Site icon R-bloggers

Gulf Stream centre detection

[This article was first published on Dan Kelley Blog/R, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

Introduction

Definitions of Gulf Stream location sometimes centre on thermal signature, but it might make sense to work with dynamic height instead. This is illustrated here, using a model for , with the distance along the transect. The idea is to select , the halfway point in the function, where the slope is maximum and where therefore the inferred geostrophic velocity peaks.

Methods and results

1
library(oce)
## Loading required package: methods
## Loading required package: mapproj
## Loading required package: maps
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
data(section)
## Extract Gulf Stream (and reverse station order)
GS <- subset(section, 109<=stationId & stationId<=129)
GS <- sectionSort(GS, by="longitude")
GS <- sectionGrid(GS)
## Compute and plot normalized dynamic height
dh <- swDynamicHeight(GS)
h <- dh$height
x <- dh$distance

par(mfrow=c(1, 3), mar=c(3, 3, 1, 1), mgp=c(2, 0.7, 0))
plot(x, h, xlab="Distance [km]", ylab="Dynamic Height [m]")

## Fit to tanh, with x0 line
m <- nls(h~a+b*(1+tanh((x-x0)/L)), start=list(a=0,b=1,x0=100,L=100))
hp <- predict(m)
lines(x, hp, col='blue')
x0 <- coef(m)[["x0"]]
abline(v=x0, col='blue')

# Temperature section, again with x0 line
plot(GS, which="temperature")
abline(v=x0, col='blue')

## Show lon and lat of x0, on a map
lon <- GS[["longitude", "byStation"]]
lat <- GS[["latitude", "byStation"]]
distance <- geodDist(lon, lat, alongPath=TRUE)
lat0 <- approxfun(lat~distance)(x0)
lon0 <- approxfun(lon~distance)(x0)
plot(GS, which="map",
     map.xlim=lon0+c(-6,6), map.ylim=lat0+c(-6, 6))
points(lon0, lat0, pch=1, cex=2, col='blue')
data(topoWorld)
## Show isobaths
depth <- -topoWorld[["z"]]
contour(topoWorld[["longitude"]]-360, topoWorld[["latitude"]], depth,
        level=1000*1:5, add=TRUE, col=gray(0.4))
## Show Drinkwater September climatological North Wall of Gulf Stream.
data("gs", package="ocedata")
lines(gs$longitude, gs$latitude[,9], col='blue', lwd=2, lty='dotted')

Exercises

From the map, work out a scale factor for correcting geostrophic velocity from cross-section to along-stream, assuming the Drinkwater (1994) climatology to be relevant.

Resources

To leave a comment for the author, please follow the link and comment on their blog: Dan Kelley Blog/R.

R-bloggers.com offers daily e-mail updates about R news and tutorials about learning R and many other topics. Click here if you're looking to post or find an R/data-science job.
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.