Shoshiro Minobe

A 50–70 year climatic oscillation over the North Pacific and North America

The chronology of interdecadal climatic regime shifts is examined, using instrumental data over the North Pacific, North America and the tropical oceans, and reconstructed climate records for North America. In the North Pacific and North America, climatic regime shifts around 1890 and in the 1920s with alternating polarities are detected, whose spatial structure is similar to that of the previously-known climatic shifts observed in the 1940s and 1970s. Sea-surface temperatures in the tropical Indian Ocean-maritime continent region exhibit changes corresponding to these four shifts. Spectra obtained by the Multi-Taper-Method suggest that these regime shifts are associated with 50–70 year climate variability over the North Pacific and North America. The leading mode of the empirical orthogonal functions of the air-temperature reconstructed from tree-rings in North America exhibits a spatial distribution that is reminiscent of instrumentally observed air-temperature differences associated with the regime shifts. The temporal evolution of this mode is characterized by a 50–70 year oscillation in the eighteenth and nineteenth centuries. This result, combined with the results of the analyses of the instrumental data, indicates that the 50–70 year oscillation is prevalent from the eighteenth century to the present in North America.

Influence of the Gulf Stream on the troposphere

The Gulf Stream transports large amounts of heat from the tropics to middle and high latitudes, and thereby affects weather phenomena such as cyclogenesis and low cloud formation. But its climatic influence, on monthly and longer timescales, remains poorly understood. In particular, it is unclear how the warm current affects the free atmosphere above the marine atmospheric boundary layer. Here we consider the Gulf Stream’s influence on the troposphere, using a combination of operational weather analyses, satellite observations and an atmospheric general circulation model. Our results reveal that the Gulf Stream affects the entire troposphere. In the marine boundary layer, atmospheric pressure adjustments to sharp sea surface temperature gradients lead to surface wind convergence, which anchors a narrow band of precipitation along the Gulf Stream. In this rain band, upward motion and cloud formation extend into the upper troposphere, as corroborated by the frequent occurrence of very low cloud-top temperatures. These mechanisms provide a pathway by which the Gulf Stream can affect the atmosphere locally, and possibly also in remote regions by forcing planetary waves. The identification of this pathway may have implications for our understanding of the processes involved in climate change, because the Gulf Stream is the upper limb of the Atlantic meridional overturning circulation, which has varied in strength in the past and is predicted to weaken in response to human-induced global warming in the future.

Resonance in bidecadal and pentadecadal climate oscillations over the North Pacific : Role in climatic regime shifts

The roles of interdecadal oscillations in climatic regime shifts, which are observed as rapid strength changes in the Aleutian low in winter and spring seasons, have been analyzed. A regime shift results from simultaneous phase reversals between pentadecadal and bidecadal variations, which synchronize with one another at a relative period of three. The pentadecadal variation, which is observed in both winter and spring seasons, provides the basic timescale of regime shifts, while the bidecadal variation, which is observed only in winter, characterizes the rapidity of the shifts. A Monte-Carlo simulation has shown that the simultaneous phase reversals or resonance between the pentadecadal and bidecadal variations reflect a physical linkage between them and do not coincide accidentally. The role of this synchronization feature for assessing and predicting regime shifts is discussed.

The Pacific Decadal Oscillation, Revisited

AbstractThe Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere–ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of ...

Spatio-temporal structure of the pentadecadal variability over the North Pacific

Abstract Using a Multi-Taper frequency domain-Singular Value Decomposition (MTM–SVD), a pentadecadal oscillation was detected in the winter–spring sea-level pressure (SLP) field over the North Pacific and surface air-temperature in North America which was significant at the 95% confidence level. The MTM–SVD captured the different SLP and air-temperature distributions between the winter and spring seasons in a consistent manner. The pentadecadal SLP signature in the spring season is centered nearer the west coast of North America than in the winter season. This zonal displacement is consistent with the prominent springtime pentadecadal air-temperature variability in mid-latitude western North America. A wavelet analysis of the Pacific Decadal Oscillation Index (PDOI) showed that the regime shifts in the 1920s, 1940s and 1970s involved simultaneous phase reversals of the bidecadal and pentadecadal variations. The two interdecadal variations are synchronized with one another such that a half period of the pentadecadal oscillation (one epoch of an individual regime) corresponds to one and half periods of the bidecadal oscillation. These results are consistent with the wavelet analysis of the North Pacific Index (NPI). Similar resonance between the bidecadal and pentadecadal variations is evident in air-temperatures over Alaska. The bidecadal and pentadecadal signals have different seasonality in these time series, suggesting that although the two interdecadal variations arise from two different physical mechanisms, they interact with each other. The most distinct seasonal difference was observed in mid-latitude western North America, where the bidecadal variation prevails only in the winter season and the pentadecadal variation only in the spring season. Alaska air-temperatures in the winter and winter–spring of 1999 were the coldest since 1977, as were springtime air-temperatures in mid-latitude western North America, in contrast to the warm anomalies that prevailed in this region during 1977–98. The NPI and PDOI also exhibited an opposite polarity in 1999 to the respective regime mean polarities. These anomalous conditions in winter and spring seasons of 1999 may signify a major regime shift in 1998–1999. In order to verify whether or not a regime shift did occur in 1998–1999, a careful examination of additional data in coming ten or so years will be necessary.

