Bidecadal Oscillations Research Articles

Global lead-lag changes between climate variability series coincide with major phase shifts in the Pacific decadal oscillation

We show that there are distinct periods when three ocean variability series in the Atlantic and the Pacific Oceans persistently lead or lag each other, as well as distinct periods when ocean variability series lead the rate of changes in global temperature anomaly (∆GTA) and in atmospheric CO2 concentration (1880–2019). The superimposed lead-lag (LL) relations that can be formed from the five climate series (three ocean series, GTA and CO2), ΣLL(10), change directions or weaken synchronously at 6 years: 1900, 1926, 1965, 1977, 1997, and 2013. During the same years, the Pacific decadal oscillation (PDO) changes between positive ( +) and negative (-) phases, but with an additional phase shift in 1947/48. We find bi-decadal oscillations in the rate of change in global temperature, ∆GTA, during the same years. Since the hiatus periods are closely related to the cold phase (-) in PDO, the hiatus periods may also be related to global changes in ocean interactions.

Anthropogenic and Climatic Contributions to Observed Carbon System Trends in the Northeast Pacific

AbstractThe ocean absorbs anthropogenic carbon, slowing atmospheric CO2 increase but driving ocean acidification. Long‐term changes in the carbon system are typically assessed from single‐point time series or from hydrographic sections spaced by decades. Using higher resolution observations (1–3 year−1) from the Line P time series, we investigate processes modulating trends in the carbon system of the northeast subarctic Pacific. Dissolved inorganic carbon (DIC) and apparent oxygen utilization (AOU) from 1990 to 2019 reveal substantial trends over most of the upper water column along the 1,500 km coastal to open ocean transect. At the surface, an increasing trend in salinity‐normalized DIC (sDIC33) (+0.5 ± 0.4 μmol kg−1 yr−1) is associated with a decrease in pH (0.01–0.02 decade−1) and a decrease in aragonite saturation state (0.04–0.08 decade−1). These observed trends are driven by anthropogenic CO2 uptake, partially offset by trends in surface salinity or temperature. Stratification associated with recent marine heat waves appears to have caused anomalously low surface pCO2. sDIC33 trends of similar magnitude were found below the seasonal thermocline on the 26.7–26.8 isopycnals (150–300 m), which are ventilated in the western Pacific. Roughly, a third (20%–50%) of the subsurface sDIC33 trend is driven by increased remineralization, likely caused by long‐term decreases in ventilation in the western Pacific. Bidecadal oscillations in the ventilation of the 26.7–26.8 isopycnals arising from the Lunar Nodal Cycle cause oscillations in sDIC33 and AOU at the offshore end of our transect. We trace the oscillations to alternating periods of higher anthropogenic carbon uptake or higher carbon remineralization.

A Proposed Exogenous Cause of the Global Temperature Hiatus

Since 1850, the rise in global mean surface temperatures (GMSTs) from increasing atmospheric concentrations of greenhouse gases (GHGs) has exhibited three ~30-year hiatus (surface cooling) episodes. The current hiatus is often thought to be generated by similar cooling episodes in Pacific or Atlantic ocean basins. However, GMSTs as well as reconstructed Atlantic and Pacific ocean basin surface temperatures show the presence of similar multidecadal components generated from a three-dimensional analysis of differential gravitational (tidal) forcing from the sun and moon. This paper hypothesizes that these episodes are all caused by external tidal forcing that generates alternating ~30-year zonal and meridional circulation regimes, which respectively increase and decrease GMSTs through tidal effects on sequestration (deep ocean heat storage) and energy redistribution. Hiatus episodes consequently coincide with meridional regimes. The current meridional regime affecting GMSTs is predicted to continue to the mid-2030s but have limited tendency to decrease GMSTs from sequestration because of continuing increases in radiative forcing from increasing atmospheric GHGs. The tidal formulation also generates bidecadal oscillations, which may generate shorter ~12-year hiatus periods in global and ocean basin temperatures. The formulation appears to assimilate findings from disciplines as disparate as geophysics and biology.

