ATOC 5051: Introduction to HW #1: answerkey Given: Jan 24; Due Feb 7, 2012

1. Ocean basins (25pts)

1a. Describe and compare the basin geometries (including the area, zonal and meridional extent, mean depth, and connection with other ocoeans) for the Pacific, Atlantic, Indian, and Arctic oceans (8pts).

The Pacific: The Pacific is the largest of all oceans. In the tropics, it spans a zonal distance of 20,000 km from Malacca strait to Panama. Its meridional extent between Bering Strait and Antarctic is over 15,000 km. With all its adjacent seas it covers 178×106 km2. Its mean depth is 4270m. It is connects to the Arctic Ocean via the Bering Strait and to the via the .

The Atlantic: In contrast to the Pacific, the has a total meridional extent: It extends from the Arctic to Antarctic. Its zonal largest extent, however, spans little more than 8300km from the Gulf of Mexico to the coast of northwest Africa. It has the largest number of adjacent seas. With all its adjacent seas, Atlantic Ocean covers 106 ×106 km2. Its mean depth is 3300m. It connects to the Arctic Ocean via 1700 km wide opening running from Greenland across to Iceland, the Faroe Islands and Scotland. In the south, it connects with the Indian Ocean at the southern tip of South Africa via the and eddies.

The Indian Ocean: The Indian Ocean is the smallest of the three major oceans. Its northern boundary is located in the tropics. Its N-S extent is 9600km from Antarctic to the inner Bay of Bengal. Zonally, it spans 7800 km between S. Africa and W. Australia. Including the SO it covers 74 ×106 km2. It connects to the Pacific Ocean via the Indonesian Throughflow, and to the Atlantic through the Agulhas current and eddies. Its mean depth is 3800m.

The Arctic Ocean: The Arctic Ocean covers the North Pole region. It is the smallest of the world’s oceans. It covers an area of about 14 x 106 km2. Much of the ocean is covered by sea ice. The Arctic Ocean is composed of Mediterranean seas and separated into a few basins by Ridges. The depth of each basin is different, ranging from 3600m to 4500m. The average depth of the Arctic Ocean is ~1038m. The major connection of the Arctic Seas with the three oceans is to the Atlantic Ocean where a 1700 km wide opening exists along a large oceanic sill running from Greenland across to Iceland, the Faroe Islands and Scotland. Minor openings to the Atlantic Ocean exist through the Canadian Archipelago. The connection with the Pacific Ocean through Bering Strait is narrow and shallow (45m deep and 85km wide).

1b. In addition to the Weddle and Ross Seas in the southern ocean, which ocean favors deep-water formation and why? (5pts)

The large N-S extent of the Atlantic allows the ocean to extend farther north, where it is cold enough to produce heavier surface water than the subsurface water and thus cause convection and deep-water

1 formation. (3pts) In addition, sea ice formation in the north Atlantic ejects salt, which increases surface water density and thus may also help to produce deep water. (2pts)

1c. In which ocean does the sea surface temperature anomaly (SSTA) associated with El Nino occur? Based on the basin geometry, why do you think this ocean favors El Nino phenomenon? (5pts)

The Pacific Ocean favors the coupled ocean/atmosphere mode like El Nino. The Pacific is the largest ocean among the four. Its vast basin size favors air-sea coupling because most region of the ocean is away from the continents. This makes the effects of land (due to land/ocean contrast) less important in affecting its climate.

1d. How does the Indian Ocean basin geometry contribute to the formation of Asian monsoons? (3pts)

The winds are commonly referred to as “monsoons”. Because the northern boundary of the Indian Ocean is located in the tropics, the strong land/ocean heating contrast facilitates the monsoons. During northern summer, land is heated up quicker than the ocean because land has much smaller heat capacity than the ocean. As a result, land is much warmer and its atmosphere is hotter and lighter than that over the ocean. Sea level pressure at the land surface is lower than that at the oceanic surface. This creates a pressure gradient force that directs from the ocean to land (from high to low), causing the southwest monsoon. As the winds move from ocean to land, it subjects to , which acts to turn the winds toward the right (left) in northern (southern) hemisphere. In contrast, during winter, land cools faster than the ocean and the pressure gradient force is from the land to ocean, driving the northeast monsoon.

