Adaptive Sampling of Marine Submesoscale Features Using Gaussian Process Regression with Unmanned Platforms

The role of sea ice in influencing the seasonal variability of sea surface temperature (SST) fronts in the Southern Ocean (SO) remains poorly understood. This study addresses this gap by utilizing Optimum Interpolation SST data and the Cayula and Cornillon algorithm to detect SST fronts. The findings demonstrate that, influenced by the seasonal expansion and contraction of sea ice, frontal activity south of 60°S varies significantly by season, peaking in summer and reaching its lowest in winter. Frontal frequency is notably higher in regions where sea ice concentration (SIC) falls below 0.2 but decreases when SIC exceeds this threshold, reflecting the impact of sea ice-driven changes in SST gradients on front formation. Additionally, the study reveals a distinct meridional pattern emerges in the distribution of SO fronts, with weaker activity south of 60°S and more intense activity in the Westerlies and further north. These results enhance our understanding of the interactions between sea ice and SST fronts, providing critical insights into the potential impacts of climate change on SO frontal systems.

The classical theory predicts that a geostrophically balanced mesoscale eddy can cause a sea surface temperature (SST) anomaly related to Ekman pumping. Previous studies show that an eddy-induced SST anomaly can result in a sea surface latent heat flux (LH) anomaly at a maximum magnitude of ∼O(10) Wm–2, decaying radially outward from the center to the margin. In this study, we investigate the LH anomalies associated with submesoscale processes within a cyclonic eddy for the first time using recent satellite-ship-coordinated air-sea observations in the South China Sea. Unbalanced submesoscale features can be identified as submesoscale SST fronts. Along the ship track, the SST strikingly decreases by 0.5°C within a horizontal distance of ∼1.5 km and increases quickly by 0.9°C with a spatial interval of ∼3.6 km. The along-track SST is decomposed into three parts: large-scale south-north fronts and anomalies induced by mesoscale and submesoscale motions. Our analysis shows that the amplitude of the LH anomaly induced by the mesoscale SST anomaly is 12.3 Wm–2, while it is 14.3 Wm–2 by unbalanced submesoscale motions. The mean (maximum) spatial gradient of the submesoscale LH anomalies is 1.7 (75.7) Wm–2km–1, which is approximately 1.5 times those (1.2 and 59.9 Wm–2km–1) in association with mesoscale eddies. The spectra of LH and SST anomalies show similar peaks at ∼15 km before sloping down with a power law between k–2 and k–3, indicating the underlying relationship between the LH variance and submesoscale processes.

Microplastics (MPs) are pervasive contaminants, yet understanding their pathways and fate in the marine environment remains unclear. A key challenge is the lack of in-situ, complementary measurements linking MP quantification with oceanographic parameters, particularly concerning submesoscale processes and density fronts. Submesoscale dynamics, including filaments, eddies, and fronts, significantly influence the transport and accumulation of MPs by creating convergence zones and sharp density gradients. Density fronts serve as critical hotspots for MP aggregation, concentrating particles through upwelling and downwelling processes. Despite their importance, these interactions remain poorly studied, emphasizing the need for integrated approaches to directly measure the interplay between MPs and the physical processes that drive their distribution.This study addresses this gap by utilizing in-situ measurements collected with an autonomous surface vehicle (ASV) in the southern North Sea, simultaneously collecting water samples for MP analysis and key oceanographic data. The ASV simultaneously sampled air, sea surface microlayer, and underlying water for MP analysis. A weather station and conductivity, temperature, and depth (CTD) sensors were deployed on the ASV to further contextualize the distribution of MPs. Additionally, CTD profiles were obtained by an accompanying research vessel to investigate the influence of stratification and temporal dynamics on MP distribution. An acoustic Doppler current profiler measured water current velocities and flow direction.The measurements underscore the pivotal role of submesoscale fronts and filaments in shaping the accumulation and distribution of MP. Upwelling and downwelling processes at these fronts and filaments concentrated MP up to 30.48 µg MP L⁻¹, and distributed MPs vertically across depth profiles and horizontally across fronts. Wind direction was found to influence the presence of MP in the atmosphere, while wind speeds appeared to enhance heterogeneity in MP composition and concentration within the water.Submesoscale fronts and filaments are highlighted as key zones for MP accumulation, driven by the interplay of horizontal and vertical water flow linked to ageostrophic circulation. The data provide novel insights into their transport mechanisms in the marine environment.

