North Pacific High Research Articles

Offshore low-level jet observations and model representation using lidar buoy data off the California coast

Abstract. Low-level jets (LLJs) occur under a variety of atmospheric conditions and influence the available wind resource for wind energy projects. In 2020, lidar-mounted buoys owned by the US Department of Energy (DOE) were deployed off the California coast in two wind energy lease areas administered by the Bureau of Ocean Energy Management: Humboldt and Morro Bay. The wind profile observations from the lidars and collocated near-surface meteorological stations (4–240 m) provide valuable year-long analyses of offshore LLJ characteristics at heights relevant to wind turbines. At Humboldt, LLJs were associated with flow reversals and north-northeasterly winds, directions that are more aligned with terrain influences than the predominant northerly flow. At Morro Bay, coastal LLJs were observed primarily during northerly flow as opposed to the predominant north-northwesterly flow. LLJs were observed more frequently in colder seasons within the lowest 250 m a.s.l. (above sea level), in contrast with the summertime occurrence of the higher-altitude California coastal jet influenced by the North Pacific High, which typically occurs at heights of 300–400 m. The lidar buoy observations also validate LLJ representation in atmospheric models that estimate potential energy yield of offshore wind farms. The European Centre for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5) was unsuccessful at identifying all observed LLJs at both buoy locations within the lowest 200 m. An extension of the National Renewable Energy Laboratory (NREL) 20-year wind resource dataset for the Outer Continental Shelf off the coast of California (CA20-Ext) yielded marginally greater captures of observed LLJs using the Mellor–Yamada–Nakanishi–Niino (MYNN) planetary boundary layer (PBL) scheme than the 2023 National Offshore Wind dataset (NOW-23), which uses the Yonsei University (YSU) scheme. However, CA20-Ext also produced the most LLJ false alarms, which are instances when a model identified an LLJ but no LLJ was observed. CA20-Ext and NOW-23 exhibited a tendency to overestimate the duration of LLJ events and underestimate LLJ core heights.

Revisiting the Relationship between the North Pacific High and Upwelling Winds along the West Coast of North America in the Present and Future Climate

Abstract The wind-driven circulation is an important driver of upwelling in the California Current System, a key factor in maintaining a productive ecosystem. In summer, the North Pacific high (NPH) dominates the atmospheric circulation, including the nearshore winds. The impact of the NPH on the surface winds along the North American west coast during summer is examined using the ECMWF Reanalysis v5 (ERA5) and the Community Earth System Model version 1 (CESM1) large ensemble of simulations. The strength, latitude, and longitude of the sea level pressure (SLP) and subsidence at 500 hPa are used to assess the NPH and its variability. While both the surface high pressure cell and subsidence are related to the interannual variability of the surface winds over the North Pacific, the strength of subsidence has a much larger effect on the coastal winds than the variability in SLP. Based on the mean of the 40 CESM simulations, future changes in upwelling also more strongly coincide with changes in subsidence than in SLP. Subsidence and southward upwelling-favorable winds increase off the Canadian coast, with the reverse occurring off the U.S. West Coast, by the end of the twenty-first century. In particular, the intermember correlation between the changes in the nearshore surface winds and the 500-hPa pressure vertical velocity reaches 0.75 and 0.87 in the southern and northern portions of the northeast Pacific, respectively. The effect of the subsidence on upwelling winds in the future is confirmed by the CESM2 large ensemble.

Validation of boreal summer tropical–extratropical causal links in seasonal forecasts

Abstract. Much of the forecast skill in the mid-latitudes on seasonal timescales originates from deep convection in the tropical belt. For boreal summer, such tropical–extratropical teleconnections are less well understood compared to winter. Here we validate the representation of boreal summer tropical–extratropical teleconnections in a general circulation model in comparison with observational data. To characterise variability between tropical convective activity and mid-latitude circulation, we identify the South Asian monsoon (SAM)–circumglobal teleconnection (CGT) pattern and the western North Pacific summer monsoon (WNPSM)–North Pacific high (NPH) pairs as the leading modes of tropical–extratropical coupled variability in both reanalysis (ERA5) and seasonal forecast (SEAS5) data. We calculate causal maps based on the Peter and Clark momentary conditional independence (PCMCI) causal discovery algorithm, which identifies causal links in a 2D field, to show the causal effect of each of these patterns on circulation and convection in the Northern Hemisphere. The spatial patterns and signs of the causal links in SEAS5 closely resemble those seen in ERA5, independent of the initialisation date of SEAS5. By performing a subsampling experiment (over time), we analyse the strengths of causal links in SEAS5 and show that they are qualitatively weaker than those in ERA5. We identify those regions for which SEAS5 data well reproduce ERA5 values, e.g. the southeastern USA, and highlight those where the bias is more prominent, e.g. North Africa and in general tropical regions. We demonstrate that different El Niño–Southern Oscillation phases have only a marginal effect on the strength of these links. Finally, we discuss the potential role of model mean-state biases in explaining differences between SEAS5 and ERA5 causal links.

