EASC1020 tut 2

EASC 1020 Introduction to Climate Sciences

Tutorial 2: General Circulation and Ocean-Atmosphere Interactions

This week we will use the NOAA website to review and explore materials related to atmospheric general circulation patterns, and ocean-atmosphere interactions including ENSO, NAO and PDO.

1. General circulation patterns

Ex 1. Make a global map of long-term mean annual sea level pressure plot. Choose variable

“sea level pressure”, level “surface” and month from Jan to Dec. What major patterns can you observe? Patterns related to general circulation (Southern Hemisphere) or secondary circulation (Northern Hemisphere)? Also check geopotential height and wind (meridional).

For sea level pressure,

In accordance with figure 1, low pressure belt and high pressure belt show significantly on the map. For low pressure belts, the low pressure areas mainly distribute at around 60ºN, 60ºS and 0 º(equator). For high pressure belts, the high pressure areas are around 30ºS, 30ºN and the two

pole (North pole and South pole). For the pattern of belts, the belts on the Northern hemisphere disperse into cells while the belts on the Southern hemisphere are continuous.

The pattern of the high and low pressure belts in southern hemisphere are related to the general circulation. The low pressure area along the equator because the equator receives the most

insolation throughout the year. Since the warmer air is less dense, the warmer air rises. Then, the relatively low pressure will cause. In accordance with the pressure gradient force, the lower

pressure air will move towards the South pole, which is higher pressure. Despite that the air

cannot move to the pole finally, because the air will cool down through the travel from low

latitude to high latitude. Thus, a great amount of cooled air will sink at around 30ºS, which

forms the high pressure belt there. At 90ºS, there is at a relatively high pressure between

1030mb and 1035mb, because a net energy deficit caused by the high albedo of ice and snow surface. Hence, the air tends to sink since air mass is cold and has a higher density. However, the sinking air moves towards lower latitudes and will diverges when arrive at around 60º S.

Then the low pressure belt formed there due to the air mass originated at 30º S meet the sinking air from 60º S and the two air masses will converge there.

Due to the characteristic of water bodies, the continuities of the belts are irregular. Because the specific heat capacity of water is relatively high and the dispersal of heat by ocean currents, the large ocean has a constant surface temperature. Therefore, the pressure belts show more

resemblance to the general circulation and less influenced by the land water contrast.

For the second circulation, the pressure cells pattern on the Northern hemisphere are highly related to the second circulation. The patterns of pressure belts are related to the land-water contrasts. Since the Northern hemisphere has a relatively high land masses than Southern

hemisphere and the height and land form are unevenly distributed, the insolation received on different area of the Northern hemisphere varies. Therefore, in Northern hemisphere, it shows

pressure cells instead of pressure belts. Moreover, the higher pressure cells tend to locate at continents and the lower pressure cells tend to located at ocean, due to the difference of

properties of water and land mentioned above.

Figure 1: Global map of long-term mean annual sea level pressure

For geopotential height, in figure 2, the lower and higher geopotential heights appears as alternate bands along the latitudes. The relatively lower geopotential height areas are mainly distributed at 60ºS, 0º and 60ºN while the relatively higher geopotential height areas are at 90ºS, 30ºS, 30ºN and 90ºN. The extreme pressure areas are situated around 60ºS, where the geopotential height is around

-100m. The geopotential height around 90ºS is 200m. The big difference of the geopotential height at the high latitude area at the Southern hemisphere leads that the extreme pressure areas are

distributed at 60ºS. Furthermore, the pattern of geopotential height on the Southern hemisphere is more continuous than the Northern hemisphere. The above geopotential height pattern is related to the general and secondary circulation, thus the relation between geopotential height and sea surface temperature is suggested that higher the pressure, higher the geopotential height, vice versa.

Figure 2: Global map of long-term mean annual geopotential height

For wind, the pattern of wind speed is also related to the sea level pressure and the geopotential

height. The wind is formed by the movement of air from high pressure areas to low pressure areas because of the pressure gradient force. In figure 3, the wind speed is highest between 60ºS and

90ºS, which is the extreme low and extreme high pressure belts area in accordance with figure 2. The pressure gradient force is large because the big difference in the pressure. Hence, the great

pressure gradient force causes the high speed wind. It is also matched the general circulation that the cold air mass direction causes the wind speed between 60ºS and 90ºS is highest. Since the

sinking air mass moves towards 60ºS to 0º according to the model of the general circulation, the

30ºS has a relatively high wind speed pattern. For the Northern hemisphere, the pattern of wind

speed is highly related to secondary circulation, which is similar to figure 1 and figure 2. The

pattern of wind speed forms cells instead of belts in the Northern hemisphere since the variation of the nature of the surface, which the Northern hemisphere has a higher proportion of land mass. The properties of land mass influence the pressure and the wind speed.

