An intimate coupling of ocean–atmospheric interaction over the extratropical North Atlantic and Pacific

The inter-basin teleconnection between the North Atlantic and the North Pacific ocean–atmosphere interaction is studied using a coupled ocean–atmosphere general circulation model. In the model, an idealized oceanic temperature anomaly is initiated over the Kuroshio and the Gulf Stream extension region to track the coupled evolution of ocean and atmosphere interaction, respectively. The experiments explicitly demonstrate that both the North Pacific and the North Atlantic ocean–atmosphere interactions are intimately coupled through an inter-basin atmospheric teleconnection. This fast inter-basin communication can transmit oceanic variability between the North Atlantic and the North Pacific through local ocean-to-atmosphere feedbacks. The leading mode of the extratropical atmospheric internal variability plays a dominant role in shaping the hemispheric-scale response forced by oceanic variability over the North Atlantic and Pacific. Modeling results also suggest that a century (two centuries) long observations are necessary for the detection of Pacific response to Atlantic forcings (Atlantic response to Pacific forcing).     

Interannual and interdecadal variabilities in the Pacific in an MRI coupled GCM

Interannual and interdecadal variabilities in the Pacific are investigated with a coupled atmosphere-ocean GCM developed at MRI, Japan. The model is run for 70 years with flux adjustments. The model shows interannual variability in the tropical Pacific which has several typical characteristics shared with the observed ENSO. A basin-scale feature of the principal SST variation for the ENSO time scale shows negative correlation in the central North Pacific with the tropical SST, similar to that of the observed one. Associated variation of the model atmosphere indicates an intensification of the Aleutian Low and a PNA-like teleconnection pattern as a response to the tropical warm SST anomaly. The ENSO time scale variability in the midlatitude ocean consists of the westward propagation of the subsurface temperature signal and the temperature variation within the shallow mixed layer forced by the anomalous atmospheric heat fluxes. For the interdecadal time scale, variation of the SST is simulated realistically with a geographical pattern similar to that for the ENSO time scale, but it has a larger relative amplitude in the northern Pacific. For the atmosphere, spatial structure of the variation in the interdecadal time scale is also similar to that in the ENSO time scale, but has smaller amplitude in the northern Pacific. Long oceanic spin-up time (>∼10 y) in the mid-high latitude, however, makes oceanic response in the interdecadal time scale larger than that in the ENSO time scale. The lagged-regression analysis for the ocean temperature variation relative to the wind stress variation indicates that interdecadal variation of the ocean subsurface at the mid-high latitudes is considered as enhanced ocean gyre spin-up process in response to the atmospheric circulation change at the mid-high latitudes, remotely forced by the interdecadal variation of the tropical SST. Received: 6 November 1995 / Accepted: 19 April 1996     

Impacts of shortwave radiation forcing on ENSO: a study with a coupled tropical ocean-atmosphere model

 We describe a coupled tropical ocean-atmosphere model that represents a new class of models that fill the gap between anomaly coupled models and fully coupled general circulation models. Both the atmosphere and ocean are described by two and half layer primitive equation models, which emphasize the physical processes in the oceanic mixed layer and atmospheric boundary layer. Ocean and atmosphere are coupled through both momentum and heat flux exchanges without explicit flux correction. The coupled model, driven by solar radiation, reproduces a realistic annual cycle and El Nino-Southern Oscillation (ENSO). In the presence of annual mean shortwave radiation forcing, the model exhibits an intrinsic mode of ENSO. The oscillation period depends on the mean forcing that determines the coupled mean state. A perpetual April (October) mean forcing prolongs (shortens) the oscillation period through weakening (enhancing) the mean upwelling and mean vertical temperature gradients. The annual cycle of the solar forcing is shown to have fundamental impacts on the behavior of ENSO cycles through establishing a coupled annual cycle that interacts with the ENSO mode. Due to the annual cycle solar forcing, the single spectral peak of the intrinsic ENSO mode becomes a double peak with a quasi-biennial and a low-frequency (4–5 years) component; the evolution of ENSO becomes phase-locked to the annual cycle; and the amplitude and frequency of ENSO become variable on an interdecadal time scale due to interactions of the mean state and the two ENSO components. The western Pacific monsoon (the annual shortwave radiation forcing in the western Pacific) is primarily responsible for the generation of the two ENSO components. The annual march of the eastern Pacific ITCZ tends to lock ENSO phases to the annual cycle. The model's deficiencies, limitations, and future work are also discussed. Received: 15 June 1999 / Accepted: 11 December 1999     