Several research cruises were conducted across the Kuroshio Extension front to examine the low-level atmospheric responses to mesoscale variations in sea surface temperature (SST). Surface meteorological observations, including sea level pressure (SLP) and SST, were collected at two moored buoys that were located on either side of the Kuroshio Extension, and from a research vessel, as it moved between the two buoys during the various cruises. Spatial perturbations in SLP along the ship transects, calculated by subtracting moored-buoy SLP from that of the moving ship, tend to be positive (negative) where SST is lower (higher) on spatial scales of about 100 km, and the magnitude of these SLP perturbations near the SST front can exceed 1.0 hPa. Radiosonde data also show that the atmospheric boundary layer thins (thickens) over lower (higher) SST. Although the contribution of across-track component in the wind cannot be calculated, the along-track component of divergence suggests low-level convergence over higher SST. The thermally induced pressure gradient is important in the momentum budget, suggesting that sea breeze-like local circulations formed over the SST fronts. Indeed, the pressure adjustment mechanism can sometimes dominate even on a scale of 100 km, although it is not always observed. The time scale in which the boundary layer thickness adjusted to SST is estimated to be 1 day or less.

Previous studies have found that sea surface temperature (SST) fronts have significant impacts on high‐wind frequency. Strong winds over the mid‐latitude North Pacific and North Atlantic Oceans occur frequently downstream of SST fronts in boreal winter. An SST front can significantly increase the instability of the marine atmospheric boundary layer, which accelerates the wind on the warmer flank. This study uses satellite observations to examine the “double acceleration” effects of closely spaced paired fronts (with interfrontal distances of 300–1,000 km) on high‐wind occurrence frequency in the Pacific subarctic frontal zone east of Japan and over the Atlantic Ocean east of Newfoundland. During boreal winter each year, mean westerly winds frequently cross two or more closely spaced SST fronts, with probabilities of 85% over the North Pacific and 87% over the North Atlantic. Over the warmer flank of the first front, the average high‐wind occurrence frequency (wind speed) increases due to the vertical‐mixing mechanism. Downstream of the second front, average high‐wind occurrence frequency (wind speed) reaches maxima of ∼9% (11.5 m s−1) over the Pacific and ∼11.5% (12 m s−1) over the Atlantic. Stronger westerly winds lead to greater high‐wind frequency. The first front contributes to the observed high winds downstream of the second front due to little friction and suppression of upward vertical momentum transfer between paired fronts.

Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as climatological imprints of oceanic fronts within the Kuroshio–Oyashio Extension (KOE) region east of Japan are investigated by taking advantage of high horizontal resolution of the ERA-Interim global atmospheric reanalysis data, for which the resolution of sea surface temperature (SST) data has been improved. These imprints, including locally enhanced sensible and latent heat fluxes and local maxima in cloudiness and precipitation in association with locally strengthened surface-wind convergence in the vicinities of SST fronts along the warm Kuroshio Extension and cool Oyashio to its north, are also identified in high-resolution satellite data. In addition to these mesoscale MABL features, meridionally confined near-surface baroclinic zones and zonally oriented sea level pressure (SLP) minima associated with the dual SST fronts are represented in ERA-Interim only in the period of high-resolution SST, but those imprints of the Oyashio front are missing in the low-resolution SST period. In the presence of the prevailing monsoonal northerlies, latitudinal displacements of the SLP trough, baroclinic zone, and the peak meridional gradient of the turbulent heat fluxes from each of the corresponding SST fronts are also found to be sensitive to the frontal width that depends on the SST resolution. The analysis herein suggests that the converging surface northerlies into the SLP minima can contribute positively to the formation of a surface baroclinic zone along the Kuroshio Extension, while a stronger baroclinic zone along the Oyashio front is maintained primarily through the pronounced cross-frontal contrast in sensible heat release from the ocean.

Sudden Stratospheric Warmings (SSWs) and Polar Stratospheric Clouds (PSCs) exhibit striking inter-hemispheric asymmetries: SSWs are frequent in the Arctic but rare in the Antarctic, while PSCs are more persistent in the Antarctic. Although land–sea thermal contrast and orography (LSCO) have been traditionally invoked to explain these asymmetries, here we show using semi-idealized model experiments that even though LSCO strongly impacts the mean state of SSWs and PSCs, it alone cannot fully account for the observed differences. Using model experiments, we reveal that midlatitude oceanic sea surface temperature (SST) fronts represent a crucial supporting additional driver of the hemispheric stratospheric differences. Like LSCO, SST fronts enhance stratospheric convergence of resolved waves, strengthening the Brewer–Dobson Circulation and inducing high-latitude adiabatic warming. This warming significantly enhances Arctic SSW frequency and strongly suppresses PSC formation. Sub-grid-scale non–resolved wave forcing modulates the stratospheric resolved waves effect. The oceanic impact is dominated by North Pacific SST fronts. Our results highlight the indispensable role of SST fronts in shaping Arctic–Antarctic asymmetries in stratospheric dynamics and associated extremes.