Where does the dust deposited over the Sierra Nevada snow come from?

Abstract. Mineral dust contributes up to one-half of surface aerosol loading in spring over the southwestern United States, posing an environmental challenge that threatens human health and the ecosystem. Using self-organizing map (SOM) analysis with dust deposition and flux data from WRF-Chem and Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), we identify four typical dust transport patterns across the Sierra Nevada, associated with the mesoscale winds, Sierra barrier jet (SBJ), North Pacific High (NPH), and long-range cross-Pacific westerlies, respectively. We find that dust emitted from the Central Valley is persistently transported eastward, while dust from the Mojave Desert and Great Basin influences the Sierra Nevada during mesoscale transport occurring mostly in winter and early spring. Asian dust reaching the mountain range comes either from the west through straight isobars (cross-Pacific transport) or from the north in the presence of the NPH. Extensive dust depositions are found on the west slope of the mountain, contributed by Central Valley emissions and cross-Pacific remote transport. In particular, the SBJ-related transport produces deposition through landfalling atmospheric rivers, whose frequency might increase in a warming climate.

Importance of the choice of heat flux parameterization in regional air–sea coupled simulations: Case studies of marine heatwaves induced by atmospheric heatwaves in 1994 and 2018

This study investigated the marine heat wave events (MHWs) that occurred near the Korean Peninsula during the summer of 1994 and 2018, using a regional air–sea coupled model. We analyzed the fifth-generation reanalysis data, ERA5, published by the European Centre for Medium-Range Weather Forecasts for both events. We found that the North Pacific High and Tibetan High were stronger than usual and were associated with warm and moist air intrusion from the subtropical regions. Air-sea interactions play an important role in the development of MHWs. Warm and moist air combined with low-level inversion and a subsequent sinking motion induced the downward latent heat flux (LHF) toward the relatively colder sea surface, resulting in increased sea surface temperatures (SSTs). To quantify the contribution of the downward LHF and evaluate the importance of the relevant physical parameters of the MHWs, we set up two coupled model experiments, namely, CPL_down and CPL_nodown. Results show that the CPL_down experiment captured the downward LHF well in both events. The model also successfully captured the observed inversion near the surface. The cold SST bias tended to be reduced as the low-level clouds decreased in the area where the downward LHF occurred. In our simulation, permitting downward LHF improved the MHW reproducibility. Therefore, we suggest that the increased downward LHF is favorable for simulating MHWs, and surface physical parameterization must be carefully performed.

Effect of Sakurajima Volcanic Eruption (July 16, 2018) on PM<sub>2.5</sub> Concentration in Busan under Summertime North Pacific High Pressure Condition

Effect of Sakurajima Volcanic Eruption (July 16, 2018) on PM<sub>2.5</sub> Concentration in Busan under Summertime North Pacific High Pressure Condition

A Lacustrine Biomarker Record From Rebun Island Reveals a Warm Summer Climate in Northern Japan During the Early Middle Holocene Due to a Stronger North Pacific High

