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1.
A review of oceanographic and climate data from the North Pacific and Bering Sea has revealed climate events that occur on two principal time scales: a) 2–7 years (i.e. El Niño Southern Oscillation, ENSO), and b) inter-decadal (i.e. Pacific Decadal Oscillation, PDO). The timing of ENSO events and of related oceanic changes at higher latitudes were examined. The frequency of ENSO was high in the 1980s. Evidence of ENSO forcing on ocean conditions in the North Pacific (Niño North conditions) showed ENSO events were more frequently observed along the West Coast than in the western Gulf of Alaska (GOA) and Eastern Bering Sea (EBS). Time series of catches for 30 region/species groups of salmon, and recruitment data for 29 groundfish and 5 non-salmonid pelagic species, were examined for evidence of a statistical relationship with any of the time scales associated with Niño North conditions or the PDO. Some flatfish stocks exhibited high autocorrelation in recruitment coupled with a significant step in recruitment in 1977 suggesting a relationship between PDO forcing and recruitment success. Five of the dominant gadid stocks (EBS and GOA Pacific cod, Pacific hake and EBS and GOA walleye pollock) exhibited low autocorrelation in recruitment. Of these, Pacific hake, GOA walleye pollock and GOA Pacific cod exhibited significantly higher incidence of strong year classes in years associated with Niño North conditions. These findings suggest that the PDO and ENSO may play an important role in governing year-class strength of several Northeast Pacific marine fish stocks. 相似文献
2.
A recently proposed method for estimating nitrate and new production from remotely sensed data (Goes and Goes) allowed us to observe significant deviations from the normal in the quantum of winter-time nitrate injected into the euphotic column and its consumption by phytoplankton in the North Pacific following the El Niño event of 1997. Results from this study allowed us to observe large differences in the ways in which the El Niño event affected the western and the eastern margins of the North Pacific basin. For the western North Pacific, a long-term (1972–1992) historical record of oceanographic data provided us with clear evidence supporting of our findings from satellite observations. In the eastern North Pacific Ocean also, our results compared well with those previously reported (Wong, Whitney, Matear, & Iseki, 1998). While it is clear from this study that El Niño/La Niña oscillations can have a major influence on interannual variations in biological processes in the North Pacific, these results also serve to highlight the value of remote sensing as a tool for studying large regional to basin-scale biological oceanographic events. 相似文献
3.
El Niño-Southern Oscillation (ENSO) encompasses variability in both the eastern and western tropical Pacific. During the warm phase of ENSO, the eastern tropical Pacific is characterized by equatorial positive sea surface temperature (SST) and negative sea level pressure (SLP) anomalies, while the western tropical Pacific is marked by off-equatorial negative SST and positive SLP anomalies. Corresponding to this distribution are equatorial westerly wind anomalies in the central Pacific and equatorial easterly wind anomalies in the far western Pacific. Occurrence of ENSO has been explained as either a self-sustained, naturally oscillatory mode of the coupled ocean–atmosphere system or a stable mode triggered by stochastic forcing. Whatever the case, ENSO involves the positive ocean–atmosphere feedback hypothesized by Bjerknes. After an El Niño reaches its mature phase, negative feedbacks are required to terminate growth of the mature El Niño anomalies in the central and eastern Pacific. Four requisite negative feedbacks have been proposed: reflected Kelvin waves at the ocean western boundary, a discharge process due to Sverdrup transport, western Pacific wind-forced Kelvin waves, and anomalous zonal advections. These negative feedbacks may work together for terminating El Niño, with their relative importance being time-dependent.ENSO variability is most pronounced along the equator and the coast of Ecuador and Peru. However, the eastern tropical Pacific also includes a warm pool north of the equator where important variability occurs. Seasonally, ocean advection seems to play an important role for SST variations of the eastern Pacific warm pool. Interannual variability in the eastern Pacific warm pool may be largely due to a direct oceanic connection with the ENSO variability at the equator. Variations in temperature, stratification, insolation, and productivity associated with ENSO have implications for phytoplankton productivity and for fish, birds, and other organisms in the region. Long-term changes in ENSO variability may be occurring and are briefly discussed. This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific. 相似文献
4.
