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1.
Summary The impact of pronounced positive and negative sea surface temperature (STT) anomalies in the tropical Pacific associated with the El Niño/Southern Oscillation (ENSO) phenomenon on the atmospheric circulation in the Northern Hemisphere extratropics during the boreal winter season is investigated. This includes both the impact on the seasonal mean flow and on the intraseasonal variability on synoptic time scales. Moreover, the interaction between the transient fluctuations on these times scales and the mean circulation is examined. Both data from an ensemble of five simulations with the ECHAM3 atmospheric general circulation model at a horizontal resolution of T42 each covering the period from 1979 through 1992 and operational analyses from ECMWF for the corresponding period are examined. In each of the simulations observed SSTs for the period of investigation are given as lower boundary forcing, but different atmospheric initial conditions are prescribed.The simulations with ECHAM3 reveal a distinct impact of the pronounced SST-anomalies in the tropical Pacific on the atmospheric circulation in the Northern Hemisphere extratropics during El Niño as well as during La Niña events. These changes in the atmospheric circulation, which are found to be highly significant in the Pacific/North American as well as in the Atlantic/European region, are consistent with the essential results obtained from the analyses. The pronounced SST-anomalies in the tropical Pacific lead to changes in the mean circulation, which are characterized by typical circulation patterns. These changes in the mean circulation are accompanied by marked variations of the activity of the transient fluctuations on synoptic time scales, that are changes in both the kinetic energy on these time scales and the atmospheric transports of momentum and heat accomplished by the short baroclinic waves. The synoptic disturbances, on the other hand, play also an important role in controlling the changes in the mean circulation associated with the ENSO phenomenon. They maintain these typical circulation patterns via barotropic, but counteract them via baroclinic processes.The hypothesis of an impact of the ENSO phenomenon in the Atlantic/European region can be supported. As the determining factor the intensification (reduction) of the Aleutian low and the simultaneous reduction (intensification) of the Icelandic low during El Niño and during La Niña events respectively, is identified. The changes in the intensity of the Aleutian low during the ENSO-events are accompanied by an alteration of the transport of momentum caused by the short baroclinic waves over the North American continent in such a way that the changes in the intensity of the Icelandic low during El Niño as well as during La Niña events are maintained.With 16 Figures 相似文献
2.
The present study aims at studying the role played by high-frequency wind variability, wave reflection and easterly wind anomalies in the western Pacific in the onset, growth and termination phases of the 1997–1998 El Niño using the Trident intermediate coupled model and observations. While the anomalous strength of the trade winds in 1996 favored the initiation of a warm event in 1997 (via western Pacific boundary Rossby wave reflection), the actual timing of the onset and the amplitude of the event resulted from the large March 1997 wind event. Once initiated, high-frequency westerly winds strongly contributed to the rapid growth of the warm event and to the displacement of the eastern edge of the warm-pool. Moreover, both easterly and westerly high-frequency wind variability in 1997–1998 contributed to the amplitude of the event, set the evolution of the warm event and potentially influenced the equatorial Pacific conditions at least one year after the El Niño event. In addition, eastern boundary reflection also significantly contributed to the amplitude and duration of the warm event, whereas its termination was a combination of various factors: reflection of upwelling Rossby waves at the western boundary and large easterly wind anomalies observed in the western Pacific from November 1997 to early 1998. These factors were sufficient to terminate the event and to switch temperature anomalies from warm to cold. To conclude, understanding the coupling between the high- and low-frequency wind variability, i.e., studying ENSO as a multi-scale phenomenon, will certainly lead to a better comprehension of the diversity of its behavior and potentially to an improvement of its predictability. 相似文献
3.
