Effects of Ekman Transport on the NAO Response to a Tropical Atlantic SST Anomaly

2006 ◽  
Vol 19 (19) ◽  
pp. 4803-4818 ◽  
Author(s):  
Shiling Peng ◽  
Walter A. Robinson ◽  
Shuanglin Li ◽  
Michael A. Alexander
Keyword(s):  
North Atlantic ◽  
Surface Heat Flux ◽  
Coupled Model ◽  
Surface Heat ◽  
Ekman Transport ◽  
Tropical Atlantic ◽  
Tropical Forcing ◽  
Mixed Layer Ocean ◽  
Sst Anomaly ◽  
Coupled Response

Abstract A recent study showed that a tropical Atlantic sea surface temperature (SST) anomaly induces a significant coupled response in late winter [February–April (FMA)] in a coupled model, in which an atmospheric general circulation model is coupled to a slab mixed layer ocean model (AGCM_ML). The coupled response comprises a dipole in the geopotential height, like the North Atlantic Oscillation (NAO), and a North Atlantic tripole in the SST. The simulated NAO response developed 1 or 2 months later in the model than in observations. To determine the possible effects of Ekman heat transport on the development of the coupled response to the tropical forcing, an extended coupled model (AGCM_EML), including Ekman transport in the slab mixed layer ocean, is now used. Large ensembles of AGCM_EML experiments are performed, parallel to the previous AGCM_ML experiments, with the model forced by the same tropical Atlantic SST anomaly over the boreal winter months (September–April). The inclusion of Ekman heat transport is found to result in an earlier development of the coupled NAO–SST tripole response in the AGCM_EML, compared to that in the AGCM_ML. The mutual reinforcement between the anomalous Ekman transport and the surface heat flux causes the tropical forcing to induce an extratropical SST response in November–January (NDJ) in the AGCM_EML that is twice as strong as that in the AGCM_ML. The feedback of this stronger extratropical SST response on the atmosphere in turn drives the development of the NAO response in NDJ. In FMA, the sign of the anomalous surface heat flux is reversed in the Gulf Stream region such that it opposes the anomalous Ekman transport. The resulting equilibrium NAO response in the AGCM_EML is similar to that in the AGCM_ML, but it is reached 1–2 months sooner in the AGCM_EML. Hence, the presence of Ekman transport causes a seasonal shift in the evolution of the coupled response. The faster development of the NAO response in the AGCM_EML suggests that tropical Atlantic SST anomalies should be able to influence the NAO, in nature, on the seasonal time scale, and that efficient interactions with the extratropical ocean play a significant role in determining the coupled response.

scholarly journals Air–Sea Interactions in the Tropical Atlantic: A View Based on Lagged Rotated Maximum Covariance Analysis

2005 ◽  
Vol 18 (18) ◽  
pp. 3874-3890 ◽  
Author(s):  
Claude Frankignoul ◽  
Elodie Kestenare
Keyword(s):  
Heat Flux ◽  
Surface Heat Flux ◽  
Surface Heat ◽  
Covariance Analysis ◽  
Atmospheric Response ◽  
Tropical Atlantic ◽  
Maximum Covariance Analysis ◽  
Wind Response ◽  
Sst Anomaly ◽  
Heat Flux Feedback

Abstract The dominant air–sea feedbacks that are at play in the tropical Atlantic are revisited, using the 1958–2002 NCEP reanalysis. To separate between different modes of variability and distinguish between cause and effect, a lagged rotated maximum covariance analysis (MCA) of monthly sea surface temperature (SST), wind, and surface heat flux anomalies is performed. The dominant mode is the ENSO-like zonal equatorial SST mode, which has its maximum amplitude in boreal summer and is a strongly coupled ocean–atmosphere mode sustained by a positive feedback between wind and SST. The turbulent heat flux feedback is negative, except west of 25°W where it is positive, but countered by a negative radiative feedback associated with the meridional displacement of the ITCZ. As the maximum covariance patterns change little between lead and lag conditions, the in-phase covariability between SST and the atmosphere can be used to infer the atmospheric response to the SST anomaly. The second climate mode involves an SST anomaly in the tropical North Atlantic, which is primarily generated by the surface heat flux and, in boreal winter, wind changes off the coast of Africa. After it has been generated, the SST anomaly is sustained in the deep Tropics by the positive wind–evaporation–SST feedback linked to the wind response to the SST. However, north of about 10°N where the SST anomaly is largest, the wind response is weak and the heat flux feedback is negative, thus damping the SST anomaly. As the in-phase maximum covariance patterns primarily reflect the atmospheric forcing of the SST, simultaneous correlations cannot be used to describe the atmospheric response to the SST anomaly, except in the deep Tropics. Using instead the maximum covariance patterns when SST leads the atmosphere reconciles the results of recent atmospheric general circulation model experiments with the observations.

