New University of Utah research shows that tropical climate patterns (especially in the Pacific and Indian Oceans) strongly influence year- to- year snowfall and surface mass balance on the West Antarctic Ice Sheet (WAIS), based on analysis of ice cores and 20th- century weather data.

The study, led by graduate student Ella Hunter (with co- authors including Summer Rupper and Court Strong), was published in Journal of Geophysical Research: Atmospheres (titled “Tropical controls on West Antarctic Ice Sheet surface mass balance interannual variability in the twentieth century”). It examined 25 ice and firn cores from the WAIS covering 1900- 1999, combined with historical climate reanalysis data.

Key findings

Tropical teleconnections via Rossby waves: Patterns such as El Niño- Southern Oscillation (ENSO) and the Madden- Julian Oscillation (MJO) generate atmospheric Rossby waves. These waves propagate southward, altering wind patterns, storm tracks, and moisture transport toward Antarctica. Tropical convection and sea-surface temperature anomalies effectively carry heat and moisture to the ice sheet.

Regional differences within the WAIS: The ice sheet does not behave uniformly. The eastern sector received roughly twice as much snow (about 350 mm water equivalent per year) as the western sector (about 175 mm). Over the 20th century, surface mass balance (SMB, the net surface gain and loss mainly from snowfall) trended upward in the east (+0.224 mm water equivalent per year) but slightly downward in the west (–0.087 mm per year). Trends were removed to focus on interannual variability.

Drivers of variability:

• East WAIS SMB variability was mainly controlled by anomalously low geopotential height over the Ross and Amundsen Seas.
• West WAIS variability was linked to ENSO and non- uniform phases of the MJO.
• Higher snowfall in some high- pressure areas occurs because storm systems are deflected inland by ridges of high pressure.

Methods and context

Shallow ice cores (upper ~50 m) act as proxies for weather records where permanent stations cannot be maintained in Antarctica’s harsh conditions. Rupper recovered some of the cores (5 cm diameter) during 2010- 2011 field seasons; samples are archived at the University of Utah.

The WAIS covers ~760,000 square miles with an average ice thickness of ~3,400 feet; complete melt would raise global sea levels by more than 15 feet. Understanding SMB variability (not just long- term trends) is important for assessing ice- sheet stability and future sea- level rise projections.

Funding came from the National Science Foundation, with additional support from the U.S. Department of Energy and NOAA programs. Related research has also examined longer- term (centennial) tropical- polar teleconnections affecting West Antarctic snowfall trends, but this study specifically focuses on interannual variability driven by tropical Rossby- wave sources.

In short, the tropics act as a remote “steering” influence on West Antarctic snowfall through large- scale atmospheric waves, with distinct effects across different parts of the ice sheet.

The Madden- Julian Oscillation (MJO) is the dominant mode of intraseasonal (roughly 30- 90 day) variability in the tropical atmosphere. It consists of a large- scale, eastward- propagating envelope of enhanced and suppressed convection (thunderstorms and rainfall) that typically originates over the tropical Indian Ocean and moves across the Maritime Continent into the western and central Pacific Ocean.

Key characteristics

• Structure: A dipole of enhanced convection (low outgoing longwave radiation, heavy rainfall, and upper-level divergence) paired with suppressed convection (clearer skies and upper-level convergence) to the east or west.
• Phases: Commonly tracked in 8 phases using the Real-time Multivariate MJO (RMM) index of Wheeler and Hendon (2004). Phases 1- 2 are usually over the Indian Ocean, 3- 5 over the Maritime Continent, and 6- 8 over the western/central Pacific.
• Timescale and speed: The convective envelope typically takes 30- 60 days to complete a cycle and propagates eastward at about 5 m/s.
• Vertical structure: Features a deep baroclinic circulation with lower- level inflow into the convective region and upper- level outflow.

Broader impacts (teleconnections)

Although the MJO is a tropical phenomenon, it generates atmospheric Rossby waves that propagate into the extratropics of both hemispheres. These waves can alter jet streams, storm tracks, temperature, and precipitation far from the tropics, including in the Southern Hemisphere high latitudes.