Interannual and interdecadal variations of tropical cyclone activity over the western North Pacific 总被引：12，自引：0，他引：12
Johnny C. L. Chan 《Meteorology and Atmospheric Physics》2005,89(1-4):143-152
Summary This paper reviews the interannual and interdecadal variations in tropical cyclone (TC) activity over the western North Pacific (WNP) and the possible physical mechanisms responsible for such variations. Interannual variations can largely be explained by changes in the planetary-scale flow patterns. Sea-surface temperatures (SSTs) in the WNP, however, do not contribute to such variations. Rather, SSTs in the central and eastern equatorial Pacific are significantly correlated with TC activity over the WNP. Causality can be established: changes in the SST in the equatorial Pacific are related to the El Niño/Southern Oscillation (ENSO) phenomenon, and modifications of the planetary-scale flow associated with ENSO alter the conditions over the WNP and hence TC activity there. Variations in annual TC activity are also associated with different phases of the stratospheric quasi-biennial oscillations due to its modification of the vertical wind shear of the environment in which TCs form. Interdecadal variations in TC activity are apparently related to the location, strength and extent of the North Pacific subtropical high. However, the mechanisms responsible for modifying these characteristics of the subtropical high have yet to be identified. 相似文献
4.
Summary Seventeen years of sea level pressure (SLP), 200-hPa zonal wind and 500-hPa geopotential height data were used to investigate the boreal winter and summer interannual (IA) circulation patterns. The IA patterns for these variables and for their zonally asymmetric (ZA) part were determined by performing empirical orthogonal function (EOF) analyses on the SLP and on ZA SLP. The corresponding patterns for the other variables were obtained by correlating their time series with the amplitude time series of these EOF analyses. For both seasons, the SLP and ZA SLP show a zonal wavenumber one pattern extending from the tropics into the winter hemisphere extratropics, which is consistent with the circulation anomalies related to the El Niño/Southern Oscillation (ENSO) cycles. The zonal wavenumber one pattern observed for the boreal winter describes the SLP and ZA SLP variations related to the mature state of the El Niño and La Niña episodes, and that for the summer, the SLP and ZA SLP variations associated with the initial or decay stages of these phenomena. The 200-hPa zonal wind and 500-hPa geopotential height patterns exhibit strong seasonal dependence, and the ZA parts of these two variables show even more pronounced seasonal differences. These results indicate that the seasonal cycle of the atmospheric circulation, in particular at the upper tropospheric levels, might play an important role in extending the IA wavetrain-like structure into the subtropics as noted for the 200-hPa zonal wind and its ZA part in the Pacific/Americas sector. This wavetrain-like structure shows its Southern Hemisphere (SH) and Northern Hemisphere (NH) branches for the boreal winter, and only its SH branch, for the boreal summer. Thus, the effects of the seasonal cycle of the atmospheric circulation on the IA patterns seem to be stronger for the NH.With 9 Figures 相似文献
5.
Remotely forced variability in the tropical Atlantic Ocean 总被引：1，自引：1，他引：1
An ensemble of eight hindcasts has been conducted using an ocean-atmosphere general circulation model fully coupled only within the Atlantic basin, with prescribed observational sea surface temperature (SST) for 1950–1998 in the global ocean outside the Atlantic basin. The purpose of these experiments is to understand the influence of the external SST anomalies on the interannual variability in the tropical Atlantic Ocean. Statistical methods, including empirical orthogonal function analysis with maximized signal-to-noise ratio, have been used to extract the remotely forced Atlantic signals from the ensemble of simulations. It is found that the leading external source on the interannual time scales is the El Niño/Southern Oscillation (ENSO) in the Pacific Ocean. The ENSO signal in the tropical Atlantic shows a distinct progression from season to season. During the boreal winter of a maturing El Niño event, the model shows a major warm center in the southern subtropical Atlantic together with warm anomalies in the northern subtropical Atlantic. The southern subtropical SST anomalies is caused by a weakening of the southeast trade winds, which are partly associated with the influence of an atmospheric wave train generated in the western Pacific Ocean and propagating into the Atlantic basin in the Southern Hemisphere during boreal fall. In the boreal spring, the northern tropical Atlantic Ocean is warmed up by a weakening of the northeast trade winds, which is also associated with a wave train generated in the central tropical Pacific during the winter season of an El Niño event. Apart from the atmospheric planetary waves, these SST anomalies are also related to the sea level pressure (SLP) increase in the eastern tropical Atlantic due to the global adjustment to the maturing El Niño in the tropical Pacific. The tropical SLP anomalies are further enhanced in boreal spring, which induce anomalous easterlies on and to the south of the equator and lead to a dynamical oceanic response that causes cold SST anomalies in the eastern and equatorial Atlantic from boreal spring to summer. Most of these SST anomalies persist into the boreal fall season.
