August 25 - September 9, 2003

Altimetry

Alongtrack measurements

XBT 

ADCP

Meteo
 

Real-time altimetric support was used to route R/V Knorr through different eddy features on her way across the Atlantic during a time period from August 25 to September 8, 2003.   The CCAR web server http://ccar.colorado.edu/~realtime/welcome/ provides near real-time data streams from Jason, Topex-2, GFO, and ERS-2 altimeters. Sea Level Anomaly (SLA) data from the CCAR server were objectively analyzed prior to and during the cruise in order to choose appropriate eddy features and navigate ship's track across them.

Thirteen eddy features were identified, labeled E1 to E13 in the map for August 26, 2003 below (click to enlarge image). E2-E5, E7-E8, E10, and E12 are negative sea level anomalies with amplitude of sea level depression 100-150 mm east of 40W, and 200-300 mm (and more) west of 40W. E1, E6, E9, E11, and E13 are positive sea level anomalies with amplitude of sea level elevation approximately 150-300 mm. 

Between August 28 and September 2, 2003  Knorr crossed through the eddy features E1-E7. As hurricane Fabian approached, the initial route was changed and Knorr steamed for the center of E8 and then on September 3 turned SSW to the center of that warm eddy below E8, at approximately 30.5N, 50W. After that Knorr steamed W toward the cold eddy centered near 30.5N 57W. At 47W Knorr commenced XBT operations, dropping a probe every 1.5 hours. An SLA map for September 3, 2003 with the actual Knorr path and XBT cast positions are displayed below.

SLA maps between July 10 and September 8, 2003 with the actual Knorr's cruise track are also available in animated format.  Vertical sections marked from AA to JJ on the map above just show turning points in ship's track and will be used as a markers in further plotting of the alongtrack measurements. To see movie press this knob O.

Alongtrack cruise measurements consist of temperature and salinity sampled at 1 min interval. A preliminary processing was applied to each time series to reduce a high-frequency noise. This was a simple 1D averaging filter with a half-width equal to 5 min. After the filter was applied each of the time-series was re-sampled with 10 min interval. Estimates show that the high frequency part of the signal removed from the time series has an average value  close to zero and accounts for  ~1% of the variance in the raw data.  Note that at the ship's speed of  ~ 10 knots this time averaging corresponds to spatial averaging approximately 3 km which value is much more smaller then the horizontal scale of the altimetry field ( Rossby deformation scale ~ 30 km). A plots of alongtrack temperature and salinity vs time along with alongtrack Sea Level Anomaly  and ship track are displayed below. 

A zonal  temperature trend is clearly evident, with temperature increasing from 23 to 29C over the distance ~ 3800 nm. Mesoscale variations are superimposed on the main trend. There is less of a large-scale trend in the salinity data, and mesoscale variations are much more pronounced (~0.5 ppt amplitude) than in the temperature record. SLA generally varies between -150mm and +100mm.

To analyze  these fields in more detail we subdivided the region into eastern and western domains. The SLA map for the Eastern part is centered on August 30, 2003, and for the Western part on September 4, 2003. Corresponding plots displayed below.

Eastern part.

Eddies E2-E5. In the altimetry field these eddies look like cyclones with surface elevation below zero. It's clearly seen that all of these eddies have a low salinity in their centers. Note, that sections CC, DD, and EE cross approximately  center of  the eddies. Center of  E2 eddy is located between sections AA and BB, closer to BB. Drop in salinity varies between  0.4-0.6 ppm. Temperature measurements indicate more uniform values, although in center of E2 and E4 eddies temperature lowered by  0.6 ^OC.  As to SLA, seem like just one single point a day is too coarse sampling interval to resolve a details of the eddy evolution but it is enough to see the tendency.

Eddy E1. In the altimetry field this eddy looks like anticyclone, or a "warm" eddy. Section AA crosses this eddy a bit west of its center and surface elevation is positive. Both, temperature and salinity show some changes in they values in that place but it's hard to interpret them as some kind of  manifestation of  anticyclone.

Western part.

Eddies E7-E8.  In the altimetry field these features look like cyclones with a surface elevation below zero. Section GG crosses eddy E8 through its center so that a dramatic drop in salinity is clearly seen on both sides of that section. Changes in temperature are much more modest but are still evident. 
A ship's track crossed eddies E6 and E7 through their periphery and a strong frontal zone between them.  Immediately west of section FF (warm feature)   a strong outburst (increase) in salinity is clearly visible. Then, in zone of cold feature, salinity drops down. A temperature is going similar way but its changes are less pronounced.
To  west of section HH  (ship's route was changed that time) there are more interesting features in the altimetry field but they are less pronounced as it was before.  There is a very dramatic drop in salinity west of section JJ -- zone of another one  cold feature. Change in temperature  is less visible due to obscuring this change by  strong temperature trend.
It worth to note that SLAs calculated relatively some mean subtracted not necessarily give us the actual sea elevation. Due to interannual, or decadal variability a zero level is still relative so that  SLA just reflects a form of elevation surface but represents its absolute value. 

In summary, note a common feature in eddies explored: fresh and cold water in the top layer over cyclonic structures. 

There were 67 XBTcasts during a cruise, locations of these casts displayed in figure below. Station # 1 was the test station performed in the Mediterranian, so it was excluded from the further analysis.
 

Vertical profiles (temperature vs depth) of all XBT casts displayed in 2 figures:  casts #1-35, and casts # 36-67   (click to see profiles).
Casts # 5,11, 23, 25, 26, 42, 47, and 59 have good values down to 200m. Below this depth temperature profiles were distorted. To maximize the number or profiles used for plotting, vertical cross-section of alongtrack XBT measurments was first drawn down  to 200 m.

Meteo set of underway measurments consists of air temperature, wind direction and wind speed. Sampling interval is ~1 min,  same as for SST and surface salinity time series. To draw these data we applied same smoothing procedure as for SST and salinity -- 10 min averaging filter. Below these data splitted in 2 parts and  displayed separetely: (1) between sections AA-FF, and (2) FF-JJ.