Heat Content Variability Research Articles

A Comparison of the Variability and Changes in Global Ocean Heat Content from Multiple Objective Analysis Products During the Argo Period

AbstractOcean heat content (OHC) is key to estimating the energy imbalance of the earth system. Over the past two decades, an increasing number of OHC studies were conducted using oceanic objective analysis (OA) products. Here we perform an intercomparison of OHC from eight OA products with a focus on their robust features and significant differences over the Argo period (2005-2019), when the most reliable global scale oceanic measurements are available. For the global ocean, robust warming in the upper 2000 m is confirmed. The 0-300 m layer shows the highest warming rate but is heavily modulated by interannual variability, particularly the El Niño–Southern Oscillation. The 300-700 m and 700-2000 m layers, on the other hand, show unabated warming. Regionally, the Southern Ocean and mid-latitude North Atlantic show a substantial OHC increase, and the subpolar North Atlantic displays an OHC decrease. A few apparent differences in OHC among the examined OA products were identified. In particular, temporal means of a few OA products that incorporated other ocean measurements besides Argo show a global-scale cooling difference, which is likely related to the baseline climatology fields used to generate those products. Large differences also appear in the interannual variability in the Southern Ocean and in the long-term trends in the subpolar North Atlantic. These differences remind us of the possibility of product-dependent conclusions on OHC variations. Caution is therefore warranted when using merely one OA product to conduct OHC studies, particularly in regions and on timescales that display significant differences.

Using Acoustic Travel Time to Monitor the Heat Variability of Glacial Fjords

AbstractMonitoring the heat content variability of glacial fjords is crucial to understanding the effects of oceanic forcing on marine-terminating glaciers. A Pressure-sensor equipped Inverted Echo Sounder (PIES) was deployed mid-fjord in Sermilik Fjord in southeast Greenland from August 2011 to September 2012 alongside a moored array of instruments recording temperature, conductivity and velocity. Historical hydrography is used to quantify the relationship between acoustic travel time and the vertically-averaged heat content, and a new method is developed for filtering acoustic return echoes in an ice-influenced environment. We show that PIES measurements, combined with a knowledge of the fjord’s two-layer density structure, can be used to reconstruct the thickness and temperature of the inflowing water. Additionally, we find that fjord-shelf exchange events are identifiable in the travel time record implying the PIES can be used to monitor fjord circulation. Finally, we show that PIES data can be combined with moored temperature records to derive the heat content of the upper layer of the fjord where moored instruments are at great risk of being damaged by transiting icebergs.

CHANGES IN THE HYDROLOGICAL CHARACTERISTICS OF THE ARCTIC OCEAN UNDER THE INFLUENCE OF INCREASING RIVER RUNOFF

The paper considers issues related to the influence of increased Arctic river runoff on the formation of the thermohaline structure of waters outside the Arctic shelf and, in particular, on the variability of heat content of the deep Arctic Ocean. The study is carried out using the three-dimensional numerical model of the ocean and sea ice SibCIOM and atmospheric reanalysis data NCEP/NCAR. Numerical simulation results showed that increased flow of Arctic rivers contributes to the stability of water stratification outside the shelf areas, which reduces the depth of autumn-winter mixing and allows Atlantic waters spreading along the continental slope to retain their heat. This is evidenced by an increase in the heat content of the upper 200 m layer waters of the eastern Eurasian basin and the rise of the Atlantic water layer upper boundary in this region.

The Arctic Subpolar Gyre sTate Estimate: Description and Assessment of a Data‐Constrained, Dynamically Consistent Ocean‐Sea Ice Estimate for 2002–2017

