1. Why Study the Southern Ocean?

As the coldest, windiest, highest, most remote continent on the planet, Antarctica's environment is in profound contrast to the majority of environments that foster modern civilization (Bargagli 2005). Still, intricate relationships exist between the polar regions and the rest of the world, and Antarctica’s Southern Ocean may provide one of the best examples of this interconnection. The Southern Ocean plays a considerable role in the global redistribution of heat surplus, regulating many important biogeochemical and ecological processes throughout the Earth’s system (Bargagli 2005). Human activities have also impacted the Antarctic region. Rising sea surface temperatures, changing ocean current patterns, and ocean acidification are all serious consequences of climate change, both presently and in the future (Bargagli 2005). Over the past fifty years, mid-winter surface atmospheric temperatures of the Antarctic Peninsula have increased by 6°C. Many species that have adapted to the cold conditions of the Antarctic now struggle to survive in this changing environment. The ice season has shortened by approximately ninety days, and at least 87% of Western Antarctic Peninsula glaciers are currently in retreat (Schofield et al. 2010). The magnitude of these changes highlights Antarctica’s sensitivity to climate change. Further research in this region will thus provide better insight into how changing patterns as a result of climate change may impact the rest of the world.

Antarctica's sensitivity makes it an important site for environmental research, not only to explore the ways that climate change has already impacted the region, but also to better predict how a changing Antarctic climate system may perpetuate future climate changes. Dramatic climate change could easily trigger either positive or negative feedback responses in Antarctica that amplify or counteract the warming effects of anthropogenic climate change (Bargagli 2005). Antarctica’s isolation and cold climate have also allowed for the preservation of a fossil record and ice core samples, which lend valuable insight into climate trends. From current and past studies performed in Antarctica, scientists have gained a broader understanding of the ways that past episodes of rapid climate change have led to shifts and reorganizations of ecosystems and biogeochemical cycles (Schofield et al. 2010). However, the number of studies conducted in Antarctica is still relatively few, in part due to the technological and logistical challenges posed by the region’s geographical isolation and extreme climate conditions. Properly measuring current and past conditions in this area is incredibly important in order to better understand current climate dynamics and to best appropriate resources to protect this sensitive system. As Schofield et al. highlight with regards to the marine ecosystem, “the complexity of food webs, combined with chronic undersampling, constrains efforts to predict their future and to optimally manage and protect marine resources” (2010). Our research in the Antarctic region is thus part of a larger research initiative to become better stewards of the polar marine environment and the Earth system.

The Southern Ocean

The Southern Ocean is located south of approximately 30°S, and plays an influential role in global ocean circulation. The Southern Ocean is influenced by the Antarctic Circumpolar Current (ACC); encircling the entire Antarctic continent, the ACC is considered the most powerful current in the ocean (Pickard and Emery 1990). Strong westerly winds cause the ACC to flow from west to east, while the Coriolis Effect causes the water to deflect to the left, and thus northwards (White et al. 1998). As the water tends to the north, a northward pressure gradient is created. Gravity then causes the inclined ocean mounds to flow back southeast as geostrophic currents, replacing the water and bringing the Antarctic Circumpolar Current into equilibrium (Hughes and Ellis 2001).

The pycnocline, or density contrast, in the Southern Ocean is weak; surface ocean temperatures are not much higher than deeper ocean temperatures as a result of low-intensity levels of solar radiation at the surface (Martinson 1990). In Austral fall, beginning in March, surface water reaches its freezing point (approximately 1.8°C), and sea ice begins to form. From Austral fall through Austral winter, sea ice grows to cover approximately eight million square miles, then rapidly decreases in Austral spring, from mid-November to mid-January (Gordon 1981). The formation of Antarctic Bottom Water (AABW) is related to the development of sea ice. It is created by deep water convection at the Antarctic coast, where the dense, salty brines that are expelled during sea ice formation sink down to the Antarctic shelf and flow downslope (Orsi et al. 1999). The highly saline waters sink, and then spread slowly at depths greater than approximately 4000 meters, diffusing northwards and eventually warming. Warmer, fresher waters from the Pacific and Indian Oceans flow to the south to replace the northward-diffusing AABW. The Southern Ocean’s AABW allows for such mixing of the global oceans, resulting in an interconnected global ocean system.

Specific Characteristics of the Southern Ocean

The water in the Southern Ocean can be characterized by several properties which also serve to demonstrate how this ocean’s behaviour differs from other oceans. Each water body has a certain density (weight per unit of volume). Water with a lower density will float on top of water of greater density. This leads to ocean stratification, or the arrangement of water masses into different layers (Bergman 2001). Water density is a function of its temperature and salinity (salt content) (Bergman 2001). Density increases with decreasing temperature and increasing salinity, that is, warm fresh water is lower in density than cold saline water. Near the equator, ocean stratification is strongly influenced by temperature, since surface water is much warmer than deep water. However, in the colder waters of the Southern Ocean, stratification is mostly caused by salinity, with surface water being less saline (Bergman 2001). Oceans in the mid-latitudes have greater salinity at the surface (Stewart 2010). Strong solar radiation increases the rate of evaporation, increasing the surface ocean salinity. In contrast, the melting of sea ice in the Southern Ocean increases fresh water input, diluting and reducing the salinity of the surface water. When ice forms, salt is left behind, creating cold, saline water. This dense water sinks to form the Antarctic Bottom Water (AABW) which plays an important role in thermohaline circulation (Stewart 2010).