scholarly journals Seasonality of Decadal Sea Surface Temperature Anomalies in the Northwestern Pacific

2006 ◽  
Vol 19 (12) ◽  
pp. 2953-2968 ◽  
Author(s):  
Takashi Mochizuki ◽  
Hideji Kida
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Mixed Layer ◽  
Surface Heat Flux ◽  
Heat Budget ◽  
Surface Heat ◽  
Sea Surface ◽  
Northwestern Pacific ◽  
Sst Anomaly

Abstract The seasonality of the decadal sea surface temperature (SST) anomalies and the related physical processes in the northwestern Pacific were investigated using a three-dimensional bulk mixed layer model. In the Kuroshio–Oyashio Extension (KOE) region, the strongest decadal SST anomaly was observed during December–February, while that of the central North Pacific occurred during February–April. From an examination of the seasonal heat budget of the ocean mixed layer, it was revealed that the seasonal-scale enhancement of the decadal SST anomaly in the KOE region was controlled by horizontal Ekman temperature transport in early winter and by vertical entrainment in autumn. The temperature transport by the geostrophic current made only a slight contribution to the seasonal variation of the decadal SST anomaly, despite controlling the upper-ocean thermal conditions on decadal time scales through the slow Rossby wave adjustment to the wind stress curl. When averaging over the entire KOE region, the contribution from the net sea surface heat flux was also no longer significantly detected. By examining the horizontal distributions of the local thermal damping rate, however, it was concluded that the wintertime decadal SST anomaly in the eastern KOE region was rather damped by the net sea surface heat flux. It was due to the fact that the anomalous local thermal damping of the SST anomaly resulting from the vertical entrainment in autumn was considerably strong enough to suppress the anomalous local atmospheric thermal forcing that acted to enhance the decadal SST anomaly.

Global Pattern Formation of Net Ocean Surface Heat Flux Response to Greenhouse Warming

2020 ◽  
Vol 33 (17) ◽  
pp. 7503-7522 ◽  
Author(s):  
Shineng Hu ◽  
Shang-Ping Xie ◽  
Wei Liu
Keyword(s):  
Heat Flux ◽  
North Atlantic ◽  
Surface Heat Flux ◽  
Surface Heat ◽  
Greenhouse Warming ◽  
Global Pattern ◽  
Passive Advection ◽  
The North ◽  
The North Atlantic ◽  
Circulation Changes

AbstractThis study examines global patterns of net ocean surface heat flux changes (ΔQnet) under greenhouse warming in an ocean–atmosphere coupled model based on a heat budget decomposition. The regional structure of ΔQnet is primarily shaped by ocean heat divergence changes (ΔOHD): excessive heat is absorbed by higher-latitude oceans (mainly over the North Atlantic and the Southern Ocean), transported equatorward, and stored in lower-latitude oceans with the rest being released to the tropical atmosphere. The overall global pattern of ΔOHD is primarily due to the circulation change and partially compensated by the passive advection effect, except for the Southern Ocean, which requires further investigations for a more definitive attribution. The mechanisms of North Atlantic surface heat uptake are further explored. In another set of global warming simulations, a perturbation of freshwater removal is imposed over the subpolar North Atlantic to largely offset the CO2-induced changes in the local ocean vertical stratification, barotropic gyre, and the Atlantic meridional overturning circulation (AMOC). Results from the freshwater perturbation experiments suggest that a significant portion of the positive ΔQnet over the North Atlantic under greenhouse warming is caused by the Atlantic circulation changes, perhaps mainly by the slowdown of AMOC, while the passive advection effect can contribute to the regional variations of ΔQnet. Our results imply that ocean circulation changes are critical for shaping global warming pattern and thus hydrological cycle changes.

