Mid-latitude North Pacific Ocean Research Articles

Heterogenous westerly shifts linked to Atlantic meridional overturning circulation slowdowns

The mid-latitude westerly winds are a major component of the global atmospheric circulation and a dominant factor in mid-latitude climate change. Understanding their behaviour and the controls on their variations under different climate background states is essential for assessing climate system feedback. Here we present a midlatitude North Pacific Ocean aeolian dust record from core NP02 through the last glacial cycle, during which extreme and abrupt climatic oscillations occurred. We find low dust contents during Heinrich stadials 2, 4, 5, and 5a that we attribute using proxy-model comparison to westerly transportation path changes associated with Atlantic meridional overturning circulation (AMOC) reductions, which caused North Atlantic cooling and modified the westerly wave train pattern, particularly over the Tibetan Plateau. The finding that AMOC variations had significant impacts on the westerlies half-way around the world, through ocean-atmosphere interactions, improves understanding of large-scale westerly sensitivity to different climate states.

The sensitivity of a mid-latitude maritime stratocumulus cloud to surface fluxes

Surface fluxes of stratocumulus clouds over mid-latitude oceans have larger variability and bigger Bowen ratios than those over subtropical oceans. This study investigates how mid-latitude maritime stratocumulus clouds respond to surface fluxes and what the underlying mechanisms are. A closed-cellular stratocumulus cloud over mid-latitude North Pacific Ocean is simulated with large eddy simulation. A control case and a reduced-flux case (with surface fluxes 50% reduced) are compared. Liquid water path (LWP) and turbulent kinetic energy (TKE) decrease as surface fluxes are reduced. It is revealed that reducing surface fluxes leads to a cooler and drier boundary layer, which directly leads to a higher cloud base. The weaker turbulence near the cloud top leads to less entrainment, resulting in a lower cloud top. Therefore, the cloud is 21% thinner with a smaller LWP in the reduced-flux case. Quadrant analysis shows that TKE production processes including surface heating, in-cloud latent heating, and cloud-top evaporative cooling are all weaker in the reduced-flux case. Meanwhile, cloud-top entrainment heating and overshooting of air parcels into the inversion, as TKE consumption processes, are also weaker in the reduced-flux case. These TKE production and consumption processes counteract with each other, leading to quite different pictures of turbulence transport in the two cases. In the subcloud layer, the control case is dominated by surface heating (indicating a turbulent heating effect), while the reduced-flux case is dominated by evaporative cooling in the downdraft region (indicating a turbulent cooling effect). In the cloud layer, the difference of heat flux between the two cases is mainly due to the difference in latent heating. At cloud top, the two cases have similar heat flux.

Decadal changes in the basin-wide heat budget of the mid-latitude North Pacific Ocean

Research has indicated that the meridional heat transport (MHT) in the North Pacific Ocean (NP) across 24°N increased in the 1980s and 1990s, resulting in different heat distributions: the ocean heat content (OHC) increased in the 1980s, while the net surface heat release was strengthened in the 1990s; however, the reasons for these differences remained unclear. The authors revisited the investigation of the heat budget in the NP mainly using hydrographic observations to understand why the heat distribution was different between these 2 decades and extend the analysis to the 2010s. The OHC in the upper 700 m north of 24°N and east of 137°E exhibited sharp increases around 1990 as well as in the 2010s, while it was nearly stable in between. The northward retreat of the subarctic gyre boundary coincided with the spin-up of the subtropical gyre in the late 1980s, thereby allowing warm anomaly from the subtropics to propagate northeastward. Meanwhile, the concurrent weakening of the wintertime westerlies resulted in the suppression of the surface heat loss in the western NP. In contrast, the southward shift of the subarctic front suppressed the OHC rise, despite the MHT increase in the late 1990s. In the 2010s, unprecedented warming occurred in the eastern NP. The MHT estimation based on hydrographic observations indicates that the net surface release must have been suppressed since the MHT did not increase; however, the latest atmospheric reanalysis datasets failed to reproduce this.

Poleward and weakened westerlies during Pliocene warmth.

