Strong Sea Surface Temperature Research Articles

Role of Japan Sea Throughflow in the spatial variability of the long-term sea surface temperature trend

AbstractThe Japan Sea shows a much stronger warming of long-term sea surface temperature (SST) than surrounding oceans. The warming trend possesses a meridionally alternating zonal band pattern, with weak trends along the paths of the Japan Sea Throughflow and strong trends in the remaining interior region. Using idealized models of the Japan Sea Throughflow and atmospheric heating, this study examines the process behind the formation of such spatial patterns in the SST trend. We find that zonal band structures form in a flat rectangular coastline model, and heat budget analysis shows that horizontal heat transport, due to throughflow, reduces the warming effect created by the surface heat flux. A weak SST trend appears around the jet, while a strong SST trend appears elsewhere. Bathymetric effects are also examined using a model with realistic coastline settings. The location of the western boundary current stabilizes, and the coastal branch begins to disconnect from the Japanese coastline toward the north, allowing a more stable SST warming region to form in the southern interior region. Lagrangian particle tracking experiments confirm that a weak (strong) SST trend corresponds to a short (long) residence time, and eddies in the Japan Sea prolong the residence time in interior regions. The model results suggest that the accumulation time of surface heating is essential to the spatial distribution of the long-term SST warming trend.

Interdecadal variations in the interannual relationship between winter tropical Pacific SST and subsequent summer Arctic sea ice in early 2000s

AbstractThe present study investigates the interdecadal change in the interannual relationship between winter sea surface temperature (SST) in the tropical central‐eastern Pacific and the subsequent summer Arctic sea ice concentration (SIC) and its possible mechanism using 1979–2023 monthly SST and SIC data from Hadley Center and atmospheric reanalysis data from NCEP/NCAR. It is found that the relationship between the winter tropical Pacific SST and the subsequent summer SIC of Beaufort Sea experienced an obvious interdecadal change from a significant negative correlation to a weak and insignificant correlation before and after the early 2000s, respectively. Before the early 2000s, when winter SST warming (cooling) anomalies occurred in the central and eastern tropical Pacific, it could sustain into summer and enhance a Rossby wave propagating from Tropic to high latitude, forming negative (positive) pressure anomalies in the North Pacific. Influenced by such anomalies, it is conducive to maintaining negative (positive) SST anomalies in the North Pacific from winter to subsequent summer. The summer SST cooling (warming) in the North Pacific could form a favourable atmospheric circulation anomaly for less (more) SIC. In contrast, after the early 21st century, the strong SST anomalies in winter over the eastern tropical Pacific would weaken with time and subsequently diminish in spring, leading to the SST anomalies in the North Pacific persist to spring, which can hardly affect summer SIC anomaly in the Beaufort Sea. Therefore, the relationship collapses after the the early 2000s due to the weakened persistence of the eastern tropical Pacific SST anomaly.

Diurnal warming rectification in the tropical Pacific linked to sea surface temperature front

Sharp and rapid changes in the sea surface temperature (SST) associated with fronts and the diurnal cycle can drive changes in the atmospheric boundary-layer stability and circulation. Here we show how a one-dimensional surface ocean model forced with either high-resolution or daily averaged surface fluxes can be used to distinguish diurnal versus frontal SST anomalies observed from an uncrewed surface vehicle. The model, forced with daily satellite fluxes, shows that the diurnal warming is largest within the equatorial Pacific cold tongue of SST. The strong persistent SST front north of the cold tongue is evident in both the oceanic and atmospheric boundary-layer stability scales and, as a consequence, in the magnitude of the diurnal ocean warming. Using SST, barometric pressure and surface wind measurements from moorings at 0°, 95° W and 2° N, 95° W, we show that the front in the SST diurnal warming results in a weakened SST front in the afternoon and a corresponding reduced meridional gradient in the barometric pressure that appears to contribute to a diurnal pulsing of the surface meridional winds. To the extent that these modulate the surface branch of the Hadley cell, these diurnal variations may have remote impacts.

