NGAO and GOADI

The Gulf of Alaska is characterized by persistent offshore upwelling and coastal downwelling, driven by cyclonic winds. This results in negative sea surface height (SSH) anomalies offshore and positive SSH anomalies on the continental shelf area.

De-trended and de-seasonalized sea surface height (m) climatology (1993-2021)

NGAO

The Northern Gulf of Alaska Oscillation (NGAO) index describes the strength of the cyclonic circulation in the Gulf of Alaska, and therefore, the intensity of the offshore upwelling in the Alaskan gyre and coastal downwelling (Hauri et al., 2021). The NGAO corresponds to the primary mode of variability, identified through Empirical Orthogonal Function (EOF) decomposition performed on SSH anomalies (with trends and monthly climatology removed). This first mode accounts for approximately 24 % of the total variance and explains about 50 % of the SSH variance in offshore areas.

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Northern Gulf of Alaska Oscillation Index (NGAO) derived from satellite-observed Absolute Dynamic Topography. Generated using E.U. Copernicus Marine Service Information https://doi.org/10.48670/moi-00149 and https://doi.org/10.48670/moi-00148.

Download the monthly NGAO index (monthly update)

A negative NGAO phase is characterized by a strong cyclonic circulation leading to strong offshore upwelling that brings cold, acidic and de-oxygenated waters to the sub-surface.

A positive NGAO phase is characterized by a weak cyclonic circulation leading to weak offshore upwelling and therefore brings less cold, acidic and de-oxygenated waters to the sub-surface.

Ecological implications

During negative phases of the NGAO, intensified upwelling brings acidic waters to the sub-surface. Combined with the long-term trend of ocean acidification, this leads to extreme acidity events. Such events have historically had adverse effects on organisms like Pteropods, which are a significant food source for Salmon (Hauri et al., 2021).

NGAO seasonal forecast

!!! WARNING !!!
This section is experimental and must used with extra caution.
Last update may 2024.

Using the forecast system model version 2 (CFS-V2) global seasonal simulation, we compute the NGAO index from January 2024 until August 2024. This product will be updated monthly.

NGAO index computed from satellite data until April 2024 and from the CFS-V2 simulation after.

Potential implications

During the 2024 fall, the NGAO index is expected to be strongly negative (i.e strong offshore upwelling).
Possible implications may be: (1) Acidic conditions offshore in the Gulf of Alaska.

GOADI

The Gulf of Alaska Downwelling Index (GOADI) quantifies the intensity of positive coastal SSH anomalies in the Gulf of Alaska, indicating the strength of coastal downwelling (Hauri et al., 2024). This index is derived from the second mode of variability identified by Empirical Orthogonal Function (EOF) decomposition, applied to SSH anomalies after removing trends and monthly climatology. While this second mode accounts for approximately 10 % of the total variance, it explains approximately 60 % of the SSH variance on the continental shelf area.

GOADI_mon.png

Gulf of Alaska Downwelling index derived from satellite-observed Absolute Dynamic Topography. Generated using E.U. Copernicus Marine Service.

Download the monthly GOADI index (monthly update)

Potential implications

The Gulf of Alaska Downwelling Index (GOADI) serves as a measure of the intensity of coastal downwelling and, consequently, acts as a proxy for the intrusion of deep water onto the continental shelf’s bottom. During a negative GOADI phase, SSH anomalies are low in the shelf area, leading to weaker downwelling that permits intrusion of cold, salty, de-oxygenated, and acidic deep water onto the bottom of the shelf. In contrast, a positive GOADI phase is characterized by high SSH anomalies and strong downwelling, making deep water intrusions less likely. These deep water intrusions, combined with the gradual pressures of climate change and ocean acidification, can lead to compound extreme events at the bottom of the shelf, adversely affecting benthic organisms. For detailed information, see Hauri et al. (2024).