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.     

Interannual and decadal variability of the subsurface thermal structure in the Pacific Ocean: 1961–90

Heat content anomalies are analyzed to understand subsurface variability on both aparticular focus on the evolving basinwide patterns and oceanic connections between the extratropics and tropics. Various analyses indicate two distinct modes, one interannual and the other decadal, that involve the tropics and the North Pacific subtropical gyre, respectively. Interannual variability is associated with El Niño in the tropics, with a prominent “see-saw” pattern alternately on and off the equator, and in the east and west, respectively. The interannual cycle features a coherent propagation of subsurface signals around the tropical Pacific, eastward along the equator but westward off the equator at 10–15?°N. Decadal signals are dominant in the subtropics and midlatitudes but also have a tropical component that appears to be independent of interannual variations. An oceanic connection can be seen between subsurface anomalies in the midlatitudes, in the subtropics and tropics on decadal time scales. Subsurface thermal anomalies associated with midlatitude decadal variability can propagate through the subtropics into the tropics, which may modulate the intensity of interannual variability in the tropics. For example, in the middle and late 1970s, a significant warm temperature anomaly appeared to penetrate into the western and central tropics at depth, warming the tropical upper ocean and depressing the thermocline. During the development of El Niño, therefore, an extratropically preconditioned subsurface state (e.g., an enhanced positive heat content anomaly) in the western and central tropical Pacific would favor a warmer sea surface temperature anomaly in the eastern equatorial Pacific, potentially increasing the intensity of ocean-atmosphere coupling. These changes in the thermocline structure and possibly in the coupling strength can further alter the very character of tropical air-sea interactions. This may help to explain decadal variability of El Niño evolution in the tropical Pacific as observed in the 1980s. Our subsurface variability analysis presents observational evidence for the detailed space-time structure of decadal oceanic links between the extratropics and the tropics.     

The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions

In this study, the effects of volcanic forcing on North Pacific climate variability, on interannual to decadal time scales, are examined using climate model simulations covering the last 600?years. The model used is the Bergen Climate Model, a fully coupled atmosphere–ocean general circulation model. It is found that natural external forcings, such as tropical strong volcanic eruptions (SVEs) and variations in total solar irradiance, play an important role in regulating North Pacific Decadal Variability (NPDV). In response to tropical SVEs the lower stratospheric pole–to–equator temperature gradient is enhanced. The North polar vortex is strengthened, which forces a significant positive Arctic Oscillation. At the same time, dipole zonal wind anomalies associated with strong polar vortex propagate downward from the lower stratosphere. Through positive feedbacks in the troposphere, the surface westerly winds across the central North Pacific are significantly weakened, and positive sea level pressure anomalies are formed in the North Pacific. This anomalous surface circulation results in changes in the net heat fluxes and the oceanic advection across the North Pacific. As a result of this, warm water converges in the subtropical western North Pacific, where the surface waters in addition are heated by significantly reduced latent and sensible heat fluxes from the ocean. In the eastern and high–latitude North Pacific the ocean loses more heat, and large–scale decreases in sea surface temperatures are found. The overall response of this chain of events is that the North Pacific enters a negative phase of the Pacific decadal oscillation (PDO), and this negative phase of the PDO is maintained for several years. It is thus concluded that the volcanic forcing plays a key role in the phasing of the PDO. The model results furthermore highlight the important role of troposphere–stratosphere coupling, tropical–extratropical teleconnections and extratropical ocean–atmosphere interactions for describing NPDV.     

On the predictability of decadal changes in the North Pacific

 The predictability of decadal changes in the North Pacific is investigated with an ocean general circulation model forced by simplified and realistic atmospheric conditions. First, the model is forced by a spatially fixed wind stress anomaly pattern characteristic for decadal North Pacific climate variations. The time evolution of the wind stress anomaly is chosen to be sinusoidal, with a period of 20 years. In this experiment different physical processes are found to be important for the decadal variations: baroclinic Rossby waves dominate the response. They move westward and lead to an adjustment of the subtropical and subpolar gyre circulations in such a way that anomalous temperatures in the central North Pacific develop as a delayed response to the preceding wind stress anomalies. This delayed response provides not only a negative feedback but also bears the potential for long-term predictions of upper ocean temperature changes in the central North Pacific. It is shown by additional experiments that once these Rossby waves have been excited, decadal changes of the upper ocean temperatures in the central North Pacific evolve without any further anomalous atmospheric forcing. In the second part, the model is forced by surface heat flux and wind stress observations for the period 1949–1993. It is shown that the same physical processes which were found to be important in the simplified experiments also govern the evolution of the upper ocean in this more realistic simulation. The 1976/77 cooling can be mainly attributed to anomalously strong horizontal advection due to the delayed response to persistent wind stress curl anomalies in the early 1970s rather than local anomalous atmospheric forcing. This decadal change could have been predicted some years in advance. The subsequent warming in the late 1980s, however, cannot be mainly explained by advection. In this case, local anomalous atmospheric forcing needs to be considered. Received: 6 July 1998 / Accepted: 16 October 1999     