The Gulf Stream Front Amplifies Large-Scale SST Feedback to the Atmosphere in North Atlantic Winter

The Gulf Stream (GS) ocean front releases intense moisture and heat to the atmosphere and regulates storm tracks and zonal jets in winter. The large-scale sea surface temperature (SST) anomaly in the central North Atlantic provides important feedback to the atmosphere in winter, but the role played in this feedback by the GS front inside the SST anomaly has not been extensively studied. In this study, two sets of ensemble experiments were conducted using a global community atmosphere model forced by SST in boreal winters from 2000 to 2013. The regional averaged SST and its variation in the experiments were identical, with the only difference being the strength of the SST front in the GS region. The large-scale SST anomaly in the central North Atlantic in our model provides feedback to the atmosphere and excites a wave train that extends across Eurasia. With the inclusion of the strong GS front, the first center of the wave train in the North Atlantic is strengthened by approximately 40%, and the wave activity flux toward downstream is highly intensified. When the large-scale SST anomaly is combined with a strong GS front, greatly increased water vapor is released from the GS region, resulting in a 50% increase in moisture transport toward Western Europe. In this scenario, precipitation and diabatic heating both increase greatly on the western Scandinavian Peninsula. With the release of deep diabatic heating, a strong upward wave activity flux is triggered, and the wave train excited by the large-scale SST variation is significantly intensified. These findings suggest that the strong SST front in the large-scale SST anomaly in the central North Atlantic significantly amplifies its feedback to the atmosphere in winter.

Diverse impacts of strong and moderate intense El Niño–Southern Oscillation events on spring precipitation over Asia

AbstractEl Niño–Southern Oscillation (ENSO) events are a major predictor of Asian spring precipitation. However, it is unclear whether ENSO events with different intensities have similar or different impacts on Asian spring precipitation. This question is explored in this study. According to their intensity, ENSO events are divided into strong and moderate (S‐ENSO and M‐ENSO) events. The analysis indicates that S‐ENSO events have stronger sea surface temperature (SST) anomalies over the tropical Pacific and Indian oceans, and warm/cold events can lead to significantly more/less precipitation over Central Asia and East China as well as less/more precipitation over the Indo‐China Peninsula, Tibetan Plateau, and Maritime Continent. The S‐ENSO events significantly affect the Walker circulation and West Pacific anticyclone and excite an eastward‐propagating wave train over the midlatitude Northern Hemisphere. The changes in these atmospheric circulations further alter the dynamic and moisture conditions over the aforementioned Asian regions, consequently leading to precipitation anomalies. In contrast, the M‐ENSO events have relatively weak SST anomalies over the tropics, and the warm/cold events can result in more/less precipitation over Central Asia, Mongolia and northern China and less/more precipitation over northeastern Siberia, South China and the Maritime Continent. The impact of M‐ENSO events on spring precipitation over the Asian regions mainly occurs due to modulating effects on the western Pacific pattern, Aleutian Low, and anomalous anticyclones over northwestern Asia. The results in this study indicate that ENSO events of different intensities may have different impacts on Asian spring precipitation, which should be considered when predicting it.

Role of North Atlantic Tripole SST in Mid‐Winter Reversal of NAO

AbstractNorth Atlantic Oscillation (NAO) has a significant impact on surrounding winter weather and climate. However, the causes of its occasional reversal between early and late winter remain unclear. This study proposes a mechanism for the mid‐winter reversal of NAO from the perspective of local midlatitude air‐sea interaction. Strong sea surface temperature (SST) tripole events, which are defined by empirical orthogonal function of winter‐mean interannual North Atlantic SST anomalies, are primarily induced by NAO in early winter and peak in January. In late winter, the persistent SST tripole exerts active feedback on atmosphere through diabatic heat and transient eddy forcing. The resulting atmospheric circulation anomalies exhibit an almost reversed NAO pattern in February, which forms a wavetrain originating above the Gulf Stream and propagating to the Middle East and weakens the former SST tripole. Consequently, significant reversals of air temperature anomalies occur in Europe and the Caspian Sea area between February and December.

Sea level pressure response to the specification of eddy-resolving sea surface temperature in simulations of Australian east coast lows