The summer climate of northern Japan since the last glacial period has likely been determined by atmospheric and oceanic dynamics, such as changes in the North Pacific High, the position of the westerlies, the Kuroshio Current, the Tsushima Warm Current (TWC), and the East Asian summer monsoon. However, it is unclear which factor has been most important. In this study, we analyzed leaf wax δ13C and δD and glycerol dialkyl glycerol tetraethers (GDGTs) in sediments from Lake Kushu, Rebun Island, northern Japan, and discuss changes in climate over the past 17,000 years. The GDGT-based temperature, the averaged chain length, δ13C and δD of long-chain n-fatty acids indicated that the climate was cold during the Oldest Dryas period ∼16 ka and warm in the early Middle Holocene from ∼9 to 6 ka. This climate change is consistent with the sea surface temperature in the Kuroshio–Oyashio transition, but inconsistent with changes in the TWC in the Sea of Japan. The results imply that the summer climate of northern Japan was controlled mainly by changes in the development of the North Pacific Highviachanges in the position of the westerly jet and East Asian summer monsoon rainfall, whereas the influence of the TWC was limited over a millennial timescale.

Effects of the Tibetan High and the North Pacific High on the Occurrence of Hot or Cool Summers in Japan

In this study, we investigated the effects of the Tibetan High near the tropopause and the North Pacific High in the troposphere on occurrences of hot or cool summers in Japan. We first classified Japan into six regions and identified hot and cool summer years in these regions from a 38-year sample (1980–2017) based on the monthly air temperature. To investigate the features of circulation fields over Asia during hot and cool summers in Japan, we calculated the composite differences (hot summer years minus cool summer years) of several variables such as geopotential height, which indicated significant high-pressure anomalies in the troposphere and lower stratosphere. These results suggest that both the North Pacific and the Tibetan Highs tend to extend to Japan during hot summer years, while cool summers seem to be associated with the weakening of these highs. We found that extension of the Tibetan High to the Japanese mainland can lead to hot summers in Northern, Eastern, and Western Japan. On the other hand, hot summers in the Southwestern Islands may be due to extension of the Tibetan High to the south. Similarly, the latitudinal direction of extension of the North Pacific High is profoundly connected with the summer climate in respective regions.

The Lagged Effect of Anthropogenic Aerosol on East Asian Precipitation during the Summer Monsoon Season

The authors investigated the lagged effect of anthropogenic aerosols (AAs) during the premonsoon season (April–May–June) on the East Asian precipitation during the postmonsoon season (July–August) using the aerosol optical depth (AOD) from a satellite dataset and reanalysis datasets. When the AOD is high in Eastern China during the premonsoon season, the amount of precipitation increases in the western North Pacific, including the Korean Peninsula and Japan, during the postmonsoon season. The amount of cloud in the western-to-central North Pacific in the premonsoon season increases during the high-AOD period. Subsequently, it cools the sea surface temperature until the postmonsoon season, which strengthens the North Pacific High. The strengthened North Pacific High in the postmonsoon season expands to the western North Pacific, which leads to the enhancement of the moisture flows from the ocean. This results in the increase in precipitation in the western North Pacific, including the Korean Peninsula and Japan, during the postmonsoon season.

Drivers of extreme wind events in Mexico for windpower applications

AbstractIn this study, we use a k‐mean clustering approach to investigate the weather patterns responsible for extreme wind speed events throughout Mexico using 40 years of the ERA‐5 atmospheric reanalysis. Generally, we find a large geographical split between the weather patterns that generate the strongest winds across the country. The highest wind power production periods therefore occur at different times in different regions across the country. In the South, these are associated with cold surge events, where an anticyclone is present in the Gulf of Mexico resulting in a strong Northerly flow across the Isthmus of Tehuantepec. In the North‐East, Easterly trade winds are responsible for the strongest wind events, whereas in the North‐West, it is the proximity of the North Pacific High. However, the weakest winds and lowest power production periods occur at the same times for all stations with the exception of Baja California Sur, meaning that low wind power production may be unavoidable at these times. The El Niño Southern Oscillation is found to influence wind speeds at some locations across Mexico at sub‐seasonal time‐scales. We report that statistically stronger wind speeds are observed during the Summer during El Niño months than during La Niña months for both sites in Chiapas and Oaxaca.

