Evenly spaced observation fields from irregularly sampled data in the Southern Ocean

Fabio Vieira Machado, PhD Candidate, School of Environment

Methods

A total of 177,028 vertical profiles unevenly sampled in time and space in the SO were arranged in two regularly spaced grid systems (1) 0.5º latitude and longitude: used for the climatological and seasonal mean fields. In this configuration, 720 (0-719) grid points were created along the longitudinal section, between 180ºE-180ºW, and 30 (0-29) along the meridional section, between 60ºS-75ºS; (2) 1º latitude and longitude: used for the climatological and seasonal mean fields. In this arrangement, 360 (0-359) grid points were created along the longitudinal section, between 180ºE-180ºW, and 15 (0-14) along the meridional section, between 60ºS-75ºS. At this phase, the code written in Python reads the geographic coordinates of each vertical profile and repositions these profiles to the corresponding grid cells. Data can be analysed by type of platform (Argo and Seal), and also combined together (Argo+Seal). The water column of 2000 decibars is divided into 200 layers with regular vertical thicknesses of 10 decibars. If there is only one vertical profile in a grid cell, the profile is averaged at 10 decibars through the water column. The mean vertical profile in that grid cell will therefore be the result of that single profile. If there are multiple measurements in a single grid cell, each vertical profile of temperature is first averaged at 10 decibars individually and hereafter the mean of averages is then taken to create a single vertical profile for the grid cell.

Table 1 – Time period of data and number of profiles collected by Argo floats and Elephant Seals in the Southern Ocean.

Results

Argo floats have covered a wider area and profiled water columns deeper than 2000 decibars in the SO (fig1a). Seal data are concentrated in the first 500 decibars and remarkably cover nearshore and offshore waters between 20ºE and 130ºE (Fig.1b). The seal data also cover the gap near the coast, from the northern tip of the Antarctic Peninsula to southeastern Amundsen Sea. Figure 2 shows the number of profiles sampled by floats Argo (fig.2a) and seals (fig.2b) within grid cells of 0.5 degree latitude and longitude. Argo data are dominant in the Belingshausen, Amundsen, Weddell, and Lazarev seas. The largest number of vertical profiles are found at the eastern contour of the Weddell Gyro and Mawson Sea, indicating more than 25 profiles per grid cell. Roughly 20 profiles per grid cell were also sampled off shore in the Ross Sea as well as over areas over the Antartic Basin. Largest numbers of vertical profiles sampled by seals were found in the Amery Ice Shelf area, between Mawson Station (67º36’S-62º52’SW) and West Ice Shelf (60º40’S-85ºW), and the Knox coast (that lies between Cape Hordern, at 100º31’E, and the Hatch Islands, at 109º16’E). A significant amount of seal data are observed near Cape Filcher, and across Davis and Mawson seas.

Fig 2. b

Figure 2 – Number of profiles sampled by (a. left) Argo floats and (b. right) Elephant Seals gridded at 0.5º latitude and longitude from 60ºS to 75ºS. Colours represent the number of profiles.

Climatology fields

Figure 3 presents the temperature climatological mean field (Argo + Seal combined) gridded at 0.5° latitude and longitude for two layers. The topmost layer features temperature ranging from near -2ºC up to 7ºC. This layer is occupied by the Antarctic Surface Water (AASW), which is recognised for temperatures typically above the freezing point, but may reach up to 2ºC near the Polar Front, or where deeper and relatively warmer water approaches the surface (Carter et al, 2008; Emery, 1986). Two large ocean water bodies, marking temperature between 2ºC and 7ºC off the Bellingshausen and Ross seas, meet with the cold AASW at about 65ºS latitude. The boundary that outlines the convergence between the AASW and the relatively warm waters from the north describes the southernmost retreat of the Polar Front in the SO.  Roughly between 42ºW and 82ºE (clockwise), AASW crosses the SO from the Antarctic continental shelf to the latitude of 60ºS indicating the northernmost extent of the temperature minimum at surface. At 300 dbar, the temperature climatological mean field (TCMF) reveals waters below 0ºC surrounding the Antarctic continent, near the shore. Offshore, roughly between 55WºW and 20ºE (clockwise), the TCMF shows relatively warm waters (1.5ºC-2.3ºC) covering the largest area in the SO. In addition, two core of warmer waters in the Bellingshausen and Ross seas present the highest temperatures at this vertical level, roughly from 2.5ºC to 4.8ºC, between 60ºS and 64ºS.  

Fig. 3 b

Figure 3. Temperature mean field (argo+seal) gridded at 05º latitude and longitude from 60ºS to 75ºS. Horizontal sections are averaged between (a) 0-10dbar and (b) 300-310dbar. Colours represent the temperature in ºC.