Decline and Bidecadal Oscillations of Dissolved Oxygen in the Oyashio Region and Their Propagation to the Western North Pacific

AbstractDuring the period 1954–2014, concentrations of dissolved oxygen (O2) in the Oyashio region of the western subarctic North Pacific has been oscillating over bidecadal timescales and significantly decreasing over the isopycnal layer spanning σθ = 26.6–27.5 kg/m3. The cycles of oscillation (16.4–19.6 years) are almost consistent and synchronized within 1 year over this density layer and are probably controlled by the nodal tidal cycle of 18.6 years and/or atmospheric forcing. The mean rate of the long‐term decrease is the highest (−0.70 ± 0.06 μmol · kg−1 · year−1) in the temperature minimum layer. The O2 decline there is predominantly attributed to a reduction of ventilation in winter due to warming and freshening. On the other hand, the O2 decline in deeper layers down to the oxygen minimum layer is attributable to a reduction in ventilation in the Sea of Okhotsk associated with a reduction in sea ice formation and propagation of its impact through diapycnal mixing adjacent to the Bussol' Strait. These trends of bidecadal oscillations and decline of O2 were also found in the downstream to the east in the 165°E section at latitudes 30°N–42.5°N on σθ = 26.8 kg/m3 and with attenuated amplitudes at latitudes of 40°N–45°N in the oxygen minimum layer on σθ = 27.4 kg/m3. These results indicate that the signal of secular declines of O2, together with bidecadal oscillations, is being propagated broadly from the Oyashio source region into the interior of the Pacific Ocean.

Wavelet-based variability of Yellow River discharge at 500-, 100-, and 50-year timescales

Water scarcity in the Yellow River, China, has become increasingly severe over the past half century. In this paper, wavelet transform analysis was used to detect the variability of natural, observed, and reconstructed streamflow in the Yellow River at 500-, 100-, and 50-year timescales. The periodicity of the streamflow series and the co-varying relationships between streamflow and atmospheric circulation indices/sunspot number were assessed by means of continuous wavelet transform (CWT) and wavelet transform coherence (WTC) analyses. The CWT results showed intermittent oscillations in streamflow with increasing periodicities of 1–6years at all timescales. Significant multidecadal and century-scale periodicities were identified in the 500-year streamflow series. The WTC results showed intermittent interannual covariance of streamflow with atmospheric circulation indices and sunspots. At the 50-year timescale, there were significant decadal oscillations between streamflow and the Arctic Oscillation (AO) and the Pacific Decadal Oscillation (PDO), and bidecadal oscillations with the PDO. At the 100-year timescale, there were significant decadal oscillations between streamflow and Niño 3.4, the AO, and sunspots. At the 500-year timescale, streamflow in the middle reaches of the Yellow River showed prominent covariance with the AO with an approximately 32-year periodicity, and with sunspots with an approximately 80-year periodicity. Atmospheric circulation indices modulate streamflow by affecting temperature and precipitation. Sunspots impact streamflow variability by influencing atmospheric circulation, resulting in abundant precipitation. In general, for both the CWT and the WTC results, the periodicities were spatially continuous, with a few gradual changes from upstream to downstream resulting from the varied topography and runoff. At the temporal scale, the periodicities were generally continuous over short timescales and discontinuous over longer timescales.

Bidecadal sea level modes in the North and South Atlantic Oceans

The relationship between the North and South Atlantic bidecadal sea level (SL) oscillations during the twentieth century is investigated for the first time using Simple Ocean Data Assimilation reanalysis. Complex empirical orthogonal function analysis of the bidecadal band gives two dominant modes: the first dominating from 1915 to 1965 and the second from 1970 onward. The long-term time-dependent change of mode dominance suggests a structural-type regime shift. The first mode is characterized by states with North and South Atlantic subtropical gyres in phase, while tropical and subpolar regions are in opposite phase relative to them. The second mode is characterized by the subpolar gyre and North subtropical gyre almost in quadrature, with North and South subtropical gyres out of phase. Thermal Rossby waves are very clear only in the second mode. These waves seem to be influenced by bottom topography. Known Atlantic meridional overturning circulation (AMOC) sea level fingerprints and AMOC strength indices are discussed in the bidecadal oscillations context.

