SST Front Research Articles

Nonstationarity in the NAO–Gulf Stream SST Front Interaction

Abstract The interaction between the North Atlantic Oscillation (NAO) and the latitudinal shifts of the Gulf Stream sea surface temperature front (GSF) has been the subject of extensive investigations. There are indications of nonstationarity in this interaction, but differences in the methodologies used in previous studies make it difficult to draw consistent conclusions. Furthermore, there is a lack of consensus on the key mechanisms underlying the response of the GSF to the NAO. This study assesses the possible nonstationarity in the NAO–GSF interaction and the mechanisms underlying this interaction during 1950–2020, using reanalysis data. Results show that the NAO and GSF indices covary on the decadal time scale but only during 1972–2018. A secondary peak in the NAO–GSF covariability emerges on multiannual time scales but only during 2005–15. The nonstationarity in the decadal NAO–GSF covariability is also manifested in variations in their lead–lag relationship. Indeed, the NAO tends to lead the GSF shifts by 3 years during 1972–90 and by 2 years during 1990–2018. The response of the GSF to the NAO at the decadal time scale can be interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the response of deep oceanic circulation, and the propagation of Rossby waves. However, there is evidence of Rossby wave propagation only during 1972–90. Here it is suggested that the nonstationarity of Rossby wave propagation caused the time lag between the NAO and the GSF shifts on the decadal time scale to differ between the two time periods.

Role of Mixed Layer Depth in Kuroshio Extension Decadal Variability

AbstractSea surface temperature (SST) anomalies in the Kuroshio Extension (KE) have been suggested to play a crucial role in decadal climate variability of the North Pacific affecting global climate and marine ecosystem variability. By analyzing the mixed layer heat budget, taking into account seasonality and mixed layer depth (MLD) variations, we here show for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulations of the effective heat capacity affecting the SST sensitivity to surface heat fluxes during 1982–2015. More specifically, when the mixed layer becomes anomalously thick (shallow) to the south (north) of the front, it becomes less (more) sensitive to wintertime surface cooling. As a result, the SST front is strengthened, with positive (negative) SST anomalies to the south (north). A heat conservation model suggests that MLD anomalies are mainly due to thermocline depth anomalies.

Ocean Mesoscale and Frontal-Scale Ocean–Atmosphere Interactions and Influence on Large-Scale Climate: A Review

Abstract Two decades of high-resolution satellite observations and climate modeling studies have indicated strong ocean–atmosphere coupled feedback mediated by ocean mesoscale processes, including semipermanent and meandrous SST fronts, mesoscale eddies, and filaments. The air–sea exchanges in latent heat, sensible heat, momentum, and carbon dioxide associated with this so-called mesoscale air–sea interaction are robust near the major western boundary currents, Southern Ocean fronts, and equatorial and coastal upwelling zones, but they are also ubiquitous over the global oceans wherever ocean mesoscale processes are active. Current theories, informed by rapidly advancing observational and modeling capabilities, have established the importance of mesoscale and frontal-scale air–sea interaction processes for understanding large-scale ocean circulation, biogeochemistry, and weather and climate variability. However, numerous challenges remain to accurately diagnose, observe, and simulate mesoscale air–sea interaction to quantify its impacts on large-scale processes. This article provides a comprehensive review of key aspects pertinent to mesoscale air–sea interaction, synthesizes current understanding with remaining gaps and uncertainties, and provides recommendations on theoretical, observational, and modeling strategies for future air–sea interaction research. Significance Statement Recent high-resolution satellite observations and climate models have shown a significant impact of coupled ocean–atmosphere interactions mediated by small-scale (mesoscale) ocean processes, including ocean eddies and fronts, on Earth’s climate. Ocean mesoscale-induced spatial temperature and current variability modulate the air–sea exchanges in heat, momentum, and mass (e.g., gases such as water vapor and carbon dioxide), altering coupled boundary layer processes. Studies suggest that skillful simulations and predictions of ocean circulation, biogeochemistry, and weather events and climate variability depend on accurate representation of the eddy-mediated air–sea interaction. However, numerous challenges remain in accurately diagnosing, observing, and simulating mesoscale air–sea interaction to quantify its large-scale impacts. This article synthesizes the latest understanding of mesoscale air–sea interaction, identifies remaining gaps and uncertainties, and provides recommendations on strategies for future ocean–weather–climate research.

