El Niño‐related vertical mixing enhancement under the winter mixed layer at western subarctic North Pacific station K2

2021 ◽  
Author(s):  
Akira Nagano ◽  
Masahide Wakita ◽  
Tetsuichi Fujiki ◽  
Hiroshi Uchida
Keyword(s):  
Mixed Layer ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
Vertical Mixing ◽  
Mixing Enhancement ◽  
Subarctic North Pacific ◽  
Winter Mixed Layer ◽  
Western Subarctic North Pacific

scholarly journals Spatial Distribution and Seasonality of Halocline Structures in the Subarctic North Pacific

2020 ◽  
Vol 50 (1) ◽  
pp. 95-109
Author(s):  
Shota Katsura ◽  
Hiromichi Ueno ◽  
Humio Mitsudera ◽  
Shinya Kouketsu
Keyword(s):  
Spatial Distribution ◽  
Mixed Layer ◽  
North Pacific ◽  
Mixed Layer Depth ◽  
Sea Surface Salinity ◽  
Late Winter ◽  
Freshwater Flux ◽  
Cooling Period ◽  
Subarctic North Pacific ◽  
Winter Mixed Layer

AbstractThe spatial distribution and seasonality of halocline structures in the subarctic North Pacific (SNP) were investigated using Argo profiling float data and various surface flux data collected in 2003–17. The permanent halocline (PH) showed zonal patterns in the spatial distributions of its depth and intensity and tended to be shallow and strong in the eastern SNP but deep and weak in the west. Mean distributions of PH depth and intensity corresponded to the winter mixed layer depth and sea surface salinity, respectively, indicating that it forms in association with the development of the winter mixed layer. In the Western Subarctic Gyre and Alaskan Gyre, where a relatively strong PH formed, PH intensity and depth showed clear seasonal variations, and deepening of the mixed layer compressed the underlying PH during the cooling period, resulting in intensification and development of the PH in late winter. In both regions, upwelling of high-salinity water also contributed to PH intensification. The summer seasonal halocline (SH) showed distinct zonal differences in frequency and intensity, which were opposite to the PH distribution. While an SH formed in the western and central SNP and coastal regions, it was seldom present in the eastern area. This zonal contrast of SH corresponded to freshening of the mixed layer during the warming period, primarily reflecting freshwater flux. Geostrophic and Ekman advection play important roles in spatial differences in SH intensity and depth. SH development contributed to PH intensification in the following winter, by decreasing salinity above the PH through entrainment.

The Role of Ekman Ocean Heat Transport in the Northern Hemisphere Response to ENSO

2008 ◽  
Vol 21 (21) ◽  
pp. 5688-5707 ◽  
Author(s):  
Michael A. Alexander ◽  
James D. Scott
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
La Niña ◽  
Ekman Transport ◽  
Layer Model ◽  
The North ◽  
La Nina

Abstract The influence of oceanic Ekman heat transport (Qek) on air–sea variability associated with ENSO teleconnections is examined via a pair of atmospheric general circulation model (AGCM) experiments. In the mixed layer model (MLM) experiment, observed sea surface temperatures (SSTs) for the years 1950–99 are specified over the tropical Pacific, while a grid of mixed layer models is coupled to the AGCM elsewhere over the global oceans. The same experimental design was used in the Ekman transport/mixed layer model (EKM) experiment with the addition of Qek in the mixed layer ocean temperature equation. The ENSO signal was evaluated using differences between composites of El Niño and La Niña events averaged over the 16 ensemble members in each experiment. In both experiments the Aleutian low deepened and the resulting surface heat fluxes cooled the central North Pacific and warmed the northeast Pacific during boreal winter in El Niño relative to La Niña events. Including Qek amplified the ENSO-related SSTs by ∼⅓ in the central and northeast North Pacific, producing anomalies comparable to those in nature. Differences between the ENSO-induced atmospheric circulation anomalies in the EKM and MLM experiments were not significant over the North Pacific. The sea level pressure (SLP) and SST response to ENSO over the Atlantic strongly projects on the North Atlantic Oscillation (NAO) and the SST tripole pattern in observations and both model experiments. The La Niña anomalies, which are stronger than during El Niño, include high pressure and positive SSTs in the central North Atlantic. Including Ekman transport enhanced the Atlantic SST anomalies, which in contrast to the Pacific, appeared to strengthen the overlying atmospheric circulation.

scholarly journals ENSO’s Modulation of Water Vapor Transport over the Pacific–North American Region

2015 ◽  
Vol 28 (9) ◽  
pp. 3846-3856 ◽  
Author(s):  
Hye-Mi Kim ◽  
Michael A. Alexander
Keyword(s):  
United States ◽  
El Niño ◽  
North Pacific ◽  
Moisture Transport ◽  
El Nino ◽  
Tropical Pacific ◽  
Vapor Transport ◽  
Water Vapor Transport ◽  
Southwestern United States ◽  
The Pacific

