Oceanic impact on the marine atmospheric circulation

An intermittent filament of cloud bisects Fig. 2.35. The picture is a northern winter geostationary satellite photograph of the Atlantic Ocean. Being winter the filament - the ITCZ - passes south of the West African bulge. Over the land surfaces of both southern Africa and Brazil the ITCZ widens into an extensive cloud band. Swirling frontal systems can be seen crossing the North and South Atlantic. West of these fronts, and in the Trade wind belts, a multitude of small white patches show extensive cumulus growth in these moist, unstable, airstreams. Large areas of mid-latitude land are practically clear of clouds. These clear regions tend to extend over the adjacent seas to the west.

Many of these visible weather features are due to, or modified by, the oceans. The ocean provides the moisture and latent heat to generate storms, both of the mid-latitudes and tropics. It provides the moisture and warm surface to create the instabilities responsible for cumulus development. The upwelling on eastern sides of sub-tropical gyres is responsible for cooler sea surface temperatures. Resulting cooling of the overlying air makes the atmosphere denser and biases the position of the sub-tropical anticyclones to the eastern side of ocean basins, as can be seen in Fig. 1.7. This reinforces the upwelling further. The cooling effect of the sea surface can also lead to extensive fog banks, as is frequently observed in the Atlantic off the west coast of Namibia and Angola.

Conversely, some of the features of Fig. 2.35 are weaker over the sea. The ITCZ, though largely fuelled by moisture gathered over the oceans, is less organized and more patchy over the sea. Heating of the land surface provides an additional mechanism to accentuate the convergence of the Trade winds. The subsidence within the sub-tropical anticyclones is less general over the ocean because of the de-stabilizing effect of a moisture supply.

We have already seen how the atmosphere and ocean can interact in mon-soonal climates (§2.11.4). In Chapter 5 we will examine changes in the marine atmosphere during El Nino events, and the preferential formation of mid-latitude cyclones over the warm western sides of ocean basins. Small-scale storms, known as mesocyclones or polar lows, are almost invariably generated over the ocean (§2.13.2). The ocean also influences the weather of adjacent land areas, giving them a milder, wetter climate than continental areas at the same latitude. This is particularly true on eastern coasts at mid-latitudes because the prevailing wind is blowing onshore. Maritime climates will also be considered further in Chapter 5. This chapter will also consider the links between ocean and atmosphere accompanying abrupt changes in the ocean.

There is, however, one atmospheric circulation feature which could not exist in the absence of the oceans. This is the hurricane.

Physical interaction between the ocean and atmosphere Table 2.3. Fatalities in historical hurricanes

Year

Death Toll

Location

1281

100 000

Kyushu, Japan

1737

300 000

Calcutta, India

1876

300 000

Chittagong, India

1881

300 000

Haiphong, India

1882

100 000

Bombay, India

1900

6000

Galveston, Texas

1942

11 000

Bengal, India

1963

6000

Cuba and Haiti

1970

300 000

Bangladesh

1977

10 000

Andra Pradesh, India

1991

140 000

Bangladesh

2.13.1 Hurricanes

Hurricanes are intense atmospheric convection systems. They originate in the tropics, over warm seas, and are among the most devastating of natural phenomena. Every few years such a great storm roars out of the Bay of Bengal to kill perhaps hundreds of thousands of people in Bangladesh, as happened in 1970 and 1991. These deaths are mostly caused by drowning in the storm surges induced by the hurricane's winds, but massive damage, with resulting fatalities, occurs from the effects of these winds, from flooding by the torrential rainfall, and from the spread of disease caused by contaminated water and food supplies. Table 2.3 gives estimates of the fatalities in a number of historical hurricanes.

