PP51A-01 INVITED
Large Scale Dynamics and High Frequency Forcing of El Niño/Southern Oscillation Variability
Year-to-year variability associated with the El Niño/Southern Oscillation (ENSO) is governed by large-scale ocean dynamics and coupled ocean-atmosphere interactions. However, the cycle between warm and cold phase ENSO conditions exhibits considerable irregularity in terms of amplitude, duration, and spatial and temporal patterns of development. One factor contributing to this irregularity is stochastic forcing in the form of weather noise, a prominent source of which is the Madden-Julian Oscillation (MJO). A simple two-predictor regression model is developed to estimate the relative influence of large-scale low frequency deterministic ocean- atmosphere dynamics and stochastic forcing on peak sea surface temperature (SST) anomalies associated with ENSO for the period 1980-2005. One predictor is equatorial warm water volume, which is an index for the role that upper ocean heat content plays in regulating ENSO variability. The other predictor characterizes stochastic forcing in the western Pacific in the form of an MJO activity index. The two-predictor model accounts for about 65% of peak Nino3.4 SST anomaly variance at 2-3 season lead times and suggests about equal influence (on average) of stochastic and deterministic processes affecting peak ENSO SST anomalies over the past 25 years. The implications of these results for ENSO prediction are discussed.
PP51A-02 INVITED
Historical Chronology of ENSO Events Based Upon Documentary Data From South America: Strengths and Limitations
The first reconstructions of past El Niño occurrences were proposed by W. Quinn twenty years ago. They were based on documentary evidence of anomalous rainfall episodes, destructive floods and other possible impacts of El Niño conditions in Peru and other South-American countries. It has been shown, later, that the El Niño chronological sequence covering the last four and a half centuries produced by Quinn needed a thorough revision since many so-called EN events had not occurred while some others had been overlooked. Beside the classical methodological problems met in historical climatology studies (reliability of data, confidence in the sources, primary and secondary information), the reconstruction of former EN events faces specific difficulties dealing with the significance of the indicators and their spatial location. For instance, strong precipitation anomalies during summer in Southern Ecuador and northern Peru and precipitation excess recorded in the preceding winter in central Chile constitute quite reliable proxies of El Niño conditions, in modern times. However this observed teleconnection pattern, which is useful to reinforce the interpretation of past EN occurrences, seems to have been inoperative before the early nineteenth century. It is interpreted that atmospheric circulation features during the Little Ice Age interfered with the teleconnection system linking the EN impacts in northern Peru and central Chile. As a consequence, how should be evaluated the significance of documented winter precipitation excess in central Chile in years during which there is drought evidence in northern Peru, during the sixteenth to eighteenth century? And vice versa, are former evidences for precipitation excess in northern Peru (prior to the nineteenth century) quite reliable indicators for EN conditions, even if the preceding winter was dry in the Valparaiso-Santiago region? Other specific problems met in the building-up of a consolidated EN chronological sequence deal with the non-homogeneity of the documentary records, the reconstruction of the paleo-events strength and the difficulty to cross check documentary data with other annually resolved records (corals, tree-rings and ice-core data). In spite of all these limitations, the documentation of former EN occurrences based upon written archives from colonial South-America remains an unavoidable and powerful tool to reconstruct a solid El Niño chronology for the last half millennium and to better understand the interplay of the different modes (at the interannual, decadal, multidecadal and centennial scales) of climate variations in this part of the world.
PP51A-03 INVITED
El Nino Studies In Mexico
Since El Niño was found to be an important modulator of interannual climate variability in the tropics and subtropics, numerous studies have documented its signal in various meteorological parameters and various regions of the world. Mexico has also been part of this effort to determine to what extent El Niño may be responsible for some of the climatic disasters in recent years. The analysis of El Niño signal in Mexico has been divided in two seasons: winter and summer. During winter, El Niño triggers the Pacific North American (PNA) pattern that modifies the mean atmospheric circulation. This anomalous mean flow leads to more frequent midlatitude systems reaching low latitudes. During El Niño, some of these extratropical cyclones result in an enhanced number of the so-called Nortes and consequently in more extreme events. During summer, El Niño affects the seas surface temperatures of the warm pools surrounding Mexico and Central America. The result of warmers than normal SSTs in the north eastern Pacific is a weaker temperature gradient between the cold tongue off Peru and the warm pool of the Pacific coast of Mexico. This causes the ITCZ to remain closer to the equator and enhanced subsidence over most of the country. Such combination of circulation anomalies translates in drought over most of Mexico during summer. However, El Niño may have effects on the Caribbean Sea climatic conditions and consequently in Mexico´s climate, since easterly waves and tropical cyclone activity may be modified. The potential mechanisms of the El Niño related processes that may lead to climate anomalies over Mexico are discussed, including the roles of quasi-stationary circulations, subsidence and transient activity in observations and models.
