Atmospheric Heat Fluxes Research Articles

Long-term monitoring of ocean deep convection using multisensors altimetry and ocean color satellite data

Deep convection occurs in oceanic regions submitted to strong atmospheric buoyancy losses and results in the formation of deep water masses (DWF) of the ocean circulation. It shows a strong interannual variability, and could drastically weaken under the influence of climate change. In this study, a method is proposed to monitor quantitatively deep convection using multi-sensors altimetry and ocean color satellite data. It is applied and evaluated for the well observed Northwestern Mediterranean Sea (NWMS) case study. For that, a coupled hydrodynamical-biogeochemical numerical simulation is used to examine the signature of DWF on sea level anomaly (SLA) and surface chlorophyll concentration. Statistically significant correlations between DWF annual indicators and the areas of low surface chlorophyll concentration and low SLA in winter are obtained, and linear relationships between those indicators and areas are established. These relationships are applied to areas of low SLA and low chlorophyll concentration computed respectively from a 27-year altimetry dataset and a 19-year ocean color dataset. The first long time series (covering the last 2 decades) of DWF indicators obtained for the NWMS from satellite observations are produced. Model biases and smoothing effect induced by the low resolution of gridded altimetry data are partly taken into account by using corrective methods. Comparison with winter atmospheric heat flux and previous modeled and observed estimates of DWF indicators suggests that those DWF indicators time series capture realistically DWF interannual variability in the NWMS. The advantages as well as the weaknesses and uncertainties of the method are finally discussed. This article is protected by copyright. All rights reserved.

Effects of Southeastern Pacific Sea Surface Temperature on the Double-ITCZ Bias in NCAR CESM1

Abstract The double intertropical convergence zone (ITCZ) is a long-standing bias in the climatology of coupled general circulation models (CGCMs). The warm biases in southeastern Pacific (SEP) sea surface temperature (SST) are also evident in many CGCMs. In this study, the role of SEP SST in the double ITCZ is investigated by prescribing the observed SEP SST in the Community Earth System Model, version 1 (CESM1). Both the double ITCZ and dry equator problems are significantly improved with SEP SST prescribed. Both atmospheric and oceanic processes are involved in the improvements. The colder SST over the SEP decreases the precipitation, which enhances the southeasterly winds outside the prescribed SST region, cooling the ocean via increased evaporation. The enhanced descending motion over the SEP strengthens the Walker circulation. The easterly winds over the equatorial Pacific enhance upwelling and shoal the thermocline over the eastern Pacific. The changes of surface wind and wind curl lead to a weaker South Equatorial Countercurrent and stronger South Equatorial Current, preventing the warm water from expanding eastward, thereby improving both the double ITCZ and dry equator. The enhanced Walker circulation also increases the low-level wind convergence and reduces the wind speed in the tropical western Pacific, leading to warmer SST and stronger convection there. The stronger convection in turn leads to more cloud and reduces the incoming solar radiation, cooling the SST. These competing effects between radiative heat flux and latent heat flux make the atmospheric heat flux secondary to the ocean dynamics in the western Pacific warming.

Local intraseasonal air–sea relationship over the North Indian Ocean and western North Pacific during the spring-to-summer transition

AbstractThis study investigates the local air–sea relationship associated with the two dominant intraseasonal oscillation (ISO) components during the spring-to-summer transition and compares their properties using multiple air–sea variables in the period 1998–2013. The amplitude of percentage variance in SST in periods of 10–20 and 30–60 days are comparable, but the locations of the maxima differ. A strong percentage variance in the 10–20-day SST is evident in the equatorial western Pacific, whereas for the 30–60-day SST the strongest ratio occurs in the North Indian Ocean (NIO), South China Sea (SCS), and North Pacific. Over the NIO, SCS, and Philippine Sea, there are significant correlations between SST and precipitation for both 10–20-day and 30–60-day ISOs. In contrast, the correlations between SST and surface heat fluxes cover a broader region and have larger coefficients. Thus, the atmospheric variables and surface heat fluxes show larger variations within the higher frequency band. However, the amp...

The phenology of Arctic Ocean surface warming.