相似文献
| B. HuangEmail: |
6.
Interannual and interdecadal oscillation patterns in sea level 总被引：3，自引：0，他引：3
Relative sea-level height (RSLH) data at 213 tide-gauge stations have been analyzed on a monthly and an annual basis to study interannual and interdecadal oscillations, respectively. The main tools of the study are singular spectrum analysis (SSA) and multi-channel SSA (M-SSA). Very-low-frequency variability of RSLH was filtered by SSA to estimate the linear trend at each station. Global sea-level rise, after postglacial rebound corrections, has been found to equal 1.62±0.38 mm/y, by averaging over 175 stations which have a trend consistent with the neighboring ones. We have identified two dominant time scales of El Niño-Southern Oscillation (ENSO) variability, quasi-biennial and low-frequency, in the RSLH data at almost all stations. However, the amplitudes of both ENSO signals are higher in the equatorial Pacific and along the west coast of North America. RSLH data were interpolated along ocean coasts by latitudinal intervals of 5 or 10 degrees, depending on station density. Interannual variability was then examined by M-SSA in five regions: eastern Pacific (25°S–55°N at 10° resolution), western Pacific (35°S–45°N at 10°), equatorial Pacific (123°E–169°W, 6 stations), eastern Atlantic (30°S, 0°, and 30°N–70°N at 5°) and western Atlantic (50°S–50°N at 10°). Throughout the Pacific, we have found three dominant spatio-temporal oscillatory patterns, associated with time scales of ENSO variability; their periods are 2, 2.5–3 and 4–6 y. In the eastern Pacific, the biennial mode and the 6-y low-frequency mode propagate poleward. There is a southward propagation of low-frequency modes in the western Pacific RSLH, between 35°N and 5°S, but no clear propagation in the latitudes further south. However, equatorward propagation of the biennial signal is very clear in the Southern Hemisphere. In the equatorial Pacific, both the quasi-quadrennial and quasi-biennial modes at 10°N propagate westward. Strong and weak El Niño years are evident in the sea-level time series reconstructed from the quasi-biennial and low-frequency modes. Interannual variability with periods of 3 and 4–8 y is detected in the Atlantic RSLH data. In the eastern Atlantic region, we have found slow propagation of both modes northward and southward, away from 40–45°N. Interdecadal oscillations were studied using 81 stations with sufficiently long and continuous records. Most of these have variability at 9–13 and some at 18 y. Two significant eigenmode pairs, corresponding to periods of 11.6 and 12.8 y, are found in the eastern and western Atlantic ocean at latitudes 40°N–70°N and 10°N–50°N, respectively. 相似文献
7.
Summary The relationships between the El-Niño phenomenon and the planetary-scale waves, and the interannual variations in the Indian monsoon (June–September) rainfall have been analysed in order to investigate how the sea surface temperature (SST) in the equatorial eastern Pacific associated with the El-Niño can produce reduced monsoon rainfall over India by teleconnections.The longitude of ridge location over the Indian region of the integrated planetary waves (numbers 1–3) along 15° N latitude circle in the height field of 200 mb pressure level in May is significantly (r=0.93, significant at 98% CL) related to the May SST anomaly at Puerto Chicama. This implies that warmer (colder) SST anomalies are associated with eastward (westward) longitude of the ridge location. The variations of the ridge location in May appear to be significantly inversely (r=–0.95, significant at 99% CL) related to the Indian monsoon rainfall, with rainfall tending to be less (more) than normal during eastward (westward) longitude of the ridge location suggesting some predictive value for the Indian monsoon rainfall. The Indian monsoon rainfall and May SST anomaly at Puerto Chicama are inversely related (r=–0.90, significant at 96% CL).In terms of the observed relationships, a plausible mechanism for linking El-Niño with the reduced Indian monsoon rainfall is discussed. The relationships noted suggest that excessive warm SST anomalies associated with El-Niño induce an eastward shift in the planetary waves which in turn reduce the Indian monsoon rainfall.With 8 Figures 相似文献
8.
