Strong Sea Surface Temperature Research Articles

The Role of Air–Sea Interactions in Atmospheric Rivers: Case Studies Using the SKRIPS Regional Coupled Model

AbstractAtmospheric rivers (ARs) play a key role in California's water supply and are responsible for most of the extreme precipitation and major flooding along the west coast of North America. Given the high societal impact, it is critical to improve our understanding and prediction of ARs. This study uses a regional coupled ocean–atmosphere modeling system to make hindcasts of ARs up to 14 days. Two groups of coupled runs are highlighted in the comparison: (1) ARs occurring during times with strong sea surface temperature (SST) cooling and (2) ARs occurring during times with weak SST cooling. During the events with strong SST cooling, the coupled model simulates strong upward air–sea heat fluxes associated with ARs; on the other hand, when the SST cooling is weak, the coupled model simulates downward air–sea heat fluxes in the AR region. Validation data shows that the coupled model skillfully reproduces the evolving SST, as well as the surface turbulent heat transfers between the ocean and atmosphere. The roles of air–sea interactions in AR events are investigated by comparing coupled model hindcasts to hindcasts made using persistent SST. To evaluate the influence of the ocean on ARs we analyze two representative variables of AR intensity, the vertically integrated water vapor (IWV) and integrated vapor transport (IVT). During strong SST cooling AR events the simulated IWV is improved by about 12% in the coupled run at lead times greater than one week. For IVT, which is about twice more variable, the improvement in the coupled run is about 5%.

Impact of the Benguela coastal low-level jet on the southeast tropical Atlantic SST bias in a regional ocean model

The current generation of coupled ocean–atmosphere climate models, which are widely used for studying the dynamics of the climate system and to make projections of future climate, suffers from erroneously warm (exceeding 5 °C) sea surface temperature (SST) simulation in the eastern boundary upwelling systems (EBUS) in the Atlantic and Pacific oceans. While recent improvements in the horizontal resolution of coupled model components helped to alleviate this issue in the North and South Pacific and North Atlantic oceans, the warm bias in the Southeast Tropical Atlantic (SETA) region still remains above 5 °C. Recent studies have highlighted the importance of inaccuracies in the near-coastal winds, especially in the Benguela low-level coastal jet (BLLCJ), in the formation of this warm SST bias. In addition, unique ocean circulation features like the poleward flowing coastal current and the presence of a strong coastal SST front were argued to contribute to the complexity in simulating realistic SST in the SETA region. In this study, we investigate how ocean model resolution and the spatial structure and strength of the BLLCJ affect the SST warm bias in the SETA. We conducted a suite of ocean model experiments with varying horizontal resolutions (81–3 km) and different atmospheric forcing products. We found that the accuracy in the magnitude of BLLCJ’s alongshore component and in the spatial structure of BLLCJ are the primary factors in reducing the warm SST bias in SETA and the ocean model horizontal resolution is of only secondary importance. A weaker alongshore component of the BLLCJ leads to a weaker upwelling through Ekman transport and fails to generate a downwind equatorward surface jet in the ocean. When the core of the BLLCJ is too far offshore, the broad wind stress curl zone gives rise to anomalous poleward depth integrated flow through Sverdrup balance and weakens the wind stress curl induced Ekman pumping near the coast. Both the presence of a poleward current, which can transport warm equatorial waters farther south, and the absence of the equatorward surface jet contributes to the warm SST bias. Heat budget analysis shows that the contribution from the anomalous poleward flow is a bigger contributor to the warm bias than the weak upwelling. With more realistic, high-resolution winds, an ocean model resolution increase from 27 to 9 km slightly reduces the extent and magnitude of SST bias. However, no such improvement occurs with coarse resolution less realistic winds. Sensitivity experiments using atmospheric forcing fields from different sources confirm that the major contribution to the warm SST bias is in fact from the erroneous winds rather than other atmospheric forcing fields like net shortwave and downward longwave radiation. Further studies are needed to understand whether eddy heat advection or mean flow heat advection primarily cause the warm SST bias.

