Climate Change

Atmospheric traffic jams halt nutrient flow to ocean phytoplankton

Journal source: Le, Chengfeng, S. Wu, C. Wu, M. W. Beck, and X. Yang.  2019.  Phytoplankton decline in the eastern North Pacific transition zone associated with atmospheric blocking.  Global Change Biology 25:3485-3493.

Imagine: It’s a crisp fall day, the sky is that perfect blue color, and the trees are showing off their hues of reds, oranges, and yellows. To top it off, a breeze rolls in, rocking the trees back and forth, leaving you with a deep sense of the autumn season. These same winds that we enjoy in our backyards have traveled far and wide, across oceans and continents, and play a vital role in ocean circulation patterns. Atmospheric wind patterns such as the westerlies and trade winds push water along the ocean’s surface, which creates currents that drive the global ocean circulation. These currents, such as the Gulf Stream, transport nutrients, such as nitrogen and phosphorus, to regions that are otherwise lacking high enough levels to support healthy food webs. This is important because floating algae called phytoplankton readily slurp up these traveling nutrients and increase in numbers, forming the foundation of ocean food webs.

An example of phytoplankton-here are a variety of tiny one-celled algae and a big diatom. Photo credit: K. Barrett.

Just as we see fluctuations in how strong the wind can be, the ocean also experiences changes in the intensity and frequency of wind patterns, which affects how surface waters from different regions intermingle. Transition zones, areas where colder, nutrient-rich waters mix with warmer, nutrient-poor waters, are especially affected by winds. One such zone is the North Pacific transition zone (NPTZ) in the North Pacific Ocean, where cooler waters flowing from polar regions around Alaska meet up with warmer waters from the tropics. This zone is nearby well-known garbage patches, which are also formed by currents. With the strong southward currents bringing in nutrient-rich water, this transition zone is known for its high levels of phytoplankton production. Because of its high productivity, the NPTZ provides critical foraging habitat for a number of marine species, such as tuna, loggerhead turtles, albatross, flying squid, sharks, and many more.

A big-picture view of the North Pacific Ocean and the wind patterns that create currents and transition zones. Photo credit: NOAA Oceans ia Wikimedia Commons Modified by K. Barrett

However, a recent decline in the amount of phytoplankton in this zone has many scientists concerned. While changes in atmospheric wind patterns may be at play, a team of researchers led an investigation into the links between wind patterns and phytoplankton.

The study

It is thought that phytoplankton productivity in the NPTZ is affected by the westerly winds and north-south currents that transport nutrient rich waters from the north to the NPTZ. In recent years, a type of atmospheric activity called atmospheric blocking was reported over Alaska, which lies in the path of those westerly winds. Atmospheric blocking occurs when the jet stream meanders too much and causes high pressure air to build up and just sit in the sky, resulting in a sort of traffic jam. These areas of congestion then block the movement of the westerlies. This stagnant air can impact the pace of ocean currents and consequently the movement of nutrients that are vital for fueling marine food webs.

Chengfeng Le of Ocean College, Zhejiang University in China and colleagues used a long-term time series of satellite observations and climate datasets to determine the link between atmospheric blocking and the declines in phytoplankton abundance in the NPTZ. To estimate changes in phytoplankton abundance, they turned to long-term changes in chlorophyll-a based on satellite color imagery. They used chlorophyll-a because it is a pigment present in the cells of phytoplankton that is used to in photosynthesis. This pigment produces a rich green color that satellite images can detect, making it a useful way to estimate phytoplankton abundance. Climate variables such as wind speed and atmospheric blocking incidences were obtained from National Center for Environmental Prediction/National Center for Atmospheric Research. The researchers determined the incidences of blocking by calculating wind and air pressure anomalies over Alaska.

Connecting wind and ocean

Le and colleagues found a strong positive relationship between phytoplankton abundance and the north-south current patterns that bring in nutrient laden waters to the NPTZ. This finding supports the notion that subarctic, high-latitude waters are the primary nutrient source supporting the high phytoplankton abundance in this transition zone. Furthermore, this suggests that changes in westerly winds are influencing phytoplankton abundance because these winds directly impact the north-south transport of nutrient-rich water.

A conceptual diagram of how winds are linked to changes in phytoplankton in the North Pacific Transition Zone. Diagram created by K. Barrett on Biorender and modified from Figure 9 in Le et al. 2019.

The researchers also found a strong relationship between atmospheric blocking over Alaska and phytoplankton. A strong blocking event over Alaska in 2014 was associated with reduced westerly wind activity and a reduction in north-south water movement to the NPTZ. Following these changes, the scientists observed a dramatic decrease in phytoplankton. For the first time ever, scientists could show the link between atmospheric wind anomalies and phytoplankton dynamics in the NPTZ.

The bigger picture

Climate change is not only directly impacting our oceans by raising surface water temperatures and causing acidification, but it also has important indirect effects by influencing how winds interact with ocean surfaces. Shifting of atmospheric winds, especially the slowing down of major wind patterns, can alter or even block the transport of nutrients from one region to another, with potentially cascading effects on food webs.

As climate change proceeds, it is likely that atmospheric blocking events will increase, and are likely to occur in other oceans. Knowledge of blocking events in the mid-latitude areas of the Pacific Ocean may help with forecasting changes in phytoplankton production in the NPTZ. Additionally, the health of the NPTZ and its rich flora and fauna may be at risk from atmospheric anomalies. Further work is needed to see if similar blocking events occur in other oceans, and whether these are also contributing to reductions in phytoplankton abundance.

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