El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Oceanic Indices <s> Abstract The “high index” response of the northeast Pacific westerlies to big positive anomalies of equatorial sea temperature, observed in the winter of 1957–58, has been found to repeat during the major equatorial sea temperature maxima in the winters of 1963–64 and 1965–66. The 1963 positive temperature anomaly started early enough to exert the analogous effect on the atmosphere of the south Indian Ocean during its winter season. The maxima of the sea temperature in the eastern and central equatorial Pacific occur as a result of anomalous weakening of the trade winds of the Southern Hemisphere with inherent weakening of the equatorial upwelling. These anomalies are shown to be closely tied to the “Southern Oscillation” of Sir Gilbert Walker. <s> BIB001 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Oceanic Indices <s> Abstract Surface marine observations, satellite data, and station observations of surface pressure and precipitation are used to describe the evolution of sea surface temperature (SST) anomalies, surface wind fields, and precipitation anomaly patterns during major warm episodes in the eastern and central tropical Pacific. The sequence of events is described in terms of composite SST and wind fields (30°N–30°S) for six warm episodes since 1949, and time series and cross-spectral analyses of mean monthly data along six shipping lanes which cross the equator between the South American coast and 170°W. During the months preceding a warm episode, the equatorial easterlies are stronger than normal west of the dateline. This and other coherent and strongly developed anomaly patterns over the western equatorial Pacific and South Pacific are associated with a South Pacific Convergence Zone (SPCZ) located southwest of its normal position. During October-November prior to El Nino, the equatorial easterly anomalies i... <s> BIB002 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> Oceanic Indices <s> Abstract El Nino–Southern Oscillation (ENSO) is a natural, coupled atmospheric–oceanic cycle that occurs in the tropical Pacific Ocean on an approximate timescale of 2–7 yr. ENSO events have been shown in previous studies to be related to regional extremes in weather (e.g., hurricane occurrences, frequency and severity of tornadoes, droughts, and floods). The teleconnection of ENSO events to extreme weather events means that the ability to classify an event as El Nino or La Nina is of interest in scientific and other applications. ENSO is most often classified using indices that indicate the warmth and coolness of equatorial tropical Pacific Ocean sea surface temperatures (SSTs). Another commonly used index is based on sea level pressure differences measured across the tropical Pacific Ocean. More recently, other indices have been proposed and have been shown to be effective in describing ENSO events. There is currently no consensus within the scientific community as to which of many indices best captures... <s> BIB003
Because ENSO is very much a phenomenon associated with ocean-atmosphere interaction, a logical parameter for monitoring its behaviour is sea surface temperature (SST) as initially recognised by Bjerknes BIB001 and later fully explored by Rasmussen and Carpenter BIB002 . The three main oceanic indices used are based on SST anomalies for a number of ocean regions distributed along the equator. These are named after numbered shipping routes as Niño 1, 2, 3 and 4 because a vast array of ships crossing the Pacific for operational reasons recorded SST over a number of decades ( Figure 5 ). Based on studies of SST variability in relation to ENSO events, Niño 1 and 2 were combined into region Niño 1+2; a new region, Niño 3.4 that straddles Niño 3 and 4, is now used. Table 2 ). Sourced and redrawn from Barnston  https://www.climate.gov/news-features/blogs/enso/why-are-there-so-manyenso-indexes-instead-just-one Niño 1+2 is the smallest Niño region. It sits directly off the coast of South America and tends to have the largest variation in SST when compared with the other Niño SST regions ( Figure  5 ). Initially Niño 3 was favoured as the key region for observing and forecasting El Niño; however, it was realised that in terms of critical ENSO related ocean-atmosphere interactions, an area further to the west, Niño 3. Figure 5 ) and is infrequently used in ENSO analyses. In addition to the Niño regionally-based based oceanic indices, the Trans-Niño Index (TNI) was developed by Trenberth and Stepaniak  , who suggest that the TNI be used in tandem with the Niño 3.4 index. The TNI is defined as difference in the standardised SST anomalies between Niño 1+ 2 and Niño 4 regions. The physical justification is that it captures the SST gradient between the central and eastern Pacific and thus may be useful for identifying El Niño Modoki events, as arise when the central to eastern Pacific SST gradient is step, for example sizeable positive (negative) SST anomalies in Niño 4 (Niño 1 + 2.) regions. However, as noted by Hanley et al. BIB003 , the TNI has non-consistent lag correlations with the Niño 3.4 index related to the transition of the Pacific Ocean from a cool to warm PDO phases in the mid-1970s. Accordingly, Hanley et al. (2003) do not include the TNI as an index for identifying individual events and comparison of ENSO years.