Detection of Sea Surface Temperature Fronts from SAR Images

Abstract Sea surface temperature (SST) fronts are important for fisheries and marine ecology, as well as upper-ocean dynamics, weather forecasting, and climate monitoring. In this paper, we propose a new approach to detect SST fronts from RADARSAT-2 ScanSAR images, based on the correlation of SAR-derived wind speeds using the gray level cooccurrence matrix (GLCM) approach. Due to the large differences between the correlation of wind speeds for SST fronts compared to other areas, SST fronts can be detected by the threshold method. To eliminate small-scale features (or noise), the 30 km scale is used as the length threshold for the detection of the SST fronts. The proposed method is effective when wind speeds are between 3 and 13 m s−1. The overall accuracy of our method is about 93.6%, which is sufficient for operational applications.

Quantifying Eddy Generation and Dissipation in the Jet Response to Upper- versus Lower-Level Thermal Forcing

Abstract The relative roles of upper- and lower-level thermal forcing in shifting the eddy-driven jet are investigated using a multilevel nonlinear quasigeostrophic channel model. The numerical experiments show that the upper-level thermal forcing is more efficient in shifting the eddy-driven jet. The finite-amplitude wave activity diagnostics of numerical results show that the dominance of the upper-level thermal forcing over the lower-level thermal forcing can be understood from their different influence on eddy generation and dissipation that affects the jet shift. The upper-level thermal forcing shifts the jet primarily by affecting the baroclinic generation of eddies. The lower-level thermal forcing influences the jet mainly by affecting the wave breaking and dissipation. The former eddy response turns out to be more efficient for the thermal forcing to shift the eddy-driven jet. Furthermore, two quantitative relationships based on the imposed thermal forcing are proposed to quantify the response of both eddy generation and eddy dissipation, and thus to help predict the shift of eddy-driven jet in response to the vertically nonuniform thermal forcing. By conducting the overriding experiments in which the response of barotropic zonal wind is locked in the model and a multiwavenumber theory in which the eddy diffusivity is decomposed to contributions from eddies and mean flow, we find that the eddy generation response is sensitive to the vertical structure of the thermal forcing and can be quantified by the imposed temperature gradient in the upper troposphere. In contrast, the response of eddy diffusivity is almost vertically independent of the imposed forcing, and can be quantified by the imposed vertically averaged thermal wind. Significance Statement Climate models predict enhanced warming over tropical upper troposphere and Arctic surface in response to greenhouse gas increases, which has competing effects on the latitudinal shift of eddy-driven jet and thus requires a better understanding of the relative roles of upper- and lower-tropospheric thermal forcing for future climate projection. We make a new quantitative comparison on responses of eddy generation and dissipation that sustain the jet shift and relate them to the imposed thermal forcing. This approach extends the fundamental eddy closure topic from vertically uniform situation to vertically nonuniform forcing. These quantitative relationships are also helpful for better understanding and predicting the jet and storm-track variabilities under other forms of thermal forcing (e.g., SST front, aerosols, latent heat release).

Decadal Variability of Southeast US Rainfall in an Eddying Global Coupled Model

AbstractOcean variability is a dominant source of remote rainfall predictability, but in many cases the physical mechanisms driving this predictability are not fully understood. This study examines how ocean mesoscales (i.e., the Gulf Stream SST front) affect decadal Southeast US (SEUS) rainfall, arguing that the local imprint of large‐scale teleconnections is sensitive to resolved mesoscale features. Based on global coupled model experiments with eddying and eddy‐parameterizing ocean, we find that a resolved Gulf Stream improves localized rainfall and remote circulation response in the SEUS. The eddying model generally improves the air‐sea interactions in the Gulf Stream and the North Atlantic Subtropical High that modulate SEUS rainfall over decadal timescales. The eddy‐parameterizing simulation fails to capture the sharp SST gradient associated with the Gulf Stream and overestimates the role of tropical Pacific SST anomalies in the SEUS rainfall.

