The North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) was a five-year scientific research program that investigated aspects of phytoplankton dynamics in ocean ecosystems, and how such dynamics influence atmospheric aerosols, clouds, and climate. The study focused on the sub-arctic region of the North Atlantic Ocean, which is the site of one of Earth's largest recurring phytoplankton blooms. The long history of research in this location, as well as relative ease of accessibility, made the North Atlantic an ideal location to test prevailing scientific hypotheses in an effort to better understand the role of phytoplankton aerosol emissions on Earth's energy budget.
NAAMES was led by scientists from Oregon State University and the National Aeronautics and Space Administration (NASA). They conducted four field campaigns from 2015-2018 that were designed to target specific phases of the annual phytoplankton cycle: minimum, climax, intermediary decreasing biomass, and increasing intermediary biomass. The campaigns were designed to observe each unique phase, in order to resolve the scientific debates on the timing of bloom formations and the patterns driving annual bloom re-creation. The NAAMES project also investigated the quantity, size, and composition of aerosols generated by primary production in order to understand how bloom cycles affect cloud formations and climate. Scientists employed multiple complementary research methods, including intensive field sampling via research ships, airborne aerosol sampling via airplane, and remote sensing via satellites.
The findings from NAAMES, while still forthcoming, have shed light on aerosols and cloud condensation nuclei, phytoplankton annual cycles, phytoplankton physiology, and mesoscale biology. Several methodological advances have also been published, including new remote sensing algorithms and advances in satellite remote sensing.
Background Competing hypotheses of plankton blooms
NAAMES sought to better understand the impact of bioaerosol emissions on cloud dynamics and climate. It also aimed to test two competing hypotheses on plankton blooms:
Critical Depth Hypothesis - a resource-based view Source:
The critical depth hypothesis is a resource-based view of the North Atlantic annual phytoplankton blooms. It is the traditional explanation for the cause of spring blooms and has been documented as a foundational concept in oceanography textbooks for over 50 years. It focuses on the environmental conditions necessary to initiate a bloom such as high nutrients, shallower mixing, increased light, and warmer temperatures.
The central argument for the critical depth hypothesis is that blooms are a consequence of increased phytoplankton growth rates resulting from shoaling of the mixed layer above the critical depth. The critical depth is a surface mixing depth where phytoplankton biomass growth equals phytoplankton biomass losses. In this hypothesis, losses are both constant and independent of growth. The decline in biomass may be due to grazing, sinking, dilution, vertical mixing, infection, or parasitism. When the surface mixed layer becomes shallower than the critical depth, initiation of the seasonal bloom occurs due to phytoplankton growth exceeding loss. There is a correlation of phytoplankton growth with springtime increases of light, temperature, and shallower stratification depths.
Climate warming may increase stratification or decrease mixed layer depth during the winter, which would enhance the vernal bloom or increase phytoplankton biomass if this hypothesis governed spring phytoplankton bloom dynamics. A primary criticism of this resource-based view is that spring blooms occur in the absence of stratification or shoaling of the mixed layer.
Dilution-recoupling Hypothesis - an ecosystem-based view Source:
The dilution-recoupling hypothesis is an ecosystem-based view of the North Atlantic annual phytoplankton bloom. This hypothesis focuses on the physical processes that alter the balance between growth and grazing. The spring bloom is considered to be one feature of an annual cycle, and other features during the cycle "set the stage" for this bloom to occur.
This ecosystem-based view is based upon a dilution experiment where the addition of seawater dilutes predators but does not change the growth of phytoplankton. Thus, growth rates increase with dilution. Although the dilution effect is transient, predator-prey interactions can be maintained if the rate of the addition of water equals the rate of growth. The deepening of the surface mixed layer dilutes the predator-prey interactions and decouples growth and grazing. When the mixed layer stops deepening, the increase in growth rate becomes apparent, but now growth and grazing become coupled again. The shoaling of the mixed layer concentrates predators, thereby increasing grazing pressure. However, the increase in light availability counters grazing pressure, which allows growth rates to remain high. In late spring, when the mixed layer is even more shallow, nutrient depletion or overgrazing ends the bloom—losses exceed growth at this point in the cycle.
Climate warming would increase stratification and suppress winter mixing that occurs with the deepening of the mixed layer. The suppression of winter mixing would decrease phytoplankton biomass under this hypothesis.
Physical oceanographic processes Mixed Layer Depth Debate Meso-scale Eddies
Mesoscale eddies play a significant role in modulating the Mixed Layer Depth (MLD). Fluctuations created by mesoscale eddies modulate nutrients in the base of the mixed layer.Gaube, P., J. McGillicuddy Jr, D., & Moulin, A. J. (2019). Mesoscale eddies modulate mixed layer depth globally. Geophysical Research Letters, 46(3), 1505-1512. These modulations, along with light availability, drive the abundance of phytoplankton in the region. The availability of phytoplankton significantly affects the marine food web and ocean health.