9.
A 50-year record of the Indonesian throughflow (ITF) was obtained using the Simple Ocean Data Assimilation (SODA) dataset to calculate a timeseries of Pacific-to-Indian Ocean pressure differences, which were calibrated to transport profiles using ARLINDO (1997) and INSTANT (2004–2006) observational data. The 50 year SODA based ITF transport average is 10.4 Sv; the transport weighted temperature (TWT) is 14.6 °C and the internal energy transport (IET) is 0.53 PW. The different configurations of the ITF transport and temperature profiles result in a dissimilarity in the variability of the IET and the TWT, with the IET more closely correlated with both the depth of the 18 °C isotherm in the western equatorial Pacific and the NINO3.4 index. As with the transport, the IET increases during La Niña and decreases during El Niño. The TWT is only weakly correlated with NINO3.4, suggesting that the El Niño-Southern Oscillation signal is transmitted from the Pacific to the Indian Ocean via changes in pressure and thus in transport rather than by changes in temperature. 相似文献
10.
Two ten-members ensemble experiments using a coupled ocean-atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event (WWE) when the tropical Pacific has initial conditions favourable to the development of a warm event. In the reference ensemble (CREF), no wind perturbation is introduced, whereas a strong westerly wind event anomaly is introduced in boreal winter over the western Pacific in the perturbed ensemble (CWWE). Our results demonstrate that an intense WWE is capable of establishing the conditions under which a strong El Niño event can occur. First, it generates a strong downwelling Kelvin wave that generates a positive sea surface temperature (SST) anomaly in the central-eastern Pacific amplified through a coupled ocean-atmosphere interaction. This anomaly can be as large as 2.5°C 60 days after the WWE. Secondly, this WWE also initiates an eastward displacement of the warm-pool that promotes the occurrence of subsequent WWEs in the following months. These events reinforce the initial warming through the generation of additional Kelvin waves and generate intense surface jets at the eastern edge of the warm-pool that act to further shift warm waters eastward. The use of a ten-members ensemble however reveals substantial differences in the coupled response to a WWE. Whereas four members of CWWE ensemble develop into intense El Niño warming as described above, four others display a moderate warming and two remains in neutral conditions. This diversity between the members appears to be due to the internal atmospheric variability during and following the inserted WWE. In the four moderate warm cases, the warm-pool is initially shifted eastward following the inserted WWE, but the subsequent weak WWE activity (when compared to the strong warming cases) prevents to further shift the warm-pool eastwards. The seasonal strengthening of trade winds in June–July can therefore act to shift warm waters back into the western Pacific, reducing the central-eastern Pacific warming. This strong sensitivity of the coupled response to WWEs may therefore limit the predictability of El Niño events, as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead. 相似文献
11.
12.
Summary This paper presents the results of the Florida State University atmospheric general circulation model that addresses the impact of sea surface temperature anomalies on an El Niño year. Northern Hemisphere winter season simulation. Specifically, our interest is in the simulation of seasonal winter monsoonal rainfall, the planetary scale divergent motions and the westerly wind anomalies of an El Niño year.The El Niño episode of 1982–1983 was interesting due to its higher than average amplitude and its overall evolution. By late 1982 the anomalous circulations associated with the sea surface temperature forcing had begun to take shape even though the anomalies did not attain their peak amplitude until February 1983. The atmosphere-ocean teleconnections set up a strong pattern of geopotential height anomalies during the Northern Hemisphere winter that coincides with El Niño conditions in the tropical Pacific Ocean.Wallace and Gutzler (1981) defined a Pacific North American (PNA) teleconnection pattern index based on data from within this region. The El Niño episode of 1982–1983 has been shown to be strong via the PNA Index and illustrates an importance for climate models to correctly simulate these teleconnections. The importance of the forced anomalies can be seen in the long-range forecasting of conditions over North America as well as the winter monsoon intensity and location.In this study, we utilize a general circulation model with a resolution of triangular truncation at 42 waves to investigate the effects of prescribed sea surface temperature anomalies. We are able to simulate the majority of the large-scale atmospheric response although on regional climatic scales some phase shifts seem apparent.With 7 Figures 相似文献
13.
