Report – ENVR210 – Midterm

Q1. a) (I) Define, then (II) describe how density (sigma-t) in seawater is used to determine water structure and (III) what factors (processes) increase/decrease sigma-t. [5 mks]

I) Definition: “An abbreviated value of the density of a seawater sample of temperature, T and salinity, S: σT = [ρ(S,T)-1] × 103, where ρ(S,T) is the value of the seawater density in centimeter*gram*second (cm*g*s) units at standard atmospheric pressure.”

– Simplified like this: (D-1.0)*1000 e.g. Density 1.02354; σT = 23.54 (Note: Unitless).

II) σT is a great tool for scientist to determine the ‘heritage’ of the water mass on a particular depth by measuring the temperature and salinity at that preferred depth.

E.g. if we took a sample at 3km depth anywhere between 50⁰N to 50⁰S in the Atlantic Ocean we most likely would find the sample to be in the range of 3-5⁰C & 34.9-35.0 PSU, which correspond to the properties of the North Atlantic Deep Water (NADW). Consequently, we’ll find cooler and slightly less salty water even further down when we enter the Antarctic Bottom Water (AABW) layer.

III) σT increases as temperature decreases and as at the same time salinity increases – i.e. warm tropical waters have a low σT, while off Greenland and Antarctica where the cold waters have a fairly constant temperature of less than 4⁰C the σT tends to be higher.

b) (I) Describe water mass formation at high (polar) latitudes and how this influences deep ocean thermohaline circulation (You can limit your discussion to circulation in the N and S Atlantic Ocean). (II) Include an explanation of Antarctic and Arctic convergent and divergent zones [10 mks]

I) Ocean circulation is mainly due to water density, often called thermohaline circulation. This all due to the unique property of water where it has its highest density at 4⁰C (freshwater) and not at it’s freezing point, this causes cooled surface water to sink during the winter months.

The cold, high density, melt waters from Greenland and Antarctica directly sink to flow along the bottom of the oceans to form the North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) respectively, where the later is the coolest and densest and thus below the NADW.

II) Since NADW flows from North to South and AABW the South to North there will be mixing at some points. These major points are called divergent or convergent zones. Since AABW, as stated, is heavier than NADW and flows below, it creates a so called Antarctic Frontal Zone roughly between 50⁰S to 65⁰S in the Atlantic Ocean.

At the Antarctic Frontal Zone the NADW is pushed up and some of it, the lower and more denser water, is cooled down by, and together with, the AABW and diverge with the AABW and gets pushed down and later North, yet again, together with the new melt water from the Antarctic.

The less dense NADW is pushed up by the denser AABW and at the surface will start to be pushed North in a new higher layer at the Antarctic Convergence and form the Antarctic Intermediate Water (AAIW).

Scientist estimate that it takes approximately 1000years for the melt water to travel from pole to pole.

Q2. (I) Discuss the major air pressure systems in the summer and the winter for the Northeast Pacific Ocean. Include in your answer what factors result in these high and low pressure systems;

(II) how geostrophic winds (include a definition) are generated from these pressure systems; and

(III) the effects of friction with the earth’s surface on these winds.

(IV) Discuss the effect on the coastal currents off the BC coast (e.g., direction of currents, upwelling and downwelling) that result from these wind systems. [10 mks]

I) Generally from the BC/Yukon boarder to about the US/Mexican boarder (60⁰N to 30⁰N) we do expect westerly winds all year. They do change somewhat over the year, with the summers it tends deflect clockwise (northwesterly winds) after hitting the the North American coast, bringing cooler air from the north down south. Some time in late fall (Oct-Nov) the winds tend to change direction after hitting the coastline and form a counter-clockwise (southwesterly) wind pattern.

This change is brought about by low and high pressure (LP/HP) systems during the year. In summer the water is cooler than the land mass, thus creating a LP zone over the continent due to the rising air, and HP zones over the North Pacific. The air from the HP zone over the pacific descends and flows outward towards the LP zone. During winter, however, the water is warmer than the land mass and the opposite happens; LP over the Pacific ocean and a HP over the continent.

