Northern Biodiversity Research Institute (NBRI)

North Pacific Marine Heatwave – September 2025 Update

summarized by Marcus Kirchner Sept. 01 2025

The North Pacific Ocean is currently experiencing one of the most severe marine heatwaves ever recorded.

• Regional averages (NOAA, Mercator Ocean International) show +2 to +3 °C above normal in broad zones of the mid-latitude Pacific and the Sea of Japan.

• Local hotspots, as visualized by Nullschool (NOAA OI SST data), exceed +5 °C above normal, particularly off Japan, around Hokkaidō, and in the western Pacific gyre.

This combination of widespread warming and extreme local peaks highlights the extraordinary intensity of the present anomaly.

Impacts:

• Ecosystems: Deoxygenation, plankton regime shifts, fish mortality, and jellyfish blooms.

• Weather: Amplified evaporation drives stronger typhoons, senjō kōsuitai (linear rainbands), and prolonged humid heatwaves in East Asia.

• Teleconnections: Altered jet stream patterns influence weather extremes in North America and Europe.

• Climate feedbacks: Persistent marine heatwaves point to long-term changes in circulation (Kuroshio Extension, Aleutian Gyre) and amplify global warming through additional water vapor.

Sources and Monitoring:

• NOAA Marine Heatwave Tracker (California Current & NE Pacific “Blob”): https://www.integratedecosystemassessment.noaa.gov/regions/california-current/california-current-marine-heatwave-tracker-blobtracker

• NOAA Coral Reef Watch – Global Daily SST Anomaly Maps: https://coralreefwatch.noaa.gov/product/5km/index_5km_ssta.php

• Mercator Ocean International – Ocean Temperature Bulletin (July 2025): https://www.mercator-ocean.eu/bulletin/ocean-tempeature-bulletin-july-205/

• Mercator Ocean International – Marine Heatwave Forecast (August 2025): https://www.mercator-ocean.eu/bulletin/marine-heatwave-forecast-9-august-2025-2/

• Visualization of local anomalies (> +5 °C): https://earth.nullschool.net
  

Interpreting the Graphic: North Pacific Sea Surface Temperature Anomalies

The image shows sea surface temperature anomalies (SSTA) in the Northwest Pacific, covering Japan, Sakhalin, the Kuril Islands, Kamchatka, and the Aleutian–Alaska region. Data source: NOAA OISST v2.1, visualized on earth.nullschool.net.

Color scale:

• Yellow / orange / red = warmer than average (+ anomalies, up to > +5 °C).

• Blue / turquoise = cooler than average (– anomalies).

Streamlines: White lines show surface currents, including swirling eddies.

Key Patterns

1. Warm hotspot east of Japan

   • Broad yellow–red zones mark +3 to +5 °C anomalies along the Kuroshio Extension.

   • Numerous eddies trap and redistribute warm water, creating localized peaks above +5 °C.

2. Sharp frontal contrasts

   • South of Hokkaidō and the Kurils, warm Kuroshio waters meet cold Oyashio waters.

   • This explains the striking juxtaposition of warm anomalies (red/yellow) next to cooler patches (blue).

3. Japan Sea and Sakhalin region

   • Mixed anomalies (warm and cool) reflect strong local dynamics, wind forcing, and exchanges through narrow straits (e.g., Sōya/La Pérouse Strait).

4. Bering Sea and Gulf of Alaska

   • Large red fields show persistent positive anomalies, reminiscent of past “Blob” events (2013–2016).

Physical Interpretation

• Heat transport & eddies: The Kuroshio carries large heat volumes eastward; eddies store and recirculate this heat, delaying cooling.

• Frontal dynamics: The Kuroshio–Oyashio boundary creates sharp thermal gradients, easily shifted by winds and currents.

• Air–sea coupling: Reduced heat loss to the atmosphere under stable weather patterns allows anomalies to accumulate; localized upwelling produces cooler spots.

Key Takeaways

• Extremely warm surface waters dominate the Northwest Pacific, with regional anomalies of +2 to +3 °C and local hotspots exceeding +5 °C.

• Frontal zones around Japan, Sakhalin, and the Kurils display strong contrasts, driven by Kuroshio–Oyashio interactions.

• Such anomalies are not absolute temperatures, but deviations from the climatological mean (1982–2010 baseline).

• Local peaks vs. averages: While NOAA and Mercator report +2 to +3 °C regional means, this visualization reveals localized extremes above +5 °C.

This visualization highlights an extraordinary contrast in global sea surface temperature anomalies (SSTA):

• Equatorial Pacific (La Niña):
  A continuous blue band across the tropical Pacific shows –1 to –2 °C anomalies. This is the clear signature of a La Niña event, driven by stronger trade winds and the upwelling of cold subsurface water.

• North Pacific Hotspot:
  In sharp contrast, the North Pacific (Japan, Bering Sea, Gulf of Alaska) displays vast red and yellow zones, with anomalies reaching +3 to +5 °C. Warm water displaced westward by La Niña accumulates in the western and northern Pacific, fueling unprecedented marine heatwaves.

• North Atlantic “Cold Blob”:
  South of Greenland, cooler-than-average waters signal a weakened Atlantic Meridional Overturning Circulation (AMOC), reducing heat transport into northern Europe.

Why this matters

• Energy shift: Heat removed from the equatorial Pacific during La Niña is redistributed to the western Pacific and North Pacific mid-latitudes, creating regional hotspots.