The prevailingmid-latitude westerlywinds, known as the westerlies,are a fundamental component of the climate system because they have a crucial role in driving surface ocean circulation1 and modulating air-sea heat, momentum and carbon exchange1-3. Recent work suggests that westerly wind belts are migrating polewards in response to anthropogenic forcing4,5. Reconstructing the westerlies during past warm periods such as the Pliocene epoch, in which atmospheric carbon dioxide (CO2) was about 350 to 450 parts per million6 and temperatures were about 2 to 4 degrees Celsius higher than today7, can improve our understanding of changes in the position and strength of these wind systems as the climate continues to warm. Here we show that the westerlies were weaker and more poleward during the warm Pliocene than during glacial periods after the intensification of Northern Hemisphere glaciation (iNHG), which occurred around 2.73 million years ago8. Our results, which are based on dust and export productivity reconstructions, indicate that major ice sheet development during the iNHG was accompanied by substantial increases in dust fluxes in the mid-latitude North Pacific Ocean, especially compared to those in the subarctic North Pacific. Following this shift, changes in dust and productivity largely track the glacial-interglacial cycles of the late Pliocene and early Pleistocene epochs. On the basis of this pattern, we infer that shifts in the westerlies were primarily driven by variations in Plio-Pleistocene thermal gradients and ice volume. By combining this relationship with other dust records9-11 and climate modelling results12, we find that the proposed changes in the westerlies were globally synchronous. If the Pliocene is predictive of future warming, we posit that continued poleward movement and weakening of the present-day westerlies in both hemispheres can be expected.

Impact of Satellite Data Assimilation in Atmospheric Reanalysis on the Marine Wind and Wave Climate

Abstract This study investigated the impact of assimilating satellite data into atmospheric reanalyses on trends in ocean surface winds and waves. Two experiments were performed using a numerical wave model forced by near-surface winds: one derived from the Japanese 55-year Reanalysis (JRA-55; experiment A) and the other derived from JRA-55 using assimilated conventional observations only (JRA-55C; experiment B). The results showed that the satellite data assimilation reduced upward trends of the annual mean of wave energy flux (WEF) in the midlatitude North Pacific and southern ocean (30°–60°S), south of Australia, from 1959 to 2012. It was also found that the assimilation of scatterometer winds reduced the near-surface wind speed in the midlatitude North Pacific after the mid-1990s, which resulted in the reduced trend in WEF from 1959 to 2012. By contrast, assimilation of the satellite radiances for 1973–94 increased near-surface wind speed in the southern ocean, south of Australia, whereas the assimilation of the scatterometer winds after the mid-1990s reduced wind speed. The latter led to the reduced trend in WEF south of Australia from 1959 to 2012.

Structure and dynamics of decadal anomalies in the wintertime midlatitude North Pacific ocean–atmosphere system

The structure and dynamics of decadal anomalies in the wintertime midlatitude North Pacific ocean–atmosphere system are examined in this study, using the NCEP/NCAR atmospheric reanalysis, HadISST SST and Simple Ocean Data Assimilation data for 1960–2010. The midlatitude decadal anomalies associated with the Pacific Decadal Oscillation are identified, being characterized by an equivalent barotropic atmospheric low (high) pressure over a cold (warm) oceanic surface. Such a unique configuration of decadal anomalies can be maintained by an unstable ocean–atmosphere interaction mechanism in the midlatitudes, which is hypothesized as follows. Associated with a warm PDO phase, an initial midlatitude surface westerly anomaly accompanied with intensified Aleutian low tends to force a negative SST anomaly by increasing upward surface heat fluxes and driving southward Ekman current anomaly. The SST cooling tends to increase the meridional SST gradient, thus enhancing the subtropical oceanic front. As an adjustment of the atmospheric boundary layer to the enhanced oceanic front, the low-level atmospheric meridional temperature gradient and thus the low-level atmospheric baroclinicity tend to be strengthened, inducing more active transient eddy activities that increase transient eddy vorticity forcing. The vorticity forcing that dominates the total atmospheric forcing tends to produce an equivalent barotropic atmospheric low pressure north of the initial westerly anomaly, intensifying the initial anomalies of the midlatitude surface westerly and Aleutian low. Therefore, it is suggested that the midlatitude ocean–atmosphere interaction can provide a positive feedback mechanism for the development of initial anomaly, in which the oceanic front and the atmospheric transient eddy are the indispensable ingredients. Such a positive ocean–atmosphere feedback mechanism is fundamentally responsible for the observed decadal anomalies in the midlatitude North Pacific ocean–atmosphere system.