In this work, we explore the seasonal relationships (i.e., the phenology) between sea ice retreat, sea surface temperature (SST), and atmospheric heat fluxes in the Pacific Sector of the Arctic Ocean, using satellite and reanalysis data. We find that where ice retreats early in most years, maximum summertime SSTs are usually warmer, relative to areas with later retreat. For any particular year, we find that anomalously early ice retreat generally leads to anomalously warm SSTs. However, this relationship is weak in the Chukchi Sea, where ocean advection plays a large role. It is also weak where retreat in a particular year happens earlier than usual, but still relatively late in the season, primarily because atmospheric heat fluxes are weak at that time. This result helps to explain the very different ocean warming responses found in two recent years with extreme ice retreat, 2007 and 2012. We also find that the timing of ice retreat impacts the date of maximum SST, owing to a change in the ocean surface buoyancy and momentum forcing that occurs in early August that we term the Late Summer Transition (LST). After the LST, enhanced mixing of the upper ocean leads to cooling of the ocean surface even while atmospheric heat fluxes are still weakly downward. Our results indicate that in the near‐term, earlier ice retreat is likely to cause enhanced ocean surface warming in much of the Arctic Ocean, although not where ice retreat still occurs late in the season.

ESA's Soil Moisture and Ocean Salinity mission: From science to operational applications

Abstract The Soil Moisture and Ocean Salinity (SMOS) mission, launched in November 2009, is the European Space Agency's (ESA) second Earth Explorer Opportunity mission. The scientific objectives of the SMOS mission directly respond to the need for global observations of soil moisture and ocean salinity, two key variables used in predictive hydrological, oceanographic and atmospheric models. SMOS observations also provide information on vegetation, in particular plant available water and water content in a canopy, drought index and flood risks, surface ocean winds in storms, freeze/thaw state and sea ice and its effect on ocean–atmosphere heat fluxes and dynamics affecting large-scale processes of the Earth's climate system. Significant progress has been made over the course of the now 6-year life time of the SMOS mission in improving the ESA provided level 1 brightness temperature and level 2 soil moisture and sea surface salinity data products. The main emphasis of this paper is to review the status of the mission and provide an overview and performance assessment of SMOS data products, in particular with a view towards operational applications, and using SMOS products in data assimilation. SMOS is in excellent technical condition with no limiting factors for operations beyond 2017. The instrument performance fulfils the requirements. The radio-frequency interference (RFI) contamination originates from man-made emitters on ground, operating in the protected L-band and adding signal to the natural radiation emitted by the Earth. RFI has been detected worldwide and has been significantly reduced in Europe and the Americas but remains a constraint in Asia and the Middle East. The mission's scientific objectives have been reached over land and are approaching the mission objectives over ocean. This review paper aims to provide an introduction and synthesis to the papers published in this RSE special issue on SMOS.

Monsoon Mixing Cycles in the Bay of Bengal: A Year-Long Subsurface Mixing Record

Based on the first year-long record of mixing collected in the eastern central Bay of Bengal, we quantify the role that subsurface turbulent heat fluxes play in upper-ocean cooling brought on by southwest (SW) and northeast (NE) monsoons. During the NE (dry, or winter) monsoon, atmospheric and subsurface turbulent heat fluxes each contribute about 50% of the net sea surface cooling. During the SW (wet, or summer) monsoon, the atmospheric heat flux varied widely due to “active” and “break” cycles of the monsoon intraseasonal oscillations, but had a net positive seasonal average. The subsurface turbulent heat flux during the SW monsoon led to surface cooling at rates more than three times greater than those measured during the NE monsoon. Since the seasonally averaged atmospheric heat flux was positive, subsurface mixing accounted for nearly all of the cooling during the SW monsoon. During the transition between the NE and SW monsoons, subsurface heat flux was near zero, and atmospheric heating rapidly warmed the sea surface. Following the SW monsoon, passage of Tropical Storm Hudhud led to O(1) m2 s–1 rates of turbulence diffusivity and strong subsurface heat flux, accounting for roughly half of the 1.4°C surface cooling that occurred over a 60-hour period.