• Atmosphere–ocean feedback: The La Niña cooling plus the North Pacific warming destabilize large-scale circulation (Walker cell, jet stream), driving weather extremes.

• Regional impacts:

  • East Asia: stronger monsoons, typhoons, and record-breaking heat.

  • North America: altered storm tracks, droughts, and heat domes.

  • Europe: the Atlantic cold anomaly increases the risk of colder winters.

Key Takeaway

This images capture a triple anomaly pattern rarely observed:

• Cold tropics (La Niña)

• Hot North Pacific (+5 °C hotspots)

• Cold North Atlantic (AMOC “cold blob”)

Together, these signals indicate that the global ocean–atmosphere system is shifting into an unstable state, with far-reaching impacts on ecosystems, regional climates, and extreme weather.

New Research Spotlight – Atlantic–Pacific Link

A 2025 study by Yamagami et al. (Gulf Stream drives Kuroshio behind the recent abnormal ocean warming) demonstrates that variability in the North Atlantic Gulf Stream region drives ocean warming in the North Pacific Kuroshio Extension.

• Atlantic → Pacific coupling: Positive SST anomalies in the Gulf Stream trigger a Northern Annular Mode-like circulation, weakening the Aleutian Low. This shifts the Kuroshio Extension northward, enhancing warm water transport and generating positive SST anomalies across the North Pacific.

• One-way influence: The study found no significant reverse effect from the Kuroshio back to the Gulf Stream.

• High-resolution models: Climate models with fine ocean resolution reproduce this coupling best, underscoring the role of mesoscale eddies.

These findings identify the North Atlantic as a key pacemaker of mid-latitude climate variability, with direct implications for understanding the current extreme North Pacific marine heatwaves observed in 2025.

Source: Yamagami et al., 2025 – Gulf Stream drives Kuroshio behind the recent abnormal ocean warming

What the graphic shows (NOAA OISST v2.1, 27 August 2025)

• North Atlantic

  • Strong red/orange anomalies along the U.S. East Coast and Gulf Stream extension → +2 to +4 °C.

  • A persistent “cold blob” south of Greenland (blue patch), again reflecting weakened heat transport by the Atlantic Meridional Overturning Circulation (AMOC).

• Caribbean & Gulf of Mexico

  • Almost uniformly +2 to +3 °C above normal, providing record-high ocean heat content.

  • These waters act as “fuel” for hurricanes → more intense tropical cyclones possible in late 2025 season.

• South Atlantic (near Brazil, West Africa)

  • Alternating warm and cool anomalies, linked to changes in the Atlantic Niño and cross-equatorial wind stress.

• Equatorial Pacific & Eastern Pacific

  • Clear blue anomaly band (–1 to –2 °C) along the equator = La Niña signature.

  • Off Peru/Ecuador → cold upwelling intensified, while just north/south of the equator the water remains warmer.

• Eastern Pacific (off California)

  • Strong positive anomalies, linked to regional upwelling changes and ongoing marine heatwave activity in the Northeast Pacific.

Why this matters

• Dual Atlantic signal:

  • Warm along U.S. East Coast → stronger hurricane potential.

  • Cold blob south of Greenland → linked to AMOC slowdown, affecting European climate and winter patterns.

• La Niña + Atlantic warm pool:

  • This combination typically shifts the Walker circulation and jet streams, altering rainfall in South America, Africa, and Asia.

  • Increases likelihood of heavier hurricanes in the Atlantic and drier conditions in parts of South America.

• Global coupling:

  • The Atlantic anomalies (warm + cold blob) interact with the Pacific (La Niña), producing large-scale atmospheric wave trains that reshape global climate extremes.

In summary:
This map shows how the Atlantic and Pacific anomalies are locked into a coupled pattern:

• La Niña cooling in the Pacific,

• North Atlantic cold blob near Greenland,

• Extreme warm pools in the Gulf of Mexico and along the U.S. East Coast.

Together, these signals strongly influence hurricane activity, monsoon rainfall, and Europe’s upcoming winter climate.

Regional Cold Anomalies – South Atlantic & Northern Australia

In addition to the dramatic warming in the North Pacific, several regions of the global ocean currently show significant cold anomalies:

• South Atlantic (west of southern Africa):
  Cooler-than-average waters (–0.5 to –2 °C) are linked to enhanced Benguela upwelling and the Atlantic Niño pattern. These anomalies influence rainfall across West Africa, with potential impacts on the Sahel rainy season.

• Northern Australia and the Maritime Continent:
  Cold anomalies (–0.5 to –1.5 °C) in the Timor and Arafura Seas reflect the influence of the ongoing La Niña. Stronger trade winds drive warm water westward into the Indonesian seas, while local upwelling cools the northern Australian margins. The result: wetter conditions over Indonesia but a tendency toward drier climate in northern Australia.

Global Context

These cold pools are part of a broader three-pole anomaly pattern:

• Tropical Pacific: La Niña cooling.

• North Pacific: Extreme marine heatwaves (+3 to +5 °C).

• North Atlantic: Persistent “cold blob” south of Greenland.

• South Atlantic & Northern Australia: Secondary cold anomalies tied to regional upwelling and ENSO dynamics.

Together, these hotspots and cold pools highlight the highly disrupted state of the global ocean–atmosphere system in 2025, with cascading effects on monsoons, storm tracks, and regional climates.