A zonal hydrographic section along 44.65°N, from the coast of Oregon to 300 km offshore, was occupied regularly (at least seasonally) from 1961 to 1971 and then sporadically until recently. Regular monitoring of this section to 160 km offshore resumed in July 1997 as part of the GLOBEC Long Term Observational Program; the recent data provide observations in Oregon coastal waters of El Niño 1997–98 and La Niña conditions that followed. The complete seasonal data from the decade 1961–1971 provide a basis for comparison with the recent temperature and salinity sections, steric height profiles, geostrophic velocity, and water mass characteristics. These data, and sporadic observations in intervening years, allow us to compare conditions during several ENSO events with the recent event and to search for evidence of climate change. The PFEL Coastal Upwelling Index, sea level from the University of Hawaii Sea Level Center, the Multivariate ENSO Index (MEI), the Pacific Decadal Oscillation (PDO), and outflow from the Columbia River are used to distinguish local and remote causes of variability in physical oceanographic conditions off Oregon. The sequence of El Niño/La Niña/El Niño in 1963–66, during a cool phase of PDO, provides a comparison to El Niño/La Niña of 1997–2000. El Niño in 1982–83 and 1997–98, during a warm phase of PDO, caused the largest oceanographic anomalies in the 40 years. The comparison indicates warming of the coastal ocean off Oregon and suggests a modulation of ENSO effects by PDO. Such modulation would mask evidence for secular climate change in our 40-year oceanographic data series. 相似文献
5.
During the boreal summer the Western Hemisphere warm pool (WHWP) stretches from the eastern North Pacific to the tropical North Atlantic and is a key feature of the climate of the Americas and Africa. In the summers following nine El Niño events during 1950–2000, there have been five instances of extraordinarily large warm pools averaging about twice the climatological annual size. These large warm pools have induced a strengthened divergent circulation aloft and have been associated with rainfall anomalies throughout the western hemisphere tropics and subtropics and with more frequent hurricanes. However, following four other El Niño events large warm pools did not develop, such that the mere existence of El Niño during the boreal winter does not provide the basis for predicting an anomalously large warm pool the following summer.In this paper, we find consistency with the hypothesis that large warm pools result from an anomalous divergent circulation forced by sea surface temperature (SST) anomalies in the Pacific, the so-called atmospheric bridge. We also find significant explanations for why large warm pools do not always develop. If the El Niño event ends early in the eastern Pacific, the Pacific warm anomaly lacks the persistence needed to force the atmospheric bridge and the Atlantic portion of the warm pool remains normal. If SST anomalies in the eastern Pacific do not last much beyond February of the following year, then the eastern North Pacific portion of the warm pool remains normal. The overall strength of the Pacific El Niño does not appear to be a critical factor. We also find that when conditions favor a developing atmospheric bridge and the winter atmosphere over the North Atlantic conforms to a negative North Atlantic Oscillation (NAO) pattern (as in 1957–58 and 1968–69), the forcing is reinforced and the warm pool is stronger. On the other hand, if a positive NAO pattern develops the warm pool may remain normal even if other circumstances favor the atmospheric bridge, as in 1991–92. Finally, we could find little evidence that interactions internal to the tropical Atlantic are likely to mitigate for or against the formation of the largest warm pools, although they may affect smaller warm pool fluctuations or the warm pool persistence. 相似文献
6.
根据1949~1998年期间8次显著El Niño事件合成的24个月年际海温异常(SSTAs)和气候平均的海温(SST),利用CCM3分别进行了3个包含10次积分的集合试验,即控制试验(CTRL),热带太平洋海洋全球大气试验(TOGA)以及整个太平洋海洋全球大气试验(TOGA-NP),通过对比分析这3个试验之间的集合模拟结果,揭示了在El Niño不同演变阶段东亚气候年际异常响应结构以及北太平洋年际SSTAs在此过程中的调制作用.结果表明:El Niño发展阶段夏季,东亚地区大气环流异常呈显著的负PJ波列,副热带高压减弱、偏东,东亚夏季风增强,东北和江淮流域降水偏多,华北和长江流域及其以南地区降水偏少;El Niño成熟阶段冬季,东亚大槽加强,东亚北部冬季风加强,西太平洋副热带地区低层有显著的反气旋式异常风场,华南地区降水显著增多;El Niño衰亡阶段夏季东亚气候年际异常型与其发展阶段夏季几乎相反.同时,北太平洋年际SSTAs对El Niño影响东亚气候年际异常有一定的调制作用,使模拟的我国降水异常分布更符合观测. 相似文献
7.
