Water exchange between the subpolar and subtropical North Pacific in an OGCM

The water exchange between the subpolar and subtropical gyres of the North Pacific is demonstrated by the simulation of chlorofluorocarbon (CFC) using an ocean general circulation model. The simulated CFC concentration in the North Pacific is in good agreement with observations. The water exchange is clearly illustrated by the tongues of CFC concentration. The subpolar waters with high CFC are transported southward into the eastern subtropical gyre, whereas the subtropical waters with low CFC are transported northward into the western subpolar gyre. The simulated exchange transport along 42°N in the layer of σθ< 26.8 indicates that the northward mass transport is about 15 Sv (1 Sv = 106 m3·s-1) west of 165°E, and about 5 Sv between 175°W and 150°W. The southward mass transport is about 5 Sv between 165°E and 175°W, and about 2 Sv east of 150°W.     

Climate fluctuations during the Holocene in NW Iberia: High and low latitude linkages

High resolution benthic foraminiferal stable isotopes (δ¹⁸O, δ¹³C) and molecular biomarkers in the sediments are used here to infer rapid climatic changes for the last 8200 years in the Ría de Muros (NW Iberian Margin). Benthic foraminiferal δ¹⁸O and δ¹³C potentially register migrations in the position of the hydrographic front formed between two different intermediate water masses: Eastern North Atlantic Central Water of subpolar origin (ENACW_sp) and subtropical origin (ENACW_st). The molecular biomarkers in the sediment show a strong coupling between continental organic matter inputs and negative δ¹³C values in benthic foraminifera. The rapid centennial and millennial events registered in these records have been compared with two well known North Atlantic Holocene records from the subtropical Atlantic sea surface temperatures (SST) anomalies off Cape Blanc, NW Africa and the subpolar Atlantic (Hematite Stained Grains percentage, subpolar North Atlantic). Comparison supports a strong link between high- and low-latitude climatic perturbations at centennial–millennial time scales during the Holocene. Spectral analyses also points to a pole-to-equator propagation of the so-called 1500 yr cycles. Our results demonstrate that during the Holocene, the NW Iberian Margin has undergone a series of rapid events which are likely triggered at high latitudes in the North Atlantic and are rapidly propagated towards lower latitudes. Conceivably, the propagation of these rapid climatic changes involves a shift in atmospheric and oceanic circulatory systems.     

Response of a Coupled Ocean�CAtmosphere Model to Greenland Ice Melting

We investigate the transient response of the global coupled ocean?Catmosphere system to enhanced freshwater forcing representative of melting of the Greenland ice sheets. A 50-year long simulation by a coupled atmosphere?Cocean general circulation model (CGCM) is compared with another of the same length in which Greenland melting is prescribed. To highlight the importance of coupled atmosphere?Cocean processes, the CGCM results are compared with those of two other experiments carried out with the oceanic general circulation model (OGCM). In one of these OGCM experiments, the prescribed surface fluxes of heat, momentum and freshwater correspond to the unperturbed simulation by the CGCM; in the other experiment, Greenland melting is added to the freshwater flux. The responses by the CGCM and OGCM to the Greenland melting have similar patterns in the Atlantic, albeit the former having five times larger amplitudes in sea surface height anomalies. The CGCM shows likewise stronger variability in all state variables in all ocean basins because the impact of Greenland melting is quickly communicated to all ocean basins via atmospheric bridges. We conclude that the response of the global climate to Greenland ice melting is highly dependent on coupled atmosphere?Cocean processes. These lead to reduced latent heat flux into the atmosphere and an associated increase in net freshwater flux into the ocean, especially in the subpolar North Atlantic. The combined result is a stronger response of the coupled system to Greenland ice sheet melting.     

Variability of the meridional overturning circulation of the North Atlantic: sensitivity to overflows of dense water masses

A numerical model of the Atlantic Ocean was used to study the low-frequency variability of meridional transports in the North Atlantic. The model shows a behaviour similar to those used in previous studies, and the temporal variability of certain variables compares favourably to observed time series. By changing the depth and width of the sills between the subpolar North Atlantic and the Nordic Seas, the mean horizontal and overturning circulation as well as some water mass properties are modified significantly. The reaction of meridional oceanic transports to atmospheric forcing fluctuations remains, however, unchanged. The critical role of the surface heat flux retroaction term for the meridional heat transport in stand-alone ocean models is discussed. The experiments underline the role of atmospheric variability for fluctuations of the large-scale ocean circulation on time scales from years to decades, and they support the hypothesis that the mean overturning strength is controlled by the model representation of the density of the overflow water masses.Responsible Editor: Dirk Olbers     

