Impact of diurnal atmosphere–ocean coupling on tropical climate simulations using a coupled GCM

The impacts of diurnal atmosphere–ocean (air–sea) coupling on tropical climate simulations are investigated using the SNU coupled GCM. To investigate the effect of the atmospheric and oceanic diurnal cycles on a climate simulation, a 1-day air–sea coupling interval experiment is compared to a 2-h coupling experiment. As previous studies have suggested, cold temperature biases over equatorial western Pacific regions are significantly reduced when diurnal air–sea coupling strategy is implemented. This warming is initiated by diurnal rectification and amplified further by the air–sea coupled feedbacks. In addition to its effect on the mean climatology, the diurnal coupling has also a distinctive impact on the amplitude of the El Nino-Southern Oscillation (ENSO). It is demonstrated that a weakening of the ENSO magnitude is caused by reduced (increased) surface net heat fluxes into the ocean during El Nino (La Nina) events. Primarily, decreased (increased) incoming shortwave radiation during El Nino (La Nina) due to cloud shading is responsible for the net heat fluxes associated with ENSO.     

The <Emphasis Type="SmallCaps">int</Emphasis>OA Experiment: A Study of Ocean-Atmosphere Interactions Under Moderate to Strong Offshore Winds and Opposing Swell Conditions in the Gulf of Tehuantepec,Mexico

The Gulf of Tehuantepec air–sea interaction experiment (intOA) took place from February to April 2005, under the Programme for the Study of the Gulf of Tehuantepec (PEGoT, Spanish acronym for Programa para el Estudio del Golfo de Tehuantepec). PEGoT is underway aiming for better knowledge of the effect of strong and persistent offshore winds on coastal waters and their natural resources, as well as performing advanced numerical modelling of the wave and surface current fields. One of the goals of the intOA experiment is to improve our knowledge on air–sea interaction processes with particular emphasis on the effect of surface waves on the momentum flux for the characteristic and unique conditions that occur when strong Tehuano winds blow offshore against the Pacific Ocean long period swell. For the field campaign, an air–sea interaction spar (ASIS) buoy was deployed in the Gulf of Tehuantepec to measure surface waves and the momentum flux between the ocean and the atmosphere. High frequency radar systems (phase array type) were in operation from two coastal sites and three acoustic Doppler current profilers were deployed near-shore. Synthetic aperture radar images were also acquired as part of the remote sensing component of the experiment. The present paper provides the main results on the wave and wind fields, addressing the direct calculation of the momentum flux and the drag coefficient, and gives an overview of the intOA experiment. Although the effect of swell has been described in recent studies, this is the first time for the very specific conditions encountered, such as swell persistently opposing offshore winds and locally generated waves, to show a clear evidence of the influence on the wind stress of the significant steepness of swell waves.     

Atmosphere–Surface Fluxes of CO<Subscript>2</Subscript> using Spectral Techniques

Different flux estimation techniques are compared here in order to evaluate air–sea exchange measurement methods used on moving platforms. Techniques using power spectra and cospectra to estimate fluxes are presented and applied to measurements of wind speed and sensible heat, latent heat and CO₂ fluxes. Momentum and scalar fluxes are calculated from the dissipation technique utilizing the inertial subrange of the power spectra and from estimation of the cospectral amplitude, and both flux estimates are compared to covariance derived fluxes. It is shown how even data having a poor signal-to-noise ratio can be used for flux estimations.     

Intercomparison of Air–Sea Interface Fluxes over the Yellow Sea and Korea Strait: Impact of Tsushima Warm Current

Surface-layer meteorological observations obtained from oceanic buoys over the Korean Strait and the Yellow Sea are used to estimate surface-layer turbulent fluxes of heat, moisture and momentum over the East-Asian Marginal Seas. Special emphasis is paid towards explanation of the impact of the Tsushima warm current flowing through the Korean Strait on air–sea interface fluxes. During the active phase of the Tsushima warm current, when the difference in sea surface temperature and air temperature becomes as large as 8°C, the sensible heat flux increases to a value of about 135 W m⁻², while the latent heat flux is around 200 W m⁻². The study attempts to broaden our understanding on the air-sea interaction processes over the Yellow Sea and Korean Strait.     