Observations of SST‐Induced Wind Perturbations in the Leeuwin Current

AbstractSea surface temperature (SST) perturbations modify low‐level winds through the influence of air‐sea heat flux across the marine atmospheric boundary layer, mostly observable in the major western boundary current systems. In this study, we investigate the effects of SST on surface winds in a poleward‐flowing eastern boundary current system, the Leeuwin Current (LC), using the Advanced Very High‐Resolution Radiometer (AVHRR) SST and wind data from Cross‐Calibrated Multi‐Platform (CCMP) during 2009–2018. There exist robust linear relationships between perturbation SST and perturbation wind speed and their derivatives in the LC. The regression slopes between SST perturbations and wind perturbations appear to be higher in austral summer when the alongshore northward winds are strongly opposing the LC, however, the relationships in winter are statistically more robust when the LC and associated cross‐shore SST fronts are the strongest. Consistent with previous studies in the western boundary currents, crosswind perturbation wind speed gradient is the largest for the winds 80° (−100°) to the right (left) of the SST gradient in the LC region. The perturbation wind stress curl induced by the SST signature of the LC causes positive wind stress curl anomalies on the offshore edge of the LC, driving anomalous downwelling and would reduce surface chlorophyll a concentrations along the LC SST front as observed in satellite data, which may have implications in regional biogeochemistry.

Variations of mesoscale eddy SST fronts based on an automatic detection method in the northern South China Sea

SST fronts at the mesoscale eddy edge (ME fronts) were investigated from 2007–2017 in the northern South China Sea (NSCS) based on an automatic method using satellite sea level anomaly (SLA) and SST data. The relative probabilities between the number of anticyclonic/cyclonic ME fronts (AEF/CEF) and the number of anticyclones/cyclones reached 20%. The northeastern and southwestern parts of these anticyclones had more fronts than the northwestern and southeastern parts, although CEFs were nearly equally distributed in all directions. The number of ME fronts had remarkable seasonal variations, while the eddy kinetic energy (EKE) showed no seasonal variations. The total EKE at the ME fronts was three times of that within the MEs, and it was much stronger in AEFs than in CEFs. The interannual variability in the number of ME fronts and EKE had no significant correlation with the El Nino-Southern Oscillation (ENSO) index. Possible mechanisms of ME fronts were discussed, but the contributions of mesoscale eddies to SST fronts need to be quantified in future studies.

Decadal coupling between storm tracks and sea surface temperature in the Southern Hemisphere midlatitudes

The relationships between the seasonal mean storm-track anomalies and sea surface temperature anomalies (SSTAs) in the midlatitude Southern Hemisphere (SH) on the decadal time scale are investigated using the lagged maximum covariance analysis (MCA). It shows that the coupling between storm-track anomalies and SSTAs is most prominent in austral summer (January–March, JFM). Firstly, large cold SSTAs in the western South Indian Ocean (SIO), corresponding to the strengthened SST front on the equator side, substantially intensify the storm-track activity manifested by the increased low-level poleward transient eddy heat flux over the entire SH. The coherent intensification of the atmospheric baroclinicity and the subtropical jet associated with such strengthened SST front provides baroclinic energy for the growth of synoptic eddies. Further, the intensified storm-track activity induces large cold SSTAs in the South Atlantic (SA), the SIO and the subtropical South Pacific (SP), primarily via anomalous net surface heat fluxes. The mean SST advection by the anomalous Ekman current also contributes to the cold SSTAs in the SA, which is related to the anomalous westerlies induced by the anomalous storm-track activity.

