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1.
Trace element distributions in the source waters of the Pacific Equatorial Undercurrent (EUC) show the existence of elevated total acid-soluble iron concentrations. This region has been suggested to contribute enough bioavailable iron to regulate interannual and interglacial variability in biological productivity downstream in the high-nitrate low-chlorophyll upwelling zone of the eastern equatorial Pacific. We investigated the advection and first-order biogeochemical impact of an imposed, data-based iron maximum in the western Pacific EUC using an ecosystem model forced by a global dynamical model. We imposed two source profiles of iron constrained by total acid-soluble iron measurements. Though the data for total acid-soluble iron included both dissolved and acid-soluble particulate iron species, we treated all of the total acid-soluble iron as if it was dissolved and bioavailable. A deeper (270 m) source was centered in the density horizon of the observed iron maximum and a shallower (180 m) source was located in the core of our model's EUC, where a dissolved iron maximum has been frequently postulated. These source runs were compared with a control run that contained no specific source of iron associated with the EUC. In the source runs elevated iron concentrations were simulated in the EUC across its entire zonal span, evident as a subsurface plume of dissolved iron slightly below the core of the EUC. In the control run there was no iron maximum associated with the EUC. Upwelling of iron-replete water in the central and eastern equatorial Pacific increased integrated primary productivity in the Wyrtki box (180°W:90°W, 5°S:5°N, 0:200 m) by 41% and 66% for the deeper and shallower iron perturbation, respectively. The source runs increased the realism of the zonal extent of HNLC conditions and the meridional distributions of biological productivity, relative to the control run. However, in the source simulations surface chlorophyll concentrations were too high by a factor of two and maximum surface nitrate concentrations were too low, relative to climatologies. The relative abundance of diatoms roughly doubled upon the input of additional iron, exceeding field observations. Though biogeochemical data are limited and we did not adjust parameters to optimize the model fits to observations, these results suggest that acid-soluble particulate iron supplied to the EUC in the western equatorial Pacific is unlikely to be entirely bioavailable.  相似文献   
2.
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3.
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   
4.
Dengue fever is the most prevalent mosquito-borne viral disease of humans in tropical lands. As an efficient vaccine is not yet available, the only means to prevent epidemics is to control mosquito populations. These are influenced by human behavior and climatic conditions and thus, need constant effort and are very expansive. Examples of succeeded prevention are rare because of the continuous reintroduction of virus or vector from outside, or growing resistance of mosquito populations to insecticides. Climate variability and global warming are other factors which may favour epidemics of dengue. During a pilot study in Claris EC project, a model for the transmission of dengue was built, to serve as a tool for estimating the risk of epidemic transmission and eventually forecasting the risk under climatic change scenarios. An ultimate objective would be to use the model as an early warning system with meteorological forecasts as input, thus allowing better decision making and prevention.  相似文献   
5.
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   
6.
The influence of chlorophyll spatial patterns and variability on the tropical Pacific climate is investigated by using a fully coupled general circulation model (HadOPA) coupled to a state-of-the-art biogeochemical model (PISCES). The simulated chlorophyll concentrations can feedback onto the ocean by modifying the vertical distribution of radiant heating. This fully interactive biological-ocean-atmosphere experiment is compared to a reference experiment that uses a constant chlorophyll concentration (0.06 mg m−3). It is shown that introducing an interactive biology acts to warm the surface eastern equatorial Pacific by about 0.5°C. Two competing processes are involved in generating this warming: (a) a direct 1-D biological warming process in the top layers (0–30 m) resulting from strong chlorophyll concentrations in the upwelling region and enhanced by positive dynamical feedbacks (weaker trade winds, surface currents and upwelling) and (b) a 2-D meridional cooling process which brings cold off-equatorial anomalies from the subsurface into the equatorial mixed layer through the meridional cells. Sensitivity experiments show that the climatological horizontal structure of the chlorophyll field in the upper layers is crucial to maintain the eastern Pacific warming. Concerning the variability, introducing an interactive biology slightly reduces the strength of the seasonal cycle, with stronger SST warming and chlorophyll concentrations during the upwelling season. In addition, ENSO amplitude is slightly increased. Similar experiments performed with another coupled general circulation model (IPSL-CM4) exhibit the same behaviour as in HadOPA, hence showing the robustness of the results.  相似文献   
7.