Air–Sea Flux Estimates And The 1997–1998 Enso Event

Bulk formulae for wind stress, sensible and latent heat flux are presented that are suitable for strong mesoscale events such as westerly wind bursts that contribute to the El Niño-Southern Oscillation (ENSO). Their exchange coefficients for heat and momentum have a simple polynomial dependence on wind speed and a linear dependence on air–sea temperature difference. The accuracy of these formulae are validated with respect to air–sea fluxes estimated using the standard algorithm adopted by the Tropical Ocean-Global AtmosphereCoupled-Ocean Atmosphere Response Experiment (TOGA COARE). The comparison ismade for observations from 96 Tropical Atmosphere Ocean (TAO) array and National Oceanographic Data Center (NODC) moorings in the equatorial and North Pacific Ocean spanning years 1990–1999. The bulk formulae are shown to have very small median root–mean-square differences with respect to the TOGA COARE estimates: 0.003 N m^-2, 1.0 W m^-2, and 10.0 W m^-2 for the wind stress, sensible heat flux, and latent heat flux, respectively.The variability of air–sea fluxes during the 1997–1998 ENSO is also examined, along with a possible relationship between air–sea fluxes and surface ocean mixed layer depth (MLD). The wind stress and latent heat flux during the 1997 El Niño are found to be greater in the warm pool of the western Pacific than in the central Pacific where the ENSO is most clearly seen. These differences disappear upon the start of La Niña. The MLD in the equatorial Pacific is found to be moderately correlated to air–sea fluxes just before the start of the 1998 La Niña and poorly correlated otherwise.     

寒潮过程中风浪对黄海海气热量通量和动量通量影响研究

采用2009—2013年CFSR(Climate Forecast System Reanalysis)大气和海洋再分析资料对黄海海气间热量通量和动量通量的特征进行统计分析,并通过FVCOMSWAVE浪流耦合模式对典型寒潮过程中风浪的影响效果进行模拟研究与对比分析。统计结果显示,通量受海表大风、海气温差及海洋环流等因子影响,秋冬季节强烈,春夏季节相对较弱,在寒潮活跃的冷季该海域的海流处于弱流期,风浪对海面通量的作用明显增强。海温特征也显示冷季的不稳定性显著强于暖季,因此该海域冷季具有更强的海气热量通量。沿岸站点的比较显示,南部吕泗站面向更开阔的东海海域,其平均波高高出北部20%左右。这与沿海南部通量强于北部特征对应。数值模拟显示,在寒潮过程中,海气界面热量通量和动量通量输送比多年月平均状态显著增强,动量通量增大1~5倍,热量通量增大1~6倍。寒潮过程入海冷锋走向、强度、移动方向显著影响海面热量通量和动量通量大值区的分布。偏北路寒潮纬向型冷锋入海,其强度东部大于西部,造成通量大值区形成在黄海东北部,而偏西路寒潮经向型冷锋入海,其强度南部大于北部,造成通量大值区形成在黄海南部。同时偏北路径寒潮强度大于偏西路径,海气动量通量响应较偏西路径强约25%,热量通量强约50%。耦合风浪作用的模拟显示,海气间热量通量和动量通量明显增大,对不同强度风浪,浪高增加1.5倍,动量通量最大值增大约2倍,热量通量增大10~160 W/m2;浪高减弱至0.5倍,动量通量最大值则减弱约40%,热量通量减小10~55 W/m2。冷锋及其驱动的风浪强烈影响区域海气通量时空特征。     

Influence of the boundary layer height on the global air–sea surface fluxes

Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by the boundary layer height, z _i , during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind profile, φ _m , has been found to be a function of both z/L and z _i /L. At constant z/L, φ _m decreases with decreasing boundary layer height. Since φ _m is directly related to the parameterizations of the air–sea surface fluxes, these results will have an influence when calculating the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979 to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase by 0.77 W m⁻² or about 1% and the mean surface stress increase by 1.4 mN m⁻² or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger response. The greatest impact is found over the tropical oceans between 30°S and 30°N. In this region the boundary layer height influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase of the mean annual heat fluxes by 2.0 W m⁻² and an increase of the mean annual surface stress by 2.6 mN m⁻².     