Abstract The vertically integrated water vapor transport (IVT) over the Pacific–North American sector during three phases of ENSO in boreal winter (December–February) is investigated using IVT values calculated from the Climate Forecast System Reanalysis (CFSR) during 1979–2010. The shift of the location and sign of sea surface temperature (SST) anomalies in the tropical Pacific Ocean leads to different atmospheric responses and thereby changes the seasonal mean moisture transport into North America. During eastern Pacific El Niño (EPEN) events, large positive IVT anomalies extend northeastward from the subtropical Pacific into the northwestern United States following the anomalous cyclonic flow around a deeper Aleutian low, while a southward shift of the cyclonic circulation during central Pacific El Niño (CPEN) events induces the transport of moisture into the southwestern United States. In addition, moisture from the eastern tropical Pacific is transported from the deep tropical eastern Pacific into Mexico and the southwestern United States during CPEN. During La Niña (NINA), the seasonal mean IVT anomaly is opposite to that of two El Niño phases. Analyses of 6-hourly IVT anomalies indicate that there is strong moisture transport from the North Pacific into the northwestern and southwestern United States during EPEN and CPEN, respectively. The IVT is maximized on the southeastern side of a low located over the eastern North Pacific, where the low is weaker but located farther south and closer to shore during CPEN than during EPEN. Moisture enters the southwestern United States from the eastern tropical Pacific during NINA via anticyclonic circulation associated with a ridge over the southern United States.

scholarly journals Local Coupled Equatorial Variability versus Remote ENSO Forcing in an Intermediate Coupled Model of the Tropical Atlantic

2006 ◽  
Vol 19 (20) ◽  
pp. 5227-5252 ◽  
Author(s):  
Serena Illig ◽  
Boris Dewitte
Keyword(s):  
Mixed Layer ◽  
El Niño ◽  
Time Scale ◽  
El Nino ◽  
Coupled Model ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Layer Model ◽  
Intermediate Coupled Model ◽  
Mixed Layer Model

Abstract The relative roles played by the remote El Niño–Southern Oscillation (ENSO) forcing and the local air–sea interactions in the tropical Atlantic are investigated using an intermediate coupled model (ICM) of the tropical Atlantic. The oceanic component of the ICM consists of a six-baroclinic mode ocean model and a simple mixed layer model that has been validated from observations. The atmospheric component is a global atmospheric general circulation model developed at the University of California, Los Angeles (UCLA). In a forced context, the ICM realistically simulates both the sea surface temperature anomaly (SSTA) variability in the equatorial band, and the relaxation of the Atlantic northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an El Niño event. The results of coupled experiments with or without Pacific ENSO forcing and with or without explicit air–sea interactions in the equatorial Atlantic indicate that the background energy in the equatorial Atlantic is provided by ENSO. However, the time scale of the variability and the magnitude of some peculiar events cannot be explained solely by ENSO remote forcing. It is demonstrated that the peak of SSTA variability in the 1–3-yr band as observed in the equatorial Atlantic is due to the local air–sea interactions and is not a linear response to ENSO. Seasonal phase locking in boreal summer is also the result of the local coupling. The analysis of the intrinsic sustainable modes indicates that the Atlantic El Niño is qualitatively a noise-driven stable system. Such a system can produce coherent interdecadal variability that is not forced by the Pacific or extraequatorial variability. It is shown that when a simple slab mixed layer model is embedded into the system to simulate the northern tropical Atlantic (NTA) SST variability, the warming over NTA following El Niño events have characteristics (location and peak phase) that depend on air–sea interaction in the equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipolelike structure of the SSTA variability that evolves at a decadal time scale. The results herein illustrate the complexity of the tropical Atlantic ocean–atmosphere system, whose predictability jointly depends on ENSO and the connections between the Atlantic modes of variability.

Future Changes to El Niño Teleconnections over the North Pacific and North America

2021 ◽  
pp. 1-43
Author(s):  
Jonathan D. Beverley ◽  
Matthew Collins ◽  
F. Hugo Lambert ◽  
Robin Chadwick
Keyword(s):  
North America ◽  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
Positive Temperature ◽  
Central Pacific ◽  
Wave Source ◽  
The North ◽  
Precipitation Anomalies ◽  
The North Pacific

AbstractThe El Niño-Southern Oscillation (ENSO) is the leading mode of interannual climate variability and it exerts a strong influence on many remote regions of the world, for example in northern North America. Here, we examine future changes to the positive-phase ENSO teleconnection to the North Pacific/North America sector and investigate the mechanisms involved. We find that the positive temperature anomalies over Alaska and northern North America that are associated with an El Niño event in the present day are much weaker, or of the opposite sign, in the CMIP6 abrupt 4×CO2 experiments for almost all models (22 out of 26, of which 15 are statistically significant differences). This is largely related to changes to the anomalous circulation over the North Pacific, rather than differences in the equator-to-pole temperature gradient. Using a barotropic model, run with different background circulation basic states and Rossby wave source forcing patterns from the individual CMIP6 models, we find that changes to the forcing from the equatorial central Pacific precipitation anomalies are more important than changes in the global basic state background circulation. By further decomposing this forcing change into changes associated with the longitude and magnitude of ENSO precipitation anomalies, we demonstrate that the projected overall eastward shift of ENSO precipitation is the main driver of the temperature teleconnection change, rather than the increase in magnitude of El Niño precipitation anomalies which are, nevertheless, seen in the majority of models.

