Strong Sea Surface Temperature Research Articles

Atmospheric Rivers over the Northwestern Pacific: Climatology and Interannual Variability

AbstractAtmospheric rivers (ARs), conduits of intense water vapor transport in the midlatitudes, are critically important for water resources and heavy rainfall events over the west coast of North America, Europe, and Africa. ARs are also frequently observed over the northwestern Pacific (NWP) during boreal summer but have not been studied comprehensively. Here the climatology, seasonal variation, interannual variability, and predictability of NWP ARs (NWPARs) are examined by using a large ensemble, high-resolution atmospheric general circulation model (AGCM) simulation and a global atmospheric reanalysis. The AGCM captures general characteristics of climatology and variability compared to the reanalysis, suggesting a strong sea surface temperature (SST) effect on NWPARs. The summertime NWPAR occurrences are tightly related to El Niño–Southern Oscillation (ENSO) in the preceding winter through Indo–western Pacific Ocean capacitor (IPOC) effects. An enhanced East Asian summer monsoon and a low-level anticyclonic anomaly over the tropical western North Pacific in the post–El Niño summer reinforce low-level water vapor transport from the tropics with increased occurrence of NWPARs. The strong coupling with ENSO and IPOC indicates a high predictability of anomalous summertime NWPAR activity.

Ensemble Prediction of Atmospheric Refractivity Conditions for EM Propagation

AbstractAn ensemble forecast system has been developed at the Naval Research Laboratory to improve the analyses and forecasts of atmospheric refractivity for electromagnetic (EM) propagation with the intention of accounting for uncertainties in model forecast errors. Algorithms for a matrix of ensemble statistics have been developed to analyze the probability, location, intensity, and structure of ducting of various types. Major parameters of ducting layers and their ensemble statistics are calculated from the ensemble forecasts. Their relationships to the large-scale and mesoscale environment are also investigated. The Wallops Island field experiment from late April to early May 2000 is selected to evaluate the system. During the spring season, this coastal region maintains a strong sea surface temperature gradient between cold shelf waters and the warm Gulf Stream, where the boundaries between land, the coastal water, and the Gulf Stream have a strong influence on marine boundary layer structures and the formation of ducting layers. Sounding profiles during the field experiment are used in the study to further understand the structures of the ducting layers and also to validate the ensemble forecast system. While some advantages of the ensemble system over the deterministic forecast for atmospheric refractivity prediction in the boundary layer are studied and demonstrated in this study, the weaknesses of the current ensemble system are revealed for future improvement of the system.

Simulated austral winter response of the Hadley circulation and stationary Rossby wave propagation to a warming climate

Ensemble simulations, using both coupled ocean–atmosphere (AOGCM) and atmosphere only (AGCM) general circulation models, are employed to examine the austral winter response of the Hadley circulation (HC) and stationary Rossby wave propagation (SRW) to a warming climate. Changes in the strength and width of the HC are firstly examined in a set of runs with idealized sea surface temperature (SST) perturbations as boundary conditions in the AGCM. Strong and weak SST gradient experiments (SG and WG, respectively) simulate changes in the HC intensity, whereas narrow (5°S–5°N) and wide (30°S–30°N) SST warming experiments simulate changes in the HC width. To examine the combined impact of changes in the strength and width of the HC upon SRW propagation two AOGCM simulations using different scenarios of increasing carbon dioxide (CO2) concentrations are employed. We show that, in contrast to a wide SST warming, the atmospheric simulations with a narrow SST warming produce stronger and very zonally extended Rossby wave sources, leading to stronger and eastward shifted troughs and ridges. Simulations with SST anomalies, either in narrow or wide latitude bands only modify the intensity of the troughs and ridges. SST anomalies outside the narrow latitude band of 5°S–5°N do not significantly affect the spatial pattern of SRW propagation. AOGCM simulations with 1 %/year increasing CO2 concentrations or 4 times preindustrial CO2 levels reveal very similar SRW responses to the atmospheric only simulations with anomalously wider SST warming. Our results suggest that in a warmer climate, the changes in the strength and width of the HC act in concert to significantly alter SRW sources and propagation characteristics.