On decadal-scale ocean-atmosphere interactions in the extended ECHAM1/LSG climate simulation

 The last 810 years of a control integration with the ECHAM1/LSG coupled model are used to clarify the nature of the ocean-atmosphere interactions at low frequencies in the North Atlantic and the North Pacific. To a first approximation, the atmosphere acts as a white noise forcing and the ocean responds as a passive integrator. The sea surface temperature (SST) variability primarily results from short time scale fluctuations in surface heat exchanges and Ekman currents, and the former also damp the SST anomalies after they are generated. The thermocline variability is primarily driven by Ekman pumping. Because the heat, momentum, and vorticity fluxes at the sea surface are correlated in space and time, the SST variability is directly linked to that in the ocean interior. The SST is also modulated by the wind-driven geostrophic fluctuations, resulting in persistent correlation with the thermocline changes and a slight low-frequency redness of the SST spectra. The main dynamics are similar in the two oceans, although in the North Pacific the SST variability is more strongly influenced by advection changes and the oceanic time scales are larger. A maximum covariance analysis based on singular value decomposition in lead and lag conditions indicates that some of the main modes of atmospheric variability in the two oceans are sustained by a very weak positive feedback between the atmosphere, SST, and the strength of the subtropical and subpolar gyres. In addition, in the North Atlantic the main surface pressure mode has a small quasi-oscillatory component at 6-year period, and advective resonance occurs for SST around 10-year period, both periods being also singled out by multichannel singular spectrum analysis. The ocean-atmosphere coupling is however much too weak to redden the tropospheric spectra or create anything more than tiny spectral peaks, so that the atmospheric and oceanic variability is dominated in both ocean sectors by the one-way interactions. Received: 2 April 1999 / Accepted: 14 October 1999     

Modulation of El Nino-Southern Oscillation by Freshwater Flux and Salinity Variability in the Tropical Pacific

The El Nin o-Southern Oscillation (ENSO) is modulated by many factors; most previous studies have emphasized the roles of wind stress and heat flux in the tropical Pacific. Freshwater flux (FWF) is another environmental forcing to the ocean; its effect and the related ocean salinity variability in the ENSO region have been of increased interest recently. Currently, accurate quantifications of the FWF roles in the climate remain challenging; the related observations and coupled ocean-atmosphere modeling involve large elements of uncertainty. In this study, we utilized satellite-based data to represent FWF-induced feedback in the tropical Pacific climate system; we then incorporated these data into a hybrid coupled ocean-atmosphere model (HCM) to quantify its effects on ENSO. A new mechanism was revealed by which interannual FWF forcing modulates ENSO in a significant way. As a direct forcing, FWF exerts a significant influence on the ocean through sea surface salinity (SSS) and buoyancy flux (Q B ) in the western-central tropical Pacific. The SSS perturbations directly induced by ENSO-related interannual FWF variability affect the stability and mixing in the upper ocean. At the same time, the ENSO-induced FWF has a compensating effect on heat flux, acting to reduce interannual Q B variability during ENSO cycles. These FWF-induced processes in the ocean tend to modulate the vertical mixing and entrainment in the upper ocean, enhancing cooling during La Nin a and enhancing warming during El Nin o, respectively. The interannual FWF forcing-induced positive feedback acts to enhance ENSO amplitude and lengthen its time scales in the tropical Pacific coupled climate system.     

