---
name: meridional-transport
description: "Meridional heat, volume, freshwater transport: section masks, overturning and gyre decomposition, RAPID comparison at 26.5N."
user-invocable: false
---

# meridional-transport

Background expertise for the METHOD of computing meridional transports
natively and framing them honestly. Dataset-specific facts (basin scope
of a given product, expected values, the RAPID-comparison discipline)
are not restated here: they live in the knowledge bundle (the
`ecco-` gotchas, recipes, and attested computations the ecco skill
lists) and are consulted per analysis by the transport-analysis
workflow and the ecco-scout agent, as the core `consult-knowledge`
skill sets out. Grid mechanics live in ocean-grids and the ecco skill's
grid reference; expected values and spreads live in the recipe and
computation concepts.

## Sections are masks, not index rows

On any curvilinear grid, a latitude circle crosses cells irregularly; a
section is a set of cell FACES with signs, not a `j = const` slice.
`ecco_v4_py.get_section_line_masks` (and `get_available_sections` for
named straits) produces the west- and south-face masks; transports sum
face fluxes times face geometry over those masks. Grid-native face
directions make rotation unnecessary in the sum (the ecco skill's grid
reference, step 8).

## Volume, heat, freshwater

- **Volume (Sv):** sum of `UVELMASS * dyG * drF` and
  `VVELMASS * dxG * drF` over the masked faces; the MASS variables
  already carry hFac and bolus is separate (below).
- **Heat (PW):** prefer the model's advective heat-flux diagnostics
  (`ADVx_TH`, `ADVy_TH` for ECCO) summed over the section, times
  rho0 * c_p on the temperature flux: these capture exactly what the
  model transported, including sub-monthly covariance that
  reconstructed v-times-theta from monthly means misses. **A heat
  transport is physically well defined only across a section with zero
  net mass transport**; where net mass transport is nonzero, the number
  is a temperature transport relative to a stated reference, and the
  reference must be stated.
- **Freshwater (Sv):** same structure with salt fluxes and a stated
  reference salinity; the reference-dependence caveat applies
  identically.

## Total means Eulerian plus bolus

In eddy-parameterized models the tracer transport includes the bolus
(GM) component; ECCO ships it as its own product (UVELSTAR family).
Eulerian-only transport labeled as total is a systematic bias; the
flux diagnostics (ADV*_TH) already include everything the model did,
which is another reason to prefer them for heat.

## Overturning and gyre decomposition

Zonally integrate meridional transport in depth space and cumulate
vertically for the overturning streamfunction (AMOC strength is its
maximum at the section, in Sv). Decompose section transport into
overturning (zonal-mean velocity times zonal-mean property) and gyre
(deviation) components to attribute mechanisms; on depth surfaces this
decomposition is convention-laden near western boundaries, so name the
convention used.

## The RAPID comparison at 26.5N

The RAPID-MOCHA array at 26.5N is the standard ground truth for
Atlantic sections. The METHOD discipline: compute the model section,
then consult the section's recipe and gotcha concepts in the knowledge
bundle for what to compare against (expected range, spread, the basin
scope the observation uses, validation provenance) and cite them; this
skill carries no expected ranges, spreads, or product-specific scope
rules of its own. Time-sampling matters generally: a single year's mean
sits inside a wider envelope than a multi-year mean, and a model window
overlaps an observational array only where their periods intersect.

## Must NOT

- Never hardcode an expected transport range or spread; read the
  recipe concept and cite it.
- Never compute transports on regridded fields (ocean-grids rule;
  native faces only).
- Never slice `j = const` as a section on a curvilinear grid.
- Never quote a heat transport across a mass-unbalanced section
  without stating the reference temperature.
- Never label Eulerian-only transport as total in an
  eddy-parameterized model.
- Never compare a model mean against RAPID without matching the
  overlap period or saying the periods differ.