scholarly journals The Impact of Extratropical Atmospheric Variability on ENSO: Testing the Seasonal Footprinting Mechanism Using Coupled Model Experiments

2010 ◽  
Vol 23 (11) ◽  
pp. 2885-2901 ◽  
Author(s):  
Michael A. Alexander ◽  
Daniel J. Vimont ◽  
Ping Chang ◽  
James D. Scott
Keyword(s):  
Heat Flux ◽  
Coupled Model ◽  
Enso Event ◽  
Surface Heat ◽  
Kelvin Waves ◽  
Heat Fluxes ◽  
Atmospheric Variability ◽  
The Tropics ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract Previous studies suggest that extratropical atmospheric variability influences the tropics via the seasonal footprinting mechanism (SFM), in which fluctuations in the North Pacific Oscillation (NPO) impact the ocean via surface heat fluxes during winter and the resulting springtime subtropical SST anomalies alter the atmosphere–ocean system over the tropics in the following summer, fall, and winter. Here, the authors test the SFM hypothesis by imposing NPO-related surface heat flux forcing in an atmospheric GCM coupled to a reduced gravity ocean model in the tropics and a slab ocean in the extratropics. The forcing is only imposed through the first winter, and then the model is free to evolve through the following winter. The evolution of the coupled model response to the forcing is consistent with the SFM hypothesis: the NPO-driven surface fluxes cause positive SST anomalies to form in the central and eastern subtropics during winter; these anomalies propagate toward the equator along with westerly wind anomalies during spring, reach the equator in summer, and then amplify, which leads to an ENSO event in the following winter. The anomalies reach the equator through a combination of thermodynamically coupled air–sea interactions, namely, the wind–evaporation–SST (WES) feedback and equatorial ocean dynamics. The initial off-equatorial anomaly propagates toward the equator through a relaxation of the climatological easterly winds south of the dominant SST anomalies, which leads to a reduction in upward latent heat flux. These westerly anomalies reach the equator during boreal summer, where they can excite downwelling equatorial Kelvin waves. The connection between off-equatorial variations and tropical ENSO-like conditions may also occur via the excitation of westward-propagating equatorial Rossby waves during spring, which reflect off of the western boundary as Kelvin waves, depressing the thermocline in the eastern Pacific during the following summer. NPO-related anomalies that form during the first winter in the tropical Pacific may also contribute to the development of an El Niño event in the following winter. The imposition of the NPO-related forcing caused warming in the ENSO region in ∼70% of the ensemble of 60 simulations; therefore, the response depends on the state of the tropical atmosphere–ocean system. For years where the control simulation was poised to develop into a neutral or negative ENSO event, the addition of the NPO heat fluxes tended to cause anomalous warming in the tropical Pacific in the following fall/winter; if the control was heading toward a warm ENSO event, the imposition of NPO forcing tends to reduce the amplitude of that event.

Midlatitude Excitation of Tropical Variability in the Pacific: The Role of Thermodynamic Coupling and Seasonality*

2009 ◽  
Vol 22 (3) ◽  
pp. 518-534 ◽  
Author(s):  
Daniel J. Vimont ◽  
Michael Alexander ◽  
Abigail Fontaine
Keyword(s):  
Heat Flux ◽  
Temporal Structure ◽  
Surface Wind ◽  
Coupled Model ◽  
Tropical Pacific ◽  
Boreal Winter ◽  
Wes Feedback ◽  
Thermodynamic Coupling ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract A set of ensemble model experiments using the National Center for Atmospheric Research Community Atmospheric Model version 3.0 (CAM3) is run to investigate the tropical Pacific response to midlatitude atmospheric variability associated with the atmospheric North Pacific Oscillation (NPO). Heat flux anomalies associated with the NPO are used to force a set of model simulations during boreal winter (when the NPO is most energetic), after which the forcing is switched off and the coupled model evolves on its own. Sea surface temperature (SST) and wind anomalies continue to amplify in the tropical Pacific after the imposed forcing has been shut off, indicating that coupled ocean–atmosphere interactions in the tropical Pacific alter the spatial and temporal structure of variability associated with midlatitude forcing. The tropical circulation evolves through feedbacks between the surface wind, evaporation, and SST (the WES feedback), as well as through changes in the shortwave radiative heat flux (caused by changes in convection). Sensitivity experiments are run to investigate how thermodynamic coupling and seasonality affect the tropical response to NPO-related forcing. Seasonality is found to affect the WES feedback through (i) altering the sensitivity of surface evaporation to changes in the low-level wind field and (ii) altering the structure and strength of the lower-level wind response to SST anomalies. Thermodynamic coupling causes an equatorward and westward development of SST anomalies and an associated equatorward shift in the lower-level zonal wind anomalies.

