Atmospheric Heat Fluxes Research Articles

Relationship of convective precipitation with atmospheric heat flux — A regression approach over an Indian tropical location

The response of atmospheric heat fluxes and sea surface temperatures on the convective precipitation over the tropics has been an important area of research in recent decades. A long-term observation (1979–2008) of the increase in convective precipitation in relation to the latent and sensible heat fluxes on a tropical location, Kolkata, has been investigated in the present study. Invigoration of convective precipitation has been caused by vertically integrated divergence of moisture flux, rise in sea surface temperatures, convective cloud cover and surface evaporation rate over the tropical region. A convective precipitation estimation (CPE) index is proposed, considering the Bowen ratio, surface evaporation rate, sea surface temperature and temperatures at 500hpa pressure level during the pre-monsoon season (March–May), to estimate the amount of convective precipitation over the tropics using multiple linear regression technique is also another aim of this study. A good agreement is obtained between the results from the proposed model and the MERRA observations during the years 2009–2013.

Heat Flux Controlled Pool Boiling of Zirconia–Water and Silver–Water Nanofluids on a Flat Plate: A Coupled Map Lattice Simulation

In the present work, the characteristic atmospheric saturated heat flux controlled pool boiling curves for zirconia–water and silver–water nanofluids have been reproduced by the coupled map lattice (CML) method using a two-dimensional (2D) boiling field model. The heater is a long horizontal flat plate of thickness 0.44 mm. The pool height is 0.7 mm. The stirring action of the bubbles is modeled by increasing the fluid thermal diffusivity by an enhancement factor. The thermal conduction in the plate is also incorporated into the model. The basic advantage of CML is that individual bubbles are not tracked, and yet the effects of bubbles are reflected qualitatively in the final solution. In the simulation of atmospheric saturated pool boiling of water minimum cavity diameter taken is 0.8 μm based on which a random distribution of cavity sizes has been specified. In the boiling of ZrO2–water nanofluid there is a deposition of nanoparticles in the cavities on the heated surface resulting in reduction of surface roughness. This feature is taken care of by proportionate decrease in minimum cavity diameter. The CML model predicts decrease in heat transfer coefficient and increase in critical heat flux (CHF) with increase in zirconia nanoparticle concentration. In the case of Ag–water nanofluid no such deposition of nanoparticles has been reported; rather surface oxidation occurs which increases the surface roughness. This is simulated by proportionately increasing the minimum cavity diameter with weight fractions of nanoparticles. The present CML model predicts increase in the heat transfer coefficient and decrease in CHF with increase in silver nanoparticle concentration. Thus, the CML results for the boiling of the aforesaid two nanofluids match qualitatively with the published experimental works.

Helicopter-borne observations with portable microwave radiometer in the Southern Ocean and the Sea of Okhotsk

AbstractAccurately measuring and monitoring the thickness distribution of thin ice is crucial for accurate estimation of ocean–atmosphere heat fluxes and rates of ice production and salt flux in ice-affected oceans. Here we present results from helicopter-borne brightness temperature (TB) measurements in the Southern Ocean in October 2012 and in the Sea of Okhotsk in February 2009 carried out with a portable passive microwave (PMW) radiometer operating at a frequency of 36 GHz. The goal of these measurements is to aid evaluation of a satellite thin-ice thickness algorithm which uses data from the spaceborne Advanced Microwave Scanning Radiometer–Earth Observing System AMSR-E) or the Advanced Microwave Scanning Radiometer-II (AMSR-II). AMSR-E and AMSR-II TB agree with the spatially collocated mean TB from the helicopter-borne measurements within the radiometers’ precision. In the Sea of Okhotsk in February 2009, the AMSR-E 36GHz TB values are closer to the mean than the modal TB values measured by the helicopter-borne radiometer. In an Antarctic coastal polynya in October 2012, the polarization ratio of 36GHz vertical and horizontal TB is estimated to be 0.137 on average. Our measurements of the TB at 36 GHz over an iceberg tongue suggest a way to discriminate it from sea ice by its unique PMW signature.