The South Atlantic Anticyclone as a key player for the representation of the tropical Atlantic climate in coupled climate models

The key role of the South Atlantic Anticyclone (SAA) on the seasonal cycle of the tropical Atlantic is investigated with a regionally coupled atmosphere–ocean model for two different coupled domains. Both domains include the equatorial Atlantic and a large portion of the northern tropical Atlantic, but one extends southward, and the other northwestward. The SAA is simulated as internal model variability in the former, and is prescribed as external forcing in the latter. In the first case, the model shows significant warm biases in sea surface temperature (SST) in the Angola-Benguela front zone. If the SAA is externally prescribed, these biases are substantially reduced. The biases are both of oceanic and atmospheric origin, and are influenced by ocean–atmosphere interactions in coupled runs. The strong SST austral summer biases are associated with a weaker SAA, which weakens the winds over the southeastern tropical Atlantic, deepens the thermocline and prevents the local coastal upwelling of colder water. The biases in the basins interior in this season could be related to the advection and eddy transport of the coastal warm anomalies. In winter, the deeper thermocline and atmospheric fluxes are probably the main biases sources. Biases in incoming solar radiation and thus cloudiness seem to be a secondary effect only observed in austral winter. We conclude that the external prescription of the SAA south of 20°S improves the simulation of the seasonal cycle over the tropical Atlantic, revealing the fundamental role of this anticyclone in shaping the climate over this region.

How distinct are the two flavors of El Niño in retrospective forecasts of Climate Forecast System version 2 (CFSv2)?

Two different flavors of El Nino-Southern Oscillation (ENSO, canonical east Pacific type and Modoki/central Pacific type) are reported in the recent decades and are found to influence the global climate in different ways. The success of a seasonal prediction system is dependent on its ability to capture these two ENSO flavors accurately, together with associated teleconnections. The present study analyses the ability of Climate Forecast System version 2 (CFSv2) in simulating the two El Nino flavors and their teleconnections. The study uses two versions of CFSv2 in which the atmospheric model horizontal resolutions are different (T126 at 100 km and T382 at 38 km) and are initialized from a calendar month, ranging from February to June. The canonical ENSO pattern is captured as prominent mode of tropical Pacific sea surface temperature (SST) by both resolutions of CFSv2. Even though the tri-polar structure of ENSO Modoki is simulated as second mode, it has some disagreement with observations. The canonical El Nino induced SST, rainfall and atmospheric circulation in the tropical Pacific in summer and fall seasons are comparable with observations in both models. Meanwhile, the teleconnections in the tropical Indian Ocean and Indian monsoon regions are close to observations in T382 only. Teleconnections associated with El Nino Modoki are proper in T382 hindcasts, in which SST bias in Indian Ocean is slightly warm and the cold bias in central Pacific is marginal (mainly for Feb IC hindcasts). The present study indicates that the distinction of ENSO flavors in summer is the major reason for the higher skill of Indian summer monsoon rainfall (ISMR) in CFSv2 T382 Feb IC hindcasts. However, teleconnections associated with two flavors of ENSO are not distinguishable in fall and winter seasons, even in higher resolution model due to the presence of strong cold SST bias in central Pacific and warm SST bias in extreme east Pacific. Thus present study confirms that, higher resolution CFSv2 is required to differentiate the flavors of ENSO and their teleconnections properly. It has better prediction skill for ISMR at a lead time of 4 months, which is significant for seasonal prediction of Indian summer monsoon.

Steady State Ocean Response to Wind Forcing in Extratropical Frontal Regions.

