Reading Notes

[andrewfagerheim]

3 June 2022 Descriptive Physical Oceanography: Chapter 1, Introduction to Descriptive Physical Oceanography

TYPES OF OCEANOGRAPHY

• Physical oceanography: generally focuses on the physical movements of the oceans on large spatial and time scales; what once consisted of individual ships/expeditions/projects is now dominated by large multinational projects that produce much more thorough datasets
• Descriptive oceanography: approach involving observations and model output to quantify fluid motions

GULF STREAM

• Gulf Stream (~100km) composes the western portion of the larger N Atlantic subtropical gyre (~1000s km)
• Strength of current and temperature gradient dissipate as it moves from the Caribbean to the E coast of N America and then out to the N Atlantic
• Mesoscale/eddy variability on the timescale of weeks

SCALE OF MIXING

• See image in Google Doc

USEFUL RATIOS

• Rossby number: ratio of timescale of Earth’s rotation to the timescale of motion for a certain phenomenon, general way to classify how much rotation (Coriolis I assume?) will effect this motion [slow fast motions: large ratio, rotation has little effect // large slow motions: small ratio, rotation has greater effect]
• Aspect ratio: ratio of vertical length to horizontal length [large scale flows: small ratio since the vertical dimension is constrained by ocean depth // surface/internal gravity waves: ratio on the order of 1]

[andrewfagerheim]

3 June 2022 Descriptive Physical Oceanography: Chapter 4, Typical Distributions of Water Characteristics

BASIC PRINCIPLES

• Ventilation: connection between surface and interior ocean, water properties are set at the surface and modified within the ocean
• Isopycnals: lines of equal density, ocean is stratified by density so interior flow generally goes along (NOT ACROSS) these lines
• Water masses: properties set by a single identifiable processes, water properties help trace the paths of flow from the surface into the interior, to identify forcing/mixing

[andrewfagerheim]

6 June 2022 Descriptive Physical Oceanography: Chapter 4, Typical Distributions of Water Characteristics

VOCABULARY

• meridional (N-S direction), zonal (E-W direction)
• equatorial (within a few degrees of the equator), Tropics (within 23N/23S of the equator)
• low latitudes (equatorial and tropical regions), subtropical/mid-latitudes, subpolar, polar (Arctic & Antarctic regions)
• Water masses are described by their identifying characteristics and the ocean processes that created them

GENERAL TRENDS

• Average ocean depth is 5km
• Variations of similar magnitude occur at much shorter distances over depth compared to spatial distances over lat/long
• Yet horizontal density differences drive horizontal circulation which is much stronger than vertical circulation since it is easier for water masses to move along lines of similar density than to cross into profiles with different densities
• Examples of water masses: Mediterranean Water/MW (highly saline layer in the N Atlantic, depth ~1,000-2,000km, high salinity is the result of evaporation in the Mediterranean Sea), Subtropical Mode Water/STMW (much thicker than surrounding layers)
• Four main layers: upper (which is in contact with the atmosphere) and intermediate, deep, and bottom (which are all below the pycnocline)

[andrewfagerheim]

17 June 2022

POTENTIAL TEMPERATURE

• Potential temperature: "temperature that a water parcel would have if moved adiabatically to another pressure. This effect has to be considered when water parcels change depth" (DPO textbook)
• Since seawater is not entire incompressible, pressure increases cause a water parcel to compress and increase in temperature, WITHOUT heat exchange with surroundings --> adiabatic compression (DPO textbook)
• "potential temperature is useful because it remains constant as a parcel undergoes an adiabatic change of pressure" (source)

CONSERVATIVE TEMPERATURE

• Not really a temperature, actually proportional to enthalpy (measurement of total internal energy)
• Potential and conservative temperature will be similar near the ocean surface and diverge with depth
• See TEOS-10 manual and the more readable explanation

[andrewfagerheim]

14 October 2022 Smith & Ferrari 2009

Takeaways:

• Spice: "the combination of temperature and salinity that does not contribute to density... a passive tracer that can be stirred by along-isopycnal velocities" (pg 2481)
• Variance in vertical profiles is set by balancing mesoscale straining and filter dissipation (pg 2487)
• Structure of tracer at submesoscales is only moderately affected by the energy and scale of the mesoscale eddies, implies that shear/strain ratio is dominated by small-scale velocity gradients (pg 2492)
• The conventional view of temperature variance says that turbulence is the result of breaking internal waves which stirs the large-scale temperature variance and dissipates to the molecular scale. However, this picture does not account for generating temperature variance by mesoscale stirring along-isopycnals (pg 2488-2489)

Questions:

• What does "forward cascade of T-S variance" mean? (pg 2478 for example), "forward potential enstrophy cascade" (pg 2491)
• What does shear mean as it pertains to temperature and salinity?
• How is spice plotted twice in Fig 3 for both stratification and gradient panels?
• Figures 11 & 12 seem similar to what we looked at over the summer, but I don't know what tracer they're looking at; what does mu represent?