Simulation of the Indian monsoon using the RegCM3–ROMS regional coupled model

A regional coupled atmosphere–ocean model was developed to study the role of air–sea interactions in the simulation of the Indian summer monsoon. The coupled model includes the regional climate model (RegCM3) as atmospheric component and the regional ocean modeling system (ROMS) as oceanic component. The two-way coupled model system exchanges sea surface temperature (SST) from the ocean to the atmospheric model and surface wind stress and energy fluxes from the atmosphere to the ocean model. The coupled model is run for four years 1997, 1998, 2002 and 2003 and the results are compared with observations and atmosphere-only model runs employing Reynolds SSTs as lower boundary condition. It is found that the coupled model captures the main features of the Indian monsoon and simulates a substantially more realistic spatial and temporal distribution of monsoon rainfall compared to the uncoupled atmosphere-only model. The intraseasonal oscillations are also better simulated in the coupled model compared to the atmosphere-only model. These improvements are due to a better representation of the feedbacks between the SST and convection and highlight the importance of air–sea coupling in the simulation of the Indian monsoon.     

Aircraft Observations of Sea-Surface Turbulent Fluxes Near the California Coast

Aircraft turbulence data from the Autonomous Ocean Sampling Network project were analyzed and compared to the Coupled Ocean–Atmosphere Response Experiment (COARE) bulk parametrization of turbulent fluxes in an ocean area near the coast of California characterized by complex atmospheric flow. Turbulent fluxes measured at about 35 m above the sea surface using the eddy-correlation method were lower than bulk estimates under unstable and stable atmospheric stratification for all but light winds. Neutral turbulent transfer coefficients were used in this comparison because they remove the effects of mean atmospheric conditions and atmospheric stability. Spectral analysis suggested that kilometre-scale longitudinal rolls affect significantly turbulence measurements even near the sea surface, depending on sampling direction. Cross-wind sampling tended to capture all the available turbulent energy. Vertical soundings showed low boundary-layer depths and high flux divergence near the sea surface in the case of sensible heat flux but minimal flux divergence for the momentum flux. Cross-wind sampling and flux divergence were found to explain most of the observed discrepancies between the measured and bulk flux estimates. At low wind speeds the drag coefficient determined with eddy correlation and an inertial dissipation method after corrections were applied still showed high values compared to bulk estimates. This discrepancy correlated with the dominance of sea swell, which was a usually observed condition under low wind speeds. Under stable atmospheric conditions measured sensible heat fluxes, which usually have low values over the ocean, were possibly affected by measurement errors and deviated significantly from bulk estimates.     

Effects of Roll Vortices on Turbulent Fluxes in the Hurricane Boundary Layer

Boundary-layer secondary circulations or ‘roll vortices’ can have a significant influence on the turbulent exchange of momentum, sensible heat and moisture throughout the hurricane boundary layer. In this study, analyses of data from a WP-3D aircraft of the National Oceanic and Atmospheric Administration (NOAA) are presented. As part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST)-hurricane experiment sponsored through the Office of Naval Research and NOAA’s annual hurricane research program, flights were conducted to investigate energy exchange across the air–sea interface. We present the first in-situ aircraft-based observations of rolls in the hurricane boundary layer and investigate their influence on energy and momentum exchange. The rolls detected in Hurricane Isidore (year 2002) have a characteristic wavelength of about 900 m, in good agreement with analyses of data from a synthetic aperture radar image captured by the Canadian Space Agency’s RADARSAT satellite in the same storm. Our analyses of the airborne data suggest that roll vortices may be a significant factor modulating the air–sea momentum exchange.     