scholarly journals A Description of Interdecadal Time-Scale Propagating North Atlantic Sea Surface Temperature Anomalies and Their Effect on Winter European Climate, 1948–2002

2006 ◽  
Vol 19 (7) ◽  
pp. 1067-1079 ◽  
Author(s):  
Bablu Sinha ◽  
Brenda Topliss
Keyword(s):  
Heat Flux ◽  
North Atlantic ◽  
Surface Air Temperature ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Western Basin ◽  
Eastern Basin ◽  
The North ◽  
Sst Anomalies

Abstract Eastward-propagating interdecadal time-scale sea surface temperature (SST) winter anomalies have been shown to exist at the North Atlantic subpolar/subtropical gyre boundary. Heat flux and surface air temperature signatures of these anomalies are investigated using satellite- and ship-based SST observations and atmospheric reanalysis. Using bandpass filter analysis, retaining periods between 9 and 25 yr, a succession of coherent propagating SST anomalies is identified. The size, speed, propagation path, and decay characteristics of propagating anomalies detected during the period 1948–2002 are documented. The behavior of the propagations changes between the periods 1948–70 and 1970–2002. In the former period, SST anomalies propagated from the east coast of North America to the British Isles in ∼10 yr. The anomalies displayed a well-defined life cycle, growing in the western basin (west of 40°W) and decaying in the eastern basin. During the period 1970–2002, SST anomalies did not propagate deep into the eastern basin, but grew in the western basin and then ceased propagating. Oceanic anomalies have a comparable marked signature in surface sensible and latent heat fluxes and in surface air temperature. Winter surface heat flux anomalies act to amplify SST anomalies during the middle of their lifetimes, normally in the west-central Atlantic. At other times, heat flux anomalies are associated with decay of anomalies. Surface heat fluxes do not always act to cause propagation, and it is likely that other processes such as advection play a role in the propagation mechanism. North European winter surface air temperatures are raised or lowered by up to ±0.5°C over decadal time scales (∼1/3 of the total variation over the United Kingdom) when an SST anomaly reaches the eastern boundary. A variety of processes can cause SST variation on decadal time scales at the eastern boundary, but in the 1950s and 1960s the variability at these periods was the signature of features that had propagated across the Atlantic from the North American coast.

scholarly journals Freshwater Flux (FWF)-Induced Oceanic Feedback in a Hybrid Coupled Model of the Tropical Pacific

2009 ◽  
Vol 22 (4) ◽  
pp. 853-879 ◽  
Author(s):  
Rong-Hua Zhang ◽  
Antonio J. Busalacchi
Keyword(s):  
Heat Flux ◽  
Interannual Variability ◽  
Coupled Model ◽  
Tropical Pacific ◽  
Freshwater Flux ◽  
Atmosphere System ◽  
Ocean Atmosphere ◽  
Hybrid Coupled Model ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract The impacts of freshwater flux (FWF) forcing on interannual variability in the tropical Pacific climate system are investigated using a hybrid coupled model (HCM), constructed from an oceanic general circulation model (OGCM) and a simplified atmospheric model, whose forcing fields to the ocean consist of three components. Interannual anomalies of wind stress and precipitation minus evaporation, (P − E), are calculated respectively by their statistical feedback models that are constructed from a singular value decomposition (SVD) analysis of their historical data. Heat flux is calculated using an advective atmospheric mixed layer (AML) model. The constructed HCM can well reproduce interannual variability associated with ENSO in the tropical Pacific. HCM experiments are performed with varying strengths of anomalous FWF forcing. It is demonstrated that FWF can have a significant modulating impact on interannual variability. The buoyancy flux (QB) field, an important parameter determining the mixing and entrainment in the equatorial Pacific, is analyzed to illustrate the compensating role played by its two contributing parts: one is related to heat flux (QT) and the other to freshwater flux (QS). A positive feedback is identified between FWF and SST as follows: SST anomalies, generated by El Niño, nonlocally induce large anomalous FWF variability over the western and central regions, which directly influences sea surface salinity (SSS) and QB, leading to changes in the mixed layer depth (MLD), the upper-ocean stability, and the mixing and the entrainment of subsurface waters. These oceanic processes act to enhance the SST anomalies, which in turn feedback to the atmosphere in a coupled ocean–atmosphere system. As a result, taking into account anomalous FWF forcing in the HCM leads to an enhanced interannual variability and ENSO cycles. It is further shown that FWF forcing is playing a different role from heat flux forcing, with the former acting to drive a change in SST while the latter represents a passive response to the SST change. This HCM-based modeling study presents clear evidence for the role of FWF forcing in modulating interannual variability in the tropical Pacific. The significance and implications of these results are further discussed for physical understanding and model improvements of interannual variability in the tropical Pacific ocean–atmosphere system.