Temperature variability in the Bay of Biscay during the past 40 years,from an in situ analysis and a 3D global simulation

A global in situ analysis and a global ocean simulation are used jointly to study interannual to decadal variability of temperature in the Bay of Biscay, from 1965 to 2003. A strong cooling is obtained at all depths until the mid-1970's, followed by a sustained warming over ～30 years. Strong interannual fluctuations are superimposed on this slow evolution. The fluctuations are intensified at the surface and are weakest at ～500 m. A good agreement is found between the observed and simulated temperatures, in terms of mean values, interannual variability and time correlations. Only the decadal trend is significantly underestimated in the simulation. A comparison to satellite sea surface temperature (SST) data over the last 20 years is also presented. The first mode of interannual variability exhibits a quasi-uniform structure and is related to the inverse winter North Atlantic Oscillation (NAO) index. Regarding the vertical structure, most cool and warm anomalies are generated at the surface, with the strongest ones penetrating down to 700 m and lasting up to 5 years. The complete heat budget from 1965 to 2004 is presented, including the contributions of vertical transport, freshwater flux and surface elevation. Interannual anomalies are mainly generated by the surface heat flux, while oceanic transports may become more important at longer time scales.     

Ocean-atmosphere dynamics changes associated with prominent ocean surface turbulent heat fluxes trends during 1958–2013

Three prominent features of ocean surface turbulent heat fluxes (THF) trends during 1958–2013 are identified based on the Objectively Analyzed air-sea Fluxes (OAFlux) data set. The associated ocean-atmosphere dynamics changes are further investigated based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. First, the THF are enhanced over the mid-latitude expansions of the subtropical western boundary currents (WBCs). An intensified oceanic heat transport, forced by stronger near-surface zonal wind, is likely to be the cause of such THF tendency. Second, the THF are reduced over the tropical eastern Pacific Ocean, which is primarily caused by the decreasing near-surface wind speed and sea surface temperature (SST), associated with a local coupled ocean-atmosphere cooling mode. Finally, the THF are reduced over the northern tropical Atlantic Ocean, which is attributed to the decreasing air-sea humidity and temperature differences as a result of the convergence of near-surface air and the divergence of ocean currents (upwelling).     

Recent changes of the thermohaline circulation in the subpolar North Atlantic

Time series of hydrographic sections in the northern North Atlantic from the period 1990 to 2004 are analyzed for changes in the characteristics and distribution of water masses that are involved in the thermohaline circulation (THC). During the 1990s, the North Atlantic Oscillation (NAO) alternates from a positive phase (strong westerlies) to a negative phase (weak westerlies). The reduced ocean heat loss confined the convection in the Labrador Sea to the upper 1,200 m, generating a new salinity minimum layer characterizing the Upper Labrador Sea Water (ULSW), and led to a warming and salinization of the older LSW below due to lateral mixing. The Lower LSW, formed in the first half of the 1990s, spread in the subpolar gyre and reached the Newfoundland and Irminger basins after about 1 to 2 years, where the associated isopycnal doming contributed to eastward frontal shifts in the upper layer. After 5 and 6 years, it arrived in the Iceland and West European basins, respectively. The collapse of the isopycnal dome in the Labrador Sea, associated with the drainage of the Lower LSW, resulted in a slowing of the cyclonic circulation of the subpolar gyre. This was accompanied in the upper layer by a westward shift of the southeastern extension of the gyre and a northward advection of warm and saline subtropical water in its eastern part, which finally reached the Labrador Sea after about 7 years. In the upper layer of the Labrador Sea, the advection of warm and saline water dominated over the heat loss to the atmosphere and the freshwater gain from melting ice and precipitation in the NAO-low period, so that no accumulation of freshwater but an increase of the heat and salt contents were observed, as in the whole eastern part of the subpolar gyre. Within 1 to 2 years after the drop of the NAO in the winter of 1995/1996, the Subarctic (Subpolar) Front shifted northward and westward north of about 50°N, favored by the retreat of the low-salinity tongue extending eastward from the southern Labrador Sea, and it shifted southward and eastward in the Newfoundland Basin. Therefore, the enhanced northward advection of subtropical waters in the northeastern North Atlantic is balanced by the enhanced southward advection of subarctic waters, including Lower LSW in the Newfoundland Basin, indicating a strong response of the gyre component of the THC.     