scholarly journals Influence of the North Atlantic SST Variability on the Atmospheric Circulation during the Twentieth Century

2015 ◽  
Vol 28 (4) ◽  
pp. 1396-1416 ◽  
Author(s):  
Guillaume Gastineau ◽  
Claude Frankignoul
Keyword(s):  
Twentieth Century ◽  
North Atlantic ◽  
Lower Troposphere ◽  
Atlantic Multidecadal Oscillation ◽  
Early Winter ◽  
Tropical Forcing ◽  
The North ◽  
Sst Variability ◽  
Sst Anomaly ◽  
The North Atlantic

Abstract The ocean–atmosphere coupling in the North Atlantic is investigated during the twentieth century using maximum covariance analysis of sea surface temperature (SST) and 500-hPa geopotential height analyses and performing regressions on dynamical diagnostics such as Eady growth rate, wave activity flux, and velocity potential. The North Atlantic Oscillation (NAO) generates the so-called SST anomaly tripole. A rather similar SST anomaly tripole, with the subpolar anomaly displaced to the east and a more contracted subtropical anomaly, which is referred to as the North Atlantic horseshoe pattern, in turn influences the atmosphere. In the fall and early winter, the response is NAO like and primarily results from subpolar forcing centered over the Labrador Sea and off Newfoundland. In summer, the largest atmospheric response to SST resembles the east Atlantic pattern and results from a combination of subpolar and tropical forcing. To emphasize the interannual to multidecadal variability, the same analysis is repeated after low-pass filtering. The SST influence is dominated by the Atlantic multidecadal oscillation (AMO), which also has a horseshoe shape, but with larger amplitude in the subpolar basin. A warm AMO phase leads to an atmospheric warming limited to the lower troposphere in summer, while it leads to a negative phase of the NAO in winter. The winter influence of the AMO is suggested to be primarily forced by the Atlantic SSTs in the northern subtropics. Such influence of the AMO is found in winter instead of early winter because the winter SST anomalies have a larger persistence, presumably because of SST reemergence.

scholarly journals Simulations of Processes Associated with the Fast Warming Rate of the Southern Midlatitude Ocean

2010 ◽  
Vol 23 (1) ◽  
pp. 197-206 ◽  
Author(s):  
Wenju Cai ◽  
Tim Cowan ◽  
Stuart Godfrey ◽  
Susan Wijffels
Keyword(s):  
Twentieth Century ◽  
Heat Flux ◽  
Surface Heat Flux ◽  
Heat Content ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Ekman Transport ◽  
Content Increase ◽  
Intergovernmental Panel ◽  
Zonal Distribution

Abstract Significant warming has occurred across many of the world’s oceans throughout the latter part of the twentieth-century. The increase in the oceanic heat content displays a considerable spatial difference, with a maximum in the 35°–50°S midlatitude band. The relative importance of wind and surface heat flux changes in driving the warming pattern is the subject of much debate. Using wind, oceanic temperature, and heat flux outputs from twentieth-century multimodel experiments, conducted for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), the authors were able to reproduce the fast, deep warming in the midlatitude band; however, this warming is unable to be accounted for by local heat flux changes. The associated vertical structure and zonal distribution are consistent with a Sverdrup-type response to poleward-strengthening winds, with a poleward shift of the Southern Hemisphere (SH) supergyre and the Antarctic Circumpolar Current. However, the shift is not adiabatic and involves a net oceanic heat content increase over the SH, which can only be forced by changes in the net surface heat flux. Counterintuitively, the heat required for the fast, deep warming is largely derived from the surface heat fluxes south of 50°S, where the surface flux into the ocean is far larger than that of the midlatitude band. The heat south of 50°S is advected northward by an enhanced northward Ekman transport induced by the poleward-strengthening winds and penetrates northward and downward along the outcropping isopycnals to a depth of over 1000 m. However, because none of the models resolve eddies and given that eddy fluxes could offset the increase in the northward Ekman transport, the heat source for the fast, deep warming in the midlatitude band could be rather different in the real world.