scholarly journals The Linear Response of ENSO to the Madden–Julian Oscillation

2005 ◽  
Vol 18 (13) ◽  
pp. 2441-2459 ◽  
Author(s):  
J. Zavala-Garay ◽  
C. Zhang ◽  
A. M. Moore ◽  
R. Kleeman
Keyword(s):  
Large Fraction ◽  
Surface Wind ◽  
Low Frequency ◽  
Coupled Model ◽  
Cumulative Effect ◽  
Enso Event ◽  
Kelvin Waves ◽  
Madden Julian Oscillation ◽  
Coupled Models ◽  
Stochastic Forcing

Abstract The possibility that the tropical Pacific coupled system linearly amplifies perturbations produced by the Madden–Julian oscillation (MJO) is explored. This requires an estimate of the low-frequency tail of the MJO. Using 23 yr of NCEP–NCAR reanalyses of surface wind and Reynolds SST, we show that the spatial structure that dominates the intraseasonal band (i.e., the MJO) also dominates the low-frequency band once the anomalies directly related to ENSO have been removed. This low-frequency contribution of the intraseasonal variability is not included in most ENSO coupled models used to date. Its effect in a coupled model of intermediate complexity has, therefore, been studied. It is found that this “MJO forcing” (τMJO) can explain a large fraction of the interannual variability in an asymptotically stable version of the model. This interaction is achieved via linear dynamics. That is, it is the cumulative effect of individual events that maintains ENSOs in this model. The largest coupled wind anomalies are initiated after a sequence of several downwelling Kelvin waves of the same sign have been forced by τMJO. The cumulative effect of the forced Kelvin waves is to persist the (small) SST anomalies in the eastern Pacific just enough for the coupled ocean–atmosphere dynamics to amplify the anomalies into a mature ENSO event. Even though τMJO explains just a small fraction of the energy contained in the stress not associated with ENSO, a large fraction of the modeled ENSO variability is excited by this forcing. The characteristics that make τMJO an optimal stochastic forcing for the model are discussed. The large zonal extent is an important factor that differentiates the MJO from other sources of stochastic forcing.

scholarly journals Assessing Surface Heat Flux Products with In Situ Observations over the Australian Sector of the Southern Ocean

2019 ◽  
Vol 36 (9) ◽  
pp. 1849-1861
Author(s):  
Vidhi Bharti ◽  
Eric Schulz ◽  
Christopher W. Fairall ◽  
Byron W. Blomquist ◽  
Yi Huang ◽  
...  
Keyword(s):  
Heat Flux ◽  
Southern Ocean ◽  
Sensible Heat Flux ◽  
Surface Heat ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Atmospheric Composition ◽  
Systematic Bias ◽  
Positive Bias

Given the large uncertainties in surface heat fluxes over the Southern Ocean, an assessment of fluxes obtained by European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) product, the Australian Integrated Marine Observing System (IMOS) routine observations, and the Objectively Analyzed Air–Sea Heat Fluxes (OAFlux) project hybrid dataset is performed. The surface fluxes are calculated using the COARE 3.5 bulk algorithm with in situ data obtained from the NOAA Physical Sciences Division flux system during the Clouds, Aerosols, Precipitation, Radiation, and Atmospheric Composition over the Southern Ocean (CAPRICORN) experiment on board the R/V Investigator during a voyage (March–April 2016) in the Australian sector of the Southern Ocean (43°–53°S). ERA-Interim and OAFlux data are further compared with the Southern Ocean Flux Station (SOFS) air–sea flux moored surface float deployed for a year (March 2015–April 2016) at ~46.7°S, 142°E. The results indicate that ERA-Interim (3 hourly at 0.25°) and OAFlux (daily at 1°) estimate sensible heat flux H s accurately to within ±5 W m−2 and latent heat flux H l to within ±10 W m−2. ERA-Interim gives a positive bias in H s at low latitudes (<47°S) and in H l at high latitudes (>47°S), and OAFlux displays consistently positive bias in H l at all latitudes. No systematic bias with respect to wind or rain conditions was observed. Although some differences in the bulk flux algorithms are noted, these biases can be largely attributed to the uncertainties in the observations used to derive the flux products.