Picture Credit:

II) Geostrophical winds are winds which follows lines of equal pressure (isobars) circulating clockwise or counter-clockwise around a high- or a low- pressure zone.

The pressure gradient flow (HP to LP) is affected by the Coriolis Effect (see definition below). This results in deflecting the wind directions’ vectors that have a clockwise direction around a HP cell in the Northern Hemisphere. These winds are called anticyclonic geostrophic winds.

During the winter months, the reversed pressure gradient, LP over the ocean and HP over land, cause the air to be pressed out towards the LP and rise; upward moving air is directed to the right (CCW) by Coreolis Force. CCW airmovement is called cyclonic geostrophic winds.

Coreolis Effect – “an apparent deflection of moving objects when they are viewed from a rotating reference frame”.

When most matter is set in motion it moves in straight paths, while the earth continues to rotate independently. E.g. a sniper firing a bullet towards a target at long distances, he has to take into account that the target is actually ‘not there’ where he aims at the time of the firing. Since the bullet is not affected by the earth rotation during its travel time.

The Coreolis effect is an apparent deflection in which a freely moving object moving in a straight line appear to travel in a curved path when viewed with the perspective of the surface of the Earth  instead of the objects’.

III) From sustained winds like the the Westerlies, surface currents are generated. The water movement accounts for about 1-3% of the wind speed. The generated currents are affected by the Coreolis force and are deflected to the right in the Northern Hemisphere. This deflection results in a water movement of 45⁰ from the wind. This friction drags along layers of water below the surface which in turn also are affected by the Coreolis force, making each subsequent layer of water move to the right of the layer above, which results in an overall spiral effect called Ekman Spiral. The effect of this is dependent on the strength of the winds but may be up to 100-150m. Since every layer is turning to the right, the bottom currents may even move in the opposite direction to the surface currents, but due to loss to friction only at a low partial force.

IV) As discussed earlier, the pressure system change over the seasons causes the cold water from the north, that is pushed south along the coast, to dwell up cold and nutrient rich waters during late summer (Sept) along the west coast. This causes all kinds of benefits for the flora and fauna in the area. The Updwelling is due to several factors including; the NW winds, Coreolis and Ekman Transport.

3.     I) Define the following ocean processes with respect to surface currents and mixing in the upper waters.

II) Describe each process, using diagrams where appropriate.

a) Western Intensification;

I) Currents along the western boundaries of ocean basins in the N. Hemisphere tend to be faster, deeper, narrower and warmer compared to the eastern boundary currents.

II) Since most of the water in the Equatorial Current is not affected by the Ekman Transport nor the Coriolis Force there is more water being forced, aided by the stronger Trade winds, against the North American continental slope which increases the geostrophical flow. The planetary vorticity also aids the increasing current flow northward in  the North Atlantic ocean gyre.

b) Eddies along the Gulf Stream;

I) Local surface water mixing process – can be described as bubbles of warmer or colder  water disconnected from the main flow of a current.

II) If you place a rubber ducky with a GPS in the waters off the coast of Florida and record its position, you would find that the rubber ducky would sometimes move in nearly circular patterns.  Some of these loops would be roughly 100 to 200 km in diameter.  These loops are known as mesoscale eddies.  These features are important because they are “hot” spots of intense biological and physical activity.

It is called a cyclonic eddy, if the rubber ducky moves in a anticlockwise direction in the N hemisphere.  The center of the eddy is likely cooler than the outer lying waters.  On the other hand, if the rotation of the ducky is clockwise, the feature is called an anti-cyclonic eddy and the center is warmer than outer waters.  The cyclonic eddy is called a cold-core eddy and forms on the east side of the main current or ring and the anti-cyclonic eddy is called a warm-core eddy and forms on the western side of the main gulf stream.

c) Coriolis Force and its influence on surface currents;

I) Coriolis Force – “an apparent deflection of moving objects when they are viewed from a rotating reference frame”. When most matter is set in motion it moves in straight paths, while the earth continues to rotate independently.