Atmospheric Response to the Gulf Stream: Seasonal Variations*

Abstract The atmospheric response to the Gulf Stream front in sea surface temperature is investigated using high-resolution data from satellite observations and operational analysis and forecast. Two types of atmospheric response are observed with different seasonality and spatial distribution. In winter, surface wind convergence is strong over the Gulf Stream proper between Cape Hatteras and the Great Banks, consistent with atmospheric pressure adjustments to sea surface temperature gradients. The surface convergence is accompanied by enhanced precipitation and the frequent occurrence of midlevel clouds. Local evaporation and precipitation are roughly in balance over the Florida Current and the western Gulf Stream proper. In summer, strong precipitation, enhanced high clouds, and increased lightning flash rate are observed over the Florida Current and the western Gulf Stream proper, without seasonal surface convergence enhancement. For the precipitation maximum over the Florida Current, local evaporation...

Permanent El Nino during the Pliocene warm period not supported by coral evidence

The El Niño/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3–5 million years ago) may have existed in a permanent El Niño state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean. This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change, ENSO variability—and related global climate teleconnections—could have been radically different from that today. Yet, owing to a lack of observational evidence on seasonal and interannual SST variability from crucial low-latitude sites, this fundamental climate characteristic of the PWP remains controversial. Here we show that permanent El Niño conditions did not exist during the PWP. Our spectral analysis of the δ18O SST and salinity proxy, extracted from two 35-year, monthly resolved PWP Porites corals in the Philippines, reveals variability that is similar to present ENSO variation. Although our fossil corals cannot be directly compared with modern ENSO records, two lines of evidence suggest that Philippine corals are appropriate ENSO proxies. First, δ18O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-δ18O events in the fossil corals closely resemble the decreases in δ18O seen in the live coral during El Niño events. Prior research advocating a permanent El Niño state may have been limited by the coarse resolution of many SST proxies, whereas our coral-based analysis identifies climate variability at the temporal scale required to resolve ENSO structure firmly.

Interannual to interdecadal changes in the Bering Sea and concurrent 1998/99 changes over the North Pacific

Abstract Sea-Surface Temperatures (SSTs), upper water Heat Storage (HS), Sea-Level Displacements (SLDs), Sea-Ice Concentration (SICs) in the Bering Seas and associated atmospheric circulations are analyzed to identify dominant interannual to interdecadal variations. As a representative time series of the SST variations, Principal Component (PC) of the first mode of a seasonally combined Empirical Orthogonal Function (EOF) is employed. The corresponding EOF (spatial pattern) exhibits the smallest amplitudes in winter and largest in summer. PC1 is characterized by a warming trend throughout the record (1921–2001) with the warmest year in 1997, which is followed by rapid cooling until 1999. The warming from 1995–1997 and cooling from 1997–1999 are commonly found in HS along the southern rim of the Bering Sea, and also accompanied by SLD rise and fall, respectively. The SIC variability corresponding to SST PC1 is prominent in the eastern Bering Sea in spring with correlations as high as 0.7, but good correlations were mainly observed prior to 1990. The correlations between the SST PC1 and sea-level pressures (SLPs) also suggest that the spring atmospheric circulation anomalies play an important role in the variations of the SST and sea-ice in the Bering Sea. The cooling and SLD fall in the late 1990s in the Bering Sea might be related with a possible major regime shift in 1998/1999, which was discussed by Minobe, 2000 , Hare and Mantua, 2000 , and Schwing and Moore (2000) . In the 1998/99 change over the North Pacific, SSTs and HS increased abruptly both in the Kuroshio/Oyashio Extension region and the central North Pacific, accompanied by cooling in the eastern North Pacific. At the same time, SLDs rose from Japan to 160°W roughly along Kuroshio Extension path with a tongue-like structure. The tongue-like SLD rise is likely forced by wintertime atmospheric anomalies associated with SLP increase in the eastern North Pacific.

