Even at the same latitude, climate is different between over oceans and over land, or between coastal and interior areas of a continent. The cause of the difference is primarily the difference of heat content between ocean and land, and next the difference of capability to supply water. In addition, the fact that the land surface is not usually flat but have mountains affects climate.
Heat capacity is the amount of energy required to warm a certain body of matter by a unit temperature (typically 1 K, i.e. 1 oC). In this context, we are concerned about the heat capacity of such layer of land and ocean, respectively, that take part to the seasonal change of temperature. For land where the only means of vertical energy transfer is conduction, we may consider a soil layer 1 meter deep. For ocean, on the other hand, energy can be transferred by vertical motion of water, i.e. convection. A layer of water approximately 100 m deep is mixed, and this layer is called the mixed layer. Actually the efficiency of convection is very different dependent on whether the layer is cooled or warmed from above. But even it is warmed, a mixed layer, tens of meters deep, is formed by mixing caused by wind stress. Heat capacity per area is the product of the depth of the layer, specific heat capacity (heat capacity per mass), and density. In a crude estimation, it is a hundred times larger for the ocean than for the land.
Accordingly, the ocean can store energy during summer and output it in winter. As a result, the annual range of temperature (difference of temperature between summer and winter) is smaller there than over land. Actually the effect that atmospheric motion mixes heat horizontally also exists, and the annual range of temperature is smaller near the ocean and larger in the interior of a continent. The share of land in the surface area of northern middle latitudes is much larger than that of southern middle latitudes. Consequently, in the northern hemisphere, the meridional gradient of temperature is much smaller in summer than in winter even averaged over both ocean and land areas, and the westerly wind is weaker. The strength of westerly wind in the southern middle latitudes in both summer and winter is similar to that of the northern middle latitudes in winter.
The temperature difference between ocean and land also causes some kinds of atmospheric circulation. They will be discussed later in the paragraph about monsoons.
The effect of land-sea distribution related to water is that the availability of water for evaporation from the surface is indefinite over the ocean but limited over the land. It consists of two factors. (1) Even the same amount of energy may be supplied, there is not enough water for evaporation over arid lands such as deserts. (2) Because of the difference of heat capacity discussed above, the ocean can store energy in summer and evaporate water in winter using it. Evaporation over the land is determined by the energy supply within the season so that it must be small in winter.
The effect of mountains on the large-scale climate can be sorted into the mechanical effect concerning the motion of the atmosphere and the effect to the water cycle. (1) The motion of air is deformed by the mountains as obstacles. Its effect propagates downwinds and forms troughs and ridges of pressure. Even the position of the mountain is the same, the pattern of effects is different between the case where the flow go over the mountain and where the flow go around the mountain. (2) As the effect to water cycle, air is forced to go up at the mountains, then water vapor condenses and precipitates near the mountain, and air depleted of water vapor is supplied to the downwind area.
Monsoons
Monsoon means the seasonal reversal of prevailing wind direction.
The English word monsoon is often used specifically to mean the phenomenon in the tropics in summer (June to September in the northern hemisphere,
December to March in the southern hemisphere).
On the other hand, the Japanese equivalent kisetsu-fû (literally `seasonal winds') is often used to mean the phenomenon in the middle latitude zone in winter.
Due to the difference of heat capacity between the ocean and the land (discussed above), the land is warmer than the ocean in summer, and colder in winter. By this, such atmospheric circulation that rising takes place over the land is formed in summer, and sinking over the land in winter. In terms of surface pressure, low pressure is formed over the land in summer and high pressure in winter. Originally the subtropical zone is a high pressure belt and the subpolar zone (the poleward part of the extratropical cyclone zone) is a low pressure zone. Therefore the contrast between the land and the ocean is better recognized in the subtropics in summer and in the subpolar zone in winter. If the circulation is coupled with cumulus convection (as discussed below), the vercital circulation cell becomes as deep as the whole troposphere. Otherwise the circulation cell is shallower and closed within the lower half of troposphere.
Tropical monsoons are clearly seen in South Asia (around India), Southeast Asia and Africa (Sahel and Sudan zones) in the northern summer, and in Indonesia and northern Australia in the southern summer. Webster (1987) gives a good explanation of the mechanism. In these areas, as season progresses, first, in the early summer (around April in the northern hemisphere), clear weather prevails, the surface is warmed, and a shallow circulation between the ocean and the land is formed. Then, maritime air with high moisture content comes to converge into the low pressure area formed over the land, and cumulus convection and precipitation over the land are activated, and the deep Monsoon circulation is formed. After the rain starts, the contrast of surface temperature between the land and the ocean becomes smaller, but the low-level convergence continues because condenstation in cumulus clouds drives upward motion of air. Among the monsoons, that of the south Asia is the most remarkable. It is considered to be due to the effect of Tibetan and Himalayan mountains that lock the position of heat source in the atmosphere.
Temperate zone monsoons appear around the 35o latitudes, where it is covered by easterly (trade) winds of Hadley circulation in summer and by westerly winds of Rossby-type circulation in winter. In winter when temperature is lower over the land, much water vapor and sensible heat (energy in the form other than water vapor) are supplied from the ocean to the atmosphere at the east coast of continents (around Japan, around the eastern United States etc.) where the westerlies flow from the land to the sea. In particular, east Asia in winter, being to the east of the surface high pressure area (Siberian High) developing over the continent, has cold and strong monsoon wind, which consists of the northwesterly wind blowing out of the Siberian High in addition to the middle latitude westerly. Mechanical barrier effects of the Eurasian mountains (including the Tibetan Plateau) contributes to the clear shape and the locked position of the east Asian winter monsoon.