BCC03We evaluated the error in PWW transport calculations arising from these data deficiencies at Stn BCC03 Based on coverage of velocity data observational periods of Stn BCC03 can be divided into two periods from October 2003 to September 2004 and from October 2004 to September 2005 First we applied the interpolation method used for the first period of Stn BCC03 to data from October 2001 to August 2002 and from October 2005 to September 2006 PWW transport 32 S 335 decreased by 0027 Sv for 2001–2002 and by 0038 Sv for 2005–2006 when compared to values calculated from real data Saline PWW transport S 335 decreased by 0020 Sv for 2001–2002 and by 0002 Sv in 2005–2006 when compared to values calculated from real data Second we applied the interpolation method used for the latter period of Stn BCC03 to data for October 2001 to August 2002 and October 2005 to September 2006 PWW transport 32 S 335 decreased by 0040 Sv for 2001–2002 and by 0062 Sv for 2005–2006 when compared to values calculated from real data Saline PWW transport S 335 calculated using these assumptions decreased by 0034 Sv for 2001–2002 and by 0022 Sv in 2005–2006 when compared to values calculated from real data The error bars on PWW transport in Fig 10 indicate the possible magnitude of the underestimation for 2003–2004 and 2004–2005
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