用SODA海洋同化和NCEP大气再分析资料,分析了热带太平洋次表层海温异常主要模态与北太平洋低纬度西边界流海域上层海洋环流和亚洲-北太平洋地区大气垂直和水平流场变化之间的关系,得到以下结果:（1） 在热带太平洋海洋次表层ENSO事件具有两种模态,二者组合构成ENSO循环。第一模态为ENSO成熟期,主要出现在冬季,第二模态为ENSO过渡期,主要出现夏季。（2） ENSO循环对北太平洋低纬度西边界流区上层海洋环流有重要影响。在El Niño发展期或La Niña 衰退期,该区出现气旋性异常环流,北赤道流（NEC）加强,NEC分叉位置北移,棉兰老海流（MC）加大,菲律宾以东黑潮（KC）减小,北赤道逆流（NECC）最强。在El Niño（La Niña）成熟期,该区气旋性（反气旋性）异常环流达最强,NEC最强（最弱）,NEC分叉位置最北（最南）,MC最大（最小）,KC最小（最大）,NECC减弱（加强）。在El Niño衰退期或La Niña发展期与El Niño发展期相反,该区出现反气旋性异常环流,由此导致相应流系异常发生反位相变化。（3） ENSO循环对北太平洋低纬度西边界流海域上层海洋环流的影响是通过ENSO事件期间热带太平洋热力状况异常改变上空大气环流来实现的。ENSO事件首先造成热带太平洋海洋热力状况异常,导致其上空对流活动异常,后者直接或间接通过“大气桥”能量传输引起相关地区大气环流场的变化,致使海面风应力场异常,进而强迫上层海洋环流场的相应变化。文章最后还分析了ENSO事件期间菲律宾附近异常反气旋或异常气旋性风场的产生和持续原因,讨论了北太平洋低纬度西边界流海域海气相互作用在ENSO循环中的贡献。 相似文献
8.
利用中国沿海台站潮位和中国近海及赤道太平洋的卫星测高、海表温度、风及气压资料,分析了中国近海海平面变化与ENSO的关系。分析结果表明:中国沿海海平面季节变化受ENSO影响明显,在厄尔尼诺事件期间,中国沿海海平面Sa分潮的振幅明显减小,其中年振幅的历史极小值均出现厄尔尼诺年,不同区域历史极小值出现的年份不同;另外,中国沿海Sa分潮的振幅对厄尔尼诺事件的响应与其强弱有关,在强事件中,响应区域和幅度较大,弱事件中,响应区域和幅度偏小。在厄尔尼诺年,中国沿海海平面多低于相邻年份,并且其年际变化存在明显的2~3a、4~7a、准9a、11a和准19a的周期,其中4~7a的周期在冬春季节震荡最显著,其震荡幅度接近2cm。中国近海海平面与赤道东太平洋区域的海表温度年际变化之间存在反相关关系,其相关系数为-0.42;同时与Nio4和Nio3.4指数序列也呈现反相关关系。针对典型的1997/1998年尼诺事件发生前后的风场和气压场分析发现,尼诺发生前的冬半年,冬季风偏强,气压梯度加强,中国沿海海平面偏低;到了厄尔尼诺的盛期,出现较强的南风异常,气压梯度反向,季风转向,过渡到了厄尔尼诺事件的衰减期,为拉尼娜事件做准备,此时海平面偏高。 相似文献
9.
The 2015/2016 El Ni?o event reached the threshold of super El Ni?o event, and was comparable to the super events in 1982/1983 and 1997/1998. Interestingly, the tropical cyclones(TCs) were found to have very late onsets in the decaying years of the super El Ni?o events. This study discusses the causes of late TC onsets related with atmospheric circulation, disturbance sources and trigger mechanisms. The analysis shows that the western North Pacific subtropical high(WNPSH) from January–June during the decaying years of the super El Ni?o events were stronger than the climatic mean, which resulted in a relatively stable atmospheric state by inhibiting deep convection. As a disturbance source, the April–June intertropical convergence zone(ITCZ) during the decaying years of the super El Ni?o events were significantly weaker than its climatic mean. The cross-equatorial flow and monsoon trough, as important TC generation triggers, were weaker from April–June during the decaying years of the super El Ni?o events, which further reduced the probability of TC generation. As for the late TC onsets, the role of atmospheric circulation anomalies(i.e., subtropical-high, the ITCZ, cross-equatorial flow, and monsoon trough) were more important. The cross-equatorial flow may take as predictor of TC onsets in the decaying years of the super El Ni?o events. 相似文献
10.