Formation of extreme surface turbulent heat fluxes from the ocean to the atmosphere in the North Atlantic

The role of extreme surface turbulent fluxes in total oceanic heat loss in the North Atlantic is studied. The atmospheric circulation patterns enhancing ocean–atmosphere heat flux in regions with significant contributions of the extreme heat fluxes (up to 60% of the net heat loss) are analyzed. It is shown that extreme heat fluxes in the Gulf Stream and the Greenland and Labrador Seas occur in zones with maximal air pressure gradients, i.e., in cyclone–anticyclone interaction zones.

Atmospheric and Oceanic Contributions to Irreducible Forecast Uncertainty of Arctic Surface Climate

Abstract Uncertainty of Arctic seasonal to interannual predictions arising from model errors and initial state uncertainty has been widely discussed in the literature, whereas the irreducible forecast uncertainty (IFU) arising from the chaoticity of the climate system has received less attention. However, IFU provides important insights into the mechanisms through which predictability is lost and hence can inform prioritization of model development and observations deployment. Here, the authors characterize how internal oceanic and surface atmospheric heat fluxes contribute to the IFU of Arctic sea ice and upper-ocean heat content in an Earth system model by analyzing a set of idealized ensemble prediction experiments. It is found that atmospheric and oceanic heat flux are often equally important for driving unpredictable Arctic-wide changes in sea ice and surface water temperatures and hence contribute equally to IFU. Atmospheric surface heat flux tends to dominate Arctic-wide changes for lead times of up to a year, whereas oceanic heat flux tends to dominate regionally and on interannual time scales. There is in general a strong negative covariance between surface heat flux and ocean vertical heat flux at depth, and anomalies of lateral ocean heat transport are wind driven, which suggests that the unpredictable oceanic heat flux variability is mainly forced by the atmosphere. These results are qualitatively robust across different initial states, but substantial variations in the amplitude of IFU exist. It is concluded that both atmospheric variability and the initial state of the upper ocean are key ingredients for predictions of Arctic surface climate on seasonal to interannual time scales.

Atmospheric Impact of Arctic Sea Ice Loss in a Coupled Ocean–Atmosphere Simulation*

Abstract The atmospheric response to an idealized decline in Arctic sea ice is investigated in a novel fully coupled climate model experiment. In this experiment two ensembles of single-year model integrations are performed starting on 1 April, the approximate start of the ice melt season. By perturbing the initial conditions of sea ice thickness (SIT), declines in both sea ice concentration and SIT, which result in sea ice distributions that are similar to the recent sea ice minima of 2007 and 2012, are induced. In the ice loss regions there are strong (~3 K) local increases in sea surface temperature (SST); additionally, there are remote increases in SST in the central North Pacific and subpolar gyre in the North Atlantic. Over the central Arctic there are increases in surface air temperature (SAT) of ~8 K due to increases in ocean–atmosphere heat fluxes. There are increases in SAT over continental North America that are in good agreement with recent changes as seen by reanalysis data. It is estimated that up to two-thirds of the observed increase in SAT in this region could be related to Arctic sea ice loss. In early summer there is a significant but weak atmospheric circulation response that projects onto the summer North Atlantic Oscillation (NAO). In early summer and early autumn there is an equatorward shift of the eddy-driven jet over the North Atlantic as a result of a reduction in the meridional temperature gradients. In winter there is no projection onto a particular phase of the NAO.

A simple model of ocean temperature re-emergence and variability

A simple stochastic one-dimensional model of interannual mid-latitude sea surface temperature (SST) variability that can be solved analytically is developed. A novel two-season approach is adopted, with the annual cycle divided into two seasons denoted summer and winter. Within each season the mixed layer depth is constant, and the transition of the mixed layer from summer to winter and vice versa is discontinuous. SST anomalies are forced by random atmospheric heat fluxes, assumed to be constant within each season for simplicity, with linear damping to represent atmospheric feedback. At the start of summer the initial SST anomaly is set equal to that at the end of the previous winter, and at the start of winter the initial temperature anomaly is found by instantaneously mixing the summer mixed layer with the heat stored below in the deeper winter mixed layer, thereby explicitly taking into account the ‘re-emergence mechanism’. Two simple auto-regressive equations for the summer and winter SST anomalies are obtained that can be easily solved. Model parameters include seasonal damping coefficients, mixed layer depths and standard deviations of the atmospheric forcing. Analytic expressions for season-to-season correlation and variability and power spectra are used to explore and illustrate the effects of the parameters quantitatively. Among the results it is found that, with regard to winter-to-winter temperature correlation, the re-emergence pathway is more influential than persistence via the summer mixed layer when the winter layer is more than twice the depth of the summer layer. With regard to winter temperature variability, the effect of a deeper winter mixed layer is to decrease the sensitivity to surface forcing and thus decrease variability, but also to increase persistence via re-emergence and thus increase variance at multidecadal scales.