Sensitivity of the tropical Pacific to a change of orbital forcing in two versions of a coupled GCM

 The changes of the variability of the tropical Pacific ocean forced by a shift of six months in the date of the perihelion are studied using a coupled tropical Pacific ocean/global atmosphere GCM. The sensitivity experiments are conducted with two versions of the atmospheric model, varied by two parametrization changes. The first one concerns the interpolation scheme between the atmosphere and ocean models grids near the coasts, the second one the advection of water vapor in the presence of downstream negative temperature gradients, as encountered in the vicinity of mountains. In the tropical Pacific region, the parametrization differences only have a significant direct effect near the coasts; but coupled feedbacks lead to a 1 °C warming of the equatorial cold tongue in the modified (version 2) model, and a widening of the western Pacific large-scale convergence area. The sensitivity of the seasonal cycle of equatorial SST is very different between the two experiments. In both cases, the response to the solar flux forcing is strongly modified by coupled interactions between the SST, wind stress response and ocean dynamics. In the first version, the main feedback is due to anomalous upwelling and leads to westward propagation of SST anomalies; whereas the version 2 model is dominated by an eastward-propagating thermocline mode. The main reason diagnosed for these different behaviors is the atmospheric response to SST anomalies. In the warmer climate simulated by the second version, the wind stress response in the western Pacific is enhanced, and the off-equatorial curl is reduced, both effects favoring eastward propagation through thermocline depth anomalies. The modifications of the simulated seasonal cycle in version 2 lead to a change in ENSO behavior. In the control climate, the interannual variability in the eastern Pacific is dominated by warm events, whereas cold events tend to be the more extreme ones with a shifted perihelion. Received: 14 December 1999 / Accepted: 24 May 2000     

A highly nonlinear coupled mode of decadal variability in a mid-latitude ocean–atmosphere model

This study examines mid-latitude climate variability in a model that couples turbulent oceanic and atmospheric flows through an active oceanic mixed layer. Intrinsic ocean dynamics of the inertial recirculation regions combines with nonlinear atmospheric sensitivity to sea-surface temperature (SST) anomalies to play a dominant role in the variability of the coupled system.Intrinsic low-frequency variability arises in the model atmosphere; when run in a stand-alone mode, it is characterized by irregular transitions between preferred high-latitude and less frequent low-latitude zonal-flow states. When the atmosphere is coupled to the ocean, the low-latitude state occurrences exhibit a statistically significant signal in a broad 5–15-year band. A similar signal is found in the time series of the model ocean's energy in this coupled simulation. Accompanying uncoupled ocean-only and atmosphere-only integrations are characterized by a decrease in the decadal-band variability, relative to the coupled integration; their spectra are indistinguishable from a red spectrum.The time scale of the coupled interdecadal oscillation is set by the nonlinear adjustment of the ocean's inertial recirculations to the high-latitude and low-latitude atmospheric forcing regimes. This adjustment involves, in turn, SST changes resulting in long-term ocean–atmosphere heat-flux anomalies that induce the atmospheric regime transitions.     