scholarly journals Surface Heat Budget over the North Sea in Climate Change Simulations

Atmosphere ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 272 ◽  
Author(s):  
Christian Dieterich ◽  
Shiyu Wang ◽  
Semjon Schimanke ◽  
Matthias Gröger ◽  
Birgit Klein ◽  
...  
Keyword(s):  
Climate Change ◽  
Heat Flux ◽  
North Sea ◽  
Heat Budget ◽  
Coupled Model ◽  
Surface Heat ◽  
The North ◽  
The North Sea ◽  
Rcp 8.5 ◽  
Surface Heat Budget

An ensemble of regional climate change scenarios for the North Sea is validated and analyzed. Five Coupled Model Intercomparison Project Phase 5 (CMIP5) General Circulation Models (GCMs) using three different Representative Concentration Pathways (RCPs) have been downscaled with the coupled atmosphere–ice–ocean model RCA4-NEMO. Validation of sea surface temperature (SST) against different datasets suggests that the model results are well within the spread of observational datasets. The ensemble mean SST with a bias of less than 1 ∘ C is the solution that fits the observations best and underlines the importance of ensemble modeling. The exchange of momentum, heat, and freshwater between atmosphere and ocean in the regional, coupled model compares well with available datasets. The climatological seasonal cycles of these fluxes are within the 95% confidence limits of the datasets. Towards the end of the 21st century the projected North Sea SST increases by 1.5 ∘ C (RCP 2.6), 2 ∘ C (RCP 4.5), and 4 ∘ C (RCP 8.5), respectively. Under this change the North Sea develops a specific pattern of the climate change signal for the air–sea temperature difference and latent heat flux in the RCP 4.5 and 8.5 scenarios. In the RCP 8.5 scenario the amplitude of the spatial heat flux anomaly increases to 5 W/m 2 at the end of the century. Different hypotheses are discussed that could contribute to the spatially non-uniform change in air–sea interaction. The most likely cause for an increased latent heat loss in the central western North Sea is a drier atmosphere towards the end of the century. Drier air in the lee of the British Isles affects the balance of the surface heat budget of the North Sea. This effect is an example of how regional characteristics modulate global climate change. For climate change projections on regional scales it is important to resolve processes and feedbacks at regional scales.

scholarly journals Assessment of Roughness Length Schemes Implemented within the Noah Land Surface Model for High-Altitude Regions

2014 ◽  
Vol 15 (3) ◽  
pp. 921-937 ◽  
Author(s):  
Donghai Zheng ◽  
Rogier van der Velde ◽  
Zhongbo Su ◽  
Martijn J. Booij ◽  
Arjen Y. Hoekstra ◽  
...  
Keyword(s):  
Heat Flux ◽  
High Altitude ◽  
Land Surface ◽  
Land Surface Model ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Surface Model ◽  
Surface Heat Fluxes ◽  
Noah Land Surface Model ◽  
Better Than

ABSTRACT Current land surface models still have difficulties with producing reliable surface heat fluxes and skin temperature (Tsfc) estimates for high-altitude regions, which may be addressed via adequate parameterization of the roughness lengths for momentum (z0m) and heat (z0h) transfer. In this study, the performance of various z0h and z0m schemes developed for the Noah land surface model is assessed for a high-altitude site (3430 m) on the northeastern part of the Tibetan Plateau. Based on the in situ surface heat fluxes and profile measurements of wind and temperature, monthly variations of z0m and diurnal variations of z0h are derived through application of the Monin–Obukhov similarity theory. These derived values together with the measured heat fluxes are utilized to assess the performance of those z0m and z0h schemes for different seasons. The analyses show that the z0m dynamics are related to vegetation dynamics and soil water freeze–thaw state, which are reproduced satisfactorily with current z0m schemes. Further, it is demonstrated that the heat flux simulations are very sensitive to the diurnal variations of z0h. The newly developed z0h schemes all capture, at least over the sparse vegetated surfaces during the winter season, the observed diurnal variability much better than the original one. It should, however, be noted that for the dense vegetated surfaces during the spring and monsoon seasons, not all newly developed schemes perform consistently better than the original one. With the most promising schemes, the Noah simulated sensible heat flux, latent heat flux, Tsfc, and soil temperature improved for the monsoon season by about 29%, 79%, 75%, and 81%, respectively. In addition, the impact of Tsfc calculation and energy balance closure associated with measurement uncertainties on the above findings are discussed, and the selection of the appropriate z0h scheme for applications is addressed.