PP51A-04 INVITED
Variability of Extreme Precipitation Events in Tijuana, Mexico During ENSO Years
We present the variability of daily precipitation extremes (top 10 percecnt) in Tijuana, Mexico during 1950-2000. Interannual rainfall variability is significantly modulated by El Nino/Southern Oscillation. The interannual precipitation variability exhibits a large change with a relatively wet period and more variability during 1976- 2000. The wettest years and the largest frequency of daily extremes occurred after 1976-1977, with 6 out of 8 wet years characterized by El Nino episodes and 2 by neutral conditions. However, more than half of the daily extremes during 1950-2000 occurred in non-ENSO years, evidencing that neutral conditions also contribute significantly to extreme climatic variability in the region. Extreme events that occur in neutral (strong El Nino) conditions are associated with a pineapple express and a neutral PNA (negative TNH) teleconnection pattern that links an anomalous tropical convective forcing west (east) of the date line with a strong subtropical jet over the study area. At regional scale, both types of extremes are characterized by a trough in the subtropical jet over California/Baja California, which is further intensified by thermal interaction with an anomalous warm California Current off Baja California, low-level moisture advection from the subtropical warm sea-surface region, intense convective activity over the study area and extreme rainfall from southern California to Baja California.
PP51A-05 INVITED
Tropical Cyclogenesis and Motion in the Eastern North Pacific
The East Pacific, north of 10oN latitude and adjacent to the west coast of Mexico, experiences one of the highest rates of tropical cyclone formation in the world. These storms tend to develop rapidly and while most of them are relatively weak (category 3 or below), they pose a significant hazard in the coastal regions of Mexico and to shipping. Also, the rapid development requires an ability to forecast the genesis of the cyclones in addition to the track and intensity, in order to provide adequate lead times for warnings. Our current understanding states that easterly waves that develop over Africa and propagate into the region serve as the seedlings of the East Pacific tropical cyclones. There are a number of open questions such as the way the waves remain coherent in the presence of Rossby dispersion. Also, the mechanism through which the waves intensify into tropical cyclones in the preferred location must be identified. One possibility is that a modification of the waves by the orography of Mexico and Central America, along with contributions from the Inter-tropical Convergence Zone provides the mesoscale circulations that develop into tropical cyclones. This talk will describe mechanisms for generating the initial circulations that develop into tropical cyclones in the East Pacific, along with a discussion of the influence of the land mass on the track of the mature storms. The influence of the East Pacific tropical cyclones on the summer climate of North America will also be discussed. These storms act as sources of moisture for thunderstorm outbreaks in the low deserts of Arizona through a mechanism known as the Gulf Surge.
PP51A-06 INVITED
Paleo-Hurricanes in the North Eastern Pacific (NEP): A Proposal for the Reconstruction and Analysis Based on Proxy Records Covering the Last Millennium
Hurricanes are one of the most destructive natural disasters, causing extensive damage to coastal regions such as the North Eastern Pacific (NEP) each year. Although the prediction of hurricanes has improved greatly in recent years, our understanding of the natural range of hurricane characteristics such as severity and frequency, and the atmospheric controls on hurricanes, is incomplete, in part because of the limited length of the instrumental record. This aspect is especially critical in the NEP where no more than 40 years of reliable records are available. However, knowledge about NEP hurricanes can be greatly augmented with paleoclimatic proxy data from both local and large-scale processes (ENSO, PDO, AMO, etc.). A variety of sources of paleoclimatic proxy data have been used to investigate paleo-hurricanes. One of the most promising sources of paleo-hurricanes data appears to be archived in historical documents. The Spanish Colonial Archives represent a wealth of information with records going back to the late 15th century. Chronicles of Aztec history contain numerous references of climatological disasters as droughts and famines. Other sources of high resolution (annual to monthly) paleo- hurricanes data may be obtained from tree rings and caves. In the NEP changes in isotopic signature can be used to document hurricane events. Rainfall from hurricanes has been found to have a distinct oxygen and hydrogen isotopic signature which can be detected in tree ring, cave, and ostracoda carapaces chemistry. In addition, sources of lower resolution at long time scales are lake and marsh sediments which provide chronologies of the most severe NEP hurricane events. On the other hand, a hypothesis about a possible teleconnection between PDO and NEP hurricanes could be tested with proxy information. A better understanding of the low frequency characteristics of NEP hurricanes and what influences low- frequency changes must come from the development and interpretation of paleo-hurricane records. This proposed project focuses on the reconstruction of paleo-hurricanes. This project addresses hurricanes, forcing factors, teleconnections, reconstructions of hurricane events from historical documents, hurricane chronologies from lake sediments, the potential for documenting hurricane events from tree rings, caves, and lakes and the practical applications of this work. One of the main results of this project will be an international and interdisciplinary research team where the possibility of sharing experiences, plans, procedures and information on this subject will be promoted.