In regions of strong sea surface temperature (SST) gradients, the surface “geostrophic” currents have a vertical shear aligned with the surface density front defined by the temperature. This surface geostrophic (“thermal wind”) shear can balance a portion of the surface wind stress, altering the classic Ekman response to wind forcing. Here we show that these frontal effects cannot be ignored in the Tropics or in strong frontal regions in the extratropics, such as found in coastal regions and in western boundary currents of all basins. Frontal effects also dominate the classic Ekman response in the regions of both hemispheres where Trade winds change to westerlies. Implications for vertical motion and global heat transport are discussed.

Indian summer monsoon rainfall variability in response to differences in the decay phase of El Niño

In general the Indian summer monsoon (ISM) rainfall is near normal or excess during the El Nino decay phase. Nevertheless the impact of large variations in decaying El Nino on the ISM rainfall and circulation is not systematically examined. Based on the timing of El Nino decay with respect to boreal summer season, El Nino decay phases are classified into three types in this study using 142 years of sea surface temperature (SST) data, which are as follows: (1) early-decay (ED; decay during spring), (2) mid-summer decay (MD; decay by mid-summer) and (3) no-decay (ND; no decay in summer). It is observed that ISM rainfall is above normal/excess during ED years, normal during MD years and below normal/deficit in ND years, suggesting that the differences in El Nino decay phase display profound impact on the ISM rainfall. Tropical Indian Ocean (TIO) SST warming, induced by El Nino, decays rapidly before the second half of the monsoon season (August and September) in ED years, but persists up to the end of the season in MD years, whereas TIO warming maintained up to winter in ND case. Analysis reveals the existence of strong sub-seasonal ISM rainfall variations in the summer following El Nino years. During ED years, strong negative SST anomalies develop over the equatorial central-eastern Pacific by June and are apparent throughout the summer season accompanied by anomalous moisture divergence and high sea level pressure (SLP). The associated moisture convergence and low SLP over ISM region favour excess rainfall (mainly from July onwards). This circulation and rainfall anomalies are highly influenced by warm TIO SST and Pacific La Nina conditions in ED years. Convergence of southwesterlies from Arabian Sea and northeasterlies from Bay of Bengal leads to positive rainfall over most part of the Indian subcontinent from August onwards in MD years. ND years are characterized by negative rainfall anomaly spatial pattern and weaker circulation over India throughout the summer season, which are mainly due to persisting El Nino related warm SST anomalies over the Pacific. Atmospheric general circulation model simulation supports our hypothesis that El Nino decay variations modulate ISM rainfall and circulation.

Understanding the double peaked El Niño in coupled GCMs

Coupled general circulation models (CGCMs) simulate a diverse range of El Nino–Southern Oscillation behaviors. “Double peaked” El Nino events—where two separate centers of positive sea surface temperature (SST) anomalies evolve concurrently in the eastern and western equatorial Pacific—have been evidenced in Coupled Model Intercomparison Project version 5 CGCMs and are without precedent in observations. The characteristic CGCM double peaked El Nino may be mistaken for a central Pacific warming event in El Nino composites, shifted westwards due to the cold tongue bias. In results from the Australian Community Climate and Earth System Simulator coupled model, we find that the western Pacific warm peak of the double peaked El Nino event emerges due to an excessive westward extension of the climatological cold tongue, displacing the region of strong zonal SST gradients towards the west Pacific. A coincident westward shift in the zonal current anomalies reinforces the western peak in SST anomalies, leading to a zonal separation between the warming effect of zonal advection (in the west Pacific) and that of vertical advection (in the east Pacific). Meridional advection and net surface heat fluxes further drive growth of the western Pacific warm peak. Our results demonstrate that understanding historical CGCM El Nino behaviors is a necessary precursor to interpreting projections of future CGCM El Nino behaviors, such as changes in the frequency of eastern Pacific El Nino events, under global warming scenarios.