Pacific interdecadal variability driven by tropical–extratropical interactions

Interactions between the tropical and subtropical northern Pacific at decadal time scales are examined using uncoupled oceanic and atmospheric simulations. An atmospheric model is forced with observed Pacific sea surface temperatures (SST) decadal anomalies, computed as the difference between the 2000–2009 and the 1990–1999 period. The resulting pattern has negative SST anomalies at the equator, with a global pattern reminiscent of the Pacific decadal oscillation. The tropical SST anomalies are responsible for driving a weakening of the Hadley cell and atmospheric meridional heat transport. The atmosphere is then shown to produce a significant response in the subtropics, with wind-stress-curl anomalies having the opposite sign from the climatological mean, consistent with a weakening of the oceanic subtropical gyre (STG). A global ocean model is then forced with the decadal anomalies from the atmospheric model. In the North Pacific, the shallow subtropical cell (STC) spins down and the meridional heat transport is reduced, resulting in positive tropical SST anomalies. The final tropical response is reached after the first 10 years of the experiment, consistent with the Rossby-wave adjustment time for both the STG and the STC. The STC provides the connection between subtropical wind stress anomalies and tropical SSTs. In fact, targeted simulations show the importance of off-equatorial wind stress anomalies in driving the oceanic response, whereas anomalous tropical winds have no role in the SST signal reversal. We further explore the connection between STG, STC and tropical SST with the help of an idealized model. We argue that, in our models, tropical SST decadal variability stems from the forcing of the Pacific subtropical gyre through the atmospheric response to ENSO. The resulting Ekman pumping anomaly alters the STC and oceanic heat transport, providing a negative feedback on the SST. We thus suggest that extratropical atmospheric responses to tropical forcing have feedbacks onto the ocean dynamics that lead to a time-delayed response of the tropical oceans, giving rise to a possible mechanism for multidecadal ocean-atmosphere coupled variability.     

SEASONAL DEPENDENCE OF LOCAL AIR-SEA INTERACTION OVER THE TROPICAL WESTERN PACIFIC WARM POOL

Based on the air-sea interface heat fluxes and related meteorological variables datasets recently released by Objectively Analyzed Air-Sea Fluxes (OA Flux) Project of Woods Hole Oceanographic Institution, as well as the outgoing longwave radiation and surface wind datasets from National Oceanic and Atmospheric Administration, the seasonal dependence of local air-sea interaction over the tropical western Pacific warm pool (referred to the region (1o-6oN, 144o-154oE)) is revealed and the probable impacts of remote forcing on the air-sea interaction are examined. The results indicated the dominance of oceanic forcing with the significant impact of ENSO in March and that of atmospheric feedback without notable influence of remote forcing in June. While the interannual variability of sea surface temperature anomaly (SSTA) is larger than that of SSTA tendency when oceanic forcing is dominant, the opposite is true when atmospheric feedback is dominant. The magnitude of the oceanic forcing of the atmosphere tends to decrease in March with the occurrence of ENSO, though ENSO has little influence on the atmospheric feedback to the ocean in June. The local air-sea interaction is substantially the same before and after the removal of the effect of Indian Oceanic Dipole. The reduction of shortwave radiation fluxes into the western Pacific warm pool, due to the enhanced overlaying convection in March associated with ENSO, leads to the decline of SST tendency that will weaken the oceanic forcing of the atmosphere.     

The global distribution of sources of atmospheric decadal variability and mechanisms over the tropical Pacific and southern North America

 Understanding natural atmospheric decadal variability is an important element of climate research, and here we investigate the geographic and seasonal diversity in the balance between its competing sources. Data are provided by an ensemble of multi-decadal atmospheric general circulation model experiments, forced by observed sea surface temperatures (SSTs), and verified against observations. First, the nature of internal atmospheric variability is studied. By assessing its spectral character, we refute the idea that internal modes may persist or oscillate on multi-annual time-scales, either through mechanisms purely internal to the atmosphere, or via coupling to the land surface; instead, they behave as a white noise process. Second, and more importantly, the role of oceanic forcing, relative to internal variability, is investigated by extending the ‘analysis of variance’ technique to the frequency domain. Significance testing and confidence intervals are also discussed. In the tropics, atmospheric decadal variability is usually dominated by oceanic forcing, although for some regions less so than at interannual time-scales. A moderate oceanic impact is also found for some extratropical regions in some seasons. Verification against observed mean sea-level pressure (MSLP) data suggests that many of these influences are realistic, although some model errors are also revealed. In other mid- and high-latitude regions, local simulated decadal variability is dominated by random processes, i.e. the integrated effects of chaotic weather systems. Third, we focus on the mechanisms of decadal variability in two specific regions (where the model is well behaved). Over the tropical Pacific, the relative impact of SSTs on decadal MSLP is strongly seasonal such that it peaks in September to November (SON). This is explained by noting that the model atmosphere is responsive to SSTs a little farther west in SON than it is in other seasons, and here it picks up relatively more decadal power from the ocean (the western Pacific being less dominated by ENSO time-scales), causing atmospheric ‘signal-to-noise ratios’ to be enhanced at decadal timescales in SON. Over southern North America, a strong SST impact is found in summer and autumn, resulting in an upward trend of MSLP over recent decades. We suggest this is caused by decadal SST variability in the Caribbean (and to some extent the tropical northeast Pacific in summer), which induces anomalous convective heating over these regions and hence the wider MSLP response. Received: 30 November 1998 / Accepted: 22 April 1999     

