Strong ENSO Research Articles

Stable isotope paleoenvironmental record of a coral from Cabo Pulmo, entrance to the Gulf of california, mexico

A 17-year-long record of δ18O and δ13C from a Cabo Pulmo coral (Pavona gigantea) was analyzed. Although the coral δ18O signal is mainly temperature controlled (~70%), the seasonal arrival of surface water masses to the Gulf of California produces an isotopic effect in the opposite direction to that produced by temperature. During summer and fall, the arrival of Equatorial water effectively reduces the annual range in the δ18O of the coral skeletons by increasing the δ18O value of water, even though this occurs during the rainy season. The incursion of Equatorial water masses to the entrance of the Gulf of California during El Niño years is so strong that even during the strongest ENSO years there is no important isotopic shift in the skeleton, even though temperature records show significant positive temperature anomaly in the region. The same effect occurs for La Niña years, but in the opposite direction. By deconvoluting the temperature signal from the isotopic record, the seasonal interplay of the water masses from the California Current and the Costa Rica Coastal Current can be effectively deduced. Changes in the coral δ13C seem to coincide with periods of upwelling events in the gulf, with the highest value coinciding with the annual maximum in pigment concentration for the region.

Changes of Probability Associated with El Niño

Away from the tropical Pacific Ocean, an ENSO event is associated with relatively minor changes of the probability distributions of atmospheric variables. It is nonetheless important to estimate the changes accurately for each ENSO event, because even small changes of means and variances can imply large changes of the likelihood of extreme values. The mean signals are not strictly symmetric with respect to El Niño and La Niña. They also depend upon the unique aspects of the SST anomaly patterns for each event. As for changes of variance and higher moments, little is known at present. This is a concern especially for precipitation, whose distribution is strongly skewed in areas of mean tropospheric descent. These issues are examined here in observations and GCM simulations of the northern winter (January–March, JFM). For the observational analysis, the 42-yr (1958–99) reanalysis data generated at NCEP are stratified into neutral, El Niño, and La Niña winters. The GCM analysis is based on NCEP atmospheric GCM runs made with prescribed seasonally evolving SSTs for neutral, warm, and cold ENSO conditions. A large number (180) of seasonal integrations, differing only in initial atmospheric states, are made each for observed climatological mean JFM SSTs, the SSTs for an observed warm event (JFM 1987), and the SSTs for an observed cold event (JFM 1989). With such a large ensemble, the changes of probability even in regions not usually associated with strong ENSO signals are ascertained. The results suggest a substantial asymmetry in the remote response to El Niño and La Niña, not only in the mean but also the variability. In general the remote seasonal mean geopotential height response in the El Niño experiment is stronger, but also more variable, than in the La Niña experiment. One implication of this result is that seasonal extratropical anomalies may not necessarily be more predictable during El Niño than La Niña. The stronger seasonal extratropical variability during El Niño is suggested to arise partly in response to stronger variability of rainfall over the central equatorial Pacific Ocean. The changes of extratropical variability in these experiments are large enough to affect substantially the risks of extreme seasonal anomalies in many regions. These and other results confirm that the remote impacts of individual tropical ENSO events can deviate substantially from historical composite El Niño and La Niña signals. They also highlight the necessity of generating much larger GCM ensembles than has traditionally been done to estimate reliably the changes to the full probability distribution, and especially the altered risks of extreme anomalies, during those events.

Unusually low airglow intensities in the Southern Hemisphere midlatitude mesopause region

During the first part of a new set of airglow observations in the OH (6-2) and the O2 (0-1) Atmospheric bands from El Leoncito, Argentina, that started in August 1997, periods of surprisingly low intensities stand out. For many nights, mean O2 intensities reach only 30%, or less, of what we have seen in previous campaigns, from 1984 to 1994, at the same site, or a similar latitude. Such an anomaly is also observed, to a lesser degree, in OH intensities. An instrumental artifact in the intensity results can be ruled out by using the signal from the stellar background for a “field calibration”. A relation to the strong ENSO event with its major climate impact in 1997/8, may, of course, be suspected, although a statistical inference would be weakened by the scarceness of historical precedents of a similar magnitude.