Figure 3: Global map of long-term mean annual meridional wind

Ex 2. Make similar plots for winter (January) and summer (July) seasons. Compare the two to see if you can find any seasonal changes.

Sea level pressure

By comparing figure 4 and figure 5, there is a significant seasonal difference of the global sea level pressure pattern. In figure 4, the land mass of the Northern hemisphere has a relatively

higher pressure during the northern winter, while the Southern hemisphere and the water bodies on the Northern hemisphere are mostly at lower pressure. Moreover, relatively high pressure

cells are found on the water bodies in Southern hemisphere. Additionally, it is worthy to

mention that an intense low pressure belt is constructed between 60ºS and 90ºS, which is around 990mb to 1005mb.

During the northern summer, the land parts of Northern hemisphere is in low pressure, while the Pacific Ocean and the Atlantic Ocean have relatively high pressure cells. In the Southern

hemisphere, the low pressure belt around 60ºS remain unchanged. However, the Antarctica has a drastically seasonal change that the extreme high pressure shows around 90ºS, which is 1035mb to 1050mb. In addition, there is a high pressure cell in Atlantic Ocean at 30ºS.

Figure 4: Global map of long-term mean annual sea level pressure in January

Figure 5: Global map of long-term mean annual sea level pressure in July

Geopotential height

The seasonal variation of geopotential height is similar to the seaonal variation of sea level

pressure above. In figure 6, during the nothern winter, most of continental parts on Northern

hemisphere are at high geopotential heights, while the low geopotential height cells shows in the Pacific Ocean and Arctic Ocean. For Southern hemisphere, extreme low geopotential height

belts are found between 60ºS and 90ºS, which is around 0m to -50m. In the Ocean part of the

Southern hemisphere, several relatively high geopotential height cells are found. In figure 7,

during the northern summer, the Northern hemisphere areas are mostly at low geopotential

height, except the relatively high geopotential height cells on Pacific Ocean and Atlantic Ocean. In contrast, the Southern hemisphere, the Antarctica is at an extreme high geopotential height, which is around 200m – 350m. Meanwhile, the relatively high geopotential cells are found at

other continents. However, the low geopotential heights at the Antarctic Ocean does not change

seasonally.

Figure 6: Global map of long-term mean annual geopotential height in January

Figure 7: Global map of long-term mean annual geopotential height in July

Meridional wind

In figure 8, during the northern winter, the wind heading north at a relatively high speed, which is around 4 m/s to 8 m/s at most of the areas, like Asia, Eastern Pacific Ocean and the Arctic

circle. The wind heading south are found at Southeast Asia and the Western Indian Ocean. In contrast, the wind directions were changed during the northern summer, which shown in figure 9. For instance, the wind directions of Asia and Eastern Pacific Ocean areas are turned to

southward during the northern summer. For the Antarctic cicrle, the wind direction remains unchange, and the wind speed has been increase from around 2 m/s to 4 m/s in January to around 2 m/s to 12 m/s in July.

Figure 8: Global map of long-term mean annual meridional wind in January

Figure 9: Global map of long-term mean annual meridional wind in July

2. Ocean-atmosphere interactions

ENSO: choose “Tropical Pacific” in Map Projection;

NAO: choose “custom” and then define latitude and longitude (suggested values: N 20-90, E -90-60);

Hong Kong: choose “custom” and then define latitude and longitude (suggested values: N 10- 40, E 90-135).

Ex 3. Construct mean climatology, positive phase (1997) and negative phase (1999) in the

ENSO region. Climate variables include SST (NOAA Extended), rainfall (GPCP) and pressure fields (sea level pressure).

SST:

Figure 10: Map of SST in the ENSO region in 1997

Figure 11: Map of SST in the ENSO region in 1999

Rainfall:

Figure 12: Map of mean rainfall in the ENSO region in 1997

Figure 13: Map of mean rainfall in the ENSO region in 1999

Pressure fields:

Figure 14: Map of pressure fields in the ENSO region in 1997

Figure 15: Map of pressure fields in the ENSO region in 1999

Ex 4. Construct mean climatology, positive phase (1990-1995) and negative phase (1960-1965) in the NAO region. Climate variables include air temperature, rainfall (GPCP) and pressure

fields (sea level pressure). Choose winter season.

Air temperature:

Figure 16: Map of mean air temperature in the NAO region in winter in 1990-1995

Figure 17: Map of mean air temperature in the NAO region in winter in 1960-1965

Rainfall:

Figure 18: Map of mean precipitation in the NAO region in winter in 1990-1995

Note that the map of mean air temperature in the NAO region in January in 1960-1965 cannot be constructed as the data in this period is out of the dataset.

Pressure fields: