Research
Improving the Predictability of Northwestern Pacific Summer Monsoon: ENSO and non-ENSO Variabilities
The East Asian Summer Monsoon (EASM) delivers essential rainfall to over a billion people. The ENSO effect on the EASM is well-established, and its influence persists into the post-ENSO summer, even after tropical Pacific SST anomalies dissipate. This long-lasting effect stems from the Indo-Western Pacific Ocean Capacitor (IPOC), a positive feedback loop involving interbasin ocean-atmospheric coupling.
We identified an asymmetry in this impact: post-ENSO anomalies are more robust following El Niño than La Niña. This is primarily because multi-year La Niña events often generate opposing atmospheric effects in consecutive summers, reducing the magnitude of the delayed influence. This apparent asymmetry is largely mitigated when concurrent ENSO variability is considered alongside the preceding winter’s state.
Recognizing that the IPOC feedback suggests substantial internal variability independent of ENSO, we conducted ensemble “forecast” experiments using a coupled model initialized from a tropical Pacific pacemaker simulation. The leading mode of internal variability identified in the forecast spread strongly resembles the post-ENSO IPOC pattern, confirming the role of the positive feedback mechanism even without antecedent ENSO forcing. This ensemble approach allowed us to identify robust non-ENSO precursors to summer IPOC variability, including specific sea surface temperature patterns in the tropical Northwestern Pacific and Northern Atlantic, and downwelling oceanic Rossby waves in the tropical Indian Ocean.
Publication:
- 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]
- 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]
Insights from Idealized Modeling: Hadley Circulation and Superrotation
My research leverages the simplicity of idealized single-layer models to disentangle the complex dynamics of the tropical atmosphere. By stripping away the noise of comprehensive climate models, these idealized frameworks allow us to isolate and quantify the fundamental “tug-of-war” between the mean meridional flow (Hadley Circulation) and the momentum transport caused by atmospheric waves (eddies).
We used this approach to explain the seasonal superrotation of Earth’s troposphere—a phenomenon where upper-tropospheric winds near the equator blow from the west (superrotate) from October to May, despite being easterly in the annual mean. Our model reveals that this cycle is driven by a delicate balance: stationary eddies accelerate the winds eastward, while the Hadley Circulation transports air across the equator and decelerates the flow . The observed northward displacement of the Intertropical Convergence Zone (ITCZ) ensures that this deceleration is strongest in boreal summer, effectively suppressing superrotation during that season.
We further applied this framework to specific Hadley cell dynamics, exploring how the circulation responds to changes in eddy strength and equatorial heating. Theoretical analysis shows that eddies flatten the tropical wind profile, shifting it from a quadratic shape (typical of an angular momentum-conserving regime) to a cubic shape. By quantifying the transition between regimes, we shed light on a key climate paradox regarding the cell’s response to heating width. We found that the nonlinearity characteristic of low-eddy regimes allows broad heating (like global warming) to drive poleward expansion, while narrow heating (like El Niño) drives contraction. Conversely, strong eddies suppreses this width-dependent expansion/contraction responses.
Publication:
Zhang, P., and Lutsko, N.J. (2022). Seasonal Superrotation in Earth’s Troposphere. Journal of the Atmospheric Sciences, 79(12), 3297-3314. [link]
Zhang, P., Lutsko, N.J., Hill, S.A., and Xie, S.-P. (2025). Hadley Cell Dynamics in an Axisymmetric Single-Layer Model: Effects of Parameterized Eddies and Equatorial Heating. , Journal of the Atmospheric Sciences, in press.
Dynamics of Atmospheric Coastal Low-Level Jets
Coastal low-level jets (CLLJs) are equatorward winds along the eastern boundaries of extratropical and subtropical oceans, extending up to about $2 \text{ km}$ in height. As a main component of the surface branch of the Hadley circulation, they significantly influence local land climate and ocean upwelling.
We investigated the occurrence and dynamics of these jets using reanalysis data and a global climate model. Analysis of the vorticity budget reveals that the stretching term is the most dominant factor in these eastern boundary regions. This stretching term is mainly balanced by a combination of the meridional advection of planetary vorticity ($\beta$-effect) and the curl of friction, both of which play a non-negligible role in the overall jet dynamics.
Antarctic Precipitation and Sea ice Response to Stratospheric Ozone Depletion and Recovery
We examined how Antarctic stratospheric ozone depletion and recovery affect snow and total precipitation using model simulations. Ozone depletion is shown to cause an overall increase in total precipitation due to cloud changes. However, this period simultaneously leads to a decrease in snowfall near the sea ice edge, as low-level air warming converts a substantial portion of the snow into rainfall. The opposite effect is observed during ozone recovery. Because of the high albedo of snow, this shift in precipitation type significantly modifies the radiation balance, thereby playing an important role in sea ice formation and melting.