AbstractA description and assessment of the first release of the Arctic Subpolar gyre sTate Estimate (ASTE_R1), a data‐constrained ocean‐sea ice model‐data synthesis, is presented. ASTE_R1 has a nominal resolution of 1/3° and spans the period 2002–2017. The fit of the model to an extensive (O(109)) set of satellite and in situ observations was achieved through adjoint‐based nonlinear least squares optimization. The improvement of the solution compared to an unconstrained simulation is reflected in misfit reductions of 77% for Argo, 50% for satellite sea surface height, 58% for the Fram Strait mooring, 65% for Ice Tethered Profilers, and 83% for sea ice extent. Exact dynamical and kinematic consistency is a key advantage of ASTE_R1, distinguishing the state estimate from existing ocean reanalyses. Through strict adherence to conservation laws, all sources and sinks within ASTE_R1 can be accounted for, permitting meaningful analysis of closed budgets at the grid‐scale, such as contributions of horizontal and vertical convergence to the tendencies of heat and salt. ASTE_R1 thus serves as the biggest effort undertaken to date of producing a specialized Arctic ocean‐ice estimate over the 21st century. Transports of volume, heat, and freshwater are consistent with published observation‐based estimates across important Arctic Mediterranean gateways. Interannual variability and low frequency trends of freshwater and heat content are well represented in the Barents Sea, western Arctic halocline, and east subpolar North Atlantic. Systematic biases remain in ASTE_R1, including a warm bias in the Atlantic Water layer in the Arctic and deficient freshwater inputs from rivers and Greenland discharge.

Climatic Variation of Maximum Intensification Rate for Major Tropical Cyclones over the Western North Pacific

To analyze the dependence of intensification rates of tropical cyclones (TCs) on the variation of environmental conditions, an index is proposed here to measure the lifetime maximum intensification rates (LMIRs) for the Saffir–Simpson scale category 4–5 TCs over the western North Pacific. To quantitatively describe the intensification rate of major TCs, the LMIR is defined as the maximum acceleration in the sustained-wind-speed over a 24-h period of an overwater TC. This new index, LMIR, is generally independent of the indices for RI frequency. The results show that the Pacific Decadal Oscillation (PDO) modulates the inter-annual relationship between the LMIR and El Niño/Southern Oscillation (ENSO). The PDO’s modulation on the ENSO’s effect on the LMIR is explored here by considering the relationship between the LMIR and the environmental conditions in different PDO phases. While the ENSO’s effect on the LMIR for the warm PDO phase is generally by affecting the variations of upper ocean heat content, ENSO mainly influences the variations of zonal wind and vertical wind shear for the cold PDO phase. Our results suggest that fast translating TCs tend to attain strong intensification during the warm PDO phase, while a warm subsurface condition may permit slow-translating TCs also to become strongly intensified during the cooling PDO phase. These findings have an important implication for both prediction of RI and the long-term projection of TC activities in the western North Pacific.

Drivers of Local Ocean Heat Content Variability in ECCOv4

AbstractVariation in upper-ocean heat content is a critical factor in understanding global climate variability. Using temperature anomaly budgets in a two-decade-long physically consistent ocean state estimate (ECCOv4r3; 1992–2015), we describe the balance between atmospheric forcing and ocean transport mechanisms for different depth horizons and at varying temporal and spatial resolutions. Advection dominates in the tropics, while forcing is most relevant at higher latitudes and in parts of the subtropics, but the balance of dominant processes changes when integrating over greater depths and considering longer time scales. While forcing is shown to increase with coarser resolution, overall the heat budget balance between it and advection is remarkably insensitive to spatial scale. A novel perspective on global ocean heat content variability was made possible by combining unsupervised classification with a measure of temporal variability in heat budget terms to identify coherent dynamical regimes with similar underlying mechanisms, which are consistent with prior research. The vast majority of the ocean includes significant contributions by both forcing and advection. However advection-driven regions were identified that coincide with strong currents, such as western boundary currents, the Antarctic Circumpolar Current, and the tropics, while forcing-driven regions were defined by shallower wintertime mixed layers and weak velocity fields. This identification of comprehensive dynamical regimes and the sensitivity of the ocean heat budget analysis to exact resolution (for different depth horizons and at varying temporal and spatial resolutions) should provide a useful orientation for future studies of ocean heat content variability in specific ocean regions.

