Ocean Circulation, Upwelling, Downwelling, and Ekman Transport

In the beginning of the lecture, there was an explanation about how ocean can circulate at one place to another in every part of this world. Wind pressure is one of the factor which is driving the ocean currents. It happens when the wind moves through the sea and causes the molecules of the air and the fluid having a collision where the energy is transferred. If there is a deflection happened between more than one different directions of large wind driven the ocean, this results a circular motion of current called a gyre. Gyres are generally caused by planetary motion of vorticity along with horizontal and vertical friction, Coriolis effect, and landmasses which is blocking the gyre to circle in one particular place. There are five most known gyres, those are Indian Ocean Gyre, North Atlantic Gyre, North Pacific Gyre, South Atlantic Gyre, and South Pacific Gyre.

As the wind is driving the surface ocean current, the total of water transported end up moving in a complete different direction from the wind. This phenomenon can be explained by Ekman spiral and transport theory. The theory clarifies that the drag at wind-water interface and Coriolis effect caused the surface current to be deflected and moves continuously in different direction around 45° to 90° as the current becoming weak in greater deep of the ocean. This illustration can be pictured as Ekman spiral, where there is a wind moving straight and the surface current is moving in 45° from the wind and Ekman current is deflected into 90° from the wind. The total of deflected currents is called Ekman transport.

Ekman current theory can explain how the surface current and the current below are having an interaction called upwelling and downwelling. Upwelling occurs where Ekman transport moves surface waters away from the coast and surface waters are replaced by water that wells up from below. In contrast, downwelling occurs where Ekman transport moves surface waters toward the coast then water piles up and sinks below the surface current. In the place where upwelling takes place, waters in the surface is becoming colder than the surface waters which have been replaced, while downwelling transports heat into the current below the surface. Apart from the temperature influence, upwelling could increase the biological productivity in the surface by helping the growth of phytoplankton and downwelling transports the dissolved oxygen from the surface to below.

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About Density, Temperature, Salinity, Mixed Layer, and Dissolved Oxygen in the Ocean

Density of a sea is distributed along with horizontal pressure gradients and ocean currents, while it is influenced by temperature and salinity. Temperature and salinity of a sea are influenced by evaporation, precipitation, melting of sea ice, and heat fluxes which can also calculate the density of the ocean. They can be interpreted by a graph of temperature based on depth of a sea, it shows there is an extreme change of temperature on the area of the surface of sea. This happens because of the radiation of the sun and could also lead to severe changes in salinity and density of the sea.

            Salinity is the total amount of dissolved material in grams in one kilogram of sea water. Salinity extend from 34.6 to 34.8 ppt and the unit has no dimension. Salinity is allocated into surface salinity which varies from 33 to 37 ppt, upper layer salinity which influenced by temperature than density, and deep water salinity which has an average from 34.6 to 34.9 ppt. In an open ocean, the salinity varies at 0.5 ppt annualy. Measuring salinity is calculated on chlorinity or conductivity of the sea.

            Temperature is allocated into three different layers such as surface temperature which has low temperature in coastal area as a cause of upwelling, upper layer and thermocline which has severe decreasing temperature on the thermocline, and deep water layer which is increasing in temperature because of the pressure. In the upper layer, temperature varies about 1-2 K in the equator and 5-10K in 40° lattitude.

            Ocean mixed layer is a 10-200m thick layer which take place on tropical and mid-lattitude where temperature and salinity must be constant. Ocean mixed layer created as wind blows the ocean and stirs the upper layer at the surface of the sea. In a mixed layer, depth and temperature varies in response to heat fluxes and turbulences. When evaporation exceeds precipitation, mixed layer is becoming saltier than the thermocline as it happens in 10° to 40° lattitude, but it is fresher in high lattitude because of rain and ice melting.

Review on Dissolved Oxygen

            Dissolved oxygen is an amount of oxygen dissolved in a body of water such as a lake, river, or stream. Dissolved oxygen is the most important indicator of the health of a water body and its capacity to support a balanced aquatic ecosystem of plants and animals. Wastewater containing organic pollutants depletes the dissolved oxygen and may lead to the death of marine organisms. Dissolved oxygen comes from the air through the plant and slowly diffuse across the water’s surface so it is mixed through aeration caused by wind or a man-made air pump.

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What is physical oceanography?

Our earth has only one ocean which is covering the planet. The ocean is divided into three named parts by agreement, Atlantic, Pacific, and Indian. Atlantic ocean is located between the continent of America and Africa, Pacific ocean can be found between the continent of Asia and America, and Indian ocean is located below the South Asia. The ocean has been our source of life and main topic of science for thousands of years. In fact, we are using the ocean to gain food, transport, obtain oil and gas, do sport and recreation, and understand weather. Thereby, some studies are needed to understand our ocean, oceanography is one of them. With oceanography, we can study the character of the ocean as an evironment so we are able to predict the future of anything that will happen in the ocean. As oceanography happens to be general, there are some studies which are more specific like biological oceanography, chemical oceanography, geological oceanography, and physical oceanography.

            Physical oceanography makes us understand about physical properties and dynamics of the ocean. Physical oceanography involves some factors which take place at the ocean, those are temperature, wind characteristic, depth, wave, tidal wave, current, biomass, salinity, etc. These factors are required to gather data and model our ocean. For instance, determining the depth of an ocean can help us to build an offshore structure with a specific type that will adjust correctly in the ocean. In this case, the depth of the ocean shall be measured first by several methods such as echo sounding and satellite altimetry. Despite the cost required for echo sounding is way more expensive than satellite altimetry, echo sounding appeared to be more accurate in measuring the depth of ocean rather than the other one. Echo sounder device works by transmitting sound pulses in to the ocean and recording the interval time as the pulses collected by the receiver. Furthermore, the data of depth of the ocean can determine the ocean floor and classify the depths in to shelf, slope, basin, canyon, plain, ridge, seamount, sill, and trench.

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