Evaluation of relative performance of QuikSCAT and NCEP re-analysis winds through simulations by an OGCM

The response of an ocean general circulation model (OGCM) to two different wind products, viz., NCEP/NCAR reanalysis and QuikSCAT scatterometer, was examined. OGCM-simulated thermodynamic variables from the two simulations, hereafter referred to as NCEP-R (NCEP/NCAR wind forced) and QS-R (QuikSCAT wind forced) were intercompared and also were compared against observations for a period of 3 years (2000–2002). In the tropical Indian Ocean (IO), the sea-level anomaly (SLA) simulated by QS-R has less root mean square error (RMSE) and higher correlation with respect to TOPEX/Poseidon SLA observations than SLA simulated by NCEP-R. Intraseasonal variability of currents observed by TRITON buoy in the IO was closely captured by QS-R, although the magnitudes are somewhat underestimated. Surface currents simulated by QS-R have less RMSE than those simulated by NCEP-R in the Pacific. However, the sub-surface currents are much weaker in magnitude in both the solutions, possibly because of deficiencies in the diffusion and viscosity parameterization. Sea-surface temperature (SST) simulated by QS-R has a cooler bias. The RMSE of SST simulated by NCEP-R is less than the RMSE of SST simulated by QS-R, with the latter capturing the variabilities more realistically. The large differences between SST simulated by QS-R and observations could be partly due to physical inconsistency between the momentum and heat fluxes. Scatterometer-forced model simulations of 20^oC thermocline depths (D20) are in better agreement with in situ-derived D20 than the D20 simulated by NCEP-R. Variations in the mixed layer depth at the TRITON buoy are better captured by QS-R than by NCEP-R. Speed of Kelvin and Rossby waves and the strength of upwelling/downwelling features in the IO are closer to observations in QS-R than in NCEP-R simulations.     

Inter-Comparison of Numerical Model Generated Surface Winds with QuikSCAT Winds over the Indian Ocean

The accurate surface wind in the equatorial Indian Ocean is crucial for modeling ocean circulation over this region. In this study, the surface wind analysis generated at the European Center for Medium Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) are compared with NASA QuikSCAT satellite derived Level2B (swath level) and Level3 (gridded) surface winds for the year 2005. It is observed that the ECMWF winds exhibit speed bias of 1.5 m/s with respect to QuikSCAT Level3 in the southern equatorial Indian Ocean. The NCEP winds are found to exhibit speed bias (1.0–1.5 m/s) in the southern equatorial Indian Ocean specifically during January–February 2005. The biases are also observed in the analysis when compared with Level2B product as well; however, it is less in comparison to Level3 products. The amplitude of daily variations of both ECMWF and NCEP wind speed in Bay of Bengal and parts of the Arabian Sea is about 80% of that in QuikSCAT, while in the equatorial Indian Ocean it is about 60% of that of QuikSCAT.     

Comparison of surface wind stress characteristics over the Tropical Atlantic (10°N–40°S) in fields derived from the UWM/COADS, NCEP/NCAR and QuikSCAT datasets

A comparison of monthly wind stress derived from winds of NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis and UWM/COADS (The University of Wisconsin-Milwaukee/Comprehensive Ocean-Atmosphere Data Set) dataset (1950–1993), and of NCEP/NCAR reanalysis and satellite-based QuikSCAT dataset (2000–2006), is made over the South Atlantic (10°N–40°S). On a mean seasonal scale, the comparison shows that these three wind stress datasets have qualitatively similar patterns. Quantitatively, in general, from about the equator to 20°S in the mid-Atlantic the wind stress values are stronger in NCEP/NCAR data than those in UWM/COADS data. On the other hand, in the Intertropical Convergence Zone (ITCZ) area the wind stress values in NCEP/NCAR data are slightly weaker than those in UWM/COADS data. In the South Atlantic, between 20° S–40°S, the QuikSCAT dataset presents complex circulation structures which are not present in NCEP/NCAR and UWM/COADS data. The wind stress is used in a numerical ocean model to simulate ocean currents, which are compared to a drifting-buoy observed climatology. The modeled South Equatorial Current agrees better with observations between March–May and June–August. Between December–February, the South Equatorial Current from UWM/COADS and QuikSCAT experiments is stronger and more developed than that from NCEP/NCAR experiment. The Brazil Current, in turn, is better represented in the QuikSCAT experiment. Comparison of the annual migration of ITCZ at 20° and 30°W in UWM/COADS and NCEP/NCAR data sources show that the southernmost position of ITCZ at 30°W in February, March and April coincides with the rainy season in NE Brazil, while the northernmost position of ITCZ at 20°W in August coincides with the maximum rainfall of Northwest Africa.     