Interannual Variability of the South Atlantic Ocean Heat Content in a High‐Resolution Versus a Low‐Resolution General Circulation Model

AbstractHigh‐ and low‐resolution coupled climate model simulations are analyzed to investigate the impact of model resolution on South Atlantic Ocean Heat Content (OHC) variability at interannual time scale and the associated physical mechanisms. In both models, ocean heat transport convergence is the main driver of OHC variability on interannual time scales. However, the origin of the meridional heat transport (MHT) convergence anomalies differs in the two models. In the high‐resolution model, OHC variability is dominated by MHT from the south. This is in contrast to the low‐resolution model, where OHC variability is largely controlled by MHT from the north. In the low‐resolution simulation, both the Ekman and geostrophic transports contribute to the OHC variability, whereas in the high‐resolution model, the geostrophic transport dominates. These differences highlight the importance of model resolution to appropriately represent ocean dynamics in the South Atlantic Ocean and associated impacts on regional and global climate.

Multidecadal preconditioning of the Maud Rise polynya region

Abstract. In this paper, we consider Maud Rise polynya formation in a long (250-year) high-resolution (ocean 0.1∘, atmosphere 0.5∘ horizontal model resolution) of the Community Earth System Model. We find a dominant multidecadal timescale in the occurrence of these Maud Rise polynyas. Analysis of the results leads us to the interpretation that a preferred timescale can be induced by the variability of the Weddell Gyre, previously identified as the Southern Ocean Mode. The large-scale pattern of heat content variability associated with the Southern Ocean Mode modifies the stratification in the Maud Rise region and leads to a preferred timescale in convection through preconditioning of the subsurface density and consequently to polynya formation.

Dominant Terms in the Freshwater and Heat Budgets of the Subpolar North Atlantic Ocean and Nordic Seas From 1992 to 2015

AbstractThe Arctic and subarctic oceans exhibit distinct decadal variations in freshwater and heat content. We describe freshwater and heat budgets with the ECCOv4 reanalysis product and compare budget variability and mechanisms within the subpolar North Atlantic Ocean, Nordic Seas, and Labrador Sea from 1992 to 2015. For all regions, changes in freshwater content are largely anticorrelated with changes in heat content. Since 1995, the subpolar North Atlantic Ocean has undergone a decade of warming and salinification followed by ongoing cooling and freshening. The recent increase in freshwater content and the reduction in heat in the subpolar North Atlantic can largely be attributed to anomalous circulation of mean salinity and temperature, respectively. Interannual variability in heat and freshwater mostly corresponds to boundary fluxes from the subtropics. Meanwhile, the Nordic Seas have undergone an overall warming and salinification from the mid‐1990s to 2015. Salinification is primarily driven by reduced sea ice flux through Fram Strait, while warming is due to changes in both sea surface heating and advective flux. In the last 5 years, Labrador Sea freshwater convergence remained unchanged, as increased inflow via the Baffin Island Current is balanced by increased outflow via the Labrador Current. Hence, the observed freshening of the Arctic Ocean is expected to be an increasingly important source of future freshwater increases in the subpolar North Atlantic. This stands in contrast to variability in freshwater flux from the subtropical North Atlantic, which is associated with variability in the Atlantic Meridional Overturning Circulation.

Evaluation of global ocean–sea-ice model simulations based on the experimental protocols of the Ocean Model Intercomparison Project phase 2 (OMIP-2)