Four east coast lows (ECLs) were simulated with the Weather Research and Forecast model to investigate the influence of the sea surface temperature (SST) distribution on the sea level pressure (SLP). Each ECL was simulated with two different SST datasets: the Bluelink SST field and NCEP skin temperature field. The former resolved eddies in the East Australian Current while the latter did not. The simulated SLP fields in the eddy-resolving SST runs were compared with those in the non-eddy-resolving SST runs. On time-scales of about 48 hours, higher SSTs were asso-ciated with lower SLPs. The spatial scale of the SLP response was similar to that of the ocean eddies, indicative of the rapidity and robustness of the response given the rapidly evolving conditions within the storms. On shorter time-scales, the SLP response to SST change can become substantially larger. The largest reductions in SLP in the eddy-resolving SST runs were associated with regions of deep atmospheric convection that warm the tropospheric column. These areas were shown to be related to the SST distribution with the greatest SLP reductions associated with convection over strong SST gradient regions. The landfall of a damaging convective mesoscale low pressure system on 8 June 2007 was also investigated. It was found that a region of strong SST gradients on the southern flank of a large warm ocean eddy was associated with lower pressures at the time of formation of this meso-low. In addition, the only case that simulated the low pressure at the correct time (albeit at not quite the correct location) was the eddy-resolved SST run. It was hy-pothesized that the development of this meso-low that impacted the coast around Newcastle, was enhanced because of the eddy-scale SST distribution at the time.

Seasonal and El Niño–Southern Oscillation-related ocean variability in the Panama Bight

Abstract. In the Panama Bight, two different seasonal surface circulation patterns coincide with a strong mean sea level variation, as observed from 27 years of absolute dynamic topography (ADT) and the use of self-organizing maps. From January to April, a cyclonic gyre with a strong southwestward Panama Jet Surface Current (PJSC) dominates the basin circulation, forced by the Panama surface wind jet that also produces upwelling, reducing sea surface temperature (SST), increasing sea surface salinity (SSS) and causing an ADT decrease. From June to December, the Choco surface wind jet enhances SST, precipitation and river runoff, which reduces SSS, causing an ADT rise, which in turn forces a weak circulation in the bight, vanishing the PJSC. Interannual variability in the region is strongly affected by El Niño–Southern Oscillation (ENSO); however this climatic variability does not modify the seasonal circulation patterns in the Panama Bight. In contrast, the positive (negative) ENSO phase increases (decreases) SST and ADT in the Panama Bight, with a mean annual difference of 0.9 ∘C and 9.6 cm, respectively, between the two conditions, while its effect on SSS is small. However, as the strong seasonal SST, SSS and ADT ranges are up to 2.2 ∘C, 2.59 g kg−1 and 28.3 cm, the seasonal signal dominates over interannual variations in the bight.

ENSO vs glacial-interglacial-induced changes in the Kuroshio-Oyashio transition zone during the Pleistocene

The subarctic front (SAF) in the pelagic North Pacific is the northernmost front of the Kuroshio-Oyashio transition zone separating the subpolar and subtropical gyres and is marked by a strong sea surface temperature gradient. A complex interplay of e.g. variations of currents, the wind system and other forcing mechanisms causes shifts of the SAF’s position on timescales from orbital to interannual. In this study, we present proxy data from the Emperor Seamount chain, which reveal a link between long-term ENSO (El Niño/Southern Oscillation) dynamics in the tropics and shifts of the SAF. Based on sediment core SO264-45-2 from Jimmu Seamount (46°33.792’N, 169°36.072’E) located close to the modern position of the SAF, we reconstruct changes in (sub)surface temperature ((sub)SSTMg/Ca) and δ18Osw-ivc (approximating salinities) via combined Mg/Ca and δ18O analyses of the shallow-dwelling foraminifera Globigerina bulloides and the near-thermocline-dwelling Neogloboquadrina pachyderma, biological productivity (XRF-based Ba/Ti ratios), and terrigenous input via dust (XRF-based Fe). From ~600 to ~280 ka BP we observe significantly higher SSTMg/Ca than after an abrupt change at 280 ka BP. We assume that during this time warmer water from the Kuroshio-Oyashio transition zone reached the core site, reflecting a shift of the SAF from a position at or even north of our study site prior to 280 ka BP to a position south of our study site after 280 ka BP. We propose that such a northward displacement of the SAF between 600-280 ka BP was induced by sustained La Niña-like conditions, which led to increased transport of tropical ocean heat into the Kuroshio-Oyashio transition zone via the Kuroshio Current. After ~280 ka BP, the change to more El Niño-like conditions led to less heat transfer via the Kuroshio Current with the SAF remaining south of the core location. In contrast, our productivity record shows a clear glacial-interglacial pattern that is common in the North Pacific. We assume that this pattern is connected to changes in nutrient supply or utilization, which are not primarily driven by changes of the Kuroshio and Oyashio Currents or the SAF.