The observed impacts of South Asian summer monsoon on the local atmosphere and the near‐surface turbulent heat exchange over the Southeast Tibet

AbstractThe Southeast Tibet is an important region of the Tibetan Plateau bearing the interaction between the Tibetan and the neighbor atmospheric systems. The South Asian summer monsoon (SASM) as a basic climate system in Asia could impact the local atmosphere and the near‐surface heat exchange process in the Southeast Tibet. An observational campaign, Observation on the Surface‐to‐air Exchange Processes in Southeast Tibet (OSEP2013), was carried out in this region during SASM in 2013. The atmospheric parameters and turbulent heat fluxes were observed and averaged over three different land surfaces of the inhomogeneous landscape during the observation campaign. Results show clear SASM impacts on the local atmosphere and near‐surface heat exchange in the Southeast Tibet. The South Asian summer monsoon was onset on 1 June 2013 and experienced a south phase and north phase during OSEP2013. The convection and humidity were increased in the Southeast Tibet by SASM, especially during the north phase. The observation domain received low radiation energy due to the convective clouds brought by SASM, and the soil and air temperatures were lowered as consequence. In addition, the air humidity was increased over this region by the wet air transportation of SASM circulation. The sensible and latent heat transfers were decreased by the low land‐air temperature difference and high air humidity during SASM. The latent heat transfer dominated the total heat transfer in the Southeast Tibet due to the low sensible heat transfer in the SASM situation, and the domination was increased as the sensible heat transfer was further decreased during the SASM north phase.

Conceptual model analysis of the influence of temperature feedbacks on polar amplification

AbstractThe role of temperature feedbacks in polar amplification of climate change is examined by comparing the response of idealized high‐ and low‐latitude atmospheric columns to greenhouse gas forcing. An analytic expression for the surface polar amplification factor is derived with a one‐layer atmospheric model and compared to a more detailed column model with two radiative transfer schemes. The modeled temperature profiles result from competition between the stabilizing influences of atmospheric heat flux convergence and atmospheric solar heating (dominant at high latitudes), and the destabilizing influence of surface solar heating (dominant at low latitudes). For a stable high‐latitude radiative‐advective atmosphere, the lapse rate increases with greenhouse gas forcing, leading to a positive feedback, and is dependent on the nature of the forcing—pointing to limitations of the traditional forcing‐feedback framework. For a low‐latitude radiative‐convective atmosphere, the lapse rate decreases, leading to a negative feedback.

Thermally Forced Transients in the Thermohaline Circulation

AbstractThe response of a convective ocean basin to variations in atmospheric temperature is explored using numerical models and theory. The results indicate that the general behavior depends strongly on the frequency at which the atmosphere changes relative to the local response time to air–sea heat flux. For high-frequency forcing, the convective region in the basin interior is essentially one-dimensional and responds to the integrated local surface heat flux anomalies. For low-frequency forcing, eddy fluxes from the boundary current into the basin interior become important and act to suppress variability forced by the atmosphere. A theory is developed to quantify this time-dependent response and its influence on various oceanic quantities. The amplitude and phase of the temperature and salinity of the convective water mass, the meridional overturning circulation, the meridional heat flux, and the air–sea heat flux predicted by the theory compare well with that diagnosed from a series of numerical model calculations in both strongly eddying and weakly eddying regimes. Linearized analytic solutions provide direct estimates of each of these quantities and demonstrate their dependence on the nondimensional numbers that characterize the domain and atmospheric forcing. These results highlight the importance of mesoscale eddies in modulating the mean and time-dependent ocean response to atmospheric variability and provide a dynamical framework with which to connect ocean observations with changes in the atmosphere and surface heat flux.

