Regional Heat Budgets Research Articles

Modulation of Western South Atlantic Marine Heatwaves by Meridional Ocean Heat Transport

AbstractMarine heatwaves and cold spells are extreme surface temperature events that have been associated with adverse societal and ecosystem impacts in several regions around the globe. Predicting these events presents a challenge because of their generally short‐lived nature and dependence on air‐sea interactions, both locally and remotely. Here we analyze oceanic propagating features that promote the occurrence of marine heatwaves and cold spells in the western subtropical South Atlantic. The main interannual feature detected from satellite sea level data since 1993 shows a westward propagating zonal pattern with a periodicity of 3–5 years. The pattern has a significant in‐phase correlation with sea surface temperature (SST) anomalies in the western South Atlantic, explaining 82% of the daily extreme warm (90th percentile) and cold (10th percentile) SST anomalies and consequently modulating interannual variations in the intensity and duration of marine heatwave and cold spell events. It is found that meridional oceanic advection plays an important role in the regional heat budget associated with the westward‐propagating mode, modulating the meridional exchange of tropical (warm) and extratropical (cold) waters in the western subtropical South Atlantic region and thereby setting a baseline for temperature extremes on interannual timescales. This propagating mode is well correlated (r > 0.6) with the strength of the meridional overturning circulation at 25°S and 30°S with a lag of approximately 3–9 months. The lagged response provides a potential source of predictability of extreme events in the western South Atlantic.

The Role of Atmospheric Noise in Decadal SST Variability

Abstract A substantial role for atmospheric noise in simulations of decadal internal variability of SST is demonstrated by a comparison of a multicentury climate model control and a corresponding interactive ensemble (IE) simulation. The IE is designed to reduce atmospheric noise in the heat flux, wind stress, and freshwater flux at the air–sea interface. This comparison suggests that nearly all SST variability on decadal time scales is forced by internal atmospheric variability. The results are examined to determine the relative roles of atmospheric surface heat flux noise and ocean dynamics in the decadal volume-averaged heat budget of the upper ocean. The regional heat budgets in two regions, the South Pacific and the midlatitude North Atlantic, show the net atmospheric surface heat flux to be approximately in equilibrium with the ocean dynamics forcing. The IE and control results are used in the equilibrium heat budget approximation to infer the atmospheric heat flux response to SST, as well as the time series of the control atmospheric noise surface heat flux and ocean dynamics forcings for several regions. The South Pacific region SST is found to be primarily forced by the atmospheric noise surface heat flux and the North Atlantic region SST is forced by the ocean dynamics. Similar strengths for the atmospheric heat flux noise and ocean dynamics forcing, with an interdecadal atmospheric heat flux noise time scale and a centennial ocean dynamics time scale, are found for an Atlantic multidecadal variability region SST.

Exploring the spatial pattern of urban heat island formation in relation to land transformation: A study on a mining industrial region of West Bengal, India

The Earth surface has transformed a lot by various deliberate and inadvertent anthropogenic activities; most profound among them are perhaps the changes in the land use land cover pattern of any area. It is considered to be responsible for many irreversible changes in the regional heat budget and water balance. Expansion of the built up area found to be modified the surface interaction of solar radiation thereby alters the surface temperature pattern of any region. Present study has been carried out in the Jamuria community development (C D.) block of West Bengal to examine the relation between land use land cover (LULC) change and its impact on the rising land surface temperature (LST) by using multi temporal satellite data. Urbanization, industrialization and mining activity in the study area found to be alters the LULC to a great extent which seems to be responsible for the general rise in the ambient temperature and micro climatic variation. Definite association has been found to be exists between LST and LULC pattern of the study area. Rise in the LST pattern are considered to be spatially associated with those areas where the intensity of land use change is maximum. Correlation between them, land use indices like normalized difference vegetation index (NDVI), normalized difference built index (NDBI) etc and transect profile also confirmed these facts.

