Evolution of El Niño–Southern Oscillation and global atmospheric surface temperatures

Abstract

The origins of the delayed increases in global surface temperature accompanying El Niño events and the implications for the role of diabatic processes in El Niño–Southern Oscillation (ENSO) are explored. The evolution of global mean surface temperatures, zonal means and fields of sea surface temperatures, land surface temperatures, precipitation, outgoing longwave radiation, vertically integrated diabatic heating and divergence of atmospheric energy transports, and ocean heat content in the Pacific is documented using correlation and regression analysis. For 1950–1998, ENSO linearly accounts for 0.06°C of global surface temperature increase. Warming events peak 3 months after SSTs in the Niño 3.4 region, somewhat less than is found in previous studies. Warming at the surface progressively extends to about ±30° latitude with lags of several months. While the development of ocean heat content anomalies resembles that of the delayed oscillator paradigm, the damping of anomalies through heat fluxes into the atmosphere introduces a substantial diabatic component to the discharge and recharge of the ocean heat content. However, most of the delayed warming outside of the tropical Pacific comes from persistent changes in atmospheric circulation forced from the tropical Pacific. A major part of the ocean heat loss to the atmosphere is through evaporation and thus is realized in the atmosphere as latent heating in precipitation, which drives teleconnections. Reduced precipitation and increased solar radiation in Australia, Southeast Asia, parts of Africa, and northern South America contribute to surface warming that peaks several months after the El Niño event. Teleconnections contribute to the extensive warming over Alaska and western Canada through a deeper Aleutian low and stronger southerly flow into these regions 0–12 months later. The 1976/1977 climate shift and the effects of two major volcanic eruptions in the past 2 decades are reflected in different evolution of ENSO events. At the surface, for 1979–1998 the warming in the central equatorial Pacific develops from the west and progresses eastward, while for 1950–1978 the anomalous warming begins along the coast of South America and spreads westward. The eastern Pacific south of the equator warms 4–8 months later for 1979–1998 but cools from 1950 to 1978.

Unfolding the relation between global temperature and ENSO

“…Thus, initially the role of El Nino is to support the positive tendency. Note that the contribution of ENSO to the long-term temperature trend in the last 50 years is about 0.06°C or 10% of the overall trend [Trenberth et al, 2002]. However, as Figure 2a indicates, after a time scale of the order of 16 (5 + 11) months, El Nino's role is to reverse that tendency.…”

“…Thus, Figure 1 displays the well-known result that El Nino tends to increase the average temperature of the planet and that in general a La Nina succeeds an El Nino. Mechanisms for this warmth can be traced to changes in cloudiness and atmospheric circulations [Trenberth et al, 2002;Klein et al, 1999;Kumar and Hoerling, 2003]. Next, we present results that add another piece to the puzzle of the relationship between ENSO and global temperature.…”

Evidence for the accelerations of sea level on multi‐decade and century timescales

“…Changes in the frequency of El Niño events are known to have occurred during the 20th century, most notably and recently during the 'climate shift' around 1976 related to a phase change of the Pacific Decadal Oscillation (PDO) (Trenberth et al, 2002). Indeed, Church et al (2004) argued that this shift is responsible for the minimum rates of sea-level rise in the western Pacific and eastern Indian oceans in the latter half of the 20th century and the regional decelerations over this period.…”

“…A property of the CW06 method is that it can produce near-global maps of sea-level trends and accelerations, even for regions with no tide gauge data, by relying on the spatial interpolations between the available data provided by the EOFs. Trenberth et al, 2002 and see below) and a minimum in the rate of sea-level rise in the western Pacific/eastern Indian Ocean in the latter half of the 20th century (Church et al, 2004). A smaller region of negative coefficients can be seen in the northern North Atlantic.…”

La Niña Modoki impacts Australia autumn rainfall variability

“…The entity including its opposite phase, La Niña Modoki, is referred to as ENSO Modoki. Similar but not identical terms describing departures from conventional ENSO include ''Dateline ENSO'' [Larkin and Harrison, 2005], or ''Trans Niño'' oscillation [Trenberth et al, 2002]. During an El Niño Modoki event, there are two anomalous Walker Circulation cells in the troposphere, instead of the single-celled pattern of the conventional El Niño.…”