Offshore Sea Surface Temperature Research Articles

High-precision retrieval of offshore sea surface temperature: A machine learning framework based on MODIS and in-situ measurements

High-precision sea surface temperature (SST) data is essential for monitoring and management of offshore environment. SST retrievals from thermal infrared remote sensing have the advantages in time frequency and space coverage over measurements from ships and buoys. However, traditional SST retrieval models are not fully applicable for offshore waters where seawater conditions change a lot, due to simple functional forms and empirical fitting of model parameters. Machine learning (ML) is introduced because it doesn’t need to define specific model rules or unknown parameters, which is more intelligent to deal with complex retrieval problems in a data-driven way. But the applicability of ML has not been investigated in the North Yellow Sea of China, and the selection of driving features for model construction has not been fully discussed. Here, we designed a high-precision SST retrieval framework based on random forest (RF) with optimal feature combination using MODIS data and in-situ SST measurements during 2013–2020 in the northern part of the North Yellow Sea. The performances of different feature combinations were assessed based on model testing accuracies and the RF output feature importance. Compared with previous ML-based SST retrieval studies, we added two time-related features (i.e., day of the year and month) and one feature as initial temperature (i.e., SST retrieval from split-window algorithm), which reduced the testing error by around 15 % and 11 %, respectively. The retrieval results were validated with in-situ measurements, showing that the RF model achieved better accuracies (R2 = 0.987, SD = 0.842 °C, MAE = 0.675 °C, RMSE = 0.841 °C) than the improved split-window algorithm (named as ISW) and deep neural network. Compared with ISW, the spatial distribution of RF retrieval map exhibited similar variation across four seasons, mainly differed in the coastal areas or at the formation of temperature fronts. Note that the constructed RF model was still reliable when applied to the independent samples in 2021. This study contributes to improve the accuracy of offshore SST retrieval under different seawater conditions and provides references for SST monitoring.

Characterizing oceanographic conditions near Coiba Island and Pacific Panama using 20 years of satellite-based wind stress, SST and chlorophyll-a measurements

Coiba Island and the associated Special Zone of Marine Protection represent an important, yet poorly studied marine reserve along the Pacific coast of Panama. While efforts have recently began to establish monitoring programs in the region, a range of historical, marine-related environmental measurements already exist, derived from satellite-based observations. The goal of this paper was to use long-term datasets for key variables to provide qualitative insights (i.e. descriptive oceanography) of climatological conditions and interannual variability in the Pacific Panama region. These are underpinned with numerical assessments, providing an important baseline for ongoing and future studies, particularly in the Coiba Island/Gulf of Chiriqui region. In particular, we examined 20 years (January 2003-December 2022) of wind stress, sea surface temperature (SST), and chlorophyll-a (Chl-a), spanning the neritic and pelagic regions of the Pacific Panama coast. During the dry season (northern winter), the well-known, seasonal, regional Panama wind jet appeared across the Gulf of Panama, leading to surface mixing and SST cooling that eventually extended across most of the Panama Bight. West of the Azuero Peninsula, SST increased and surface warming extended further offshore from January through April. The SST in the Gulf of Chiriqui during this period was about 1 °C warmer on average than east of Coiba Island. By July and August, offshore SST gradients became largely longitudinal, cooling occured across the season, and the SST on either side of Coiba Island was nearly the same. The influence of the Panama jet in the Gulf of Panama was clear in the Chl-a data as well, with upwelling-driven values peaking in February/March (up to 11 mg m-3, with a monthly climatological value of around 2 mg m-3 during this period). During the rest of the year, the Chl-a concentration in this region averaged around 0.5-1.0 mg m-3. In the Gulf of Chiriqui and the region east of Coiba Island, the climatological monthly averages were roughly 0.3-0.5 mg m-3 and 0.4-0.6 mg m-3, respectively. Somewhat surprisingly, very high Chl-a values were present in the satellite data for the Gulf of Chiriqui during May 2007 and June 2008, peaking at 16 mg m-3 and 32 mg m-3 at a location just west of Coiba Island, respectively. It remains unclear as to the cause of these apparent blooms. Even when the high Chl-a values were excluded in the calculation of climatological averages in the Gulf of Chiriqui, however, there is a suggestion of modest seasonality in Chl-a values, with slightly elevated values (~ 0.4 mg m-3) peaking around May and October. During the extreme El Niño event of 2015-2016, the monthly-averaged SST along the Panama Pacific coast was warmer than average, with elevated levels of up to + 2 °C and lasting 12 months in the Gulf of Chiriqui. In the Gulf of Panama, the monthly-averaged SST anomalies were up to + 1.7 °C, although the temperatures returned to near-seasonal averages after roughly 5 months.