A predictability study of simulated North Atlantic multidecadal variability

 The North Atlantic is one of the few places on the globe where the atmosphere is linked to the deep ocean through air–sea interaction. While the internal variability of the atmosphere by itself is only predictable over a period of one to two weeks, climate variations are potentially predictable for much longer periods of months or even years because of coupling with the ocean. This work presents details from the first study to quantify the predictability for simulated multidecadal climate variability over the North Atlantic. The model used for this purpose is the GFDL coupled ocean-atmosphere climate model used extensively for studies of global warming and natural climate variability. This model contains fluctuations of the North Atlantic and high-latitude oceanic circulation with variability concentrated in the 40–60 year range. Oceanic predictability is quantified through analysis of the time-dependent behavior of large-scale empirical orthogonal function (EOF) patterns for the meridional stream function, dynamic topography, 170 m temperature, surface temperature and surface salinity. The results indicate that predictability in the North Atlantic depends on three main physical mechanisms. The first involves the oceanic deep convection in the subpolar region which acts to integrate atmospheric fluctuations, thus providing for a red noise oceanic response as elaborated by Hasselmann. The second involves the large-scale dynamics of the thermohaline circulation, which can cause the oceanic variations to have an oscillatory character on the multidecadal time scale. The third involves nonlocal effects on the North Atlantic arising from periodic anomalous fresh water transport advecting southward from the polar regions in the East Greenland Current. When the multidecadal oscillatory variations of the thermohaline circulation are active, the first and second EOF patterns for the North Atlantic dynamic topography have predictability time scales on the order of 10–20 y, whereas EOF-1 of SST has predictability time scales of 5–7 y. When the thermohaline variability has weak multidecadal power, the Hasselmann mechanism is dominant and the predictability is reduced by at least a factor of two. When the third mechanism is in an extreme phase, the North Atlantic dynamic topography patterns realize a 10–20 year predictability time scale. Additional analysis of SST in the Greenland Sea, in a region associated with the southward propagating fresh water anomalies, indicates the potential for decadal scale predictability for this high latitude region as well. The model calculations also allow insight into regional variations of predictability, which might be useful information for the design of a monitoring system for the North Atlantic. Predictability appears to break down most rapidly in regions of active convection in the high-latitude regions of the North Atlantic. Received: 28 October 1996 / Accepted: 21 March 1997     

Atmospheric bridge on orbital time scales

Large-scale atmospheric patterns are examined on orbital timescales using a climate model which explicitly resolves the atmosphere–ocean–sea ice dynamics. It is shown that, in contrast to boreal summer where the climate mainly follows the local radiative forcing, the boreal winter climate is strongly determined by modulation of circulation modes linked to the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) and the Northern/Southern Annular Modes. We find that during a positive phase of the AO/NAO the convection in the tropical Pacific is below normal. The related atmospheric circulation provides an atmospheric bridge for the precessional forcing inducing a non-uniform temperature anomalies with large amplitudes over the continents. We argue that this is important for mechanisms responsible for multi-millennial climate variability and glacial inception.     

