5. Comparing to the big picture

All of the following satellite data was obtained from MODIS-Aqua, SeaWiFS and SSM/I, in order to draw comparisons with our results. All MODIS-Aqua and SeaWiFS analyses and visualizations used in our analysis were produced with the Giovanni online data system, development and maintained by the NASA GES DISC. SSM/I visualizations were produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project.

Winds

As the SSM/I data indicates, the wind speed for the week of February 20-26 was similar to the monthly average at approximately

10 m/s. However, these wind speeds are slightly higher than the average February wind speeds from previous years (2005, 2007, 2009) which tended to be around 5 m/s.

Figure 59: Average SSM/I wind speed of Antarctic Peninsula during February 2009.

Figure 60: Average SSM/I wind speed of Antarctic Peninsula during February 20-26, 2011, the time of our data collection.

Sea surface temperatures

According to MODIS-Aqua, the sea surface temperatures around the Antarctic Peninsula tended to stay between 0°C and 3°C, with coastal waters being slightly cooler, around 0°C. The trends for February 2011 were similar to previous years in this pattern.

Figure 61: Average MODIS-Aqua sea surface temperature in the Antarctic Peninsula in February 2011.

A time-series graph produced with MODIS-Aqua data also revealed that February temperatures have been decreasing slightly since 2006, which had an average temperature of 2°C. Until this data is compared on longer time scales, it is difficult to determine if 2006 was a temperature anomaly or part of a larger climate trend.

Figure 62: Time series graph demonstrating MODIS-Aqua sea surface temperatures in the Antarctic Peninsula from 2002-2011.

A comparison of sea surface temperatures (SST) since 2003 also reveals that each year consistently shows warming temperatures farther away from the poles, typically beginning around 66°S. However, the years of 2007 and 2008 demonstrated warmer temperatures (between 0-3°C) starting at 68.75°S.

Figure 63: Average MODIS-Aqua sea surface temperature for different longitudes in the Southern Ocean for 2001-2011.

Chlorophyll-a concentrations

Algae blooms occur during Antarctica’s spring and summer, followed by periods of low primary productivity during Antarctica’s winter due to sea ice cover (April-August). Peak chlorophyll-a concentrations typically occur in December. Since our research was conducted in February, the peak of the algae bloom had already passed, although chlorophyll-a concentrations were still high. As the time-series graph from SeaWiFS data depicts, 2002, 2006, and 2007 had higher concentrations of algae than other years.

Figure 64: Time series graph demonstrating SeaWiFS chlorophyll-a concentrations in the Antarctic Peninsula from 1997-2009.

Time-series data from MODIS-Aqua also indicate that spring 2009 and 2010 were years with lower concentration of chlorophyll-a, indicating that our research was conducted in a year with a more-contained algae bloom.

Figure 65: Time series graph demonstrating MODIS-Aqua chlorophyll-a concentration in the Antarctic Peninsula from 2002-2011.

Longitude and latitude time-series graphs with MODIS-Aqua data also support the expansive algae bloom that occurred in 2006 and 2007, and also indicate that algae blooms occur primarily along the coastlines.

Figure 66: Average MODIS-Aqua chlorophyll-a concentrations for different latitudes in the Southern Ocean for 2001-2011.

Figure 67: Average MODIS-Aqua chlorophyll-a concentrations for different longitudes in the Southern Ocean for 2001-2011.

Though SeaWiFS data was unavailable for chlorophyll-a concentrations in February 2011, average chlorophyll-a levels in September-December 2010 show algae blooms around Brown Bluff and west of the Gerlache Strait. In comparison to September-December 2011, high chlorophyll-a concentrations were found further south of Gerlache Strait on the western side of the Antarctica peninsula.

Figure 68: Average SeaWiFS chlorophyll-a concentrations surrounding the Antarctic peninsula in September -December 2009.

Figure 69: Average SeaWiFS chlorophyll-a concentration surrounding the Antarctic peninsulsa in September- December 2010.