These storms can travel great distances, especially if their path does not cross over large areas of land. Atlantic hurricanes have been known to wreak havoc in the northeastern United States. It is not unknown for the remnants of an intense hurricane to reach the United Kingdom, bringing heavy rain and strong winds, as shown by Hurricane Charlie over the August Bank Holiday weekend of 1986. In the western Pacific near Japan these storms are known as typhoons, while in the Philippines they are called bagiuos. Near Australia, and in the Indian Ocean, such storms are known as tropical cyclones.

The maritime nature of hurricanes is shown by their trajectories in Fig. 2.41. Also shown on this diagram is the July limit of the 26°C sea surface temperature isotherm. Hurricanes clearly form over very warm water between 5° and 25° from the equator. The energy supply is thus latent heat from evaporation, and the Coriolis force is necessary to induce spin in a formative instability in the atmosphere. Hurricanes tend to form within the convergence regions on the eastern flank of travelling low pressure troughs in the Trade winds. The atmosphere needs to be convectively unstable, with little vertical shear in the wind. If such conditions are combined with a ready moisture supply from a warm ocean, and a divergent air flow aloft to accentuate the convection, a hurricane should develop.

Fig. 2.42 shows the main features of a mature hurricane. At the centre of the storm there is a small region without convection known as the eye. The wind speed here is much lower than in the main body of the hurricane, although not

Fig. 2.41. Typical trajectories of hurricanes over the global ocean. The dashed line is the 26° C sea surface temperature in July. Note that very few storms form in cooler waters, or less than 10° from the equator.

Fig. 2.41. Typical trajectories of hurricanes over the global ocean. The dashed line is the 26° C sea surface temperature in July. Note that very few storms form in cooler waters, or less than 10° from the equator.

Fig. 2.42. Model of a mature tropical hurricane (vertical cross-section), showing principal cloud formations and regions of strong vertical motion. The spiral rain bands are indicated by hatching under the clouds.

often the dead calm of some literary storms. There may also be little cloud. The eye is some 20 km in diameter and has the lowest surface pressure of the storm. This is typically well below 1000 mb, and has been recorded as low as 870 mb in Typhoon Tip on 12 October 1979. Just outside the eye is a narrow region of intense rainfall and wind speed in the wall. This is the zone where wind speeds reach their highest values, possibly more than 200 kmhr-1. Away from the centre of the hurricane the wind decreases, and the pressure rises, steadily. At radii of 100 km the wind speed may still be over 60 kmhr-1 (~20 ms-1). Another typical feature is the set of spiral bands of strongly convective cloud. The existence of these was not known until the use of radar for hurricane research at the end of World War II. These spirals are regions of localized enhanced instability, producing massive thunderstorms of cumulonimbus clouds, and intense rainfall.

A number of theories exist to explain the presence of the eye at the centre of the storm. They can be conveniently combined as follows. As the convection within a tropical storm intensifies, the air that is thus transported to the upper troposphere tends to spread out. This can be seen on a small scale in individual mid-latitude thunderstorms from the presence of a layer of thin and high ice cloud (cirrus) spreading in an anvil-shape ahead of the storm. Of course, within a developing hurricane there is considerably more air to be spread, and a positive pressure anomaly is produced at high levels because of the ring of convection about the storm centre. This accelerates the downflow, or subsidence, on the edge of the convection. Subsidence warms the air by compression; try touching the valve on your bicycle pump after you next inflate your tyres. This warming creates buoyancy and acts to counter the subsidence. Eventually a state of negligible vertical motion is achieved.

The spiral bands also show some interesting physics. The bands revolve about the storm centre, but much slower than the wind speeds would suggest. They are not, therefore, composed of the same air throughout their existence. Parcels of air enter the unstable region, form thunderstorms, and then leave. A typical passage time for the air is only 40 minutes. During its transit time the air parcel may lose water by precipitation at a rate of 30 mmhr-1.