The interannual variability of the South Pacific Convergence Zone (SPCZ) and its influence on tropical cyclone (TC) genesis in the South Pacific are investigated using observations and ERA40 reanalysis over the 1979?C2002 period. In austral summer, the SPCZ displays four typical structures at interannual timescales. The first three are characterized by a diagonal orientation of the SPCZ and account for 85% of the summer seasons. One is close to climatology and the other two exhibit a 3° northward or southward departure from the SPCZ climatological position. In contrast, the fourth one, that only encompasses three austral summer seasons (the extreme 1982/1983 and 1997/1998 El Ni?o events and the moderate 1991/1992 El Ni?o event), displays very peculiar behaviour where the SPCZ largely departs from its climatological position and is zonally oriented. Variability of the western/central Pacific equatorial sea surface temperature (SST) is shown to modulate moisture transport south of the equator, thereby strongly constraining the location of the SPCZ. The SPCZ location is also shown to strongly modulate the atmospheric circulation variability in the South Pacific with specific patterns for each class. However, independently of its wide year-to-year excursions, the SPCZ is always collocated with the zero relative vorticity at low levels while the maximum vorticity axis lies 6° to the south of the SPCZ position. This coherent atmospheric organisation in the SPCZ region is shown to constrain tropical cyclogenesis to occur preferentially within 10° south of the SPCZ location as this region combines all the large-scale atmospheric conditions that favour the breeding of TCs. This analysis also reveals that cyclogenesis in the central Pacific (in the vicinity of French Polynesia) only occurs when the SPCZ displays a zonal orientation while this observation was previously attributed to El Ni?o years in general. Different characteristics of El Ni?o Southern Oscillation (ENSO)-related Pacific equatorial warming are shown to impact differently on the SPCZ position, suggesting that for regional scales, such as the South Pacific, the SPCZ classification is more appropriate than a simple ENSO index to characterize TC interannual variability. These findings suggest that forecasting the strength of El Ni?o through SST variations in the eastern Pacific may not be sufficient to accurately predict cyclogenesis in the South Pacific, especially east of the dateline.  相似文献   
8.
This paper evaluates the performances of four cyclogenesis indices against observed tropical cyclone genesis on a global scale over the period 1979–2001. These indices are: the Genesis Potential Index; the Yearly Genesis Parameter; the Modified Yearly Convective Genesis Potential Index; and the Tippett et al. Index (J Clim, 2011), hereafter referred to as TCS. Choosing ERA40, NCEP2, NCEP or JRA25 reanalysis to calculate these indices can yield regional differences but overall does not change the main conclusions arising from this study. By contrast, differences between indices are large and vary depending on the regions and on the timescales considered. All indices except the TCS show an equatorward bias in mean cyclogenesis, especially in the northern hemisphere where this bias can reach 5°. Mean simulated genesis numbers for all indices exhibit large regional discrepancies, which can commonly reach up to ±50%. For the seasonal timescales on which the indices are historically fitted, performances also vary widely in terms of amplitude although in general they all reproduce the cyclogenesis seasonality adequately. At the seasonal scale, the TCS seems to be the best fitted index overall. The most striking feature at interannual scales is the inability of all indices to reproduce the observed cyclogenesis amplitude. The indices also lack the ability to reproduce the general interannual phase variability, but they do, however, acceptably reproduce the phase variability linked to El Ni?o/Southern Oscillation (ENSO)—a major driver of tropical cyclones interannual variations. In terms of cyclogenesis mechanisms that can be inferred from the analysis of the index terms, there are wide variations from one index to another at seasonal and interannual timescales and caution is advised when using these terms from one index only. They do, however, show a very good coherence at ENSO scale thus inspiring confidence in the mechanism interpretations that can be obtained by the use of any index. Finally, part of the gap between the observed and simulated cyclogenesis amplitudes may be attributable to stochastic processes, which cannot be inferred from environmental indices that only represent a potential for cyclogenesis.  相似文献   
9.
A biogeochemical model of the tropical Pacific has been used to assess the impact of interannual variability in a western Pacific iron source on the iron-limited ecosystem of the central and eastern Pacific during the 1997–1998 El Niño. A reference simulation and two simulations with an iron source in the western Pacific have been performed. The two “source” simulations differed only in the temporal variability of the iron source. In the variable source simulation, the iron concentration in the source region was proportional to the velocity of the New Guinea Coastal Undercurrent (NGCUC). In the constant source simulation, the same time-averaged concentration of iron was imposed with no temporal variability. The variable source was designed to mimic variations of iron flux from the northeast slope of New Guinea to the NGCUC due to modulation of sedimentary iron resuspension as previously hypothesized. Through the comparison of these simulations, it appeared that: (i) an iron source in the NGCUC, regardless of its source variability, increases biomass in the eastern equatorial Pacific because of the greater eastward iron flux by the Equatorial Undercurrent and (ii) a variable NGCUC iron source does not change the temporal variability of eastern Pacific chlorophyll, and in particular the timing and intensity of the June 1998 bloom. To explain eastern Pacific biological variability, local rather than remote processes are needed, such as wind-driven upwelling, the local depth of the thermocline, tropical instability waves and biological processes such as high grazing pressure. Therefore, while the western Pacific sources of dissolved iron are important in our model to sustain annually integrated equatorial Pacific production, they are unlikely to strongly constrain the timing of blooms in the central and eastern Pacific such as during the 1998 La Niña.  相似文献   
10.
Dutheil  Cyril  Menkes  C.  Lengaigne  M.  Vialard  J.  Peltier  A.  Bador  M.  Petit  X. 《Climate Dynamics》2021,56(1-2):87-108
Climate Dynamics - Global climate models projections indicate no clear future rainfall changes over the Southwestern Pacific islands in response to anthropogenic forcing. Yet, these models have low...  相似文献   
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