Anomalous Heat Advection Research Articles

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.

Large and local-scale features associated with heat waves in the United States in reanalysis products and the NARCCAP model ensemble

This study investigates the role of large-scale and local-scale processes associated with heat waves using two reanalysis products, and evaluates the performance of the regional climate model ensemble used in the North America Regional Climate Change Program (NARCCAP) in reproducing these processes. The Continental US is divided into eight different climate divisions to investigate different mechanisms associated with heat waves. At the large scale, heat waves are associated with terrestrial warm air advection to the Northeast, Midwest and Northern Great Plains, and with oceanic warm air advection for the Southeast and Southern Great Plains. Over the western US, reduced maritime cool air advection results in local warming. At the local scale, antecedent precipitation deficits lead to the continuous drying of soil, more net radiative energy is partitioned into sensible heat flux and acts to warm surface air temperature, especially over the Great Plains. NARCCAP-simulated large-scale meteorological patterns and temporal evolution of antecedent local scale terrestrial conditions are shown to be very similar to those of the reanalysis products. Even though extreme temperature is overestimated by the NARCCAP ensemble over the US, different heat wave metrics are realistically represented both in terms of the inter-annual variability and spatial representation. However, NARCCAP overestimates the magnitude of heatwaves over the Northeast, Midwest and Northern Great Plains, partially due to anomalous heat advection through large-scale forcing.

Warm Greenland during the last interglacial: the role of regional changes in sea ice cover

Abstract. The last interglacial, also known as the Eemian, is characterized by warmer than present conditions at high latitudes. This is implied by various Eemian proxy records as well as by climate model simulations, though the models mostly underestimate the warming with respect to proxies. Simulations of Eemian surface air temperatures (SAT) in the Northern Hemisphere extratropics further show large variations between different climate models, and it has been hypothesized that this model spread relates to diverse representations of the Eemian sea ice cover. Here we use versions 3 and 4 of the Community Climate System Model (CCSM3 and CCSM4) to highlight the crucial role of sea ice and sea surface temperatures changes for the Eemian climate, in particular in the North Atlantic sector and in Greenland. A substantial reduction in sea ice cover results in an amplified atmospheric warming and thus a better agreement with Eemian proxy records. Sensitivity experiments with idealized lower boundary conditions reveal that warming over Greenland is mostly due to a sea ice retreat in the Nordic Seas. In contrast, sea ice changes in the Labrador Sea have a limited local impact. Changes in sea ice cover in either region are transferred to the overlying atmosphere through anomalous surface energy fluxes. The large-scale spread of the warming resulting from a Nordic Seas sea ice retreat is mostly explained by anomalous heat advection rather than by radiation or condensation processes. In addition, the sea ice perturbations lead to changes in the hydrological cycle. Our results consequently imply that both temperature and snow accumulation records from Greenland ice cores are sensitive to sea ice changes in the Nordic Seas but insensitive to sea ice changes in the Labrador Sea. Moreover, the simulations suggest that the uncertainty in the Eemian sea ice cover accounts for 1.6 °C of the Eemian warming at the NEEM ice core site. The estimated Eemian warming of 5 °C above present day based on the NEEM δ15N record can be reconstructed by the CCSM4 model for the scenario of a substantial sea ice retreat in the Nordic Seas combined with a reduced Greenland ice sheet.

Effects of Kuroshio Intrusions on the atmosphere northeast of Taiwan Island

AbstractThe Kuroshio loses bathymetric support off northeast Taiwan Island, causing large variability in its path. The resultant covariability of sea surface temperature (SST) and the lower atmosphere is investigated using satellite observations. In winter and spring off northeast Taiwan Island, the intrusions of warm Kuroshio water onto the continental shelf cause a large increase in local SST, intensify the northeasterly monsoonal winds, and lead to the increases in water vapor and rainfall. Key to this air‐sea interaction is the existence of anomalous heat advection by the Kuroshio intrusions. The Kuroshio intrusions are partly due to westward propagating ocean eddies east of Taiwan Island with a lead time of 3 weeks, hinting at the possibility of improved weather prediction near northeast Taiwan Island by considering ocean variability east of Taiwan Island.