Hurricanes will decay when removed from their copious source of moisture. Thus, if one moves polewards over colder seas this supply lessens and the storm slowly decays. This is accentuated if it travels far enough to be diluted with mid-latitude air, although hurricanes can gain energy for a short time at this stage through interaction with the instabilities that cause extra-tropical cyclones. Even over tropical latitudes, if a hurricane moves over land it will lose energy rapidly. The central pressure can rise by several mb per hour. The moisture, and hence energy, supply is removed but the stored water is released, converting the hurricane into a very wet, if more conventional, depression.

Hurricanes feed from the warm tropical oceans. Their strong winds and rainfall will, in turn, have an impact on the ocean. The wind stress stirs the sea to deeper levels than normal. It has been suggested that the fisheries off the western Australian coast exist only because of the nutrients mixed into the surface waters by this occasional deep stirring. Such effects remain a moot point, however, as confirmatory observations are practically impossible in such extreme weather, and other work suggests that wind mixing alone may not have such penetrative power. This stirring will, however, lower the sea surface temperature, as will the rainfall, acting to dampen the storm development.

The high wind speeds also create local circulations in the ocean. This can be most important as the storm approaches land and pushes water in front of it. In shoaling seas this creates a storm surge, with water levels reaching as much as 5-10 m above normal sea level. These surges account for many of the fatalities from hurricanes, particularly in the Bay of Bengal where the seas are quite shallow near the Ganges delta. Storm surges are also produced by intense mid-latitude storms over confined basins, but the strong winds of hurricanes generally give higher floods. The worst surges in both cases are caused by a coincidence of the surge with high spring tides. The North Sea experiences minor surges in most winters from extra-tropical cyclones, with occasional disasters such as the 2 m surge in 1953 when 2000 people were killed in Holland and Great Britain. Surges are generally caused by strong alongshore winds, creating Ekman transport towards the coast, but interaction of this transport with the shoaling ocean floor can generate surges which move along the coast.

Fig. 2.43. Infra-red satellite image of four mesocyclones behind a vigorous depression centred over Iceland in the northern Atlantic, 20 December 1994. [Photo courtesy of University of Dundee.]

Fig. 2.43. Infra-red satellite image of four mesocyclones behind a vigorous depression centred over Iceland in the northern Atlantic, 20 December 1994. [Photo courtesy of University of Dundee.]

2.13.2 Mesocyclones

In sub-polar and polar latitudes intense, small-scale storms called polar lows or mesocyclones can form that have properties reminiscent of hurricanes. These storms frequently are accompanied by strong winds, intense localized convection, sometimes in spiral bands, heavy rain or snowfall and are typically formed within a larger scale airstream. Occasionally, these storms have even been known to have central eye-like structures. The majority of these storms are under 100 km in diameter, but may extend over several hundred kilometres. They usually rely on a latent heat supply from the ocean for their energy. Thus such storms frequently occur in off-ice flow, where cold, dry air is suddenly heated at its base, and supplied with water vapour through evaporation encouraged by strong winds and the boundary layer warming. Another frequent mechanism for their generation is through local instability in convection in the cold air behind a cold front. This convection is again set off by surface warming as air travels over a warm ocean. Thus, most storms crossing the northern Atlantic, particularly in winter, have 2-3 mesocyclones that form in their wake (Fig. 2.43). North Atlantic mesocyclones tend to be stronger than their Southern Ocean cousins because of the warmer ocean over which north Atlantic synoptic systems travel and the warmer waters that penetrate into the sea-ice latitudes.

Sometimes mesocyclones have a noticeable cloud signature - either a spiral or a comma shape - but no detectable surface signature. Almost half of the mesocyclones observed, by satellite, to cross a weather observing ship in the Norwegian Sea showed no change to surface wind or pressure fields. The systems thus may be generated by interaction between the surface and atmosphere, but the net impact can be in just the latter. Nevertheless, the near-hurricane strength winds, and thus oceanic heat loss, that sometimes accompany mesocy-clones in the northeast Atlantic may well help pre-condition the ocean for later deep convection, or even cause these spatially and temporally isolated events to occur.

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