ITP Buoy-70 Homepage （Last Update 2014-11-05 21:18:22）

Workshop 8 Surface Velocity, Probabilities, and Scales

Time series of buoy velocity, estimated from the trajectory of the buoy, are showed below.

From the histogram of the perturbation velocities (), we can find that the distribution of that is close to Gaussian distribution. Two times standard derivation

From the time-lagging auto-correlation function, we can find that the time-series of velocity repeats with a major period of approximate 750 hours (~30 days). It shows the monthly variations of velocity, which may due to the wind variability. Moreover, several smaller scale peaks can be figure out, e.g., with periods of ~100 hours.

The degree of freedoms of u- and v- components are 190 and 238. Because the observation is long enough, the degree of freedoms are large.

Workshop 7 Geostrophic

The density profiles from ITP-70 buoy was used to estimate the geostrophic current perpendicular to the direction of the buoy movement (i.e. the ice drifting). Following so-called “thermal wind relation”, one could compute the relative geostrophic velocity (i.e., vertical shear of baroclinic geostrophic velocity) through the horizontal pressure gradients [Cushman-Roisin and Beckers, 2011].

To compute geostrophic velocity, we need clean gridded section of density . The density profiles, which are five-day averaged, are show below. The white crosses and dots indicate the computed geostrophic velocities are perpendicularly inward and outward, respectively. And size of them show the relative magnitude of them according to the slope of the density surface.

We start from the assumption, in which the flow is stable, geostrophic, hydrostatic and non-homogenous:

Combine above two equations, thermal wind equation could be derived, as:

With the thermal wind equation, we can compute the geostrophic velocity. In the computation through ITP-70 temperature and salinity data, first-order central difference algorithm was applied, while the averaged location of 5 days was used as x. The result is presented below.

To focus on the near-surface layers where geostrophic velocity are large, we zoomed in the upper 300 m layers. Principally, the geostrophic velocities are small in magnitude (<0.1 m s^-1), except in some specially locations. For instance, near day 30, the geostrophic velocity shows a sudden change from positive to negative values. From the density map, it is also discernable that the isopycnals show abnormal uplift above 130 m, while penetrating downward beneath 150 m. The vertical displacements are as large as ~50 m. This abnormal displacement results in geostrophic velocity ~ 0.2 m/s.

The actual trajectory (black line) and geostrophic velocity (dots, both size and color show the magnitude, unit: m s^-1) of the buoy. Meanwhile, time series comparing the geostrophic velocity at -10 m layer and drifting velocity (from the trajectory of ice) are also shown.

From the above plot, one can figure out that: The geostrophic velocity is smaller at order-of-magnitude. It seems that the two velocities are in covariance. The near surface agrees with the variation of buoy velocity at some points. In other points, inverse phase pattern can be recognized, indicating complex flow structure.

Workshop 6 Acoustics (or download .docx file)

Here we employ the simplified formula (1) to calculate sound speed profiles from temperature, salinity and depth.

C(T,P,S)=1449.2+4.6T+0.055 T²+1.39 (S−35)+0.016 D (1)

From the formula, we can find that the temperature and salinity are the dominant factor to influent the sound speed. For instance, the typical range of temperature difference in Arctic region could be about [-2, 2], which will result in speed anomaly differs from -9 to 9 (m s^-1). In contrast, salinity ranged within [25, 35] only contribute to sound speed difference from -14 to 0 (m s^-1).

Typically, the collaboration effects from temperature and salinity would form a subsurface minimum of sound speed profile, as is shown in the following figure ([). The sound speed minimum, or SOFAR channel, generally located in 500-1000m depth.

However, the sound speed profiles of ITP Buoy-70 show abnormal structure compared with typical sound speed profiles (see below). The near surface structure is mainly due to the fresh and high temperature water input from the Pacific Ocean.

The profiles show surface minimum. It means that within the water column where ITP-70 buoy is located, the sound wave will be accumulated within shallow surface layer (~100m). Although a secondary sound speed minimum is also discernable in the profiles (~100-200m), it is unlikely that sound wave will be trapped in this layer because it is too close to surface minimum.

With respect to time, the surface layer with minimum sound speed deepens from ~10m to ~40m. The variation show similar pattern with that of N² which is likely due to the intensified wind-induced mixing. In terms of spatial variation, no significant pattern is recognized.

To further illustrate the property of sound profiles, I also calculate the second sound minimum (SSM), which is defined by the minimum sound speed under 70m. The SSM depth is as:

From the time series, it is obvious that the SSM is relative stable at ~125m. It is probably because the SSM is well below the thermocline, being free of surface mixing signal.

References:

Talley, L. D., G. L. Pickard, W. J. Emery, and J. H. Swift (2011), Descriptive physical oceanography: an introduction, Academic Press.

Wikipedia, http://en.wikipedia.org/wiki/SOFAR_channel

Talley, L. D., G. L. Pickard, W. J. Emery, and J. H. Swift (2011), Descriptive physical oceanography: an introduction, Academic Press.

Workshop 5 Stratification (II)

Because of the water property in ITP Buoy-70 (near Alaska), so the depth of second N² peak (hereafter, SNP) is identified, as following:

It is located around ~250 m depth, which is much deeper than that of central Arctic basin.

The spatial distribution of SNP is shown below, which is with a lot of high frequency signal:

The new T-S diagram is as:

Workshop 4 Stratification (I)

Buoyancy frequency (N²), or the Brunt–Väisälä frequency, computed from the ITP-70 profile data, with temperature and quality controlled salinity profiles, from the equation of:

Where is discretized with a first-order upward central difference (means that computed from surface to deep). Negative values of N² are neglected, which means that the instability state of the flow. The profile of N² is showing below, where the blue scatters are all profiles, and the red bold curve is the averaged N² over time.

For all profiles, the vertical distribution pattern are similar, with two N² extremum. First one is near 50m depth, while the other is ~250m.

Further, the time-depth map of N² is as follows, with the depth of maximum N² below:

It is quite interesting that the ~50m core seems to own two separate sub-cores. And the halocline (or more accurate, pycnocline) shows different trend before and after day 400. Before day 400, the pycnocline deepens from ~25m to ~45m. After that day, the pycnocline is relatively stable in vertical structure. It may be related to the seasonal variability of wind-induced mixing.

Two typical profiles data are available in day 392.7514 and day 545.3764. The former is in two-core regime while the latter has single core.

Workshop 1~3

3174 profiles | T-S diagram | 8.8 km/day | map | t-series | section | Freshwater Content (Quality Control)|

Combination FWC Map from ITP60, 61, 68 and 70;

- Wenfang Lu

- Dual PhD Degree Candidate, Physical Oceanography

- Xiamen University and University of Delaware