北太平洋冬季海-气耦合的主模态及其与瞬变扰动异常的联系

利用45年的ECMWF再分析资料,使用SVD方法研究了冬季北太平洋地区表层海温(SST)异常与大气环流异常间的主要耦合模态,探讨了大尺度海-气耦合型与天气尺度瞬变扰动的相互关系。分析结果表明,中纬度北太平洋地区冬季存在两种主要的海-气耦合型,第1种耦合型反映了与ENSO紧密相关的中纬度北太平洋冬季海温异常分布型以及大气的PNA型,第2种耦合型SST异常集中在东亚沿海以及中纬度北太平洋海流区,相应的大气场则为暖(冷)SSTA上空东西向带状区域内位势高度偏高(低),明显独立于ENSO型。进一步的合成分析表明,在第1种耦合型SST正(负)异常年里,冬季阿留申低压主体位置偏西南(东北),从东北亚到北美西海岸的西北—东南向带状区域内是低层大气温度正(负)异常区和高层西风负(正)异常区,西风负(正)异常中心位于西风急流出口处的北太平洋中东部,而西风急流主体区的风速变化很小。在第2种耦合型东亚沿海至中纬度北太平洋海流区SST偏暖(冷)时,阿留申低压整体偏弱(强),SST暖(冷)异常上空的大气温度偏暖(冷),高层西风急流区西风偏弱(强)。两种耦合型均显示出在北太平洋中纬度地区大气和海洋的异常相关中心有很好的空间对应性。在两种耦合型下,中纬度北太平洋冬季的大气斜压性也发生截然不同的改变,引起中纬度天气尺度瞬变扰动活动异常。瞬变扰动异常的动力强迫作用对北太平洋西风异常的形成存在正反馈作用,而其热力作用则试图破坏与两种海-气耦合模态相关的大气温度异常型。     

与冬季北太平洋大范围海温异常相联系的海气特征

采用1948—2014年NCEP/NCAR大气再分析资料以及延伸重建海温资料,基于大气海洋间不同的主导关系对冬季北太平洋大范围海温异常进行分类,探究其相应的海气结构特征。结果表明:1)大气影响海洋的个例多于海洋影响大气的个例,即在冬季北太平洋大气强迫海洋占主要地位,但也存在海洋对大气的反馈作用。2)对于大气影响海洋而言,SST(Sea Surface Temperature)暖异常区上空主要伴随着东北—西南走向的相当正压高低压异常(东北高西南低),对应东南风异常以及显著的深厚暖异常,表现出相当正压暖/脊结构,冷异常情况与此相反。SST异常为净热通量异常与风速异常共同作用引起。3)对于海洋影响大气而言,在SST暖异常区上空西部为南北向高低压异常(北高南低),东部为低压异常,对应偏东风异常。在SST冷异常区上空为偶极型的南北向高低压异常(南高北低),对应偏西风异常;位势高度异常表现出相当正压结构且较大气影响海洋时相对偏弱,大气暖(冷)温度异常比较浅薄且主要局限于对流层低层。4)海洋温度结构异常主要表现为,在大气影响海洋时海温异常由表层下传,海洋影响大气时为上下一致的温度异常。     

一种预测西北太平洋热带气旋逐月演变的新统计模型

本文提出一种月尺度西北太平洋热带气旋生成频数(Western North Pacific Tropical Cyclone Frequency, WNPTCF)预测的新方法。该方法利用全球次表层海温(Subsurface Sea Temperature Anomaly, SubSSTA)和中国气象局国家气候中心发布的130项监测指数,构建了既考虑热力强迫因子、又考虑大气动力因子,既考虑同期海洋强迫,又考虑前期海洋和大气影响的集成预测模型。利用该预测模型对2011—2020年6—10月逐月WNPTCF进行独立样本检验预测,准确率达70%以上,说明该预测模型对WNPTCF的逐月演变预测的效果良好。该预测模型对ENSO信号较强年份的WNPTCF预测效果要好于ENSO信号不强的年份,原因在于在ENSO信号不强的年份,SubSSTA可预报性较低,非线性变率大,海洋对WNPTC的强迫作用弱。     

Can local linear stochastic theory explain sea surface temperature and salinity variability?

