March 3, 2026 – A new tool for predicting Eastern Mediterranean winter rainfall, the Aegean Sea Heat Uptake Anomaly (AQA) Index, has been developed by researchers at the Hebrew University of Jerusalem (HU).

The new Index measures deviations in the net heat exchange between the sea and the atmosphere in the Aegean region during August, which impacts the Eastern Mediterranean region. The study, published in the international journal Weather and Climate Dynamics, was led by Prof. Ori Adam and research student Ofer Cohen and included Dr. Assaf Hochman, Prof. Hezi Gildor, and Prof. Dorit Rostkier-Edelstein, all from the Hebrew University of Jerusalem Fredy and Nadine Herrmann Institute of Earth Sciences.

“The Mediterranean Sea acts as the primary moisture source and driver of weather systems for our region,” the researchers say. “This research demonstrates that summer processes in the sea can predict winter outcomes several months ahead—a remarkable result given the inherent chaotic nature of weather systems. The AQA index explains approximately one-third of the year-to-year variation in Levant rainfall, and integrating this into existing seasonal models could significantly improve our ability to forecast water availability months in advance.”

The Mediterranean as a Seasonal Barometer

The Eastern Mediterranean is a recognized global warming “hotspot,” currently facing severe trends of warming and drying. In this water-stressed region, improving seasonal forecasting is critical for resource management and drought preparation.

By analyzing sea surface temperature (SST) and heat uptake data from 1979–2023, the team identified three primary patterns of variability across the Mediterranean basin. Two of these patterns, characterized by an east-west temperature “dipole,” were found to be directly linked to winter precipitation in the Levant.

Introducing the AQA Index

Based on these findings, the researchers developed the Aegean Sea Heat Uptake Anomaly (AQA) index. This index measures deviations in the net heat exchange between the sea and the atmosphere in the Aegean region during August.

Key findings regarding the AQA index include:

• Strong Correlation: August AQA values show a significant correlation (R = -0.6) with rainfall during the subsequent December–February period.
• The “Negative” Effect: When the Aegean Sea shows a “negative anomaly” in August, meaning it emits more heat into the atmosphere than average, the following winter tends to be significantly rainier in Israel.
• Data Verification: This link was confirmed using both global observational interpolated data (ERA5) and long-term ground measurements from the Israel Meteorological Service (IMS).

The Physical Mechanism: More Persistent Storms

The study reveals that these August Sea conditions trigger a lagged atmospheric response. Specifically, years with negative August AQA values lead to:

• Persistent Cyprus Lows: The primary rain-bearing systems for Israel, known as “Cyprus Lows,” last longer and occur more frequently.
• Jet Stream Intensification: The regional subtropical jet stream strengthens, creating more “baroclinic” or unstable conditions that favor storm development.
• Regional Dynamics: Interestingly, this regional index (AQA) proved to be a more accurate predictor for the Levant than well-known global climate drivers like the North Atlantic Oscillation (NAO) or El Niño (ENSO).

The research paper titled “Mediterranean Sea heat uptake variability as a precursor to winter” is now available in Weather and Climate Dynamics and can be accessed here.

Funding