The Seasonality of Convective Events in the Labrador Sea

Abstract Modeling deep convection is a key challenge for climate science. Here two simulations of the Labrador Sea circulation obtained with the Regional Ocean Modeling System (ROMS) run at a horizontal resolution of 7.5 km are used to characterize the response of convection to atmospheric forcing and its seasonal variability over the period 1980–2009. The integrations compare well with the sparse observations available. The modeled convection varies in three key aspects over the 30 years considered. First, its magnitude changes greatly at decadal scales. This aspect is supported by the in situ observations. Second, the initiation and peak of convection (i.e., initiation and maximum) shift by 2–3 weeks between strong and weak convective years. Third, the duration of convection varies by approximately one month between strong and weak years. The last two changes are associated with the variability of the time-integrated surface heat fluxes over the Labrador Sea during winter and spring, while the first results from changes in both atmospheric heat fluxes and oceanic conditions through the lateral inflow of warm Irminger Water from the boundary current system to the basin interior. Changes in surface heat fluxes over the convective region are linked to large-scale modes of variability, the North Atlantic Oscillation and Arctic Oscillation. Implications for modeling the climate variability of the Labrador basin are discussed.

Recent climatic trends in the tropical Atlantic

A homogeneous monthly data set of sea surface temperature (SST) and pseudo wind stress based on in situ observations is used to investigate the climatic trends over the tropical Atlantic during the last five decades (1964–2012). After a decrease of SST by about 1 °C during 1964–1975, most apparent in the northern tropical region, the entire tropical basin warmed up. That warming was the most substantial (>1 °C) in the eastern tropical ocean and in the longitudinal band of the intertropical convergence zone. Surprisingly, the trade wind system also strengthened over the peirod 1964–2012. Complementary information extracted from other observational data sources confirms the simultaneity of SST warming and the strengthening of the surface winds. Examining data sets of surface heat flux during the last few decades for the same region, we find that the SST warming was not a consequence of atmospheric heat flux forcing. Conversely, we suggest that long-term SST warming drives changes in atmosphere parameters at the sea surface, most notably an increase in latent heat flux, and that an acceleration of the hydrological cycle induces a strengthening of the trade winds and an acceleration of the Hadley circulation. These trends are also accompanied by rising sea levels and upper ocean heat content over similar multi-decadal time scales in the tropical Atlantic. Though more work is needed to fully understand these long term trends, especially what happens from the mid-1970’s, it is likely that changes in ocean circulation involving some combination of the Atlantic meridional overtuning circulation and the subtropical cells are required to explain the observations.

Coherent sea level variation in and around the Sea of Okhotsk

We investigated the seasonal and interannual variations of the sea level in and around the Sea of Okhotsk and their causes, based on tide gauge and satellite altimeter data. The sea level all along the coastal region of the Sea of Okhotsk is found to be dominated by the seasonal variation with a maximum in winter and a minimum in summer, which cannot be explained by the annual cycle of atmospheric heat flux and pressure. This sea level variation appears to reflect ocean current variations. Both the Arrested Topographic Waves (ATWs) caused by alongshore wind stress and the Sverdrup transport by wind stress curl show corresponding seasonal variations. Seasonal amplitude of the sea level is relatively large along Sakhalin Island with a tendency of a larger amplitude toward the south. This meridional dependence is consistent with the ATWs, but not with the Sverdrup transport in the Sea of Okhotsk. Seasonal variation of the geostrophic current velocity expected from the sea level variation is comparable to that of the observed nearshore current and is consistent with the theoretical ATW transport. It is also revealed that, on an interannual timescale, the wintertime sea level fluctuates quite coherently all around the Sea of Okhotsk and further along the East Kamchatka and Oyashio coasts in the North Pacific. The altimeter data clearly show that this coherent sea level variation is trapped over the coastal and continental shelf regions with depths shallower than 1000m. The wintertime sea levels have a higher correlation with the ATW transport than with the Sverdrup transport in the Sea of Okhotsk and the upstream East Kamchatka coast. All these suggest that the interannual sea level variation along the coastal and shelf regions in winter, as well as the seasonal variation, is mainly caused by the ATWs (coastal trapped current forced by the alongshore wind stress). The wintertime Sverdrup transport, raised by the previous studies, is the secondary contributor to these variations.