Multidecadal climate variability in the subtropics/mid‐latitudes of the Southern Hemisphere oceans

Observations of multidecadal variability in sea surface temperature (SST), surface air temperature and winds over the Southern Hemisphere are presented and an ocean general circulation model applied towards investigating links between the SST variability and that of the overlying atmosphere. The results suggest that the dynamical effect of the wind stress anomalies is significant mainly in the neighbourhood of the western boundary currents and their outflows across the mid‐latitudes of each Southern Hemisphere basin (more so in the South Indian and South Atlantic than in the South Pacific Ocean) and in the equatorial upwelling zones. Over most of the subtropics to mid‐latitudes of the Southern Hemisphere oceans, changes in net surface heat flux (particularly in latent heat) appear to be more important for the SST variability than dynamical effects. Implications of these results for modelling and understanding low frequency climate variability in the Southern Hemisphere as well as possible links with mechanisms of decadal/interdecadal variability in the Northern Hemisphere are discussed.     

基于浮标、NCEP2及OAFlux产品的新海表感热和潜热通量对比分析

New satellite-derived latent and sensible heat fluxes are performed by using Wind Sat wind speed, Wind Sat sea surface temperature, the European Centre for Medium-range Weather Forecasting(ECMWF) air humidity, and ECMWF air temperature from 2004 to 2014. The 55 moored buoys are used to validate them by using the 30 min and 25 km collocation window. Furthermore, the objectively analyzed air-sea heat fluxes(OAFlux) products and the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis 2(NCEP2) products are also used for global comparisons. The mean biases of sensible and latent heat fluxes between Wind Sat flux results and buoy flux data are –0.39 and –8.09 W/m~2, respectively. In addition, the rootmean-square(RMS) errors of the sensible and latent heat fluxes between them are 5.53 and 24.69 W/m~2,respectively. The RMS errors of sensible and latent heat fluxes are observed to gradually increase with an increasing buoy wind speed. The difference shows different characteristics with an increasing sea surface temperature, air humidity, and air temperature. The zonal average latent fluxes have some high regions which are mainly located in the trade wind zones where strong winds carry dry air in January, and the maximum value centers are found in the eastern waters of Japan and on the US east coast. Overall, the seasonal variability is pronounced in the Indian Ocean, the Pacific Ocean, and the Atlantic Ocean. The three sensible and latent heat fluxes have similar latitudinal dependencies; however, some differences are found in some local regions.     

Validation of a decadal OGCM simulation for the tropical Pacific

An ocean general circulation model is forced with NCEP reanalysis over the 1948–1999 period. The simulated dynamic height and sea level are compared respectively to the dynamic height computed from hydrological data and to the sea level measured by tide gauges in the tropical Pacific. The model is shown to capture important features of the temporal structure of variability in the tide gauge data over the last several decades. However, the comparison reveals a largely artificial trend in the simulation, which consists of a decreasing dynamic height and sea level in the southwest and northwest of the tropical Pacific. Model sensitivity experiments show this trend is controlled by the NCEP surface wind stresses and more precisely by a weakening in the trade winds and a trend in the off-equatorial wind curl, with this trend existing mainly before the mid 1970s. For studies of decadal variability, the simple removal of a linear trend is an inadequate way to solve the problem, due to the inhomogeneities in the data used in reanalysis products and the non-linearity of models.     