Using MODIS-Aqua data, a correlation graph also demonstrates higher concentrations of chlorophyll-a in areas with relatively higher sea surface temperatures along the coastlines. This correlation is weaker farther away from the coast because phytoplankton growth requires many nutrients that are only available on the coastline, thus restricting the distribution of chlorophyll-a to these areas. A negative correlation can be seen in the ocean, which may be an indicator of offshore iron that resulted from deep upwelling. This iron would have stimulated growth in the ocean when it was still quite cold, and would have been exhausted when temperatures started to rise in Antarctica’s spring, thus showing the unusual negative correlation pattern of lower chlorophyll-a concentrations with warmers temperatures. Generally this graph supports that higher chlorophyll-a concentrations are associated with warmer temperatures. This relationship supports the higher chlorophyll-a concentrations seen in 2006, which was also marked by higher SST.

Figure 70: Correlation graph between MODIS-Aqua sea surface temperatures and MODIS-Aqua chlorophyll-a concentrations in the Antarctic peninsula.

Argo

Three sites have been characterized using Argo probes. The data from these measurements have been retrieved from Coriolis. The first site (brown) is located in the Drake passage, the second (green) is in the Weddell Sea and the last site (purple) is located further east into the Southern Ocean. The Weddell Sea site has the freshest surface water, perhaps from freshwater input from the peninsula; it then reaches its pycnocline around the same depth as the others Argo sites, though they have more saline surface waters.

In terms of temperature, the green site, representing the Argo in the Weddell Sea, has the coldest temperature, around -1.8°C at the surface; it then increases to 0.5°C in its thermocline, and decreases again below the thermocline to just below 0°C. The brown site, in the Drake Passage, has the warmest surface temperatures, 2.4°C, which decreases to below -0.5°C in its thermocline, it then increases to 2.2°C at around 400 decibars of pressure. It then decreases again to about 1°C as the pressure drops. The purple site has a surface temperature of 1.6°C, and it decreases to -0.3°C in its thermocline, and increases again to 0.5°C, and stays fairly consistent with this temperature with decreasing pressure.

Argo Data and Site Comparisons

Robert Point and Pendulum Cove, the two sites which are the closest to the Argo station in the Drake passage present similar warmer temperatures in the range of approximately 1 to 2.5°C . This is caused by the Antarctic circumpolar current (ACC) and the warmer water that it carries from the Pacific Ocean by the Westerly winds. For sites near the Peninsula, temperatures are colder. The Weddell Sea temperature is also characterized by a gyre which traps the melting sea ice. Gyres are large systems of rotating ocean currents caused by wind movement, the Coriolis Effect and friction. We also noticed that the Weddell Sea contained many icebergs. The Argo site in the Weddell Sea are comparable to the ones in Brown Bluff and the Antarctic Sound with water temperatures below zero. Most of the Antarctic bottom water is formed in the Weddell sea, which has the strongest stratification at the surface and the weakest at greater depth. No measurements were done further east during our expedition, but looking at the Argo data in the last Argo site, it can be estimated that temperatures would be slightly warmer than the Weddell Sea but not as warm as the Drake. This can be attributed to eddies, which are swirls and reverse currents created when water flows past an obstacle, in our case, the Antarctic Peninsula. These eddies contribute to the mixing of the ACC with some cold water from the Weddell sea. Finally, according to the Argo data interpolation Section plot, measurements taken on the western side of the peninsula should be situated between 0 and 1°C. Though they are located more south, it is interesting to note that our sampling sites on the western side of the peninsula (Paradise Bay, Neko Harbor and Errera Channel) also have temperatures between 0 and 1.5°C.

Looking at the salinity section of the Argo data, the Drake Passage has a fresh water layer at the surface similar to both Pendulum Cove and Robert Point. Brown Bluff and the Antarctic Sound are both slightly saltier than the Weddell sea. The Argo measurements further East have less of a fresh surface layer. All sites have a more constant salinity with increasing depth. The Argo section, taken near the northern part of the peninsula, presents higher salinity than the southern part of the peninsula where some of our measurements were taken.

Figure 71: Station Plot with the Argo data

Figure 72: Section plot with the Argo data. The Drake Passage site is the farthest to the left, the Weddell Sea site is in the middle, and the remaining site is in the far right of the plot.