Response of the Pacific and Atlantic oceans to interannual variations in net atmospheric freshwater

The response of the Pacific and Atlantic Oceans to net atmospheric freshwater (evaporation minus precipitation (EmP)) between 1988 and 2000 was studied using a global ocean general circulation model developed in the Massachusetts Institute of Technology. Evaporation estimates from the Goddard Satellite Surface Turbulent Fluxes and precipitation estimates from the Global Precipitation Climatology Project were used. Model simulations showed that the spatial distribution of the average sea surface salinity (SSS) changes during the 1988–2000 period resembled that of average EmP changes, because SSS changes were primarily associated with anomalous vertical mixing forced by the anomalous EmP. The spatial distribution of average near‐surface temperature anomalies, however, was different from those of average EmP and SSS anomalies. Analyses indicated that temperature changes in the subtropical North and South Pacific resulted from anomalous heat advection which, in turn, resulted from changes in the subtropical gyre circulations. Temperature changes in the tropical Pacific were associated with anomalous heat advection due to changes in the South Equatorial Current (SEC). Furthermore, the changes in the subtropical gyres and the SEC resulted largely from anomalous EmP in the subtropical Pacific. Temperature changes in the Atlantic, however, were largely associated with vertical mixing changes due to anomalous EmP. Observed interannual variations of SSS in the Western Pacific Warm Pool were simulated successfully. A large difference between simulated and observed SSS, however, was found in the central equatorial Pacific. Simulated SSS and temperature varied at interannual and longer timescales in most of the Pacific and Atlantic Oceans. Large changes in SSS and temperature due to anomalous EmP in the Pacific and Atlantic strongly suggest that EmP variability can play an important role in ocean and climate variabilities at interannual and longer timescales.

Analytical study of anomalous heat advection accompanying an NAO/NAM signature over the Eurasian continent during winter and spring

Analytical study of anomalous heat advection accompanying an NAO/NAM signature over the Eurasian continent during winter and spring

Decadal Variability in Coupled Sea-Ice–Thermohaline Circulation Systems*

An interdecadal oscillation in a coupled ocean‐ice system was identified in a previous study. This paper extends that study to further examine the stability of the oscillation and the sensitivity of its frequency to various parameters and forcing fields. Three models are used: (i) an analytical box model; (ii) a two-dimensional model for the ocean thermohaline circulation (THC) coupled to a thermodynamic ice model, as in the authors’ previous study; (iii) a three-dimensional ocean general circulation model (OGCM) coupled to a similar ice model. The box model is used to elucidate the essential feedbacks that give rise to this oscillation and to identify the most important parameters and processes that determine the period. Numerical experiments in the 2D THC‐ice model show that the model stability is sensitive to the ocean‐ice coupling coefficient, the eddy diffusivity, and the strength of the thermohaline-circulation feedback per unit surface-polar density perturbation. The coupled model becomes more stable toward low coupling, greater diffusion, and weaker THC feedback. The period of the oscillation is less sensitive to these parameters. Nonlinear effects in the sea-ice model become important in the higher ocean‐ice coupling regime where the effective sea-ice damping associated with this nonlinearity stabilizes the model. Surface Newtonian damping is also tested. The 3D OGCM, which includes both wind stress and buoyancy forcings, is used to test this coupled ocean‐ice mechanism in a more realistic model setting. This model generates an interdecadal oscillation whose characteristics and phase relations among the model variables are similar to the oscillation obtained in the 2D models. The major difference is that the oscillation frequency is considerably lower. This difference can be explained in terms of the analytical box model solution in which the period of the oscillation depends on the rate of anomalous density production by melting/cooling of sea ice per SST anomaly, times the rate of warming/cooling by anomalous THC heat advection per change in density anomaly. The 3D model has a smaller THC response to high-latitude density perturbations than in the 2D model, and anomalous velocities in the 3D case tend to follow the mean isotherms so the anomalous heat advection is reduced. This slows the ocean‐ice feedback process, leading to the longer oscillation period.

The Differences between Early and Midwinter Atmospheric Responses to Sea Surface Temperature Anomalies in the Northwest Atlantic

Using an atmospheric global spectral model, it is shown that the winter atmosphere in the midlatitudes is capable of reacting to prescribed sea surface temperature (SST) anomalies in the northwest Atlantic with two very different responses. The nature of the response is determined by the climatological conditions of the winter regime. Experiments are performed using either the perpetual November or January conditions with or without the prescribed SST anomalies. Warm SST anomalies in November result in a highly significant anomalous ridge downstream over the Atlantic with a nearly equivalent barotropic structure; in January, the response is a statistically less significant trough. The presence of the SST anomalies also causes a northward (southward) shift of the Atlantic storm track in the November (January) cases. A diagnostic analysis of the anomalous heat advection in the simulations reveals that in the January cases, the surface heating is offset primarily by the strong horizontal cold advection in the lower troposphere. In the November cases, there is a vitally important vertical heat advection through which a potential positive ocean-atmosphere feedback was found. The positive air temperature anomalies exhibit a deep vertical penetration in the November cases but not in the January cases. The simulated atmospheric responses to the warm SST anomalies in the November and January cases are found to be in qualitative agreement with the observational results using 50-yr ( 1930-1979) records. The atmospheric responses to the cold SST anomalies in the simulations are found to be insignificant.