Sea surface height anomaly and upper ocean temperature over the Indian Ocean during contrasting monsoons

Recent research emphasizes the importance of the oceanic feedback to monsoon rainfall over the Asian landmass. In this study, we investigate the differences in the sea surface height anomaly (SSHA) and upper ocean temperature over the tropical Indian Ocean during multiple strong and weak monsoons. Analysis of satellite derived SSHA, sea surface temperature (SST) and ocean reanalysis data reveals that patterns of SSHA, SST, ocean temperature, upper ocean heat content (UOHC) and propagations of Kelvin and Rossby waves differ during strong and weak monsoon years. During strong monsoons positive SSH, SST and UOHC anomalies develop over large parts of north Indian Ocean whereas during weak monsoons much of the north Indian Ocean is covered with negative anomalies. These patterns can be used as a standard tool for evaluating the performance of coupled and ocean models in simulating & forecasting strong and weak monsoons. The rainfall over central India is found to be significantly correlated with SSHA over the regions (Arabian Sea and West central Indian Ocean and Bay of Bengal) where SSHA is positively large during strong monsoons. The SST-SSHA correlation is also very strong over the same area. The study reveals that much convection takes place over these regions during strong monsoons. In contrast during weak monsoons, convection takes place over eastern equatorial region. These changes in SST are largely influenced by oceanic Kelvin and Rossby waves. The Rossby waves initiated in spring at the eastern boundary propagate sub-surface heat content in the ocean influencing SST in summer. The SST anomalies modulate the Hadley circulation and the moisture transport thereby contributing to rainfall over central India. Therefore oceanic Kelvin and Rossby waves influence the rainfall over central India.

Response of the Baiu Rainband to Northwest Pacific SST Anomalies and Its Impact on Atmospheric Circulation

Abstract The Pacific–Japan (PJ) teleconnection pattern leads to a meridional precipitation dipole between the subtropics and East Asia in the summer of El Niño decaying years. However, observational analysis and model experiments indicate that increased sea surface temperature (SST) in the Kuroshio–Oyashio Extension (KOE) region, where a strong northward-decreasing SST gradient occurs, induces a northward shift of baiu rainfall with the subtropical jet, forming a tripolar precipitation anomaly pattern over the northwest Pacific. In July, the leading empirical orthogonal function (EOF) mode for precipitation represents the meridional dipole between subtropical and baiu rainfalls, while EOF-2 features the northward-shifted baiu rainband. The PJ atmospheric forcing increases KOE SST, which enhances northward shifts in the subtropical jet and baiu rainband, resulting in oceanic forcing over the KOE. In experiments using a regional climate model the subtropical jet response to KOE SST forcing explains about 20% of the anomalies in the control experiment. The northward-shifted baiu condensational heating also enhances the northward shift of the subtropical jet with a northeastward extension. In August when the baiu rainband disappears, the northward shift of the subtropical jet is enhanced mainly by KOE SST variability. During the summer, as a result of increased KOE SST and the northward-shifted baiu heating, cyclonic anomalies intensify over the Sea of Okhotsk and anticyclonic anomalies intensify over the northwest Pacific, affecting the western Pacific subtropical high. The results suggest that air–sea interaction over the KOE must be considered to better understand the summer northwest Pacific climate.

Daily States of the March–April East Pacific ITCZ in Three Decades of High-Resolution Satellite Data

Abstract Zonally elongated areas of cloudiness that make up the east Pacific intertropical convergence zone (ITCZ) can take on several configurations in instantaneous observations. A novel statistical model is used to automatically assess the daily state of the east Pacific ITCZ using infrared satellite images from 1980 to 2012. Four ITCZ states are defined based on ITCZ location relative to the equator: north (nITCZ) and south (sITCZ) of the equator, simultaneously north and south of the equator (dITCZ, for double ITCZ), and over the equator (eITCZ). A fifth ITCZ state is used to classify days when no zonally elongated area of cloudiness is present (aITCZ, for absent ITCZ). The ITCZ states can occur throughout the year (except for the eITCZ, which is not present during June–October), with the nITCZ state dominating in terms of frequency of occurrence. Interannual variability of the state distribution is large. The most striking variability in ITCZ states is observed in spring. During March–April, the dITCZ state occurs on average 34% of the time, second only to the nITCZ state (39%). Composites of observed infrared temperature and precipitation by ITCZ state reveal distinct spatial configurations of cloudiness and rainfall. Strong sea surface temperature anomalies are associated only with eITCZ and sITCZ and they correspond to El Niño and La Niña, respectively. However, all five ITCZ states are associated with distinct atmospheric circulation patterns. A connection is found between the ITCZ and the South Pacific convergence zone (SPCZ), such that activity in the SPCZ is enhanced when the ITCZ is absent in the east Pacific.