1e. Give an example that demonstrates how the Arctic Ocean affects global climate. (4pts) The Arctic Ocean's sea ice melting/freezing can affect the water property (salinity). The export of Arctic sea ice and fresher water into the North Atlantic can affect the stratification and thus deep water formation in the Atlantic Ocean, which in turn may influence global and thus affect global climate. [On the other hand, sea ice formation/melting also affects albeido, and thus radiative fluxes and climate.]

2. Properties of sea water and observational method (40 pts)

2a. What property of sea water distinguishes it from pure water? (2pts) How is this property measured in the past and present? [8 points]

The property of sea water that distinguishes it from pure water is salinity.

(a) Oldest method: Evaporate 1kg of sea water and weigh the residual.

(b) Laboratory, Classical (Knudsen) method: Determine the amount of chlorine, bromine and iodine to give "chlorinity", through titration with silver nitrate. Then relate salinity to chlorinity: S = 1.80655 Cl.

(c) Measure conductivity: Conductivity of sea water depends strongly on temperature, somewhat less strongly on salinity, and very weakly on pressure. If the temperature is measured, then conductivity can

2 be used to determine the salinity. Seawater standard is used to calibrate the conductivity based measurement. (i) For a seawater sample in the laboratory, an ``autosalinometer'' is used, which gives the ratio of conductivity of the seawater sample to a standard solution that has a known salinity. (ii) CTD - Conductivity, Temperature, and Depth. From an electronic instrument on board a ship, either inductive or capacitance cells are used to measure conductivity. Temperature must also be measured, from a thermistor mounted close to the conductivity sensor. From conductivity, T, and depth, we obtain salinity with depth.

(d) Satellite. NASA Aquarius salinity mission.

2b. Briefly describe the a few methods that measure temperature in the ocean. [10 points]

(a) Old method: Bathythermograph. Provides a continuous curve of temperature versus depth. Less accurate. (b) Expendable Bathythermograph (XBT). Also provides a continuous curve of temperature with depth. Less accurate. (c) CTD--Conductivity, temperature, and depth (actually pressure)}. T is measured uses a thermistor mounted close to the conductivity sensor. Accurate. (d) Protected reversing mercury thermometer. Because the mercury glass is protected from the sea water pressure, the length of mercury in glass is determined by temperature. Accurate. (e) Thermistor chain. on moorings (or thermistor on drifting buoys). Accurate. (f) Satellite. Advanced Very High Resolution Radiometer (AVHRR) on board of NOAA satellite. Problem: inaccurate when there are clouds due to cloud vapor absorption. Tropical Rainfall Measuring Mission (TRMM)--Microwave Imager (TMI) also measures SST. TMI can penetrate clouds but data can be contaminated by strong rainfall. (g) Acoustic tomography. Use changes in sound speed as a function of temperature along paths between acoustic sources and receivers.

2c. What is SOFAR channel? (2pts) How is it formed? (3pts) Give two examples for the use of SOFAR channel in the observations of physical oceanography [5 pts].

Sound speed is a function of temperature and pressure: the higher the temperature, the higher the speed; the higher the pressure, the higher the speed. In most areas of the ocean (tropics, subtropics and mid- latitude), the warm water at the surface and the high pressure at the bottom produce a sound speed profile which is maximum at the surface and bottom, with a minimum in between.

This sound speed minimum (near surface at high latitudes to over 1000m in mid- and low-latitudes) is referred to as the SOFAR (SOund Fixing And Ranging) channel.

Two examples in the observations of physical oceanography: (i) Acoustic Thermometry of Ocean Climate (ATOC project) - measuring global warming using the speed of sound in the ocean. Scientists proposed to place sound sources across the ocean using the SOFAR channel. As we learned, the speed of sound in water depends heavily on temperature. So how fast the sound arrives at the other side would depend on the temperature of the ocean basin. In this way scientists can measure temperatures of oceans.