The surface water layer is usually quite homogeneous in salinity, density and temperature. Below this surface layer, properties vary with depth (Stewart 2010). The region where we observe a rapid change in water temperature with depth is called the thermocline. Similarly, the region where we observe rapid salinity and density changes are referred to as the halocline and the pycnocline, respectively (Stewart 2010). Water pressure also increases with depth. This leads to higher internal energy from molecular compression, causing a temperature increase of approximately 1°C with a few kilometres of depth (Stewart 2010). This means colder water resides on top of warmer water. The concepts of potential temperature and potential density have been introduced for stratified fluids in order to adjust the temperature and density measurements to eliminate the effects of increasing pressure with depth.

A variety of processes, for example wind driven currents and the breaking of surface waves, are responsible for the homogeneity in the surface layer since they increase turbulence. Turbulence in surface water is also increased by convection movement caused by cooling (seasonal air temperature changes), as well as by evaporation and sea ice formation, which increase salinity (Mann & Lazier 2006). This layer is defined as the mixed layer. This zone may vary in depth depending on the time of the year or the geographic location. A greater homogeneity in temperature and salinity will result in a deeper mixed layer. This layer is responsible for nutrient mixing, and typically determines the average amount of light that reaches marine organisms. Deep mixed layers reduce the availability of light for phytoplankton. Such water will not be as biologically productive as a shallow mixed layer, which will often lead to plankton blooms.

The precursors for water turbulence are also involved in upwelling. This phenomenon describes a rising body of deep, nutrient-rich cold water towards the surface to replace warmer unproductive water (Mann & Lazier 2006). Upwelling is essential for the marine food chain. The Southern Ocean is characterized by large-scale upwelling. Westerly winds blowing towards Antarctica drive significant water flow, then draw up water from greater depths.

The Antarctic Peninsula

The Antarctic Peninsula is the northernmost part of the Antarctic mainland, and is the closest part of the continent to any other continent. It thus provides the easiest access to Antarctica for researchers and other visitors. One of the major research topics with regard to the Antarctic Peninsula is the effect that climate change will have on ice shelves. Recent studies have shown that the Antarctic Peninsula’s mean annual air temperature has warmed about 2.5°C in the last 50 years (Scambos et al. 2000) and is considered to be Antarctica’s most rapidly changing area. With this increase, many of the ice shelves and glaciers on the peninsula have retreated or disappeared (Vaughan and Doake 1996). One of the more serious implications resulting from the increased melting of the glaciers and ice shelves is its contribution to sea level rise. It has been estimated that if the West Antarctic Ice Sheet (WAIS) were to fully melt, it would result in a sea level rise of about 5 meters, which would displace 17 million people (IPCC 2007). This prediction highlights the importance of understanding the processes that influence these ice shelves and glaciers on the Antarctic Peninsula.

Previous studies relating to the Antarctic Peninsula have shown increased upwelling of warm Upper Circumpolar Deep Water onto the shelf, influencing the distribution of ocean heat and subsequently affecting ice-melting processes (Martinson et al. 2008). The increased availability of heat to the shelf around the Antarctic Peninsula is suggested to play a role in several phenomena, including recent atmospheric warming, the accelerated glacier melting on the western side of the Peninsula, decreased sea ice concentration, and decreased length of the sea-ice season (Cook et al. 2005; Martinson et al. 2008).

The Motivation Behind the Study

During our study of the Antarctic Peninsula, we intended to look at differences in temperature, salinity, density, and water quality at various sites in the Southern Ocean. We were also curious to see if we would be able to observe any significant currents traveling through the water column through our analysis of these factors. As we traveled through the South Shetland Islands and near the coast of the Antarctic Peninsula, we were interested in observing whether there were warm water masses at any location, and whether they played a role in melting sea ice or the Antarctic ice shelves. We tried to analyze as broad a range of the water column as possible, by conducting a variety of zodiac-based shallow casts, as well as ship-based deep casts, while also comparing our results to similar data collected in the area prior to our arrival. We were also interested in observing the influence that surrounding glaciers and sea ice had on the ocean salinity at our different study sites, as well as the influence that solar radiation had on sea surface temperature. Another factor motivating our research was to determine whether the proximity to land and the amount of shelter at a site influenced the water column's homogeneity, since we knew that turbulence often affects the depth of the mixed layer. The stratification of the water column at various sites was of particular interest to us, as this information can be used as an indication of how the ocean in a particular area mixes vertically, and how nutrients travel to the surface from the deep ocean.