Shineng Hu

Exceptionally strong easterly wind burst stalling El Niño of 2014

Significance The El Niño–Southern Oscillation is the dominant mode of climate variability in the tropical Pacific, with pronounced global teleconnections. Predicting El Niño and understanding its progression still present a challenge to climate scientists. At the beginning of 2014, many in the scientific community anticipated that a strong El Niño could potentially develop by year end, but the observed warm event barely reached the El Niño threshold. Here, we analyze satellite-based data and numerical simulations to show that an intense easterly wind burst that occurred in June of 2014 was the key dynamical factor that stalled the El Niño development. We discuss our findings in the context of limited El Niño predictability and explore links between intraseasonal wind bursts and decadal climate change. Intraseasonal wind bursts in the tropical Pacific are believed to affect the evolution and diversity of El Niño events. In particular, the occurrence of two strong westerly wind bursts (WWBs) in early 2014 apparently pushed the ocean–atmosphere system toward a moderate to strong El Niño—potentially an extreme event according to some climate models. However, the event’s progression quickly stalled, and the warming remained very weak throughout the year. Here, we find that the occurrence of an unusually strong basin-wide easterly wind burst (EWB) in June was a key factor that impeded the El Niño development. It was shortly after this EWB that all major Niño indices fell rapidly to near-normal values; a modest growth resumed only later in the year. The easterly burst and the weakness of subsequent WWBs resulted in the persistence of two separate warming centers in the central and eastern equatorial Pacific, suppressing the positive Bjerknes feedback critical for El Niño. Experiments with a climate model with superimposed wind bursts support these conclusions, pointing to inherent limits in El Niño predictability. Furthermore, we show that the spatial structure of the easterly burst matches that of the observed decadal trend in wind stress in the tropical Pacific, suggesting potential links between intraseasonal wind bursts and decadal climate variations.

The impact of westerly wind bursts on the diversity and predictability of El Niño events: An ocean energetics perspective

In this study, we apply ocean energetics as a diagnostic tool to investigate the impact of westerly wind bursts (WWBs) on the evolution, diversity, and predictability of El Nino events. Following Fedorov et al. (2014), we add an observed WWB to simulations within a comprehensive coupled model and explore changes in the available potential energy (APE) of the tropical Pacific basin. We find that WWB impacts strongly depend on the ocean initial state and can range from a Central Pacific (CP) to Eastern Pacific (EP) warming, which is closely reflected by the ocean energetics. Consequently, the APE can be used to quantify the diversity of El Nino events within this continuum—higher negative APE values typically correspond to EP events, lower values to CP events. We also find that a superimposed WWB enhances El Nino predictability even before the spring predictability barrier, if one uses the APE as a predictor.

Glacial to Holocene changes in trans-Atlantic Saharan dust transport and dust-climate feedbacks

Variations in long-range Saharan dust transport may have amplified past Atlantic ITCZ and West African monsoon changes. Saharan mineral dust exported over the tropical North Atlantic is thought to have significant impacts on regional climate and ecosystems, but limited data exist documenting past changes in long-range dust transport. This data gap limits investigations of the role of Saharan dust in past climate change, in particular during the mid-Holocene, when climate models consistently underestimate the intensification of the West African monsoon documented by paleorecords. We present reconstructions of African dust deposition in sediments from the Bahamas and the tropical North Atlantic spanning the last 23,000 years. Both sites show early and mid-Holocene dust fluxes 40 to 50% lower than recent values and maximum dust fluxes during the deglaciation, demonstrating agreement with records from the northwest African margin. These quantitative estimates of trans-Atlantic dust transport offer important constraints on past changes in dust-related radiative and biogeochemical impacts. Using idealized climate model experiments to investigate the response to reductions in Saharan dust’s radiative forcing over the tropical North Atlantic, we find that small (0.15°C) dust-related increases in regional sea surface temperatures are sufficient to cause significant northward shifts in the Atlantic Intertropical Convergence Zone, increased precipitation in the western Sahel and Sahara, and reductions in easterly and northeasterly winds over dust source regions. Our results suggest that the amplifying feedback of dust on sea surface temperatures and regional climate may be significant and that accurate simulation of dust’s radiative effects is likely essential to improving model representations of past and future precipitation variations in North Africa.

The physics of orographic elevated heating in radiative-convective equilibrium

AbstractElevated heating of the atmosphere by large plateaus has been argued to influence regional climate in Asia and other regions, but the mechanisms that cause the troposphere to equilibrate at warmer temperatures over elevated terrain are not well understood. This paper quantitatively describes the physics that controls temperatures over elevated terrain in radiative–convective equilibrium (RCE). First, a cloud-system-resolving model (CSRM) is used to simulate RCE states over surfaces with various elevations. Then, a theory for the influence of surface elevation on temperatures in RCE is presented. Together with offline radiative transfer calculations, this theory is used to quantitatively attribute the magnitude of the elevated heating effect to top-of-atmosphere radiative flux changes caused by decreases in longwave absorption, shortwave scattering, and the moist lapse rate that occur as surface pressure drops. Sensitivity functions obtained through these offline calculations suggest that elevated ...

Competing effects of surface albedo and orographic elevated heating on regional climate

All else being equal, a given atmospheric pressure-level is thought to be warmer over a plateau than over surrounding non-elevated terrain because of orographic “elevated heating”. However, elevated surfaces are also typically brighter due to reduced vegetation and increased ice cover. Here we assess the degree to which surface albedo compensates for orographic elevated heating. We confirm that land surface albedo generally increases with surface elevation in observations. Using a cloud system-resolving model, we show that increased surface albedo strongly compensates for orographic elevated heating in radiative-convective equilibrium. A non-elevated surface with the albedo of modern India would enter a runaway greenhouse regime without ventilation by monsoonal winds, while a surface with the albedo and elevation of Tibet would achieve a cooler radiative-convective equilibrium. Surface albedo changes may thus be just as important as surface elevation changes for the evolution of low-latitude regional climate throughout Earths history.

Cross-equatorial winds control El Niño diversity and change

Over the past two decades, El Niño events have weakened on average and their sea surface temperature (SST) anomalies shifted westward towards the central Pacific. Moreover, the intertropical convergence zone (ITCZ), which typically migrates southward from its northerly position during El Niño events, has not crossed the Equator since 1998. The causes of these changes remain under debate1–5. Here, using in situ, satellite and atmospheric reanalysis data, we show they can be related to a multidecadal strengthening of cross-equatorial winds in the eastern Pacific. This gradual strengthening of meridional winds is unlikely to be caused by El Niño/Southern Oscillation (ENSO) changes, and contains signals forced both locally and from outside the tropical Pacific, probably from the tropical North Atlantic. Coupled model simulations in which the observed cross-equatorial wind strengthening is superimposed successfully reproduce the key features of the recent changes in tropical climate. In particular, the tropical mean state experiences a ‘La Niña-like’ change, the ENSO amplitude weakens by about 20%, the centre of the SST anomalies shifts westward and the ITCZ now rarely crosses the Equator. Thus, cross-equatorial winds are found to modulate tropical Pacific mean state and variability, with implications for quantifying projected changes in ENSO under anthropogenic warming.In recent years, El Niño sea surface temperature anomalies have weakened and shifted westward. Observational and model analyses reveal these changes can be related to a multidecadal strengthening of cross-equatorial winds, forced both locally and from the tropical Atlantic.