Northeast Channel inflow

Transport across the Northeast Channel, between Georges Bank and Browns Bank. Positive values are into the Gulf of Maine.

Usage

northeast_channel_inflow(
  env_dat,
  u = "UO",
  v = "VO",
  spacing = NULL,
  min_coverage = 0.5,
  name = "channel_inflow"
)

Arguments

env_dat: an sf POINT object with one row per location and time step, as datamatch's access functions return, on a regular lon/lat grid, with eastward and northward velocity columns
u: name of the eastward velocity column, in m/s
v: name of the northward velocity column, in m/s
spacing: sample spacing along the section, in km. NULL uses half a grid cell.
min_coverage: fraction of sample points that must carry a velocity for the step to return a value
name: name for the new column

Value

env_dat with a channel_inflow column, in m^2/s, positive into the Gulf of Maine

Details

A separate index from scotian_shelf_inflow(), not a variant of it. The Northeast Channel is the deep connection through which slope water enters the Gulf, and it is the Gulf's main source of dissolved inorganic nutrients (Ramp et al. 1985; Townsend et al. 2015). Scotian Shelf inflow at Cape Sable is shallow, fresh, and nutrient-poor. The two vary independently and episodically, so combining them into one number would hide the contrast worth having.

A caution specific to this section

Ramp et al. (1985) measured the deep flow here, below 75 m, and found persistent in-channel transport. Surface velocities over the same section can run the other way, because the channel is strongly baroclinic. An index built from surface currents is therefore not comparable in sign or magnitude to the moored estimates, and is best read as relative variability. If depth-resolved velocities are available, fetch them at channel depth and pass them here instead.

The inflow regime is not stationary

Slope water entering here is modulated by Gulf Stream warm-core rings. Du et al. (2022) found interannual ring activity off the Gulf tracks bottom salinity in the Northeast Channel, through coastal-trapped waves excited when a ring meets the shelf edge.

Silver et al. (2023) then showed the forcing itself changed: ring formation nearly doubled after 2000, from about 18 a year to 33, and salinity-maximum intrusions onto the Northeast Shelf quadrupled, with 72% of observed intrusions coinciding with a ring offshore.

So a long record of this index spans two regimes rather than one, and a model fitted across the break may be averaging over a change in the mechanism. This matters most for anything interannual: check whether a relationship holds before and after 2000 separately, rather than assuming it is stable.

It also bears on interpretation. A high value here can mean the same circulation carrying more slope water because more rings are present, rather than the circulation itself having strengthened.

References

Ramp SR, Schlitz RJ, Wright WR (1985). The deep flow through the Northeast Channel, Gulf of Maine. Journal of Physical Oceanography 15(12), 1790-1808.

Townsend DW, Pettigrew NR, Thomas MA, Neary MG, McGillicuddy DJ, O'Donnell J (2015). Water masses and nutrient sources to the Gulf of Maine. Journal of Marine Research 73, 93-122.

Du J, Zhang WG, Li Y (2022). Impact of Gulf Stream warm-core rings on slope water intrusion into the Gulf of Maine. Journal of Physical Oceanography 52(8). doi:10.1175/JPO-D-21-0288.1

Silver A, Gangopadhyay A, Gawarkiewicz G, Fratantoni P, Clark J (2023). Increased Gulf Stream warm core ring formations contributes to an observed increase in salinity maximum intrusions on the Northeast Shelf. Scientific Reports 13, 7538. doi:10.1038/s41598-023-34494-0

Examples

if (FALSE) { # \dontrun{
env <- northeast_channel_inflow(env)
} # }