Patterns and Dynamics of SST Fronts in the California Current System

AbstractThe California Current System (CCS) is one of the four major Eastern Boundary Current Systems, where major coastal upwelling is observed in the ocean. Oceanic fronts are abundant in the CCS, and are known to be associated with coastal upwelling. Analyses of satellite observations (altimetry and sea surface temperature, SST) of the CCS highlight the presence of SST fronts in the southern part of the CCS outside of the coastal upwelling season, which will be investigated. The submesoscale‐permitting and tidal‐resolving LLC4320‐simulation was used to analyze the nature of these features, through the study of the surface density gradients, the vertical velocities and the horizontal strain. An algorithm was also developed to track SST fronts in the model. The classical scenario of coastal upwelling is confirmed in the central CCS (off California), with a strain field correlated to vertical velocities associated with the SST fronts, and collocated density fronts. In contrast, in the southern CCS (off Baja California), vertical velocities were much weaker and density gradients were not always observed where SST fronts were detected, leading to the assumption that the fronts there may be affected by thermohaline compensation occurring in the region. These findings confirm the existence of SST fronts in the CCS outside of the traditional coastal upwelling framework, with part of the fronts in the southern CCS being passive and advected by the background flow.

Prevailing Surface Wind Direction during Air–Sea Heat Exchange

AbstractWhile the climatological-mean sensible and latent heat fluxes are remarkably well described using climatological-mean fields in the bulk flux formulas, this study shows that a significant fraction of the climatological-mean wind speed in the midlatitudes is associated with wind variations on synoptic time scales. Hence, the prevailing wind direction associated with the most intense air–sea heat exchange can differ from the mean wind direction. To pinpoint these striking differences between the climatological and synoptic viewpoint, this study presents a global climatology of the prevailing surface wind direction during air–sea heat exchanges calculated for instantaneous and time-averaged reanalysis data. The interpretation of the fluxes in the lower latitudes is basically unaffected by the different time averages, highlighting the time-mean nature of the circulation in the lower latitudes. In the midlatitudes, however, the prevailing wind direction features a significant equatorward component for subweekly time averages and reverts to pure westerlies for longer time averages. These findings pinpoint the necessity to consider subweekly time scales, in particular along the midlatitude SST fronts, to describe the air–sea heat exchange in a physically consistent way.

Cross-spectral analysis of the SST/10-m wind speed coupling resolved by satellite products and climate model simulations

This study aims to determine the spatial–temporal scales where the SST forcing of the near-surface winds takes places, and its relationship with the action of coherent ocean eddies. Here, cross-spectral statistics are used to examine the relationship between satellite-based SST and 10-m wind speed (w) fields at scales between 10 $$^2$$ –10 $$^4$$ km and 10 $$^1$$ –10 $$^3$$ days. It is shown that the transition from negative SST/w correlations at large-scales to positive at oceanic mesoscales occurs at wavelengths coinciding with the atmospheric first baroclinic Rossby radius of deformation; and that the dispersion of positively-correlated signals resembles tropical instability waves near the equator, and Rossby waves in the extratropics. Transfer functions are used to estimate the SST-driven w response in physical space ( $$w_c$$ ), a signal that explains 5–40% of the mesoscale w variance in the equatorial cold tongues, and 2–15% at extratropical SST fronts. The signature of ocean eddies is clearly visible in $$w_c$$ , accounting for 20–60% of its variability in eddy-rich regions. To provide further insight on the role of ocean eddies in the SST-driven coupling, the analysis is repeated for two climate model (CCSM) simulations using ocean grid resolutions of $$1^\circ$$ (eddy-parameterized, LR) and $$0.1^\circ$$ (eddy-resolving, HR). The lack of resolved eddies in LR leads to a significantly underestimated mesoscale w variance relative to HR. Conversely, the $$w_c$$ variability in HR can exceed the satellite estimates by a factor of two at extratropical SST fronts and underestimate it by a factor of almost six near the equator, reflecting shortcomings of the CCSM to be addressed in its future developments.