Origin of Decadal-Scale, Eastward-Propagating Heat Content Anomalies in the North Pacific*

Abstract Upper ocean heat content (OHC) is at the heart of natural climate variability on interannual-to-decadal time scales, providing climate memory and the source of decadal prediction skill. In the midlatitude North Pacific Ocean, OHC signals are often found to propagate eastward as opposed to the frequently observed westward propagation of sea surface height, another variable that represents the ocean subsurface state. This dichotomy is investigated using a 150-yr coupled GCM integration. Simulated OHC signals are distinguished in terms of two processes that can support eastward propagation: higher baroclinic Rossby wave (RW) modes that are associated with density perturbation, and spiciness anomalies due to density-compensated temperature and salinity anomalies. The analysis herein suggests a unique role of the Kuroshio–Oyashio Extension (KOE) region as an origin of the spiciness and higher mode RW signals. Wind-forced, westward-propagating equivalent barotropic RWs cause meridional shifts of the subarctic front in the KOE region. The associated anomalous circulation crosses mean temperature and salinity gradients and thereby generates spiciness anomalies. These anomalies are advected eastward by the mean currents, while the associated surface temperature anomalies are damped by air–sea heat exchange. The accompanying surface buoyancy flux generates higher baroclinic, eastward-propagating RWs. The results suggest that the large OHC variability in the western boundary currents and their extensions is associated with the spiciness gradients and axial variability of oceanic fronts.

Local origins of interdecadal Pacific variability in the tropical and North Pacific Ocean: evidence from a comparative study of coral oxygen isotope records

Interdecadal Pacific variability (IPV) is commonly observed in both the tropical and mid-latitude Pacific Ocean, and has a widespread influence on surface climate in the Pan-Pacific Basin. This variability is recorded by climate proxies such as geochemical parameters preserved in corals. However, the origins of IPV remain uncertain. To shed light on this, interdecadal variations in two long coral δ18O records from Nauru Island and the South China Sea (SCS), respectively located in the tropical Pacific and the mid-latitude North Pacific Ocean, were investigated. The interdecadal fluctuations in the δ18O series from Nauru Island (tropical Pacific) match those of the NINO3.4 index reasonably well (r=–0.30, n=96, p=0.0015), but are not correlated with those of the Pacific decadal oscillation (PDO) index (r=–0.17, n=96, p=0.05). The δ18O time series from the SCS (northwestern Pacific), by contrast, co-vary with the PDO index (r=–0.30, n=156, p=0.0007), but are out of phase with the NINO3.4 index at the interdecadal timescale (r=0.04, n=156, p=0.31). The impact on the interdecadal variability of processes occurring outside the growth region of corals is generally weak. The results thus do not support a tropical origin of IPV, but demonstrate that the interdecadal variability in the tropical Pacific and the North Pacific originates predominantly from local coupled ocean–atmosphere processes within these regions. The results also suggest that tropical–extratropical interactions played a role in IPV between 1920 and 1940, which indicates that IPV is a complex climatic phenomenon that involves multiple forcing mechanisms.