Estimation of the Surface Heat Flux Response to Sea Surface Temperature Anomalies over the Global Oceans

2005 ◽  
Vol 18 (21) ◽  
pp. 4582-4599 ◽  
Author(s):  
Sungsu Park ◽  
Clara Deser ◽  
Michael A. Alexander
Keyword(s):  
Heat Flux ◽  
North Atlantic ◽  
Surface Heat Flux ◽  
Surface Wind ◽  
Surface Heat ◽  
Radiative Flux ◽  
Turbulent Heat ◽  
Global Oceans ◽  
Sst Anomalies ◽  
Heat Flux Feedback

Abstract The surface heat flux response to underlying sea surface temperature (SST) anomalies (the surface heat flux feedback) is estimated using 42 yr (1956–97) of ship-derived monthly turbulent heat fluxes and 17 yr (1984–2000) of satellite-derived monthly radiative fluxes over the global oceans for individual seasons. Net surface heat flux feedback is generally negative (i.e., a damping of the underlying SST anomalies) over the global oceans, although there is considerable geographical and seasonal variation. Over the North Pacific Ocean, net surface heat flux feedback is dominated by the turbulent flux component, with maximum values (28 W m−2 K−1) in December–February and minimum values (5 W m−2 K−1) in May–July. These seasonal variations are due to changes in the strength of the climatological mean surface wind speed and the degree to which the near-surface air temperature and humidity adjust to the underlying SST anomalies. Similar features are observed over the extratropical North Atlantic Ocean with maximum (minimum) feedback values of approximately 33 W m−2 K−1 (9 W m−2 K−1) in December–February (June–August). Although the net surface heat flux feedback may be negative, individual components of the feedback can be positive depending on season and location. For example, over the midlatitude North Pacific Ocean during late spring to midsummer, the radiative flux feedback associated with marine boundary layer clouds and fog is positive, and results in a significant enhancement of the month-to-month persistence of SST anomalies, nearly doubling the SST anomaly decay time from 2.8 to 5.3 months in May–July. Several regions are identified with net positive heat flux feedback: the tropical western North Atlantic Ocean during boreal winter, the Namibian stratocumulus deck off West Africa during boreal fall, and the Indian Ocean during boreal summer and fall. These positive feedbacks are mainly associated with the following atmospheric responses to positive SST anomalies: 1) reduced surface wind speed (positive turbulent heat flux feedback) over the tropical western North Atlantic and Indian Oceans, 2) reduced marine boundary layer stratocumulus cloud fraction (positive shortwave radiative flux feedback) over the Namibian stratocumulus deck, and 3) enhanced atmospheric water vapor (positive longwave radiative flux feedback) in the vicinity of the tropical deep convection region over the Indian Ocean that exceeds the negative shortwave radiative flux feedback associated with enhanced cloudiness.

scholarly journals Extratropical cyclone induced sea surface temperature anomalies in the 2013/14 winter

2019 ◽  
Author(s):  
Helen F. Dacre ◽  
Simon A. Josey ◽  
Alan L. M. Grant
Keyword(s):  
Heat Flux ◽  
North Atlantic ◽  
Surface Heat Flux ◽  
Cold Front ◽  
Winter Season ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Extratropical Cyclones ◽  
Sst Cooling