Environmental drivers of fish population dynamics in an estuarine ecosystem of south‐eastern Australia

AbstractEstuaries are dynamic environments and often support rich biodiversity and important fisheries. Linking environmental drivers or events to observed biological changes in these systems provides critical information that can facilitate their management. Historical fish and fisheries data for Western Port, a large embayment in south‐east Australia that supports rich biodiversity, were collated, including catch records, recruitment indices and two novel growth time series. Dynamic factor analysis (DFA) identified three common patterns associated with the biological time‐series data, which in turn were related to both local and regional environmental drivers. On a local scale, nitrogen loads and chlorophyll a concentration affected fish metrics, potentially through the food web and via changes to critical seagrass habitat. On a regional scale, offshore sea surface temperature in Bass Strait was important. Step changes (change points) in the three common patterns were predominantly associated with significant El Niño and La Niña events and, to a lesser extent, recruitment pulses and the cessation of commercial netting.

Subtropical Eastern North Pacific SST Bias in Earth System Models

AbstractThis study systematically evaluates the warm sea surface temperature (SST) bias in the Subtropical Eastern North Pacific, a problem plaguing most Coupled Model Intercomparison Project Phase 6 models, using the Energy Exascale Earth System Model version 1 (E3SM). In the model at its standard resolution (1° atmosphere, 30–60 km ocean), the SST bias, exceeding several degrees, is mainly concentrated along the coast between 25°N and 40°N. In the high‐resolution (0.25° atmosphere, 18–6 km ocean) version of the model, the nearshore SST bias improves considerably with a better representation of coastal upwelling. However, the offshore SST bias, approximately centered at 125°W and 25°N, is relatively stronger in the high‐resolution version. To better understand the offshore warm bias in the model, a mixed‐layer heat budget analysis is performed. While errors in surface radiative fluxes occur at both resolutions, positive biases in horizontal heat advection also play a role in the SST bias at high‐resolution. Analysis of HighResMIP models indicates that the shift in the location of the prominent SST bias from nearshore to offshore with an increase in model spatial resolution, is not native to E3SM alone.

Temperate Oligocene surface ocean conditions offshore of Cape Adare, Ross Sea, Antarctica

Abstract. Antarctic continental ice masses fluctuated considerably during the Oligocene “coolhouse”, at elevated atmospheric CO2 concentrations of ∼600–800 ppm. To assess the role of the ocean in the Oligocene ice sheet variability, reconstruction of past ocean conditions in the proximity of the Antarctic margin is needed. While relatively warm ocean conditions have been reconstructed for the Oligocene offshore of Wilkes Land, the geographical extent of that warmth is unknown. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from Deep Sea Drilling Project (DSDP) Site 274 offshore of the Ross Sea continental margin. This site, located offshore of Cape Adare is ideally situated to characterise Oligocene regional surface ocean conditions, as it is situated between the colder, higher-latitude Ross Sea continental shelf and the warm-temperate Wilkes Land margin in the Oligocene. We first improve the age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse organic walled dinoflagellate cyst assemblages and lipid biomarkers (TEX86, TetraEther indeX of 86 carbon atoms) to reconstruct surface palaeoceanographic conditions during the Oligocene (33.7–24.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10–17 ∘C ± 5.2 ∘C) surface ocean conditions at Site 274 throughout the Oligocene. Oceanographic conditions between the offshore Wilkes Land margin and Cape Adare became increasingly similar towards the late Oligocene (26.5–24.4 Ma); this is inferred to be the consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. Maintaining marine terminations of terrestrial ice sheets in a proto-Ross Sea with offshore SSTs that are as warm as those suggested by our data requires a strong ice flux fed by intensive precipitation in the Antarctic hinterland during colder orbital states but with extensive surface melt of terrestrial ice during warmer orbital states.