Rates and Mechanisms of Turbulent Mixing in a Coastal Embayment of the West Antarctic Peninsula

AbstractQuantifying and understanding the processes driving turbulent mixing around Antarctica are key to closing the Southern Ocean's heat budget, an essential component of the global climate system. In 2016, a glider deployed in Ryder Bay, West Antarctic Peninsula, collected hydrographic and microstructure data, obtaining some of the first direct measurements of turbulent kinetic energy dissipation off West Antarctica. Elevated dissipation O(10−8) W kg−1 is found above a topographic ridge separating the 520‐m‐deep bay, where values are O(10−10) W kg−1, from a deep fjord of the continental shelf, suggesting the ridge is important in driving upward mixing of warm Circumpolar Deep Water. The 12 glider transects reveal significant temporal variability in hydrographic and dissipation conditions. Mooring‐based current and nearby meteorological data are used to attribute thermocline shoaling (deepening) to Ekman upwelling (downwelling) at Ryder Bay's southern boundary, driven by ∼3‐day‐long south‐westward (north‐westward) wind events. Anticyclonic winds generated near‐inertial shear in the bay's upper layers, causing elevated bay‐wide shear and dissipation ∼1.7 days later. High dissipation over the ridge appears to be controlled hydraulically, being co‐located (and moving) with steeply sloping isopycnals. These are observed in ∼60% of the transects, with a corresponding mean upward heat flux of ∼2.4 W m−2. The ridge, therefore, provides sustained heat to the base of the thermocline, which can be released into overlying waters during the bay‐wide, thermocline‐focused dissipation events (mean heat flux of ∼1.3 W m−2). This highlights the role of ridges, which are widespread across the West Antarctic Peninsula, in the regional heat budget.

Seasonal Evolution of the Surface Layer Heat Balance in the Eastern Subtropical Indian Ocean

AbstractThe South Indian Ocean (SIO) is a region of strong air‐sea heat loss due to the unique ocean circulation pattern influenced by the Indonesian Throughflow. In this study, the seasonal variation of the surface layer heat budget in the eastern SIO is investigated using 2 years of measurements from a mooring at 25°S, 100°E, the only colocated upper ocean and surface meteorology time series in the subtropical Indian Ocean. The mooring data are combined with other in situ and satellite data to examine the role of air‐sea fluxes and ocean heat transport on the evolution of mixed layer temperature using heat budget diagnostic models. Results show that on seasonal time scales, mixed layer heat storage in the eastern SIO is mostly balanced by a combination of surface fluxes and turbulent entrainment with a contribution from horizontal advection at times. Solar radiation dominates the seasonal cycle of net surface heat flux, which warms the mixed layer during austral summer (67 W/m2) and cools it during austral winter (−44 W/m2). Entrainment is in good agreement with the heat budget residual for most of the year. Horizontal advection is spatially variable and appears to be dominated by the presence of mesoscale eddies and possibly annual and semiannual Rossby waves propagating from the eastern boundary. Results from the 2‐year mooring‐based data analysis are in reasonably good agreement with a 12‐year regional heat budget analysis around the mooring location using ocean reanalysis products.

Adequacy of the Ocean Observation System for Quantifying Regional Heat and Freshwater Storage and Change

Considerable advances in the global ocean observing system over the last two decades offers an opportunity to provide more quantitative information on changes in heat and freshwater storage. Variations in these storage terms can arise through internal variability and also the response of the ocean to anthropogenic climate change. Disentangling these competing influences on the regional patterns of change and elucidating their governing processes remains an outstanding scientific challenge. This challenge is compounded by instrumental and sampling uncertainties. The combined use of ocean observations and model simulations is the most viable method to assess the forced signal from noise and ascertain the primary drivers of variability and change. Moreover, this approach offers the potential for improved seasonal-to-decadal predictions and the possibility to develop powerful multi-variate constraints on climate model future projections. Regional heat storage changes dominate the steric contribution to sea level rise over most of the ocean and are vital to understanding both global and regional heat budgets. Variations in regional freshwater storage are particularly relevant to our understanding of changes in the hydrological cycle and can potentially be used to verify local ocean mass addition from terrestrial and cryospheric systems associated with contemporary sea level rise. This White Paper will examine the ability of the current ocean observing system to quantify changes in regional heat and freshwater storage. In particular we will seek to answer the question: What time and space scales are currently resolved in different regions of the global oceans? In light of some of the key scientific questions, we will discuss the requirements for measurement accuracy, sampling, and coverage as well as the synergies that can be leveraged by more comprehensively analyzing the multi-variable arrays provided by the integrated observing system.