The Effect of the Sea-ice Zone on the Development of Boundary-layer Roll Clouds During Cold Air Outbreaks

High latitude air–sea interaction is an important component of the earth’s climate system and the exchanges of mass and energy over the sea-ice zone are complicated processes that, at present, are not well understood. In this paper, we perform a series of numerical experiments to examine the effect of sea-ice concentration on the development of high latitude boundary-layer roll clouds. The experiments are performed at sufficiently high spatial resolution to be able to resolve the individual convective roll clouds, and over a large enough domain to be able to examine the roll’s downstream development. Furthermore the high spatial resolution of the experiments allows for an explicit representation of heterogeneity within the sea-ice zone. The results show that the sea-ice zone has a significant impact on the atmospheric boundary-layer development, which can be seen in both the evolution of the cloud field and the development of heat and moisture transfer patterns. In particular, we find the air-sea exchanges of momentum, moisture and heat fluxes are modified by the presence of the roll vortices (typically a 10% difference in surface heat fluxes between updrafts and downdrafts) and by the concentration and spatial distribution of the sea-ice. This suggests that a more realistic representation of processes over the sea-ice zone is needed to properly calculate the air-sea energy and mass exchange budgets.     

Weakened SST variability in the tropical Atlantic Ocean since 2000

A prominent weakening in equatorial Atlantic sea surface temperature (SST) variability, occurring around the year 2000, is investigated by means of observations, reanalysis products and the linear recharge oscillator (ReOsc) model. Compared to the time period 1982–1999, during 2000–2017 the May–June–July SST variability in the eastern equatorial Atlantic has decreased by more than 30%. Coupled air–sea feedbacks, namely the positive Bjerknes feedback and the negative net heat flux damping are important drivers for the equatorial Atlantic interannual SST variability. We find that the Bjerknes feedback weakened after 2000 while the net heat flux damping increased. The weakening of the Bjerknes feedback does not appear to be fully explainable by changes in the mean state of the tropical Atlantic. The increased net heat flux damping is related to an enhanced response of the latent heat flux to the SST anomalies (SSTa). Strengthened trade winds as well as warmer SSTs are suggested to increase the air–sea specific humidity difference and hence, enhancing the latent heat flux response to SSTa. A combined effect of those two processes is proposed to be responsible for the weakened SST variability in the eastern equatorial Atlantic. The ReOsc model supports the link between reduced SST variability, weaker Bjerknes feedback and stronger net heat flux damping.     

ESTIMATES OF SEA SURFACE FLUXES DURING TOGA-COARE IOP

With the marine ship observation data set obtained by three cruise-phases of Chinese Xi-angyanghong 5 from November 5,1992 to February 19,1993 in the TOGA-COARE IOP at 2°S,156°E.the sea surface fluxes at this point are estimated by three different bulk schemes.Firstly.aquasi-linear relation is found between the neutral drag coefficients and wind speed.Then,the sta-bility-dependent drag and heat transfer coefficients are solved in the iterative method.Based onthus-derived transfer coefficients,the momentum,sensible and latent heat fluxes are calculated.In the warm pool region,the fluxes corresponding to the westeily winds are much greater thanthose of the easterly trade winds.The magnitude of sea surface fluxes depends upon the atmo-spheric stability as well,in particular in the case of weak wind condition.The estimated sea sur-face net heat budget shows that considerable amount of heat transport from ocean to atmosphere ismainly produced by the effective longwave radiation,latent and sensible heat fluxes.Among themthe value of latent heat flux is the largest and the sensible heat flux is the smallest.Finally,an ac-curacy analysis is made by direct measurements with the eddy-correlation method on the JapaneseR/V Hakuho board at the same time.It is shown that the bulk-derived fluxes are acceptable withmuch confidence.The estimated effective longwave radiation is used to compare with those by di-rect observations on Xiangyanghong 5.This research is compared with TOGA investigation in thetropical western Pacific.     