Significant Inverse Influence of Tropical Indian Ocean SST on SIF of Indian Vegetation during the Summer Monsoon Onset Phase

Sea surface temperature (SST) substantially influences the land climate conditions through the co-variability of multiple climate variables, which in turn affect the structural and functional characteristics of terrestrial vegetation. Our study explored the varying responses of vegetation photosynthesis in India to the SST variations in the tropical Indian Ocean during the summer monsoon. To characterise the terrestrial photosynthetic activity, we used solar-induced chlorophyll fluorescence (SIF). Our results demonstrated a significant negative SST-SIF relationship during the onset phase of the summer monsoon: the SIF anomalies in the northern and central Indian regions decrease when strong warm SST anomalies persist in the tropical Indian Ocean. Further, SIF anomalies increase with cold anomalies of SST. However, the negative SST anomalies in the tropical Indian Ocean are less impactful on SIF anomalies relative to the positive SST anomalies. The observed statistically significant SST–SIF link is feasible through atmospheric teleconnections. During monsoon onset, positive SST anomalies in the tropical Indian Ocean favour weakened monsoon flow, decreasing moisture transport from the ocean to the Indian mainland. The resultant water deficiency, along with the high air temperature, created a stress condition and reduced the photosynthetic rate, thus demonstrating negative SIF anomalies across India. Conversely, negative SST anomalies strengthened monsoon winds in the onset period and increased moisture availability across India. Negative air temperature anomalies also dampen water stress conditions and increased photosynthetic activity, resulting in positive SIF anomalies. The identified SST-SIF relationship would be beneficial to generate a simple framework that aids in the detection of the probable impact on vegetation growth across India associated with the rapidly varying climate conditions in the Indian Ocean.

On the importance of moisture conveyor belts from the tropical eastern Pacific for wetter conditions in the Atacama Desert during the mid-Pliocene

Abstract. Geomorphic and sedimentologic data indicate that the climate of today's hyper-arid Atacama Desert (northern Chile) was more humid during the mid-Pliocene to Late Pliocene. The processes, however, leading to increased rainfall in this period are largely unknown. To uncover these processes we use both global and regional kilometre-scale model experiments for the mid-Pliocene (3.2 Ma). We found that the PMIP4–CMIP6 (Paleoclimate Modelling Intercomparison Project–Coupled Model Intercomparison Project) model CESM2 (Community Earth System Model 2) and the regional model WRF (Weather Research and Forecasting) used in our study simulate more rainfall in the Atacama Desert for the mid-Pliocene in accordance with proxy data, mainly due to stronger extreme rainfall events in winter. Case studies reveal that these extreme winter rainfall events during the mid-Pliocene are associated with strong moisture conveyor belts (MCBs) originating in the tropical eastern Pacific. For present-day conditions, in contrast, our simulations suggest that the moisture fluxes rather arise from the subtropical Pacific region and are much weaker. A clustering approach reveals systematic differences between the moisture fluxes in the present-day and mid-Pliocene climates, both in strength and origins. The two mid-Pliocene clusters representing tropical MCBs and occurring less than 1 d annually on average produce more rainfall in the hyper-arid core of the Atacama Desert south of 20∘ S than what is simulated for the entire present-day period. We thus conclude that MCBs are mainly responsible for enhanced rainfall during the mid-Pliocene. There is also a strong sea-surface temperature (SST) increase in the tropical eastern Pacific and along the Atacama coast for the mid-Pliocene. It suggests that a warmer ocean in combination with stronger mid-tropospheric troughs is beneficial for the development of MCBs leading to more extreme rainfall in a +3 ∘C warmer world like in the mid-Pliocene.

Impacts of Sea Surface Temperature Gradient on Monsoon Intraseasonal Oscillation

Abstract The northward-propagating monsoon intraseasonal oscillation (MISO) is the most pronounced variability over the tropical Indian Ocean during the Indian summer monsoon (June–September). MISO is accompanied by significant air–sea interactions; however, the mechanism of the oceanic feedback to MISO is still a great scientific challenge. In this study, the role of the intraseasonal sea surface temperature (SST) gradient in MISO is diagnosed using reanalysis products and model sensitivity experiments. It is found that the positive meridional gradient of intraseasonal SST induces positive wind convergence in the planetary boundary layer (PBL) and leads convection by about 1–2 days. This accounts for approximately half of the total wind convergence in the PBL to the north of convection during MISO. The warm SST anomalies before convection accelerate the intraseasonal northerly wind in the PBL due to the enhanced downward transport of momentum from aloft. By contrast, the cold SST anomalies behind deep convection weaken the downward vertical momentum transport, thereby inducing a deceleration in the intraseasonal northerly. Consequently, changes in the speed of intraseasonal northerly along the meridional direction strengthen the PBL wind convergence ahead of deep convection. It finally results in the intensification of intraseasonal rainfall associated with MISO over the summer monsoon region. In addition, a strong (weak) SST meridional gradient at an intraseasonal time scale amplifies (lessens) the MISO intensity in the model simulation. Thus, this study highlights the role of SST meridional gradient in the feedback to MISO, which differs from the weak contribution of the warm SST itself to the wind convergence mentioned in previous studies. Collectively, these findings indicate that consideration of oceanic feedback is necessary to improve the understanding and simulation of MISO.