 Sea surface temperature (SST) and salinity (SSS) time series from four ocean weather stations and data from an integration of the GFDL coupled ocean-atmosphere model are analyzed to test the applicability of local linear stochastic theory to the mixed-layer ocean. According to this theory, mixed-layer variability away from coasts and fronts can be explained as a ‘red noise’ response to the ‘white noise’ forcing by atmospheric disturbances. At one weather station, Papa (northeast Pacific), this stochastic theory can be applied to both salinity and temperature, explaining the relative redness of the SSS spectrum. Similar results hold for a model grid point adjacent to Papa, where the relationships between atmospheric energy and water fluxes and actual changes in SST and SSS are what is expected from local linear stochastic theory. At the other weather stations, this theory cannot adequately explain mixed-layer variability. Two oceanic processes must be taken into account: at Panulirus (near Bermuda), mososcale eddies enhance the observed variability at high frequencies. At Mike and India (North Atlantic), variations in SST and SSS advection, indicated by the coherence and equal persistence of SST and SSS anomalies, contribute to much of the low frequency variability in the model and observations. To achieve a global perspective, TOPEX altimeter data and model results are used to identify regions of the ocean where these mechanisms of variability are important. Where mesoscale eddies are as energetic as at Panulirus, indicated by the TOPEX global distribution of sea level variability, one would expect enhanced variability on short time scales. In regions exhibiting signatures of variability similar to Mike and India, variations in SST and SSS advection should dominate at low frequencies. According to the model, this mode of variability is found in the circumpolar ocean and the northern North Atlantic, where it is associated with the irregular oscillations of the model’s thermohaline circulation. Received: 11 March 1996 / Accepted: 6 September 1996     

Climate variability in millennium integrations with coupled atmosphere-ocean GCMs: a spectral view

 Climate variations in four millennium integrations obtained with coupled GCMs are studied from a spectral point of view. It is shown that the bulk of these variations can be described by two distinctly different types of spectra. The type-I spectra, characterized by a high-frequency ω⁻² slope (with ω being frequency) and a low-frequency plateau, indicate the dominance of short-term fluctuations in generating climate variations. They are obtained for many atmospheric variables and variables representing predominantly the upper ocean and the high-latitude part of the deep ocean. The time scale, at which the spectra level off, varies from a few days for grid-point time series of atmospheric variables, to a few months for time series of large-scale atmospheric patterns, several years for SST anomalies in the tropical Pacific, and a few decades for variables describing oceanic baroclinic waves. The type-II spectra are obtained in the ocean interior, which is shielded from the fluctuating forcing at the surface. Since the ocean model does not produce oceanic eddies, the disappearance of type-I spectra in the deep ocean indicates that the fluctuating surface forcing does not fully penetrate into the deep ocean. While type-I spectra are supported by observations, type-II spectra might describe a model specific phenomenon and the realism of these spectra is still a open question. Received: 12 January 2000 / Accepted: 14 June 2000     

On the phase propagation and relationship of interannual variability in the tropical Pacific climate system

