Historical Temperature Records Around the Great Barrier Reef
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Historical temperature records around the Great Barrier Reef J.D. Vlok & J.J. Marohasy 30 June 2020 1 Introduction This report provides an overview of historical temperature measurement around the Great Barrier Reef (GBR), including • sea surface temperature (SST) data recorded by the Australian Institute of Marine Science (AIMS), and • surface air temperature (SAT) data recorded by the Australian Bureau of Meteo- rology (BoM). Special attention is given to SST measurements taken around Lizard Island, off the coast of North-Eastern Queensland. Long SAT records from weather stations near the GBR are also considered, including Willis Island and Gayndah. The Argo float system is also considered briefly at the end of this report. Fig. 1 shows a map of the GBR and locations for which temperature data are publicly available, including BoM and AIMS measurement sites. The map shows 596 AIMS and 189 BoM sites. 2 Figure 1: Locations where temperature is measured near the Great Barrier Reef, including 596 AIMS SST and 189 BoM SAT measurement sites. 3 2 AIMS SST data The AIMS records SST data at several locations around Australia as shown on the map in Fig. 2. Figure 2: Locations of 749 AIMS SST stations and data loggers around Australia. AIMS runs two different programs to collect and record SST data: 1. A network of weather stations [1] providing in-situ data in near real time via satellite relay to the AIMS online data platform [2]. The most recent data can be viewed online, and older data are archived which can be downloaded in bulk as part of the SST datasets, discussed below. 2. A network of temperature loggers [3], also deployed in-situ although the data are extracted and processed up to two years later when each logger is physically ex- tracted from the ocean. These loggers do not have a direct wireless communication link to disseminate data in real time. The archived AIMS SST data are made available at the AIMS data centre [4] and the Australian Ocean Data Network (AODN) [5]. These two sources differ in terms of the number of recording stations, update schedule, and data fields included. Each SST dataset therefore contains a mixture of historical data from both programs mentioned above. Following are short overviews of each dataset. 4 2.1 AIMS data centre dataset The AIMS SST dataset is described as follows on the AIMS website [4]. The data provided here are from a number of sea water temperature monitoring programs conducted in tropical and subtropical coral reefs environments around Australia. Data are available from approximately 80 GBR sites, 16 Coral Sea sites, 7 sites in North West Western Australia (WA), 8 Queensland regional ports, 13 sites in the Solitary Islands, 4 sites in PNG and 10 sites in the Cocos (Keeling) Islands. Data are obtained from in-situ data loggers deployed on the reef. Data loggers instantaneously record sea temperatures every 30 minutes and are exchanged and downloaded approximately every 12 months. Temperature loggers on the reef-flat are generally placed just below Lowest Astronomical Tide level. Reef- slope (or where specified as Upper reef-slope) generally refers to depths 5 - 9 m while Deep reef-slope refers to depths of ∼20 m. The AIMS temperature data are made available in a single file, temp-logger.zip with size 810 MB, at [6]. The zip file contains a single data file, temp-logger.csv with size 14.1 GB. Loading all the contents of such a large file into memory at once would not be possible on a typical computer, and hence the data needs to be processed or extracted in batches (or line by line). The Python computer code developed to perform this is available at [7]. The file temp-logger.csv contains 141,012,622 lines of data, with each line containing the data fields listed in Table 1 below. The data from the first line in the file is used as example. Table 1: Data fields used in the AIMS file temp-logger.csv with the first line used as example. TIME 2012-06-18 17:30:33 +10:00 SITE Bramble Cay LOCATION BRAMBLEFL1 LATITUDE -9.4732166666667 LONGITUDE 143.876733333333 NOMINAL DEPTH 3 WATER TEMPERATURE 28.130714 The time field contains the date, time and timezone, mostly as indicated above. There are however exceptions where the time is not given in the local timezone (e.g. in UTC instead) or where the timezone is given as a text description. Only the time and temperature values change in each new line for a given site location. The depth value also changes for sites where temperature measurements were taken at multiple depth levels. All the other data fields are repeated on every line (for a fixed location), and the data file hence