Impact of the tropical Pacific SST biases on the simulation and prediction of Indian summer monsoon rainfall in CFSv2, ECMWF-System4, and NMME models

Present study analyses the role of tropical Pacific sea surface temperature (SST) biases in simulating the mean and inter-annual variability of Indian summer monsoon rainfall (ISMR) using the hindcasts from Monsoon Mission CFSv2 (MMCFS) and ECMWF-SYSTEM4 (ES4) prediction systems. ES4 simulates the mean and annual cycle of ISMR better than MMCFS when initialized with February initial conditions (3-month lead, Feb IC). At the same time, interannual variability (IAV) and skill (correlation between the ensemble mean hindcast and observations) are the highest (least) for MMCFS (ES4). May IC (0-month lead) hindcasts of both the models have similar mean, annual cycle, IAV and skill. Further analysis shows that ES4_Feb IC hindcasts exhibit a strong cold bias (~ 2 °C) in the equatorial central Pacific, which is close to zero for MMCFS_Feb IC. Meanwhile, the 0-month lead (May IC) hindcast has a similar cold bias (~ 1.5 °C) in the equatorial Pacific for both the model hindcasts. Thus, model hindcasts with a better mean ISMR are having a strong cold bias in the equatorial Pacific and that having very less SST bias has higher IAV and skill for ISMR simulations. Additional investigation using North American National Multi-Model Ensemble Project (NMME) models is carried out and the following conclusion on the different roles of Pacific mean state bias is drawn. Hindcasts with strong cold SST bias in the tropical Pacific (ES4 hindcast and May IC hindcasts of MMCFS) tend to mimic the teleconnections associated with La Nina conditions reducing the dry bias over India, resulting in mean ISMR closer to the observed value, still less than the observed mean. At the same time, due to the same strong cold bias, the El Nino-Southern Oscillation (ENSO) induced rainfall and circulation pattern in the Pacific are weak and extended further northwestward. This weakens the ENSO induced heat sources over the Indian Ocean and monsoon region, resulting in reduced ENSO-ISMR teleconnections. These factors result in a poor simulation of interannual variability and skill of ISMR. On the other hand, model hindcasts with less cold bias in the equatorial Pacific (MMCFS_Feb IC) suffer from strong dry bias over India; however, the skill of ISMR is higher as a result of strong ENSO-Monsoon teleconnections. Thus the study confirms the differential role of SST bias in the tropical Pacific in simulating mean and IAV of ISMR in seasonal prediction models. A close to observed mean SST in the tropical Pacific and proper ENSO-Monsoon teleconnection is essential for the better skill of ISMR in the present generation seasonal prediction models.

MJO induced diurnal sea surface temperature variations off the northwest shelf of Australia observed from Himawari geostationary satellite

High frequency coupling between the tropical upper ocean and atmosphere at diurnal time scales is being recognised to be important for the behaviour of large scale climate modes such as the Madden Julian Oscillation (MJO). Sea surface temperature (SST) variability in the Eastern Indian Ocean warm pool and on the Northwest Shelf of Australia is strongly influenced by the MJO activity. In this study, we use the hourly Himawari-8 geostationary satellite SST data to understand the modulation of the amplitude of SST diurnal variations in the region by the MJO during two austral summer seasons of 2015–16 and 2016–17. The high temporal and spatial resolutions of the Himawari-8 SST data are able to capture the temporal and spatial variability of diurnal SST amplitude. Our results show that the MJO onset (active phase) is preceded by strong diurnal SST variations during the MJO suppressed phase. The amplitudes of the SST diurnal variations are mainly modulated by surface wind variability in the region, as denoted by their spatial and temporal correlations. The very strong El Niño event in 2015–16 may have significantly weakened the MJO activities in the region, resulting in prolonged MJO suppressed phase, high diurnal SST variation and mean SST. This study provides background knowledge for future research on how the diurnal SST variation in the region may influence the progression of the MJO events.

Cyclone intensification in the Kuroshio region and its relation to the sea surface temperature front and upper‐level forcing

AbstractThe Northwest Pacific features strong sea surface temperature (SST) gradients providing favourable conditions for wintertime cyclone intensification in the midlatitudes. To estimate the relative contribution of the SST front to the evolution of cyclones and identify the mechanisms for cyclone intensification, we track individual cyclones and categorise them depending on their propagation relative to the SST front. We focus on cyclones remaining on either the cold or warm side of the SST front, as well as those crossing the SST front from the warm to the cold side. Cyclones crossing the SST front or remaining on its warm side propagate near the left exit region of the jet and are associated with higher precipitation, consistent with higher moisture availability and cyclone intensity. Comparing the different cyclone categories, there is no direct effect of the SST front on cyclone intensification. However, the SST front contributes to the climatological low‐level baroclinicity, providing a conducive environment for cyclone intensification for the cyclones crossing the SST front. Compared with the Gulf Stream region, the land–sea contrast plays a less prominent role for the low‐level baroclinicity in the Kuroshio region.