热带印度洋海温的年际变化与ENSO 总被引：22，自引：3，他引：22
文中讨论了热带印度洋海表温度距平空间分布的年际变化与赤道中东太平洋海温的关系。EOF分析的结果表明 ,印度洋海温的变化主要存在全区符号一致的单极型和西部与东南部符号相反的偶极型 ,它们具有显著的年际变化。小波凝聚谱揭示了单极、偶极的变化与Nino3区海表温度距平存在密切关系 ,印度洋海温距平从偶极到单极的变化对应着ElNi no事件从发展到衰减的过程。平均而言 ,印度洋偶极超前Nino3区海温距平约 4个月 ,单极滞后约 6个月。整个热带印度洋 -太平洋地区海气耦合特征的演变表明 ,与ElNino从发展到衰减相联系的热带西太平洋海气耦合相互作用在印度洋海温距平从偶极到单极的演变过程中起着非常重要的作用。 相似文献
14.
Seasonality and time scales in the relationship between global SST and African rainfall 总被引：1，自引：0，他引：1
The main goal of this study is to determine the oceanic regions corresponding to variability in African rainfall and seasonal differences in the atmospheric teleconnections. Canonical correlation analysis (CCA) has been applied in order to extract the dominant patterns of linear covariability. An ensemble of six simulations with the global atmospheric general circulation model ECHAM4, forced with observed sea surface temperatures (SSTs) and sea ice boundary variability, is used in order to focus on the SST-related part of African rainfall variability. Our main finding is that the boreal summer rainfall (June–September mean) over Africa is more affected by SST changes than in boreal winter (December–March mean). In winter, there is a highly significant link between tropical African rainfall and Indian Ocean and eastern tropical Pacific SST anomalies, which is closely related to El Niño-Southern Oscillation (ENSO). However, long-term changes are found to be associated with SST changes in the Indian and tropical Atlantic Oceans, thus, showing that the tropical Atlantic plays a critical role in determining the position of the intertropical convergence zone (ITCZ). Since ENSO is less in summer, the tropical Pacific and the Indian Oceans are less important for African rainfall. The African summer monsoon is strongly influenced by SST variations in the Gulf of Guinea, with a response of opposite sign over the Sahelian zone and the Guinean coast region. SST changes in the subtropical and extratropical oceans mostly take place on decadal time scales and are responsible for low-frequency rainfall fluctuations over West Africa. The modelled teleconnections are highly consistent with the observations. The agreement for most of the teleconnection patterns is remarkable and suggests that the modelled rainfall anomalies serve as suitable predictors for the observed changes. 相似文献
15.
RECENT TRENDS IN TREE-RING RECORDS FROM HIGH ELEVATION SITES IN THE ANDES OF NORTHERN PATAGONIA 总被引：12，自引：0，他引：12
RICARDO VILLALBA JOSE A. BONINSEGNA THOMAS T. VEBLEN ANDREA SCHMELTER SIGFRIDO RUBULIS 《Climatic change》1997,36(3-4):425-454
A new set of tree-ring records from the Andes of northern Patagonia, Argentina (41° S) was used to evaluate recent (i.e., last 250 years) regional trends in tree growth at upper treeline. Fifteen tree-ring chronologies from 1200 to 1750 m elevation were developed for Nothofagus pumilio, the dominant subalpine species. Samples were collected along three elevational transects located along the steep west-to-east precipitation gradient from the main Cordillera (mean annual precipitation >4000 mm) to an eastern outlier of the Andes (mean annual precipitation >2000 mm). Ring-width variation in higher elevation tree-ring records from the main Cordillera is mainly related to changes in temperature and precipitation during spring and summer. However, the response to climatic variation is also influenced by local site factors of elevation and exposure. Based on the relationships between Nothofagus growth and climate, we reconstructed changes in snow cover duration in late spring and variations in mean annual temperature since A.D. 1750. Abrupt interannual changes in the mean annual temperature reconstruction are associated with strong to very strong El Niño-Southern Oscillation events. At upper treeline, tree growth since 1977 has been anomalously high. A sharp rise in global average tropospheric temperatures has been recorded since the mid-1970s in response to an enhanced tropical hydrologic cycle due to an increase in temperature of the tropical Pacific. Temperatures in northern Patagonia have been anomalously high throughout the 1980s, which is consistent with positive temperature anomalies in the tropical Pacific and along the western coast of the Americas at c.a. 40° S latitude. Our 250-year temperature reconstruction indicates that although the persistently high temperatures of the 1980s are uncommon during this period, they are not unprecedented. Tropical climatic episodes similar to that observed during the 1980s may have occurred in the recent past under pre-industrial carbon dioxide levels. 相似文献
16.