The fast-moving currents in the Gulf Stream meander and pinch-off to create eddies. These eddies retain the physical properties of their parent water mass (e.g. temperature, density, salinity, and other ocean dynamic properties) when they separate. As the eddies migrate, their physical properties change as they mix with the surrounding water. In the Gulf Stream, migrating eddies are known as anticyclonic or cyclonic eddies based on the direction in which they spin (clockwise vs. counter-clockwise). The two eddies differ in motion, physical properties, and, consequently, their effects on biology and chemistry of the ocean.
The Coriolis force combined with high velocity currents drive eddy motion. This motion creates a 'bulge,' i.e., high sea surface height (SSH) in the center of the Anticyclonic eddies. In contrast, cyclonic eddies exhibit a low SSH in the center. The SSH in both anticyclonic and cyclonic decreases and increases, respectively, as the distance from the center increases.Gaube, P., Chelton, D. B., Samelson, R. M., Schlax, M. G., & O'Neill, L. W. (2015). Satellite observations of mesoscale eddy-induced Ekman pumping. Journal of Physical Oceanography, 45(1), 104-132. Upwelling and downwelling processes in the eddies create a cold and warm core.Chi, P. C., Chen, Y., & Lu, S. (1998). Wind-driven South China Sea deep basin warm-core/cool-core eddies. Journal of Oceanography, 54(4), 347-360.
Chicago Downwelling in the anticyclonic eddy prevents colder water from entering the surface, thus creating a warm-core in the center. Whereas in the cyclonic eddy, the upwelling entrains deep cold water and forms a cold-core.
Previous studies show the deepening effects of MLD under anticyclonic eddies and shoaling of MLD in cyclonic eddies.Klein, P., Treguier, A. M., & Hua, B. L. (1998). Three-dimensional stirring of thermohaline fronts. Journal of marine research, 56(3), 589-612.Kunze, E. (1986). The mean and near-inertial velocity fields in a warm-core ring. Journal of physical oceanography, 16(8), 1444-1461. These phenomena may be due to increased heat loss to the atmosphere in anticyclonic eddies. This loss of heat causes the sinking of dense water, referred to as convective mixingTalley, L. D. (2011). Descriptive physical oceanography: an introduction. Academic press., and the deepening of the MLD. In contrast, in cyclonic eddies the water temperature at the core is less cold than the Anticyclonic eddy. This therefore does not lead to deepening of the MLD. Studies conducted in the region via a network of Argo floats and model simulations created through satellite data have shown cases of the opposite phenomena. The deepening and shoaling of MLD via eddies is ubiquitous and varies seasonally. Such anomalies are most significant in the winter. Thus, the role of meso-scale eddies in MLD is complex, and a function of simultaneous processes where enhanced wind shear induced currents contribute to a shallowing of the MLD in anticyclonic eddies.
Relevant Atmospheric Processes Marine Boundary Layer The marine boundary layer (MBL) is the part of the atmosphere in direct contact with the ocean surface. The MBL is influenced by the exchange of heat, moisture, gases, particulates, and momentum, primarily via turbulence. The MBL is characterized by the formation of convective cells (or vertical flow of air) above the ocean surface, which perturbs the direction of the mean surface wind and generates texture, roughness, and waves on the sea's surface. Two types of boundary layers exist. One is a stable, convective layer found between the lower 100m of the atmosphere extending up to approximately 3 km in height, and is referred to as the convective boundary layer (CBL). The other boundary layer forms as a result of a surface atmospheric inversion. This generally occurs closer to the surface in the absence of turbulence and vertical mixing, and is determined through the interpretation of vertical humidity and temperature profiles. The MBL is often a localized and temporally dynamic phenomenon, and therefore its height into the air column can vary considerably from one region to another, or even across the span of a few days. The North Atlantic is a region where diverse and well-formed MBL clouds are commonly formed, and where MBL layer height can be between 2.0-and 0.1 km in height
Regional Atmospheric Processes The westerlies are prevailing winds in the middle latitudes (between 35 and 65 degrees latitude), which blow in regions north or southward of the high-pressure sub-tropical regions of the world. Consequently, aerosols sampled over the North Atlantic Ocean will be influenced by air masses originating in North America, and therefore be characterized by both the natural terrestrial and anthropogenic inputs. Relevant to NAAMES are the emissions from industry and urban environments in eastern North America, which emit substantial quantities of sulfates, black carbon, and aromatic compounds. Such substances can be transported hundreds of kilometers over the sea. This contribution of continental influences may create a false positive signal in the biological fluorescence signals being measured and could affect cloud microphysical properties in the open North Atlantic Ocean. Furthermore, aerosols such as black carbon mixed with carbon dioxide and other greenhouse gases are emitted through the impartial combustion of fossil fuels from ship engines. These unburned hydrocarbons are present in the marine boundary layer of the North Atlantic and most other remote oceanic regions. As these particles age or are chemically transformed as a function of time in the air, they may alter microphysical and chemical properties as they react with other airborne particles.
Role of aerosols
Aerosols