Spring Dust Mass Flux over the Tibetan Plateau during 2007–19 and Connections with North Atlantic SST Variability

AbstractDust is the major aerosol type over the Tibetan Plateau (TP), and the TP plays an important role in forming the spring dust belt across the Northern Hemisphere in the upper troposphere. Estimated spring dust mass flux (DMF) showed a significant declining trend over the TP during 2007–19. The total spring DMF across the TP (TDMFTP) was mainly affected by DMFs over the Tarim Basin, while the spring DMF across the TP in the midtroposphere was also connected with DMFs over the northwest Indian Peninsula and central Asia. Interannual variability of spring TDMFTP was strongly correlated with the North Atlantic winter sea surface temperature (SST) tripole. A cold preceding winter induced by the North Atlantic winter SST tripole over midlatitude Eurasia promotes dust activities in the subsequent spring. The North Atlantic winter SST tripole anomalies persist into the subsequent spring and induce a corresponding atmosphere response. Enhanced atmospheric baroclinicity develops over northwest China and the northern Indian Peninsula during spring, which is attributed to surface thermal forcing induced by the positive winter SST tripole phase. A strong positive North Atlantic winter SST tripole anomaly strengthens the upper-level westerly jets, enhancing airflow toward the TP midtroposphere; together, these circulation patterns cause anomalous cyclonic conditions in the lower troposphere, especially over the Tarim Basin, via the eastward propagation of a Rossby wave train. These atmospheric circulation conditions are likely to increase the frequency of dust occurrence and promote the transport of dust onto the TP.

Diversity of North Pacific Meridional Mode and Its Distinct Impacts on El Niño‐Southern Oscillation

AbstractNumerous studies have demonstrated that the North Pacific Meridional Mode (NPMM) plays an important role in determining El Niño‐Southern Oscillation (ENSO) events in the following winter season. However, little attention has been given to significant differences among its spatial patterns. Here we show that the NPMM exhibits a large diversity in spatial patterns, leading to distinct impacts on ENSO. Based on objective clustering analysis, two distinct spatial patterns of NPMM are detected. Cluster 1 (C1) NPMM exhibits a strong sea surface temperature dipole over the subtropical eastern Pacific and midlatitude central Pacific whereas Cluster 2 (C2) features a dipole over the subtropical eastern Pacific and equatorial cold tongue region. We find that the C1 NPMM is strongly linked to following ENSO events while the C2 NPMM has no statistically significant relation. This gives new implications for ENSO dynamics and predictions.

Seasonal Movements and Habitat Use of Juvenile Smooth Hammerhead Sharks in the Western North Atlantic Ocean and Significance for Management

Upper trophic level predators dramatically impacted by fisheries include the large-bodied hammerhead sharks, which have become species of conservation concern worldwide. Implementing spatial management for conservation of hammerhead populations requires knowledge of temporal distribution patterns and habitat use, identification of essential habitat for protection, and quantification of interactions with human activities. There is little such information for the smooth hammerhead shark, Sphyrna zygaena. We used fin-mounted satellite tags to examine the movements and habitat use of juvenile smooth hammerheads, a demographic segment particularly threatened by exploitation. Six sharks were tagged off the US mid-Atlantic and tracked for 49–441 d (mean 187 d). Sharks consistently showed area-restricted movements within a summer core area in waters of the New York Bight and a winter core area off Cape Hatteras, North Carolina, with directed movements between them in autumn. There was high overlap of shark winter core area use and the Mid-Atlantic Shark Area (MASA) - a seven-month per year, bottom-longline fishery closure - indicating that this area closure offers seasonal reduction in fishing pressure for this species. Based on timing of shark movements and the MASA closure, protection for juvenile smooth hammerheads may be increased by beginning the closure period one month earlier than currently scheduled. Generalized additive mixed models revealed that area-restricted movements of sharks in their summer and winter core areas coincided with high primary productivity, strong sea surface temperature fronts and elevated sea surface temperature. Consistency in use of summer and winter core areas suggests that the coastal waters of the New York Bight and Cape Hatteras, North Carolina could be considered for Essential Fish Habitat designation for this species. This study reveals the first high resolution movements and habitat use for smooth hammerheads in the western North Atlantic to inform management planning for this population.

