Gulf Stream centre detection

June 22, 2014
By

(This article was first published on Dan Kelley Blog/R, and kindly contributed to R-bloggers)

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')

center

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

  • Source code: 2014-06-22-gulf-stream-center.R

  • K. F. Drinkwater, R. A Myers, R. G. Pettipas and T. L. Wright, 1994. Climatic data for the northwest Atlantic: the position of the shelf/slope front and the northern boundary of the Gulf Stream between 50W and 75W, 1973-1992. Canadian Data Report of Fisheries and Ocean Sciences 125. Department of Fisheries and Oceans, Canada.

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

R-bloggers.com offers daily e-mail updates about R news and tutorials on topics such as: visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git, hadoop, Web Scraping) statistics (regression, PCA, time series, trading) and more...



If you got this far, why not subscribe for updates from the site? Choose your flavor: e-mail, twitter, RSS, or facebook...

Comments are closed.