convection in the atmosphere and oceans

1. convection in the atmosphere and oceans AOS 101 Weather and Climate Lisha M. Roubert University of Wisconsin-Madison Department of Atmospheric & Oceanic Sciences

2. Differences between the Atmosphere and Oceans • Forcing in the oceans is different from forcing in the atmosphere. • Heating destabilizes the atmosphere and stabilizes the ocean. • In the atmosphere: the sun heats the surface, which transfers heat and causes air it to rise. • In the ocean: when the surface of the ocean is heated nothing happens • Buoyancy forcing is important as well.

3. Buoyancy •  Buoyancy is an upward acting force that opposes an object's weight.   • This force is caused by fluid pressure. • If the object is either less dense than the liquid or is shaped appropriately (as in a boat), the force can keep the object afloat.

4. Structure of the Ocean • The oceans is stratified into layers of water of different density. The less dense water will be at the surface and the density increases with depth. • Density in the ocean is determined by salinity and temperature. • Warm and less salty=lower density • Cold and very salty=higher density • Density is a measure of Buoyancy. Water masses with less density will be more buoyant.

5. Profile of how Temperature, Salinity and Density change with Depth

6. Convection in the Oceans Normal Ocean in Summer heat • Convection in the oceans varies with seasons. • In warm months: the sun warms the upper layer of the ocean and keeps it well mixed so that we have warm water at the surface of the ocean and cold water at the bottom. Not much convection. • In cold months: cold wind blows over the surface. This cools the water at the surface of the ocean and make it more dense causing it to sink deeper. This causes deep convection. Shallow convection warm cold

7. Mixed Layers • Buoyancy and stratification determine mixing. • The thermocline is the transition layer between the mixed layer at the surface and the deep water layer. • The definitions of these layers are based on temperature. • mixed layer- it is near the surface where the temperature is roughly that of surface water. • thermocline-transition zone between mixed layer and deep water. The temperature decreases rapidly from the mixed layer to the deep water • Deep water-water at the bottom with very cold temperatures

8. Differences between convection in the atmosphere and oceans ATMOSPHERE • Convection propagates upwardinto the atmosphere. • In warm months there is more convection. The ground is heated. This heat is transferred to the air, the air will get warm and rise and cold air descends to replace the air that ascended. • In cold months the air at the surface will be very cold and will not rise as much which indicates less convection. OCEAN • Convection propagates downward into the ocean. • In warm months there is less convection. The water is heated but the heated water will not reach great depths. • In cold months we have deep convection because water at the surface becomes more dense (less buoyant) and thus sinks.

9. Today in the Lab • Satellites can only take measurements at the surface of the ocean, that’s why we need probes instead to take measurements at a much greater depth. • The data you will be using today was taken from an XCTD probe. • XCTD probes take measurements of temperature, salinity, conductivity, density at different depths. (In our case from 2m-999m)