The seasonal footprinting mechanism in large ensemble simulations of the second generation Canadian earth system model: uncertainty due to internal climate variability

Previous studies indicated that the wintertime North Pacific Oscillation (NPO) could exert marked impacts on the following winter El Niño-Southern Oscillation (ENSO) via the seasonal footprinting mechanism (SFM). Here, we examine this winter NPO-ENSO relationship in a 50-member ensemble of historical simulations conducted with the Canadian Centre for Climate Modeling and Analysis second generation Canadian Earth System Model (CanESM2) over the period of 1950–2005. The observed NPO pattern, featured by a meridional dipole atmospheric anomaly over the North Pacific, can be well reproduced by all of the 50 ensemble members. The multi-member ensemble (MME) mean can well simulate the observed NPO-ENSO relationship, as well as the SFM process. However, there exists a large spread of the results among the 50 members due to internal climate variability. Internal climate variability influences the winter NPO-ENSO relationship through modulating the subtropical center of the NPO. Specifically, the ensemble members with high NPO-ENSO correlations tend to have strong atmospheric anomalies over the subtropical North Pacific in winter. The atmospheric circulation anomaly brings strong sea surface temperature and precipitation anomalies in the tropical central Pacific and westerly wind anomalies over the tropical western Pacific in the following spring. These anomalies sustain in the following seasons and eventually lead to ENSO events in the following winter.

Warming Trends in the Central Equatorial Indian Ocean and the Associated Coupled Feedback Processes

Mohapatra, S. and Gnanaseelan, C., 2020. Warming trends in the central equatorial Indian Ocean and the associated coupled feedback processes. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 39-45. Coconut Creek (Florida), ISSN 0749-0208.The recent increasing trends in the tropical Indian Ocean (TIO) temperature have strong impact on the regional as well as global climate variability. In this work we studied the changes in the boreal summer monsoon winds and the associated monsoon currents on modulating the surface and subsurface temperature of the equatorial Indian Ocean. The reduction in the meridional sea surface temperature (SST) gradient and strengthening of zonal SST gradient over the Indian Ocean reduced the northward propagation of south-westerly wind, thereby enhancing the surface westerlies over the equator. The strengthened surface westerlies increased the equatorial downwelling Kelvin waves and deepened the thermocline. The present study unravels the causes for the observed confinement of strong SST trend over the central equatorial Indian Ocean (CEIO) even though surface westerlies and the associated downwelling extend upto the eastern equatorial Indian Ocean (EEIO). In contrast to the CEIO surface warming, EEIO experienced strong subsurface warming at thermocline. Our analysis suggests that the upper ocean stratification plays an important role in the observed warming trend of both CEIO surface and EEIO subsurface. The eastward equatorial currents bring warm salty water from Arabian Sea and WEIO towards east and warms CEIO, however the high saline surface water is found to sink in the EEIO, thereby warming the subsurface instead of surface. The composite of temperature anomaly for the period 1978-2002, the period of warm Pacific Decadal Oscillation, shows patterns similar to temperature trends, which clearly suggests the dominance of decadal variability on the TIO temperature trends.

Stratocumulus in the Cold and Warm Sides of the Spring Kuroshio Sea Surface Temperature Front in the East China Sea

AbstractThe characteristics and formation mechanisms of stratocumulus (Sc) clouds were investigated in this study based on satellite and sounding data from March and April 2007–2010. The effects of the intensity of the Kuroshio sea surface temperature (SST) front on the amount and height of Sc clouds were also assessed by composite analysis. Sc clouds mainly occurred lower than 3 km as single‐layer clouds, and there was a larger amount of clouds in the warm side than in the cold side of the Kuroshio area. This difference occurred because the Kuroshio SST front produced different dynamics and thermal structures in the planetary boundary layer (PBL) of the warm and cold sides. Both the strong local sensible heat flux forced by strong northwesterly winds and the large relative humidity in the PBL favored the formation of Sc clouds in the warm side. In the cold side, both the strong temperature inversion caused by downdrafts and the PBL top height hindered cloud development, similar to the cloud formation conditions in the subtropical oceans near California. Furthermore, the amount and height of clouds were influenced by the Kuroshio SST front intensity. In the warm side, more abundant clouds and a higher cloud height were associated with a strong SST front background than with a weak SST front background.

