Physical drivers of the summer 2019 North Pacific marine heatwave

The study investigates why a major marine heatwave, dubbed Blob 2.0, intensified during summer 2019 in the Northeast Pacific—unlike the original Blob (2013/2014) which developed in winter under a persistent atmospheric ridge that reduced winds and mixing. The central research question is to identify the physical drivers (local and remote) of summertime North Pacific SST extremes, including the roles of atmospheric circulation (North Pacific High), remote SST teleconnections (e.g., central Pacific ENSO, Pacific Meridional Mode), and local air–sea feedbacks (notably low-cloud feedback). Understanding these mechanisms is important for predicting the intensity, persistence, and ecological impacts of summertime marine heatwaves.

Prior work linked winter marine heatwaves and the original Blob to persistent atmospheric ridging that weakened the Aleutian Low, reducing Ekman advection and mixing, and allowing surface fluxes to warm the ocean. Teleconnections from remote SSTs, particularly ENSO, have been shown to affect North Pacific winter variability, while fewer studies addressed summer teleconnections. The North Pacific Oscillation (NPO) and connections to central Pacific (Modoki) ENSO have been discussed, as well as the Pacific Meridional Mode (PMM) influencing the ITCZ position and subtropical circulation via the summer deep convection (SDC) response. Low-cloud feedbacks are known to amplify SST anomalies in the subtropical North Pacific during summer, though their role differed for Blob 1.0. These studies motivate examining both remote forcing and local feedbacks for the summer 2019 event.

The authors combined reanalysis and satellite observations with atmospheric modeling. Observational/reanalysis datasets: GODAS ocean reanalysis (1980–2019) for mixed layer depth, temperature, and heat budget terms; NCEP Reanalysis 2 for sea level pressure (SLP) and latent heat flux; NCEP Reanalysis 1 for wind speed; OISSTv2 for SST; MODIS Level 3 for low-cloud fraction; CERES-EBAF for surface radiation; GPCP for precipitation. Anomalies were computed relative to 1982–2018 (MODIS relative to 2001–2018). They computed a North Pacific High Intensity index (after Schmidt et al.) indicating NPSH strength, a PMM SST index (Chiang and Vimont), and an NPO index (second EOF of SLP in 20°N–60°N, 120°E–80°W). Mixed layer heat budget was evaluated over 34°N–47°N, 147°W–128°W, from surface to GODAS mixed layer depth, using May–August averages; the August–May mixed layer temperature tendency and contributions from net surface heat flux, horizontal advection, and entrainment/residual were quantified, with flux decomposition from supplementary analyses. Wind-driven mixing was characterized via wind speed cubed. Atmospheric modeling used GFDL-AM2.1 AGCM (2°×2.5°, 24 levels) in 20-member ensembles for January 2018–August 2019 (first 17 months spun up; JJA 2019 analyzed). Experiments: (1) global SST forcing with observed OISSTv2; (2) tropical-only forcing (10°S–10°N); (3) North Pacific-only forcing (>15°N to coasts). Outside forcing regions, climatological repeating SST seasonal cycle was used; GHGs fixed at 1860 levels. Additional ensembles isolated tropical Pacific vs tropical Indian contributions. Ensemble means and best-member patterns (based on pattern correlation over a North Pacific box) were compared to reanalysis SLP anomalies. Low-cloud fraction anomalies and radiative flux changes were examined in observations and the North Pacific–forced AGCM to assess local cloud–SST feedbacks. Statistical relationships included correlations between indices and SST anomalies, and significance testing (95%). Potential errors and uncertainties in GODAS heat budgets and OISSTv2 biases were acknowledged.

