A Seasonal Journey at 170°W
The Antarctic Circumpolar Current is a force of nature that holds crucial clues to understanding our planet's climate and biological productivity.
The Southern Ocean is a realm of extremes, home to the world's most powerful ocean current—the Antarctic Circumpolar Current (ACC). This relentless, eastward-flowing current connects the Atlantic, Pacific, and Indian Oceans, playing a critical role in global climate regulation and marine ecosystems.
During 1997-1998, an ambitious scientific mission ventured into these treacherous waters to unravel one of oceanography's persistent mysteries. Despite swimming in nutrient-rich waters, the ACC seemed strangely devoid of life, a phenomenon known as High Nutrient Low Chlorophyll (HNLC) conditions. This article explores the groundbreaking research that tracked how hydrographic properties—the physical and chemical characteristics of seawater—evolve throughout the seasons in this remote but globally significant region.
The Antarctic Circumpolar Current moves more water than any other current—approximately 140 million cubic meters per second—carrying more than 100 times the flow of all the world's rivers combined.
The Antarctic Circumpolar Current is the planet's only ocean current that flows completely uninterrupted by continents, circling Antarctica from west to east. As early as 1937, Deacon observed that the poleward rise of density surfaces in the ACC occurred in a series of clear step-like patterns called fronts 1 .
The research along 170°W focused on several key frontal systems, each with unique characteristics:
The northern boundary of the ACC
A major current core within the ACC
Separates the northern and southern water masses
The southernmost front of the ACC system
These narrow jets are interspersed with broader zones of reduced or even reversed flow, creating enormous shear forces within the water column 1 . The 170°W study area was strategically selected because these fronts run parallel to each other in this region, minimizing the complicating effects of convergence and divergence found in other sectors.
The US Joint Global Ocean Flux Study (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) was a comprehensive research program designed to understand how biological and physical processes interact to control the cycling of carbon in the Southern Ocean 1 .
The ACC component of this program involved four dedicated research cruises along the 170°W meridian during the austral spring, summer, and fall of 1997-1998 1 . This marked one of the first efforts to systematically track seasonal changes in this dynamic region within a single year.
| Cruise Designation | Time Period | Seasonal Phase |
|---|---|---|
| R06 | October/November 1997 | Austral spring |
| R07 | November/December 1997 | Austral spring |
| R08 | January/February 1998 | Austral summer |
| R09 | February/March 1998 | Austral fall |
A central puzzle driving the research was the High Nutrient Low Chlorophyll (HNLC) condition of the ACC 1 . Unlike most ocean regions where abundant nutrients lead to plentiful phytoplankton growth, the ACC waters remained curiously barren despite high concentrations of essential nutrients like nitrate, phosphate, and silicate.
This paradox puzzled scientists because phytoplankton—the base of the marine food web—typically flourish when nutrients are abundant. Understanding this anomaly was crucial since phytoplankton growth directly influences how much carbon dioxide the ocean can absorb from the atmosphere.
In the austral winter, the Southern Ocean presents a starkly uniform environment. Early spring measurements from October/November 1997 revealed a well-mixed water column with nearly constant values of temperature, salinity, and nutrients from surface to depth 1 .
As sunlight returned in spring, a dramatic transformation began. The research team observed the initial stages of stratification—the development of distinct water layers with different densities 1 . Freshwater from melting sea ice further enhanced this stratification.
By austral summer (January/February 1998), the researchers documented the full force of the biological response. The phytoplankton bloom was in full swing, drawing down nutrients and accumulating chlorophyll and particulate organic carbon (POC) in surface waters 1 .
As autumn approached, biological activity began to decline. The water column started mixing again, and nutrient levels at the surface began to recover as the seasonal stratification weakened.
| Season | Water Column Structure | Biological Activity | Nutrient Status |
|---|---|---|---|
| Winter | Well-mixed, uniform properties | Minimal | High, uniform with depth |
| Spring | Initial stratification developing | Bloom initiation | Beginning to decrease at surface |
| Summer | Strongly stratified | High phytoplankton biomass | Significantly drawn down at surface |
| Fall | Mixing beginning | Declining activity | Starting to recover |
Conducting research in the violent seas of the Southern Ocean requires specialized equipment and meticulous protocols. The AESOPS program employed a comprehensive suite of oceanographic tools to capture the dynamic changes occurring throughout the year.
| Method/Equipment | Function |
|---|---|
| CTD/Rosette System | Measures Conductivity, Temperature, Depth and collects water samples at precise depths |
| Nutrient Analysis | Quantifies concentrations of nitrate, phosphate, silicate essential for plant growth |
| Chlorophyll Measurements | Tracks phytoplankton biomass and distribution |
| Particulate Organic Carbon (POC) Analysis | Measures carbon contained in particles, indicating biological productivity |
| Total CO2 (TCO2) and pCO2 Measurements | Quantifies carbon dioxide distribution and air-sea exchange |
| Sediment Traps | Collects sinking particles to measure carbon export from surface waters |
The research vessel Roger Revelle covered an extensive study area along 170°W, crossing multiple frontal systems from the Subantarctic Zone to the Antarctic Zone 1 . At each station, scientists collected detailed measurements through the entire water column, with particular focus on the upper 400 meters where most biological activity occurs.
The comprehensive seasonal coverage allowed scientists to observe clear patterns in how the ACC functions throughout the year. The data revealed that the seasonal biological response to summer stratification was evident in the reduction of nutrient concentrations and buildup of chlorophyll and POC 1 .
Interestingly, the research showed that different zones within the ACC had distinct characteristics. The Polar Front separated the Antarctic Zone to the south—with high winter silicate concentrations from upwelling Circumpolar Deep Water—from the Polar Frontal Zone to the north, where silicate-depleted Antarctic Intermediate Water sinks and begins its northward transit 1 .
Perhaps most significantly, the research documented how these seasonal biological cycles influence the global carbon cycle. The drawing down of carbon dioxide during phytoplankton blooms and its subsequent release during decay periods affects how much atmospheric CO₂ the Southern Ocean can absorb 1 .
This finding has profound implications for understanding the ocean's role in climate regulation. The Southern Ocean is known to be a critical sink for anthropogenic carbon dioxide, and these seasonal patterns help scientists predict how this might change in future climate scenarios.
The Southern Ocean absorbs approximately 40% of the anthropogenic carbon dioxide taken up by the global oceans, making it a crucial component of Earth's climate system.
The 1997-1998 AESOPS study of the Antarctic Circumpolar Current at 170°W provided an unprecedented window into the seasonal workings of one of Earth's most powerful natural systems. By tracking changes across multiple seasons, researchers revealed the intimate connections between physical water properties, chemical cycles, and biological productivity.
These findings extend far beyond academic interest—they help us understand how our planet functions as an integrated system. The data collected during this ambitious project continues to inform climate models and policy decisions decades later. As climate change accelerates, understanding the delicate balance revealed by this research becomes ever more critical to predicting and preparing for our planetary future.
The seasonal evolution of hydrographic properties in the ACC reminds us that even in the most remote corners of our world, dynamic processes are constantly unfolding that affect life across the globe—processes that science must continue to unravel if we are to be wise stewards of our changing planet.