—Upper ocean thermal data and surface marine observations are used to describe the three-dimensional, basinwide co-evolution of interannual variability in the tropical Pacific climate system. The phase propagation behavior differs greatly from atmosphere to ocean, and from equatorial to off-equatorial and from sea surface to subsurface depths in the ocean. Variations in surface zonal winds and sea surface temperatures (SSTs) exhibit a standing pattern without obvious zonal phase propagation. A nonequilibrium ocean response at subsurface depths is evident, characterized by coherent zonal and meridional propagating anomalies around the tropical North Pacific: eastward on the equator but westward off the equator. Depending on geographic location, there are clear phase relations among various anomaly fields. Surface zonal winds and SSTs in the equatorial region fluctuate approximately in-phase in time, but have phase differences in space. Along the equator, zonal mean thermocline depth (or heat content) anomalies are in nonequilibrium with the zonal wind stress forcing. Variations in SSTs are not in equilibrium either with subsurface thermocline changes in the central and western equatorial Pacific, with the former lagging the latter and displaced to the east. Due to its phase relations to SST and winds, the basinwide temperature anomaly evolution at thermocline depths on an interannual time scale may determine the slow physics of ENSO, and play a central role in initiating and terminating coupled air-sea interaction. This observed basinwide phase propagation of subsurface anomaly patterns can be understood partially as water discharge processes from the western Pacific to the east and further to high latitudes, and partially by the modified delayed oscillator physics. Received: 17 January 1997 / Accepted: 10 March 1998     

A Hybrid Coupled Ocean–Atmosphere Model and Its Simulation of ENSO and Atmospheric Responses

A new hybrid coupled model(HCM) is presented in this study, which consists of an intermediate tropical Pacific Ocean model and a global atmospheric general circulation model. The ocean component is the intermediate ocean model(IOM)of the intermediate coupled model(ICM) used at the Institute of Oceanology, Chinese Academy of Sciences(IOCAS). The atmospheric component is ECHAM5, the fifth version of the Max Planck Institute for Meteorology atmospheric general circulation model. The HCM integrates its atmospheric and oceanic components by using an anomaly coupling strategy. A100-year simulation has been made with the HCM and its simulation skills are evaluated, including the interannual variability of SST over the tropical Pacific and the ENSO-related responses of the global atmosphere. The model shows irregular occurrence of ENSO events with a spectral range between two and five years. The amplitude and lifetime of ENSO events and the annual phase-locking of SST anomalies are also reproduced realistically. Despite the slightly stronger variance of SST anomalies over the central Pacific than observed in the HCM, the patterns of atmospheric anomalies related to ENSO,such as sea level pressure, temperature and precipitation, are in broad agreement with observations. Therefore, this model can not only simulate the ENSO variability, but also reproduce the global atmospheric variability associated with ENSO, thereby providing a useful modeling tool for ENSO studies. Further model applications of ENSO modulations by ocean–atmosphere processes, and of ENSO-related climate prediction, are also discussed.     

A new tool for evaluating the physics of coupled atmosphere�Cocean variability in nature and in general circulation models

Intermediate models of the coupled tropical atmosphere?Cocean system have been used to illuminate the physics of interannual climate phenomenon such as El Ni?o Southern Oscillation (ENSO) in the tropical Pacific and to explore how the tropics might respond to a forcing such as changing insolation (Milankovitch) or atmospheric carbon dioxide. Importantly, most of the intermediate models are constructed as anomaly models: models that evolve on a prescribed climatological mean state, which is typically prescribed and done so on a rather ad hoc basis. Here we show how the observed climatological mean state fields [ocean currents and upwelling, sea surface temperature (SST) and atmospheric surface winds] can be incorporated into a linearized intermediate model of the tropical coupled atmosphere?Cocean system: called Linear Ocean?CAtmosphere Model (LOAM), it is a linearized version of the Zebiak and Cane model. With realistic, seasonally varying mean state fields, we find that the essential physics of the ENSO mode is very similar to that in the original model and to that in the observations and that the observed mean fields support an ENSO mode that is stable to perturbations. Thus, our results provide further evidence that ENSO is generated and maintained by stochastic (uncoupled) perturbations. The method that we have outlined can be used to assimilate any set of ocean and atmosphere climatological data into the linearized atmosphere?Cocean model. In a companion paper, we apply this same method to incorporate mean field output from two global climate models into the linearised model. We use the latter to diagnose the physics of the leading coupled mode (ENSO) that is supported by the climate models, and to illuminate why the structure and variance in the ENSO mode changes in the models when they are forced by early Holocene and Last Glacial Maximum boundary conditions.