Double-celled subtropical gyre in the South Atlantic Ocean: Means, trends, and interannual changes

[1] We present here a study of the mean upper layer structure of the South Atlantic Subtropical Gyre (SASG) and its modes of variability based on Archiving, Validation, and Interpretation of Satellite Oceanographic data sea level anomaly data, Gravity Recovery and Climate Experiment-based mean dynamic topography (MDT) models, Reynolds sea surface temperatures, and two tide gauge time series from Permanent Service for Mean Sea Level. Analysis of the MDTs shows that the SASG mean structure is represented by a double-cell circulation, the southern cell being a narrow quasi-zonal strip centered around 33°S. Its southern limb is limited by the recently discovered Saint Helena Current/Tristan da Cunha Current (StHC/TCC) and the North Subtropical Front around 35°S. This system is shown to be well separated from the South Atlantic Current/South Subtropical Front around 42–45°S. A method based on singular spectrum analysis was used to determine the principal modes of variability through the decomposition of the data into band-passed data sets. The altimeter sea level trend analysis suggests that the SASG pattern is slowly migrating southward and has been growing in amplitude between 1994 and 2006 with local growth rates of 5–10 mm/yr occurring in the southern cell. Flow intensifications in this period (2–3 cm/s in the South Equatorial Current and North Brazil Undercurrent, 3.5 cm/s in the Benguela Current), and weakening by 4.5 cm/s in the StHC/TCC are recorded. The altimeter-derived sea level trend is coherent with the growing phase of a bidecadal oscillation found from both tide gauges.

Recent change in the oceanic uptake rate of anthropogenic carbon in the North Pacific subpolar region determined by using a carbon-13 time series

[1] Applying the δ13C approach to time series of observations in the North Pacific subpolar region (Station KNOT; 44°00′N, 155°00′E), we demonstrated time series of vertical distributions of oceanic anthropogenic carbon. We found that the vertical distributions of oceanic anthropogenic carbon during 1999–2006 were almost consistent with those estimated by the other carbon-based quasi-conservative tracer approach (ΔC*). Comparing the oceanic anthropogenic carbon contents and the water-column inventories among 1999, 2000, and 2006, we found the recent oceanic uptake rate of anthropogenic carbon above 27.3 σθ to be 0.86 ± 0.12 μmol kg−1 yr−1, which was 1.2 times higher than the expected value derived from oceanic equilibration with increasing atmospheric CO2. Considering the strengthened ocean stratification with a bidecadal oscillation and the recent increase in alkalinity from the Sea of Okhotsk, it was possible to explain the difference in the recent oceanic uptake rate of anthropogenic carbon between our result and the expected one.

Variability of summer precipitation over Eastern China during the last millennium

Abstract. We use measurements of recent decades, 1500-yr proxy data, and millennium model simulations with a variety of climate facings to study the temporal and spatial variability of summer precipitation over eastern China. Spectral analysis of the proxy data using multi-taper method reveals three statistically significant bidecadal (15–35-yr), pendadecadal (40–60-yr), and centennial (65–170-yr) oscillation bands. The results of wavelet filtering show that the amplitudes of these bands vary substantially through time depending on the temperature regimes. Weak centennial oscillation and strong pentadecadal oscillation occur in warm conditions, whereas both the centennial and pentadecadal oscillations are strong in cold conditions. A model/data intercomparison suggests that pentadecadal and bidecadal oscillations could be associated with internal variability of the climate system. It is also found that the increased frequency of drought-in-north/flood-in-south spatial pattern over eastern China during the last two decades is unusual in the past five centuries.