- SST anomalies during JJA 2019 in the Blob 2.0 region exceeded those of 2013–2015 and were the warmest in at least 40 years; peak anomalies exceeded 2.5 °C and ~3.5 standard deviations locally. - The North Pacific High (NPH) was the weakest in the last 40 years; the NPH Intensity index correlated significantly with JJA SST anomalies in the Blob 2.0 region (R = −0.39; detrended R = −0.48; both 95% significant), indicating a weakened High is a primary driver of summer marine heatwaves there. - Mixed layer heat budget (May–August): record mixed layer warming of 7.9 °C in 2019 (difference anomaly of 2.2 °C, ~40% above mean), driven primarily by positive net surface heat fluxes (dominated by increased shortwave radiation with additional contribution from reduced latent heat loss) and reduced entrainment cooling; horizontal advection was negligible. - Wind-driven mixing (wind speed cubed) was strongly reduced; mixed layer depth was 62% shallower than average (record minimum), enhancing the impact of surface heat fluxes on SST. - Remote SST forcing contributed to the weakened NPH: AGCM ensemble means reproduced a cyclonic SLPA pattern over the subtropical North Pacific when forced by (a) global SSTs, (b) tropical SSTs, and (c) North Pacific SSTs. The tropical Pacific (central Pacific El Niño; elevated Niño4) accounted for most of the tropical-forced signal; tropical Indian SSTs contributed minimally. North Pacific SSTs (associated with a strong PMM) produced a pattern consistent with a PMM-driven SDC response and northward ITCZ shift. - Magnitudes: observed JJA 2019 NPH Intensity = −1.94 hPa; ensemble mean values were −1.58 (global), −1.01 (tropical), and −1.07 hPa (North Pacific). Best single members achieved pattern correlations R = 0.50 (global), 0.57 (tropical), and 0.71 (North Pacific) with reanalysis SLPAs. Counts of members matching or exceeding observed weakening: 9 (global), 3 (tropical), 3 (North Pacific). This indicates internal atmospheric variability was crucial for amplitude, while SST forcing shaped persistence and structure. - Local low-cloud feedback amplified and helped maintain the heatwave: observed reductions in low-cloud fraction co-located with warm SSTs led to +11.37 W m−2 net shortwave anomalies in the Blob 2.0 region, more than twice the magnitude of latent heat flux anomalies (+5.09 W m−2). The North Pacific–forced AGCM reproduced negative low-cloud anomalies, supporting a positive cloud–SST feedback, especially important in July–August after SSTAs exceeded ~1 °C. - Additional local effect: using a Lindzen–Nigam framework, ~1 hPa of negative SLPAs equatorward of 40°N could be linked to boundary-layer warming over ~20°N SSTAs, further weakening the NPH. - Decadal context: positive trends in JJA SSTAs and surface heat flux and negative trends in mixed layer depths suggest increased stratification potentially related to anthropogenic warming; interannual variance of detrended JJA SSTAs doubled from 0.24 °C² (1980–2000) to 0.49 °C² (2000–2019), a 106% increase, possibly influenced by both anthropogenic forcing and internal decadal variability (e.g., PDO).

The findings demonstrate that the summer 2019 Blob 2.0 was primarily driven by a pronounced weakening of the North Pacific High, which reduced surface winds, evaporative cooling, and wind-driven mixing. This allowed strong downward surface heat fluxes to act on an anomalously thin mixed layer, rapidly amplifying SST anomalies. Remote SST forcing from central Pacific El Niño conditions and subtropical North Pacific PMM-related SSTs helped establish and sustain the weakened NPH pattern through teleconnections (including an SDC response), while internal atmospheric variability determined the event’s amplitude. Local air–sea interactions, especially low-cloud reductions increasing shortwave flux, further maintained and amplified the anomalies into mid/late summer. These mechanisms clarify why a summer heatwave can intensify differently from winter events (e.g., Blob 1.0), emphasizing seasonal contrasts in mixed layer depth, flux impacts, and feedbacks. The results underscore the importance of accounting for both remote teleconnections and local feedbacks to assess the intensity, persistence, and ecological risks of summertime North Pacific marine heatwaves.

The study identifies the key physical drivers of the 2019 summer North Pacific marine heatwave: a record-weak North Pacific High that reduced winds and upper-ocean mixing, strong positive surface heat fluxes acting on a record-shallow mixed layer, reinforcement by remote SST teleconnections from central Pacific El Niño and subtropical North Pacific PMM anomalies, and amplification via a positive low-cloud feedback. Together these processes explain the rapid growth and persistence of Blob 2.0. The work highlights critical seasonal differences from wintertime events and their implications for ecological impacts. Future research should: (1) quantify the relative roles of internal variability versus forced trends in summertime North Pacific variability; (2) assess sensitivity to observational products and model choices; (3) further disentangle local versus remote SST influences (including ITCZ shifts and boundary-layer pressure effects); and (4) improve seasonal forecasts of marine heatwaves to support fisheries and ecosystem management.

- Mixed layer heat budget uncertainties arise from reliance on GODAS reanalysis, which may include nonphysical adjustments and offline budget residuals (entrainment) that are difficult to interpret. - OISSTv2 SSTs used to force AGCM exhibit regional biases (~±0.1–0.5 °C) relative to in situ data; while small relative to 2.5+ °C anomalies, they and model physics limitations may affect details of atmospheric responses. - AGCM ensemble means could not fully reproduce the amplitude of observed SLP anomalies, indicating a substantial role for internal atmospheric variability; best-member matches suggest sensitivity to initial conditions. - North Pacific–only AGCM runs cannot conclusively separate PMM-driven SDC teleconnections from purely local SST-forced circulation effects. - Limited temporal scope (focus on JJA 2019) constrains inferences about longer-term persistence and seasonal transitions.