Regimes of seasonal air–sea interaction and implications for performance of forced simulations

Sea surface temperature (SST) anomalies can induce anomalous convection through surface evaporation and low-level moisture convergence. This SST forcing of the atmosphere is indicated in a positive local rainfall–SST correlation. Anomalous convection can feedback on SST through cloud-radiation and wind-evaporation effects and wind-induced oceanic mixing and upwelling. These atmospheric feedbacks are reflected in a negative local rainfall–SST tendency correlation. As such, the simultaneous rainfall–SST and rainfall–SST tendency correlations can indicate the nature of local air–sea interactions. Based on the magnitude of simultaneous rainfall–SST and rainfall–SST tendency correlations, the present study identifies three distinct regimes of local air–sea interactions. The relative importance of SST forcing and atmospheric forcing differs in these regimes. In the equatorial central-eastern Pacific and, to a smaller degree, in the western equatorial Indian Ocean, SST forcing dominates throughout the year and the surface heat flux acts mainly as a damping term. In the tropical Indo-western Pacific Ocean regions, SST forcing and atmospheric forcing dominate alternatively in different seasons. Atmospheric forcing dominates in the local warm/rainy season. SST forcing dominates with a positive wind-evaporation feedback during the transition to the cold/dry season. SST forcing also dominates during the transition to the warm/rainy season but with a negative cloud-radiation feedback. The performance of atmospheric general circulation model simulations forced by observed SST is closely linked to the regime of air–sea interaction. The forced simulations have good performance when SST forcing dominates. The performance is low or poor when atmospheric forcing dominates.     

Variance Method to Determine Turbulent Fluxes of Momentum And Sensible Heat in The Stable Atmospheric Surface Layer

Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined from the standard deviations of longitudinal wind velocity and temperature, σ _u and σ _T respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements that can be obtained with two-dimensional sonic anemometers. These instruments are increasingly being used at official weather stations, where they replace the standard cup anemometer–wind vane system. With methods such as the one described in this note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It is shown that a ‘variance’ dimensionless height (ζ _σ) defined from σ _u and σ _T is highly related to the ‘conventional’ dimensionless stability parameter ζ=z/L, where z is height and L is the Obukhov length. Empirical functions for ζ _σ are proposed that allow direct calculation of heat and momentum fluxes from σ _u and σ _T. The method performs fairly well also during a night of intermittent turbulence.     

A DIAGNOSTIC ANALYSIS OF THE RELATIONSHIP BETWEEN THE FLUXES AND METEOROLOGICAL ELEMENTS OVER TROPICAL WESTERN PACIFIC

Through the use of the hourly wind, air temperature and humidity, sea surface temperature data measured on board the observing vessel Moana Wave and buoy in the warm pool of western Pacific during the IOP of TOGA COARE, we compute the fluxes over sea surface and analyze the characteristics of the variation ofthe latent heat flux with sea surface temperature. During weak rather than strong wind periods a maximum valueof latent heat flux appears at some points of SST, which is caused mainly by the variations of wind, then by the humidity difference between air and sea and the transfer coefficient with SAT. Using correlation analysis. we also analyze the relationship between the fluxes and meteorological elements during weak wind periods. wester lywind burst periods, and convective disturbed periods etc. The main conclusions are that the latent heat flux ismainly determined by wind, sensible heat flux by the potential temperature difference between air and sea and the momentum flux by wind. The precipitation affects the sensible heat flux through the potential temperature difference and wind.     