Seasonal mean fields

Figures 4 and 5 show the temperature seasonal mean fields (Argo + Seal combined), gridded at 0.5° latitude and longitude for two levels in the water column (left-300dbar; right-750dbar). For these examples, the data are classified as if there were only two long seasons during the year: a six months’ summer season (October through March); and six months’ winter season (April through September). There is a large volume of data sampled by seals in the winter, filling important gaps by expanding data coverage between 15°E and 130°E. However, deeper into the water column, the volume of data collected by seals decreases significantly. Only a few profiles exceed the depth of 760dbar. In summary, both seals and Argo floats have played an important role in the acquisition of ocean observations that will help estimate a winter climatology for the SO, which has previously been unreachable because of the seasonally ice-covered section of this ocean. 

Fig 5. b

Figure 5.  Winter mean temperature field (argo+seal). Horizontal sections are gridded at 05º latitude and longitude from 60ºS to 75ºS, and averaged between (a. left) 300-310dbar and (b. right) 750-760dbar. Colours represent the temperature in ºC.

Monthly mean fields

The dataset was sorted by months (January, February, …, December) to arrange the monthly temperature mean fields according to the 1° horizontal resolution grid system. This grid system (1ºLat x1ºLon) was initially chosen due to lack of data at monthly resolution.  Figures 6 and 7 show examples of the monthly temperature mean fields for the topmost layer (0-10dbar) of the SO. The monthly mean fields for June and December are presented for Argo floats (left) and seals (right). It is noteworthy that there are more vertical profiles collected by Argo in December than in June. On the other hand, there is more data collected by seals in June than in December.

Fig 7. b

Figure 7 – Monthly mean temperature fields for December. Temperature data are from (a. left) Argo floats and (b. right) elephant seals. Horizontal sections are gridded at 1º latitude and longitude from 60ºS to 75ºS, and vertically averaged between 0-10dbar. Colours represent the temperature in ºC.

Discussion

The amount of data currently available has contributed significantly to the studies of the physical properties of seawater in the SO. However, these observations are irregularly sampled in space and time. Spatial and temporal interpolation techniques were used to provide evenly arranged data fields in time and space from irregularly sampled data.There are more profiles sampled by seals than Argo. Although, Argo data cover a wider area and deeper waters in the SO. Argo floats are the dominant platform in collecting oceanographic data off shore the Bellingshausen, Amundsen, Weddell and Ross seas. Seal data fill important gaps in nearshore waters around Antarctica, including the northern tip and western coast of the Antarctica Peninsula, and particularly off shore waters between 15ºE and 130ºE. In general, most profiles sampled by seals reach roughly the first 600 decibars only. A few vertical profiles exceeded the depth of 1000 decibars and there are no records for the grid’s deepest layer (1990-2000 dbar).

The mean climatological and seasonal fields in the SO are well represented by the grid system designed at half a degree of latitude and longitude. It is important to assess whether the application of specialised interpolation methods will alter the mean climatological fields compared to the use of data in the grid by means of a simple average as has been done so far. Despite efforts by the scientific community and massive investments made to increase direct observations in the SO, the amount of data available is still insufficient to create a monthly climatology at 1º. It is necessary to reassess that chosen grid system and create a coarser resolution grid system for the estimation of the monthly mean fields. Instead of creating a monthly climatology using data sampled by Argo floats and elephant seals, it is recommended that a slightly reduced temporal scale (seasonal scale) be used. This will ensure a more consistent and coherent means for analysis due to lack of monthly data and will create a more robust winter and summer climatology for the SO. Last, but by no means least, this study also contributes toward seeking better strategies to indicate where data acquisition platforms should be launched in order to play a more directive role in future studies.

ACKNOWLEGMENT

The author thanks e-research group, HackyHour@Auckland, and The Carpentries (UoA) on the introductory computational skills needed for data management. The author also thanks Nick Young for his crucial support and exchanges in regards to code in Python and GitHub.

BIBLIOGRAPHY

1. Carter, L. and McCave, I. N. and Williams, M. J. M. (2008) Circulation and water masses of the Southern Ocean: a review. In: Antarctic Climate Evolution. Developments in earth and environmental sciences (8). Elsevier, Amsterdam. ISBN 978-0-444-52847-6
2. Emery, W. J., and Meincke, J. (1986). Global water masses: summary and review. Oceanologica acta, 9(4), 383-391.
3. Roemmich, D., and Gilson, J. (2009). The 2004–2008 mean and annual cycle of temperature, salinity, and steric height in the global ocean from the Argo Program. Progress in oceanography, 82(2), 81-100.
4. Roquet, F., et al. (2013). Estimates of the Southern Ocean general circulation improved by animal‐borne instruments. Geophysical Research Letters, 40(23), 6176-6180.

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