东亚冬季风异常对西北太平洋海温的影响

利用1950—1998年的月平均海温资料和NCEP/NCAR月平均大气环流再分析资料,研究了东亚冬季风的异常对西北太平洋海温的作用过程。结果表明,南海—台湾附近海域—日本南部以南海域(简称东亚邻海)是海-气热通量异常的显著区。弱东亚冬季风在东亚邻海有偏南风距平,抑制相应海域海-气界面上由海表向大气释放的热通量,从而使得海表温度出现正距平。强冬季风则反之。这种大气-热通量-海温的异常影响过程所需的响应时间约为1个月。东亚邻海冬季发生的海温异常可持续到下一年的夏季。     

The Indian Ocean’s asymmetric effect on the coupling of the Northwest Pacific SST and anticyclone anomalies during its spring–summer transition after El Niño

By comparing different climatologies in El Niño decaying summer with regard to the presence of Indian Ocean Basin (IOB) warming, we studied the effect of IOB warming on the Northwest Pacific sea surface temperature (SST) anomalies and the coupling process with the surface wind. Zonal asymmetric coupling feedback in the west and east of the Northwest Pacific were caused by the asymmetric spring–summer transition of the background tropical atmospheric circulation. Although the westward wind anomaly caused by the remote effect of IOB warming is found in the whole Northwest Pacific, reversal of the mean background surface winds in the western part leads to negative wind-evaporation SST (WES), whereas sustained trade winds lead to positive WES in the eastern part. The east–west SST gradient resulting from this zonal asymmetric evolution of SST sets off more positive feedback that strengthens the local anticyclone easterly anomalies.     

Comparing satellite and meteorological data on wind velocity over the Black Sea

Wind-velocity data obtained from in situ measurements at the Golitsyno-4 marine stationary platform have been compared with QuikSCAT scatterometer data; NCEP, MERRA, and ERA-Interim global reanalyses and MM5 regional atmospheric reanalysis. In order to adjust wind velocity measured at a height of 37 m above the sea surface to a standard height of 10 m with stratification taken into account, the Monin–Obukhov theory and regional atmospheric reanalysis data are used. Data obtained with the QuikSCAT scatterometer most adequately describe the real variability of wind over the Black Sea. Errors in reanalysis data are not high either: the regression coefficient varies from 0.98 to 1.06, the rms deviation of the velocity amplitude varies from 1.90 to 2.24 m/s, and the rms deviation of the direction angle varies from 26° to 36°. Errors in determining the velocity and direction of wind depend on its amplitude: under weak winds (<3 m/s), the velocity of wind is overestimated and errors significantly increase in determining its direction; under strong winds (>12 m/s), its velocity is underestimated. The influence of these errors on both spatial and temporal estimates of the characteristics of wind over the Black Sea is briefly considered.     

Modeling of mesoscale coupled ocean–atmosphere interaction and its feedback to ocean in the western Arabian Sea

Observations of the western Arabian Sea over the last decade have revealed a rich filamentary eddy structure, with large horizontal SST gradients in the ocean, developing in response to the southwest monsoon winds. This summertime oceanic condition triggers an intense mesoscale coupled interaction, whose overall influence on the longer-term properties of this ocean remains uncertain. In this study, a high-resolution regional coupled model is employed to explore this feedback effect on the long-term dynamical and thermodynamical structure of the ocean.The observed relationship between the near-surface winds and mesoscale SSTs generate Ekman pumping velocities at the scale of the cold filaments, whose magnitude is the order of 1 m/day in both the model and observations. This additional Ekman-driven velocity, induced by the wind-eddy interaction, accounts for approximately 10–20% of oceanic vertical velocity of the cold filaments. This implies that Ekman pumping arising from the mesoscale coupled feedback makes a non-trivial contribution to the vertical structure of the upper ocean and the evolution of mesoscale eddies, with obvious implications for marine ecosystem and biogeochemical variability.Furthermore, SST features associated with cold filaments substantially reduce the latent heat loss. The long-term latent heat flux change due to eddies in the model is approximately 10–15 W/m² over the cold filaments, which is consistent with previous estimates based on short-term in situ measurements. Given the shallow mixed layer, this additional surface heat flux warms the cold filament at the rate of 0.3–0.4 °C/month over a season with strong eddy activity, and 0.1–0.2 °C/month over the 12-year mean, rendering overall low-frequency modulation of SST feasible. This long-term mixed layer heating by the surface flux is approximately ±10% of the lateral heat flux by the eddies, yet it can be comparable to the vertical heat flux. Potential dynamic and thermodynamic impacts of this observed air–sea interaction on the monsoons and regional climate are yet to be quantified given the strong correlation between the Somalia upwelling SST and the Indian summer monsoons.     