Climatological Large-Scale Circulation Patterns over The Middle Americas Region

In this study, twenty large-scale circulation patterns are identified to generate a synoptic classification of Weather Types (WT) over a region that comprises Mexico, the Intra-Americas Seas, Central America, and northern South America. This classification is performed using Self-Organizing Maps (SOMs) with mean sea-level pressure standardized anomalies from reanalysis. The influence of quasi-permanent pressure centers over the region, such as North Atlantic Subtropical High (NASH) and North Pacific High (NPH) are well captured. Seasonal variability of high-pressure centers for dry (November–April) and wet (May–October) periods over the entire region are also well represented in amplitude and pattern among the WTs. The NASH influence and intensification of the Caribbean low-level jet and the North American monsoon system is well captured. During the dry period, a strong trough wind advects cold air masses from mid-latitudes to the subtropics over the western Atlantic Ocean. High-frequency transitions among WTs tend to cluster around the nearest neighbors in SOM space, while low-frequency transitions occur along columns instead of rows in the SOM matrix. Low-frequency transitions are related to intraseasonal and seasonal scales. The constructed catalog can identify near-surface atmospheric circulation patterns from a unified perspective of synoptic climate variability, and it is in high agreement with previous studies for the region.

Study on the Association of Casualties and Classification of Heat Wave Weather Patterns in South Korea Using K-Means Clustering Analysis

In this study, weather patterns (WPs) associated with the heat wave in South Korea are objectively classified by applying <i>K</i>-means clustering analysis. The representative weather patterns that caused the heat wave were divided into three WPs, namely WP 1, WP 2, and WP 3. The heat wave over the Korean Peninsula was mainly related to the expansion of the North Pacific High (NPH). Moreover, we analyzed the relationship between casualties and WPs of the heat wave. In WP 1, the isobar of NPH was located in the southern part of South Korea. Most casualties (18 people) occurred in this region. In WP 2, NPH was distributed throughout South Korea, with nationwide casualties of 44 people. Moreover, the duration of the heat wave for WP 2 was the longest, at 4.5 days. WP 3 occurred mainly in June, when the NPH was not yet developed, presenting the smallest number of casualties.

Linking large-scale circulation patterns to low-cloud properties

Abstract. The North Pacific High (NPH) is a fundamental meteorological feature present during the boreal warm season. Marine boundary layer (MBL) clouds, which are persistent in this oceanic region, are influenced directly by the NPH. In this study, we combine 11 years of reanalysis and an unsupervised machine learning technique to examine the gamut of 850 hPa synoptic-scale circulation patterns. This approach reveals two distinguishable regimes – a dominant NPH setup and a land-falling cyclone – and in between a spectrum of large-scale patterns. We then use satellite retrievals to elucidate for the first time the explicit dependence of MBL cloud properties (namely cloud droplet number concentration, liquid water path, and shortwave cloud radiative effect – CRESW) on 850 hPa circulation patterns over the northeast Pacific Ocean. We find that CRESW spans from −146.8 to −115.5 W m−2, indicating that the range of observed MBL cloud properties must be accounted for in global and regional climate models. Our results demonstrate the value of combining reanalysis and satellite retrievals to help clarify the relationship between synoptic-scale dynamics and cloud physics.

Physical drivers of the summer 2019 North Pacific marine heatwave

Summer 2019 observations show a rapid resurgence of the Blob-like warm sea surface temperature (SST) anomalies that produced devastating marine impacts in the Northeast Pacific during winter 2013/2014. Unlike the original Blob, Blob 2.0 peaked in the summer, a season when little is known about the physical drivers of such events. We show that Blob 2.0 primarily results from a prolonged weakening of the North Pacific High-Pressure System. This reduces surface winds and decreases evaporative cooling and wind-driven upper ocean mixing. Warmer ocean conditions then reduce low-cloud fraction, reinforcing the marine heatwave through a positive low-cloud feedback. Using an atmospheric model forced with observed SSTs, we also find that remote SST forcing from the central equatorial and, surprisingly, the subtropical North Pacific Ocean contribute to the weakened North Pacific High. Our multi-faceted analysis sheds light on the physical drivers governing the intensity and longevity of summertime North Pacific marine heatwaves.

Thermal Convection in the North Pacific High Pressure Cell

If it is accepted that thermal convection consistently takes place inside the North Pacific High, as proposed here, then the existence of the NPH, as well as its seasonal variation, will be explained simultaneously, building on an earlier attempt. More observations than available at present would help prove that thermal convection happens and pin down its characteristics, since it is not visible. Also the physics of how thermal convection produces relatively high pressure at sea level needs work.