Origin of the Intraseasonal Variability over the North Pacific in Boreal Summer*

Abstract The spatial structure and temporal evolution of the intraseasonal oscillation (ISO) in boreal summer over the midlatitude North Pacific Ocean are investigated, through the diagnosis of NCEP reanalysis data. It is found that the midlatitude ISO has an equivalent-barotropic structure, with maximum amplitude at 250 hPa. Initiated near 120°W, the ISO perturbation propagates westward at a phase speed of about 2.4 m s−1 and reaches a maximum amplitude at 150°W. A diagnosis of barotropic energy conversion shows that the ISO gains energy from the summer mean flow in the ISO activity region. A center-followed column-averaged vorticity budget analysis shows that the nonlinear eddy meridional vorticity transport plays a major role in the growth of the ISO perturbation. There is a two-way interaction between ISO flows and synoptic eddies. While a cyclonic (anticyclonic) ISO flow causes synoptic-scale eddies to tilt toward the northwest–southeast (northeast–southwest) direction, the tilted synoptic eddies then exert a positive feedback to reinforce the ISO cyclonic (anticyclonic) flow through eddy vorticity transport. The reanalysis data and numerical simulations show that the midlatitude ISO is primarily driven by local processes and the tropical forcing accounts for about 20% of total intraseasonal variability in midlatitudes. However, 20% might be an underestimate given that the tropical intraseasonal forcing is not fully included in the current observational analysis and modeling experiment.

Late Miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing

Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO(2), p(co(2))). Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (∼12-5 Myr ago), an interval with p(co(2)) values of only 200-350 parts per million by volume but nearly ice-free conditions in the Northern Hemisphere and warmer-than-modern temperatures on the continents. Here we present quantitative geochemical sea surface temperature estimates from the Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth persisted throughout the interval of low p(co(2)) ∼12-5 Myr ago. We also present new stable isotope measurements from the western equatorial Pacific that, in conjunction with previously published data, reveal a long-term trend of thermocline shoaling in the equatorial Pacific since ∼13 Myr ago. We propose that a relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapour feedbacks may help to explain the warmth of the late Miocene. Additional shoaling of the thermocline after 5 Myr ago probably explains the stronger coupling between p(co(2)), sea surface temperatures and climate that is characteristic of the more recent Pliocene and Pleistocene epochs.

Eddy correlation measurements of the air/sea flux of dimethylsulfide over the North Pacific Ocean

Shipboard measurements of air/sea fluxes and sea surface concentrations of dimethylsulfide (DMS) were made over the tropical and midlatitude North Pacific Ocean. Atmospheric pressure chemical ionization mass spectrometry was used to measure DMS levels in ambient air and in air equilibrated with surface seawater drawn from a depth of 5 m. Air/sea fluxes were obtained using eddy covariance. Corrections and uncertainties involved in the calculation of fluxes from shipboard data are discussed. The surface ocean DMS concentrations measured during this study ranged from 1 to 10 nM, and atmospheric mixing ratios ranged from 20 to 1000 ppt. Air/sea fluxes ranged from 0.47 to 39 μmol m−2 d−1. Most of the variance in the fluxes can be accounted for by variations in sea surface concentration (37%) and wind speed (19%). Gas transfer coefficients derived from the measurements are generally consistent with those derived from deliberate inert gas tracer experiments and eddy covariance measurements of CO2. The gas transfer coefficients exhibit wind speed dependence, but the variance in the data is sufficiently large that they do not constrain the functionality of the wind speed dependence of gas exchange.

Estimation of the Surface Heat Flux Response to Sea Surface Temperature Anomalies over the Global Oceans

Abstract The surface heat flux response to underlying sea surface temperature (SST) anomalies (the surface heat flux feedback) is estimated using 42 yr (1956–97) of ship-derived monthly turbulent heat fluxes and 17 yr (1984–2000) of satellite-derived monthly radiative fluxes over the global oceans for individual seasons. Net surface heat flux feedback is generally negative (i.e., a damping of the underlying SST anomalies) over the global oceans, although there is considerable geographical and seasonal variation. Over the North Pacific Ocean, net surface heat flux feedback is dominated by the turbulent flux component, with maximum values (28 W m−2 K−1) in December–February and minimum values (5 W m−2 K−1) in May–July. These seasonal variations are due to changes in the strength of the climatological mean surface wind speed and the degree to which the near-surface air temperature and humidity adjust to the underlying SST anomalies. Similar features are observed over the extratropical North Atlantic Ocean with maximum (minimum) feedback values of approximately 33 W m−2 K−1 (9 W m−2 K−1) in December–February (June–August). Although the net surface heat flux feedback may be negative, individual components of the feedback can be positive depending on season and location. For example, over the midlatitude North Pacific Ocean during late spring to midsummer, the radiative flux feedback associated with marine boundary layer clouds and fog is positive, and results in a significant enhancement of the month-to-month persistence of SST anomalies, nearly doubling the SST anomaly decay time from 2.8 to 5.3 months in May–July. Several regions are identified with net positive heat flux feedback: the tropical western North Atlantic Ocean during boreal winter, the Namibian stratocumulus deck off West Africa during boreal fall, and the Indian Ocean during boreal summer and fall. These positive feedbacks are mainly associated with the following atmospheric responses to positive SST anomalies: 1) reduced surface wind speed (positive turbulent heat flux feedback) over the tropical western North Atlantic and Indian Oceans, 2) reduced marine boundary layer stratocumulus cloud fraction (positive shortwave radiative flux feedback) over the Namibian stratocumulus deck, and 3) enhanced atmospheric water vapor (positive longwave radiative flux feedback) in the vicinity of the tropical deep convection region over the Indian Ocean that exceeds the negative shortwave radiative flux feedback associated with enhanced cloudiness.

