Research

Improving the Predictability of Northwestern Pacific Summer Monsoon: ENSO and non-ENSO Variabilities

The East Asian summer monsoon supplies vital rainfall for over one billion people. ENSO markedly affects the EASM, and its influence can persist through the post-ENSO summer, after the SST anomalies in the tropical Pacific have dissipated. The long persistence of coherent post-ENSO anomalies is caused by a positive feedback due to interbasin ocean-atmospheric coupling, known as the Indo-Western Pacific Ocean Capacitor (IPOC) effect. The post-ENSO anomalies are more robust following El Niño than La Niña. This asymmetry arises from the asymmetry in ENSO evolution: though most El Niño events last for one year, La Niña events often persist for 2-3 years. In the summers between consecutive La Niña events, the concurrent La Niña opposes the delayed effect of the preceding winter La Niña on the EASM, causing a reduction in the magnitude and coherence of climate anomalies. The apparent asymmetry in the impacts of the ENSO on the EASM can be reduced by considering the concurrent ENSO, in addition to the ENSO state in the preceding winter.

The feedback mechanism behind the IPOC effect does not necessarily rely on the antecedence of ENSO events, suggesting the potential for substantial internal variability independent of ENSO. We investigate the respective role of ENSO forcing and non-ENSO internal variability by conducting ensemble “forecast” experiments with a coupled atmosphere-ocean model initialized from a multi-decadal tropical Pacific pacemaker simulation. The leading mode of internal variability as represented by the forecast-ensemble spread resembles the post-ENSO IPOC, despite the absence of antecedent ENSO forcing by design. The persistent atmospheric and oceanic anomalies in the leading mode highlight the positive feedback mechanism in the internal variability. The large sample size afforded by the ensemble spread allows us to identify robust non-ENSO precursors of summer IPOC variability, including a cool SST patch over the tropical Northwestern Pacific, a warming patch in the tropical Northern Atlantic, and downwelling oceanic Rossby waves in the tropical Indian Ocean south of the equator.

Publication:

1. Zhang, P., Xie, S.-P., Kosaka, Y., and Lutsko, N.J. (2024). Non-ENSO Precursors for Northwestern Pacific Summer Monsoon Variability with Implications for Predictability. Journal of Climate, 37(1), 199-212. [link]
2. Zhang, P., Xie, S.-P., Kosaka, Y., Lutsko, N.J., Okumura, Y.M., and Miyamoto A., 2024: Why East Asian Monsoon Anomalies Are More Robust in Post El Niño than in Post La Niña Summers. Nature Communications, 15, 7401. [link]

Seasonal Superrotation in Earth’s Troposphere

Although Earth’s troposphere does not superrotate in the annual-mean, for most of the year – from October to May – the winds of the tropical upper troposphere are westerly. We investigate this seasonal superrotation using reanalysis data and a single-layer model for the winds of the tropical upper troposphere. We characterize the temporal and spatial structures of the tropospheric superrotation, and quantify the relationships between the superrotation and the leading modes of tropical interannual variability. We also find that the strength of the superrotation has remained roughly constant over the past few decades, despite the winds of the tropical upper troposphere decelerating (becoming more easterly) in other months. The monthly zonal-mean zonal momentum budget and numerical simulations with an axisymmetric, single-layer model of the tropical upper troposphere are used to study the underlying dynamics of the seasonal superrotation. Momentum flux convergence by stationary eddies accelerates the superrotation, while cross-equatorial easterly momentum transport associated with the Hadley circulation decelerates the superrotation. The seasonal modulations of these two competing factors shape the superrotation. The single-layer model is able to qualitatively reproduce the seasonal progression of the winds in the tropical upper troposphere, and highlights the northward displacement of the Intertropical Convergence Zone in the annual-mean as a key factor responsible for the annual cycle of the tropical winds.

Publication:

1. Zhang, P., & Lutsko, N. J. (2022). Seasonal Superrotation in Earth’s Troposphere, Journal of the Atmospheric Sciences, 79(12), 3297-3314. [link]

Dynamics of Atmospheric Coastal Low-Level Jets

Coastal low-level jets are equatorward lower tropospheric winds along the eastern boundaries of extratropical and subtropical oceans extending from the surface to about 2 km height. These jets are one of the main components of the surface branch of the Hadley circulation, and have a profound influence on local land climate conditions and local ocean circulation and upwelling. In this study, we examine the occurrence and distribution of these jets using both reanalysis data and a global climate model simulation, and use their vorticity budget to understand their dynamics. We find that in the regions of these eastern boundary jets, the most important term in the vorticity budget is the stretching term. The stretching term is balanced by a combination of meridional advection of the planetary vorticity ($\beta$ effect) as well as the curl of friction, which also play a non-negligible role in the vorticity budget of these jets.

Antarctic Precipitation and Sea ice Response to Stratospheric Ozone Depletion and Recovery

The snow and total precipitation responses to Antarctic stratospheric ozone depletion and recovery are studied with model simulations. The total precipitation is shown increasing during the ozone depletion period, yet snow fall decreases near the edge of sea ice. Our research shows that the cloud changes induced by ozone depletion causes precipitation to increase, and the low-level air warming turns a large part of snowfall into rainfall. The situation is well opposite when the ozone layer recovers. With relatively high albedo, this variation of snowfall can significantly modify radiation balance and further plays a non-negligible role in sea ice forming and melting.