Interannual variations of the air-sea carbon dioxide exchange in the different regions of the Pacific Ocean

Interannual variations of the air-sea CO 2 exchange from 1965 to 2000 in the Pacific Ocean are studied with a Pacific Ocean model.Two numerical experiments are performed,including the control run that is forced by climatological monthly mean physical data and the climate-change run that is forced by interannually varying monthly mean physical data.Climatological monthly winds are used in both runs to calculate the coefficient of air-sea CO 2 exchange.The analysis through the differences between the two runs shows that in the tropical Pacific the variation of export production induced by interannual variations of the physical fields is negatively correlated with that of the air-sea CO 2 flux,while there is no correlation or a weak positive correlation in the subtropical North and South Pacific.It indicates that the variation of the physical fields can modulate the variation of the air-sea CO 2 flux in converse ways in the tropical Pacific by changing the direct transport and biochemical process.Under the interannually varying monthly mean forcing,the simulated EOF1 of the air-sea CO 2 flux is basically consistent with that of sea surface temperature(SST) in the tropical Pacific,but contrary in the two subtropical Pacific Ocean.The correlation coefficient between the regionally integrated air-sea CO 2 flux and area-mean SST shows that when the air-sea CO 2 flux lags SST by about 5 months,the positive coefficient in the three regions is largest,indicating that in the tropical Pacific or on the longer time scale in the three regions,physical processes control the flux-SST relationship.     

Numerical simulation of the tropical Pacific response to equatorial wind stress anomalies

The results of the tropical Pacific response to the sudden onset of the equatorial wind stress anomalies are discussed. The ocean model is a barotropic, non-linearized one that includes reduced-gravity and an equation for the temperature of the ocean mixed-layer. The experiments are based on a state of equilibrium reached through a long running under the action of annual mean wind stress. There are two kinds of westward wind intensity regions: the whole tropical Pacific and the western tropical Pacific, which are all between latitude 6. 8癗 and 6. 8癝.In these cases, the results show that the positive sea surface temperature (SST) anomalies in the Eastern Pacific and the negative SST anomalies in the Western Pacific are produced, and the positive SST anomalies propagate eastward, just as those observed during the actual El Nino phenomena. The propagations of the Kelvin waves and Rossby waves in the ocean are discussed.Another experiment is also carried out in simulating the process of the decay of El Ni     

赤道海洋对罕见台风“画眉”的响应

利用GHRSST L4、QuikSCAT、OAFLUX以及SeaWiFS L3资料分析了近赤道罕见台风"画眉"生成前后海表温度SST及其感热通量、潜热通量和叶绿素a浓度的变化。在台风"画眉"生成之前,中南半岛沿岸海表平均温度较其他区域低,并且在南海盛行东北风,在台风生成区有一明显的气旋性涡旋存在。南海北部地区潜热通量和感热通量均较大,而在台风的生成区域仅感热通量较大。台风"画眉"使其路径右侧的区域发生海表温度降低,相对于其他强度较强的台风降温较小,海表温度在马来半岛以东洋面以及马六甲海峡降低明显,降低约2—2.5℃。与高纬度的台风类似,台风"画眉"使中南半岛沿岸以及马来半岛与苏门答腊岛之间的地区叶绿素a浓度相对于台风前增大0.6 mg.m 3以上。     

Modeling of mesoscale coupled ocean–atmosphere interaction and its feedback to ocean in the western Arabian Sea