scholarly journals Weakened Anomalous Western North Pacific Anticyclone during an El Niño–Decaying Summer under a Warmer Climate: Dominant Role of the Weakened Impact of the Tropical Indian Ocean on the Atmosphere

2018 ◽  
Vol 32 (1) ◽  
pp. 213-230 ◽  
Author(s):  
Chao He ◽  
Tianjun Zhou ◽  
Tim Li
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Tropical Indian Ocean ◽  
Tropospheric Temperature ◽  
Coupled Models ◽  
Subtropical Anticyclone ◽  
Mean State

Abstract The western North Pacific subtropical anticyclone (WNPAC) is the most prominent atmospheric circulation anomaly over the subtropical Northern Hemisphere during the decaying summer of an El Niño event. Based on a comparison between the RCP8.5 and the historical experiments of 30 coupled models from the CMIP5, we show evidence that the anomalous WNPAC during the El Niño–decaying summer is weaker in a warmer climate although the amplitude of the El Niño remains generally unchanged. The weakened impact of the sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) on the atmosphere is essential for the weakened anomalous WNPAC. In a warmer climate, the warm tropospheric temperature (TT) anomaly in the tropical free troposphere stimulated by the El Niño–related SSTA is enhanced through stronger moist adiabatic adjustment in a warmer mean state, even if the SSTA of El Niño is unchanged. But the amplitude of the warm SSTA over TIO remains generally unchanged in an El Niño–decaying summer, the static stability of the boundary layer over TIO is increased, and the positive rainfall anomaly over TIO is weakened. As a result, the warm Kelvin wave emanating from TIO is weakened because of a weaker latent heating anomaly over TIO, which is responsible for the weakened WNPAC anomaly. Numerical experiments support the weakened sensitivity of precipitation anomaly over TIO to local SSTA under an increase of mean-state SST and its essential role in the weakened anomalous WNPAC, independent of any change in the SSTA.

scholarly journals Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions

2013 ◽  
Vol 10 (9) ◽  
pp. 5947-5965 ◽  
Author(s):  
M. Girault ◽  
H. Arakawa ◽  
A. Barani ◽  
H. J. Ceccaldi ◽  
F. Hashihama ◽  
...  
Keyword(s):  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
Subtropical Gyre ◽  
La Niña ◽  
North Pacific Subtropical Gyre ◽  
Heterogeneous Distribution ◽  
La Nina ◽  
The Kuroshio ◽  
Kuroshio Region

Abstract. The distribution of ultraphytoplankton was investigated in the western North Pacific Subtropical Gyre (NPSG) during La Niña, a cold phase of El Niño Southern Oscillation (ENSO). Observations were conducted in a north-south transect (33.6–13.25° N) along the 141.5° E meridian in order to study the ultraplankton assemblages in various oligotrophic conditions. Analyses were performed at the single cell level by analytical flow cytometry. Five ultraphytoplankton groups (Prochlorococcus, Synechococcus, picoeukaryotes, nanoeukaryotes and nanocyanobacteria-like) defined by their optical properties were enumerated in three different areas visited during the cruise: the Kuroshio region, the subtropical Pacific gyre and a transition zone between the subtropical Pacific gyre and the Warm pool. Prochlorococcus outnumbered the other photoautotrophs in all the investigated areas. However, in terms of carbon biomass, an increase in the relative contribution of Synechococcus, picoeukaryotes and nanoeukaryotes was observed from the centre of the subtropical gyre to the Kuroshio area. In the Kuroshio region, a peak of abundance of nanoeukaryotes observed at the surface suggested an increase in nutrients likely due to the vicinity of a cold cyclonic eddy. In contrast, in the salinity front along the isohaline 35 and anticyclonic eddy located around 22.83° N, the mainly constant distribution of Prochlorococcus from the surface down to 150 m characterised the dominance by these microorganisms in high salinity and temperature zone. Results suggested that the distribution of nanocyanobacteria-like is also closely linked to the salinity front rather than low phosphate concentration. The maximum abundance of ultraphytoplankton was located above the SubTropical Counter Current (STCC) at depths > 100 m where higher nutrient concentrations were measured. Finally, comparison of the ultraphytoplankton concentrations during El Niño (from the literature) and La Niña (this study) conditions seems to demonstrate that La Niña conditions lead to higher concentrations of Synechococcus in the Subtropical gyre and a lower abundance of Synechococcus in the Kuroshio region. Our results suggest that the west part of NPSG is a complex area, where different water masses, salinity fronts and eddies lead to a heterogeneous distribution of ultraphytoplankton assemblages in the upper layer of the water column.

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