A control algorithm for statistically stationary large-eddy simulations of thermally stratified boundary layers

Large-eddy simulations (LESs) of thermally stratified atmospheric boundary layers (ABLs) typically involve imposed heating or cooling at the ground surface, leading to slowly varying temperature distributions in the domain. While for some applications an interpretation of the resulting fields as quasi-stationary is possible, there are certain applications for which the slow temporal variations are a serious drawback. An example occurs when using LES for a systematic analysis of effects of large wind farms on atmospheric heat fluxes, where analysis of stationary statistics greatly facilitates understanding. In order to obtain statistically stationary temperature fields in LES of thermally stratified boundary layers, in this article we introduce a control algorithm, and test its effectiveness for various levels of surface heating and cooling. The method uses a ‘proportional-integral’ (PI) control algorithm, designed to keep constant the horizontally averaged temperature in a computational fringe layer placed above the ABL region of interest. For flows driven by a geostrophic wind, another controller is used to adjust the mean flow angle within the ABL at a specified height to achieve a prescribed direction. This is done by controlling a source term (in the form of an additional Coriolis force) in the momentum equations. This term is deactivated once the statistics become stationary and the flow aligns with the desired direction. Together, these two control schemes enable very long-time simulations of thermally stratified heated or cooled boundary layers displaying fully stationary distributions of mean velocity and temperature.

Analysis of the Slab Ocean El Nino atmospheric feedbacks in observed and simulated ENSO dynamics

In a recent study it was illustrated that the El Nino Southern Oscillation (ENSO) mode can exist in the absence of any ocean dynamics. This oscillating mode exists just due to the interaction between atmospheric heat fluxes and ocean heat capacity. The primary purpose of this study is to further explore these atmospheric Slab Ocean ENSO dynamics and therefore the role of positive atmospheric feedbacks in model simulations and observations. The positive solar radiation feedback to sea surface temperature (SST), due to reduced cloud cover for anomalous warm SSTs, is the main positive feedback in the Slab Ocean El Nino dynamics. The strength of this positive cloud feedback is strongly related to the strength of the equatorial cold tongue. The combination of positive latent and sensible heat fluxes to the west and negative ones to the east of positive anomalies leads to the westward propagation of the SST anomalies, which allows for oscillating behavior with a preferred period of 6-7 years. Several indications are found that parts of these dynamics are indeed observed and simulated in other atmospheric or coupled general circulation models (AGCMs or CGCMs). The CMIP3 AGCM-slab ensemble of 13 different AGCM simulations shows unstable ocean-atmosphere interactions along the equatorial Pacific related to stronger cold tongues. In observations and in the CMIP3 and CMIP5 CGCM model ensemble the strength and sign of the cloud feedback is a function of the strength of the cold tongue. In summary, this indicates that the Slab Ocean El Nino dynamics are indeed a characteristic of the equatorial Pacific climate that is only dominant or significantly contributing to the ENSO dynamics if the SST cold tongue is sufficiently strong. In the observations this is only the case during strong La Nina conditions. The presence of the Slab Ocean ENSO atmospheric feedbacks in observations and CGCM model simulations implies that the family of physical ENSO modes does have another member, which is entirely driven by atmospheric processes and does not need to have the same spatial pattern nor the same time scales as the main ENSO dynamics.

