El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Blended Indices <s> Abstract This note documents two indices of the Southern Oscillation based on sea-surface temperature (SST) and rainfall, and compares their statistics with those of two pressure indices. The indices are almost equivalent on an annual basis, and the SST and rainfall indices may be used with monthly time resolution without smoothing. <s> BIB001 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Blended Indices <s> Many research workers require indices of the Southern Oscillation that are continuous for a long period, homogeneous and easily updated. This paper aims to provide such indices. The method used is to compare indices based on sea-surface temperature (SST), rainfall and pressure, and thereby to make some corrections to improve homogeneity. To fill data gaps, substitutions are proposed based on regressions between indices over 1950–1979. Tabulations are presented of the recommended long-period indices: SST monthly 1872–1985; rainfall monthly 1893–1983; and pressure seasonal 1851–1984. The most extreme years according to the two monthly indices are listed. <s> BIB002 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Blended Indices <s> The effect of the El Nino–Southern Oscillation (ENSO) on the probability distribution of daily surface air temperature over the Pacific–North American sector is investigated using the US National Centers for Environmental Prediction (NCEP) Reanalysis data for 1959–1998. The El Nino response is characterized by reduced intraseasonal variance over most of the US, western Canada and the Gulf of Alaska. Conversely, there is an increase of variance during La Nina over the US and the west coast, northward to Alaska. The sign of this response is consistent for most individual El Nino/La Nina years in regions with a strong signal. The response is also robust with respect to differing definitions of ENSO or choice of dataset. Finally, a similar response is evident in station data for an earlier period. The change of variance is associated both with altered skewness, and a change in high and low extremes. Extremes of both signs are reduced during El Nino, and are slightly increased during La Nina. These results are consistent with other studies, suggesting an increased incidence of blocking along the west coast of North America during El Nino, leading to less storm activity and less incursions of warm and cold air over the eastern US. While an understanding of the changed variance is important in itself, it also has implications for changes in exceedence statistics (e.g. heating degree days) and the occurrence of extreme values. Copyright © 2000 Royal Meteorological Society <s> BIB003 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Blended Indices <s> Abstract. Column Ozone measured in tropical latitudes from Nimbus 7 total ozone mapping spectrometer (TOMS), Earth Probe TOMS, solar backscatter ultraviolet (SBUV), and Aura ozone monitoring instrument (OMI) are used to derive an El Nino-Southern Oscillation (ENSO) index. This index, which covers a time period from 1979 to the present, is defined as the "Ozone ENSO Index" (OEI) and is the first developed from atmospheric trace gas measurements. The OEI is constructed by first averaging monthly mean column ozone over two broad regions in the western and eastern Pacific and then taking their difference. This differencing yields a self-calibrating ENSO index which is independent of individual instrument calibration offsets and drifts in measurements over the long record. The combined Aura OMI and MLS ozone data confirm that zonal variability in total column ozone in the tropics caused by ENSO events lies almost entirely in the troposphere. As a result, the OEI can be derived directly from total column ozone instead of tropospheric column ozone. For clear-sky ozone measurements a +1 K change in Nino 3.4 index corresponds to +2.9 Dobson Unit (DU) change in the OEI, while a +1 hPa change in SOI coincides with a −1.7 DU change in the OEI. For ozone measurements under all cloud conditions these numbers are +2.4 DU and −1.4 DU, respectively. As an ENSO index based upon ozone, it is potentially useful in evaluating climate models predicting long term changes in ozone and other trace gases. <s> BIB004 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Blended Indices <s> El Nino/Southern Oscillation (ENSO) remains the most important coupled ocean–atmosphere phenomenon to cause global climate variability on seasonal to interannual time scales. This paper addresses the need for a reliable ENSO index that allows for the historical definition of ENSO events in the instrumental record back to 1871. The Multivariate ENSO Index (MEI) was originally defined as the first seasonally varying principal component of six atmosphere–ocean (COADS) variable fields in the tropical Pacific basin. It provides for a more complete and flexible description of the ENSO phenomenon than single variable ENSO indices such as the SOI or Nino 3.4 SST. Here we describe our effort to boil the MEI concept down to its most essential components (based on SLP, SST) to enable historical analyses that more than double its period of record to 1871–2005. The new MEI.ext confirms that ENSO activity went through a lull in the early- to mid-20th century, but was just about as prevalent one century ago as in recent decades. We diagnose strong relationships between peak amplitudes of ENSO events and their duration, as well as between their peak amplitudes and their spacing (periodicity). Our effort is designed to help with the assessment of ENSO conditions through as long a record as possible to be able to differentiate between ‘natural’ ENSO behaviour in all its rich facets, and the ‘Brave New World’ of this phenomenon under evolving GHG-related climate conditions. So far, none of the behaviour of recent ENSO events appears unprecedented, including duration, onset timing, and spacing in the last few decades compared to a full century before then. Copyright © 2011 Royal Meteorological Society <s> BIB005 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Blended Indices <s> Analysis of the first 26 months of data from the Aquarius satellite confirms the existence of a sharp sea surface salinity (SSS) front along the equator in the western equatorial Pacific. Following several earlier studies, we use the longitudinal location of the 34.8-psu isohaline as an index, termed Niño-S34.8, to measure the zonal displacement of the SSS front and consequently the eastern edge of the western Pacific warm pool. The on-going collection of the Array for Real-time Geostrophic Oceanography (ARGO) program data shows high correlations between Niño-S34.8 and the existing indices of El Niño, suggesting its potential important role in ENSO evolution. Further analysis of the ARGO data reveals that SSS variability in the southeastern tropical Pacific is crucial to identify the type of El Niño. A new SSS index, termed the southeastern Pacific SSS index (SEPSI), is defined based on the SSS variability in the region (0°–10°S, 150°–90°W). The SEPSI is highly correlated with the El Niño Modoki index, as well as the Trans-Niño index, introduced by previous studies. It has large positive anomalies during central Pacific El Niño or El Niño Modoki events, as a result of enhanced zonal sea surface temperature gradients between the central and eastern tropical Pacific, and can be used to characterize the type of El Niño. The processes that possibly control these SSS indices are also discussed. <s> BIB006
Blended indices are a combination of a number of single variables with the blending technique achieved through a variety of methods. The justification for blended indices is that ENSO is a multivariate phenomenon [26] , thus any index should be comprised of more than one variable. Most commonly, blended indices are used to diagnose ENSO events. While researchers recognised the physical complexity of ENSO events and the challenges associated with diagnosing them BIB001 BIB002 it wasn't until the early 21 st century that attempts to produce blended indices first appeared in the literature. For example, a Bivariate ENSO Timeseries (BEST) was produced by Smith and Sardeshmukh BIB003 by combining the SOI and Niño 3.4 SST, while Allan  BIB005 were able to detect strong associations between ENSO amplitude and duration plus amplitude and periodicity using the MEI.ext. Although not strictly blended in nature, a number of alternative ENSO indices based on variables other than the more traditional ones (e.g. SLP, SST, wind and OLR) have emerged recently including an ozone-based ENSO index BIB004 , an atmospheric electrical index  , and an ENSO salinity index BIB006 .