Anomalies in the tropics associated with the heavy rainfall in east asia during the summer of 1998

1.IntroductionInthesummerof1998,theYangtzeRiverbasin,includingNenjiangRiverValleyinNortheastChinasufferedaseverelarge--scalefloodonlynexttothatinthesummerof1954inthiscentury.Thefloodcausedapproximatelythedeathof3000individualsandthedirecteconomicdamageof250billionRMByuans(Yan,1998).Thisextremedisasterpromptedaseriesofimmediatestudiesonit(e.g.,Iluangetal.,1998;Taoetal.,1998).TheevolutionoftheEastAsiansupsmermonsoonshowsagreatvariabilityfromyeartoOThisstudywassupportedbytheNationalNatura…     

Roles of Indian and Pacific Ocean air–sea coupling in tropical atmospheric variability

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.     

Equivalent forcing depth in tropical oceans

The forcing efficiency for the first and the second baroclinic modes by the wind stress in tropical oceans has been discussed by calculating equivalent forcing depth from annual mean, seasonal, and pentadal density profiles of the observational data. In the annual mean field, the first mode is forced preferentially in the western Pacific and the Indian Ocean, whereas the second mode is more strongly excited in the Atlantic and the eastern Pacific. This difference is mostly due to the pycnocline depth; the second mode is more dominantly forced where the pycnocline depth is shallower. We also revealed large seasonal variations of the second mode's equivalent forcing depth in the western Indian Ocean. The first mode is more dominantly forced during boreal spring and fall in the western Indian Ocean, while the second mode becomes more dominantly forced during boreal summer and winter. Those are due to seasonal variations of both the zonal wind and the pycnocline depth. Moreover, we show that the excitation of the second mode in the western Pacific increases after the late 1970s, which is associated with the decreasing trend of the zonal pycnocline gradient. Revealing the variation of the equivalent forcing depth will be useful for understanding the oceanic response to winds in tropical oceans and the improvement in the predictability of air-sea coupled climate variability in the tropics.     

Dynamic Mechanism of Interannual Sea Surface Height Variability in the North Pacific Subtropical Gyre

In this study, the dynamic mechanisms of interannual sea surface height (SSH) variability are investigated based on the first-mode baroclinic Rossby wave model, with a focus on the effects of different levels of wind stress curl (WSC). Maximum covariance analysis (MCA) of WSC and SSH anomalies displays a mode with significant WSC anomalies located primarily in the mid-latitude eastern North Pacific and central tropical Pacific with corresponding SSH anomalies located to the west. This leading mode can be attributed to Ekman pumping induced by local wind stress and the westward-propagating Rossby wave driven by large- scale wind stress. It is further found that in the middle latitudes, the SSH anomalies are largely determined by WSC variations associated with the North Pacific Gyre Oscillation (NPGO), rather than the Pacific Decadal Oscillation (PDO). The sensitivity of the predictive skill of the linear first-mode baroclinic model to different wind products is also examined.     

Extratropical control of recent tropical Pacific decadal climate variability: a relay teleconnection

Observations indicate that recent tropical Pacific decadal climate variability tends to be associated with the extratropical North Pacific through a relay teleconnection of a fast coupled ocean-atmosphere bridge and a slow oceanic tunnel. A coupled ocean-atmosphere model, forced by the observed decadal wind in the extratropical North Pacific, explicitly demonstrates that extratropical decadal sea surface temperature (SST) anomalies may propagate to the tropics through a coupled wind-evaporative-SST (WES) feedback. The WES feedback cannot only lead to a nearly synchronous change of tropical SST, but also force a delayed adjustment of the meridional overturning circulation in the upper ocean to further sustain the tropical SST change. The study further suggests that the extratropical–tropical teleconnection provides a positive feedback to sustain the decadal changes in both the tropical and extratropical North Pacific.     

