Atlantic waters in the western arctic ocean
Figure 2: Differing proposed AW circulation schemes (black or red arrows) showing (top)
Download 2.02 Mb.
|
- Navigate this page:
- (bottom)
- 1.3 Recent observational and theoretical advances
- Double-diffusive zigzags
- (top left)
| Figure 2: Differing proposed AW circulation schemes (black or red arrows) showing (top) anticyclonic (clockwise) flow in the southern interior Canada Basin and a cyclonic (anticlockwise) counter flow (the boundary current) along the Alaskan coast or (bottom) only a cyclonic (anticlockwise) circulation. Flow from the north Northwind Ridge into the interior (top left and middle) is also inferred from tracer studies [Smith et al., 1999; Smethie et al., 2000].
Interestingly, this confusion is mirrored in modeling results, with 10 major arctic models (part of the AOMIP –Arctic Ocean Model Intercomparison Project [Proshutinsky et al., 2005]) split almost equally in their simulation of flow directions in the Canadian Basin, possibly due to different subgrid-scale parameterizations [Holloway et al., 2007]. Zhang and Steele [2007] show that “tuning” of a model’s vertical mixing coefficients (to reflect low arctic mixing rates) can switch the direction of Canadian Basin mid-depth circulation. Thus, to tune a model realistically, it is obviously essential to know the correct circulation. There is still no generally accepted theory of the driving mechanism for AW circulation in the Arctic. Indeed, similar to the arctic models, recent theoretical studies also disagree about the sense of circulation they are explaining, e.g., Nost and Isachsen [2003] seek a cyclonic boundary current and an anticyclonic interior flow as per Aagaard [1989] (top right, Figure 2), but Yang [2005] seeks cyclonic flow in both the boundary current and in the interior of the Canadian Basin [e.g., as per Rudels et al., 1994] (bottom left, Figure 2). Neither is there a basin-wide observational description of the flow properties. Near the Eurasian end of the Lomonosov Ridge, moorings suggest an equivalent barotropic [Killworth, 1992] flow structure, consistent with the strong link between flow and topography, with velocity decreasing with depth [Woodgate et al., 2001]. However, the AW velocity structure over the Beaufort slope appears rather different, with moored observations showing eastward flow increasing with depth [Aagaard, 1984]. Although the presence of the boundary current is clear in tracer data [Smethie et al., 2000], there are remarkably few published estimates of volume transport, current width, or the range of isobaths over which the core of the current is found [Woodgate et al., 2001; Woodgate et al., 2007]. The latter is extremely important for the placing of moorings in the upcoming Arctic Observing Network (AON). There are significant, unresolved issues as to how AWs escape topographic trapping and ventilate the interior basins. One suggested mechanism relates to the remarkable double-diffusive interleaving layers which are found to line-up (in T-S space) throughout the Arctic [Carmack et al., 1997; Carmack et al., 1998; Rudels et al., 1999a]. Although these intrusions may be advected throughout the Arctic by the boundary current flow, double-diffusive theory suggests instead that these intrusions spread across a front (i.e., here across isobaths) with zero velocity in the along-front direction (i.e., here along isobaths) [McDougall, 1985]. It is suggested that intrusions may propagate away from the boundary current into the interior at ~1-2 mm/s [Walsh and Carmack, 2003], an order of magnitude less than the measured speed of the boundary current (1-5 cm/s [Woodgate et al., 2001]). Another candidate for transfer away from the boundary current is eddy formation, which may be caused by the sharp topography of the northern tip of the Chukchi Rise [Smethie et al., 2000; Woodgate et al., 2007]. Although Pacific Water eddies are common in the Canada Basin, there are (to date) far fewer observations of AW eddies in the Canada or Makarov basins [D'Asaro, 1988; Woodgate et al., 2001], and the pan-arctic importance of this process is still to be quantified. The influence of AW is not confined to the physical oceanography of the slopes and deep basin. Observations from the Chukchi Sea show that upper layers of