Wind-induced subduction at the South Atlantic subtropical front

The South Atlantic Subtropical Front, associated with the eastward-flowing South Atlantic Current, separates the colder, nutrient-rich waters of the subpolar gyre from the warmer, nutrient-poor waters of the subtropical gyre. Perturbations to the quasi-geostrophic, eastward flow generate meanders and filaments which induce cross-frontal exchange of water properties. Down-front winds transport denser waters from the South over warm waters from the North, inducing convective instability and subduction. Such processes occur over spatial scales of the order of 1 km and thus require high horizontal spatial resolution. In this modeling study, a high-resolution (4 km) regional grid is embedded in a basin-wide configuration (12 km) of the South Atlantic Ocean in order to test the importance of submesoscale processes in water mass subduction along the subtropical front. Stronger and more numerous eddies obtained in the high-resolution run yield more intense zonal jets along the frontal zone. Such stronger jets are more susceptible to instabilities, frontogenesis, and the generation of submesoscale meanders and filaments with \(\mathcal {O}(1)\) Rossby number. As a consequence, vertical velocities larger than 100 md ¹ are obtained in the high-resolution run, one order of magnitude larger than in the low-resolution run. Wind-driven subduction occurs along the frontal region, associated with negative Ertel potential vorticity in the surface layer. Such processes are not observed in the low-resolution run. A passive tracer experiment shows that waters with density characteristics similar to subtropical mode waters are preferentially subducted along the frontal region. The wind-driven buoyancy flux is shown to be much larger than thermal or haline fluxes during the wintertime, which highlights the importance of the frictional component in extracting PV from the surface ocean and inducing subduction, a process that has been overlooked in subtropical mode water formation in the region.     

北极海冰与夏季欧亚遥相关型年际变化的联系及对我国夏季降水的影响

本文利用NCEP/NCAR等再分析资料和CAM3.1数值模式研究了夏季欧亚中高纬遥相关型年际变率与前期春季北极海冰变化的联系及其对我国夏季降水影响的可能机制.结果表明,夏季北大西洋-欧亚中高纬地区500 hPa位势高度场自然正交分解第二模态表现为"-+-+"遥相关波列,其中格陵兰岛-北大西洋和乌拉尔山地区为异常高空槽区所控制,而欧洲和贝加尔湖附近地区则为异常高压脊区,这种波列分布与欧亚中高纬EU型遥相关型十分类似.当遥相关波列为"-+-+"（"+-+-"）型分布时,前期春季巴伦支海北部和巴芬湾一带海冰偏少（多）,同期夏季巴伦支海北部一带海冰亦持续偏少（多）,同时在我国东北北部地区、长江和黄河之间地区降水明显偏少（多）.深入分析发现,巴伦支海北部和巴芬湾一带海冰偏少后,由于该地区湍流热通量明显偏强,在动力过程影响方面会形成异常Rossby波源,准定常Rossby波活动通量将向东亚地区传播,使得夏季欧亚中高纬"-+-+"遥相关波列出现.另外,海冰异常偏少后,在热动力过程影响方面,4-5月欧亚中高纬乌拉尔山-贝加尔湖以北地区积雪会出现"西少东多"偶极子型异常分布,其通过影响后期土壤湿度及下垫面热通量异常,也有利于夏季欧亚中高纬遥相关波列的维持.伴随着欧亚中高纬"-+-+"遥相关波列的出现,乌山阻塞高压偏弱,东亚槽偏浅,且亚洲副热带急流随之加强,贝加尔湖以北的副极地地区出现西风异常,东亚副热带急流北侧出现东风异常,贝加尔湖以南地区为异常反气旋控制,南下冷空气活动减弱.受到上述环流形势影响,我国东北北部地区、黄河和长江之间地区降水明显偏少.当巴伦支海北部和巴芬湾区域海冰偏多时,结论则反之.最后,基于春季海冰指数和晚春偶极子型积雪指数,我们建立了江淮流域夏季降水的预测模型,回报结果表明其对江淮流域夏季降水的年际变率具有较高的预测技巧.     