An Overview of Atmospheric Features Over the Western North Atlantic Ocean and North American East Coast—Part 2: Circulation, Boundary Layer, and Clouds

AbstractThe Western North Atlantic Ocean (WNAO) is a complex land‐ocean‐atmosphere system that experiences a broad range of atmospheric phenomena, which in turn drive unique aerosol transport pathways, cloud morphologies, and boundary layer variability. This work, Part 2 of a 2‐part paper series, provides an overview of the atmospheric circulation, boundary layer variability, three‐dimensional cloud structure, and precipitation over the WNAO; the companion paper (Part 1) focused on chemical characterization of aerosols, gases, and wet deposition. Seasonal changes in atmospheric circulation and sea surface temperature explain a clear transition in cloud morphologies from small shallow cumulus clouds, convective clouds, and tropical storms in summer, to stratus/stratocumulus and multilayer cloud systems associated with winter storms. Synoptic variability in cloud fields is estimated using satellite‐based weather states, and the role of postfrontal conditions (cold‐air outbreaks) in the development of stratiform clouds is further analyzed. Precipitation is persistent over the ocean, with a regional peak over the Gulf Stream path, where offshore sea surface temperature gradients are large and surface fluxes reach a regional peak. Satellite data show a clear annual cycle in cloud droplet number concentration with maxima (minima) along the coast in winter (summer), suggesting a marked annual cycle in aerosol‐cloud interactions. Compared with satellite cloud retrievals, four climate models qualitatively reproduce the annual cycle in cloud cover and liquid water path, but with large discrepancies across models, especially in the extratropics. The paper concludes with a summary of outstanding issues and recommendations for future work.

A Comparative Study on the Offshore Intensification of Supertyphoon Rammasun (2014) and Typhoon Rumbia (2013): the Role of Summer Monsoon

The offshore intensification mechanisms of Supertyphoon Rammasun (2014) and Typhoon Rumbia (2013) are analyzed and compared based on the best track typhoon data provided by the Shanghai Typhoon Institute of China Meteorological Administration (STI/CMA), ECMWF (CMA and JMA) objective (subjective) predicted typhoon track and intensity, and reanalysis data of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). Results show that the offshore sea surface temperature over 28 °C was conducive to the intensification of the two typhoons in the coastal area prior to their landfall in South China. Rammasun was embedded in an extremely active phase of a 10–20-day oscillation in the summer monsoon over the South China Sea and western North Pacific prior to its landfall. The low-level jet transported warm and moist air to the inner core of Rammasun and the deepening of the southwesterly monsoon flow up to 200 hPa reduced vertical wind shear, both favorable for the offshore rapid intensification of Rammasun. In 2013, the summer monsoon over the South China Sea and western North Pacific experienced quasi-20-day and quasi-10-day oscillations. The out of phase of the two oscillations resulted in relatively weak tropical and subtropical summer monsoon flow prior to the landfall of Rumbia. As a result, the low-level monsoon flow transported relatively less moist air into Rumbia, and the shallow monsoon flow was not conducive to the weakening of vertical wind shear, resulting in a less rapid offshore intensification of Rumbia than Rammasun.

Increased chlorophyll-a concentration in the South China Sea caused by occasional sea surface temperature fronts at peripheries of eddies

ABSTRACTThis study investigates the processes of occasional sea surface temperature (SST) fronts and their impacts on chlorophyll-a concentration (chl-a) in the South China Sea (SCS), based on satellite remote sensing and in situ observations in 2009–2013. The SST fronts were detected by an entropy-based edge detection algorithm method from satellite-derived SST images with a 0.011° grid size. Three offshore SST front case studies (S1, S2 and S3) at the peripheries of eddies in the northern SCS were studied. In case S1 in September 2013, two SST fronts were detected with gradient magnitudes (GMs) greater than 0.06°C km–1 in the cyclonic eddy and 0.08°C km–1 in the periphery waters, and the fronts only existed for one and two days, respectively. After three and seven days, the high chl-a was found in the strong SST front waters which were about 51 and 54% higher than the concentration in the surrounding waters. The depth of the maximum chl-a elevated from the subsurface (50 m) to the surface. In case S2 in August 2013, two SST fronts were detected at the periphery of an anti-cyclonic eddy with GM stronger than 0.06°C km–1 and only existed for one day. After two days, the chl-a in the SST front waters was about 40% higher than the levels in the surrounding waters. In case S3 in June 2009, the GM of the eddy-feature SST front was stronger than 0.12°C km–1 and existed for three days, which was generated by tropical cyclone Linfa. The chl-a in the eddy-feature phytoplankton bloom was 6 times higher than in the surrounding waters. The results show that, in general, occasional offshore SST fronts at peripheries of eddies have stronger influence on surface chl-a, comparing to those seasonal coastal and permanent offshore SST fronts, via ‘Wind Pump’ effects.

