Upwelling and Downwelling

What have we learned about water movement in the Western Durham nearshore?

In the nearshore, upward and downward water movement can occur if certain wind conditions exist. Relatively strong winds will cause the surface waters to either move away from the shore causing an upwelling, or the winds will cause surface waters to move towards the shore causing a downwelling. According to Csanady (1972), water up to 5km from the shoreline can be involved in these events!

Along the north shores of Lake Ontario:

• Strong winds from the west to the east are ideal for upwellings to occur.
• Strong winds from the east to the west are ideal for downwellings to occur.

Upwelling

• Occurs when dense cool nutrient rich water from the bottom of the water column offshore replaces the nutrient depleted surface water in the nearshore.
• Is driven by wind, the Coriolis effect, and Ekman transport
  • Wind blows across the lake.
  • Water is transported 90 degrees from the direction of wind (Coriolis forces/Ekman transport).
  • Friction between the surface water and water underneath surface layer causes both water parcels to move in the same direction.
  • As water moves away from the shore, the lost water is replaced by upwelling of deep waters.
• Upwelled nutrient-rich water can provide nutrients (nitrate, total phosphorus, soluble reactive phosphorus, etc) for biological growth in the nearshore.

Downwelling

• Occurs when surface water becomes more dense and sinks to the bottom of the lake.
• Is driven by wind, the Coriolis effect, and Ekman transport.
  • Wind blows across the lake.
  • Water is transported 90 degrees from the direction of wind (Coriolis forces/Ekman transport).
  • As water moves towards the shore, the water already present accumulates or “piles up” and the pressure of this water causes it to sink down to deeper waters.
• Transports dissolved oxygen to deeper waters, affecting decomposition in surface waters.

Identifying Upwellings and Downwellings: Instruments of Use

We can use the temperature at the surface and at the bottom of the lake to identify upwellings and downwellings. Temperature can be retrieved from:

a) Thermisters (temperature chains that are suspended in the water column).

b) The Land Ocean Biophysical Observatory (LOBO). The Ontario Ministry of the Environment, Conservation and Parks has deployed this instrument in the Ajax region from 2008-present during the ice-free months. It collects surface and bottom information on a variety of parameters including temperature, conductivity, chlorophyll a and turbidity.

Identifying Upwellings and Downwellings

By Temperature Graphs:

If we compare the 2009 surface and bottom temperatures in the lake, we see that there are times when:

1. Surface waters suddenly drop and match the bottom temperatures: upwelling
2. Bottom waters suddenly increase and match the surface temperatures: downwelling

The graph below shows the 2009 surface and bottom temperatures at the LOBO station offshore from Duffins Creek (see Figure 8 below for the location of the LOBO). An example of an upwelling and a downwelling event are pointed out by the blue arrows on the following graph.

By Temperature Differences:

To make it easier to see, we can calculate when there is a large temperature change within 24 hours. The graph below calculates the difference between the top and the bottom water temperatures.

If the bar falls above the dotted line at 4 degrees Celsius, we know that a downwelling has occurred. If the bar falls below the dotted line at -4 degrees Celsius, we know an upwelling has occurred.

By Spatial Maps:

Another way that we can see upwellings is using spatially interpolated nutrient concentrations from the surveys completed by the Ontario Ministry of the Environment, Conservation and Parks in 2008.

The maps below are from a published article and show that temperature is lower by the shoreline than it is 5 km from the shoreline. This is because the water by the shoreline was pushed away from the shore, and was replaced by bottom waters in an upwelling event.

During this event, chlorophyll a disappeared by the shore, but nitrate rich waters appeared by the shore transported from the nutrient rich waters at the bottom of the lake.

Since wind, water temperature, and circulation play an important role in upwellings and downwellings, the number of upwellings and downwelling in the Ajax nearshore can potentially change year to year.

If we look at the number of upwellings and downwelling from 2008-2012, we see that although different climatic conditions exist, the number of upwellings and downwellings were coincidentally similar.

Table 1: Number of upwellings and downwellings calculated from the LOBO or thermisters in 2008-2012:

NOTE: the time frame that the LOBO was out changed from year to year, however, it was generally logging temperature from April until November.

1. The LOBO malfunctioned in 2008; upwelling value from Howell et al., 2012, downwelling calculated from the deepest meter available
2. The surface LOBO temperature recorder malfunctioned; no thermisters logging temperature

Understanding Water Movement in the Nearshore

Richardson Numbers: Mixing between surface and bottom waters

Richardson numbers are another way to look at the mixing that occurs between surface and bottom waters. In technical terms, Richardson numbers use current velocities and temperature differences to describe the stability of a parcel of water in a water column.

If the ratio between the stabilizing forces due to stratification and the destabilizing forces due to vertical shear is above a critical value, then the water is stable and the surface and bottom waters do not mix. If the number is below the critical value, then the water mixes.

What this means is that if we calculate this number and it is greater than a specific value, then no mixing can occur between the layers of water. If the calculated number is below a specific value, then the water layers can mix.

In this case, the top and bottom waters can mix because they can overcome the forces between them.

Near Duffins Creek, Richardson numbers shown for the entire year tell us that most of the time the top and bottom waters are different. However, in the fall there is a lot of mixing between the top and bottom waters.

There are two graphs placed on the map below. The graph on the left is from a water depth of ~15 meters, while the graph on the right is from a water depth of ~18m.

There are differences in the two graphs: the left graph has more points below the red line showing that more mixing is occurring. This tells us that there are changes occurring between the two sites and the deeper that you are and that in deeper waters, the surface waters are not mixing with bottom waters as much.

Note that in the graphs on the image below are calculating Richardson numbers between approximately 4-5 m below the water surface, and 13-15 m below the water surface.

Understanding how the nearshore mixes is important to understand as it will tell us the path that nutrients will follow when they enter the lake.

Take Home Messages

1. Upwellings and Downwellings naturally move nutrients in the nearshore.