Modeling studies of the upper ocean response to a tropical cyclone

A coupled ocean and boundary layer flux numerical modeling system is used to study the upper ocean response to surface heat and momentum fluxes associated with a major hurricane, namely, Hurricane Dennis (July 2005) in the Gulf of Mexico. A suite of experiments is run using this modeling system, constructed by coupling a Navy Coastal Ocean Model simulation of the Gulf of Mexico to an atmospheric flux model. The modeling system is forced by wind fields produced from satellite scatterometer and atmospheric model wind data, and by numerical weather prediction air temperature data. The experiments are initialized from a data assimilative hindcast model run and then forced by surface fluxes with no assimilation for the time during which Hurricane Dennis impacted the region. Four experiments are run to aid in the analysis: one is forced by heat and momentum fluxes, one by only momentum fluxes, one by only heat fluxes, and one with no surface forcing. An equation describing the change in the upper ocean hurricane heat potential due to the storm is developed. Analysis of the model results show that surface heat fluxes are primarily responsible for widespread reduction (0.5°–1.5°C) of sea surface temperature over the inner West Florida Shelf 100–300 km away from the storm center. Momentum fluxes are responsible for stronger surface cooling (2°C) near the center of the storm. The upper ocean heat loss near the storm center of more than 200 MJ/m² is primarily due to the vertical flux of thermal energy between the surface layer and deep ocean. Heat loss to the atmosphere during the storm’s passage is approximately 100–150 MJ/m². The upper ocean cooling is enhanced where the preexisting mixed layer is shallow, e.g., within a cyclonic circulation feature, although the heat flux to the atmosphere in these locations is markedly reduced.     

全球海洋碳循环模式MOM4_L40对碳和营养物自然分布的模拟

本文介绍了国家气候中心发展的一个全球海洋碳循环环流模式,并分析评估了该模式的基本性能.该模式是在美国地球物理流体动力学实验室(GFDL,Geophysical Fluid Dynamics Laboratory)的全球海洋环流模式MOM4(Modular Ocean Model Version 4)基础上发展的一个垂直方向40层、包含生物地球化学过程的全球三维海洋碳循环环流模式,简称为MOM4_L40(Modular Ocean Model Version 4 With 40Levels).该模式在气候场强迫下长期积分1000年,结果分析表明,与观测相比,模式较好地模拟了海洋温度、盐度、总二氧化碳、总碱、总磷酸盐的表面和垂直分布特征.模拟的海洋总二氧化碳分布与观测基本相符,表层为低值区,其下为高值区,高值区域位于10°S—60°N之间,但2000m以上模拟值较观测偏小,2000m以下模拟值较观测偏大.总体来说,MOM4_L40模式是一个可信赖的海洋碳循环过程模拟研究工具.     

Air–sea interactions during strong winter extratropical storms

A high-resolution, regional coupled atmosphere–ocean model is used to investigate strong air–sea interactions during a rapidly developing extratropical cyclone (ETC) off the east coast of the USA. In this two-way coupled system, surface momentum and heat fluxes derived from the Weather Research and Forecasting model and sea surface temperature (SST) from the Regional Ocean Modeling System are exchanged via the Model Coupling Toolkit. Comparisons are made between the modeled and observed wind velocity, sea level pressure, 10 m air temperature, and sea surface temperature time series, as well as a comparison between the model and one glider transect. Vertical profiles of modeled air temperature and winds in the marine atmospheric boundary layer and temperature variations in the upper ocean during a 3-day storm period are examined at various cross-shelf transects along the eastern seaboard. It is found that the air–sea interactions near the Gulf Stream are important for generating and sustaining the ETC. In particular, locally enhanced winds over a warm sea (relative to the land temperature) induce large surface heat fluxes which cool the upper ocean by up to 2 °C, mainly during the cold air outbreak period after the storm passage. Detailed heat budget analyses show the ocean-to-atmosphere heat flux dominates the upper ocean heat content variations. Results clearly show that dynamic air–sea interactions affecting momentum and buoyancy flux exchanges in ETCs need to be resolved accurately in a coupled atmosphere–ocean modeling framework.     

Transport of salt and freshwater in the Atlantic Subpolar Gyre

Transport of salt in the Irminger Current, the northern branch of the Atlantic Subpolar Gyre coupling the eastern and western subpolar North Atlantic, plays an important role for climate variability across a wide range of time scales. High-resolution ocean modeling and observations indicate that salinities in the eastern subpolar North Atlantic decrease with enhanced circulation of the North Atlantic subpolar gyre (SPG). This has led to the perception that a stronger SPG also transports less salt westward. In this study, we analyze a regional ocean model and a comprehensive global coupled climate model, and show that a stronger SPG transports more salt in the Irminger Current irrespective of lower salinities in its source region. The additional salt converges in the Labrador Sea and the Irminger Basin by eddy transports, increases surface salinity in the western SPG, and favors more intense deep convection. This is part of a positive feedback mechanism with potentially large implications for climate variability and predictability.     

