General Comment on Lab Reports: v. good + corresponds to a lab report that: has structure (Intro., Method, Results, Discussion, an Abstract would be a bonus) is well written (take your time to edit) shows all figures with figure captions (Fig.1: Time series of atmospheric pressure at Buoy for the year 1997) the content of the figures is discussed and referenced in the main text there is a minimum amount analysis of the observations – i.e. analysis of what you see in the figures in the context of what you know or you are now learning in the course (From the curves shown in Fig X. we can see that there is a lag between the annual temperature cycle for the atmosphere and the ocean.)

Chapter 4: The N.Atlantic Gyre Exploration of the east coast of America, colonization, trading and whaling led to the Western N.A. being charted earlier than most other ocean regions The Gulf stream was the most notable current and has been depicted on some of the earliest maps. The Gulf Stream consists of water from equatorial regions and re-circulated water from the subtropical gyre

Maury’s Chart of the Gulf Stream Subtropical gyres: intense western currents diffuse eastern currents Maury: GS not wind driven PGF resulting from hydraulic head in the western side of the basin (forced by NE trade winds) gives the jet like quality to the GS (2 m/s in some areas) GS follows continental slope up to Cape Hatteras then moves into deeper water, meanders and forms eddies (Paul Gayes Story)

The Subtropical Gyres: setting the stage for generating a more realistic gyre Ekman used an ideal, infinite ocean, no slopes in sea level, or variations in salinity Western Boundary Currents fast, deep, narrow, warm Gulf Stream, Kuroshio O(100 km) width, u = 2 m/s Eastern Boundary Currents slow, shallow, broad, cool Canary, California O(1000 km) width, u = 0.25 m/s

Dynamic Systems: conservation of momentum (linear and angular) Conservation of a tendency to rotate Conservation of vorticity Current shear – change in the velocity in the direction perpendicular to the flow direction. Current shear will induce a tendency to rotate in a water parcel. Positive Vorticity: anticlockwise Negative Vorticity: clockwise

All rotating objects on the Earth possess: relative vorticity: rotation in relation the surface of the Earth – ζ Planetary vorticity: vorticity associated with the rotation of the Earth – f VORTICITY is a measurement of the rotation of a small air/water parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists/oceanographers are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.

Planetary Vorticity and the Coriolis force: Poleward decrease in linear eastward velocity The angular velocity of the surface of the Earth increases with latitude - anticlockwise rotation around a local vertical axis N.H. - clockwise rotation around a local vertical axis S.H.  sin(  ) = angular velocity 2  sin(  ) = Planetary Vorticity (f) Q4.3

Conservation of Vorticity: relative to fixed space Absolute Vorticity = relative + planetary vorticity Absolute vorticity of a parcel of fluid (its vorticity as a result of being on the Earth plus any vorticity relative to the Earth) must remain constant in the absence of any of external forces such as wind stress and friction (f +  ) X Q4.4

Angular Momentum: m x r 2  m = mass r = distance to axis of rotation  = angular velocity D: could be from the sea surface to the top of the permanent thermocline, or in the case of deep and bottom waters from the permanent thermocline to the sea floor. The depth of the column of water will change as angular velocity changes.

For much of the ocean (away from coastal boundaries and large current shear) f >>  This results in ‘topographic steering’. Conservation of Vorticity  Conservation of Potential Vorticity (f +  )/D D Q4.5

Question 4.6

Sverdrup model – flow due to horizontal pressure gradients and wind stress on the surface. But it is not wind stress that matters, rather its torque Ahh here we go again! Velocity (linear & angular), Force, Momentum (linear & angular), Work, Energy, Torque and much more!! Torque is a force that tends to rotate or turn things. (You generate a torque any time you apply a force using a wrench, for example) – torque equals force multiplied by distance. Remember ‘work’? Work is simply the application of a force over a distance. The catch: the distance only counts if it is in the direction of the applied force – lifting an object, for example. And doing work is using energy! The difference between torque and work is the concept of rotation. Torque is work (distance times force) which causes rotation. (think ‘vorticity!) Why Is There A Gulf Stream

Sverdrup’s Results: the net amount of water transported by a given pattern of wind stress depends not on the absolute value of the wind stress but on its torque (its tendency to cause rotation, its ability to supply relative vorticity to the ocean) Did not account for a western boundary current.

Thinking About Meridional Transport: M = b x torque of  M: meridional transport kgs -1 m -1 b: latitude dependent constant M = b x curl(  ) M = 1/  x curl(  )  : the rate of change of f with latitude In the end: the total meridional flow is determined by the rate of change of f at the latitude concerned and the torque, or curl, of the wind stress.

1948: Henry Stommel onto the problem of the Gulf Stream Considered the effects of a symmetrical wind field on a rectangular basin for three different scenarios: 1.non rotating Earth 2.rotating with constant Coriolis parameter f 3.rotating and Coriolis parameter varies with latitude Stommel added friction – Sverdrup did not have it equilibrium achieved so forces must balance (steady state) ‘lines’ of wind

1.non rotating Earth 2.rotating with constant Coriolis parameter f 3.rotating and Coriolis parameter varies with latitude Stommel’s Results

Munk: extends Stommel’s domain up to 60 N, and improves the representation of both wind and friction. Considered friction with boundaries and friction associated with both lateral and vertical current shear (eddy viscosity in both horizontal and vertical dimensions; Ah and Az) Result: reproduced more realistic ocean circulation features

Equations of Motion

Gulf Stream: Extends from Straits of Florida to Grand Banks off Newfoundland Flow structure on either side of Cape Hatteras is different Shallow (800 m), follows continental slope, well defined T,S (mixture of water from Antilles Current and water recirculated in the Sargasso Sea meanders on the order of 55km

North of Cape Hatteras the Gulf Stream pulls away from the cont. slope and moves into deeper water (4-5 km) meanders can exceed 350 km Flow around Cape Hatteras svd Flow reaches a maximum around 65W, 150 svd (10 6 m 3 /s) Flow begins to be more diffuse as water enters the Azores Current and the recirculatory system

Continuity: The formation of the NADW (part of the recirculatory system), which is most noticeable in winter, strengthens the flow of the Gulf Stream Think about the link between thermohaline and wind driven circulation

Distribution of Temperature and Salinity Through the Straits of Florida Geostrophic flow calculation performs reasonably well Question 4.10

Geostrophic Currents: E-W transect extending from Cape Hatteras Flow consists of narrow, deep vertical filaments separated by counter currents Up to the 1960, the idea of counter currents were rejected by the scientific community, data was sparse leaving room for interpretation velocities (m/s)

T,S Sections Chesapeake Bay – Bermuda Measurements in 1932, could not capture the deep filaments of the Gulf Stream or the deeper counter currents. Where is the Gulf Stream? Note: the Sargasso Sea water is warmer and more saline than the water on the landward side. Salinity alone would make the system unstable, the temperature distribution has a greater effect on the density distribution and slope of the isopycnals

Insights From MODE (Mid-Ocean Dynamics Experiment Deployed neutrally buoyant floats and Sofar Drifters Resolution of mesoscale eddies, which were found to dominate flow with periods greater than the tidal and inertial periods Fastest currents occurred near the surface, however the eddies persisted down to at least 1500m. The axes of rotation were not always vertical

Gulf Stream Eddies cold core - warm core rings Current shear generated meanders once the eddy has sheared off the energy source becomes the sloping isopycnals ‘dynamic topography’

Back to Chapter 2: Icelandic Low & Azores High