Statistical Characteristics of the Multiscale SST Fractal Structure over the Kuroshio Extension Region Using VIIRS Data

The ocean behaves as a typical multiscale fractal structure, whose dynamic and thermal variabilities extend over a wide range of spatial scales, r, spanning from 10−3 to 107 m. Studying the statistical characteristics of multiscale fractal structures is crucial to understanding the interactions and energy cascade processes between different spatial scales. Remote sensing data are one of the best choices for revealing these statistical characteristics. This work analyzes the multiscale (1–1000 km) fractal structures of sea surface temperature (SST) from the Level-2+ (L2P) satellite orbit Visible Infrared Imaging Radiometer Suite (VIIRS) products over the Kuroshio Extension (KE) region (145°E–160°W, 20°N–50°N), using a conventional method (second-order structure function, D(r)) and a newly developed statistical method (spatial variance, V(r)). The results show that both the power-law distribution slopes of D(r) and V(r) are close to 2/3, which is equivalent to the −5/3 wavenumber spectrum. V(r) is found to be more robust when depicting the fractal structure and variance density, V’(r), compared with D(r). V’(r) is slightly larger at the mesoscale (50–150 km) than at the large scale (higher than 150 km) and is much smaller than that at the submesoscale (smaller than 50 km). Additionally, V’(r) has an indiscernible diurnal variation but remarkable seasonal and latitudinal variations. For the seasonal variability, the maximum V’(r) appears in winter at the large scale and mesoscale, and gradually shifts towards spring at the submesoscale, which implies that a forward energy cascade process may occur during this period. The maximum of the latitude-dependent V’(r) appears around 40°N for all the scales. It is consistent with the latitude of the strongest background SST gradient, indicating that the background SST front is the main source of the strong SST multiscale spatial variabilities over the KE region. This work benefits the application of other high-resolution remote sensing data in this research field, including the forthcoming Surface Water Ocean Topography (SWOT) satellite product.

On the Rapid Weakening of Typhoon Trami (2018): Strong Sea Surface Temperature Cooling Associated with Slow Translation Speed

Abstract This work investigates the rapid weakening (RW) processes of Typhoon Trami (2018) by examining sea surface temperature (SST) cooling based on air–sea coupled simulations during typhoon passage. The cold wake and Trami’s RW occurred as the storm was moving at a very slow translation speed. A marked structural change of Trami is found in a three-dimensional ocean-coupled model experiment during the RW stage, in which the convective clouds and convective bursts in the inner core of the simulated TC dramatically decrease, resulting in the loss of diabatic heating and leading to weakening of the TC. In the simulation, the enthalpy flux dramatically decreases in the inner core because of the SST cooling during the RW period, while a stable boundary layer (SBL) is formed in the TC’s inner-core region. The expanding SBL coverage stabilizes the atmosphere and suppresses convection in the inner core, leading to weakening of the storm. A more stable atmosphere in the cold wake is also identified by the inner-core dropsonde data from the field program of Tropical Cyclones-Pacific Asian Research Campaign for Improvement of Intensity Estimations/Forecasts. The strong SST cooling also changes the evolution of Trami’s eyewall replacement cycle (ERC) and limits the eyewall contraction after the ERC.

