We've talked about how heat is distributed through the Earth's atmosphere. Now let's take a look at the other primary means of heat transport to the polar regions, the ocean. The Earth receives the most energy at the equator And evaporation from equatorial oceans helps to drive global atmospheric circulation. Circulation of the earth's oceans is also driven by the energy imbalance between the poles and the equator. On a global scale, ocean circulation is driven by the temperature and density of ocean water, along with surface winds. Given what you have learned about how the Earth receives heat from the Sun, how might the global ocean surface temperature field be structured? Select only the most correct answer. A) Probably reflecting latitude with warmer temperatures and the highest latitudes. B) Broadly reflecting latitudes with warmest temperatures at lowest latitudes. C) broadly reflecting longitude with warmest temperatures to the east due to the Coriolis effect. D) set by the circulation in each ocean. Answer B is correct. The temperature of the ocean water at the surface broadly reflects latitude, as more solar radiation is received at lower latitudes. Thus, the warmest water is found near the equator. Salinity, the saltiness of water, is influenced by the amount of evaporation and by fresh water fluxes, mainly from rivers and precipitation. The easterly, equatorial trade winds and temperate westerly winds, help to set up the direction of global upper ocean circulation. Thermohaline circulation or the THC It's the global pattern of ocean circulation linking the upper and deep oceans. It is density driven and is set by the temperature and salinity. It is sometimes called the ocean conveyor belt or the meridional overturning circulation or MOC. In a simplified form, warm water flows north at the Atlantic Ocean near the surface. Evaporation and outflow of salty water from the Mediterranean Sea increases the salinity as it moves northward. Within the North Atlantic, the North Atlantic current allows this warm, salty water to reach high latitudes in subpolar regions and the Nordic seas. As the warm water flows north, it loses heat to the atmosphere. This plays a role in the poleward transfer of heat. This cooling leads to the northward flowing water, transforming to denser and denser along its path. Eventually, very strong heat loss due to winter cooling in a few limited locations leads to the density becoming as high as that of the deep water. This then triggers deep convection as the surface water is rapidly and violently. Mixed deep down into the interior of the ocean. The mixing also transports oxygen, carbon dioxide, and other dissolved gases deep into the ocean interior. This is the oceanic analog of a thunderstorm. This cool, salty bottom water flows southward along the east coasts of North and South America. Given that such flow is along the western side of the ocean, the current that transports this newly formed deep water is called the deep western boundary current. Some of this deep water is lost to the conveyor by mixing into the interior of the Atlantic ocean. However, some will reach the region of the Antarctic continent, in the southern ocean. This allows for deep exchange into the Indian and Pacific oceans in concert with additional intermediate and bottom waters formed Antarctica. Eventually, each branch wells up to the surface. This occurs in all of the world's oceans. Recent research suggests that much of this upwelling is produced by strong mixing occurring over regions of rough topography such as undersea mountain ranges. In the southern ocean, upwelling also occurs because of strong surface winds. As well as density-driven mixing, resulting from sea ice formation. According to this simple conveyor belt model, waters need to return to the Atlantic Ocean in the upper ocean. This occurs by various routes, including south of the southern tip of South America, Through the passages of Indonesia. And then across the Indian Ocean. The surface then flows north to the Atlantic. If an individual water parcel was to follow this entire route. It has been suggested that the transit time from north Atlantic sinking to upwelling in the north Pacific. Maybe up to 1,000 years. However, this is a simplified model and is an amalgamation of many, much more complicated, circulation patterns. And it is doubtful any individual water parcel would follow the entire route. You may have noticed that we haven't discussed how the Arctic Ocean is involved. We'll get to that basin in short order. What do you think could hypothetically interrupt the circulation of this global conveyor belt system? A) increased freshwater influx from rivers. B) decreased delivery of icebergs from