Recent and dramatic changes in Pacific storm trajectories recorded in δ18O from Bristlecone Pine tree ring cellulose

A 300‐year stratigraphy of annual cellulosic δ18OVSMOW is presented for the long‐lived Bristlecone Pine (Pinus longaeva) from the White Mountains of California. The δ18OVSMOW stratigraphy exhibits two distinctive characteristics: a bidecadal oscillation during the 20th century with peak excursions of 4‰ and a dramatic 9‰ shift in mean δ18O values in the mid‐19th century. The bidecadal δ18O variability during the 20th century does not correlate well with the bidecadal pattern of drought recurrence as reflected in the Palmer Drought Severity Index. Geochemical modeling of the cellulose δ18OVSMOW indicates local climate conditions, particularly humidity and rainfall amount, should have been important influences on the cellulose δ18O variability, but these influences cannot explain the larger bidecadal excursions or the shift in values during the mid‐19th century. The large isotope variations must reflect sustained shifts in the frequency of subtropical versus polar storms that affect the isotopic composition of rainfall at the site. The mid‐19th century isotopic shift correlates with the major climate shift across the northern hemisphere as documented in a wide range of proxy records. We hypothesize that the large isotopic shift in the 19th century represents the hydrologic response to a regime shift in mean wintertime atmospheric circulation that changed storm trajectories brought about by a more southerly position of the midlatitude jet, increased trade wind strength, and a change in the frequency of the Pacific Decadal Oscillation. The extent to which the hydrologic changes influenced the growth behavior of the high‐altitude Bristlecone Pine in the White Mountains remains an important unanswered question.

Temperature and precipitation changes in China during the Holocene

We review here proxy records of temperature and precipitation in China during the Holocene, especially the last two millennia. The quality of proxy data, methodology of reconstruction, and uncertainties in reconstruction were emphasized in comparing different temperature and precipitation reconstruction and clarifying temporal and spatial patterns of temperature and precipitation during the Holocene. The Holocene climate was generally warm and wet. The warmest period occurred in 9.6–6.2 cal ka BP, whereas a period of maximum monsoon precipitation started at about 11.0 cal ka BP and lasted until about 8.0–5.0 cal ka BP. There were a series of millennial-scale cold or dry events superimposed on the general trend of climate changes. During past two millennia, a warming trend in the 20th century was clearly detected, but the warming magnitude was smaller than the maximum level of the Medieval Warm Period and the Middle Holocene. Cold conditions occurred over the whole of China during the Little Ice Age (AD 1400–AD 1900), but the warming of the Medieval Warm Period (AD 900–AD 1300) was not distinct in China, especially west China. The spatial pattern of precipitation showed significant regional differences in China, especially east China. The modern warm period has lasted 20 years from 1987 to 2006. Bi-decadal oscillation in precipitation variability was apparent over China during the 20th century. Solar activity and volcanic eruptions both were major forcings governing the climate variability during the last millennium.

06/01827 Potential for improvement in estimation of solar diffuse irradiance: Muneer, T. and Munawwar, S. Energy Conversion and Management, 2006, 47, (1), 68–86