Abstract. We present a new framework for global ocean–sea-ice model simulations based on phase 2 of the Ocean Model Intercomparison Project (OMIP-2), making use of the surface dataset based on the Japanese 55-year atmospheric reanalysis for driving ocean–sea-ice models (JRA55-do). We motivate the use of OMIP-2 over the framework for the first phase of OMIP (OMIP-1), previously referred to as the Coordinated Ocean–ice Reference Experiments (COREs), via the evaluation of OMIP-1 and OMIP-2 simulations from 11 state-of-the-science global ocean–sea-ice models. In the present evaluation, multi-model ensemble means and spreads are calculated separately for the OMIP-1 and OMIP-2 simulations and overall performance is assessed considering metrics commonly used by ocean modelers. Both OMIP-1 and OMIP-2 multi-model ensemble ranges capture observations in more than 80 % of the time and region for most metrics, with the multi-model ensemble spread greatly exceeding the difference between the means of the two datasets. Many features, including some climatologically relevant ocean circulation indices, are very similar between OMIP-1 and OMIP-2 simulations, and yet we could also identify key qualitative improvements in transitioning from OMIP-1 to OMIP-2. For example, the sea surface temperatures of the OMIP-2 simulations reproduce the observed global warming during the 1980s and 1990s, as well as the warming slowdown in the 2000s and the more recent accelerated warming, which were absent in OMIP-1, noting that the last feature is part of the design of OMIP-2 because OMIP-1 forcing stopped in 2009. A negative bias in the sea-ice concentration in summer of both hemispheres in OMIP-1 is significantly reduced in OMIP-2. The overall reproducibility of both seasonal and interannual variations in sea surface temperature and sea surface height (dynamic sea level) is improved in OMIP-2. These improvements represent a new capability of the OMIP-2 framework for evaluating process-level responses using simulation results. Regarding the sensitivity of individual models to the change in forcing, the models show well-ordered responses for the metrics that are directly forced, while they show less organized responses for those that require complex model adjustments. Many of the remaining common model biases may be attributed either to errors in representing important processes in ocean–sea-ice models, some of which are expected to be reduced by using finer horizontal and/or vertical resolutions, or to shared biases and limitations in the atmospheric forcing. In particular, further efforts are warranted to resolve remaining issues in OMIP-2 such as the warm bias in the upper layer, the mismatch between the observed and simulated variability of heat content and thermosteric sea level before 1990s, and the erroneous representation of deep and bottom water formations and circulations. We suggest that such problems can be resolved through collaboration between those developing models (including parameterizations) and forcing datasets. Overall, the present assessment justifies our recommendation that future model development and analysis studies use the OMIP-2 framework.

Budgets for Decadal Variability in Pacific Ocean Heat Content

AbstractA slowdown in the rate of surface warming in the early 2000s led to renewed interest in the redistribution of ocean heat content (OHC) and its relationship with internal climate variability. We use the Community Earth System Model version 1 to study the relationship between OHC and the interdecadal Pacific oscillation (IPO), a major mode of decadal sea surface temperature variability in the Pacific Ocean. By comparing the relative contributions of surface heat flux and ocean dynamics to changes in OHC for different phases of the IPO, we try to identify the underlying physical processes involved. Our results suggest that during IPO phase transitions, changes of 0–300-m OHC across the northern extratropical Pacific are positively contributed by both surface heat flux and oceanic heat transport. By contrast, oceanic heat transport appears to drive the OHC changes in equatorial Pacific whereas surface heat flux acts as a damping term. During a positive IPO phase, weakened wind-driven circulation acts to increase the OHC in the equatorial Pacific while the enhanced evaporation acts to damp OHC anomalies. In the Kuroshio–Oyashio Extension region, a dipole anomaly of zonal heat advection amplifies an OHC dipole anomaly that moves eastward, while strong turbulent heat fluxes act to dampen this OHC anomaly. In the northern subtropical Pacific, both the wind-driven evaporation change and the change of zonal heat advection along Kuroshio Extension contribute to the OHC change during phase transition. For the northern subpolar Pacific, both surface heat flux and enhanced meridional advection contribute to the positive OHC anomalies during the positive IPO phase.

A method for constructing synthetic parallel tracks for augmenting in-situ data with satellite observations

In-situ observations have significant importance for calibration and validation of satellite data, processes studies etc, however in-situ observations are often available with less spatio-temporal coverage due to practical limitations in making observations, as well as cost effectiveness. Albeit satellite observations have better spatial and temporal coverage, they have certain limitations like obstacles of cloud coverage, retrieval algorithms etc. Hence worthy inferences on the processes (that was intended to study) can be obtained only through skillful utilization of both in-situ observations, satellite and/or reanalysis products. We have illustrated a method for deriving synthetic parallel tracks to in-situ track/survey. Using this method, the satellite and/or reanalysis products can be extracted over these synthetic parallel tracks to derive worthy understanding/conclusions on the processes. The points on the original track are reproduced in terms of slope and distance, which can be utilized to construct a new synthetic parallel track initiating at a predetermined distance to the starting point of original track. As an example, we have demonstrated variations in oceanic physical properties along synthetic parallel tracks to the track of the Khai-muk cyclone. It is easy to understood using this technique that the variations in sub-surface heat content (as evidenced by variations in sea level along Khai-muk cyclone track) have a relatively major role in controlling the intensity of the system, compared to the changes in surface temperature. The method illustrated here is applicable to any geophysical track/survey with varying spatial and/or temporal extension.