MOM4_L40模式对全球海洋CFC-11分布的模拟及其通风能力评估

CFC-11是评估全球海洋环流模式的一个重要工具,海水中溶解的CFC-11被用来分析全球海洋模式的通风模拟.本文在中国气象局国家气候中心发展的40层全球海洋环流模式(MOM4_L40)增加了示踪物CFC-11模块,然后利用该模式研究了CFC-11在全球海洋中的分布,并评估了模式的通风能力.对CFC-11的海表浓度、柱总含量以及大洋剖面的垂直浓度分布和渗透深度进行了分析,结果表明,与观测相比,模式较好地再现了CFC-11在海洋表面的水平分布特征,CFC-11主要储存区位于西北大西洋、副热带北太平洋及南大洋,其浓度分布与温度分布梯度相反.沿三个大洋的5个剖面的CFC-11垂直分布模拟也与观测基本吻合.模式模拟的CFC-11分布情况与全球平均经向流函数吻合,在南大洋模拟效果更加接近观测值,深海模拟效果较好,渗透深度接近观测.同时,模拟与观测相比也存在偏差.比如在北大西洋主要的存储区域,模式低估了CFC-11的吸收,这与高纬的CFC-11向低纬过度输送有关,可能是受温盐环流和强迫资料的影响.总体来说,MOM_L40模式模拟大洋吸收的CFC-11总量是理想的,通过模拟被动示踪物CFC-11很好地再现了海洋的通风能力.     

Interannual variability in the Biannual Rossby waves in the tropical Indian Ocean and its relation to Indian Ocean Dipole and El Nino forcing

The interannual variability of the tropical Indian Ocean is studied using Simple Ocean Data Assimilation (SODA) sea surface height anomalies (SSHA) and Hadley Centre Ice Sea Surface Temperature anomalies. Biannual Rossby waves (BRW) were observed along the 1.5° S and 10.5° S latitudes during the Indian Ocean Dipole (IOD) years. The SODA SSHA and its BRW components were comparable with those of Topex/Poseidon. The phase speed of BRW along 1.5° S is −28 cm/s, which is comparable with the theoretical speed of first mode baroclinic (equatorially trapped) Rossby waves. This is the first study to show that no such propagation is seen along 1.5° S during El Nino years in the absence of IOD. Thus the westward propagating downwelling BRW in the equatorial Indian Ocean is hypothesized as a potential predictor for IOD. These waves transport heat from the eastern equatorial Indian Ocean to west, long before the dipole formation. Along 10.5° S, the BRW formation mechanisms during the El Nino and IOD years were found to be different. The eastern boundary variations along 10.5° S, being localized, do not influence the ocean interior considerably. Major portion of the interannual variability of the thermocline, is caused by the Ekman pumping integrated along the characteristic lines of Rossby waves. The study provides evidence of internal dynamics in the IOD formation. The positive trend in the downwelling BRW (both in SODA and Topex/Poseidon) is of great concern, as it contributes to the Indian Ocean warming.     

Study of seasonal variability and heat budget of the East Australian Current using two eddy-resolving ocean circulation models

Two mutually exclusive ocean models, Ocean general circulation model for the Earth Simulator (OFES) and the Bluelink ReANalysis (version 2.1; BRAN2.1), and the spin-up model (SPINUP4) of BRAN2.1 were used to investigate seasonal variability of the East Australian Current (EAC). These model outputs were tested against satellite and in situ data. The seasonally averaged sea surface temperature (SST) in the OFES and SPINUP4 shows a negative bias of 1 °C. However, the OFES, SPINUP4, and BRAN2.1 have a similar seasonal cycle in SST. The annual mean EAC transport computed at 28°S from the three models shows a good agreement with annual mean transport computed using the in situ data. However, they have considerable differences in terms of annual cycle. A better performance of the BRAN2.1 in simulating the temperature field is a result of data assimilation. The advection of heat across the open boundaries contributes ～50 % of the heat content change in the region. This study suggests that the advection by the EAC plays a significant role in heat content change of the region.     