[andrewfagerheim]

6 June 2023 Steinberg et al 2022

General methods & findings:

• MLD database: http://mixedlayer.ucsd.edu/
• Defined a framework for calculating variance (in the form of EKE): use a taper kernel when filtering, calculate the amount of energy at small scales (EKE), and how to bound energy within a range of scales
• EKE is plotted by month for different regions at 3 filter scales: 60m, 140m, 300m; alongside estimates for PE--> EKE conversion
• In certain regions (midlatitude gyres), we see little lag between conversion and small scale (0-2 months) and longer lag to larger scales (1-4 months) which suggests an inverse cascade (energy moves from small scales to large)
• Interesting plots and their relevance are discussed here: https://github.com/ocean-transport/argo-intern/issues/10#issuecomment-1574128413

Questions:

• What is the source of PE in the ocean? I'm imagining something to do with density gradients, but I don't know how.
• What is the deformation radius and why did this limit their analysis at small scales?
• Is the taper kernel built into scipy?
• Why do they need to account for "time-varying instrument error" and is this limited to satellite measurements?

[andrewfagerheim]

6 June 2023 Roullet et al 2014

Notes:

• Created a new term to study energetics of the interior ocean -- eddy available potential energy (EAPE), which mainly illuminates mesoscale turbulence, but also may include the signature of internal waves.
• One of the things they plot/consider is isopycnal displacement in the North Atlantic (Figures 1b and 4b), and the magnitude and structure seems very comparable to the CCS plot.

Questions:

• What are the advantages/disadvantages of looking at energy budgets compared with tracer structure? It seems like their analysis framework is similar (variance of density structure), but they're interpreting it within an energy framework.

[andrewfagerheim]

6 June 2023 Dove et al 2022

Notes:

• This felt harder to read than others, especially around page 7 as the methods were unfamiliar.
• More intermediate water is subducted in high EKE regions, largest difference in oxygen utilization between high and low EKE regions is between 200m-700m.
• Evidence that submesoscale motions move surface properties below the mixed layer, then along isopycnal stirring transports them to depth.

Questions:

• What is being shown in Figure 4? I'm not familiar with Lyapunov exponents so it's not obvious to me what role this figure plays in the story.

[andrewfagerheim]

7/8 June 2023 Serazin et al

Notes:

• Analysis of near-surface density structure, and variation across both seasonality and regions using the following metrics:
  • Upper Ocean Pycnocline (UOP): closest stratification peak of buoyancy frequency to the surface, region just below the mixed layer which facilitates tracer transport to depth; specifically analyzed by plotting the intensity, depth, and thickness of the UOP layer where thickness is relatively consistent (~23m) but intensity and depth vary more significantly
  • Upper Ocean Stratification Index (UOSI): stratification index computed at the lower boundary of the UOP
• Compared the upper boundary of the UOP to the lower boundary of the ML and found relative agreement outside of the tropics during the summer, but more inconsistencies in winter.

Questions:

• How would the intensity, depth, and stratification of the mixed layer and UOP affect our filtering analysis?
• Within this framework, what are we trying to quantify? Tracer transport below the mixed layer or below the UOP? Or the energies that make this transport possible? Or do we want a stratification index for below the UOP?

[andrewfagerheim]

8 June 2023 Chung 2019

Notes:

• Motivation is to constrain ocean heat uptake through improving the metric used to quantify isopycnal heaving, and more specifically to separate the influences of temperature and salinity which are masked by using potential density.
• The process is very methodically built from definitions to applications, following this rough structure:
  • defining Dynamic Depth Warping (DDW)
  • conducting synthetic tests to compare conventional density heave, warping of potential density, and warping of combined temperature-salinity
  • comparing these methods using station data in Hawaii (HOT) and Bermuda (BATS)
  • comparing the station data with Argo data
  • analyzing heave in the Pacific basin
  • calculate heat uptake in the high latitude North Pacific
• A general summary of their findings so far:
  • The conventional method had the most error in synthetic tests because it doesn't account for the way heaving impacts spice. While density warping and T-S warping often have similar results, in certain regions where there are horizontal intrusions of water masses with different characteristics, these features are apparent in T-S warp but not in density.
  • Argo seems to show similar heaving trends to the HOT station, with heave residual around 20-50m.
  • Using a linear model that accounts for heave trends, ENSO, and seasonality accounts for most change in most regions, but a more robust model would be helpful.
  • Combining this model with coherence/phase analysis helps explain the connections between these three factors across the basin.
  • It's possible to convert heave trends to heat uptake, with an answer that's in the same order of magnitude as other studies.

Questions:

• I'm having a hard time conceptualizing the specifics of singular value decomposition (due to a shaky grasp of linear algebra generally) and I also think the spectra plots are very noisy and difficult to pull any concrete conclusions from.
• While I conceptually understand the linearization and coherence methods more that SVD, I wouldn't be able to implement them in a notebook myself currently.
• What are Rossby waves? That keeps coming up and I keep forgetting to investigate further.