本文使用SODA(simple ocean data assimilation)海洋同化资料,系统分析了厄尔尼诺-南方涛动(El Ni?o-Southern Oscillation,ENSO)循环中冷暖位相期间热带太平洋上层海洋环流的演变规律,探讨了形成海洋环流异常的新机制。结果表明,在厄尔尼诺成熟期,热带中东太平洋赤道潜流最弱,赤道两侧出现反气旋性环流异常;西太平洋赤道外热带海域出现气旋性环流异常,该区南、北赤道流、棉兰老流、黑潮、新几内亚沿岸潜流及南赤道逆流增强;北赤道逆流区出现异常气旋性环流串,北赤道逆流接近正常。在厄尔尼诺衰退期和拉尼娜发展期,热带中西太平洋赤道潜流达到极强,赤道两侧出现气旋性环流异常;西太平洋赤道外热带海域异常环流减弱,该处主要流场的强度减弱或处于正常状态;北赤道逆流区反转为异常西向流。结果表明, ENSO循环期间的上层海洋环流异常受到热带太平洋温跃层深度异常产生的压强梯度力异常调控,在赤道外热带海洋温跃层深度异常和科里奥利力共同作用产生大尺度海洋环流异常,而在赤道海域,海洋温跃层深度异常和Gill效应造成赤道潜流异常以及关于赤道对称的气旋或反气旋性环流异常。 相似文献
11.
On the basis of more than 200-year control run, the performance of the climate system model of Chinese Academy of Sciences(CAS-ESM-C) in simulating the El Ni?o-Southern Oscillation(ENSO) cycle is evaluated, including the onset, development and decay of the ENSO. It is shown that, the model can reasonably simulate the annual cycle and interannual variability of sea surface temperature(SST) in the tropical Pacific, as well as the seasonal phase-locking of the ENSO. The model also captures two prerequisites for the El Ni?o onset, i.e., a westerly anomaly and a warm SST anomaly in the equatorial western Pacific. Owing to too strong forcing from an extratropical meridional wind, however, the westerly anomaly in this region is largely overestimated. Moreover, the simulated thermocline is much shallower with a weaker slope. As a result, the warm SST anomaly from the western Pacific propagates eastward more quickly, leading to a faster development of an El Ni?o. During the decay stage, owing to a stronger El Ni?o in the model, the secondary Gill-type response of the tropical atmosphere to the eastern Pacific warming is much stronger, thereby resulting in a persistent easterly anomaly in the western Pacific. Meanwhile, a cold anomaly in the warm pool appears as a result of a lifted thermocline via Ekman pumping. Finally, an El Ni?o decays into a La Ni?a through their interactions. In addition, the shorter period and larger amplitude of the ENSO in the model can be attributed to a shallower thermocline in the equatorial Pacific, which speeds up the zonal redistribution of a heat content in the upper ocean. 相似文献
12.
The physical, chemical and biological perturbations in central California waters associated with the strong 1997–1998 El Niño are described and explained on the basis of time series collected from ships, moorings, tide gauges and satellites. The evolution of El Niño off California closely followed the pattern observed in the tropical Pacific. In June 1997 an anomalous influx of warm southerly waters, with weak signatures on coastal sea level and thermocline depth, marked the onset of El Niño in central California. The timing was consistent with propagation from the tropics via the equatorial and coastal wave-guide. By late 1997, the classical stratified ocean condition with a deep thermocline, high sea level, and warm sea surface temperature (SST) commonly associated with El Niño dominated the coastal zone. During the first half of 1998 the core of the California Current, which is normally detected several hundred kilometers from shore as a river of low salinity, low nutrient water, was hugging the coast. High nutrient, productive waters that occur in a north–south band from the coast to approximately 200 km offshore during cool years disappeared during El Niño. The nitrate in surface waters was less than 20% of normal and new production was reduced by close to 70%. The La Niña recovery phase began in the fall of 1998 when SSTs dropped below normal, and ocean productivity rebounded to higher than normal levels. The reduction in coastal California primary productivity associated with El Niño was estimated to be 50 million metric tons of carbon (5×10 13 g C). This reduction certainly had deleterious effects on zooplankton, fish, and marine mammals. The 1992–1993 El Niño was more moderate than the 1997–1998 event, but because its duration was longer, its overall chemical and biological impact may have been comparable. How strongly the ecosystem responds to El Niño appears related to the longer-term background climatic state of the Pacific Ocean. The 1982–1983 and 1992–1993 El Niños occurred during the warm phase of the Pacific Decadal Oscillation (PDO). The PDO may have changed sign during the 1997–1998 El Niño, resulting in weaker ecological effects than would otherwise have been predicted based on the strength of the temperature anomaly. 相似文献
13.