Intraseasonal variability in the far-east pacific: investigation of the role of air�Csea coupling in a regional coupled model

Intraseasonal variability in the eastern Pacific warm pool in summer is studied, using a regional ocean?Catmosphere model, a linear baroclinic model (LBM), and satellite observations. The atmospheric component of the model is forced by lateral boundary conditions from reanalysis data. The aim is to quantify the importance to atmospheric deep convection of local air?Csea coupling. In particular, the effect of sea surface temperature (SST) anomalies on surface heat fluxes is examined. Intraseasonal (20?C90?day) east Pacific warm-pool zonal wind and outgoing longwave radiation (OLR) variability in the regional coupled model are correlated at 0.8 and 0.6 with observations, respectively, significant at the 99% confidence level. The strength of the intraseasonal variability in the coupled model, as measured by the variance of outgoing longwave radiation, is close in magnitude to that observed, but with a maximum located about 10° further west. East Pacific warm pool intraseasonal convection and winds agree in phase with those from observations, suggesting that remote forcing at the boundaries associated with the Madden?CJulian oscillation determines the phase of intraseasonal convection in the east Pacific warm pool. When the ocean model component is replaced by weekly reanalysis SST in an atmosphere-only experiment, there is a slight improvement in the location of the highest OLR variance. Further sensitivity experiments with the regional atmosphere-only model in which intraseasonal SST variability is removed indicate that convective variability has only a weak dependence on the SST variability, but a stronger dependence on the climatological mean SST distribution. A scaling analysis confirms that wind speed anomalies give a much larger contribution to the intraseasonal evaporation signal than SST anomalies, in both model and observations. A LBM is used to show that local feedbacks would serve to amplify intraseasonal convection and the large-scale circulation. Further, Hovm?ller diagrams reveal that whereas a significant dynamic intraseasonal signal enters the model domain from the west, the strong deep convection mostly arises within the domain. Taken together, the regional and linear model results suggest that in this region remote forcing and local convection?Ccirculation feedbacks are both important to the intraseasonal variability, but ocean?Catmosphere coupling has only a small effect. Possible mechanisms of remote forcing are discussed.     

Low Frequency Characteristics of Tropical Pacific Wind Stress Anomalies in Observations and Simulations from a Simple and a Comprehensive Models

Low frequency characteristics of tropical Pacific wind stress anomalies in observation and simulations; from the CZ simple atmospheric model and COLA R15 AGCM are analyzed. The results show that ENSO event may be a multi-scale process, that is, ENSO time scale has the period longer than three yean; biennial oscillation and annual variability Dynamical characteristics are involved in the evolution process of wind stress anomaly with ENSO time scale: 1) the development and eastward movement of a cyclonic anomaly circulation in subtropical northwestern Pacific and weakening of Southern Oscillation result in the eastward propagation of westerly anomaly along the equator, there?fore, interactions between flows in subtropics and in tropics play an important role in the evolution of wind stress anomaly with ENSO time scale; 2) easterly and westerly anomalies with ENSO time scale are one kind of propagating wave, which differs from Barnett’s (1991). It is interesting that the evolution of observed and simulated wind stress anomalies with biennial time scale bears a strong resemble to that with ENSO time scale although their period it dif?ferent. Observed annual variability it weak during 1979-1981 and intensified after 1981, especially it reaches to max?imum during 1982-1984, and the spatial structure of the first mode is the ENSO-like pattern.     

Coupled atmosphere�Cmixed layer ocean response to ocean heat flux convergence along the Kuroshio Current Extension

The winter response of the coupled atmosphere?Cocean mixed layer system to anomalous geostrophic ocean heat flux convergence in the Kuroshio Extension is investigated by means of experiments with an atmospheric general circulation model coupled to an entraining ocean mixed layer model in the extra-tropics. The direct response consists of positive SST anomalies along the Kuroshio Extension and a baroclinic (low-level trough and upper-level ridge) circulation anomaly over the North Pacific. The low-level component of this atmospheric circulation response is weaker in the case without coupling to an extratropical ocean mixed layer, especially in late winter. The inclusion of an interactive mixed layer in the tropics modifies the direct coupled atmospheric response due to a northward displacement of the Pacific Inter-Tropical Convergence Zone which drives an equivalent barotropic anomalous ridge over the North Pacific. Although the tropically driven component of the North Pacific atmospheric circulation response is comparable to the direct response in terms of sea level pressure amplitude, it is less important in terms of wind stress curl amplitude due to the mitigating effect of the relatively broad spatial scale of the tropically forced atmospheric teleconnection.     