Observations of the western Arabian Sea over the last decade have revealed a rich filamentary eddy structure, with large horizontal SST gradients in the ocean, developing in response to the southwest monsoon winds. This summertime oceanic condition triggers an intense mesoscale coupled interaction, whose overall influence on the longer-term properties of this ocean remains uncertain. In this study, a high-resolution regional coupled model is employed to explore this feedback effect on the long-term dynamical and thermodynamical structure of the ocean.The observed relationship between the near-surface winds and mesoscale SSTs generate Ekman pumping velocities at the scale of the cold filaments, whose magnitude is the order of 1 m/day in both the model and observations. This additional Ekman-driven velocity, induced by the wind-eddy interaction, accounts for approximately 10–20% of oceanic vertical velocity of the cold filaments. This implies that Ekman pumping arising from the mesoscale coupled feedback makes a non-trivial contribution to the vertical structure of the upper ocean and the evolution of mesoscale eddies, with obvious implications for marine ecosystem and biogeochemical variability.Furthermore, SST features associated with cold filaments substantially reduce the latent heat loss. The long-term latent heat flux change due to eddies in the model is approximately 10–15 W/m² over the cold filaments, which is consistent with previous estimates based on short-term in situ measurements. Given the shallow mixed layer, this additional surface heat flux warms the cold filament at the rate of 0.3–0.4 °C/month over a season with strong eddy activity, and 0.1–0.2 °C/month over the 12-year mean, rendering overall low-frequency modulation of SST feasible. This long-term mixed layer heating by the surface flux is approximately ±10% of the lateral heat flux by the eddies, yet it can be comparable to the vertical heat flux. Potential dynamic and thermodynamic impacts of this observed air–sea interaction on the monsoons and regional climate are yet to be quantified given the strong correlation between the Somalia upwelling SST and the Indian summer monsoons.     

South China Sea wave characteristics during typhoon Muifa passage in winter 2004

The responses to tropical cyclones of ocean wave characteristics in deep water of the western Atlantic Ocean have been investigated extensively, but not the regional seas in the western Pacific such as the South China Sea (SCS), due to a lack of observational and modeling studies there. Since monsoon winds prevail in the SCS but not in the western Atlantic Ocean, the SCS is unique for investigating wave characteristics during a typhoon’s passage in conjunction with steady monsoon wind forcing. To do so, the Wavewatch-III (WW3) is used to study the response of the SCS to Typhoon Muifa (2004), which passed over not only deep water but also the shallow shelf of the SCS. The WW3 model is forced by the NASA QuikSCAT winds and tropical cyclone wind profile model during Typhoon Muifa’s passage from 0000UTC 16 on November to 1200UTC on 25 November 2004. The results reveal the unique features of the SCS wave characteristics in response to Muifa, such as non-decaying, monsoon-generated swell throughout the typhoon period and strong topographic effects on the directional wave spectrum.     

The heat balance on the sea surface in the mature phase of 1982/83 El Nino event

Using the air-sea data set of January, 1983 (the mature phase of the 1982/83 El Nino event), the net radiation on the sea surface, the fluxes of the latent and the sensible heat from ocean to the atmosphere and the net heat gain of the sea surface are calculated over the Indian and the Pacific Oceans for the domain of 35°N-35°S and 45°E-75°W. The results indicate that the upward transfer of the latent and the sensible heat fluxes over the winter hemisphere is larger than that over the summer hemisphere. The sensible heat over the tropical mid Pacific in the Southern Hemisphere is transported from the atmosphere to the ocean, though its magnitude is rather small. The latent heat flux gained by the air over the eastern Pacific is less than the mean value of the normal year. The net radiation, on which the cloud amount has considerable impact, is essentially zonally distributed. Moreover, the sea surface temperature (SST) has a very good correlation with the net radiation, the region of warm SST coinci     

Sea surface height variability in the Rio de la Plata estuary from synoptic to inter-annual scales: Results of numerical simulations