A numerical experiment on the influence of the interannual variation of sea surface temperature on terrestrial precipitation in northern Japan during the cold season

The role of sea surface temperature (SST) anomaly in modulating the terrestrial precipitation in winter around Japan was investigated using a regional atmospheric model. Large amount of snowfall occurs as the westerly winter monsoon carries abundant moisture from the Japan Sea. An experiment with realistic SST gives improved representation of terrestrial precipitation distribution compared to the reanalysis. The standard deviation of interannual variation of precipitation in the experiment was approximately 20% larger over the Kuroshio extension than that in the experiment with climatology SST, suggesting that the SST variability enhances the oceanic precipitation variability. For the role of Japan Sea, the terrestrial precipitation over the Japan Sea side (JSS) region in northern Japan was sensitive to the offshore SST anomaly through affecting moisture flux toward Japan. Since the offshore SST was clearly warmer in the 2000s relative to the 1980s, the effect of the long‐term SST variation on the terrestrial precipitation trend was examined. The experiment with realistic SST simulated the observed trend in terrestrial precipitation in the JSS region. In contrast, the precipitation trend was significantly reduced in the experiment with climatology SST. Therefore, the long‐term SST trend is an important factor for the precipitation trend in the region of Japan and the adjacent oceans where SST has significant trends. Precipitation in southern Japan facing the Pacific Ocean indicated increasing trend even without the SST trend. This suggests that the long‐term variations in extratropical cyclones are also an important factor for precipitation trends around the Kuroshio extension.

Processes of India’s offshore summer intraseasonal sea surface temperature variability

Active and break phases of the Indian summer monsoon are associated with sea surface temperature (SST) fluctuations at 30–90 days timescale in the Arabian Sea and Bay of Bengal. Mechanisms responsible for basin-scale intraseasonal SST variations have previously been discussed, but the maxima of SST variability are actually located in three specific offshore regions: the South-Eastern Arabian Sea (SEAS), the Southern Tip of India (STI) and the North-Western Bay of Bengal (NWBoB). In the present study, we use an eddy-permitting 0.25° regional ocean model to investigate mechanisms of this offshore intraseasonal SST variability. Modelled climatological mixed layer and upper thermocline depth are in very good agreement with estimates from three repeated expendable bathythermograph transects perpendicular to the Indian Coast. The model intraseasonal forcing and SST variability agree well with observed estimates, although modelled intraseasonal offshore SST amplitude is undere-stimated by 20–30 %. Our analysis reveals that surface heat flux variations drive a large part of the intraseasonal SST variations along the Indian coastline while oceanic processes have contrasted contributions depending of the region considered. In the SEAS, this contribution is very small because intraseasonal wind variations are essentially cross-shore, and thus not associated with significant upwelling intraseasonal fluctuations. In the STI, vertical advection associated with Ekman pumping contributes to ∼30 % of the SST fluctuations. In the NWBoB, vertical mixing diminishes the SST variations driven by the atmospheric heat flux perturbations by 40 %. Simple slab ocean model integrations show that the amplitude of these intraseasonal SST signals is not very sensitive to the heat flux dataset used, but more sensitive to mixed layer depth.

Links between Patagonian Ice Sheet fluctuations and Antarctic dust variability during the last glacial period (MIS 4-2)

Antarctic and Greenland ice-core records reveal large fluctuations of dust input on both orbital and millennial time-scales with potential global climate implications. At least during glacial periods, the Antarctic dust fluctuations appear to be largely controlled by environmental changes in southern South America. We compare dust flux records from two Antarctic ice-cores to variations in the composition of the terrigenous supply at ODP Site 1233 located off southern Chile and known to record fluctuations in the extent of the northern part of the Patagonian ice-sheet (NPIS) during the last glacial period (Marine Isotope Stage, MIS, 4 to 2). Within age uncertainties, millennial-scale glacial advances (retreats) of the NPIS correlate to Antarctic dust maxima (minima). In turn, NPIS fluctuations were closely related to offshore sea surface temperature (SST) changes. This pattern suggests a causal link involving changes in temperature, in rock flour availability, in latitudinal extensions of the westerly winds and in foehn winds in the southern Pampas and Patagonia. We further suggest that the long-term trend of dust accumulation is partly linked to the sea-level related changes in the size if the Patagonian source area due to the particular morphology of the Argentine shelf. We suggest that sea-level drops at the beginning of MIS 4 and MIS 2 were important for long-term dust increases, while changes in the Patagonian dust source regions primarily control the early dust decrease during the MIS 4/3 transition and Termination 1.