Predictability of SST forced climate signals in two atmospheric general circulation models

 The predictability of atmospheric responses to global sea surface temperature (SST) anomalies is evaluated using ensemble simulations of two general circulation models (GCMs): the GENESIS version 1.5 (GEN) and the ECMWF cycle 36 (ECM). The integrations incorporate observed SST variations but start from different initial land and atmospheric states. Five GEN 1980–1992 and six ECM 1980–1988 realizations are compared with observations to distinguish predictable SST forced climate signals from internal variability. To facilitate the study, correlation analysis and significance evaluation techniques are developed on the basis of time series permutations. It is found that the annual mean global area with realistic signals is variable dependent and ranges from 3 to 20% in GEN and 6 to 28% in ECM. More than 95% of these signal areas occur between 35 °S–35 °N. Due to the existence of model biases, robust responses, which are independent of initial condition, are identified over broader areas. Both GCMs demonstrate that the sensitivity to initial conditions decreases and the predictability of SST forced responses increases, in order, from 850 hPa zonal wind, outgoing longwave radiation, 200 hPa zonal wind, sea-level pressure to 500 hPa height. The predictable signals are concentrated in the tropical and subtropical Pacific Ocean and are identified with typical El Ni?o/ Southern Oscillation phenomena that occur in response to SST and diabatic heating anomalies over the equatorial central Pacific. ECM is less sensitive to initial conditions and better predicts SST forced climate changes. This results from (1) a more realistic basic climatology, especially of the upper-level wind circulation, that produces more realistic interactions between the mean flow, stationary waves and tropical forcing; (2) a more vigorous hydrologic cycle that amplifies the tropical forcing signals, which can exceed internal variability and be more efficiently transported from the forcing region. Differences between the models and observations are identified. For GEN during El Ni?o, the convection does not carry energy to a sufficiently high altitude, while the spread of the tropospheric warming along the equator is slower and the anomaly magnitude smaller than observed. This impacts model ability to simulate realistic responses over Eurasia and the Indian Ocean. Similar biases exist in the ECM responses. In addition, the relationships between upper and lower tropospheric wind responses to SST forcing are not well reproduced by either model. The identification of these model biases leads to the conclusion that improvements in convective heat and momentum transport parametrizations and basic climate simulations could substantially increase predictive skill. Received: 25 April 1996 / Accepted: 9 December 1996     

A Linear Diagnosis of the Coupled Extratropical Ocean–Atmosphere System in the GFDL GCM

Summary:Diagnosing a coupled system with linear inverse modelling (LIM) can provide insight into the nature and strength of the coupling. This technique is applied to the cold season output of the GFDL GCM, forced by observed tropical Pacific SSTs and including a slab mixed layer ocean model elsewhere. It is found that extratropical SST anomalies act to enhance atmospheric thermal variability and diminish barotropic variability over the east Pacific in these GCM runs, in agreement with other theoretical and modelling studies. North-west Atlantic barotropic variability is also enhanced. However, all these feedbacks are very weak. LIM results also suggest that North Pacific extratropical SST anomalies in this model would rapidly decay without atmospheric forcing induced by tropical SST anomalies.     

An analysis on the physical process of the influence of AO on ENSO

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Niño-like warming in the tropical eastern Pacific in winter.     

Southern Hemisphere extra-tropical forcing: a new paradigm for El Ni?o-Southern Oscillation

The main goal of this paper is to shed additional light on the reciprocal dynamical linkages between mid-latitude Southern Hemisphere climate and the El Ni?o-Southern Oscillation (ENSO) signal. While our analysis confirms that ENSO is a dominant source of interannual variability in the Southern Hemisphere, it is also suggested here that subtropical dipole variability in both the Southern Indian and Atlantic Oceans triggered by Southern Hemisphere mid-latitude variability may also provide a controlling influence on ENSO in the equatorial Pacific. This subtropical forcing operates through various coupled air?Csea feedbacks involving the propagation of subtropical sea surface temperature (SST) anomalies into the deep tropics of the Atlantic and Indian Oceans from boreal winter to boreal spring and a subsequent dynamical atmospheric response to these SST anomalies linking the three tropical basins at the beginning of the boreal spring. This atmospheric response is characterized by a significant weakening of the equatorial Atlantic and Indian Inter-Tropical Convergence Zone (ITCZ). This weakened ITCZ forces an equatorial ??cold Kelvin wave?? response in the middle to upper troposphere that extends eastward from the heat sink regions into the western Pacific. By modulating the vertical temperature gradient and the stability of the atmosphere over the equatorial western Pacific Ocean, this Kelvin wave response promotes persistent zonal wind and convective anomalies over the western equatorial Pacific, which may trigger El Ni?o onset at the end of the boreal winter. These different processes explain why South Atlantic and Indian subtropical dipole time series indices are highly significant precursors of the Ni?o34 SST index several months in advance before the El Ni?o onset in the equatorial Pacific. This study illustrates that the atmospheric internal variability in the mid-latitudes of the Southern Hemisphere may significantly influence ENSO variability. However, this surprising relationship is observed only during recent decades, after the so-called 1976/1977 climate regime shift, suggesting a possible linkage with global warming or decadal fluctuations of the climate system.     