A long term simulation of the barotropic circulation in the Río de la Plata estuary was performed with the aim of identifying the characteristic patterns of sea surface height (SSH) variability from synoptic to inter-annual time scales and their forcing mechanisms. Hamburg Shelf Ocean Model (HamSOM), forced by tides, monthly mean runoffs and 4-daily 10 m winds and surface atmospheric pressure from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis was run. The solution was analyzed for the period 1965–2004. Inter-annual variability accounts for almost 10% of the variance. The first EOF mode of SSH variability on this time scale is associated with a mean anomaly of approximately 0.25 m at the upper estuary forced by both runoff and winds, which seems to be strongly associated with the ENSO cycles. Other two modes, with periodicities around 2.5 and 10 years were also found. Even though they are linked to weaker SSH anomalies, they are consistent with inter-annual modes of wind variability reported by other authors. Those modes are important, particularly if they act in phase, because they can provide a background for stronger surges. In contrast with the salinity field, SSH variability on seasonal time scales accounts for a very small percentage of variance and it is the combination of an annual and a semi-annual signal forced by winds and runoff, respectively. Approximately 90% of the variance is due to wind driven variability on sub-annual time scales. The most significant SSH anomalies in this band are associated with cyclogenetic events in the atmosphere, occurring either over Uruguay or over the Patagonian Shelf, whereas the strengthening or weakening of the semi-permanent South Atlantic anticyclone displays a relatively smaller influence. In agreement with previous publications, the estuary's spatial patterns in response to short-scale wind variability seem to be determined by wind direction more than by wind speed.     

全球海洋环流模式中上层海洋对表面强迫的响应和调整Ⅱ.年代际变率

利用一个较高分辨率的全球海洋环流模式在COADS 1945～1993年逐月平均资料的强迫下对海温和环流场进行了模拟,分析了北太平洋海温和环流场的年代际变化特征,同时诊断了1976-77年代际跃变过程中海温场变化的机制.模式模拟出了北太平洋海温年代际异常的主要模态以及1976-77年跃变前后的演变特征,模拟的北太平洋中部、加州沿岸和KOE区的海温异常的强度和演变趋势均和观测比较一致;同时,模式重现了分别始于20世纪70和80年代的中纬度海温异常信号沿等密度面向低纬地区的两次潜沉过程.在表层,流场的异常主要表现为与风应力异常基本符合Ekman关系的一个异常海洋涡旋,而整个上层海洋平均的流场异常则表现为两个海洋涡旋的异常,其中副热带海洋涡旋的异常的强度要显著于副极地海洋涡旋的异常,而副极地海洋涡旋异常出现的时间比副热带海洋涡旋晚3a左右的时间.对1976-77年前后3个区域上层海温各贡献项的诊断结果表明,北太平洋中部变冷主要是水平平流和热通量异常贡献的结果;而加州沿岸变暖主要归因于热通量的贡献;在KOE区,垂直平流、热通量和水平平流三者都起了重要作用,其中水平平流异常对这一区域海温年代际跃变出现的时间起了至关重要的作用.     

一种QuikSCAT大风地球物理模型及其在台风IOKE风场反演中的应用

The geophysical model function (GMF) describes the relationship between a backscattering and a sea surface wind, and enables a wind vector retrieval from backscattering measurements. It is clear that t...     

春季南海南部上混合层数值模拟与数值实验

采用一维湍动能模式对南海南部的 SST及混合层进行数值模拟和数值试验。结果表明 :TKE模式能够模拟南海南部的海表面温度 SST以及除南海南部 5月中旬以外的上混合层深度随时间变化基本特征。在 5～ 6月 ,SST的日振荡主要依赖于短波辐射的日变化 ,风的混合作用抑制了 SST的日周期振荡。春季夏季风爆发期间 ,南海海面潜热通量和感热通量与短波辐射和风应力相比较 ,是一个对 SST和混合层影响较小的量。在春季南海南部 ,短波辐射作用能使 SST升高的最大值约为 4℃ ;潜热和感热通量能使 SST的下降的最大值为 3℃。风应力对南海混合层深度随时间变化趋势起着决定的作用 ,并能使其深度加深 2 0～ 30 m,而短波辐射则使混合层的深度变浅2～ 3m,潜热和感热通量会使混合层的深度加深 1～ 2 m。在春季南海南部 ,热通量对混合层深度的影响与风应力相比要小得多