High-resolution temperature sensing in the Dead Sea using fiber optics

The thermal stratification of the Dead Sea was observed in high spatial and temporal resolution by means of fiber-optics temperature sensing. The aim of the research was to employ the novel high-resolution profiler in studying the dynamics of the thermal structure of the Dead Sea and the related processes including the investigation of the metalimnion fluctuations. The 18 cm resolution profiling system was placed vertically through the water column supported by a buoy 450 m from shore, from 2 m above to 53 m below the water surface (just above the local seafloor), covering the entire seasonal upper layer (the metalimnion had an average depth of ∼20 m). Temperature profiles were recorded every 5 min. The May to July 2012 data set allowed quantitative investigation of the thermal morphology dynamics, including objective definitions of key locations within the metalimnion based on the temperature depth profile and its first and second depth derivatives. Analysis of the fluctuation of the defined metalimnion locations showed strong anticorrelation to measured sea level fluctuations. The slope of the sea level versus metalimnion depth was found to be related to the density ratio of the upper layer and the underlying main water body, according to the prediction of a two-layer model. The heat content of the entire water column was calculated by integrating the temperature profiles. The vertically integrated apparent heat content was seen to vary by 50% in a few hours. These fluctuations were not correlated to the atmospheric heat fluxes, nor to the momentum transfer, but were highly correlated to the metalimnion and the sea level fluctuations (r = 0.84). The instantaneous apparent heat flux was 3 orders of magnitude larger than that delivered by radiation, with no direct correlation to the frequency of radiation and wind in the lake. This suggests that the source of the momentary heat flux is lateral advection due to internal waves (with no direct relation to the diurnal cycle). In practice, it is shown that snap-shot profiles of the Dead Sea as obtained with standard thermal profilers will not represent the seasonal typical status in terms of heat content of the upper layer.

Diagnosing the warming of the Northeastern U.S. Coastal Ocean in 2012: A linkage between the atmospheric jet stream variability and ocean response

The temperature in the coastal ocean off the northeastern U.S. during the first half of 2012 was anomalously warm, and this resulted in major impacts on the marine ecosystem and commercial fisheries. Understanding the spatiotemporal characteristics of the warming and its underlying dynamical processes is important for improving ecosystem management. Here, we show that the warming in the first half of 2012 was systematic from the Gulf of Maine to Cape Hatteras. Moreover, the warm anomalies extended through the water column, and the local temperature change of shelf water in the Middle Atlantic Bight was largely balanced by the atmospheric heat flux. The anomalous atmospheric jet stream position induced smaller heat loss from the ocean and caused a much slower cooling rate in late autumn and early winter of 2011–2012. Strong jet stream intraseasonal oscillations in the first half of 2012 systematically increased the warm anomalies over the continental shelf. Despite the importance of advection in prior northeastern U.S. continental shelf interannual temperature anomalies, our analyses show that much of the 2012 warming event was attributed to local warming from the atmosphere.

Turbulent fluxes of momentum and heat over land in the High-Arctic summer: the influence of observation techniques

Different observation techniques for atmospheric turbulent fluxes of momentum and sensible heat were tested in a High-Arctic valley in Svalbard during two consecutive summers (June–August in 2010 and 2011). The gradient method (GM) and the bulk method (BM) have been compared to the more direct eddy covariance method (ECM) in order to evaluate if relatively robust and cheap instrumentation with low power consumption can be used as a means to increase the number of observations, especially at remote locations where instruments need to be left unattended for extended periods. Such campaigns increase knowledge about the snow-free surface exchange processes, an area which is relatively little investigated compared to snow-covered ground. The GM agreed closely to the ECM, especially for momentum flux where the two methods agree within 5%. For sensible heat flux, the GM produces, on average, approximately 40% lower values for unstable stratification and 67% lower for stable stratification. However, this corresponds to only 20 and 12 W m −2 , respectively. The BM, however, shows a greater scatter and larger differences for both parameters. In addition to testing these methods, radiation properties were measured and the surface albedo was found to increase through the summer, from approximately 0.1 to 0.2. The surface energy budget shows that the sensible heat flux is usually directed upwards for the whole summer, while the latent heat flux is upwards in June, but becomes downward in July and August. Keywords : Turbulent fluxes; Svalbard; Arctic; observation techniques; surface energy budget. (Published: 30 June 2014) Citation: Polar Research 2014, 33 , 21567, http://dx.doi.org/10.3402/polar.v33.21567

Northern Hemisphere Winter Atmospheric Transient Eddy Heat Fluxes and the Gulf Stream and Kuroshio–Oyashio Extension Variability