1998年に発生した陸上生態系からの大量の炭素放出

The relationship between the carbon balance of terrestrial ecosystems and climate anomalies in 1998, the warmest year since 1860, was investigated by means of a mechanistic global biosphere model (Sim-CYCLE) based on the plant-production theory.We simulated the monthly time-series and spatial distribution of atmosphere-biosphere CO2 exchange from 1961 to 1998, based on the Sim-CYCLE by inputting the NCEP/NCAR-reanalysis climate data at the spatial resolution of T62 (containing 5828 land pixels after the Matthews biome map). The peculiar climate conditions in 1998, i. e., strong ENSO event and global warming (+0.58°C above the long-term mean), resulted in a remarkable anomaly (as much as 2.7PgC yr-1 of net emission) via the accelerated plant respiration and soil decomposition. The strong sources were in eastern Siberia, northern South America, and South Africa. This anomaly, equivalent to +1.26ppmv yr-1 of an increase rate of atmospheric CO2 concentration, can be a major cause of the extraordinarily high CO2 increase rate in 1998.

ENSO Influence on Intraseasonal Extreme Rainfall and Temperature Frequencies in the Contiguous United States: Implications for Long-Range Predictability

Potential ENSO-related predictability of wintertime daily extreme precipitation and temperature frequencies is investigated. This is done empirically using six decades of daily data at 168 stations distributed over the contiguous United States. ENSO sensitivity in the extreme ranges of intraseasonal precipitation and temperature probability density functions is demonstrated via a compositing technique. Potential predictability of extremes is then investigated with a simple statistical model. Given a perfect forecast of ENSO, the frequency of intraseasonal extremes is specified as the average frequency of occurrence during similar-phased ENSO winters on record. Specification skill is assessed as the cross-validated proportion of local variance explained by this method. The skill depends on varying ENSO sensitivity in different geographic regions and quantile ranges and on consistency or variability from one like-phased ENSO event to another. ENSO sensitivity also varies according to the intensity of the tropical forcing; however, not always in the expected sense. Good predictability is likely for variables and in regions displaying a strong and consistent ENSO signal. This is found in some coherent regions of the United States for various combinations of frequency variable and ENSO phase. ENSO-based predictability of heavy and extreme precipitation frequency is potentially good along the Gulf Coast, central plains, Southwest, and in the Ohio River

Global oceanic climate anomalies in 1980’s

The climate in the 1980’s is characterized by the appearance of two strong ENSO events and by the warmest decade in global mean temperature. Whether there is a linkage between ENSO and global warming ? This paper shows the climate anomaly patterns over the global ocean in the 1980’s and their comparison with that of ENSO composite mode and that simulated by 2 × CO2, indicating the role of super ENSO in the establishment of new climate regime in thel980’s.

The 1997 ENSO event and implication for North American Laurentian Great Lakes winter severity and ice cover

Mild winters and below‐average annual maximum ice cover for the Laurentian Great Lakes occur during strong warm ENSO events. During the six strongest warm ENSO events since 1950 a Great Lakes winter severity temperature index and simulated annual maximum ice cover averaged 1.2°C above and 15% below a 1950–1994 average, respectively. The observed differences between the average of the strong ENSO event years and the base period average are statistically significant.

Vertical velocity in the central tropical Pacific: a circulation model perspective for JGOFS

Results from a variety of experiments with a primitive equation ocean general circulation model are examined to provide insight about variations in vertical velocity ( W) that may have occurred during the JGOFS Equatorial Pacific field program. Measurements of W during this period are not available, so a model offers the only plausible, dynamically consistent way to obtain this information. Variability associated with four different aspects of the tropical Pacific circulation is discussed: the seasonal average; the tropical instability waves (TIW); the interannual variability in strong and modest ENSO events; and the Kelvin response associated with remote forcing from westerly wind events in the western Pacific. Results from 140°W are presented. According to the model, TIW provide the greatest source of W variability. Near and somewhat north of the equator W can range from about 1500 cm day −1 upward to 1000 cm day −1 downward over a typical 30-day period, with significant structure in both the zonal and meridional directions. Remotely forced response can be very large on the day-to-day timescale, but diminishes typically to, at most, a few hundred cm day −1 up or down when averaged over 10 days. Interannual variation associated with a major ENSO event can involve complete disruption of the seasonal cycle, the disappearance of TIWs and great diminution or disappearance of the normal equatorial upwelling. The seasonal cycle variations are of the smallest amplitude; the maximum variation is perhaps 100 cm day −1 near the equator and somewhat more just north of the equatorial waveguide in the downwelling part of the seasonal circulation. The field program period was studied using the different available wind analyses to force the model over the 1991–1992 period. Because the wind analyses have substantial differences, they force substantially different W fields in the ocean model. Unfortunately, no wind analysis can be shown to be substantially superior to any other during this period. Thus, estimates of the net vertical transport of any quantity of JGOFS interest must take into account the wide range of possible W values.