The role of circulation and heat fluxes in the formation of stratification leading to hypoxia in Green Bay, Lake Michigan

Abstract Summertime bottom water hypoxia has been a recurring water quality issue in Green Bay for decades. Evidence suggests that the magnitude and duration of hypoxia is highly variable from year to year, despite the fact that nutrient loading has been relatively constant for at least 20 years. The bay's size, orientation, and morphology (~ 20 × 200 km) with high riverine inflow and restricted mixing at the southern end, combined with extensive water mass exchange with Lake Michigan at the northern end, results in a changing hydrodynamic structure that plays a key role in the set up and persistence of stratification that leads to hypoxic conditions. Using both observed and modeled data, this study represents the first attempt to examine the spatial and temporal characteristics of the interactions among the atmospheric heat flux across the water surface, the advective heat flux driven by the circulation, and the intrusion of cold Lake Michigan bottom water into the hypolimnion. With the onset of positive solar heat fluxes in April, stratification generally exists continuously between late June and early September in regions deeper than 15–20 m, and develops discontinuously at depths less than 15 m. Critical improvements in developing realistic simulations of stratification in the bay were obtained via consideration of water clarity conditions that impact heat adsorption and varying default parameters, particularly in the vertical diffusivity, in the Lake Michigan model framework.

Summertime atmosphere–ocean preconditionings for the Bering Sea ice retreat and the following severe winters in North America

Atmospheric responses to sea ice retreat in the Bering Sea have been linked to recent extreme winters in North America. We investigate the leading factor for the interannual variability of Bering sea ice area (SIA) in early winter (November–December), using canonical correlation analysis based on seasonally resolved atmosphere and ocean data for 1980–2014. We found that the 3-month leading (August–September) geopotential height at 500 hPa (Z500) in the Northern Hemisphere explains 29% of SIA variability. The spatial pattern of Z500 for positive (negative) sea ice anomalies is associated with negative (positive) anomalies over the Gulf of Alaska related to the Pacific transition (PT) pattern. The heat budget analysis indicates that summertime atmospheric conditions influence SIA through the ocean temperature anomalies of the Alaskan Coastal Current forced by atmospheric turbulent heat fluxes. The PT pattern highly correlates with convective precipitation in the western subtropical Pacific, implying that weakened subtropical forcing is the likely cause for the recent extreme winters in North America. Our results present a major factor for interannual variability in the Bering SIA, and further would contribute to the improvement of forecasting winter climate in North America.

Quantifying and improving sub‐grid diffusion in the boundary‐layer grey zone

Operational high‐resolution numerical weather prediction models are now able to partially resolve turbulent motions due to increased computing power. The partitioning of resolved and parametrized fluxes becomes important in the representation of turbulent transfer that determines the state of the atmospheric boundary layer. In this study, successive simulations of a convective boundary layer using the Met Office Large Eddy Model from the large‐eddy simulation to the mesoscale limit are compared with the corresponding coarse‐grained fields to examine model behaviour over the grey zone. The differences in the turbulent kinetic energy partitioning between coarse‐grained (reference) and actual fields are identified and used to quantify sub‐grid diffusion in the grey zone under different atmospheric forcings and surface heat flux. It is shown that the excessive mixing due to the large values of mixing length at coarse resolutions results in the cut‐off of resolved turbulence in the grey zone. The damping of resolved motions comes earlier for wind shear runs. In contrast, coarse‐grained fields exhibit a smooth transition of the resolved turbulent kinetic energy across the scales. Decreasing numerical dissipation through the sub‐grid scheme leads to the increase of resolved turbulence, but fails to reproduce the reference transition pattern that imposes some physical limitations to the partially resolved turbulence simulations. Pragmatic blending of mixing length values maintains a more realistic turbulent kinetic energy transition from fine to coarse resolutions and potential temperature profiles in the grey zone. Bounding vertical diffusion to its effective values, an approach based on maintaining inherent properties of the flow across the scales, is able to match the coarse‐grained fields from the highly resolved to the almost unresolved state, regardless of the forcing. Finally, the complexity of modelling in the grey zone is exhibited in the dependence of turbulence onset on time and vertical resolution.