Aerosol deposition velocities on the Pacific and Atlantic oceans calculated from7Be measurements

The concentrations of⁷Be have been measured in Pacific and Atlantic ocean water for the past several years to determine the deposition velocity of aerosol particles on the ocean surface.⁷Be is produced at a relatively constant rate in the atmosphere by spallation reactions of cosmic rays with atmospheric nitrogen and oxygen. Immediately after its formation⁷Be becomes attached to aerosol particles, and therefore can serve as tracers of the subsequent behavior of these particles. Isopleths of⁷Be surface water concentrations,⁷Be inventory in the ocean, and deposition velocity have been prepared for the Pacific Ocean from 30°S to 60°N and for the Atlantic Ocean from 10°N to 55°N. The concentrations, inventories and deposition velocities tended to be higher in regions where precipitation was high, and generally increased with latitude. The average flux of⁷Be across the ocean surface was calculated to be 0.027 atoms cm^?2 s^?1 which is probably not significantly greater than the worldwide average⁷Be flux across land and ocean surfaces of 0.022 atoms cm^?2 s^?1 calculated by Lal and Peters. The average deposition velocity was calculated to be 0.80 cm s^?1. This value may be 10–50% too low, since it was calculated using atmospheric⁷Be concentrations which were measured at continental stations. Measurements of atmospheric⁷Be concentrations at ocean stations suggest that the concentrations at the continental stations averaged 10–50% higher than the concentrations over the ocean.     

Relationship between sea surface salinity and ocean circulation and climate change

Based on Argo sea surface salinity(SSS) and the related precipitation(P), evaporation(E), and sea surface height data sets, the climatological annual mean and low-frequency variability in SSS in the global ocean and their relationship with ocean circulation and climate change were analyzed. Meanwhile, together with previous studies, a brief retrospect and prospect of seawater salinity were given in this work. Freshwater flux(E-P) dominated the mean pattern of SSS, while the dynamics of ocean circulation modulated the spatial structure and low-frequency variability in SSS in most regions. Under global warming, the trend in SSS indicated the intensification of the global hydrological cycle, and featured a decreasing trend at low and high latitudes and an increasing trend in subtropical regions. In the most recent two decades, global warming has slowed down, which is called the"global warming hiatus". The trend in SSS during this phase, which was different to that under global warming, mainly indicated the response of the ocean surface to the decadal and multi-decadal variability in the climate system, referring to the intensification of the Walker Circulation. The significant contrast of SSS trends between the western Pacific and the southeastern Indian Ocean suggested the importance of oceanic dynamics in the cross-basin interaction in recent decades. Ocean Rossby waves and the Indonesian Throughflow contributed to the freshening trend in SSS in the southeastern Indian Ocean, while the increasing trend in the southeastern Pacific and the decreasing trend in the northern Atlantic implied a long-term linear trend under global warming. In the future, higher resolution SSS data observed by satellites, together with Argo observations, will help to extend our knowledge on the dynamics of mesoscale eddies, regional oceanography, and climate change.     

Solar activity and dynamic processes in the atmosphere and world ocean heat content

The scenario of climatic changes in the 20th century has been presented in the scope of the developed model concerning the effect of solar activity on the parameters of the climatic system governing the energy flux, outgoing from the Earth into space in the high-latitude regions. The regularities of changes in the circulation in the atmosphere and ocean are discussed. Specific attention is paid to the causes of a “cold snap” in 1940–1976 in the Northern Hemisphere and the nature of an anomalous increase in the heat content in the Earth climatic system (ocean) in 1969–1980. It has been indicated that these phenomena result from changes in the circulation in the atmosphere and ocean (specifically, a change in the thermohaline circulation in the Northern Atlantic), heat exchange between the ocean and the atmosphere and cryosphere.     

A composite trans-Atlantic heat flow profile between 20°S and 35°S

Sixty new measurements together with published heat flow data in the South Atlantic between 20°S and 35°S latitude have been analyzed. Heat flux is greater through the eastern Mid-Atlantic Ridge flank and basin than their counterparts on the west but the standard deviation or spatial variation is greater on the west, contrary to expectation based on sediment thickness. The variance in the data indicates that this asymmetry in mean heat flux is statistically significant at 87% confidence level. A pair of ridge-flank minima appear in a composite trans-Atlantic profile of heat flux versus sea-floor age, suggesting hydrothermal circulation in the young oceanic crust. The Walvis Ridge has a mean excess heat flux of 28 mW m^?2 (0.7 μcal cm^?2 sec^?1) above the surrounding Cape and Angola Basins, and decreases along the ridge towards the northeast. Consistent with the apparent asymmetric distribution in the South Atlantic, it is also significantly higher than that of the Rio Grande Rise. We hypothesize that the trend and larger mean heat flux of the Walvis Ridge is best explained by a hot-spot origin, perhaps combined with higher radioactivity in the crust. However, the morphologic and heat flow differences between the Walvis Ridge and Rio Grande Rise suggest that these features have different geologic histories.