Water scarcity in the Yellow River, China, has become increasingly severe over the past half century. In this paper, wavelet transform analysis was used to detect the variability of natural, observed, and reconstructed streamflow in the Yellow River at 500-, 100-, and 50-year timescales. The periodicity of the streamflow series and the co-varying relationships between streamflow and atmospheric circulation indices/sunspot number were assessed by means of continuous wavelet transform (CWT) and wavelet transform coherence (WTC) analyses. The CWT results showed intermittent oscillations in streamflow with increasing periodicities of 1–6 years at all timescales. Significant multidecadal and century-scale periodicities were identified in the 500-year streamflow series. The WTC results showed intermittent interannual covariance of streamflow with atmospheric circulation indices and sunspots. At the 50-year timescale, there were significant decadal oscillations between streamflow and the Arctic Oscillation (AO) and the Pacific Decadal Oscillation (PDO), and bidecadal oscillations with the PDO. At the 100-year timescale, there were significant decadal oscillations between streamflow and Niño 3.4, the AO, and sunspots. At the 500-year timescale, streamflow in the middle reaches of the Yellow River showed prominent covariance with the AO with an approximately 32-year periodicity, and with sunspots with an approximately 80-year periodicity. Atmospheric circulation indices modulate streamflow by affecting temperature and precipitation. Sunspots impact streamflow variability by influencing atmospheric circulation, resulting in abundant precipitation. In general, for both the CWT and the WTC results, the periodicities were spatially continuous, with a few gradual changes from upstream to downstream resulting from the varied topography and runoff. At the temporal scale, the periodicities were generally continuous over short timescales and discontinuous over longer timescales.

06/01828 Potential of quarterwave interference stacks for colored thermal solar collectors: Schüler, A. et al. Solar Energy, 2005, 79, (2), 122–130

Water scarcity in the Yellow River, China, has become increasingly severe over the past half century. In this paper, wavelet transform analysis was used to detect the variability of natural, observed, and reconstructed streamflow in the Yellow River at 500-, 100-, and 50-year timescales. The periodicity of the streamflow series and the co-varying relationships between streamflow and atmospheric circulation indices/sunspot number were assessed by means of continuous wavelet transform (CWT) and wavelet transform coherence (WTC) analyses. The CWT results showed intermittent oscillations in streamflow with increasing periodicities of 1–6 years at all timescales. Significant multidecadal and century-scale periodicities were identified in the 500-year streamflow series. The WTC results showed intermittent interannual covariance of streamflow with atmospheric circulation indices and sunspots. At the 50-year timescale, there were significant decadal oscillations between streamflow and the Arctic Oscillation (AO) and the Pacific Decadal Oscillation (PDO), and bidecadal oscillations with the PDO. At the 100-year timescale, there were significant decadal oscillations between streamflow and Niño 3.4, the AO, and sunspots. At the 500-year timescale, streamflow in the middle reaches of the Yellow River showed prominent covariance with the AO with an approximately 32-year periodicity, and with sunspots with an approximately 80-year periodicity. Atmospheric circulation indices modulate streamflow by affecting temperature and precipitation. Sunspots impact streamflow variability by influencing atmospheric circulation, resulting in abundant precipitation. In general, for both the CWT and the WTC results, the periodicities were spatially continuous, with a few gradual changes from upstream to downstream resulting from the varied topography and runoff. At the temporal scale, the periodicities were generally continuous over short timescales and discontinuous over longer timescales.

Bidecadal variability in the intermediate waters of the northwestern subarctic Pacific and the Okhotsk Sea in relation to 18.6‐year period nodal tidal cycle

On the basis of historical oceanographic data, we investigated the long‐term variations of the intermediate waters in the four regions in the northwestern subarctic Pacific: Oyashio, Okhotsk Sea Mode Water, Upstream Oyashio and East Kamchatka Current. We found bidecadal oscillations in these water properties that are synchronized with the 18.6‐year period nodal cycle. In periods when the diurnal tide is strong, the following characteristics are found: Apparent oxygen utilization and phosphate are low in Oyashio and Okhotsk Sea Mode Water. The thickness of the intermediate layers is large, and thus potential vorticity is correspondingly low, in Oyashio, Okhotsk Sea Mode Water, and Upstream Oyashio. Around the mesothermal (temperature maximum) water, isopycnal potential temperature are low in the areas on the Pacific side, and high in the intermediate layer of Okhotsk Sea Mode Water. The mixing ratio of Okhotsk Sea Mode Water in the Upstream Oyashio water is high. These bidecadal oscillations can be explained by changes in the vertical mixing around the Kuril Straits induced by the diurnal tide whose amplitude is modulated with the 18.6‐year nodal cycle. Higher sea surface salinity water around the Kuril Straits caused by stronger tidal mixing is possibly transported northward along the cyclonic Okhotsk Sea Gyre, and possibly enhances the formation of the dense shelf water. This makes apparent oxygen utilization, phosphate, and potential vorticity lower in Okhotsk Sea Mode Water and Oyashio.