Coral record of southeast Indian Ocean marine heatwaves with intensified Western Pacific temperature gradient.

Increasing intensity of marine heatwaves has caused widespread mass coral bleaching events, threatening the integrity and functional diversity of coral reefs. Here we demonstrate the role of inter-ocean coupling in amplifying thermal stress on reefs in the poorly studied southeast Indian Ocean (SEIO), through a robust 215-year (1795–2010) geochemical coral proxy sea surface temperature (SST) record. We show that marine heatwaves affecting the SEIO are linked to the behaviour of the Western Pacific Warm Pool on decadal to centennial timescales, and are most pronounced when an anomalously strong zonal SST gradient between the western and central Pacific co-occurs with strong La Niña's. This SST gradient forces large-scale changes in heat flux that exacerbate SEIO heatwaves. Better understanding of the zonal SST gradient in the Western Pacific is expected to improve projections of the frequency of extreme SEIO heatwaves and their ecological impacts on the important coral reef ecosystems off Western Australia.

Arabian Sea SST evolution during spring to summer transition period and the associated processes in coupled climate models

ABSTRACTMany climate models have problems in simulating the sea surface temperature (SST) in the tropical Indian Ocean (TIO). The Coupled Model Inter‐comparison Project Phase 5 (CMIP5) models, in general, underestimate SST over the entire TIO region. This study examines the SST evolution during spring to summer transition months (May and June) over the Arabian Sea (AS) region in the historical simulations of 13 CMIP5 models and the Climate Forecasting System coupled models CFSv1 and CFSv2. The annual cycle of SST shows that the summer monsoon cooling is not adequately captured by many models. Based on the state of June SST tendency, models have been divided in to three groups, the first group (G1) consists of models having stronger than observed cooling, second group (G2) considers models having closer to observed cooling and the third group (G3) includes models having lesser than observed cooling. Mixed layer heat budget analysis revealed that atmospheric flux is mainly responsible for unrealistic SST warming in most of the G3 models during June. The vertical mixing and horizontal advection contribute considerably to the SST cooling in summer (June) especially for G1 and G2 models. On the other hand, spring warming in all the models is consistently forced by the surface heat flux. It is also found that the monsoon low‐level jet (LLJ) is not accurately represented in most of the models. The misrepresentation of LLJ causes bias in the oceanic processes leading to unrealistic SST evolution in many models. One way of LLJ affecting the oceanic processes is by modulating mixed layer depth (MLD). It is observed in general that the models with deeper MLD display strong SST cooling. The model deficiency in representing AS SST is speculated to be a major limiting factor in capturing the monsoon rainfall in the current coupled models. The proper simulation of AS SST is therefore very crucial for the accurate representation of Indian summer monsoon precipitation.