3 (ii) Subsurface floats: SOFAR floats, RAFOS floats, and pop-up floats that measure subsurface currents. SOFAR floats are sound sources and are tracked by moored receivers, and RAFOS floats receive sound from moored sound sources. Pop-up floats are tracked periodically by satellite navigation when they pop to the surface.

2d. Write down the light attenuation law (1pt). Assume the mixed layer depth is 10 m everywhere in the ocean. For clear open ocean water where the light attenuation coefficient , how many percent of light is left at the bottom of the mixed layer? (2pts) For turbid coastal water where , how many percent of light is left at the mixed layer bottom? [2pts]

The light attenuation law is:

where is the ``shortwave radiation'' at the surface, and is the shortwave radiation at depth z, and is the vertical attenuation coefficient of the water.

For clear ocean water with and

So 82% of light is left at the 10m mixed layer bottom.

For cloudy coastal water with

For and

So almost no light is left at the 10m mixed layer bottom.

3. Geostrophic method (15pts)

Give a detailed discussion on ``geostrophic method'' for measuring the geostrophic currents. (15pts)

The ``geostrophic method'' is an indirect way to measure oceanic currents. It calculates the geostrophic current component that is perpendicular to the line connecting a two station pair. The theoretical basis for the geostropic method is that in the ocean interior (away from the surface, bottom, and side boundaries), large-scale ocean circulation obeys geostrophy, which is a balance between pressure gradient force and Coriolis force.

Step 1: Use observed T,S, P derive density and thus at stations A and B; Step 2: Calculate from to by integrating

p(z) p(r) Φ = − αdp = αdp; ∫ p(r) ∫ p(z)

Step 3: Calculate geostrophic current € 4

(i) Reference level is the level of no motion. (ii) Current at the reference level can be inferred from observations or measured by direct methods. Reference levels at 500m, 1000m and 1500m are often used.

4. Observed ocean circulation (15pts). 4a. Identify the major gyres and current systems associated with them, as well as the equatorial currents in the Pacific Ocean in Figure 1. [10 points] NH STG: NEC, Kuroshio, NPC, California C NH SPG: NPC, Alaskan current and stream, East Kamchatka current, Oyashio SH STG: East Australian C. (WBC), SPC (adjacent to ACC), Peru/Chile C., SEC. EQ: ECC

East KamchatkaAlaskan C current and stream C

Oyashio SPG

Kuroshio NPC California C STG

NEC ECC

SEC Peru/Chile C STG East Australian C South Pacific C

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Figure 1.

4b. Figure 2 shows 100dbar currents measured by geostrophic method in the Atlantic. Label the (1pt). The subpolar Gyre in this figure is much weaker than the directly measured currents at the same depth. Can you explain why? (4pts)

In the North Atlantic, currents below 700dbar have significant amplitudes due to the active deep water formation and recirculation. The assumption of level of no motion at 700dbar in Figure 2 causes large errors in the inferred geostrophic currents. Additionally, interaction with Arctic Ocean also obscures the SPG.

The Gulf Stream

Figure 2.

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A reference current figure for grading 4a.

5. Visit the TAO website: http://www.pmel.noaa.gov/tao/. From there go to either the RAMA site or the combined Display and Delivery Page. Using their data display feature, produce a plot for an oceanographic variable, such as SST, salinity, current, etc. from the TAO or RAMA data (5pts). Please provide a detailed description of your figure: including the variable name, observed location and depth, time span, etc. (5pts).

The figure below shows the time series of daily SST (temperature at 1m depth; blue curves) from RAMA array of the Indian Ocean at various locations along the 90E longitude for the period of 2002- present: From top to bottom: (90E,15N), (90E, 12N), (90E, 8N), (90E,4N), (90E, 1.5N), (90E,0N) and (90E,1.5S).

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