Thermal Convection Mechanics

Assuming thermal convection takes place continuously inside the North Pacific High, an application of physics to the rising sea-warmed air plumes increases understanding of the relatively high air pressure at the sea surface and at the bases of the plumes. Since the ocean is warmer than the air under the NPH, heat is transferred upward decreasing the air density, which should then accelerate upward when no other forces are around to stop it. By Newton’s 3rd law the upward accelerating air will cause an equal but opposite (downward) reaction resulting in high pressure in the air under the rising column. That is the central proposal offered, which is consistent with available observations. New data that confirm the upward acceleration of the heated air are especially needed. Perhaps dye or neutrally buoyant particles could be released from a ship or buoy to make the upward air motion visible.

Thermal Convection Mechanics

Assuming thermal convection takes place continuously inside the North Pacific High, an application of physics to the rising sea-warmed air plumes increases understanding of the relatively high air pressure at the sea surface and at the bases of the plumes. Since the ocean is warmer than the air under the NPH, heat is transferred upward decreasing the air density, which should then accelerate upward when no other forces are around to stop it. By Newton’s 3rd law the upward accelerating air will cause an equal but opposite (downward) reaction resulting in high pressure in the air under the rising column. That is the central proposal offered, which is consistent with available observations. New data that confirm the upward acceleration of the heated air are especially needed. Perhaps dye or neutrally buoyant particles could be released from a ship or buoy to make the upward air motion visible.

Changes in the North Pacific Current divergence and California Current transport based on HadGEM2-ES CMIP5 projections to the end of the century

This study aims to evaluate the potential changes to the strength of the North Pacific Current (NPC) and its position of divergence, which would alter the variability of the Alaska (AC) and California (CC) currents throughout the century. For this, the volume transports of these currents were evaluated using the historical CMIP5 and RCP4.5 experimental results of the HadGEM2-ES model from 1985 to 2100. In general, the NPC will migrate to the north, in accord to the northwestward expansion of North Pacific High, which would alter the positioning and the amount of flow entering the AC and CC systems by 2100. Along with the bifurcation shift, changes in the two main modes of variability, i.e., the bifurcation and breathing modes, are expected. The bifurcation mode will become responsible for only 11% of the total variability while presenting a positive anomaly throughout the century. These results indicate the role of NPC migration on the southward transport of enriched waters and the possible impact on the primary productivity along the North American coast.

A Snowpack Forecasting Model for the Eastern Sierra Nevada Based on Cointegration With the North Pacific High Sea-Level Pressure Anomaly

A cointegrated relationship has been identified between the January sea level pressure anomaly at the climatological location of the North Pacific High (NPH) and seasonal precipitation throughout California (Costa-Cabral et al., 2016). This cointegration can be used for forecasting precipitation or snowpack indices at California locations. Here we develop a cointegration model, termed Vector Error Correcting Model (VECM), for issuing a forecast, in early February, for April 1 snow water content at snow stations in the Eastern Sierra Nevada mountain range of California. Snowmelt from this region flows into the Owens River and serves as a major source of freshwater for the Los Angeles metropolitan area. The VECM relies on the cointegration between three variables: the January NPH sea level pressure, the February 1 snow water content, and the April 1 snow water content. Forecasts based on this VECM model have higher measures of skill compared to linear correlation methods. The statistical tool presented can be applied to other California watersheds and may provide reservoir operators the needed insight for making storage decisions in early February.

Rising synchrony controls western North American ecosystems.

Along the western margin of North America, the winter expression of the North Pacific High (NPH) strongly influences interannual variability in coastal upwelling, storm track position, precipitation, and river discharge. Coherence among these factors induces covariance among physical and biological processes across adjacent marine and terrestrial ecosystems. Here, we show that over the past century the degree and spatial extent of this covariance (synchrony) has substantially increased, and is coincident with rising variance in the winter NPH. Furthermore, centuries-long blue oak (Quercus douglasii) growth chronologies sensitive to the winter NPH provide robust evidence that modern levels of synchrony are among the highest observed in the context of the last 250years. These trends may ultimately be linked to changing impacts of the El Niño Southern Oscillation on midlatitude ecosystems of North America. Such a rise in synchrony may destabilize ecosystems, expose populations to higher risks of extinction, and is thus a concern given the broad biological relevance of winter climate to biological systems.