THE MoDERATiNG iNFLuENcE of the ocean

Perhaps the most obvious effect that the ocean has on climate is its moderating effect on extremes of temperature, both diurnally (i.e., the day-night contrast) and annually (the seasonal cycle). We focus on the effects on the annual cycle because these tend to be on a larger scale and more befitting a book with climate in the title, but much the same principles and effects apply to the diurnal cycle. First we take a look at the observations to confirm that there is a moderating influence from the ocean. Fig. 5.1 shows the annual cycle of temperatures of San Francisco and New York. The two cities have similar latitudes (San Francisco is at about 38° N and New York is at about 41° N) and both are on the coast, yet we see from the figure that the range is enormously larger in New York—the highs are higher and the lows are lower. (One wonders if the respective climate extremes affect or even effect the different personalities of New Yorkers and Californians.)

The difference is mainly caused by the fact that the climate of San Francisco is maritime, meaning that it

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New York

San Francisco

New York

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 5.1. The seasonal cycle of temperature (°c) in San Francisco and New York. For each city, we plot the average low temperature and the average high temperature for each month. note the much bigger range in New York and the maximum earlier in the year, in July rather than September.

is influenced by the ocean, whereas the climate of New York is, in spite of it being on the Atlantic coast, essentially continental. A city that is truly land-locked, such as Moscow, has a climate much more like New York than San Francisco. new york's climate is continental because the mean winds come primarily from the west, so they blow over land and take up its temperatures before they reach the city. In contrast, the winds have blown over the Pacific ocean before arriving at San Francisco. So why does the ocean moderate the climate? It is in part because water has a relatively high heat capacity, compared to the material that makes land (e.g., soil and concrete), and in larger part because the upper ocean is in constant motion, and so the depth of ocean being heated and cooled over the seasonal cycle is much larger than the depth of land that is being heated and cooled. Let's explain that in a bit more detail.

First of all, the higher the heat capacity of a body, the more heat is needed to change its temperature. Thus, if an object is being cyclically heated and cooled, as in a seasonal cycle, then the change in its temperature is much smaller if its heat capacity is higher. Now, to what depth in the land and ocean does the heat penetrate over the course of a seasonal cycle? Plainly not all of the ocean's great depth is heated during the day or cooled during the night, or even over the course of a season. Rather, regarding the ocean, just that part that is turbulently mixed by the effects of wind (and in part by heating and cooling itself) fully partakes in the annual cycle, namely the mixed layer, which we discussed in chapter 2. Although its character varies from place to place in the ocean, it has a typical depth of about 50-100 m. That is, the effective heat capacity of the ocean is approximately that of a body of water 50-100 m deep. This heat capacity is quite large, and for comparison, the heat capacity of the atmosphere corresponds to a depth of just 3 m of water.

What is the effective heat capacity of land? Two effects make it much less than that of water. First, the specific heat of dry land is about 4 times less than that of water (for wet land, the factor is about 2). Second, because land is, rather obviously, not in motion in the same way as the ocean is, the penetration of heat into the land is much less than it is into the ocean. The heat can penetrate only by conduction, and because the earth (soil) has rather low thermal conductivity, only the top few meters are significantly heated and cooled over the course of a season. The same can be said for the major ice sheets over land, such as those over Greenland and Antarctica: they have large mass but low thermal conductivity. Thus, combining the effects of a larger heat capacity and a larger effective depth, the ocean has an effective heat capacity that is about 100 times greater than that of land. This high heat capacity considerably attenuates the seasonal cycle and is a good part of the reason for the large difference between San Francisco and New York.

San Francisco is a rather extreme case because not only is the summer temperature moderated by the ocean, but also the interminable fog that blows in from the ocean and covers the city like a wet blanket keeps the summer temperatures miserably low and makes them seem even lower, as Mark Twain perhaps felt. However, the heat capacity effect does occur on very large scales. The surface of the Southern Hemisphere is about 80% ocean whereas the Northern Hemisphere is only about 60% ocean, and as a consequence the seasonal cycle is much more pronounced over the Northern Hemisphere than the Southern, as we see in figure 5.2. Between 40° N and 60° N, the amplitude of the annual cycle is about 12°C, whereas between the corresponding latitudes in the Southern Hemisphere, the annual cycle has an amplitude of only 3°C.

Zero Annual Amplitude
Figure 5.2. Amplitude and lag of the annual cycle in the Northern and Southern hemispheres, as a function of latitude. The lag is the time, in days, from the maximum solar insolation to the maximum temperature. Source: Trenberth, 1983.
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