Spatiotemporal Decreases of Nutrients and Chlorophyll-a in the Surface Mixed Layer of the Western North Pacific from 1971 to 2000

µmol-PO 4 l -1 yr -1 and 0.01~0.04 µmol-NO 3 l -1 yr -1 with the decrease of density. The water column Chl-a (CHL) and the net community production (NCP) had also declined by 0.27~0.48 mg-Chl m -2 yr -1 and 0.08~0.47 g-C-NCP m -2 yr -1 with a clear oscillation of 20.8 ± 0.8 years. These changes showed a strong negative correlation with the Pacific Decadal Oscillation Index (PDO) with a time lag of 2 years (R = 0.89 ± 0.02). Considering the recent significant decrease of O 2 over the North Pacific subsurface water, these find- ings suggest that the long-term decreasing trend of surface-deep water mixing has caused the decrease of marine biological activity in the surface mixed layer with a bidecadal oscillation over the western North Pacific.

Moisture Balance for Bidecadal Variability of Wintertime Precipitation in the North Pacific Using NCEP/NCAR Reanalysis

Moisture budgets are analyzed using National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data over the North Pacific for a Bi-Decadal Oscillation (BDO) in precipitation that was reported by Minobe and Nakanowatari (2002). BDO in wintertime precipitation is mostly associated with moisture flux convergence, with a minor contribution from evaporation. The moisture convergence is mainly due to anomalies of wind and moisture on time scales longer than a month, except at high-latitudes where transient eddy (time scales shorter than a month) contributions are greater. When the Aleutian low strengthens (weakens), the anomalous moisture flux convergence is due to a cyclonic (anti-cyclonic) wind circulation over 30°-60&deg:N and anti-cyclonic (cyclonic) wind circulation over 0°-30°N. This pair of anomalous circulations is also observed on interannual time scales, but they appear independent of El Ni&ntildeo;/Southern Oscillation (ENSO). The anomalous circulation over 0°-30°N is associated with sea-level pressure anomalies in the tropics (20°S-20°N) and this result is confirmed with the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) and the NCEP real-time marine data. This low-latitude anomalous circulation plays a dominant role in precipitation variability in Hawaii on the bi-decadal and interannual time scales.

Climate precursors of multidecadal drought variability in the western United States

Low‐frequency (periodicities lower than 20 years) hydrologic variability in the western United States over the past 500 years is studied using available tree‐ring reconstructions of Palmer Drought Severity Index (PDSI), streamflow, and climate indices. Leading rotated principal component (RPC) scores of a gridded tree‐ring reconstruction of the PDSI from 1525 to 1975 are significantly correlated with indices representing large‐scale climate variations from the Pacific and Atlantic Oceans. RPC1 (31%) is related to the influence of North Pacific sea surface temperature (SST) variations, indexed by the Pacific Decadal Oscillation (PDO). RPC2 (24%) is apparently related to North Atlantic SST variations, indexed by the Atlantic Multidecadal Oscillation (AMO). RPC3 (19%) is moderately correlated with a smoothed version of the Southern Oscillation Index. Consistent with recent studies of instrumental data, RPC1 (PDO) and RPC2 (AMO) explain a large part of the multidecadal hydrologic variability of the interior western United States. Western U.S. PDSI variability exhibits significant pentadecadal (and longer) oscillations in the epochs from circa 1525 to 1650 and 1850 to 1975, while bidecadal oscillations are prevalent in the middle epoch from circa 1650 to 1850. The changes in spectral characteristics of western U.S. PDSI were related to similar changes in the PDO (and therefore in RPC1). In contrast, RPC2 had a regular periodicity of 51 years for the past ∼500 years. This regularity is intriguing, and although RPC2 was primarily related to the AMO in this study, the influence from Pacific climate cannot be discarded.