Abstract. The 2013/14 winter averaged sea surface temperature (SST) was anomalously cool in the mid-North Atlantic region. This season was also unusually stormy with extratropical cyclones passing over the mid-North Atlantic every 3 days. However, the processes by which cyclones contribute towards seasonal SST anomalies are not fully understood. In this paper a cyclone identification and tracking method is combined with ECMWF atmosphere and ocean reanalysis fields to calculate cyclone-relative net surface heat flux anomalies and resulting SST changes. Anomalously large negative heat fluxes are located behind the cyclones cold front resulting in anomalous cooling up to 0.2 K/day when the cyclones are at maximum intensity. This extratropical cyclone induced cold wake extends along the cyclones cold front but is small compared to climatological variability. To investigate the potential cumulative effect of the passage of multiple cyclone induced SST cooling in the same location we calculate Earth-relative net surface heat flux anomalies and resulting SST changes for the 2013/2014 winter period. Anomalously large winter averaged negative heat fluxes occur in a zonally orientated band extending across the North Atlantic between 40–60° N. The anomaly associated with cyclones is estimated using a cyclone masking technique which encompasses each cyclone centre and its trailing cold front. North Atlantic extratropical cyclones in the 2013/14 winter season account for 78 % of the observed net surface heat flux in the mid- North Atlantic and net surface heat fluxes in the 2013/14 winter season account for 70 % of the observed cooling in the mid-North Atlantic. Thus extratropical cyclones play a major role in determining the extreme 2013/2014 winter season SST cooling.

An Evaluation of ENSO Asymmetry in the Community Climate System Models: A View from the Subsurface

2009 ◽  
Vol 22 (22) ◽  
pp. 5933-5961 ◽  
Author(s):  
Tao Zhang ◽  
De-Zheng Sun ◽  
Richard Neale ◽  
Philip J. Rasch
Keyword(s):  
Heat Flux ◽  
Surface Heat Flux ◽  
Deep Convection ◽  
Surface Heat ◽  
Climate System ◽  
Eastern Pacific ◽  
Enso Asymmetry ◽  
Sst Anomaly ◽  
The Asymmetry

Abstract The asymmetry between El Niño and La Niña is a key aspect of ENSO that needs to be simulated well by models in order to fully capture the role of ENSO in the climate system. Here the asymmetry between the two phases of ENSO in five successive versions of the Community Climate System Model (CCSM1, CCSM2, CCSM3 at T42 resolution, CCSM3 at T85 resolution, and the latest CCSM3 + NR, with the Neale and Richter convection scheme) is evaluated. Different from the previous studies, not only is the surface signature of ENSO asymmetry examined, but so too is its subsurface signature. By comparing the differences among these models as well as the differences between the models and the observations, an understanding of the causes of the ENSO asymmetry is sought. An underestimate of the ENSO asymmetry is noted in all of the models, but the latest version with the Neale and Richter scheme (CCSM3 + NR) is getting closer to the observations than the earlier versions. The net surface heat flux is found to damp the asymmetry in the SST field in both the models and observations, but the damping effect in the models is weaker than that in the observations, thus excluding a role of the surface heat flux in contributing to the weaker asymmetry in the SST anomalies associated with ENSO. Examining the subsurface signatures of ENSO—the thermocline depth and the associated subsurface temperature for the western and eastern Pacific—reveals the same bias; that is, the asymmetry in the models is weaker than that in the observations. The analysis of the corresponding Atmospheric Model Intercomparison Project (AMIP) runs in conjunction with the coupled runs suggests that the weaker asymmetry in the subsurface signatures in the models is related to the lack of asymmetry in the zonal wind stress over the central Pacific, which in turn is due to a lack of sufficient asymmetry in deep convection (i.e., the nonlinear dependence of the deep convection on SST). In particular, the lack of a westward shift in the deep convection in the models in response to a cold phase SST anomaly is found as a common factor that is responsible for the weak asymmetry in the models. It is also suggested that a more eastward extension of the deep convection in response to a warm phase SST anomaly may also help to increase the asymmetry of ENSO. The better performance of CCSM3 + NR is apparently linked to an enhanced convection over the eastern Pacific during the warm phase of ENSO. Apparently, either a westward shift of deep convection in response to a cold phase SST anomaly or an increase of convection over the eastern Pacific in response to a warm phase SST anomaly leads to an increase in the asymmetry of zonal wind stress and therefore an increase in the asymmetry of subsurface signal, favoring an increase in ENSO asymmetry.

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