The North China/Northeastern Asia Severe Summer Drought in 2014

Abstract In summer 2014, north China and large areas of northeastern Asia (NCNEA) suffered from the most severe drought of the past 60 years. This study indicates that the East Asian summer precipitation in 2014 exhibited a tripole anomaly, with severe negative anomalies in NCNEA, strong positive anomalies in south China, South Korea, and Japan, and intense negative anomalies in the western North Pacific. Along with the severe tripole precipitation anomalies, there were strong intensities of the Silk Road pattern, the Pacific–Japan pattern, and the Eurasian teleconnection pattern, which were responsible for the strong precipitation anomaly in 2014 through changes to the western Pacific subtropical high (WPSH) and the East Asian trough. Further analysis indicates that the sea surface temperature (SST) in the North Pacific was nearly the warmest in the past 60 years and, together with the strong SST warming in the warm pool region, thus caused the strong Pacific–Japan teleconnection pattern, southward positioning of the WPSH, and weakened East Asian summer monsoon. Additionally, the summertime sea ice cover in the Arctic Ocean was anomalous, resulting in high SST in the Laptev–Kara Sea and, hence, triggering a strong Eurasian teleconnection pattern and contributing to the severe drought of NCNEA. Furthermore, the intense warming over the European Continent and Caspian Sea favored the Silk Road pattern, also contributing to the southward positioning of the WPSH and the NCNEA drought. The NCNEA severe drought was therefore the joint result of Pacific SST anomalies, Arctic sea ice anomalies, and warming over the European Continent and Caspian Sea.

Sensitivity Difference in the Extratropical Atmosphere to Two Types of El Niño Events

AbstractA comparison of sensitivity in extratropical circulation in the Northern Hemisphere (NH) and Southern Hemisphere (SH) is conducted through observational analyses and diagnostic linear model experiments for two types of El Nino events, the traditional El Nino with the strongest warmth in the eastern tropical Pacific (EP El Nino) and the El Nino Modoki with the strongest warmth in the central tropical Pacific (CP El Nino). It is shown that CP El Nino favors the occurrence of a negative-phase Northern Annular Mode (NAM), while EP El Nino favors that of the Pacific-North American (PNA) pattern. In SH, both EP and CP El Nino induce a negative phase Southern Annular Mode (SAM). However, the former has a greater amplitude, which is consistent with the stronger sea surface temperature (SST) warmth. The difference in the two types of El Nino events in NH may originate from the dependence of heating-induced extratropical response on the location of initial heating, which may be associated with activity of t...

On the year-to-year changes of the Iberian Poleward Current

The results of a 20 year high-resolution simulation that spans from 1989 to 2008 are analyzed to study the year-to-year changes of the Iberian Poleward Current (IPC), and its effects on the temperature and salinity variability on the Western Iberian Margin. The model results are compared with satellite data and with data measured at two moored multiparametric buoys. The model reproduces the events of strong sea surface temperature (SST) anomalies described in the literature, for the northern and western coasts, and it helps to explain the connection between the IPC intensity, temperature, and salinity. By analyzing some specific winters, with different characteristics, it is confirmed that years of stronger IPC result in higher transport of heat and salt, and the development of positive anomalies of temperature and salinity. However, local air-sea fluxes are also important and explain the temperature and salinity anomalies observed in some of the winters. The interplay between the IPC transport, or advection, and the local heat and salt fluxes explain why the temperature and salinity anomalies may be in antiphase or uncorrelated with the IPC magnitude. It is shown that from November to January, the IPC magnitude depends mostly on the intensity of the southerly winds, and it has a significant negative correlation with the NAO index.

Influence of extratropical sea‐surface temperature on the Indian summer monsoon: an unexplored source of seasonal predictability