Large-Scale Variability in the Midlatitude Subtropical and Subpolar North Pacific Ocean: Observations and Causes

Altimetric data from the 8-yr TOPEX/Poseidon (T/P) mission (Oct 1992–Jul 2000) are used to investigate large-scale circulation changes in the three current systems of the midlatitude North Pacific Ocean: the North Pacific Current (NPC), the Alaska gyre, and the western subarctic gyre (WSG). To facilitate the understanding of the observed changes, a two-layer ocean model was adopted that includes first-mode baroclinic Rossby wave dynamics and barotropic Sverdrup dynamics. The NPC intensified steadily over the T/P period from 1992 to 1998. Much of this intensification is due to the persistent sea surface height (SSH) drop on the northern side of the NPC. A similar SSH trend is also found in the interior of the Alaska gyre. Both of these SSH changes are shown to be the result of surface wind stress curl forcing accumulated along the baroclinic Rossby wave characteristics initiated from the eastern boundary. In addition to the interior SSH signals, the intensity of the Alaska gyre is shown to depend also on the SSH anomalies along the Canada/Alaska coast, and these anomalies are shown to be jointly determined by the signals propagating from lower latitudes and those forced locally by the alongshore surface winds. The WSG changed interannually from a zonally elongated gyre in 1993–95 to a zonally more contracted gyre in 1997–99. This structural change is due to the interannual SSH anomalies within the WSG as a result of the baroclinic Rossby wave adjustment attenuated by eddy dissipation. Along the western boundary of the subpolar North Pacific, variability of the East Kamchatka Current (EKC) and Oyashio is in balance with that of the interior Sverdrup flow on the annual and year-to-year timescales. On the multiyear timescales, the EKC/Oyashio variability is shown to be determined by the baroclinic SSH signals.

Meridional Ekman Heat Transport: Estimates from Satellite Data

Analyses of satellite-derived SSM/I winds and AVHRR sea surface temperatures are used to compute weekly estimates of global meridional ocean Ekman heat transport for the 4-year period 1987–1991. The heat transport is consistently poleward throughout the year over the Atlantic and much of the Pacific between 30°S and 30°N and equatorward at higher latitudes. The zonally integrated Ekman heat transport in the Pacific was weak and equatorward at 10°N in September 1989 and 1990, whereas in other years it is poleward throughout the year. In the Indian Ocean, equatorward heat transport was strongest in Northern Hemisphere summer 1990. The weekly time series provides better temporal resolution than previous studies that at best used monthly averages. The higher-frequency variations are explored through rotated empirical orthogonal functions (REOFs) of nonseasonal heat transport anomalies. The REOFs show large-scale coherence across the tropical and subtropical Pacific and Indian Oceans. The first REOF has a strong spectral peak at periods of 50–60 days and is dominated by the variability in the Southern Hemisphere Indian Ocean. The second REOF is dominated by the variance in the Northern Hemisphere Indian Ocean and western tropical North Pacific. The fist four REOFs, which explain 22.5% of the nonseasonal variance, have spectral peaks at periods consistent with the 30–60 day atmospheric Madden–Julian oscillation. These periods were not resolved in the monthly averaged data from COADS. Singular spectrum analysis has been used as a filter to show the long timescale variations. The early phase of the 1988–89 La Niña has a strong influence on the heat transport, indicating enhanced poleward heat transport in the eastern Indian Ocean, tropical North Pacific, and tropical and subtropical South Pacific and enhanced equatorward heat transport in the midlatitude North Pacific and western tropical Indian Ocean.