We have used NCEP/NCAR Reanalysis data and a Northern Hemisphere snow cover data set to analyze changes in freezing level heights and snow cover for the past three to five decades. All the major continental mountain chains exhibit upward shifts in the height of the freezing level surface. The pattern of these changes is generally consistent with changes in snow cover, both over the course of the year and spatially. We examined different free-air temperature parameters (dry bulb temperature, virtual temperature, and 700–500 hPa thickness) using the Reanalysis grid point valueslocated over the different mountain areas as defined in this study. The different trend values were in reasonably good agreement with each other, particularly over the second half of the record.Freezing level changes in the American Cordillera are strongly modulated by the El Niño/Southern Oscillation (ENSO) phenomenon and the freezing level heights (FLH) respond to both interannual and decadal-scale change in tropical Pacific sea surface temperature (SST). The 0.5 °C increase in SST recorded in the tropical Pacific since the 1950s accounts for approximately half of the increase in FLH in tropical and subtropical latitudes of the Cordilleran region during that same time. 相似文献
17.
This paper uses recent gridded climatological data and a coupled general circulation model (GCM) simulation in order to assess the relationships between the interannual variability of the Indian summer monsoon (ISM) and the El Niño-Southern Oscillation (ENSO). The focus is on the dynamics of the ISM-ENSO relationships and the ability of the state-of-the-art coupled GCM to reproduce the complex lead-lag relationships between the ISM and the ENSO. The coupled GCM is successful in reproducing the ISM circulation and rainfall climatology in the Indian areas even though the entire ISM circulation is weaker relative to that observed. In both observations and in the simulation, the ISM rainfall anomalies are significantly associated with fluctuations of the Hadley circulation and the 200 hPa zonal wind anomalies over the Indian Ocean. A quasi-biennial time scale is found to structure the ISM dynamical and rainfall indices in both cases. Moreover, ISM indices have a similar interannual variability in the simulation and observations. The coupled model is less successful in simulating the annual cycle in the tropical Pacific. A major model bias is the eastward displacement of the western North Pacific inter-tropical convergence zone (ITCZ), near the dateline, during northern summer. This introduces a strong semiannual component in Pacific Walker circulation indices and central equatorial Pacific sea surface temperatures. Another weakness of the coupled model is a less-than-adequate simulation of the Southern Oscillation due to an erroneous eastward extension of the Southern Pacific convergence zone (SPCZ) year round. Despite these problems, the coupled model captures some aspects of the interannual variability in the tropical Pacific. ENSO events are phase-locked with the annual cycle as observed, but are of reduced amplitude relative to the observations. Wavelet analysis of the model Niño34 time series shows enhanced power in the 2–4 year band, as compared to the 2–8 year range for observations during the 1950–2000 period. The ISM circulation is weakened during ENSO years in both the simulation and the observations. However, the model fails to reproduce the lead-lag relationship between the ISM and Niño34 sea surface temperatures (SSTs). Furthermore, lag correlations show that the delayed response of the wind stress over the central Pacific to ISM variability is insignificant in the simulation. These features are mainly due to the unrealistic interannual variability simulated by the model in the western North Pacific. The amplitude and even the sign of the simulated surface and upper level wind anomalies in these areas are not consistent with observed patterns during weak/strong ISM years. The ISM and western North Pacific ITCZ fluctuate independently in the observations, while they are negatively and significantly correlated in the simulation. This isolates the Pacific Walker circulation from the ISM forcing. These systematic errors may also contribute to the reduced amplitude of ENSO variability in the coupled simulation. Most of the unrealistic features in simulating the Indo-Pacific interannual variability may be traced back to systematic errors in the base state of the coupled model. 相似文献
18.