The Role of Wave Dynamics and Small-Scale Topography for Downslope Wind Events in Southeast Greenland

In Ammassalik, in southeast Greenland, downslope winds can reach hurricane intensity and represent a hazard for the local population and environment. They advect cold air down the ice sheet and over the Irminger Sea, where they drive large ocean–atmosphere heat fluxes over an important ocean convection region. Earlier studies have found them to be associated with a strong katabatic acceleration over the steep coastal slopes, flow convergence inside the valley of Ammassalik, and—in one instance—mountain wave breaking. Yet, for the general occurrence of strong downslope wind events, the importance of mesoscale processes is largely unknown. Here, two wind events—one weak and one strong—are simulated with the atmospheric Weather Research and Forecasting (WRF) Model with different model and topography resolutions, ranging from 1.67 to 60 km. For both events, but especially for the strong one, it is found that lower resolutions underestimate the wind speed because they misrepresent the steepness of the topography and do not account for the underlying wave dynamics. If a 5-km model instead of a 60-km model resolution in Ammassalik is used, the flow associated with the strong wind event is faster by up to 20 m s−1. The effects extend far downstream over the Irminger Sea, resulting in a diverging spatial distribution and temporal evolution of the heat fluxes. Local differences in the heat fluxes amount to 20%, with potential implications for ocean convection.

Understanding long-term (1982–2013) multi-decadal change in the equatorial and subtropical South Atlantic climate

High-resolution observations along with atmospheric and oceanic reanalyses are diagnosed to understand how and why southeastern Atlantic SSTs have changed over the 1982–2013 period. Multiple datasets are used to evaluate confidence. Results indicate significant SST warming trends (0.5–1.5 K per 32-years) along the Guinean and Angolan/Namibian Coasts, and a cooling trend (−0.10 to −0.60 K per 32-years) over the subtropical South Atlantic between 18°S and 28°S. SST trends are shown to vary over the annual cycle with the greatest changes occurring during November–January. Analysis of the ocean surface heat balance reveals that the austral summer SST warming trend along the Angolan/Namibian Coast is associated with an increase in the net downward atmospheric heat flux. In addition, there is a decrease in coastal upwelling due to circulation changes related to a poleward shift of the South Atlantic subtropical anticyclone and an intensification of the southwestern African thermal low. The cooling trend over the subtropical South Atlantic is also associated with the poleward shift of the South Atlantic anticyclone, as stronger surface winds enhance latent heat loss from the ocean over this region. Positive SST trends along the Guinean coast are found to be primarily associated with changes internal to the ocean, specifically, reduced coastal upwelling, diffusion, and enhanced horizontal transport of warmer water. These results highlight the need to better understand South Atlantic subtropical anticyclone and the continental thermal low interactions and their implications for present day climate variability and future climate change.

Regional estimation and validation of remotely sensed evapotranspiration in China

Accurate estimates of evapotranspiration (ET) are important to skillful predictions of runoff, soil moisture and crop productivity. In this study, ET was simulated over three years in China by integrating remotely-sensed data with the simulation model SEBS-China. SEBS-China was developed from the Surface Energy Balance System (SEBS) model that is used to estimate atmospheric turbulent heat fluxes and evaporative fraction through satellite-derived radiation fluxes and land surface temperatures coupled with near-surface meteorological variables. Simulated regional ET was first evaluated using experimental data from the Changbai Mountains and five river basins in China. Average annual simulated ET agreed well with the observed data, based on the detailed results through comparative validation. Regional ET derived from SEBS-China was then compared with the simulated ET from the Common Land Model (CoLM). Both of the models simulated regional ET well, with a few overestimated areas such as in the Qinghai–Tibet Plateau, YanShan and South Daxing'anling Mountains. We conclude that more efficient ET estimation can be achieved through combining remotely-sensed information with land surface models.

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.