Midlatitude Sea Surface Temperature Anomalies: A Numerical Hindcast

Abstract A multilevel primitive equation, ocean circulation model with surface layer physics is used to study the interannual variability of sea surface temperatures (SST) in the central midlatitude North Pacific Ocean. Results from a 10-year model simulation (hindcast) driven by observed winds are analyzed and compared with observations. The hindcast SSTs exhibit a significant amount of nonseasonal variability despite being damped toward a regular annual cycle by the thermal boundary condition at the surface. This variability is due to the horizontal and vertical redistribution of heat by currants and by parameterized turbulence processes caused by the winds. The resulting hindcast SST anomalies are correlated with observed SST anomalies at a statistically significant level over a large part of the central midlatitude North Pacific Ocean. This suggests that wind forcing by itself, through the mechanisms noted before, makes an important contribution to the development of SST anomalies in this area. The hi...

A cyclo-stationary model of sea surface temperatures in the Pacific Ocean

Sea surface temperature anomalies in mid-latitude north Pacific Ocean are compared with a cyclo-stationary stochastic model in which the anomalies are forced by atmospheric “weather” disturbances. The results show that the model explains the observed two-time auto-covariances of the anomalies for almost all (98%) of the studied region. DOI: 10.1111/j.1600-0870.1985.tb00266.x

Response of the NCAR General Circulation Model to Prescribed Changes in Ocean Surface Temperature. Part II: Midlatitude and Subtropical Changes

Abstract The sensitivity of a six-layer NCAR atmospheric general circulation model (GCM) to a variety of idealized, very large amplitude, midlatitude and subtropical North Pacific Ocean surface temperature (OST) anomalies is analyzed. In the Pacific sector, the model exhibits a differential sensitivity depending on the latitudinal position of the imposed anomaly. Typically, the model response is a combination of a relative direct thermal circulation, an alteration in the pattern of cyclonic activity, and a selective wave response dependent on the planetary waves present in the unmodified control case. The extent to which the background planetary waves affect the model response is dependent on several factors including latitude-dependent features of the control case and the relative position of the anomaly. Analogous experiments with a simple quasi-geostrophic model are useful in isolating important physical and dynamical processes, and thereby assist in the interpretation of the GCM results. An analysis o...

Sea surface temperature anomalies in the North Pacific Ocean

Sea surface temperature anomalies in the mid-latitude North Pacific Ocean were compared with a first-order autoregression model in which the anomalies are forced by local atmospheric white noise. The results showed that the model can explain the power spectrum of the anomalies for a little over 50% of the investigated regions, mainly in the central regions of the Pacific, but fails, not surprisingly, in regions of strong oceanographic processes.

Sea surface temperature anomalies in the North Pacific Ocean

Sea surface temperature anomalies in the mid-latitude North Pacific Ocean were compared with a first-order autoregression model in which the anomalies are forced by local atmospheric white noise. The results showed that the model can explain the power spectrum of the anomalies for a little over 50% of the investigated regions, mainly in the central regions of the Pacific, but fails, not surprisingly, in regions of strong oceanographic processes.

Zonal Variability in the Distribution of Eddy Energy in the Mid-Latitude North Pacific Ocean

Hydrographic and expendable bathythermograph (XBT) data from several sources have been drawn together to summarize the distribution of baroclinc mesoscale eddy energy in the mid-latitude North Pacific Ocean. A striking order of magnitude decrease in the energy distribution at 500–1000 km wave-lengths is observed to occur abruptly east of the 170°W meridian. Mean zonal wavenumber spectra, constructed from 16 zonal XBT sections on either side of 170°W, confirm these observations.