Impact of Kuroshio Extension dipole mode variability on the North Pacific storm track

An index of a large-scale Kuroshio Extension (KE) sea surface height dipole (KED) mode is constructed using satellite altimeter sea level anomaly observations from January 1993 to December 2015 based on previous work of the second author. It is found that the KED mode that undergoes a decadal variation from a negative phase (a positive-over-negative dipole, KED−) to a positive phase (a negative-over-positive dipole, KED+) can affect the variability of the oceanic SST front and the North Pacific storm track. The results show that the oceanic SST fronts in the north of the KE region and in the KE region — referred to as the NSST and KSST fronts, respectively — are closely correlated with the KED mode. In the NSST front region, the SST front is stronger for KED− than for KED+, and the opposite is the case in the KSST region. It is further revealed that the decadal phase transition of the KED mode can change the location and strength of the North Pacific storm track, with the North Pacific storm track being slightly weaker and moving more northwards as a whole during the KED− mode than during the KED+ mode. The westerly wind associated with the storm track on the downstream side of the KE region intensifies and shifts northwards under KED− compared to KED+. Furthermore, the transition of the KED mode gives rise to changes in the North Pacific storm track by changing the NSST and KSST fronts and meridional heat flux.

Atmospheric Response to a Midlatitude SST Front: Alongfront Winds

AbstractSatellite observations and modeling studies show that midlatitude SST fronts influence the marine atmospheric boundary layer (MABL) and atmospheric circulation. Here, the Weather Research and Forecasting (WRF) mesoscale model is used to explore the atmospheric response to a midlatitude SST front in an idealized, dry, two-dimensional configuration, with a background wind oriented in the alongfront direction.The SST front excites an alongfront wind anomaly in the free atmosphere, with peak intensity just above the MABL. This response is nearly quasigeostrophic, in contrast to the inertia–gravity wave response seen for cross-front background winds. The free-atmosphere response increases with the background wind , in contrast to previously proposed SST frontal MABL models.The MABL winds are nearly in Ekman balance. However, a cross-front wind develops in the MABL as a result of friction and rotation such that the MABL cross-front Rossby number ε ≈ 0.2. The MABL vorticity balance and scaling arguments indicate that advection plays an important role in the MABL dynamics. Surface wind convergence shows poor agreement with MABL depth-integrated convergence, indicating that the MABL mixed-layer assumption may not be appropriate for SST frontal zones with moderate to strong surface winds.

Impact of the Pacific–Japan teleconnection pattern on July sea fog over the northwestern Pacific: Interannual variations and global warming effect

The northwestern Pacific (NWP) is a fog-prone area, especially the ocean east of the Kuril Islands. The present study analyzes how the Pacific–Japan (PJ) teleconnection pattern influences July sea fog in the fog-prone area using independent datasets. The covariation between the PJ index and sea fog frequency (SFF) index in July indicates a close correlation, with a coefficient of 0.62 exceeding the 99% confidence level. Composite analysis based on the PJ index, a case study, and model analysis based on GFDL-ESM2M, show that in high PJ index years the convection over the east of the Philippines strengthens and then triggers a Rossby wave, which propagates northward to maintain an anticyclonic anomaly in the midlatitudes, indicating a northeastward shift of the NWP subtropical high. The anticyclonic anomaly facilitates the formation of relatively stable atmospheric stratification or even an inversion layer in the lower level of the troposphere, and strengthens the horizontal southerly moisture transportation from the tropical–subtropical oceans to the fog-prone area. On the other hand, a greater meridional SST gradient over the cold flank of the Kuroshio Extension, due to ocean downwelling, is produced by the anticyclonic wind stress anomaly. Both of these two aspects are favorable for the warm and humid air to cool, condense, and form fog droplets, when air masses cross the SST front. The opposite circumstances occur in low PJ index years, which are not conducive to the formation of sea fog. Finally, a multi-model ensemble mean projection reveals a prominent downward trend of the PJ index after the 2030s, implying a possible decline of the SFF in this period.