本文选取ECMWF ORAS4再分析数据对1959-2015年热带太平洋海平面的低频变化进行了分析。热带太平洋海平面年际变化第一模态反映了ENSO爆发阶段的海平面变化,热带东、西太平洋变化反相,其时间序列与Niño3.4指数高度相关。海平面第二模态则体现了El Niño爆发前后热带太平洋暖水的输运过程。El Niño爆发前热带西太平洋暖水聚集的位置,以及爆发后暖水向赤道外输运的位置在两类El Niño事件中均有所不同。此外,ENSO的周期在近半个世纪发生了显著的年代际变化,这一变化与热带太平洋的年代际变化有关。热带太平洋的年代际变化对海平面趋势变化也有着显著的影响。卫星高度计观测到的近20年海平面的快速上升（下降）正是由20世纪90年代后热带太平洋频繁的位相转换引起的。 相似文献
14.
Long-term monthly sea level and sea surface temperature (SST) anomalies from central California show that during winter months, positive anomalies are associated with El Niño events and the negative ones with La Niña events. There is no significant impact on monthly mean anomalies associated with Pacific decadal oscillations, although there is a tendency for more extreme events and greater variance during positive decadal oscillations. The very strong 1997–1998 El Niño was analyzed with respect to the long-term historic record to assess the forcing mechanisms for sea level and SST. Beginning in the spring of 1997, we observed several long-period (>30 days) fluctuations in daily sea level with amplitudes of over 10 cm at San Francisco, California. Fluctuations of poleward long-period alongshore wind stress anomalies (AWSA) are coherent with the sea level anomalies. However, the wind stress cannot entirely account for the observed sea level signals. The sea level fluctuations are also correlated with sea level fluctuations observed further south at Los Angeles and Tumaco, Columbia, which showed a poleward phase propagation of the sea level signal. We suggest that the sea level fluctuations were, to a greater degree, forced by the passage of remotely generated and coastally trapped waves that were generated along the equator and propagated to the north along the west coast of North America. However, both local and remote AWSA can significantly modulate the sea level signals. The arrival of coastally trapped waves began in the spring of 1997, which is earlier than previous strong El Niño events such as the 1982–1983 event. 相似文献
15.
A time-series sediment trap was deployed from October 2007 to May 2011 in the western subtropical Pacific with the aim of understanding the seasonal and inter-annual variability on particle flux in response to El Niño-Southern Oscillation (ENSO) events. Total mass fluxes varied from 3.04 mg m −2 day −1 to 31.1 mg m −2 day −1, with high fluxes during February–April and low fluxes during other months. This seasonal variation was also characterized by a distinct change in the CaCO 3 flux between the two periods. The marked increase in particle flux during February–April may be attributed to enhanced biological productivity in surface waters caused by strong wind-driven mixing in response to the western North Pacific monsoon system. The 2009/10 strong El Niño was accompanied by a significant reduction in particle flux, whereas the La Niña had no recognizable effect on particle flux in the subtropical Pacific. In particular, in the mature phase of the 2009/10 strong El Niño, the fluxes of organic carbon and biogenic silica decreased by 70–80% compared with those during the normal period, implying that the El Niño acted to suppress biological productivity in surface waters. The suppression of biological productivity during the 2009/10 strong El Niño is attributed to the decrease in precipitation due to the shift in the western Pacific warm pool. This finding is opposite that of other studies of the western equatorial Pacific, where El Niño events were observed to result in an increase in biological productivity and particle flux. The difference in particle flux between the western equatorial and subtropical Pacific is attributed to the regional differences in oceanic and atmospheric circulation systems generated by the strong El Niño. 相似文献
16.
