2 The atmospheric and ocean flows

PBDEs end up in the Arctic through their physical transport by the winds, the ocean and the rivers of the world. All three mechanisms are important, but the most rapid carrier is the wind.

The basic principle of global atmospheric circulation is simple: warm air rises and cold air sinks. The warming effect of the Sun is much greater at the equator than at higher latitudes, so the air is much warmer and rises. At high latitudes the air cools and sinks. This drives a horizontal wind. To help picture this, imagine a room with a radiator on one wall, and at the other end of the room an open fridge (Figure 4).

Figure 4 (a) A room with a radiator on one wall and an open fridge on the other will cause air to rise and sink at opposite ends; (b) horizontal winds are set up to replace this ascending and descending air

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A schematic picture showing how horizontal flows are initiated by warm air rising and cold air sinking. There are two parts to the figure. Part (a) shows a cross section through a room. On the left wall of the room is a radiator above which warm air is rising. At the opposite, right, end of the room is an open fridge, adjacent to which cold air is sinking to the floor. Part (b) shows the same cross section some time later. The rising and sinking air has set up a clockwise circulation pattern around the room.

Figure 4 (a) A room with a radiator on one wall and an open fridge on the other will cause air to rise and sink at opposite ends; (b) ...

The radiator heats up the air around it, and the air rises in what is called a convection current all the way to the ceiling and starts to spread. At the other end, the fridge is doing the opposite and cooling the air, which sinks and spreads across the floor. To replace the air that has risen, the air beneath the radiator is pulled upwards and then heated and rises, while the opposite is happening at the other end of the room.

At the most basic level, on Earth the same process is happening, with warm air rising from lower latitudes and sinking at higher, colder latitudes. High-level winds therefore tend to blow from the hotter regions to the colder ones. This general pattern is modified by the rotation of the Earth, which deflects the wind flow away from the apparently direct path.

These wind flows are further complicated by the distribution of continents and their mountain ranges across the globe. Winds are modified as they move around and over mountain ranges. They are also affected as they travel over land and sea surfaces, where the air is warmed to different extents. This is because of two additional processes: land and sea surfaces reflect different amounts of solar energy falling on them, and materials such as rocks and water need different amounts of heat to warm them up. You will look at the impact of these processes next.