Conductors of Heat Content Changes of the North Atlantic Ocean Layers

It is considered that changes of the North Atlantic’s heat content can be a source of surprises in the evolution of the present-day climate, especially the climate of Europe. To investigate this problem, the RAPID monitoring program has been organized. However, results of such monitoring still cover too short a time interval to reach any particular conclusions. In this study, using wavelets, heat content variations in the 0–700 and 0–2000 m oceanic layers were analyzed since the mid-20th century. Their relationships with mean temperature variations of the 0–100 m layer, as well as with earlier analyzed temperature variations of the North Atlantic sea-surface temperature, also are analyzed. It is found that, against a total increase in heat content, variations of all specified characteristics reveal a periodicity similar to the known approximately 22‑year-long Hale heliomagnetic activity cycle. In the 0–700 m layer, this periodicity leading on its phase not only the corresponding periodicity in the 0–2000 m layer, which corresponds well to the generally accepted opinion that heat content anomalies extend from top ocean layers down, but also in the mean temperature of the 0–100 m layer, as well as in the sea-surface temperature. This makes it possible to consider that heliomagnetic activity immediately influences the upper 0–700 m layer of the ocean, thence extending down and up.

What Drives Upper-Ocean Temperature Variability in Coupled Climate Models and Observations?

AbstractA key question in climate modeling is to what extent sea surface temperature and upper-ocean heat content are driven passively by air–sea heat fluxes, as opposed to forcing by ocean dynamics. This paper investigates the question using a climate model at different resolutions, and observations, for monthly variability. At the grid scale in a high-resolution climate model with resolved mesoscale ocean eddies, ocean dynamics (i.e., ocean heat flux convergence) dominates upper 50 m heat content variability over most of the globe. For deeper depths of integration to 400 m, the heat content variability at the grid scale is almost totally controlled by ocean heat flux convergence. However, a strong dependence on spatial scale is found—for the upper 50 m of ocean, after smoothing the data to around 7°, air–sea heat fluxes, augmented by Ekman heat transports, dominate. For deeper depths of integration to 400 m, the transition scale becomes larger and is above 10° in western boundary currents. Comparison of climate model results with observations show that the small-scale influence of ocean intrinsic variability is well captured by the high-resolution model but is missing from a comparable model with parameterized ocean-eddy effects. In the deep tropics, ocean dynamics dominates in all cases and all scales. In the subtropical gyres at large scales, air–sea heat fluxes play the biggest role. In the midlatitudes, at large scales >10°, atmosphere-driven air–sea heat fluxes and Ekman heat transport variability are the dominant processes except in the western boundary currents for the 400 m heat content.

Analyzing the Influence of the North Atlantic Ocean Variability on the Atlantic Meridional Mode on Decadal Time Scales

Important features of the Atlantic meridional mode (AMM) are not fully understood. We still do not know what determines its dominant decadal variability or the complex physical processes that sustain it. Using reanalysis datasets, we investigated the influence of the North Atlantic Ocean variability on the dominant decadal periodicity that characterizes the AMM. Statistical analyses demonstrated that the correlation between the sea surface temperature decadal variability in the Atlantic Ocean and the AMM time series characterizes the Atlantic multidecadal oscillation (AMO). This corroborates previous studies that demonstrated that the AMO precedes the AMM. A causal inference with a newly developed rigorous and quantitative causality analysis indicates that the AMO causes the AMM. To further understand the influence of the subsurface ocean on the AMM, the relationship between the ocean heat content (0–300 m) decadal variability and AMM was analyzed. The results show that although there is a significant zero-lag correlation between the ocean heat content in some regions of the North Atlantic (south of Greenland and in the eastern part of the North Atlantic) and the AMM, their cause-effect relationship on decadal time scales is unlikely. By correlating the AMO with the ocean heat content (0–300 m) decadal variability, the former precedes the latter; however, the causality analysis shows that the ocean heat content variability drives the AMO, corroborating several studies that point out the dominant role of the ocean heat transport convergence on AMO.