Abstract Spatial and temporal covariability between the atmospheric transient eddy heat fluxes (i.e., 〈υ′T′〉 and 〈υ′q′〉) in the Northern Hemisphere winter (January–March) and the paths of the Gulf Stream (GS), Kuroshio Extension (KE), and Oyashio Extension (OE) are examined based on an atmospheric reanalyses and ocean observations for 1979–2009. For the climatological winter mean, the northward heat fluxes by the synoptic (2–8 days) transient eddies exhibit canonical storm tracks with their maxima collocated with the GS and KE/OE. The intraseasonal (8 days–3 months) counterpart, while having overall similar amplitude, shows a spatial pattern with more localized maxima near the major orography and blocking regions. Lateral heat flux divergence by transient eddies as the sum of the two frequency bands exhibits very close coupling with the exact locations of the ocean fronts. Linear regression is used to examine the lead–lag relationship between interannual changes in the northward heat fluxes by the transient eddies and the meridional changes in the paths of the GS, KE, and OE, respectively. One to three years prior to the northward shifts of each ocean front, the atmospheric storm tracks shift northward and intensify, which is consistent with wind-driven changes of the ocean. Following the northward shifts of the ocean fronts, the synoptic storm tracks weaken in all three cases. The zonally integrated northward heat transport by the synoptic transient eddies increases by ~5% of its maximum mean value prior to the northward shift of each ocean front and decreases to a similar amplitude afterward.

Variability of Iberian upwelling implied by ERA-40 and ERA-Interim reanalyses

The Regional Ocean Modeling System ocean model is used to simulate the decadal evolution of the regional waters in offshore Iberia in response to atmospheric fields given by ECMWF ERA-40 (1961–2001) and ERA-Interim (1989–2008) reanalyses. The simulated sea surface temperature (SST) fields are verified against satellite AVHRR SST, and they are analysed to characterise the variability and trends of coastal upwelling in the region. Opposing trends in upwelling frequency are found at the northern limit, where upwelling has been decreasing in recent decades, and at its southern edge, where there is some evidence of increased upwelling. These results confirm previous observational studies and, more importantly, indicate that observed SST trends are not only due to changes in radiative or atmospheric heat fluxes alone but also due to changes in upwelling dynamics, suggesting that such a process may be relevant in climate change scenarios.

Variability in a fjord-like coastal estuary I: Quantifying the circulation using a formal multi-tracer inverse approach

During 2002–2005, a comprehensive set of observations covering physical, biological, radiative and atmospheric parameters was obtained from the southern Strait of Georgia (SoG), Western Canada by the STRATOGEM program. Monthly time series of estuarine layer transports over 2002–2005 were estimated using a time-dependent 2-box model in a formal inverse approach. The formal inverse approach builds up upon a so-called “pseudo-inverse” and the Singular Value Decomposition methodology. The transports are then consistent with the temperature and salinity fields, as well as riverine freshwater inflow (R) and detailed atmospheric heat fluxes. Uncertainty was analyzed by resampling observations using bootstrap methods.Analysis of these time series suggests that the SoG estuarine circulation is not very sensitive to the seasonal changes of R. Comparison of the surface layer transport (U1) and R yields the first observational relationship between the SoG estuarine circulation and R. The analysis of this relationship shows that U1 has a fractional form as R to the power of 1/n with n < 1. Such fractional relationship shows that the flows change only slightly with the freshet. A 5-fold change in R results only in a 40% change in U1. However, freshwater range and uncertainty in the data prevented us from clearly determining the fraction n.Analysis of the transports in light of the residuals in the mass, heat and salt budgets suggests that our inversion procedure works properly and improves on the SVD inverse procedure. Analysis of the transports sensitivity to inversion parameters shows that the transports are close to both the a priori and true transports and that they are dependent on both a priori information and data.

Amplified Inception of European Little Ice Age by Sea Ice–Ocean–Atmosphere Feedbacks