Environmental drivers of oceanic foraging site fidelity in central place foragers

Finding food is crucial to the survival and reproductive success of individuals. Fidelity to previous profitable foraging sites may bring benefits to individuals as they can allocate more time to foraging rather than searching for prey. We studied how environmental conditions influence when lactating long-nosed fur seals (Arctocephalus forsteri) adopt a risky (low fidelity) or conservative (high fidelity) foraging strategy at two intra-annual temporal scales when foraging in a highly variable oceanic environment. Core foraging areas (CFAs; n = 534; 30 × 30 km cells) of consecutive foraging trips were obtained from geolocation tracks of 12 females from summer to winter in 2016 (n = 5) and 2017 (n = 7). We used the spatial variability (standard deviation) of CFAs between or among oceanic foraging trips as a proxy for individual foraging site fidelity (IFSF). Over the entire oceanic foraging period (n = 12), IFSF in the latitudinal axis increased with stronger sea-surface temperature gradient (SSTgrad), but decreased with greater SSTgrad and sea-surface height gradient variability. Over a period of two consecutive oceanic foraging trips (n = 66), IFSF decreased with greater SSTgrad variability in the earlier foraging trip. LNFS show evidence that they use IFSF as a strategy to potentially optimise food acquisition, and that this behaviour is influenced by mesoscale oceanographic parameters.

The ocean-atmosphere interaction over a summer upwelling system in the South China Sea

Abstract The ocean-atmosphere interaction over a summer upwelling system in the South China Sea (SCS) is investigated. A strong positive correlation between sea surface temperature (SST) and wind stress exists near the southeast coast of Vietnam, where strong SST fronts occur during summertime. This air-sea coupling relationship indicates that the ocean drives the atmosphere and receives feedback at the oceanic mesoscale, in contrast with the case for large scales. The coupling process off the southeast coast of Vietnam is initiated by coastal upwelling, which is driven by wind-induced offshore Ekman transport in early summer. Cold upwelled water reduces the wind stress aloft and results in a positive wind stress curl. The Ekman pumping induced by the wind stress curl is on average 18% stronger than that of the offshore Ekman transport and occupies a larger extent. As the upwelling season terminates by the end of summer, the air-sea coupling also diminishes. Our results reveal that the SST-induced wind stress curl anomalies have a positive feedback on SST change and that wind stress curl anomalies can further reduce the SST magnitude by as much as 0.5 °C. The factors influencing the strength and interannual variability in ocean-atmosphere interaction are also explored. The wind stress directional steadiness (WSDS) plays an important role in the air-sea coupling over the SCS SST frontal regions. The coupling between the wind stress and SST can fully develop under high WSDS. Furthermore, the interannual variabilities in air-sea coupling are largely impacted by the El Nino events by changing the regional wind. The influence of a strong El Nino event on the summer upwelling system is characterized by a warmer SST and declining winds, which are associated with weakened SST gradients and mesoscale air-sea coupling. The present study reveals that the interannual climate signals have a significant impact on the mesoscale ocean-atmosphere interaction in a typical upwelling system. It provides an example for investigation of seasonal and interannual variability of mesoscale air-sea coupling in other coastal regions.