In the context of the recent University of Utah study on West Antarctic Ice Sheet (WAIS) surface mass balance:

• West WAIS interannual snowfall variability is linked to both ENSO and non- uniformity in the phase distribution of the MJO.
• More frequent MJO phases 1- 3 and 8 tend to favor higher West WAIS snowfall, while phases 5- 6 are associated with lower snowfall.
• The MJO modulates tropical convection that sources the Rossby waves influencing high- latitude circulation and moisture transport toward Antarctica.

The MJO is distinct from ENSO (which operates on interannual timescales of 2- 7 years) but can interact with it. Recent research also highlights lower- frequency (interannual and longer) variations in MJO phase preference, which can affect year- to- year climate patterns.

In short, the MJO is a traveling “pulse” of tropical convection that acts as an important bridge between tropical weather and global atmospheric circulation.

Rossby waves (also called planetary waves) are large- scale, meandering waves in the atmosphere (and oceans) that arise from the conservation of potential vorticity on a rotating planet. They are named after Swedish- American meteorologist Carl- Gustaf Rossby, who first described them in the 1930s.

Basic physics

On a rotating Earth, the Coriolis force varies with latitude (stronger toward the poles). When air parcels are displaced north or south, this variation causes them to regain their original latitude, producing a restoring force. The result is slow, westward- propagating waves relative to the mean flow.

• Wavelength: Typically, thousands of kilometers (planetary scale).
• Speed: Much slower than the winds that carry them; they can appear quasi-stationary or move slowly.
• Structure: In the atmosphere they appear as large ridges (high- pressure meanders) and troughs (low-pressure meanders) in the jet stream and upper- level flow.

Atmospheric Rossby waves and tropical- extratropical teleconnections

Tropical convection (from phenomena such as the Madden- Julian Oscillation or ENSO) creates regions of heating and upper- level divergence. These act as wave sources that generate Rossby- wave trains. The waves then propagate poleward and eastward (or sometimes westward) into the mid- and high latitudes of both hemispheres.

In the context of the University of Utah study on West Antarctic snowfall:

• Tropical convection in the Indian Ocean and central tropical Pacific launches Rossby- wave trains that travel south toward Antarctica.
• These waves alter geopotential height patterns, wind fields, and storm tracks over the Southern Ocean.
• The resulting changes in atmospheric circulation control how much moisture reaches different parts of the West Antarctic Ice Sheet (WAIS), producing the observed year- to- year differences in surface mass balance between the East and West sectors.

Key effects in the Southern Hemisphere

• They can deepen or shift the Amundsen Sea Low.
• They create blocking highs or low- pressure anomalies that steer storms onshore or offshore.
• They influence temperature, precipitation, and sea- ice patterns over West Antarctica on timescales ranging from days to seasons (and, via modulation by ENSO and MJO, on interannual timescales).

Rossby waves are fundamental to understanding how weather and climate in the tropics can “teleconnect” to remote regions such as Antarctica, Europe, or North America.

Tropical Controls on West Antarctic Ice Sheet Surface Mass Balance Interannual Variability in the Twentieth Century

The West Antarctic Ice Sheet (WAIS) holds ice equivalent to 5.3 m of global mean sea- level rise. This study focuses on interannual variability of surface mass balance (SMB, primarily snowfall and wind redistribution) linked to Rossby wave trains from tropical convection in the Indian Ocean and central tropical Pacific (rather than just long- term trends or well-known indices like the Southern Annular Mode).