The spatial and temporal variation characteristics of the waves in the South China Sea(SCS) in the boreal winter during the period of 1979/1980–2011/2012 have been investigated based on the European Centre for Medium-range Weather Forecasts interim(ERA-Interim) reanalysis dataset. The results show that the leading mode of significant wave height anomalies(SWHA) in the SCS exhibits significant interannual variation and a decadal shift around the mid-1990 s, and features a basin-wide pattern in the entire SCS with a center located in the west of the Luzon Strait. The decadal change from a weak regime to a strong regime is mainly associated with the enhancement of winter monsoon modulated by the Pacific decadal oscillation(PDO). The interannual variation of the SWHA has a significant negative correlation with the El Ni?o Southern Oscillation(ENSO) in the same season and the preceding autumn. For a better understanding of the physical mechanism between the SCS ocean waves and ENSO, further investigation is made by analyzing atmospheric circulation. The impact of the ENSO on the SWHA over the SCS is bridged by the East Asian winter monsoon and Pacific-East Asian teleconnection in the lower troposphere. During the El Ni?o(La Ni?a), the anomalous Philippine Sea anticyclone(cyclone) dominates over the Western North Pacific, helps to weaken(enhance) East Asian winter monsoon and then emerges the negative(positive) SWHA in the SCS. 相似文献
17.
利用Hadley中心逐月海表温度、欧洲中心ERA-40的10 m风场及CMAP降水资料探讨了年循环对热带太平洋El Niño海气相互作用过程的影响。尽管El Niño对应的海表温度异常主要出现在赤道东太平洋,经向上呈南北对称分布,然而其对应的大气响应在El Niño年衰减阶段却有着强的向南移动特征。在El Niño发展年的11月之前,强的西风和降水异常主要出现在赤道中太平洋;在12月份之后,赤道上的西风和降水异常迅速南移至5°S,随后西风一直维持在该位置直至衰亡。同时,西太平洋负降水和反气旋异常向北移动。这种SST异常与其大气响应的经向移动不一致,主要是由热带中太平洋气候态SST的季节性南移导致的。由于对流与海温之间存在非线性关系,即当总SST超过一定的阈值,对流降水才会迅速增强;因此相应的对流响应也随着总海温的南移而南移,风场响应也同时南移。此外,南半球增强的对流会通过经向环流进一步抑制北半球的降水,从而使西太平洋负降水和反气旋异常增强并北移。通过分析有/无年循环的两组数值试验结果验证了上述结论,即有年循环的试验较真实地模拟出了观测中异常西风南移和西北太平洋反气旋异常的出现;无年循环试验尽管能模拟出El Niño年赤道中太平洋的西风异常,但其却没有南北向的移动,西北太平洋的反气旋也没有出现。因此,热带中太平洋气候态暖海温的季节循环对El Niño事件大气响应有着至关重要的作用。 相似文献
18.
Several species of migratory, warm-water, oceanic fishes invaded Oregon waters during the summer of 1997. Also, the jumbo squid ( Dosidicus gigas), common in the eastern tropical Pacific, was reported for the first time in 1997 and was caught in large numbers. The occurrence of these oceanic nekton was associated with inshore advection of anomalously warm water. During 1998, after arrival of the main El Niño signal, some warm-water coastal fishes appeared off Oregon. However, unlike observations off California, fewer species of warm-water coastal fishes were noted during the 1997–98 El Niño than during the 1982–83 El Niño. 相似文献
19.
The seasonal variability of sea surface height (SSH) and currents are defined by analysis of altimeter data in the NE Pacific Ocean over the region from Central America to the Alaska Gyre. The results help to clarify questions about the timing of seasonal maxima in the boundary currents. As explained below, the long-term temporal mean of the SSH values must be removed at each spatial point to remove the temporally invariant (and large) signal caused by the marine geoid. We refer to the resulting SSH values, which contain all of the temporal variations, as the ‘residual’ SSH. Our main findings are: - 1. The maximum surface velocities around the boundaries of the cyclonic Alaska Gyre (the Alaska Current and the Alaska Stream) occur in winter, at the same time that the equatorward California Current is weakest or reversed (forming the poleward Davidson Current); the maximum surface velocities in the California Current occur in summer. These seasonal maxima are coincident with the large-scale atmospheric wind forcing over each region.
- 2. Most of the seasonal variability occurs as strong residuals in alongshore surface currents around the boundaries of the NE Pacific basin, directly connecting the boundaries of the subpolar gyre, the subtropical gyre and the Equatorial Current System.
- 3. Seasonal variability in the surface velocities of the eastward North Pacific Current (West Wind Drift) is weak in comparison to seasonal changes in the surface currents along the boundaries.
