Advanced Very High Resolution Radiometer Sea Surface Temperature Research Articles

Seasonal and Inter‐Annual Variability of SST off the East Coast of the South Island, New Zealand

This study examined the temporal variability of sea surface temperature (SST) of three water masses east of the South Island, New Zealand, where two of these water masses are part of the global Subtropical Front (STF). The STF is well defined and can be detected easily using remote sensing Advanced Very High Resolution Radiometer (AVHRR) SST data. Ten sites were used from a ten year AVHRR SST data set and they were compared with a manually sampled 45 year SST data set from Portobello Marine Laboratory, Dunedin. This investigation decomposed each SST series into a periodic seasonal component and an interannual trend. The latter component was further decomposed into low and mid-frequency bands using smoothing techniques. The results showed that the periodic component of offshore waters were in phase and reached their maximum temperature 10 days after the inshore neritic waters. The amplitudes of seasonal variations of Subantarctic and Subtropical water masses were similar while the neritic waters showed considerably higher seasonal variation. El Niño has a clear effect and lowers the temperatures of all water masses by 1.4°C at Portobello and 2.2-3.4°C for all offshore waters.

The sea surface temperature field in the Eastern Mediterranean from advanced very high resolution radiometer (AVHRR) data: Part I. Seasonal variability

A ten-year dataset of Advanced Very High Resolution Radiometer-Sea Surface Temperature (AVHRR-SST) with 18-km space resolution and weekly frequency is used to study the seasonal variability of the Eastern Mediterranean Sea surface field. Three main objectives are addressed in this study. The first is to define the time and space scales of the surface temperature distributions. The second objective is to relate the SST features to the upper thermocline circulation and the third is to compare these features with the observational evidence of the Physical Oceanography of the Eastern Mediterranean (POEM) Programme. The time analysis reveals the presence of a strong seasonal signal characterized by two main seasonal extremes, winter and summer. The transition between the overall zonal distribution of the isotherms (winter) and the mostly meridional pattern of the fronts (summer) occurs very rapidly in May and October. The space analysis shows that the dominant scale is the sub-basin scale and the sub-basin gyres are very well resolved allowing the identification of permanent and semipermanent structures. The results for the two further objectives can be summarized together. The seasonal and monthly SST distributions are strongly correlated with the dynamical structure of the basin upper thermocline circulation. A direct comparison of the September 1987 SST pattern with the corresponding surface temperature map of the POEM-87 survey proves this correlation quantitatively. Furthermore, comparison of the SST monthly climatologies with the POEM circulation scheme shows that all the major currents and the sub-basin gyres are also found consistently in our patterns, with the only exception of the anticyclonic Mersa-Matruh Gyre.

Aircraft validation of ERS-1 ATSR and NOAA-14 AVHRR sea surface temperature measurements

Infrared brightness temperature (BT) measurements obtained from the UK Meteorological Research Flight C-130 Hercules aircraft, spatially coincident and near contemporaneous with ERS-1 Along Track Scanning Radiometer (ATSR) and NOAA-14 Advanced Very High Resolution Radiometer (AVHRR), are presented. These data have been used to obtain much needed on-going validation of the ERS-1 ATSR prior to the de-commissioning of the ATSR instrument in March 1997. BT comparisons between the ATSR and AVHRR nadir channels show negligible differences of 0.3 deg K, indicating that both radiometers are well calibrated. However, significant differences are found when common sea surface temperature (SST) algorithms are applied to the BT data. The original dual view ERS-1 ATSR skin SST (SSST) algorithm has a 0.6K cool bias relative to the in situ observations, which is consistent with other in situ validation studies. New SSST coefficients derived using the same atmospheric transmission model show that, when the appropriate pixel noise contribution is considered in the algorithm derivation, substantially improved SSST is derived from the ERS-1 ATSR. Comparing the NOAA-14 AVHRR non-linear SST (NLSST) and multi-channel SST (MCSST) algorithms to the aircraft data, the non-linear nature of the NLSST algorithm results in a small bias of 0.3 deg K compared to a substantial cool bias of 1 deg K in the MCSST case. This result highlights deficiencies in the MCSST.

Seasonal variation of sea surface temperature in the Bay of Bengal during 1992 as derived from NOAA-AVHRR SST data

Monthly maps of sea surface temperature (SST) derived from NOAA (National Oceanic and Atmospheric Administration)-AVHRR (Advanced Very High Resolution Radiometer) data during 1992 for the Bay of Bengal are analysed and compared with the available/compiled monthly seatruth (bucket thermometer) data of this region. It was noticed that the computed SST bias (AVHRR SST minus Seatruth SST), in general, varied between 2.0 and 2.5 C with smaller bias values (1.5 to 1.5 C) during January-June and December. Larger bias values were noticed in the south-eastern Bay in July and in the Andaman Sea in October. The large SST biases suggested the necessity for improvement of SST algorithms by properly removing the clouds. The spatial variation of Standard Deviation of SST bias was particularly high (0.7) in the western Bay when compared to other parts of the Bay of Bengal. The monthly maps of AVHRR SST clearly depicted the seasonal cycle of SST showing the well known bi-modal SST distribution of the study region with winter cooling, summer heating, monsoonal cooling and post-monsoon warming phases. The seasonal cycle of SST further revealed the persistence of Warm Pool (SST 28 C) in the Bay of Bengal from March through October.