the boundary current are also involved in ventilating the arctic shelves, via upwelling up canyons in the continental slope and mixing with shelf waters [e.g., Bourke and Paquette, 1976; Aagaard et al., 1981; Woodgate et al., 2005a]. By this mechanism, heat from AWs affects shelf waters and possibly sea-ice. (Sea-ice in Barrow sometimes melts from below, possibly due to upwelling of oceanic heat [Eichen, pers.comm.].) The AWs are also a climate connection to the world ocean – AWs entering the Arctic emerge modified after a significant delay (order years/decades), and affect the Greenland Sea, feeding also into the overflow waters to the Atlantic [e.g., Mauritzen, 1996]. Furthermore, climate models suggest this arctic part of the meridional overturning circulation strengthens significantly under reduced ice scenarios [Bitz et al., 2006]. By providing continuity from lower latitudes into the high Arctic, AWs are a transport mechanism for pollutants into the Arctic (e.g., radionuclides from European nuclear reprocessing plants [Smith et al., 1999]), and by their water properties, they offer a warm corridor for invasive species. Accurate knowledge of the AW pathways in the Canadian Basin is critical for understanding transit times, contaminant pathways, and ocean ventilation, and for assessing climate connections between the Arctic and the global ocean. This knowledge is urgently required for advances in arctic modeling and theoretical studies. Thus, we propose an observationally-based study of Atlantic Water circulation in the western Arctic, which will collate and synthesize hydrographic data sets from the Canadian Basin over the last 2 decades; provide a basin-wide assessment of flow pathways and essential flow properties; and consider processes of exchange between the boundary current and the interior and the arctic shelves. The proposed work is timely for several reasons. Firstly, since the previous basin-wide observational syntheses [Aagaard, 1989; Rudels et al., 1994], there have been many major observational projects within the Canadian Basin that have primarily been investigated individually, mostly focusing on the upper layers. This (international) collection of data is generally unexploited as a set, and as we will show below, is well suited to our objectives. Secondly, these datasets span a period when the pathways of the boundary current are well marked by a dramatic warming of FSBW advecting through the western Arctic [Carmack et al., 1995; Shimada et al., 2004], simplifying the task of tracing boundary current pathways. Thirdly, recent work on double-diffusive intrusions [Woodgate et al., 2007] has suggested a new technique (discussed below) for “fingerprinting” the boundary current and inferring the T-S properties of newer and older waters, which can mitigate for lower spatial coverage of the older data. Finally, the modeling (arctic and global) and theoretical communities are in urgent need of this synthesis, and the results (e.g., major branches and isobath range for the boundary current) will be important information for designing an effective, efficient Arctic Observing Network. 1.3 Recent observational and theoretical advances Our study will use techniques and theory unavailable during the previous synthesis efforts. For example, many modern data sets include dissolved oxygen profiles, which can resolve ambiguities in interpretation of T-S data [Falkner et al., 2005; Woodgate et al., 2005a], and nutrient data can address issues of Pacific versus Atlantic origin [Jones et al., 1998]. We will use a new technique based on the T-S characteristics of double-diffusive intrusions to trace pathways and infer prior and new water properties [Woodgate et al., 2007]. We will also draw on theoretical results. Double-diffusive zigzags: Double-diffusive intrusions (nicknamed “zigzags”, Figure 3) form from an interleaving of two parent water columns. Using T and S conservation requirements, Woodgate et al., [2007], argue that the water properties of the points of the zigzags in T-S space may indicate the water properties of the parents, and that the T-S form of the zigzags (large zigzags or small zigzags) also conveys information about differences between the parent water columns. For example (Figure 4), T-S structures in data from the Chukchi Borderland