El Niño-Southern Oscillation (ENSO)

The 2015/16 extreme El Niño. Anomalies in sea surface temperature and surface winds in December 2015. From Hu and Fedorov (2017).

ENSO arises from rigorous ocean-atmosphere interactions in the tropical Pacific and it dominates the Earth’s year-to-year climate variability with substantial climatic and socioeconomic impacts worldwide. Each El Niño is unique in amplitude, pattern, evolution, and impact. What causes the diversity of El Niño events? How will El Niño change under global warming? The main goal of this project is to better understand the ENSO dynamics, diversity, predictability, and future changes.

A westerly wind burst (WWB) in the W. Pacific. Anomalies in sea surface temperature and surface winds on March 10, 2015. From Hu and Fedorov (2017).

A key element of the ENSO cycle is the westerly and easterly wind bursts that typically last for a week to a month. They can significantly influence and are in turn modulated by the development of an ENSO event. Combining observations, theory, and climate modeling, we aim to address: What are the mechanisms of those wind bursts? How are they connected to the tropical weather extremes (e.g. tropical cyclones, MJOs, etc.)? How do they interact with the underlying sea surface temperature?

Global warming pattern formation

A Qnet-centered framework to understand the global warming pattern formation. From Hu et al. (2020).

Global warming is not spatially uniform. Both oceanic and atmospheric processes can influence the warming pattern, and a central factor that connects the two is the net ocean surface heat flux, Qnet. How does the global warming pattern affect atmospheric circulation and the hydrological cycle? How do the atmospheric changes affect ocean heat transport? How do they interact on various timescales? The main goal of this project is to explore the mechanisms of global warming pattern formation in an ocean-atmosphere coupled framework.

Observed trends in sea surface temperature during 1950-2015. From Hu and Fedorov (2019).

For example, the subpolar North Atlantic has experienced a suppressed warming in the past century. This is at least partially caused by the surface wind strengthing related to the concurrent enhanced Indian Ocean warming. Why has the Indian Ocean been warming faster than the tropical Pacific and Atlantic? Will the enhanced Indian Ocean warming continue through the coming decades? What does this imply for regional climate change? Those questions remain to be addressed.

Ocean inter-basin connections

Effect of Indian Ocean warming on the Atlantic meridional overturning circulation (AMOC). From Hu and Fedorov (2019).

Different ocean basins are closely connected through not only “oceanic tunnels”, but also “atmospheric bridges”. The main goal of this project is to better understand the connections among the global oceans via ocean-atmosphere coupled processes. We aim to address: What is the atmospheric role in the heat and freshwater budgets of each ocean basin? How do different ocean basins interact and lead to climate variations? How do they change in future and past climates?

Impact of tropical North Atlantic warming on the Pacific. Anomalies in sea surface temperature and surface wind. From Hu and Fedorov (2018).

Here we provide two examples of ocean inter-basin connections. First, Indian Ocean warming can strengthen the Atlantic meridional overturning circulation by suppressing the tropical Atlantic rainfall. Second, tropical North Atlantic warming can weaken the ENSO variability by strengthening the easterly Trade wind and the southerly cross-equatorial wind. Both examples highlight the importance of atmospheric bridges in connecting the global oceans.

Past climates

Past reconstructions and future projections of global mean surface temperature. From Burke et al. (2018).

Paleo-proxies recorded a lot of important information about our climate system in the past. The main goal of this project is, by modeling the past climates, to test the theory of climate and explore the implications for future climate changes. We are interested in how the ocean and atmospheric circulations were different and what was the role of ocean-atmosphere interactions in past climates.

Global temperature reconstructed from oxygen isotope records. From Wikipedia.

We are particularly fascinated by the mystery of glacial-interglacial cycles. Around 3 Ma, the Earth’s climate experienced a transition from the warm, stable Pliocene to the cold, fluctuating Pleistocene. We are hoping to answer: What has caused this Plio-Pleistocene transition? Why did the glacial cycles change around 1 Ma from a 41-kyr world to a 100-kyr world? How did the ocean and atmospheric circulations respond to the orbital forcing (obliquity, precession, eccentricity)?