II) Surface currents move in 45⁰ angle to the right of the direction of the wind in the northern hemisphere. Due to the Trade winds and the Westerlies blowing in opposite directions the Coriolis force (together with the Ekman transport) moves the water in circular motion. Since the Coriolis force ‘pushes’ the force 45⁰ to the right, it creates a huge s.k. gyre in the large oceans where the force presses the water inward and in fact builds a mound of the water of about a meter in height.

d) open ocean (e.g., not coastal) regions of convergence and divergence. [4×5=20 mks]

Ia) Convergence; due to a number of factors like the great ocean gyres, the Coriolis force, Ekman transport water can pile up and cause a water mound. In the middle of this mound the water have only one way to go due to gravity and that is down causing down dwelling e.g. bringing high oxygenated waters into the deep. It can happen locally as well, due to on-shore winds and sinking of polar waters.

Newton’s third law states: To every action there is always an equal and opposite reaction. Basically, what goes down, must come up…

Ib) Divergence; when the North Atlantic Deep Water (NADW) has reached about 60⁰S on its way south in the Atlantic it is pushed all the way up to the surface by the denser and colder Antarctic Deep Water (AADW) and the Antarctic continent. With it the NADW drags lots of decayed phytoplankton and other nutrients from the depth and causes a so called updwelling. These areas are very important in both mixing of the oceans and also for marine life, as they are the feeding stations for e.g. the marine mega fauna like whales.

Basically, water of same temperature and salinity move over the great oceans, when they meet up water of a different temperature and/or salinity it is pushed either down or up, the lighter/warmer water is pushed up, much like tectonic plates, when they collide and creates an updwelling which in turn turns into a divergence zone on the surface.

4. a) I) Describe the differences between Holomictic and Meromictic lakes.

Holomictic lakes are lakes that at some time during the year, have a uniform temperature and density from top to bottom, allowing the lake waters to completely mix. Although the permanent ice-cover of a mictic lake prevents mixing, they are also regarded as being holomictic. The mixing of the more common holomictic lakes can be driven by wind, which creates waves and turbulence at the lake’s surface, but wind is only effective at times of the year when the lake’s deep waters are not much colder than its surface waters, please see discussion below about the build-up and breakdown of a thermocline.

A meromictic lake (e.g. McGinnis Lake in Petroglyphs Provincial Park, Ontario), on the contrary to holomictic, has layers of water that do not intermix.

II) Then describe the differences between Dimictic, Monomictic and Polymictic types of Holomictic Lakes. Give an example of each type (i.e., geographic place name from anywhere in the world). [10 mks]

In monomictic lakes (e.g. Lake Washington, WA) the mixing occurs once a year [mono]; in dimictic lakes (e.g. Kootenay Lake, BC) the mixing occurs twice a year [di] (typically together with spring and autumn storms), and in polymictic lakes (e.g. Southern Indian Lake, MB) the mixing occurs several times a year [poly].

b) Describe the development and the breakdown of a thermocline in the top 100 metres of the Pacific Ocean offshore of Canada. Include a diagram with your answer. [10 mks]

Starting the annual circle in March where we have a uniform temperature of this the top 100m portion of the ocean, which tend to be at the time of storms and lower temperatures than January which in turn will contribute to complete mixing of this portion. This is essential that both happen at the same time, i.e. a late summer storm (Sept/Oct) will only stir the portion above the thermocline.

With increasing surface water temperatures a thermocline starts to develop with a beginning in April/May it’ll grow more pronounced during the summer months, from the summer warmth, with it being fully developed in August.

A second less defined thermocline can also develop in the middle of the summer at greater depths (45m on the diagram). Temperatures quickly decreases from that point, from about 8⁰C to around 4.5⁰C as we descend further down to 100m.

