The most costly natural disaster the United States has experienced occurred in August 1992 when Hurricane Andrew hit Florida. Hurricanes (Huracan = the Taino god of the wind) and typhoons (Taifung = Chinese for "great wind") are both tropical cyclones; Hurricanes occur in the Atlantic Ocean and typhoons in the Pacific Ocean. To qualify as a hurricane or typhoon the storm must have sustained winds exceeding 64 knots (74 miles per hour). Though the high winds inflict damage when these storms hit land, it is the huge waves; storm surge and associated flooding that cause most of the destruction. These ferocious atmospheric storms cannot survive without the moisture supplied by warm ocean water. They are vivid examples of the coupling between the earth's oceans and atmosphere.
Ocean currents and variations in oceanic water temperature have been known for many centuries. Benjamin Franklin was the first person to chart the Gulf Stream. He did this in 1769 based on his observations and those of the whalers off the coast of New England. Benjamin Franklin also made some careful observations of the atmosphere and was no doubt aware that interactions existed between the atmosphere and the ocean. This chapter explores the links between the atmosphere and oceans.
Oceanography is the study of the oceans. An understanding of weather and climate requires knowledge of oceanography because of the interaction between the atmosphere and the oceans. There are three important roles the oceans play in relation to weather and climate: 1) They are a source of atmospheric water vapor and 2) They exchange energy with the atmosphere and 3) They transfer heat poleward.
Each year approximately 396,000 cubic kilometers (94,644 cubic miles) of water vapor circulates through the atmosphere through the hydrological cycle. Most of the water that recycles into the atmosphere comes from the oceans (Chapter 1). About 334,000 cubic kilometers (79,826 cubic miles) of water evaporates from the oceans every year. At any given time, the atmosphere contains approximately 15,3000 cubic kilometers (3,580 cubic miles) of water. It takes approximately two weeks for all of the water in the atmosphere to be recycled. The oceans provide the majority of water needed to form precipitation.
Exchanges of heat and moisture occur at the interface between the atmosphere and the ocean. Figure 8.1 shows the net energy gains and losses at this interface for winter and summer. These energy exchanges include latent heat, sensible heat, and radiative exchanges. On average, the ocean gains energy during summer and losses energy to the atmosphere in winter. So, on average the oceans warm in the summer and cool during the winter. These energy budget maps (Figure 8.1) are a measure of the average interaction between the atmosphere and ocean. Maximum exchanges of energy occur in the Northern Hemisphere winter to the east of North America and Asia. Warm ocean currents flow northward in these regions (Chapter 3) and supply energy to winter storms that are leaving these continents heading eastward. As we will learn later in this chapter, warm ocean waters also supply the energy that fuels hurricanes.
The rate of heat and moisture transfer, as discussed in Chapter 2, depends on the temperature difference between the air and the water and the winds. Warm sea surface temperatures and strong winds are favorable for large heat exchanges between the ocean and atmosphere. So, it is important to understand where the warm waters of the oceans are and to know why they are there. We learned in Chapter 3 that warm currents of the middle and high latitudes border the East Coast of some landmasses. Countries such as Norway have mild winters and small annual temperature ranges. The West Coasts of landmasses that boarder cool waters, such as San Francisco, have low average temperatures and small annual ranges of temperature (Chapter 3). This Chapter will discuss the origins of these cool and warm currents
Like the winds, the ocean currents transport heat from the tropics to higher latitudes. Figure 8.2 shows the poleward transport of heat in the Northern Hemisphere by the atmospheres and oceans required to balance the radiation budget, as discussed in Figure 2.20 (reproduced as an inset of Figure 8.2). The poleward transport of heat by the atmosphere is discussed in the previous chapter. At about 30 latitude, the atmosphere and ocean each transport about the same amount of heat. Equatorward of 30 the ocean transfers the majority of heat required to maintain a balance. The atmospheric transport has a broad peak at around 30N and 60N. This transport of heat by the atmosphere and ocean is coupled to our current climate. Any changes in the ocean circulation patterns may change the heat transport by the oceans and thus impact climate.
Changes in the ocean and its circulation patterns affect the atmosphere and visa versa. Solar energy is the primary source of energy that warms the oceans. Atmospheric conditions determine the amount of solar energy reaching the ocean. Cloud free conditions result in more energy entering the oceans while cloudy conditions reduce the solar energy reaching the surface. Winds also play an important role in determining ocean temperatures. The solar energy that is absorbed at the surface of the water can be mixed to deeper layers by the winds, which stir the upper layer of the ocean.
Chapter 1 described the atmosphere based on vertical distribution of temperature. In the oceans, we are interested in the vertical distribution of water density. Observations of temperature with depth in the ocean at a given location reveal a vertical distribution as demonstrated in Figure 8.3. Based on measurements of temperature we can classify the ocean into three vertical zones.
The characteristic feature of the top 100 meters of the ocean at a given location is a constant temperature. The uniform layer is called the surface zone or mixed layer. Wind driven waves and current mix this layer so that the temperature is relatively uniform. Only about 2% of the world's ocean waters are within the surface zone. The bottom layer, or the deep zone, lies below about 1000 meters and is composed of cold water with temperature between -1C to 3C (30.5-37.5F). The temperature is uniform in this deep layer. The transition zone between the surface and deep layers is the thermocline (therm means heat and clinare, to lean). In the thermocline temperature decreases rapidly with depth down to a depth of about 3000 feet.
Thermocline is a zone of the ocean in which the temperature and decrease rapidly with depth.
Figure 8.3 is a typical temperature distribution of the ocean with depth. This temperature variation with depth is a function of latitude. Figure 8.4 shows the temperature profile with depth for oceans typical of tropical, midlatitude and polar regions. The deep zone temperatures are similar for all three regions. The tropical waters have the warmest surface zone temperatures and the polar regions are the coldest. This is because the polar regions receives less solar energy. The tropical and midlatitude regions have the strongest thermocline.
Sea surface temperature (SST) distributions
Interactions between the atmosphere and ocean occur at the surface transfer heat and moisture. The distribution of sea surface temperatures is therefore important in meteorology. Indeed, as we shall see later in this chapter, the sea surface temperature, or SST, determines the distribution of hurricanes.
Figure 8.5 shows the SST distribution in the world's oceans, with red regions representing warm water and blue regions cold. There are a few general conclusions we can draw from this figure, in addition to the one already made regarding tropical waters being warmer than the polar waters. These additional observations are:
Western coasts in the subtropics and middle latitudes are bordered by cool-water.
Western coasts in tropical and subtropical latitudes are bordered by warm-water.
Eastern coasts in the mid latitudes are bordered by warm-water currents
Eastern coasts in polar regions are bordered by cool-water.
In addition to the temperature distribution patterns, it is important to know in which direction these warm and cold waters are moving. We know from energy transport principals that on average cloud waters move equatorward and warm waters poleward.