Self‐Aggregation of Convection in Spatially Varying Sea Surface Temperatures

AbstractThe phenomenon of self‐aggregation of convection was first identified in convection‐permitting simulations of radiative convective equilibrium, characterized by homogeneous boundary conditions and in the absence of planetary rotation. In this study, we expose self‐aggregation of convection to more complex, nonhomogeneous boundary conditions and investigate its interaction with convective aggregation, as forced by large‐scale variations in sea surface temperatures (SSTs). We do this by conducting radiative convective equilibrium simulations on a spherical domain, with SST patterns that are zonally homogeneous but meridionally varying. Due to the meridional contrast in SST, a convergence line first forms, mimicking the Intertropical Convergence Zone. We nevertheless find that the convergence line breaks up and contracts zonally as a result of the self‐aggregation of convection. The contraction is significant, being here more than 50% of the original extent. The stability of the convergence line is controlled by the strength of the meridional circulation, which depends upon the imposed SST contrast. However, the process of self‐aggregation, once it is initiated, is insensitive to the strength of the SST contrast. The zonal contraction is accompanied by a slight meridional expansion and a moistening of the high latitudes, where SSTs are low. The moistening of the high latitudes can be understood from the fact that the convective cluster intensifies and expands its moist meridional low‐level outflow when it self‐aggregates zonally. Overall, our results suggest that the Intertropical Convergence Zone may be unstable to the self‐aggregation of convection, that self‐aggregation may serve as a precursor to the formation of atmospheric rivers, and that longer convergence lines are more likely to exist in regimes with strong SST gradients.

Impact of El Niño and El Niño Modoki Events on Indonesian Rainfall

On an interannual time-scale, the Indonesian climate is strongly influenced by the dynamics of Indo-Pacific climate modes. This study aims to investigate the possible impact of different types of El Niño events, namely, typical El Niño and El Niño Modoki, on Indonesian rainfall variability. Seasonal composite analysis was used to evaluate the magnitude and significance level of the influence of El Niño on the rainfall variability over the Indonesian region. Typical El Niño (hereafter El Niño) has a stronger influence on Indonesian rainfall than the El Niño Modoki during boreal autumn in September, October, and November when the events almost reach their peak. Cold sea surface temperature (SST) anomalies were observed in the Indonesian sea and in the western Pacific during El Niño years, while cold SST anomalies covered only the eastern Indonesian sea during El Niño Modoki years. Strong cold SST anomalies during El Niño years led to stronger low-level wind divergence over the Maritime Continent compared with that during El Niño Modoki years. In addition, the Walker circulation during El Niño years revealed a stronger downward motion over the Maritime Continent than that observed during El Niño Modoki years. This stronger downward motion (low-level wind divergence) during El Niño years reduced lower atmospheric water vapor and suppressed atmospheric convection over the Maritime Continent, leading to a significant decrease in rainfall. Similar situations were also observed during El Niño Modoki years but with a much weaker amplitude.

A New Picture of the Global Impacts of El Nino-Southern Oscillation

The El Nino-Southern Oscillation (ENSO) is the dominant interannual variability of Earth’s climate system and plays a central role in global climate prediction. Outlooks of ENSO and its impacts often follow a two-tier approach: predicting ENSO sea surface temperature anomaly in tropical Pacific and then predicting its global impacts. However, the current picture of ENSO global impacts widely used by forecasting centers and atmospheric science textbooks came from two earliest surface station datasets complied 30 years ago, and focused on the extreme phases rather than the whole ENSO lifecycle. Here, we demonstrate a new picture of the global impacts of ENSO throughout its whole lifecycle based on the rich latest satellite, in situ and reanalysis datasets. ENSO impacts are much wider than previously thought. There are significant impacts unknown in the previous picture over Europe, Africa, Asia and North America. The so-called “neutral years” are not neutral, but are associated with strong sea surface temperature anomalies in global oceans outside the tropical Pacific, and significant anomalies of land surface air temperature and precipitation over all the continents.

The Collective Contribution of Atmospheric and Oceanic Components to ENSO Asymmetry

Four cross-coupled models were used to investigate the relative contributions of atmospheric and oceanic components to the asymmetry of the El Niño–Southern Oscillation (ENSO). Strong El Niño and La Niña events related to the negative heat flux feedbacks were found to be determined mainly by the atmospheric component, and the stronger sea surface temperature (SST) anomalies in the warm phase did not lead to an increased SST asymmetry. The skewness of the four models could be affected by both atmospheric and oceanic components; the atmospheric component determines the strength of positive and negative SST anomalies, and the oceanic component affects the strength of the negative SST anomalies in the cold phase under the same atmospheric component group. The Bjerknes stability index (BJ index) of warm and cold phases contributed to the El Niño–La Niña SSTA asymmetries in observation, but the BJ index did not necessarily explain the El Niño–La Niña SSTA asymmetries in climate model simulations. The SST asymmetries in these four models were closely associated with convective precipitation and wind stress asymmetries, which are also determined by both the atmospheric and oceanic components.