Starting in August it gets a little bit cooler and the thermocline starts to sink. In September we can see the thermocline being almost 10m deeper as in August and the second level has been leveled out. This trend continues in November and January, where just after the new-year in January the temperature is fairly homogeneous throughout the top 100m with the thermocline sinking even further and can be found just around the 100m mark. One reason for it not getting cooler during January, and having March as the coolest month, could be ice-coverage and thus not allowing winter storms circulate and mix the water.

5. Describe the events (offshore and in the water basins of Juan de Fuca Strait, Haro Strait and Satellite Channel) which results in complete or partial turnover of bottom water in Saanich Inlet.

[10 mks ]

The major components of the oceanography of the Saanish Inlet is quite complicated and it’s unusual, if not unique, in that there is little freshwater input and therefore no estuarine circulation.
The currents in the Pacific ocean changes along the west coast of North America during the year, due to the high and low pressure systems change as discussed in Q2. In summery; a semi-permanent low pressure over the Aleutian Islands (Alaska) causes a strong northbound current, warmer water with lower salinity, named Davidson Current along the west coast of Canada during the winters. This causes downdwelling along the coast of Vancouver Island.
Juan de Fuca Strait is fairly uniform in shape and thus the water is widely mixed. It’s not until the water hits Haro Strait and San Juan Islands that the obstacles starts.
Columbia River influences the movement of warmer, lower salinity and water with higher dissolved oxygen. The mixing in Haro Strait with very cold surface waters causes the warmer water to form an intermediate water layer but doesn’t always result in renewal or mixing with the remnants of the warm, high salinity waters from the previous summer.
The freshwater discharge, from mainly Fraser River, with a massive 1.000m3/s to 10.000m3/s evidently helps to stir the Strait of Georgia during maximum discharge in May (melt waters) and during the fall storm run-offs.
During the summers in the Pacific ocean, we normally experience a Pacific high pressure system and we experience updwelling offshore and with colder, saltier and lower in dissolved oxygen deepwater coming in the Juan de Fuca Strait in the late summer/early fall. At Haro Strait it mixes with warmer, lower salinity surface layer but even after this mixing the resulting water is dense enough to flow up the Strait of Georgia and into the Saanish Inlet replacing existing remnants of cold, low salinity winter waters.
Since Saanish Inlet has a ‘sill’ of more shallow waters (Satellite Channel) the water from Haro Strait don’t manage to mix more than annually as a result from high density oceanic water which is higher in dissolved oxygen from the mixing in Haro Strait during intense spring tidal mixing. The complete or partial mixing is dependent on the density of the water flows through Satellite Channel. If it is not of high enough density the water it might not renew the existing water in the inlet.

6. You are planning to conduct a geological and biological survey along a stretch of shoreline at Point No Point (which is just before Jordan River), and again along the shoreline near Crofton. The survey can only be carried out during the two hours before and two hours after low tide. The work is to be done during the lowest tides of the summer. What should be the two days that you work and during what hours each day?

[Use the July to September, 2001 Sooke and Fulford Harbour tide tables, and the appropriate corrections for Secondary Ports; copies are available from the instructor and on the D2L site.] [5 mks]

I found that for
    • Crofton (Fulford Harbour) the low water is +0.14min and mean water level is +0.3ft.
    • Point No Point (Sooke) the low water was -0.50min and the mean water level is +0.0ft
Browsing through the tide tables I could only found three times with a 0.0ft tide (I couldn’t see any negative low tides). For Crofton I found that both the 20th and the 21st of July had 0.0ft low tides and at Point No Point only had 0.0ft tides the 21st.
Due to logistics I would go to Crofton the 20th July and start surveying at 07:51 until 11:51 (low tide at 10:05 – 0.14 = 09:51am +/-2h) then, conveniently, go to Point No Point the day after 21st July and start surveying at 06:05 until 10:05 (low tide at 0855 – 0.50 = 08:05am +/-2h).

Leave a Reply