YAN Bangliang 《大气科学进展》2005,22(5):741-750
The mechanism of the locking of the E1 Nino event onset phase to boreal spring （from April to June） in an intermediate coupled ocean-atmosphere model is investigated. The results show that the seasonal variation of the zonal wind anomaly over the equatorial Pacific associated with the seasonal variation of the ITCZ is the mechanism of the locking in the model. From January to March of the E1 Nino year, the western wind anomaly over the western equatorial Pacific can excite the downwelling Kelvin wave that propagates eastward to the eastern and middle Pacific by April to June. From April to December of the year before the E1 Nifio year, the eastern wind anomaly over the equatorial Pacific forces the downwelling Rossby waves that modulate the ENSO cycle. The modulation and the reflection at the western boundary modulate the time of the transition from the cool to the warm phase to September of the year before the E1 Nifio year and cause the strongest downwelling Kelvin wave from the reflected Rossby waves at the western boundary to arrive in the middle and eastern equatorial Pacific by April to June of the E1 Nino year. The superposition of these two kinds of downwelling Kelvin waves causes the El Nino event to tend to occur from April to June. 相似文献
19.
Interannual variations of subsurface influence on SST in the Indian Ocean show strong seasonality. The subsurface influence on SST confines to the southern Indian Ocean (SIO) in boreal winter and spring; it is observed on both sides of the equator in boreal summer and fall. Interannual long Rossby waves are at the heart of this influence, and contribute significantly to the coupled climate variability in the tropical Indian Ocean (TIO). Principal forcing mechanism for the generation of these interannual waves in the Indian Ocean and the relative influence of two dominant interannual signals in the tropics, namely El Niño and Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), are also discussed. Two distinct regions dominated by either of the above climate signals are identified. IOD dominates the forcing of the off-equatorial Rossby waves, north of 10°S, and the forcing comes mainly from the anomalous Ekman pumping associated with the IOD. However, after the demise of IOD activity by December, Rossby waves are dominantly forced by ENSO, particularly south of 10°S.It is found that the subsurface feedback in the northern flank of the southern Indian Ocean ridge region (north of 10°S) significantly influences the central east African rainfall in boreal fall. The Indian Ocean coupled process further holds considerable capability of predicting the east African rainfall by one season ahead. Decadal modulation of the subsurface influence is also noticed during the study period. The subsurface influence north of 10°S coherently varies with the IOD, while it varies coherently with the ENSO south of this latitude. 相似文献
20.
Summary This paper is to promote a further understanding of the interdecadal mode of the South Pacific. With this focus, we will specifically
aim at better understanding the difference between interannual and interdecadal SSTA modes over South Pacific. We define the
difference of the normalization area-averaged SSTA in the southern extratropical Pacific (160° W–110° W, 40° S–25° S) and
the south subpolar Pacific (150° W–110° W, 60° S–45° S) as the South Pacific interdecadal index (I
spd). It is found that the interannual mode is more coherent than the interdecadal mode in the central and eastern tropical Pacific,
and the interdecadal mode is significant only during boreal winter (DJF). The interdecadal variation of SSTA firstly occurring
in the extratropic South Pacific propagates to the western boundary of the South Pacific, then moves northeast to cross the
equator, and finally reaches the central tropic Pacific. It takes about 8 years to propagate from southeast subtropical Pacific
to the north hemisphere. The previous studies have suggested the mechanism of waves in the subsurface in the South Pacific.
Our study also highlights the Rossby waves play important roles in linkage between the extratropics-tropics South Pacific
SSTA on interdecadal time scales. Moreover, the paper shows that the interdecadal variability originated in the extrotropic
southeast Pacific is mainly induced by interannual variability in the tropic Pacific. 相似文献