Ocean Heat Content and the Intraseasonal Oscillation

AbstractIntraseasonal sea surface temperature anomalies generally cool during the convectively active phase of the intraseasonal oscillation in the Indian Ocean, but the behavior of intraseasonal ocean heat content anomalies is quite different. This is demonstrated using satellite observations and ocean reanalysis data. Ocean heat content anomalies increase during the convectively active phase of the intraseasonal oscillation and decrease during the convectively suppressed phase. Much of the intraseasonal variability of ocean heat content is westward propagating, moving in the opposite direction of the intraseasonal oscillation's convective envelope. While sea surface temperature anomalies are strongly regulated by variations in surface fluxes, their out of phase relationship with ocean heat content suggests that different processes are modulating the reservoir of warm water in the upper ocean. We hypothesize that oceanic equatorial waves are the primary forcing of intraseasonal ocean heat content anomalies during intraseasonal oscillation events.

The Importance of Systematic Spatial Variability in the Surface Heat Flux of a Large Lake: A Multiannual Analysis for Lake Geneva

AbstractThe spatiotemporal surface heat flux (SurHF) distribution over Lake Geneva, the largest lake in Western Europe, was estimated for a 7‐year period (2008–2014). Data sources included hourly maps of over‐the‐lake assimilated meteorological data from a validated numerical weather model and lake surface water temperature (LSWT) from satellite imagery. A set of bulk algorithms, previously optimized and calibrated at two locations in Lake Geneva, was used. Results indicate a systematic long‐term average spatial range of >40 Wm‐2 between different parts of the lake and little year‐to‐year variability. This variability is mainly due to topographically induced wind sheltering over parts of the lake, which in turn produces spatial variability in the sensible and latent heat fluxes. These results are supported by a systematic spatial heat content variability obtained from long‐term temperature profile measurements in the lake. During spring, a lower SurHF spatial range was evident. Unlike other seasons, the spring spatial variability of air‐water temperature differences and, to a lesser extent, the global radiation variability resulting from sheltering by the mountainous topography were the main drivers of the SurHF spatial variability. Analysis of the atmospheric thermal boundary layer showed stable conditions from March to early June and unstable conditions for the rest of the year. This regime change can explain the low SurHF spatial variability observed during spring. The results emphasize that spatial variability in meteorological and LSWT patterns, and consequently in the spatiotemporal SurHF data, should be considered when assessing the time evolution of the heat budget of large lakes.

Heat budget analysis during the ice-melting season in the Chukchi Sea based on a model simulation

Rapid changes of sea ice in the Chukchi Sea are substantially influencing the marine ecosystem and carbon cycling in the region. In this study, sea ice variations during 1994–2015 are simulated with a coupled ocean and sea ice model covering the North Atlantic-North Pacific-Arctic Oceans (NAPA) with a nominal horizontal resolution of 1/4° in latitude/longitude. The model is based on version 3.6 of the Nucleus for European Modelling of the Ocean (NEMO) and version 3 of Louvain-la-Neuve Sea Ice Model (LIM). The analysis is focused on the heat budget of sea ice and sea water in the Chukchi Sea during the ice-melting season, defined as the duration with positive net water flux from sea ice to sea water. Averaged over 1994–2015, sea ice melting in the Chukchi Sea occurs from mid-May to mid-October. The relative contributions of melting at the surface and bottom of the sea ice vary spatially in relationship to the variations of sea ice concentration, in particular from June to August when the ice melts relatively rapidly. The rate of melting at the ice surface is much higher in areas with ice concentration greater than 80%, while ice bottom melting is more important elsewhere. Bottom melting accounts for 68% of the total seasonal ice loss. Further analysis of the heat budget of the sea water column reveals the contributions of net heat flux at sea water surface (including open waters, ice leads, and under ice covered areas) and Pacific inflow heat flux to ice bottom melting. First, during the entire ice-melting season, the Pacific inflow heat flux contributes ~67% of the total heat input to the sea water in the study area. Among the total heat flux input to the sea water, about 58% is used to heat the sea water, 28% is used to melt ice bottom, and 14% is transported downstream. Secondly, during the early and peak stages of ice melting (from mid-May to early August), net heat flux at sea water surface makes the dominant contribution to ice bottom melting. During the late and final stages (from mid-August to mid-October), the Pacific inflow heat flux plays a leading role and continues to provide heat during the early freezing stage, acts to slow down the freezing rate, and extends the ice-free season. Finally, ~60% of the net heat flux at ice surface is used for ice surface melting. Very little of the net heat flux at ice surface could penetrate the sea ice into the sea water for ice bottom melting. During the ice-melting season of 1994–2015, the time-mean and standard deviation of interannual variations of the following three variables in the Chukchi Sea are: (3.16 ±0.67)×1020 J for the heat content variation in the Chukchi Sea, (3.65±0.69)×1020 J for the Pacific inflow heat flux, and (1.81 ±0.46)×1020 J for the net heat flux at sea water surface. Interannual variation of heat flux causing ice bottom melting is negatively correlated with the heat content variation in the Chukchi Sea ( R =−0.62; P R =0.96; P R =1.0; P 6.06×1018 J per year, while the heat content variation in the Chukchi Sea has been increasing by 6.03×1018 J per year. Overall, the present model results suggest that variations upstream of the Pacific inflow play an important role in variations of sea ice area in the Chukchi Sea.