Synchronous bidecadal periodic changes of oxygen, phosphate and temperature between the Japan Sea deep water and the North Pacific intermediate water

From time series of dissolved oxygen (O2), phosphate (PO4) and water temperature (T) during the last forty years in the Japan Sea Deep Water and the North Pacific Intermediate Water, we found that O2, PO4 and T show a clear bidecadal oscillations of about 18 years superimposed on the linear trends of decrease of 0.47 μmol‐O2 kg−1 yr−1 and increase of 0.003 μmol‐PO4 kg−1 yr−1 and 0.005°C yr−1 in both two oceanic areas. The changes of O2, PO4 and T in the two oceanic areas were synchronized despite the water formation systems of each area being independent. Both the linear trends and the oscillations of O2, PO4 and T in the two oceanic areas also showed a strong correlation with the anomaly of the sea surface level pressure in the North Pacific that possibly affects the change of surface oceanic condition.

Global structure of Bidecadal precipitation variability in boreal winter

[1] Three global monthly precipitation datasets, including gauge measurements, gauge/satellite merged analysis (CMAP), and NCEP/NCAR reanalysis, are analyzed with respect to Bi-Decadal Oscillation (BDO) in boreal winter during the 20th century. Correlation and coherency analyses between the precipitations and the strength of the Aleutian low, which is an action center of the BDO, reveal substantial impacts of the BDO on precipitations around the Pacific Ocean. The covariability of the precipitations is prominent for Hawaii, mid-latitude eastern North America, Florida and eastern-China/Southern-Japan, and at some other regions including the Southern Hemisphere. The most coherent BDO precipitation pattern common to the CMAP and reanalysis features significant anomalies over the tropical (10°–30°N), central (30°–50°N) and northern (50°–70°N) North Pacific with alternating polarities. The influence of the BDO on Hawaii winter droughts and salinity in the North Pacific is discussed.

Maximal Wavelet Filter and Its Application to Bidecadal Oscillation over the Northern Hemisphere through the Twentieth Century

Based on a wavelet transform, a new method referred to as maximal wavelet filter (MWF) is proposed to extract temporal structure changes of a climatic oscillation, which varies its pattern corresponding to the changes of the oscillation period. The MWF is a bandpass filter having a narrow pass band, the central frequency of which temporally varies according to the periods of maximal wavelet amplitudes for a specific region. MWF is applied to wintertime sea level pressures (SLPs) in the Northern Hemisphere from 1899 to 2000 to extract SLP changes associated with the bidecadal oscillation (BDO), which distributes globally but has the strongest amplitudes in the North Pacific. In the Pacific sector, the BDO center of action captured by the MWF was located over Alaska in the first few decades of the record, and then moved southward to the central North Pacific from 1920 to 1950, with maximal BDO amplitudes in the middle of the century. The southward migration was accompanied by the previously reported increase of the oscillation period from 15 to 20 years. On the other hand, Atlantic SLP variations coherent with the Pacific BDO had large amplitudes in midlatitudes (high latitudes) in the early (late) part of the twentieth century. In association with these spatial structure changes, the pattern of the recent BDO resembles the pattern of the Arctic Oscillation. The analysis of the sea surface temperatures (SSTs) gridded from the Comprehensive Ocean‐Atmosphere Data Set (COADS) and the newly digitized Kobe collections suggests that BDO pattern in the SSTs also shifted toward the south between the first and last few decades of the twentieth century. Furthermore, covariability between the land‐air temperatures and Aleutian low strength is observed through the twentieth century for Alaska, but only after 1940 for the midlatitudes of western North America and Hawaii, indicating that the BDO influence was limited to the high latitudes in the first few decades of the twentieth century in these regions, consistent with the spatial structure changes in the SLP field over the North Pacific.