Distribution and transport of Fish larvae at the entrance of the Gulf of California (September, 2016)

Fish larvae distribution and transport of dominant species were analyzed at the entrance to the Gulf of California in September 2016. Using the Sea Surface Temperature (SST) gradient, a frontal system was defined at the entrance to the Gulf. The most relevant gradient (∼0.06 °C km−1) in the region was transverse to the gulf axis (W-E), from south of Cabo San Lucas to the mainland, south of the Pescadero Basin. This SST gradient was associated with an anticyclonic flow (∼0.4 m s−1) that was connected with a cyclonic eddy outside the Gulf. Fish larvae of mesopelagic species such as Vincigueria lucetia and Benthosema panamense were widely distributed and increasing in abundance from north to south, while the mesopelagic Bregmaceros Bathymaster and the pelagic Auxis spp. showed an inverse abundance gradient. Most of these larvae were absent from the zone of strong SST gradient. A Lagrangian particle tracking model applied at the sampling stations where the fish larvae were collected, showed that during the first 5 days of trajectory, the transport was weak. But for the next 5 days, most of the particles were trapped by mesoscale flows that dominated the region. An unexpected result was that no particles crossed the transverse front, coinciding with the absence or low abundance of larvae there. The particles near to this front were transported west out the Gulf. We concluded that when the mesoscale flows are extended along the transverse axis of the Gulf entrance, these might trap inert particles and zooplankton organisms with little mobility, such as fish larvae in early phases of development, preventing their transport along the Gulf parallel axis (NW-SE), and therefore, affecting the species distribution.

A Monthly Index for the Large‐Scale Sea Surface Temperature Gradient Across the Separated Gulf Stream

AbstractThe strong sea‐surface temperature (SST) gradient associated with the Gulf Stream (GS) is widely acknowledged to play an important role in shaping mid‐latitude weather and climate. Despite this, an index for the GS SST gradient has not yet been standardized in the literature. This paper introduces a monthly index for the large‐scale SST gradient across the separated GS based on the time‐varying GS position detected from sea‐surface height. Analysis suggests that the variations in the monthly average SST gradient throughout the year result primarily from SST variability to the north of the GS, with little contribution from SST to the south. The index exhibits a weak periodicity at ∼2 years. Sea level pressure and turbulent heat flux patterns suggest that variability in the large‐scale SST gradient is related to atmospheric (rather than oceanic) forcing. Ocean‐to‐atmosphere feedback does not persist throughout the year, but there is some evidence of wintertime feedback.

Variability and Predictability of Basinwide and Sub-Basin Tropical Cyclone Genesis Frequency in the Northwest Pacific

Abstract The variability and predictability of tropical cyclone genesis frequency (TCGF) during 1973–2010 at both basinwide and sub-basin scales in the northwest Pacific are investigated using a 100-member ensemble of 60-km-resolution atmospheric simulations that are forced with observed sea surface temperatures (SSTs). The sub-basin regions include the South China Sea (SCS) and the four quadrants of the open ocean. The ensemble-mean results well reproduce the observed interannual-to-decadal variability of TCGF in the southeast (SE), northeast (NE), and northwest (NW) quadrants, but show limited skill in the SCS and the southwest (SW) quadrant. The skill in the SE and NE quadrants is responsible for the model’s ability to replicate the observed variability in basinwide TCGF. Above-normal TCGF is tied to enhanced relative SST (i.e., local SST minus tropical-mean SST) either locally or to the southeast of the corresponding regions in both the observations and ensemble mean for the SE, NE, and NW quadrants, but only in the ensemble mean for the SCS and the SW quadrant. These results demonstrate the strong SST control of TCGF in the SE, NE, and NW quadrants; both empirical and theoretical analyses suggest that ensembles of ∼10, 20, 35, and 15 members can capture the SST-forced TCGF variability in these three sub-basin regions and the entire basin, respectively. In the SW quadrant and the SCS, TCGF contains excessive noise, particularly in the observations, and thus shows low predictability. The variability and predictability of the large-scale atmospheric environment and synoptic-scale disturbances and their contributions to those of TCGF are also discussed.

Integrating a UAV-Derived DEM in Object-Based Image Analysis Increases Habitat Classification Accuracy on Coral Reefs