Regional Radiocarbon Effect Due to Thawing of Frozen Earth

Accelerator mass spectrometry (AMS) measurement of 25 single-year tree rings from AD 1861–1885 at ca. ±3.5‰ precision shows no evidence of an anomalous 11-yr cycle of 14C near the Arctic Circle in the Mackenzie River area. However, the Δ14C measurements are lower on average by 2.7 ± 0.9 (ō)‰ relative to 14C measurements on tree rings from the Pacific Northwest (Stuiver and Braziunas 1993). We attribute this depression of Δ14C to thawing of the ice and snow cover followed by melting of frozen earth that releases trapped 14C-depleted CO2 to the atmosphere during the short growing season from May through August. Correlation of Δ14C with May–August estimated temperatures yields a correlation index of r = 0.60. The reduction in Δ14C is dominated by seven years of anomalous depletion. These years are 1861, 1867–1869, 1879–1880 and 1883. The years 1867–1869 are coincident with a very strong ENSO event.

State of coral reefs in the Galápagos Islands: Natural vs anthropogenic impacts

Before the 1982–1983 El Niño disturbance event low diversity coral communities and small, actively accreting coral reefs were present on Galápagos shallow shelves in areas protected from strong upwelling. Prolonged El Niño 1982–1983 sea warming resulted in 95–99% coral mortality, virtually eliminating corals throughout the archipelago. The population size of an ubiquitous, large sea urchin species ( Eucidaris thouarsii) was unaffected by the warming event. Urchins later showed increased abundance on dead coral colonies and frameworks, and caused bioerosion that exceeded the net calcification capacity of disturbed reefs. Known human impacts on corals result mainly from anchor damage, the collection of corals for sale as curios, and mechanical damage resulting from the activities of fishermen. An evaluation of natural and anthropogenic damage to Galápagos coral communities indicates that natural disturbances, especially strong ENSO episodes, are of greatest concern. However, rapidly increasing human pressures could exacerbate the recovery capacity of naturally perturbed coral communities.

Variability in latent heat flux over the tropical pacific in association with recent two ENSO events

This paper analyzed the variations of latent heat flux (LHF) over the tropical Pacific in the period 1978-1988 by using COADS (Comprehensive Ocean and Atmospheric Data Set). It has been founded that the interannual variability of LHF exhibits strong ENSO signal, with the significant increasing LHF during the recent two warm events, i.e., 1982 / 83 and 1986 / 87 and decreasing LHF in the cold episodes. However the longitudinal distribution of the LHF departures varies from event to event. In the eastern Pacific, the specific humidity difference at air-sea interface(q s - q a ) makes a dominant contribution to the interannual variability of LHF ( r = 0.73 ), while in the western Pacific the surface wind speed,W and theq s - q a make nearly equal contribution to that of LHF.

Tropical-Extratropical Teleconnections during the Northern Hemisphere Winter. Part II: Relationships between Monthly Mean Northern Hemisphere Circulation Patterns and Proxies for Tropical Convection

Abstract Monthly mean teleconnections during the northern winter between proxies for tropical heating (OLR and SST data) and Northern Hemisphere 700 mb circulation patterns (PNA, TNH, and WPO) are examined, principally with correlation analysis. In particular it is found that positive projections on all three patterns are highly probable during certain strong ENSO winters but the means to predict their relative strengths was not discovered, although the absolute strength of the TNH pattern is directly related to SST anomalies in the central Pacific. Other ENSO winters also have a tendency for positive PNA and WPO projections, but for a negative TNH projection. For other winters the importance of an area north of the equator and 25 degrees to the west of the date line is confirmed as a probable source region for the PNA pattern. Another area about 25 degrees to the east of the date line is singled out as a possible tropical response to the PNA pattern. Implications for current and future GCM experiments an...