• Data: 25 WAIS ice/firn cores (1900- 1999), clustered into East and West sectors along a topographic ridgeline using k-means. Trends were removed to isolate year-to-year variations. Combined with 20th Century Reanalysis (20CR) precipitation, geopotential height, winds, outgoing longwave radiation (OLR), HadISST sea-surface temperatures (SSTs), and wave activity flux.
• Opposite trends: East WAIS mean SMB increased ~0.224 mm water equivalent (w.e.) per year; West decreased ~0.087 mm w.e. per year (both significant at p ≤ 0.1). East received roughly twice the snowfall of West.
• East WAIS variability: Controlled mainly by anomalously low geopotential height over the Ross and Amundsen Seas (bringing warm, moist marine air). Key months: June and November (highest correlations with annual SMB). Linked to convection near the Maritime Continent and western Australia (sometimes with weak La Niña- like signals). MJO influence is weaker and sporadic.
• West WAIS variability: Driven by ENSO (El Niño- like SST warming in the central and eastern tropical Pacific) and non-uniform phase distribution of the Madden- Julian Oscillation (MJO). High-pressure ridges (blocking highs) deflect storms inland, enhancing onshore moisture flow. Significant positive correlations with more frequent MJO phases 1- 3 and 8; negative with phases 5- 6. Leading EOFs of MJO phase frequency explain substantial variance and correlate with West SMB.

These tropical Rossby- wave teleconnections help explain regional differences in snowfall and can improve models of future WAIS mass balance and sea-level contributions.

Methods Highlights

• Annual core SMB correlated with monthly 20CR precipitation to identify climatically important months.
• Regressions of SSTs, OLR, 200- hPa geopotential height (Z₂₀₀), streamfunction, and wave activity flux onto standardized sector SMB.
• MJO analysis used unfiltered multivariate indices (to retain interannual variability) and correlations with phase frequency.
• Composite synoptic evolution for top 5% precipitation events (day- before, day- of and day- after).
• Sensitivity checks with ERA- 20C and ERA5 reanalyses yielded consistent main results.

Discussion and Implications

East and West WAIS respond differently to tropical forcing, underscoring the need for regional (not continent- wide) analysis. Low pressure over the Ross Sea sector (related to the semi- annual oscillation) drives East precipitation via meridional moisture transport. In the West, El Niño- like patterns and MJO phase preferences create ridges that steer storms onshore. Understanding these interannual drivers is important because SMB variability can influence ice-sheet dynamics and stability on timescales relevant to sea-level rise projections.

The full paper (including figures of core locations, regression maps, MJO correlations, synoptic composites, and supporting information) is available via the DOI link above. A University of Utah news summary of the work is also available.

Journal information: Journal of Geophysical Research – Atmospheres Volume 131, Issue 11, e2025JD045664.

DOI: 10.1029/2025jd045664

First published: 28 May 2026 (corrected 7 July 2026).

Provided: University of Utah

Authors: Ella K. Hunter, Courtenay Strong, Summer Rupper, Husile Bai, Joey Krueger, Christopher Mitchell

Ella K. Hunter, Courtenay Strong (corresponding), Summer Rupper, Husile Bai, Joey Krueger, Christopher Mitchell (University of Utah and collaborators).

Abstract

The West Antarctic Ice Sheet (WAIS) contains an ice volumetric equivalent of 5.3 m of global mean sea-level rise. Understanding the climate mechanisms that influence WAIS surface mass balance (SMB) can help reduce uncertainties in sea-level rise projections. Previous work has focused on trends related to temperature and the roles of well-known climate phenomena in the Pacific sector, including the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode. Here, the focus is on interannual variability of precipitation associated with Rossby wave trains emanating from tropical convection regions in the Indian Ocean and the central tropical Pacific. Objective clustering of SMB measurements from 25 WAIS ice and firn cores from 1900 through 1999 indicated oppositely signed mean SMB trends in the East and West sectors of WAIS, and these trends were removed to focus on year-to-year variations. Climatically important months for each sector were identified by correlating reanalysis precipitation with ice core SMB. Tropical Rossby wave source regions that produced anomalous WAIS SMB were identified by analyzing composite convection, geopotential height, and wave activity flux. East WAIS SMB variability was controlled by anomalously low geopotential height over the Ross and Amundsen Seas. West WAIS SMB variability was driven by ENSO and non-uniformity in the phase distribution of the Madden-Julian Oscillation. These results provide a deeper understanding of the drivers of WAIS SMB variability and may improve projections of future sea-level rise.