Very shallow coral reefs (<5 m deep) are naturally exposed to strong sea surface temperature variations, UV radiation and other stressors exacerbated by climate change, raising great concern over their future. As such, accurate and ecologically informative coral reef maps are fundamental for their management and conservation. Since traditional mapping and monitoring methods fall short in very shallow habitats, shallow reefs are increasingly mapped with Unmanned Aerial Vehicles (UAVs). UAV imagery is commonly processed with Structure-from-Motion (SfM) to create orthomosaics and Digital Elevation Models (DEMs) spanning several hundred metres. Techniques to convert these SfM products into ecologically relevant habitat maps are still relatively underdeveloped. Here, we demonstrate that incorporating geomorphometric variables (derived from the DEM) in addition to spectral information (derived from the orthomosaic) can greatly enhance the accuracy of automatic habitat classification. Therefore, we mapped three very shallow reef areas off KAUST on the Saudi Arabian Red Sea coast with an RTK-ready UAV. Imagery was processed with SfM and classified through object-based image analysis (OBIA). Within our OBIA workflow, we observed overall accuracy increases of up to 11% when training a Random Forest classifier on both spectral and geomorphometric variables as opposed to traditional methods that only use spectral information. Our work highlights the potential of incorporating a UAV’s DEM in OBIA for benthic habitat mapping, a promising but still scarcely exploited asset.

Grain size controls on long-chain diol distributions and proxy signals in marine sediments

Long chain alkyl diols (LCDs) are lipid biomarkers that occur ubiquitously in sediments. Their abundance and distributions are increasingly used as the basis of molecular proxies for environmental parameters such as sea surface temperature (SST) via the Long chain Diol Index (LDI), and upwelling intensity and nutrient conditions (parametrized as diol indices, DI-2, and Nutrient Diol Index, NDI, respectively). Their marine producers remain the subject of debate, but in cultures, they can be found within the outer wall (algaenan) of eustigmatophytes or in Proboscia diatoms. LCDs appear to be well preserved in sediments, potentially as a result of their association with algaenan and/or minerals, but little is known of their pre-depositional histories, in particular transport dynamics. Here, 15 surface continental margin sediments as well as one high-deposition-rate sediment core (50 cm, spanning the last ~30 years) were analyzed in order to evaluate the impact of organo-mineral associations, lateral transport, and hydrodynamic sorting on sedimentary LCD signals. The abundance and distribution of LCDs in bulk sediments and corresponding grain-size fractions was determined. The highest proportion of all LCD isomers is found in the fine fraction (2 – 10 µm), which also holds the highest proportion of organic matter in relation to the other grain-size fractions. However, LCDs are also found in the other fractions (sand, coarse silt, and clay), and their concentrations are not correlated with bulk organic carbon content, indicating different preservation or transport mechanisms. LDI-SST in the bulk sediment is comparable to the mean annual SST at all sites except those influenced by upwelling and characterized by strong seasonal SST gradients. To the contrary of other biomarker-related proxies (e.g., alkenones), lateral transport does not appear to strongly affect LDI-SST in size fractions, suggesting that the intimate relationship of LCD with the algaenan may counteract the influence of hydrodynamic mineral sorting processes on related proxy signals. The difference between the fraction-weighted LCD concentration and bulk sedimentary LCD concentration indicates potential release of LCD during laboratory fractionation, suggesting degradation of algaenan or dissolution of opal frustules.

Sensitivity of ECMWF coupled forecasts to improved initialization of the ocean mesoscale

AbstractIt has been hypothesized that initial conditions derived from eddy‐resolving ocean models could benefit eddy‐permitting forecast systems at subseasonal lead times (2 weeks to 2 months) by providing improved ocean initial conditions and reduced sea‐surface temperature (SST) errors associated with a more realistic representation of mesoscale features and ocean fronts such as the Gulf Stream. Here, we test this hypothesis by running a series of coupled 32‐day reforecasts with ocean initial conditions derived from native resolution (i.e., eddy‐permitting) and higher resolution (i.e., eddy‐resolving) ocean reanalyses. Initial conditions derived from the eddy‐resolving reanalysis reproduce observed temperature and sea‐level variability associated with mesoscale ocean eddies more accurately. Furthermore, forecasts of ocean variables are significantly improved in the extratropics, particularly in regions of observed intense mesoscale variability. This result is relevant for the marine forecasting community and demonstrates the potential for improved probabilistic forecasts of ocean variables in affordable ensemble forecast systems. However, the impacts on the mean state and forecast skill in the atmosphere are more limited. Based on an evaluation of model biases as a function of lead time, we conclude that there is room for further improvements to the initialization of the Gulf Stream that could lead to more significant impacts on atmospheric skill. Furthermore, we demonstrate that our reforecasts underestimate the strength of mesoscale air–sea interaction in the vicinity of strong SST fronts by 20–40%. We speculate that the impacts of improved ocean eddy initialization on atmospheric forecast skill may be larger in coupled models with higher ocean and atmosphere resolutions that can simulate air–sea interaction in the midlatitudes more realistically.