Long‐Term Monitoring of the Brazil Current Transport at 22°S From XBT and Altimetry Data: Seasonal, Interannual, and Extreme Variability

AbstractThe seasonal and interannual variability of the Brazil Current (BC) at 22°S is examined using expendable bathythermograph (XBT) transect and satellite altimetry data from 1993 to 2017. The XBT‐based mean absolute geostrophic transport of the BC is estimated as 4.7 ± 1.9 Sv, with additional 0.9 ± 0.9 Sv along the shelf. The strong agreement between the absolute dynamic height and altimetric sea surface height is used in two methods to reconstruct a daily time series of the BC transport since 1993. The altimetry‐based methods can represent well the BC transport seasonal cycle, whereas the XBT‐based estimates are slightly aliased by the strong regional mesoscale variability. At interannual timescales, the BC transport is significantly correlated (r = 0.43) with the wind stress curl in the western half of the basin with a lag of 19 months, which is consistent with baroclinic adjustment timescales. Other sources of variability can be observed in a case study of the summer 2009/2010 event, which was characterized by strong sea surface temperature anomalies of approximately 3 °C. During the event, the BC reached 11 Sv for nearly 3 months, partly driven by an increased coastal upwelling from a cyclonic wind stress anomaly, a standing eddy along the section, and thermosteric anomalies that reached the offshore side of section in February. Heat anomalies were transported southward along the subtropical gyre following the BC path in a period of 2 months, which is consistent with advective timescales. Potential implications for extreme sea level and summer precipitation events in South America are discussed.

Alternating Decision Trees for Cloud Masking in MODIS and VIIRS NASA Sea Surface Temperature Products

AbstractIdentification and exclusion of clouds from satellite-based infrared fields is critical to achieve accurate retrievals of sea surface temperature (SST). Historically, identification of clouds has been driven primarily by a few uniformity tests involving a small number of pixels, brightness temperature range tests, and comparisons to low-resolution gap-free reference fields. Collectively these tests are adequate at identifying large, upper-level, very cold cumulus clouds, and uniformity tests identify moderately sized patchy cumulus clouds. But the efficacy of cloud identification often decreases at cloud edges, for small or thin cirrus clouds, and for the lower, more uniform stratus clouds, for which cloud-top temperature can be comparable to that of the sea surface, particularly at high latitudes. The heavy reliance on stringent uniformity thresholds often also has the unintended consequence of eliminating strong SST frontal regions from the pool of best-quality retrievals. This paper presents results for an ensemble cloud classifier based on a machine-learning approach, boosted alternating decision trees (ADtrees), applied to NASA MODIS and VIIRS SST imagery. The ADtree algorithm relies on the use of a majority vote from a collection of both “weak” and “strong” classifiers. This approach offers the potential to identify more cloud types and improve the retention of SST gradients in best-quality SST retrievals and also provides a per pixel confidence estimate in the classification.

Co-variation of the surface wind speed and the sea surface temperature over mesoscale eddies in the Gulf Stream region: momentum vertical mixing aspect

The co-variation of surface wind speed and sea surface temperature (SST) over the Gulf Stream frontal region is investigated using high-resolution satellite measurements and atmospheric reanalysis data. Results show that the pattern of positive SST-surface wind speed correlations is anchored by strong SST gradient and marine atmospheric boundary layer (MABL) height front, with active warm and cold-ocean eddies around. The MABL has an obvious transitional structure along the strong SST front, with greater (lesser) heights over the north (south) side. The significant positive SST-surface wind-speed perturbation correlations are mostly found over both strong warm and cold eddies. The surface wind speed increases (decreases) about 0.32 (0.41) m/s and the MABL elevates (drops) approximate 55 (54) m per 1°C of SST perturbation induced by warm (cold) eddies. The response of the surface wind speed to SST perturbations over the mesoscale eddies is mainly attributed to the momentum vertical mixing in the MABL, which is confirmed by the linear relationships between the downwind (crosswind) SST gradient and wind divergence (curl).