Reynolds sea surface temperature (SST) data showing tropical instability waves (TIWs) in the tropical Pacific are analyzed along with current measurements from the Tropical Atmosphere–Ocean (TAO) buoy array and wind speeds from the European Remote Sensing Satellite (ERS) -1 and -2 scatterometers. TIWs are visible as undulations in the SST cold fronts that delineate the northern and southern boundaries of the cold tongue in the equatorial Pacific. The SST pattern results from advection of the SST front by instabilities in the near-surface equatorial currents. Although the waves are seen on both sides of the Pacific cold tongue and north of the equator in the Atlantic, they are most intense, and thereby most observable, in the north equatorial Pacific. The combination of data used in this analysis provides information about these waves, the factors controlling them, and their coupling to the atmosphere on annual and interannual timescales. On annual timescales, the TIWs generally establish a strong signal in July east of about 140°W with a westward phase speed of about 0.5 m s−1. By August, the waves usually occupy the longitudes between 160° and 100°W and continue to propagate west at roughly the same speed. With the onset of the warm season in the equatorial cold tongue (spring), the signal typically weakens and the propagation speeds show large variations. On interannual timescales, activity is strongest during the cold phase of the ENSO cycle (La Niña) when the cold tongue is most pronounced; the waves are weak or nonexistent during the warm phase of ENSO (El Niño) when the SST front is weak. The TIW signature in SST is noticeable throughout all seasons of the year provided that the gradient in SST at 140°W is greater than about 0.25°C (100 km)−1. In addition, analysis of the currents underlines the importance of the background currents to the zonal propagation speeds.

Neglected High-frequency Variability events of the sea surface Wind Coupled with Precipitation (HV-WCP) events are revealed over the Kuroshio Sea Surface Temperature (SST) front in the East China Sea (ECS) during wintertime. This phenomenon is characterized as modulation events of the surface wind directions, whereby the southerly anomaly changes into a northwest anomaly, accompanied with increasing wind speed, and coupled with decreasing precipitation from morning to night on the day with the peak variability. The events are associated with significant climate effects on the rainfall anomaly over southeastern China and adjacent ocean areas of the ECS. To sustain these events, the large-scale circulation needs to exhibit declines in the upstream monsoon and weakening vertical mixing in the planetary boundary layer (PBL). The Kuroshio thermal effect on the underlying atmosphere becomes prominent due to weakening vertical mixing, which results in pronounced PBL height gradient over the Kuroshio SST front. More plentiful moisture transported from PBL to the free atmosphere, generates the initialization of rainfall and the cyclone anomaly. Meanwhile, the latent heat releasing caused by this precipitation forces positive vorticity anomaly within the PBL, which again strengthens the cyclone and the upward moisture transport. Therefore, a complement between local circulation and rainfall is developed over the Kuroshio region. Due to the significant effect of the Kuroshio SST front on the PBL structure, the HV-WCP intensity is able to maintain its strength over the Kuroshio region. However, because of the northwestern background monsoon, the HV-WCP events vanish quickly when the coupled system moves out of Kuroshio SST frontal region.

Nowadays, high resolution sea surface temperature (SST) observations recorded from orbital satellites are available. Because SST fronts appearing at the ocean surface convey information about the dynamics of deeper ocean layers, their study is of high interest in oceanography. In this paper we present a variational method for fronts tracking in SST images. The proposed method integrates into the variational data assimilation framework a variational method for fronts detection using the level set formulation. This allows our method to extract temporally consistent fronts in SST images sequences. The proposed method is validated on two sequences of SST images of two regions, the region of Malvinas and the region of Aghulas-Benguela, which host very active oceanic fronts.

Satellite-derived sea surface temperature fronts in a river-influenced coastal upwelling area off central–southern Chile

Turbulent flow in a weakly convective marine atmospheric boundary layer (MABL) driven by geostrophic winds Ug = 10 m s−1 and heterogeneous sea surface temperature (SST) is examined using fine-mesh large-eddy simulation (LES). The imposed SST heterogeneity is a single-sided warm or cold front with temperature jumps Δθ = (2, −1.5) K varying over a horizontal distance between [0.1, −6] km characteristic of an upper-ocean mesoscale or submesoscale regime. A Fourier-fringe technique is implemented in the LES to overcome the assumptions of horizontally homogeneous periodic flow. Grid meshes of 2.2 × 109 points with fine-resolution (horizontal, vertical) spacing (δx = δy, δz) = (4.4, 2) m are used. Geostrophic winds blowing across SST isotherms generate secondary circulations that vary with the sign of the front. Warm fronts feature overshoots in the temperature field, nonlinear temperature and momentum fluxes, a local maximum in the vertical velocity variance, and an extended spatial evolution of the boundary layer with increasing distance from the SST front. Cold fronts collapse the incoming turbulence but leave behind residual motions above the boundary layer. In the case of a warm front, the internal boundary layer grows with downstream distance conveying the surface changes aloft and downwind. SST fronts modify entrainment fluxes and generate persistent horizontal advection at large distances from the front.