Mechanisms of Ocean Heat Anomalies in the Norwegian Sea

AbstractOcean heat content in the Norwegian Sea exhibits pronounced variability on interannual to decadal time scales. These ocean heat anomalies are known to influence Arctic sea ice extent, marine ecosystems, and continental climate. It nevertheless remains unknown to what extent such heat anomalies are produced locally within the Norwegian Sea, and to what extent the region is more of a passive receiver of anomalies formed elsewhere. A main practical challenge has been the lack of closed heat budget diagnostics. In order to address this issue, a regional heat budget is calculated for the Norwegian Sea using the ECCOv4 ocean state estimate—a dynamically and kinematically consistent model framework fitted to ocean observations for the period 1992–2015. The depth‐integrated Norwegian Sea heat budget shows that both ocean advection and air‐sea heat fluxes play an active role in the formation of interannual heat content anomalies. A spatial analysis of the individual heat budget terms shows that ocean advection is the primary contributor to heat content variability in the Atlantic domain of the Norwegian Sea. Anomalous heat advection furthermore depends on the strength of the Atlantic water inflow, which is related to large‐scale circulation changes in the subpolar North Atlantic. This result suggests a potential for predicting Norwegian Sea heat content based on upstream conditions. However, local surface forcing (air‐sea heat fluxes and Ekman forcing) within the Norwegian Sea substantially modifies the phase and amplitude of ocean heat anomalies along their poleward pathway, and, hence, acts to limit predictability.

Early Spring Oceanic Heat Fluxes and Mixing Observed from Drift Stations North of Svalbard*

Abstract From several drifting ice stations north of Svalbard, Norway, observations were made in early spring of the ocean turbulent characteristics in the upper 150 m using a microstructure profiler and close to the under-ice surface using eddy correlation instrumentation. The dataset is used to obtain average heat fluxes at the ice–water interface, in the mixed layer, across the main pycnocline, as well over different water masses in the region. The results are contrasted with proximity to the branches of the warm and saline Atlantic water current, the West Spitsbergen Current (WSC), which is the main oceanic heat and salinity source both to the region and to the Arctic Ocean. Hydrographic properties show that the surface water mass modification is typically due to atmospheric cooling with relatively less influence of ice melting. Surface heat fluxes of O(100) W m−2 are found within the branches of the WSC and over shelf areas with elevated levels of mixing due to strong tides. Away from the shelves and WSC, however, ocean-to-ice turbulent heat fluxes are typical of the central Arctic. Deeper in the water column, entrainment from below together with equally important horizontal advection and diffusion increase the heat content of the mixed layer and contribute to the heat flux maximum in the upper layers. The results in this study emphasize the importance of mixing along the boundaries, over shelves, and topography for the cooling of the Atlantic water layer in the Arctic in general, and for the regional heat budget, hence the ice cover and cooling of the WSC north of Svalbard, in particular.

Mixing on the continental slope of the southern Weddell Sea

Shipboard hydrography and current profiles collected in 2003 and time series from moored current meters deployed in late 1990s are analyzed to study the variability of mixing in the southeastern Weddell Sea. Profiles of eddy diffusivity Kρ are inferred from fine‐scale shear (vertical derivative of horizontal velocity) and strain (vertical derivative of isopycnal displacement) variance using parameterizations which relate the internal wave energy to the dissipation rate at small scales. The highest mixing rates are seen near the bottom where the eddy diffusivities are elevated by 1 order of magnitude from those in the interior and exceed 10−4 m2 s−1. The observations show latitudinal variability in Kρ, particularly near the bottom, where Kρ significantly increases near 74° 28′S, the critical latitude for lunar semidiurnal (M2) tides. In this region, the critical latitude coincides with near‐critical topography on the upper continental slope, a situation which favors generation of M2 internal waves. Consistent with the results from fine‐scale shear and strain parameterizations, which indicate highest bottom diffusivities near the critical latitude, independent analysis of current time series from moored instruments shows a thickening of the frictional bottom boundary layer near the critical latitude. Semidiurnal tidal dynamics at the upper continental slope together with the critical latitude effects lead to mixing that might significantly affect the regional heat budget and the circulation in the study area.