Abstract The inception of the Little Ice Age (~1400–1700 AD) is believed to have been driven by an interplay of external forcing and climate system internal variability. While the hemispheric signal seems to have been dominated by solar irradiance and volcanic eruptions, the understanding of mechanisms shaping the climate on a continental scale is less robust. In an ensemble of transient model simulations and a new type of sensitivity experiments with artificial sea ice growth, the authors identify a sea ice–ocean–atmosphere feedback mechanism that amplifies the Little Ice Age cooling in the North Atlantic–European region and produces the temperature pattern suggested by paleoclimatic reconstructions. Initiated by increasing negative forcing, the Arctic sea ice substantially expands at the beginning of the Little Ice Age. The excess of sea ice is exported to the subpolar North Atlantic, where it melts, thereby weakening convection of the ocean. Consequently, northward ocean heat transport is reduced, reinforcing the expansion of the sea ice and the cooling of the Northern Hemisphere. In the Nordic Seas, sea surface height anomalies cause the oceanic recirculation to strengthen at the expense of the warm Barents Sea inflow, thereby further reinforcing sea ice growth. The absent ocean–atmosphere heat flux in the Barents Sea results in an amplified cooling over Northern Europe. The positive nature of this feedback mechanism enables sea ice to remain in an expanded state for decades up to a century, favoring sustained cold periods over Europe such as the Little Ice Age. Support for the feedback mechanism comes from recent proxy reconstructions around the Nordic Seas.

Forecasting river temperatures in real time using a stochastic dynamics approach

[1] We address the growing need for accurate water temperature predictions in regulated rivers to inform decision support systems and protect aquatic habitats. Although many suitable river temperature models exist, few simultaneously model water temperature dynamics while considering uncertainty of predictions and assimilating observations. Here, we employ a stochastic dynamics approach to water temperature modeling that estimates both the water temperature state and its uncertainty by propagating error through a physically based dynamical system. This method involves converting the governing hydrodynamic and heat transport equations into a state space form and assimilating observations via the Kalman Filter. This model, called the River Assessment for Forecasting Temperature (RAFT), closes the heat budget by tracking heat movement using a robust semi-Lagrangian numerical scheme. RAFT considers key thermodynamic processes, including advection, longitudinal dispersion, atmospheric heat fluxes, lateral inflows, streambed heat exchange, and unsteady nonuniform flow. Inputs include gridded meteorological forecasts from a numerical weather prediction model, bathymetric cross-sectional geometry, and temperature and flow measurements at the upstream boundary and tributaries. We applied RAFT to an ∼100 km portion of the Sacramento River in California, downstream of Keswick Dam (a regulatory dam below Shasta Dam), at a spatial resolution of 2 km and a temporal resolution of 15 min. Model prediction error over a 6 month calibration period was on the order of 0.5°C. When temperature and flow gage data were assimilated, the mean prediction error was significantly less (0.25°C). The model accurately predicts the magnitude and timing of diel temperature fluctuations and can provide 72 h water temperature forecasts when linked with meteorological forecasts and real-time flow/temperature monitoring networks. RAFT is potentially scalable to model and forecast fine-grained one-dimensional temperature dynamics covering a broad extent in a variety of regulated rivers provided that adequate input data are available.

Atmospheric forcing intensifies the effects of regional ocean warming on reef‐scale temperature anomalies during a coral bleaching event

We investigate how local atmospheric conditions and hydrodynamic forcing contributed to local variations in water temperature within a fringing coral reef‐lagoon system during the peak of a marine heat wave in 2010–2011 that caused mass coral bleaching across Western Australia. A three‐dimensional circulation model Regional Ocean Modeling System (ROMS) with a built‐in air‐sea heat flux exchange module Coupled Ocean Atmosphere Experiment (COARE) was coupled with a spectral wave model Simulating Waves Nearshore (SWAN) to resolve the surface heat exchange and wave‐driven reef circulation in Coral Bay, Ningaloo Reef. Using realistic oceanic and atmospheric forcing, the model predictions were in good agreement with measured time series of water temperature at various locations in the coral reef system during the bleaching event. Through a series of sensitivity analyses, we found that the difference in temperature between the reef and surrounding offshore waters (ΔT) was predominantly a function of both the daily mean net heat flux ( ) and residence time, whereas diurnal variations in reef water temperature were dependent on the diurnal fluctuation in the net heat flux. We found that reef temperatures were substantially higher than offshore in the inner lagoon under normal weather conditions and over the entire reef domain under more extreme weather conditions (0.7°C–1.5°C). Although these temperature elevations were still less than that caused by the regional ocean warming (2°C–3°C), the arrival of peak seasonal temperatures in the summer of 2010–2011 (when net atmospheric heat fluxes were positive and abnormally high) caused substantially higher thermal stresses than would have otherwise occurred if offshore temperatures had reached their normal seasonal maxima in autumn (when net atmospheric heat fluxes were negative or cooling). Therefore, the degree heating weeks calculated based on offshore temperature substantially underestimated the thermal stresses experienced by the reef in the period leading up to the observed bleaching event (3 versus 11°C‐weeks).