Fourteen years (September 2002 to August 2016) of high-resolution satellite observations of sea surface temperature (SST) data are used to describe the frontal pattern and frontogenesis on the southeastern continental shelf of Brazil. The daily SST fronts are obtained using an edge-detection algorithm, and the monthly frontal probability (FP) is subsequently calculated. High SST FPs are mainly distributed along the coast and decrease with distance from the coastline. The results from empirical orthogonal function (EOF) decompositions reveal strong seasonal variability of the coastal SST FP with maximum (minimum) in the astral summer (winter). Wind plays an important role in driving the frontal activities, and high FPs are accompanied by strong alongshore wind stress and wind stress curl. This is particularly true during the summer, when the total transport induced by the alongshore component of upwelling-favorable winds and the wind stress curl reaches the annual maximum. The fronts are influenced by multiple factors other than wind forcing, such as the orientation of the coastline, the seafloor topography, and the meandering of the Brazil Current. As a result, there is a slight difference between the seasonality of the SST fronts and the wind, and their relationship was varying with spatial locations. The impact of the air-sea interaction is further investigated in the frontal zone, and large coupling coefficients are found between the crosswind (downwind) SST gradients and the wind stress curl (divergence). The analysis of the SST fronts and wind leads to a better understanding of the dynamics and frontogenesis off the southeastern continental shelf of Brazil, and the results can be used to further understand the air-sea coupling process at regional level.

Spectral analyses of the sea surface temperature (SST) in the Simple Ocean Data Analysis (SODA) reanalysis for the past half-century identify prominent and statistically significant interannual oscillations in two regions along the Gulf Stream front over the North Atlantic. A model of the atmospheric marine boundary layer coupled to a baroclinic quasigeostrophic model of the free atmosphere is then forced with the SST history from the SODA reanalysis. Two extreme states are found in the atmospheric simulations: 1) an eastward extension of the westerly jet associated with the front, which occurs mainly during boreal winter, and 2) a quiescent state of very weak flow found predominantly in the summer. This vacillation of the oceanic-front-induced jet in the model is found to exhibit periodicities similar to those identified in the observed Gulf Stream SST front itself. In addition, a close correspondence is found between interannual spectral peaks in the observed North Atlantic Oscillation (NAO) index and the SODA-induced oscillations in the atmospheric model. In particular, significant oscillatory modes with periods of 8.5, 4.2, and 2.8 yr are found in both observed and simulated indices and are shown to be highly synchronized and of similar energy in both time series. These oscillatory modes in the simulations are shown to be suppressed when either (i) the Gulf Stream front or (ii) its interannual oscillations are omitted from the SST field. Moreover, these modes also disappear when (iii) the SST front is spatially smoothed, thus confirming that they are indeed induced by the oceanic front.

In recent years, the accuracy of ocean forecasts has significantly improved thanks to spatial altimetry. However, despite their importance, forecasting accurate currents is still a challenge. Therefore, satellite missions are proposed to provide measurements of surface velocity on a global coverage. The Ocean DYnamics and Surface Exchange with the Atmosphere (ODYSEA) mission of the National Aeronautics and Space Administration (NASA) and Centre National d’Etudes Spatiales (CNES) is currently competing for selection in the Earth System Explorers program. As part of the preliminary studies for ODYSEA, we have carried out Observing System Simulation Experiments (OSSEs) to assess the impact of assimilating these prospective observations in terms of Root Mean Square differences of the model state variables to a reference run. We focus on disentangling the impact of the ODYSEA observations from the impact of the observations provided by altimetry, on large scale, and checking the complementarity of these networks to avoid redundant information. We show that zonal velocity out of the Equator, sea surface height and surface salinity are mainly constrained by altimetry. Conversely, the meridional velocity is mainly constrained by velocity observations. Moreover, these latter observations help better prescribing both components of velocity at the Equator as well as the sea surface temperature in the Eastern Pacific. They also tend to significantly improve surface salinity in some regions where freshwater input occurs. Altimetry and surface velocity observations are complementary, and when they are assimilated together, all the model state variables are improved in all regions compared to assimilating altimetry only.