The relative roles of coastal and oceanic processes in determining physical and chemical characteristics of an intensively sampled nearshore system

We compared the extent to which offshore and remote‐sensing measurements of sea surface temperature (SST), upwelling, and chlorophyll a (Chl a) were concordant with in situ measurements of temperature, Chl a, and water nutrients at Tatoosh Island, Washington for the past 8 yr. Offshore SSTs were significantly correlated with water temperatures at Tatoosh, though consistently 2°C to 3°C warmer. Sea‐viewing wide field‐of‐view sensor Chl a estimates were poor predictors of Chl a at Tatoosh Island measured with an anchored fluorometer. Nitrate and phosphorus estimates at Tatoosh Island were positively correlated with an upwelling index and negatively correlated with SST, as would be expected from an upwelling source. In contrast, ammonium and nitrite were uncorrelated with the upwelling index or SST and showed elevated levels immediately adjacent to Tatoosh Island, suggesting strong local effects of marine invertebrates, birds, and mammals on nutrient dynamics and cycling in coastal ecosystems.

1993年夏季のヤマセ気流の気団変質

Yamase Air-Flow (YAF) is an outflow of the summertime maritime polar airmass over the North Pacific toward the San-Riku district of Japan (SRJ) and forms along the southern or southeastern edge of an anticyclone developed over the Okhotsk Sea (Okhotsk Sea high). In most summers, the Okhotsk Sea high appears intermittently, but the high and associated YAF persisted for an abnormally long period between mid July and mid August in 1993. Due to the continuous YAF, an abnormal low temperature accompanied with synoptic-scale (several or ten days) variation was observed in the SRJ during this period. We studied mechanisms of the AirMass Transformation (AMT) of the YAF in 1993 over the western North Pacific for both relatively warm and cool spells of the variation. The temperature of the YAF at the coast of the SRJ varied over a narrow range around the offshore Sea Surface Temperature (SST). The temperature sometimes exceeded the SST plus 3C only for a short period in the warm spells but never fell below the SST minus 3C in the cool ones. Namely, the offshore SST controlled the lower limit of the YAF's temperature. The temperature variation was related to trajectories of air-parcels in the YAF over the western North Pacific. The YAF was directed southwestward toward the SRJ from Kuril Islands during the cool spells, while directed southwestward over the North Pacific and turned northwestward off the SRJ during the warm spells. Since SST gradient is large off the SRJ, the meridional direction of the YAF largely affected the AMT of the YAF. Trajectories of the YAF were related to the southeastward extension of the Okhotsk Sea high into the North Pacific, which was stronger in the warm spells. During the cool spells, an atmospheric mixed layer accompanied with low-level clouds appeared in the YAF at the coast of the SRJ. The ocean supplied sensible heat of -30Wm-2 and latent heat of -80Wm-2 to the YAF on its way from the neighborhood of Kuril Islands to the SRJ. This heating compensated strong radiative cooling at the top of the low-level clouds (-70Wm-2) and kept the temperature of the YAF higher than the offshore SST minus 3C. During the warm spells, a stable layer attached to the sea surface accompanied with low-level clouds appeared in the YAF. Off the SRJ, the temperature of the YAF exceeded the offshore SST by several degrees and the YAF was cooled (-10 ti -20Wm-2) and lost its moisture (0-20Wm-2) at its bottom. The low-level clouds also contributed to the cooling of the YAF through the radiative process (N -70Wm-2). The cooling of the YAF on its way to the SRJ, however, sometimes may not have been enough to adjust its temperature to the SST when northwestward advection of the YAF off the SRJ was rapid or the radiative cooling was weakened by upper-level clouds. Even during the cool spells, the AMT of the YAF was much weaker than that of the Asian winter monsoon breaking out over the Kuroshio area. In the AMT of the winter monsoon, radiative cooling at the top of the low-level clouds can be neglected in comparison with the dominant heat supply from the ocean. In the AMT of the YAF, on the other hand, the radiative cooling was comparable to the oceanic heating or cooling.

Sea surface temperature variations at coastal sites around New Zealand

Abstract Sea surface temperature (SST) observations from 16 coastal sites around New Zealand are analysed along with some corresponding air temperatures. Day‐to‐day variations in SST show weak periodicity over an 8–16 day range. Air temperatures are generally cooler and short‐term fluctuations have 3–4 times the standard deviation of the SSTs. Seasonal SST variations are described and coastal SSTs are compared with offshore SST data. At Farewell Spit the coastal SSTs were always cooler than offshore SSTs which supports the view that upwelling is persistent in the Cape Farewell region. Inter‐annual variations in SST are found to be correlated with Southern Oscillation atmospheric pressure anomalies. In particular we find that the El Nino phenomenon which is accompanied by warm SSTs in the central and eastern tropical Pacific is also accompanied by lowered SSTs throughout New Zealand's coastal waters.