Based on extensive analysis of observations and a series of climate model experiments, here we establish that slow variations of northern hemispheric extratropical sea‐surface temperature (SST) anomalies can augment seasonal predictability of the south Asian monsoon. The SST conditions and performance of the south Asian monsoon during 2013 boreal summer months (June–September) led us to hypothesize that the strong extratropical SST anomalies in the North Pacific and North Atlantic in conjunction with weak tropical SST anomalies (weak La Niña) were responsible for the above‐normal rainfall over India during 2013. We also argue that the 2013 SST pattern and above‐normal monsoon condition are not unique but occurred on several occasions in the past. Further, we show that there is a complementary pattern of strong extratropical SST and weak tropical SST that is associated with below‐normal south Asian monsoon rainfall. We also show that the extratropical SST pattern in the Northern Hemisphere is associated with a low‐frequency interdecadal mode of variability indicating potential predictability associated with such extratropical SST forcing. Extensive experiments with an atmospheric general circulation model forced by such SST conditions elucidate the mechanism through which the extratropical SSTs influence the Indian monsoon. The SST anomalies affect the north–south temperature gradient and lead to a local displacement of the jet stream, setting up a quasi‐stationary wave. Such a stationary wave, in turn, affects the tropospheric temperature (TT) over southern Eurasia, influencing the north–south TT gradient in the region and thereby the Indian monsoon. Our discovery of this additional source of potential predictability together with the fact that the new‐generation coupled ocean–atmosphere models are capable of predicting the extratropical SST anomalies brightens the prospect of south Asian monsoon prediction.

The influence of shelf-sea fronts on winter monsoon over East China Sea

Strong sea surface temperature fronts in open seas are known to affect the atmosphere. Shelf-sea fronts in winter have comparable strengths, yet their impacts on winds have not been studied. In January of 2012, a persistent, narrow band of cloud stretching 600–1,000 km was observed along the front of East China Sea (ECS). Numerical and analytical models show that the cloud was formed atop a recirculating cell induced by the front and, more generally, that β-plumes of low and high pressures emanate and spread far from fronts. Consistent with the theory, observations show that in ECS at inter-annual time scales, strong fronts co-vary with on-shelf convergent wind, strong northeasterly monsoon, and alongshelf alignment of clouds with low clouds near the coast and higher clouds offshore. Our results suggest that shelf-sea fronts are potentially an important dynamic determinant of climate variability of East Asia.

Dynamic and thermal responses of the Kuroshio to typhoon Megi (2004)

AbstractRelatively weak hurricane‐induced cooling (<1°C) is often observed in the Loop Current, while observed sea surface temperature (SST) cooling is much stronger in the Kuroshio. This was previously attributed to strong advection effects by the Loop Current. Based on a budget analysis, this study quantitatively diagnoses the dynamic and thermal responses of the Kuroshio to typhoon Megi and four additional tropical cyclones (TCs), which produce up to ~3°C SST cooling within the Kuroshio itself. The results show that the surface thermal response of the Kuroshio is a wind‐induced temperature cooling primarily due to vertical mixing (up to 96%). The dynamic response of the Kuroshio depends not only on TC conditions (intensity and wind direction) but also on prestorm ocean conditions (e.g., ocean mixed layer depth, baroclinicity, and current velocity). Differently from the open ocean where the ocean gains kinetic energy mainly from wind stress work, the Kuroshio can obtain greater kinetic energy from its baroclinic pressure work, especially in the upper ocean. The strong SST cooling observed in the Kuroshio region is therefore likely associated with the kinetic energy extracted from the Kuroshio baroclinic potential energy, which greatly enhances local vertical mixing.

Modelling the impact of Baltic Sea upwelling on the atmospheric boundary layer

Coastal upwelling, with a strong sea-surface temperature (SST) signal, is extremely common in the Baltic Sea during the summer months. Although the spatial scale of upwelling is small, its high frequency of occurrence in the semi-enclosed basin may allow the SST signature to have significant feedback onto the lower atmosphere. In this paper, we develop a method to remove the signature of upwelling from SST fields, and use these modified SST fields as the lower boundary condition of an atmospheric model, allowing us to evaluate the importance of coastal upwelling to the mean summer conditions over the Baltic. The contribution of upwelling is found to be small generally, although significant locally over areas where upwelling is most common. This includes 2 m air temperature reductions of up to 2°C, wind speed reductions of up to 0.25 m s−1, and reductions in the mean boundary-layer height of up to 100 m. High-resolution simulations with artificially enhanced upwelling suggest that upwelling may become increasingly important as winds slacken at the end of an upwelling period.