A new 2 km Sea Surface Temperature Atlas of the Australian Regional Seas (SSTAARS) Summary We use 25 years of Advanced Very High-Resolution Radiometer (AVHRR) data from NOAA Polar Orbiting Environmental Satellites received by six Australian and two Antarctic receiving stations to construct a detailed climatology of sea surface temperature (SST) at 20 cm depth around Australasia. • Raw scans have been accurately navigated • Only night-time nearly cloud-free data are used to reduce diurnal bias and cloud contamination • Irradiances have been processed and fitted to in situ drifter data following international GHRSST protocols to help reduce instrument bias • A pixel-wise (2km) climatology (with four annual sinusoids) and linear trend are fit to the data using a robust fitting technique • Monthly non-seasonal percentiles are also derived Validation • Used temperatures from coastal tide gauges and wave-rider buoys, not used to de-bias the satellite data (Figure 1a) • Climatology based on in situ observations was compared to SSTAARS • The major driver of difference is the distance between the in situ site and the nearest Atlas pixel (Figure 1b). • The more offshore wave-rider sites show very good agreement with the Atlas as they can be most closely collocated with an Atlas pixel • For co-locations within 3km, RMS errors are 0.2o C or less, and within 1km, 0.1o C or less, comparable to the average drifter matchup errors Susan E. Wijffels1,2, Helen Beggs3, Christopher Griffin3, John F. Middleton4, Madeleine Cahill1, Edward King1, Emlyn Jones1, Ming Feng1, Jessica A. Benthuysen5, Craig R. Steinberg5 and Phil Sutton6 Key phenomena revealed • Indonesian Throughflow warm advection (Figures 4 and 5) • Summer tidal drawdown over tropical shelves (Figures 6 and 7) • Tidally-driven reef-flat mixing (Figures 6 and 7) • Boundary current flows and their seasonality (Figures 8 and 9) • Cross-shelf gradients of variance due to mesoscale eddies (Figure 9) • Suppression of the annual cycle by warm advection along the Lleeuwin Current pathway (Figures 8 and 9) Data Access SSTAARS Atlas files: https://portal.aodn.org.au/search?uuid=79c8eea2-4e86-4553-8237-4728e27abe10 IMOS AVHRR daily night-time L3S data used as input: https://portal.aodn.org.au/search?uuid=79c8eea2- 4e86-4553-8237-4728e27abe10 Figure 1: a) Locations of long and relatively continuous near surface in situ temperature records from either coastal tide gauge sites (SL) or offshore wave-rider sites (WR); b) RMS difference in the seasonal cycle between the in situ and Atlas estimates as a function of distance between the in situ measurement site and the nearest Atlas pixel. 1 CSIRO Oceans and Atmosphere, GPO Box 1538 Hobart, TAS 7001 Australia 2Woods Hole Oceanographic Institution, MS #29, 266 Woods Hole Road, MA 02540, USA 3 Bureau of Meteorology, Docklands, VIC 3008, Australia 4 South Australian Research and Development Institute, West Beach, SA 5024, Australia 5 Australian Institute of Marine Science, Townsville, QLD 4810, Australia 6NIWA, Wellington, New Zealand Coverage • The coverage for June and December is shown in Figure 3 in observation count per pixel • Sampling is very seasonally dependent, due to cloudiness • Pixels near Australia have more observations, with sampling dropping off with distance from Australian satellite receiving stations due to reduced reception of direct broadcast NOAA satellite data by these receivers. Figure 2: Estimates of the linear SST trend from 3/1992-12/2016: a) from the IMOS SST archive and b) from the NOAA OI V2 weekly SST. Units are oC/decade and the contour interval is 0.2C/decade. Grey stippled regions mark where the trend is smaller than 2 times the standard error (95% significance level). 25 Year Trends Figure 4 (top left): Climatological SST from the Atlas for the Indonesian Seas shown every 2 months. The colour scale is adaptive and changes every month to help better reveal spatial contrasts. Grey contours mark the 50m (thin) and 200m (thic