in 2002 show large zigzags (blue panel, top right of Figure 4) in the FSBW at the northern end of the Chukchi Rise, signifying intrusions between two very different water masses - new boundary current water (the 1990s FSBW T-maximum) and older (colder, fresher) basin water. West of the Mendeleev Ridge, smaller zigzags (red panel, left of Figure 4) imply interleaving between two more similar water masses, i.e., between the 1990s FSBW T-maximum and newer boundary current water, which is slightly cooler [Gunn and Muench, 2001]. These conclusions are reinforced by dissolved oxygen and CFC data [Woodgate et al., 2007]. The analysis suggests that the FSBW transitioned from a cold T-maximum (pre 1990s) to a warm T-maximum (the 1990s warming) and then to a slightly cooler T-maximum (after the 1990s warming), a result consistent with other studies [Gunn and Muench, 2001; Polyakov et al., 2005]. Similar arguments for BSBW zigzags suggest that BSBW has become monotonically cooler on similar timescales [Woodgate et al., 2007]. The presence of zigzags indicates a change in AW properties. Thus, mapping changes in zigzag distribution in data from different years will inform us of AW pathways and bound advection speeds. In the western Canadian Basin, T-S zigzags were first observed in 1993 at the Eurasian end of the Mendeleev Ridge [Carmack et al., 1995]. Between 1993 and 1996, the zigzags were absent from the Canada Basin [Carmack et al., 1997]. In 2002, they were observed in the Chukchi Borderland [Woodgate et al., 2007] and extending into the central Canada Basin [McLaughlin et al., 2005], in conjunction with the FSBW warming propagating through the region. The zigzag analysis yields more information than simply tracking the T-maximum. Firstly, the zigzag form can indicate which part of the boundary current is being sampled (e.g., Figure 4), information that can mitigate for poor spatial coverage. Secondly, it suggests the T-S of the parent water masses. Thirdly, the zigzag form yields a qualitative measure of age, which is valuable when other age tracers are not available (e.g., low CFC, “relic” waters within a topographically isolated zone of the Chukchi Borderland had characteristically “ragged”, i.e., decaying, zigzags structures, Figure 4 and Woodgate et al., [2007]). 
Figure 3: Examples of T-S zigzags in potential temperature (theta) – salinity plots for the Chukchi Borderland (CBL) 2002 data [Woodgate et al., 2007]. Oblique dotted lines are sigma-0 in kg/m3. Colors indicate station location as per map (top left), depth contours from IBCAO, interval 500m. Grey dots show the entire CBL data, with locations as per map. Panels show different T-S scales, indicating (top right) the Atlantic Water temperature maximum (AW Tmax); (bottom left) the T-S regimes for the Fram Strait Branch Waters (FSBW) and the Barents Sea Branch Waters (BSBW); and (bottom right) details of the FSBW zigzags. (Adapted from Woodgate et al., [2007]). 
Figure 4: Schematic of AW circulation in the Chukchi Borderland, indicating (by color) the T-S zigzags “fingerprinting” the FSBW in each region, i.e., small zigzags of the newest boundary current waters (red, in the west); large zigzags of the 1990s warming FSBW interleaving with older basin water (blue, in the northeast); a “point and bump” zigzag structure related to the boundary current core (magenta); ragged zigzags (low CFCs) of relic waters (cyan, in the southeast). Figure also shows regions where shelf/slope mixing processes have smoothed the zigzag structure (green, labeled shelf influence). Black arrows label hypothesized pathways. Black dots show CTD stations. (Adapted from Woodgate et al., [2007].) Directory: rebecca rebecca -> Education 2005 Masters of Fine Art, California Institute of the Arts, Valencia, Los Angeles, usa. 2002 Honors year, Bachelor of Fine Arts, The Victorian College of the Arts, Melbourne, aus. 2001 Bachelor of Fine Arts, Photography Major, V rebecca -> Irina Khasin, Legal Counsel and Secretary rebecca -> Education Ph. D. American Studies, University of Maryland, College Park, 2010 Fields: humor studies, performance studies, visual/popular culture, women’s history, cultural studies and studies in race/ethnicity, gender, disability rebecca -> The Unbelievable Truth Download 2.02 Mb. Share with your friends:
|