Island-induced eddies in the Canary islands

Cyclonic and anticyclonic eddies were observed downstream of Gran Canaria (Canary Islands), an almost circular island of about 50 km diameter, located in the path of the Canary Current. Temperature data were obtained from five AXBT and one CTD surveys carried out during two years (April, May and December, 1989; February, May and June, 1990), and from NOAA AVHRR (Advanced Very High Resolution Radiometer) sea surface temperature (SST) satellite images. The presence of eddies in most of the surveys and in many SST images suggests that they are common mesoscale features in the flow past the island througout the year. In general, eddy diameter is similar to the width of the island, while the vertical extent is from the near-surface layers down to at least 400 m depth. However, vertical sections across the eddies show distinct patterns in their structures which could correspond to different stages of development. Wakes of relatively warm surface water develop in the lee of the island, interacting with the eddies and affecting their upper mixed layer structure. It is hypothesized that eddies are sequentially spun off from the island with a period ranging from several days to two weeks. If this is the case, they could contribute to the high levels of eddy kinetic energy observed recently downstream of the Canarian archipelago from moored current meters.

Sea surface temperature variability of the Iceland‐Faeroe front: Correction

In a previous study by Niiler et al. (1992), advanced very high resolution radiometer (AVHRR) images were used for locating the Iceland‐Faeroe front temperature distributions and gradients. In that study the satellite‐sensed sea surface temperatures were found to contain an error after an atmospheric correction algorithm was applied. This was determined through a regression fit of AVHRR data against in situ aircraft‐deployed expendable bathythermograph (AXBT) and conductivity‐temperature‐depth measurements. Recently, Minnett (1993) pointed out that an error occurred in that fit. As a result, we decided to repeat the comparisons of AVHRR sea surface temperature (SST) data against in situ measurements. After correcting several errors, we still find a shift of about 2°C in the scatter of AVHRR against AXBT SST data. The bias (mean difference) was 1.73°C, and the standard deviation was 0.3°C.

A correction for Saharan dust effects on satellite sea surface temperature measurements

Sea surface temperature (SST) retrievals obtained from NOAA advanced very high resolution radiometer (AVHRR) data can determine SST within 0.7°C on a global basis. However, retrievals have been severely limited in the presence of tropospheric aerosol particles because no adequate aerosol correction algorithm exists. High tropospheric aerosol concentrations in the atmosphere significantly increase infrared signal attenuation and prevent the retrieval of accurate satellite SSTs. This paper presents tropospheric aerosol correction algorithms derived from NOAA AVHRR SST and optical depth measurements and from drifting buoy in situ SSTs. We show that the correction algorithms perform better than the current global algorithm in an atmosphere that contains high Saharan dust concentrations and generate statistical accuracy comparable to satellite SST performance globally. These corrections would significantly increase the quantity and quality of satellite retrievals under such atmospheric conditions, and the observations would greatly benefit both global climate research and maritime operations conducted in such regions.

Global differences between skin and bulk sea surface temperatures

Infrared satellite imagery provides one of the best means of mapping changes in global sea surface temperature (SST). A series of workshops [Hilland et al., 1985], held to compare the various methods of remotely sensing SST, concluded that of the presently available systems, the Advanced Very High Resolution Radiometer (AVHRR), carried on National Oceanic and Atmospheric Administration (NOAA) polar‐orbiting weather satellites, yielded the smallest rms error when compared with ship and buoy in situ data. The widely used Multi Channel SST (MCSST) method, described by McClain et al. [1983], was used to calibrate the AVHRR SST for this workshop intercomparison. This method uses the differences between the radiances of the two AVHRR thermal infrared channels to correct for atmospheric signal attenuation due primarily to water vapor absorption. To provide absolute temperature calibration, the MCSST coefficients are derived by matching the AVHRR observations with simultaneous SSTs measured in situ by freely drifting ocean buoys [McClain et al., 1985]. These drifting buoys typically measure the ocean temperature 0.5–1 m below the sea surface.

Sea surface temperature measurement from satellites: Validation and accuracy

A review of the latest published results concerning the accuracy of satellite derived sea surface temperature (SST) estimation is presented. Two types of platforms are considered : orbiting satellites and geosynchronous satellites and the accuracies that may now be expected from such systems are reported. This review emphasizes the impressive improvement in global mapping of SST obtained from the Advanced Very High Resolution Radiometer (AVHRR) on NOAA's operational polar satellites. Tests of the AVHRR SST's against a high reliability data set consisting of buoys, bathythermographs and research ship reports indicate biases of < 0.1°C and RMS differences of < 0.75°C (McClain [1]). Particular attention is also paid to a method adding along track scanning capability to the present multichannel AVHRR technique. This method is demonstrated owing to the coupling of an orbiting satellite (TIROS-N) and a geosynchronous satellite (METEOSAT). Another type of coupling of two such platforms is also presented in connection with the advent of geostationary satellites equipped with a vertical sounding capability, such as GOES-4.