Atmospheric Responses to Kuroshio SST Front in the East China Sea under Different Prevailing Winds in Winter and Spring

AbstractAtmospheric responses to the Kuroshio SST front in the East China Sea under different prevailing winds are examined using high-resolution observations and numerical modeling. Satellite data reveal a significant in-phase relationship between SST and surface wind speed, indicative of ocean-to-atmosphere influence. The atmospheric response varies according to the relative surface wind direction with respect to the SST front orientation. Under the alongfront condition, low (high) SLP anomalies are found on the warmer (colder) flank of the front, accompanied by surface wind convergence (divergence). Enhanced precipitation and frequent cumulus convection appear over the warm Kuroshio, suggesting an atmospheric response extending into the free troposphere. Under the cross-front condition, when the air blows from cold to warm (warm to cold) SST, divergence (convergence) is located directly over the SST front, and its magnitude is proportional to the downwind SST gradient. Under such prevailing winds, the SST front has little effect on the SLP and precipitation.The Weather Research and Forecasting (WRF) Model is used to investigate the mechanism responsible for the atmospheric adjustment. The results show that under the alongfront condition, large temperature and pressure perturbations in the boundary layer are caused by SST gradients, while stability and turbulent mixing are less affected. By contrast, under the cross-front condition, the perturbations of temperature and pressure are small and shifted downstream, while the SST gradient exerts stronger impact on vertical mixing. The modeling results confirm that the pressure adjustment mechanism contributes more to the atmospheric response under alongfront prevailing winds, while the vertical mixing mechanism dominates the atmospheric adjustment under cross-front winds.

Investigating the Local Atmospheric Response to a Realistic Shift in the Oyashio Sea Surface Temperature Front

Abstract The local atmospheric response to a realistic shift of the Oyashio Extension SST front in the western North Pacific is analyzed using a high-resolution (HR; 0.25°) version of the global Community Atmosphere Model, version 5 (CAM5). A northward shift in the SST front causes an atmospheric response consisting of a weak surface wind anomaly but a strong vertical circulation extending throughout the troposphere. In the lower troposphere, most of the SST anomaly–induced diabatic heating is balanced by poleward transient eddy heat and moisture fluxes. Collectively, this response differs from the circulation suggested by linear dynamics, where extratropical SST forcing produces shallow anomalous heating balanced by strong equatorward cold air advection driven by an anomalous, stationary surface low to the east. This latter response, however, is obtained by repeating the same experiment except using a relatively low-resolution (LR; 1°) version of CAM5. Comparison to observations suggests that the HR response is closer to nature than the LR response. Strikingly, HR and LR experiments have almost identical vertical profiles of . However, diagnosis of the diabatic quasigeostrophic vertical pressure velocity (ω) budget reveals that HR has a substantially stronger response, which together with upper-level mean differential thermal advection balances stronger vertical motion. The results herein suggest that changes in transient eddy heat and moisture fluxes are critical to the overall local atmospheric response to Oyashio Front anomalies, which may consequently yield a stronger downstream response. These changes may require the high resolution to be fully reproduced, warranting further experiments of this type with other high-resolution atmosphere-only and fully coupled GCMs.