Variability of Sea Level and Upper-Ocean Heat Content in the Indian Ocean: Effects of Subtropical Indian Ocean Dipole and ENSO

Abstract In this study, the Indian Ocean upper-ocean variability associated with the subtropical Indian Ocean dipole (SIOD) is investigated. We find that the SIOD is associated with a prominent southwest–northeast sea level anomaly (SLA) dipole over the western-central south Indian Ocean, with the north pole located in the Seychelles–Chagos thermocline ridge (SCTR) and the south pole at southeast of Madagascar, which is different from the distribution of the sea surface temperature anomaly (SSTA). While the thermocline depth and upper-ocean heat content anomalies mirror SLAs, the air–sea CO2 flux anomalies associated with SIOD are controlled by SSTA. In the SCTR region, the westward propagation of oceanic Rossby waves generated by anomalous winds over the eastern tropical Indian Ocean is the major cause for the SLAs, with cyclonic wind causing negative SLAs during positive SIOD (pSIOD). Local wind forcing is the primary driver for the SLAs southeast of Madagascar, with anticyclonic winds causing positive SLAs. Since the SIOD is correlated with ENSO, the relative roles of the SIOD and ENSO are examined. We find that while ENSO can induce significant SLAs in the SCTR region through an atmospheric bridge, it has negligible impact on the SLA to the southeast of Madagascar. By contrast, the SIOD with ENSO influence removed is associated with an opposite SLA in the SCTR and southeast of Madagascar, corresponding to the SLA dipole identified above. A new subtropical dipole mode index (SDMI) is proposed, which is uncorrelated with ENSO and thus better represents the pure SIOD effect.

Insights into Decadal North Atlantic Sea Surface Temperature and Ocean Heat Content Variability from an Eddy-Permitting Coupled Climate Model

Abstract An ocean mixed layer heat budget methodology is used to investigate the physical processes determining subpolar North Atlantic (SPNA) sea surface temperature (SST) and ocean heat content (OHC) variability on decadal to multidecadal time scales using the state-of-the-art climate model HadGEM3-GC2. New elements include development of an equation for evolution of anomalous SST for interannual and longer time scales in a form analogous to that for OHC, parameterization of the diffusive heat flux at the base of the mixed layer, and analysis of a composite Atlantic meridional overturning circulation (AMOC) event. Contributions to OHC and SST variability from two sources are evaluated: 1) net ocean–atmosphere heat flux and 2) all other processes, including advection, diffusion, and entrainment for SST. Anomalies in OHC tendency propagate anticlockwise around the SPNA on multidecadal time scales with a clear relationship to the phase of the AMOC. AMOC anomalies lead SST tendencies, which in turn lead OHC tendencies in both the eastern and western SPNA. OHC and SST variations in the SPNA on decadal time scales are dominated by AMOC variability because it controls variability of advection, which is shown to be the dominant term in the OHC budget. Lags between OHC and SST are traced to differences between the advection term for OHC and the advection–entrainment term for SST. The new results have implications for interpretation of variations in Atlantic heat uptake in the CMIP6 climate model assessment.