Towards closure of regional heat budgets in the North Atlantic using Argo floats and surface flux datasets

Abstract. The upper ocean heat budget (0–300 m) of the North Atlantic from 20°–60° N is investigated using data from Argo profiling floats for 1999–2005 and the NCEP/NCAR and NOC surface flux datasets. Estimates of the different terms in the budget (heat storage, advection, diffusion and surface exchange) are obtained using the methodology developed by Hadfield et al. (2007a, b). The method includes optimal interpolation of the individual profiles to produce gridded fields with error estimates at a 10°×10° grid box resolution. Closure of the heat budget is obtained within the error estimates for some regions – particularly the eastern subtropical Atlantic – but not for those boxes that include the Gulf Stream. Over the whole range considered, closure is obtained for 13 (9) out of 20 boxes with the NOC (NCEP/NCAR) surface fluxes. The seasonal heat budget at 20–30° N, 35–25° W is considered in detail. Here, the NCEP based budget has an annual mean residual of −55±35 Wm−2 compared with a NOC based value of −4±35 Wm−2. For this box, the net heat divergence of 36 Wm−2 (Ekman=−4 Wm−2, geostrophic=11 Wm−2, diffusion=29 Wm−2) offsets the net heating of 32 Wm−2 from the NOC surface heat fluxes. The results in this box are consistent with an earlier evaluation of the fluxes using measurements from research buoys in the subduction array which revealed biases in NCEP but good agreement of the buoy values with the NOC fields.

An Assessment of the Southern Ocean Mixed Layer Heat Budget

Abstract The mixed layer heat balance in the Southern Ocean is examined by combining remotely sensed measurements and in situ observations from 1 June 2002 to 31 May 2006, coinciding with the period during which Advanced Microwave Scanning Radiometer-Earth Observing System (EOS) (AMSR-E) sea surface temperature measurements are available. Temperature/salinity profiles from Argo floats are used to derive the mixed layer depth. All terms in the heat budget are estimated directly from available data. The domain-averaged terms of oceanic heat advection, entrainment, diffusion, and air–sea flux are largely consistent with the evolution of the mixed layer temperature. The mixed layer temperature undergoes a strong seasonal cycle, which is largely attributed to the air–sea heat fluxes. Entrainment plays a secondary role. Oceanic advection also experiences a seasonal cycle, although it is relatively weak. Most of the seasonal variations in the advection term come from the Ekman advection, in contrast with western boundary current regions where geostrophic advection controls the total advection. Substantial imbalances exist in the regional heat budgets, especially near the northern boundary of the Antarctic Circumpolar Current. The biggest contributor to the surface heat budget error is thought to be the air–sea heat fluxes, because only limited Southern Hemisphere data are available for the reanalysis products, and hence these fluxes have large uncertainties. In particular, the lack of in situ measurements during winter is of fundamental concern. Sensitivity tests suggest that a proper representation of the mixed layer depth is important to close the budget. Salinity influences the stratification in the Southern Ocean; temperature alone provides an imperfect estimate of mixed layer depth and, because of this, also an imperfect estimate of the temperature of water entrained into the mixed layer from below.

Hydrological climate in Japan (4): Representative wind speed, annual variation of the surface heat balance, and evaporation from a paved surface and a lawnland

An arrangement of the representative wind speed at 66 meteorological stations in Japan is made, which is available to the regional heat budget analysis. Annual evapotranspiration from the lawnland of the meteorological observation field is estimated by the heat budget method with use of routine data of the ground surface temperature. The lawnland evapotranspiration is smaller than the forest evapotranspiration by 300mmy-1. Approximate estimation of evaporation from a paved surface is made, being about 100mmy-1 in northern Japan and about 200mmy-1 in southern Japan. This value is the lower limit of the land evaporation.

Polynyas and leads: An overview of physical processes and environment

Polynyas and leads are openings in pack ice due to divergences in ice drift and to local melting. They are the vents and windows to the polar oceans. In winter they are a major source of brine during freezing and a locus for gas exchange. Large sensible heat fluxes, together with evaporation and longwave radiation from a very small percentage of open water and thin ice, dominate regional heat budgets. In summer, solar radiation is absorbed by open water but is reflected from snow‐covered pack ice. Experiments and models describing these processes are reviewed.

Modeling distinct vortices of the ocean

On March 23–25, 1987, with the support of the Office of Naval Research, a 3‐day colloquium was held at the Florida State University, Tallahassee, to discuss and summarize the recent developments in modeling of oceanic distinct vortices, such as Gulf Stream rings, isolated eddies, and solitary waves. The continuing interest in these oceanic features stems from their ubiquity, large amplitudes, and significant effects. Indeed, they not only contribute, with mesosca–20 days in time) but also affect the general circulation, regional heat budgets, and open ocean air‐sea interactions, not to mention their biological and other impacts.