the Greenland Ice Sheet. C) decreased freshwater input from lakes. D) the melting of the Greenland Ice Sheet. More than one answer may be correct, so select all that you think apply. Answers A and D are correct. The THC is driven by salinity and temperature differences. One of the most important regions for bottom water formation is the far north Atlantic ocean adjacent to Arctic ocean. The THC may be weakened by the addition of fresh water. Which decreases salinity and therefore density, inhibiting overturning. Fresh water could be added to the ocean via rivers. The melting of glacial ice which consists of fresh water, can also lower the salinity. Icebergs are pieces of glacial ice broken off from glaciers or ice sheets. That subsequently float in the ocean. Therefore, a decrease delivery of them to the ocean would not contribute to additional fresh water. We'll look into this in more detail in chapter four. As we've just seen, the most important region for deepwater formation Is the far North Atlantic Ocean, adjacent to the Arctic Ocean. In addition to being important to the ocean conveyor belt, the Arctic Ocean serves to connect the Pacific and Atlantic Oceans in the northern hemisphere through narrow gateways. The Arctic Ocean is globally important and unique. Here we will focus on it's circulation and water masses. The Arctic Ocean is surrounded by the continents of the Circumpolar North. It is the shallowest of the worlds oceans with an average depth of just over 1000 meters and a maximum depth of about 5000 meters. It has extensive shelf seas along the coast lines particularly north of Eurasia. These shelves play a key role in the biology of the arctic and in the formation of sea ice. The arctic ocean consists of two basins. Separated by the relatively shallow Lomonosov Ridge which ranges from 500 to 900 metres in depths. The two basins are the Amerasian basin and Eurasian basin. The Amerasian basin is further divided into the Canada basin nearest North America. And the Makarov Basin, nearest to Asia. The Eurasian Basin is subdivided into the Nansen Basin, adjacent to northern Europe and the Amundsen Basin, north of the Nansen Basin. The Arctic Ocean covers 14 million square kilometers, centered on the North Pole. As you can see, there is no land at the North Pole. In fact, there are 4,000 meters of water under the sea ice. the nearest inhabited location is the Alert weather station on northern Ellesmere Island in Canada some 800 kilometers away. For most of the year the arctic ocean is covered by sea ice. Sea ice is frozen sea water which can take many forms as we will see in lesson three. Ice is moved by both ocean currents and wind and sometimes can pile up in some areas. This sea ice waxes and wanes seasonally, growing during fall and winter, and melting during spring and summer. Sea ice can play a major role in the large scale ocean circulation. When it forms, it expels salt-rich brine. When it melts low salinity water is added to the ocean, helping to establish stratification in the ocean water column. Sea ice also plays a crucial role in the global climate system through it's high albedo. If you remember from the introduction, albedo is the proportion of incoming solar energy that is reflected. So, high albedo sea ice, keeps the underlying ocean cool. Arctic sea ice has experienced dramatic changes in the last few decades, diminishing in both extent And thickness. As the ice melts an no longer covers the oceans surface, albedo declines substantially. This means instead of having a white ice surface that reflects most of the insolation, the dark ocean surface now absorbs most of the suns energy warming up the ocean. In turn, this makes it much more difficult for sea ice to form. This is another example of a positive feedback amplifying the initial effect of a small change. The salinity of the Arctic Ocean is relatively low compared to the Pacific or Atlantic Oceans. Based on the map on your screen, what factor or factors, might contribute to this low salinity? A) the high latitude in which the Arctic Ocean is situated. B) the restricted connection to the Pacific and Atlantic oceans. C) The inflow of many sizeable rivers to the Arctic Ocean. D) The high albedo of sea ice. In this case, both B and C are factors that contribute to low salinity. The salinity of the Arctic Ocean Remains low compared to other ocean because of the volume of fresh water entering from Arctic rivers as we discussed in lesson one. Low salinity is also maintained because of the few narrow gateways to the other oceans, in particular the inflow of low salinity water from the Pacific Ocean. The high latitude of the Arctic does not low salinity and neither does the albedo of sea ice.
