Subantarctic zone oceanography - SAZ Project 1997-1998 - Oceanography Data

The circumpolar Subantarctic Zone (SAZ) is a globally significant region of water mass formation and carbon dioxide uptake from the atmosphere.
Here we synthesise the results of nine voyages over 8 years to describe the seasonal variation in mixed layer properties in the SAZ south of Australia for comparison with biogeochemical process studies carried out in late summer (March 1998) as part of the SAZ Project. Winter mixing extends to depths greater than 400 m, resulting in the formation of Subantarctic Mode Water. In summer the mixed layer shoals to 75-100 m, depths which are still sufficiently deep that phytoplankton growth may be light limited. Nitrate and phosphate concentrations are reduced in summer( e.g., nitrate decreases from greater than 15 to less than 5/ micro mol kg- 1) but remain well above limiting levels.
Silicate in contrast, is low throughout the year (4/micro mol kg- 1 in winter and less than 2 / micro mol kg- 1 in summer). Water mass properties along a north-south hydrographic section in March 1998 suggest that near-surface waters spread from south to north across the Subantarctic Front (SAF), supplying cool, fresh, nutrient-rich water to the SAZ. As a consequence, the properties of the southern SAZ differ from those farther north: the mixed layer in the south is cooler, fresher, deeper, higher in nutrients, and bounded below by a halocline (rather than by a seasonal thermocline, as in the northern SAZ). The contrast between the northern and southern SAZ persists throughout the year, suggesting the cross-front exchange occurs year-round and likely contributes to the differences in seasonal thermal amplitude and algal biomass accumulation seen in satellite images. Density-compensated horizontal gradients of temperature and salinity are common in the mixed layer of the SAZ and the northern SAF, consistent with the hypothesis that the vigour of lateral mixing in the mixed layer is a strong function of the magnitude of the lateral density gradient.

CTD Data are available for access via the provided URL. Data from the following voyages of the Aurora Australis were used:

1991-1992 Voyage 1
1992-1993 Voyage 9
1994-1995 Voyage 4
1994-1995 Voyage 7
1995-1996 Voyage 1
1996-1997 Voyage 1
1997-1998 Voyage 6

环极亚南极海域（Subantarctic Zone, SAZ）是全球重要的水团形成与大气二氧化碳吸收区域。
本研究整合了8年间9次科考航次的成果，旨在描述澳大利亚以南亚南极海域的混合层属性季节变化，并与1998年晚夏（1998年3月）作为亚南极海域项目（SAZ Project）一部分开展的生物地球化学过程研究进行对比。
冬季混合层可延伸至400米以深，由此形成亚南极模态水（Subantarctic Mode Water）。
夏季混合层抬升至75~100米，该深度仍足够深，致使浮游植物生长受到光照限制。
夏季硝酸盐与磷酸盐浓度均有所降低（例如硝酸盐浓度从大于15 μmol·kg⁻¹降至5 μmol·kg⁻¹以下），但仍远高于限制阈值。
相比之下，硅酸盐浓度全年均处于较低水平（冬季为4 μmol·kg⁻¹，夏季低于2 μmol·kg⁻¹）。
1998年3月南北向水文断面的水团属性数据显示，近表层海水自南向跨越亚极锋（Subantarctic Front, SAF）向北扩散，为亚南极海域输送低温、低盐且富含营养盐的水体。
因此，亚南极海域南部的水团属性与北部区域存在显著差异：南部混合层温度更低、盐度更小、厚度更深、营养盐含量更高，其下界为盐跃层（halocline），而非北部亚南极海域的季节温跃层。
南北亚南极海域的属性差异全年持续存在，这表明跨锋面交换终年发生，且可能是卫星影像中观测到的季节温度振幅与藻类生物量积累差异的成因之一。
亚南极海域及北部亚极锋的混合层中，温度与盐度的密度补偿水平梯度普遍存在，这与“混合层内侧向混合的强度是侧向密度梯度大小的强函数”这一假说相符。
温盐深（CTD）数据可通过提供的链接获取。
本研究使用了“南极光号”（Aurora Australis）以下航次的观测数据：
1991-1992年第1航次
1992-1993年第9航次
1994-1995年第4航次
1994-1995年第7航次
1995-1996年第1航次
1996-1997年第1航次
1997-1998年第6航次