Relations between isobaric and level surfaces

1. Relations between isobaric and level surfaces • A level of no motion is usually selected at about 1000 meter depth • In the Pacific, the uniformity of properties in the deep water suggests that assuming a level of no motion at 1000m or so is reasonable, with very slow motion below this • In the Atlantic, there is evidence of a level of no motion at 1000-2000 m (between the upper waters and the North Atlantic Deep Water) with significant currents above and below this depths • That the deep ocean current is small does not mean the deep ocean transport is small

2. Level of No Motion (LNM) Atlantic: A level of no motion at 1000-2000m above North Atlantic Deep Water Pacific: Deep water is uniform, current is weak below 1000m. “Slope current”: Relative geostrophic current is zero but absolute current is not. May occurs in deep ocean (barotropic). The situation is possible in deep ocean where T and S change is small Current increases into the deep ocean, unlikely in the real ocean

3. Barotropic flow:  p and  surfaces are parallel => =(p) For a barotropic flow, we have is geostrophic current. Since  Given a barotropic and hydrostatic conditions, and Therefore, By definition,  and t are both constant in barotropic flow And So (≈0 in Boussinesq approximation) The slope of isopycnal is small and undetectable (as isobar) for V=0.1 m/s, slope~10-6, i.e., 0.1m height change in 100 km

4. Relations between isobaric and isopycnal surfaces and currents Baroclinic Flow: and There is no simple relation between the isobars and isopycnals. slope of isobar is proportional to velocity slope of isopycnal is proportional to vertical current shear. With a barotropic of mass the water may be stationary but with a baroclinic field, having horizontal density gradients, such situation is not possible In the ocean, the barotropic case is most common in deep water while the baroclinic case is most common in the upper 1000 meters where most of the faster currents occur.

5. 1 and 1/2 layer flow Simplest case of baroclinic flow: Two layer flow of density 1 and 2. The sea surface height is =(x,y) (In steady state, =0). The depth of the upper layer is at z=d(x,y)<0. The lower layer is at rest. For z > d, For z ≤ d, If we assume  The slope of the interface between the two layers (isopycnal)= times the slope of the surface (isobar). The isopycnal slope is opposite in sign to the isobaric slope.

9. σt A B diff 26.8 50m --- >50m 27.0 130m 280m -150m 27.7 579m 750m -180m Isopycnals are nearly flat at 100m Isobars ascend about 0.13m between A and B for upper 150m Below 100m, isopycnals and isobars slope in opposite directions with 1000 times in size.

10. Relation between fluctuations of SSH () and thermocline depth (h) 20oC isotherm anomalies (m) COLA ODA December,1997-February, 1998 SSH anomalies (cm) TOPEX/Poseidon

13. Example: sea surface height and thermocline depth“Warm water on the right”

14. Comments on the geostrophic equation • If the ocean is in “real” geostrophy, there is no water parcel acceleration. No other forces acting on the parcel. Current should be steady • Present calculation yields only relative currents and the selection of an appropriate level of no motion always presents a problem • One is faced with a problem when the selected level of no motion reaches the ocean bottom as the stations get close to shore • It only yields mean values between stations which are usually tens of kilometers apart • Friction is ignored • Geostrophy breaks down near the equator (within 0.5olatitude, or ±50km) • The calculated geostrophic currents will include any long-period transient current

15. Review Topics and Questions Properties of Sea Water • What unit of pressure is very similar to meter in depth in ocean? • What is salinity? Why can we use a single chemical constituent (which one?) to determine it? What other physical property of seawater is used to determine salinity? • What properties of seawater determine its density? What is an equation of state? • What happens to the temperature of a parcel of water (or any fluid or gas) when it is compressed adiabatically? What is potential temperature? Is it larger or smaller than the subsurface temperature? • What are the two effects of adiabatic compression on density? • What are t and ? How do they different from the in situ density? • Why do we use different reference pressure levels for potential density?

16. Typical distribution of water properties • Major characteristics of the sea surface temperature (distributions of warm pool and cold tongue) and its seasonal cycle • Distribution of the surface salinity and its relation to the precipitation and evaporation • Typical vertical distribution of temperature. What is main thermocline? • What is seasonal thermocline? What are the major processes cause its formation?

17. Conservation laws • Mass conservation (continuity equation), volume conservation • Salt conservation (evaporation, river run-off, and precipitation) • Heat conservation (short and long-wave radiative fluxes, sensible and evaporative heat fluxes, basic factors controlling the fluxes, parameterizations) • Meridional heat and freshwater transports • Qualitative explanations of the major distributions of the surface heat flux components

18. Basic Dynamics • What are the differences between the centrifugal force and the Coriolis force? Why do we treat them differently in the primitive equation? • How to do a scale analysis for a fluid dynamics problem? • What is the hydrostatic balance? Why does it hold so well in the scales we are interested in? • What is the geostrophic balance? • What is the definition of dynamic height? • In geostrophic flow, what direction is the Coriolis force in relation to the pressure gradient force? What direction is it in relation to the velocity? • Why do we use a method to get current based on temperature and salinity instead of direct current measurements for most of the ocean? • How are temperature and salinity information used to calculate currents? What are the drawbacks to this method? • What is a "level of no motion"? Why do we need a "level of known or no motion" for the calculation of the geostrophic current? (What can we actually compute about the velocity structure given the density distribution and an assumption of geostrophy?) • What are the barotropic and baroclinic flows? Is there a “thermal wind” in a barotropic flow? • What can you expect about the relation between the slopes of the thermocline depth and the sea surface height, based on a 1 and 1/2 layer model? • What factors determine the static stability?