- 4. There is an initial appearance next to the coast and offshore migration of seasonal highs and lows in SSH, alongshore velocity and eddy kinetic energy (EKE) in the Alaska Gyre, similar to the previously-described seasonal offshore migration in the California Current.
- 5. The seasonal development of high SSH and poleward current residuals next to the coast appear first off Central America and mainland Mexico in May–June, prior to their appearance in the southern part of the California Current in July–August and their eventual spread around the entire basin in November–December. Similarly, low SSH and equatorward transport residuals appear first off Central America and Mexico in January–February before spreading farther north in spring and summer.
- 6. The maximum values of EKE occur when each of the boundary currents are maximum.
Article Outline- 1. Introduction and background
- 2. Data and methods
- 2.1. Altimeter and tide gauge data
- 2.2. Atmospheric forcing—sea level pressure
- 2.3. Statistical gridding
- 3. Results
- 4. Summary and discussion
- 4.1. Alaska Gyre
- 4.2. Connections around the boundaries of the subarctic and subtropical gyres
- 4.3. Connections to the North Pacific Current
- 4.4. Offshore ‘propagation’ of the seasonal height and transport signals
- 4.5. Connections to the equatorial current systems along the boundaries
- Acknowledgements
- References
1. Introduction and backgroundThis is the first of a two-part analysis of temporal variability of the NE Pacific Ocean’s surface circulation, as measured by satellite altimeters. Here we examine the seasonal variability. In Part 2 (Strub & James, 2002) we analyze the non-seasonal anomalies of the surface circulation over the 1993–1998 period, during which the 1997–1998 El Niño creates the largest signal. Formation of the seasonal cycles discussed here is the first step in creating the non-seasonal anomalies. The seasonal cycles themselves, however, provide new information on the response of the NE Pacific to strong seasonal forcing, on scales not previously addressed. This analysis quantifies the degree of connection, on seasonal time scales, between the boundary currents in the eastern subarctic and subtropical gyres, as well as the connection between the boundaries and the interior NE Pacific. It further shows a connection to the equatorial current system.Numerous papers describe aspects of the seasonal cycles for certain parameters in subregions of our larger domain. Chapters in Robinson and Brink (1998) review some of the past results from the coastal ocean in the regions between the Equator and the Alaska Gyre ( Badan; Hickey and Royer). Fig. 1 presents the climatological surface dynamic height field (relative to 500 m) in the NE Pacific, calculated from the long-term mean climatological temperature and salinity data of Levitus and Gelfeld (1992). The 500 m reference level is used to concentrate on the surface flow seen by altimeters. Although this climatology is overly smooth, it shows the major currents in the area. The broad, eastward North Pacific Current (also called the West Wind Drift) splits into the counterclockwise Alaska Gyre and the equatorward California Current. South of 20°N in summer, the California Current turns westward and flows into the North Equatorial Current, while in winter–spring, part of it continues along the Mexican mainland before turning westward ( Badan; Fiedler and Fiedler). The long-term climatology shows both paths. The North Equatorial Countercurrent (NECC) flows eastward between 5° –10°N to approximately 120°W, but is only weakly seen in the annual climatology from there to the cyclonic flow around the Costa Rica Dome near 8°N, 92°W. The NECC is a shallow current (found in the upper 200 m) and might appear more strongly if a shallower reference were used, but it is also seasonally intermittent. When the Intertropical Convergence Zone (ITCZ) is in its northern location near 10°N (summer), surface divergences and upwelling create a zonal trough in surface height, driving the NECC along the southern side of the trough. When the ITCZ moves south in winter, the NECC weakens or reverses. 相似文献
20.
利用一个斜压两层海洋模式解析地研究了赤道东、西太平洋对信风张弛的响应特征.研究表明:当赤道上空偏东信风张弛或转为西风时,由于打破了海洋原来的平衡关系,结果在赤道东、西太平洋的温跃层附近产生了扰动并开始传播.西太平洋温跃层附近的扰动向东传播的速度远大于东太平洋扰动向西传播的速度,而且与东太平洋温跃层扰动向西传播的狭窄范围和小振幅相比,西太平洋温跃层扰动向东传播的范围和强度均很大.这与最近几次强厄尔尼诺增暖事件暖水从赤道西太平洋向赤道中、东太平洋的迅速传播特征是一致的. 相似文献
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