scholarly journals Air–Sea Turbulent Heat Fluxes in Climate Models and Observational Analyses: What Drives Their Variability?

2019 ◽  
Vol 32 (8) ◽  
pp. 2397-2421 ◽  
Author(s):  
R. Justin Small ◽  
Frank O. Bryan ◽  
Stuart P. Bishop ◽  
Robert A. Tomas
Keyword(s):  
Heat Flux ◽  
Climate Models ◽  
Climate Model ◽  
Turbulent Heat Flux ◽  
Heat Fluxes ◽  
Small Scale ◽  
Turbulent Heat ◽  
Realistic Representation ◽  
Frontal Zones ◽  
The Tropics

Abstract A traditional view is that the ocean outside of the tropics responds passively to atmosphere forcing, which implies that air–sea heat fluxes are mainly driven by atmosphere variability. This paper tests this viewpoint using state-of-the-art air–sea turbulent heat flux observational analyses and a climate model run at different resolutions. It is found that in midlatitude ocean frontal zones the variability of air–sea heat fluxes is not predominantly driven by the atmosphere variations but instead is forced by sea surface temperature (SST) variations arising from intrinsic oceanic variability. Meanwhile in most of the tropics and subtropics wind is the dominant driver of heat flux variability, and atmosphere humidity is mainly important in higher latitudes. The predominance of ocean forcing of heat fluxes found in frontal regions occurs on scales of around 700 km or less. Spatially smoothing the data to larger scales results in the traditional atmosphere-driving case, while filtering to retain only small scales of 5° or less leads to ocean forcing of heat fluxes over most of the globe. All observational analyses examined (1° OAFlux; 0.25° J-OFURO3; 0.25° SeaFlux) show this general behavior. A standard resolution (1°) climate model fails to reproduce the midlatitude, small-scale ocean forcing of heat flux: refining the ocean grid to resolve eddies (0.1°) gives a more realistic representation of ocean forcing but the variability of both SST and of heat flux is too high compared to observational analyses.

Two- and Three-Dimensional Cloud-Resolving Model Simulations of the Mesoscale Enhancement of Surface Heat Fluxes by Precipitating Deep Convection

2006 ◽  
Vol 19 (1) ◽  
pp. 139-149 ◽  
Author(s):  
Xiaoqing Wu ◽  
Stephen Guimond
Keyword(s):  
Heat Flux ◽  
Latent Heat ◽  
Deep Convection ◽  
Three Dimensional ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Total Flux ◽  
Surface Heat Fluxes ◽  
Cloud Resolving Model ◽  
2D And 3D

Abstract Two-dimensional (2D) and three-dimensional (3D) cloud-resolving model (CRM) simulations are conducted to quantify the enhancement of surface sensible and latent heat fluxes by tropical precipitating cloud systems for 20 days (10–30 December 1992) during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The mesoscale enhancement appears to be analogous across both 2D and 3D CRMs, with the enhancement for the sensible heat flux accounting for 17% of the total flux for each model and the enhancement for the latent heat flux representing 18% and 16% of the total flux for 2D and 3D CRMs, respectively. The convection-induced gustiness is mainly responsible for the enhancement observed in each model simulation. The parameterization schemes of the mesoscale enhancement by the gustiness in terms of convective updraft, downdraft, and precipitation, respectively, are examined using each version of the CRM. The scheme utilizing the precipitation was found to yield the most desirable estimations of the mean fluxes with the smallest rms error. The results together with previous findings from other studies suggest that the mesoscale enhancement of surface heat fluxes by the precipitating deep convection is a subgrid process apparent across various CRMs and is imperative to incorporate into general circulation models (GCMs) for improved climate simulation.

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