New Aegean index unlocks breakthrough in Mediterranean seasonal rainfall forecasting

A new study reveals that heat uptake fluctuations in the Aegean Sea during August provide a powerful new tool for predicting Eastern Mediterranean winter rainfall. Researchers developed the Aegean Sea Heat Uptake Anomaly (AQA) index, showing that when the sea releases more heat in late summer, the following winter tends to bring more frequent and persistent “Cyprus Low” storm systems across the Eastern Mediterranean. The regional index captures a substantial share of year-to-year rainfall variability and significantly improves seasonal forecasting skill compared to traditional global climate indicators such as ENSO and the North Atlantic Oscillation (NAO). By identifying a localized ocean signal with strong predictive power, the study marks a major advance in Mediterranean seasonal rainfall forecasting.

A new study has identified a distinct climate precursor in the Mediterranean Sea that can predict winter precipitation levels in the Levant months in advance. The study, published in the international journal Weather and Climate Dynamics, was led by Prof. Ori Adam and research student Ofer Cohen from the Institute of Earth Sciences at Hebrew University. The research team also included Dr. Assaf Hochman, Prof. Hezi Gildor, and Prof. Dorit Rostkier-Edelstein from Hebrew University, along with Dr. Ehud Strobach from the Volcani Institute.

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 Mediterranean Sea acts as the primary moisture source and driver of weather systems for our region," say the researchers. "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. Integrating this into existing seasonal models could significantly improve our ability to forecast water availability months in advance".