Projection of subtropical gyre circulation and associated sea level changes in the Pacific based on CMIP3 climate models

For all of the IPCC Special Report on Emission Scenarios (SRESs), sea level is projected to rise globally. However, sea level changes are not expected to be geographically uniform, with many regions departing significantly from the global average. Some of regional distributions of sea level changes can be explained by projected changes of ocean density and dynamics. In this study, with 11 available Coupled Model Intercomparison Project Phase 3 climate models under the SRES A1B, we identify an asymmetric feature (not recognised in previous studies) of projected subtropical gyre circulation changes and associated sea level changes between the North and South Pacific, through analysing projected changes of ocean dynamic height (with reference to 2,000 db), depth integrated steric height, Sverdrup stream function, surface wind stress and its curl. Poleward expansion of the subtropical gyres is projected in the upper ocean for both North and South Pacific. Contrastingly, the subtropical gyre circulation is projected to spin down by about 20 % in the subsurface North Pacific from the main thermocline around 400 m to at least 2,000 m, while the South Pacific subtropical gyre is projected to strengthen by about 25 % and expand poleward in the subsurface to at least 2,000 m. This asymmetrical distribution of the projected subtropical gyre circulation changes is directly related to differences in projected changes of temperature and salinity between the North and South Pacific, forced by surface heat and freshwater fluxes, and surface wind stress changes.     

The impact of combined ENSO and PDO on the PNA climate: a 1,000-year climate modeling study

This study analyzes the atmospheric response to the combined Pacific interannual ENSO and decadal–interdecadal PDO variability, with a focus on the Pacific-North American (PNA) sector, using a 1,000-year long integration of the Canadian Center for Climate Modelling and Analysis (CCCma) coupled climate model. Both the tropospheric circulation and the North American temperature suggest an enhanced PNA-like climate response and impacts on North America when ENSO and PDO variability are in phase. The anomalies of the centers of action for the PNA-like pattern are significantly different from zero and the anomaly pattern is field significant. In association with the stationary wave anomalies, large stationary wave activity fluxes appear in the mid-high latitudes originating from the North Pacific and flowing downstream toward North America. There are significant Rossby wave source anomalies in the extratropical North Pacific and in the subtropical North Pacific. In addition, the axis of the Pacific storm track shifts southward with the positive PNA. Atmospheric heating anomalies associated with ENSO variability are confined primarily to the tropics. There is an anomalous heating center over the northeast Pacific, together with anomalies with the same polarity in the tropical Pacific, for the PDO variability. The in-phase combination of ENSO and PDO would in turn provide anomalous atmospheric energy transports towards North America from both the Tropical Pacific and the North Pacific, which tends to favor the occurrence of stationary wave anomalies and would lead to a PNA-like wave anomaly structure. The modeling results also confirm our analysis based on the observational record in the twentieth century.     

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     

Meridional Wind Stress Anomalies over Tropical Pacific and the Onset of El Nino. Part Ⅰ: Data Analysis

对热带太平洋海表经向风应力异常与EI Nino事件之间的关系进行了诊断分析。结果表明，超前的经向风应力距平场与NINO3区(150°-90°W，5°S-5°N)的海面温度异常(SSTA)有显著的超前相关，这种相关性在超前6个月甚至更早一些就有显示。利用奇异值分解方法分析超前的经向风应力距平场与太平洋海表温度异常场之间的耦合模，结果表明对应于赤道中东太平洋的海面温度异常升高，大气风应力场在超前6个月甚至更早的时候，在赤道中东太平洋表现为辐合的经向异常风应力场，即赤道以北为北风异常应力，赤道以南为南风异常应力。这种耦合模的时间系数与NINO3 SSTA指数所表示的El Nino事件有很好的对应关系，表明这种耦合模反映的正是超前的经向风应力异常与El Nino事件所对应的海表温度异常之间的相关模态。通过与热带西太平洋纬向风应力异常的比较，赤道中东太平洋辐合的经向风应力异常与El Nino事件发生的同样具有重要的联系。