contains a high level of redundancy. The data can therefore be compressed by a factor of over 17, as is evident by comparing the sizes of temp-logger.csv and temp-logger.zip. 5 Repeating every data field on every line enables the gathering of any record from any site in any order into the data file. Seemingly, this was the approach followed as the data are not uniformly sorted according to either site or time. However, several large batches of recordings from the same site do appear together chronologically in the file. Processing is nevertheless required to extract and sort the data properly for each unique station. Furthermore, temp-logger.csv contains data from 770 unique sites, although 20 do not contain any temperature values. 130,432,208 lines (92.5% of the file) do however contain temperature data. The locations of 749 of these sites are shown in Fig. 2, whereas the remaining one is located at Puerto Morelos on the east coast of Mexico. All SST data from temp-logger.csv (excluding a total of 13 outliers) in the AIMS dataset are shown in Fig. 3 below. The overall date range spans 1987-07-31 to 2018-05-15. Although the dataset description on [4] states that temperatures are recorded every 30 minutes, the sample period Ts varies within and over different stations as shown in Ap- pendix B. Figure 3: AIMS SST data, with 130,432,195 temperature values shown. The data displayed in Fig. 3 can be refined and displayed as in Fig. 4, showing all monthly mean Tmean series of all AIMS stations, that has sufficient data such that at least one monthly mean could be calculated.1 1 Of the 750 stations, only 569 has sufficient data spanning at least one month. As shown in Appendix B several stations have very little data available, with the number of samples N often less than 10. Seemingly only a few measurements were taken over several years at some of these sites. 6 Figure 4: Monthly mean series of all sufficiently long AIMS SST stations. 2.2 AODN dataset The SST data can also be downloaded from the AODN platform [8]. The dataset is described on this platform as follows (similarly as for the AIMS data centre dataset). The data provided here are from a number of sea water temperature monitoring programs conducted in tropical and subtropical coral reefs environments around Australia. Data are available from approximately 80 GBR sites, 16 Coral Sea sites, 7 sites in North West Western Australia (WA), 8 Queensland regional ports, 13 sites in the Solitary Islands, 4 sites in PNG and 10 sites in the Cocos (Keeling) Islands. Data are obtained from in-situ data loggers deployed on the reef. Temperature instruments sample water temperatures every 5-10 minutes (typically) and are exchanged and downloaded approximately every 12 months. Temperature loggers on the reef-flat are generally placed just below Lowest Astronomical Tide level. Reef-slope (or where specified as Upper reef-slope) generally refers to depths 5 - 9 m while Deep reef-slope refers to depths of 20 m. Reefs are under threat from climate change. Elevated sea temperature is a major stress to reefs, capable of causing widespread coral bleaching and mortality. Small excursions of temperature outside normal summer levels can cause stress and bleaching in corals, especially sensitive species. Temperature data from this program are used to correlate with bleaching events and derive local bleaching thresholds. Data are also used in a wide variety of other marine research programs, including fisheries, balast water, turtles, sea birds, seagrass, coral disease, oceanography, process studies, validation of proxy climate records etc. The AODN platform allows downloading data by selecting stations or data loggers on an interactive map. The data of all selected stations are combined into a single CSV file, totalling 30.5 GB if all AIMS SST stations (totalling 550, including less stations than for the AIMS data made available on the platform discussed in Section 2.1) are selected. As mentioned in Section 2.1, the file size presents a challenge in terms of downloading and processing the data; loading the contents of a collection of stations with data in the GB 7 range into memory at once would not be possible on a typical computer, and hence the data needs to be processed or extracted in batches (or line by line). In total, the 550 stations contain 110,031,431 lines of data (downloaded on 23 May 2020), with each line containing the data fields listed in Table 2 below. The fields are populated with an example. Clearly, the AODN dataset includes additional data not available in the AIMS dataset discussed in Section 2.1 above, with data fields shown in Table 1. Table 2: Data fields used in the AIMS data files (AODN) populated with an example.