‘Modelling the Arctic Boundary Layer: An Evaluation of Six Arcmip Regional-Scale Models using Data from the Sheba Project’

A primary climate change signal in the central Arctic is the melting of sea ice. This is dependent on the interplay between the atmosphere and the sea ice, which is critically dependent on the exchange of momentum, heat and moisture at the surface. In assessing the realism of climate change scenarios it is vital to know the quality by which these exchanges are modelled in climate simulations. Six state-of-the-art regional-climate models are run for one year in the western Arctic, on a common domain that encompasses the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment ice-drift track. Surface variables, surface fluxes and the vertical structure of the lower troposphere are evaluated using data from the SHEBA experiment. All the models are driven by the same lateral boundary conditions, sea-ice fraction and sea and sea-ice surface temperatures. Surface pressure, near-surface air temperature, specific humidity and wind speed agree well with observations, with a falling degree of accuracy in that order. Wind speeds have systematic biases in some models, by as much as a few metres per second. The surface radiation fluxes are also surprisingly accurate, given the complexity of the problem. The turbulent momentum flux is acceptable, on average, in most models, but the turbulent heat fluxes are, however, mostly unreliable. Their correlation with observed fluxes is, in principle, insignificant, and they accumulate over a year to values an order of magnitude larger than observed. Typical instantaneous errors are easily of the same order of magnitude as the observed net atmospheric heat flux. In the light of the sensitivity of the atmosphere–ice interaction to errors in these fluxes, the ice-melt in climate change scenarios must be viewed with considerable caution.     

Spatio-temporal Variability of Surface-layer Turbulent Fluxes Over the Bay of Bengal and Arabian sea During the ICARB Field Experiment

We report the spatio-temporal variability of surface-layer turbulent fluxes of heat, moisture and momentum over the Bay of Bengal (BoB) and the Arabian Sea (AS) during the Integrated Campaign for Aerosols, gases Radiation Budget (ICARB) field experiment. The meteorological component of ICARB conducted during March – May 2006 onboard the oceanic research vessel Sagar Kanya forms the database for the present study. The bulk transfer coefficients and the surface-layer fluxes are estimated using a modified bulk aerodynamic method, and then the spatio-temporal variability of these air-sea interface fluxes is discussed in detail. It is observed that the sensible and latent heat fluxes over the AS are marginally higher than those over the BoB, which we attribute to differences in the prevailing meteorological conditions over the two oceanic regions. The values of the wind stress, sensible and latent heat fluxes are compared with those obtained for the Indian Ocean Experiment (INDOEX) period. The variation of drag coefficient (C _D ), exchange coefficients of sensible heat and moisture (C _H = C _E ) and neutral drag coefficient (C _DN ) with wind speed is also discussed.

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Bulk Transfer Relations for the Roughness Sublayer

In the roughness sublayer (RSL), Monin–Obukhov surface layer similarity theory fails. This is problematic for atmospheric modelling applications over domains that include rough terrain such as forests or cities, since in these situations numerical models often have the lowest model level located within the RSL. Based on empirical RSL profile functions for momentum and scalar quantities, and scaling the height with the RSL height z _*, we derive a simple bulk transfer relation that accounts for RSL effects. To verify the validity of our approach, these relations are employed together with wind speed and temperature profiles measured over boreal forest during the BOREAS experimental campaign to estimate momentum and heat fluxes. It is demonstrated that, when compared with observed flux values, the inclusion of RSL effects in the transfer relations yields a considerable improvement in the estimated fluxes.     

Turbulence intensities and bulk coefficients in the surface layer above the sea

The structure of the atmospheric surface layer above the sea is analysed from aircraft turbulence measurements. The data are issued from two experiments performed in 1990 above the Mediterranean sea: Crau and PYREX, and correspond to moderately unstable conditions and to wind velocities ranging from 6 to 20 m/s. Low-altitude straight and level runs were used to compute the variances of the wind components, as well as of the temperature and moisture. Their dependence on the stability index —z/L is analysed. The turbulent fluxes of momentum, sensible heat and latent heat, calculated by the eddy-correlation technique, are used to estimate the neutral bulk coefficients: drag coefficient, Stanton number and Dalton number. The neutral drag coefficient clearly exhibits a dependence on the windspeed, which could be well fitted by the Charnock relation, with a constant of 0.012.