Energy budget of first‐year Arctic sea ice in advanced stages of melt

During an 8 day drift in July–August 2012 in the Nansen Basin, all components of the energy budget of melting first‐year sea ice were observed. Absorption of solar radiation by the ice and ponds was the largest source of energy to the ice at almost all times during the drift. However, oceanic heat flux also provided significant heating and dominated during one wind event. Longwave fluxes provided a relatively small cooling effect, and atmospheric heat fluxes were negligible. The aggregate scale albedo of this younger, thinner ice was significantly lower than at Surface Heat Budget of the Arctic Ocean (SHEBA), and the transmittance was significantly higher here, despite similar pond and open water fractions. The oceanic heat flux was only half of the solar flux through the ice to the water, producing warm water near the surface that might delay ice growth in autumn, an important effect of the transition to thinner first‐year ice in the high Arctic.

Seasonal to decadal variability of Arctic Ocean heat content: A model‐based analysis and implications for autonomous observing systems

A high‐resolution global ocean/sea ice model is used to investigate the modes of Arctic Ocean heat content variability for the period 1968–2007. A rotated empirical orthogonal function analysis is performed on the monthly mean vertically integrated heat content to investigate the mechanisms governing its spatiotemporal variations. In the model, 28% of the heat content variability is driven by the seasonal and interannual fluctuations of the atmospheric heat flux in the seasonally ice free regions. The heat flux variability associated with Atlantic Water advected through Fram Strait drives 31% of the heat content variability. Changes of temperature and circulation drive Fram Strait heat transport variability, and these two effects project on different modes and thus drive heat content variations in different parts of the Eurasian Basin. A second branch of Atlantic Water is modified in the Barents Sea and the variations of the heat flux associated with the Barents Sea water branch penetrating the deep Arctic yield heat content variations in the Eurasian Basin. The effect of the Bering Strait heat flux variations remains limited to the Chukchi Sea. Autonomous observing system may be able to capture the Arctic heat content variability. Sea surface temperature satellite observations combined with temperature profiles of the top 800 m in the deep Arctic covered by sea ice are sufficient to capture most of the variability signal. The results emphasize the crucial need for measurements in the Eurasian Basin.

Processes of India’s offshore summer intraseasonal sea surface temperature variability

Active and break phases of the Indian summer monsoon are associated with sea surface temperature (SST) fluctuations at 30–90 days timescale in the Arabian Sea and Bay of Bengal. Mechanisms responsible for basin-scale intraseasonal SST variations have previously been discussed, but the maxima of SST variability are actually located in three specific offshore regions: the South-Eastern Arabian Sea (SEAS), the Southern Tip of India (STI) and the North-Western Bay of Bengal (NWBoB). In the present study, we use an eddy-permitting 0.25° regional ocean model to investigate mechanisms of this offshore intraseasonal SST variability. Modelled climatological mixed layer and upper thermocline depth are in very good agreement with estimates from three repeated expendable bathythermograph transects perpendicular to the Indian Coast. The model intraseasonal forcing and SST variability agree well with observed estimates, although modelled intraseasonal offshore SST amplitude is undere-stimated by 20–30 %. Our analysis reveals that surface heat flux variations drive a large part of the intraseasonal SST variations along the Indian coastline while oceanic processes have contrasted contributions depending of the region considered. In the SEAS, this contribution is very small because intraseasonal wind variations are essentially cross-shore, and thus not associated with significant upwelling intraseasonal fluctuations. In the STI, vertical advection associated with Ekman pumping contributes to ∼30 % of the SST fluctuations. In the NWBoB, vertical mixing diminishes the SST variations driven by the atmospheric heat flux perturbations by 40 %. Simple slab ocean model integrations show that the amplitude of these intraseasonal SST signals is not very sensitive to the heat flux dataset used, but more sensitive to mixed layer depth.