Quad‐polarization SAR features of ocean currents

AbstractA methodology is demonstrated to exploit the polarization sensitivity of high‐resolution radar measurements to interpret and quantify upper ocean dynamics. This study particularly illustrates the potential of quad‐polarization synthetic aperture radar (SAR) measurements. The analysis relies on essential characteristics of the electromagnetic scattering mechanisms and hydrodynamical principles. As the relaxation scale of centimeter‐scale ocean surface scatters is typically small, radar signal anomalies associated with surface manifestations of the upper ocean dynamics on spatial scales exceeding 100 m are mostly dominated by nonresonant and nonpolarized scatters. These “scalar” contributions can thus efficiently trace local breaking and near‐breaking areas, caused by surface current variations. Using dual copolarized measurements, the polarized Bragg‐type radar scattering is isolated by considering the difference (PD) between vertically and horizontally polarized radar signals. The nonpolarized (NP) contribution associated with wave breaking is then deduced, using the measured polarization ratio (PR) between polarized signals. Considering SAR scenes depicting various surface manifestations of the upper ocean dynamics (internal waves, mesoscale surface current features, and SST front), the proposed methodology and set of decompositions (PD, PR, and NP) efficiently enable the discrimination between surface manifestation of upper ocean dynamics and wind field variability. Applied to quad‐polarized SAR images, such decompositions further provide unique opportunities to more directly assess the cross‐polarized (CP for HV or VH) signal sensitivity to surface roughness changes. As demonstrated, such an analysis unambiguously demonstrates and quantitatively evaluates the relative impact of breakers on cross‐polarized signals under low to moderate wind conditions.

Boundary Layer Convergence Induced by Strong Winds across a Midlatitude SST Front*

Abstract Recent studies indicate that the influence of midlatitude SST fronts extends through the marine atmospheric boundary layer (MABL) into the free atmosphere, with implications for climate variability. To better understand the mechanisms of this ocean-to-atmosphere influence, SST-induced MABL convergence is explored here with the Weather Research and Forecasting mesoscale model in an idealized, dry, two-dimensional configuration, for winds crossing from cold to warm SST and from warm to cold SST. For strong cross-front winds, O(10 m s−1), changes in the turbulent mixing and MABL depth across the SST front lead to MABL depth-integrated convergence in the cold-to-warm case and depth-integrated divergence in the warm-to-cold case. The turbulent stress divergence term changes over a shorter length scale than the pressure gradient and Coriolis terms, such that the MABL response directly above the SST front is governed by nonrotating, internal boundary layer–like physics, which are consistent with the vertical mixing mechanism. An important consequence is that the increment in the cross-front surface stress diagnoses the vertical motion at the top of the MABL. These physics are at variance with some previously proposed SST frontal MABL models in which pressure adjustments determine the MABL convergence. The SST-induced MABL convergence results in vertical motion that excites a stationary internal gravity wave in the free atmosphere, analogous to a mountain wave. For a 15 m s−1 cross-front wind, the gravity wave forced by an SST increase of 3°C over 200 km is comparable to that forced by an 80-m change in topography.

Relationship of Northeast Atlantic albacore juveniles with surface thermal and chlorophyll-a fronts

When the spring seasonal warming starts, North Atlantic albacore (Thunnus alalunga) juveniles and pre-adults perform a trophic migration to the northeastern Atlantic, to the Bay of Biscay, and to the southeast of Ireland. During this migration, albacore juveniles are mainly exploited by Spanish trolling and baitboat fleets. The present study analyzes the relationship between the albacore spatio-temporal distribution and the upper surface horizontal fronts in their migration paths and destinations. For this, albacore catches from personal fishing logbooks from Basque trolling and live-bait fleets and daily MODIS AQUA Chlorophyll-a and SST products covering the period 2003–2005 have being used. Gradients have been calculated with the front algorithm proposed by Belkin and O'Reilly (2009). The approach used to study the relationship of catches location with frontal areas is based in the comparison of distributions of gradient magnitude around catch locations versus gradient magnitudes in a Monthly Occupation Area. The results obtained show that there is a high spatio- temporal variability of SST and Chl-a fronts in the area. SST and Chl-a fronts are not coincident in time or in space. However, there is a clear seasonal pattern of SST and Chl-a frontal activity in the area with a peak in July for SST gradient magnitudes and a peak in April for Chl-a gradient magnitudes. The relationship of albacore juvenile catches with high gradient magnitude areas is different according to the months and fleets analysed. In general, there is no evidence of consistent adherence of albacore catches to areas with higher SST gradients. However, results suggest a potential association between both fleets catches and Chl-a high gradient magnitude areas in August and September.