Interdecadal modulation of interannual atmospheric and oceanic variability over the North Pacific

Abstract The interdecadal modulation of interannual variability of the atmosphere and ocean is examined over the North Pacific by using Wavelet Transform combined with Empirical Orthogonal Function (EOF) or Singular Value Decomposition (SVD) analysis. For the period of record 1899–1997, the interannual variability of the wintertime Aleutian Low, identified by either the North Pacific Index or the leading eigenvector (EOF-1) of North Pacific sea level pressure (SLP), exhibits an interdecadal modulation. Interannual variance in the strength of the Aleutian Low was relatively large from the mid-1920s to mid-1940s and in the mid-1980s, but relatively small in the periods from 1899 to the mid-1920s and from the mid-1940s to the mid-1970s. The periods of high (low) interannual variability roughly coincide with pentadecadal regimes having a time averaged relatively intense (weak) Aleutian Low. Consistent with this SLP variability the interannual variance in the zonal wind stress is strengthened in the central North Pacific after the 1970s. The SLP EOF-2, which is related to the North Pacific Oscillation, exhibited a strengthening trend from the beginning of this century to the mid-1960s. After the 1970s, the interannual variance of SLP EOF-2 is generally smaller than that in the period from 1930 to 1970. Similar interdecadal changes in interannual variance are found in expansion coefficients for the first two EOFs of the Pacific sector 500 hPa height field for the period 1946–1993. EOF-1 of Pacific sector 500 hPa corresponds to the Pacific/North American (PNA) teleconnection pattern, while EOF-2 is related to the Western Pacific (WP) pattern. The relative influence of the atmospheric PNA and WP interannual variability on North Pacific SSTs appears to have varied at pentadecadal time scales. Results from an SVD analysis of winter season (December–February) 500 hPa and North Pacific spring season (March–May) SST fields demonstrate that the PNA-related SST anomaly exhibited larger interannual variance after the 1970s, whereas the interannual variance of the WP related SST anomaly is larger before the 1970s. Correlations between the coastal North Pacific SST records and gridded atmospheric field data also change on interdecadal time scales. Our results suggest that the SST records from both the northwest and northeast Pacific coasts were more closely coupled with the PNA teleconnection pattern during the periods of 1925–1947 and 1977–1997 than in the regime from 1948 to 1976. Teleconnections between ENSO and preferred patterns of atmospheric variability over the North Pacific also appear to vary on interdecadal time scales. However, these variations do not reflect a unique regime-dependent influence. Our results indicate that ENSO is primarily related to the PNA (WP) pattern in the first (last) half of the present century. Correlation coefficients between indices for ENSO and PNA-like atmospheric variability are remarkably weak in the period from 1948 to 1976.

Interannual to Interdecadal Variability in the Japan Sea Based on a New Gridded Upper Water Temperature Dataset

A new gridded water temperature dataset of upper 400-m depths (0, 50, 100, 200, 300, and 400 m) for the Japan Sea (or East Sea) is produced by using an optimal interpolation technique from 1930 to 1996, based on oceanographic observations collected in the World Ocean Database 1998. The temperature data are analyzed by a complex empirical orthogonal function (CEOF) with six levels combined using the data for a period from 1957 to 1996, during which most of gridded data are available. Before calculating the CEOFs, low-pass or highpass filters (cutoff period at 7 yr) are applied to separate interannual and decadal temperature changes, respectively. One interannual and two decadal CEOF modes are identified. The interannual first CEOF mode is characterized by the energetic variability around and south of the subpolar front in the western Japan Sea, accompanied by northward and northeastward phase propagations emanating from the Tsushima Strait. The decadal first CEOF mode exhibits a broad structure prevailing over the whole Japan Sea, but large amplitudes are trapped by the subpolar front, with 608‐908 phase lags between the northeastern and southwestern Japan Sea. The decadal second CEOF mode has a localized structure with strong correlations in the Yamato Basin. The relation between the atmosphere and ocean is analyzed by a correlation analysis of wintertime sea level pressures (SLPs) onto the temporal coefficients of the CEOF modes. The interannual first CEOF mode is accompanied by the SLP anomalies over the western North Pacific Ocean with steep SLP gradients over the Japan Sea, suggesting that this mode is forced by local wind anomalies associated with the SLP changes over the western North Pacific. The decadal first CEOF mode is likely to be caused by changes of the east Asian winter monsoon due to the SLP variability of the northern part of the Siberian high, which is closely associated with the decadal fluctuations of the Arctic Oscillation and the North Atlantic Oscillation. The second decadal CEOF mode is accompanied by high SLP correlations over the central North Pacific associated with strength changes of Aleutian lows, suggestive of remote forcing from the central North Pacific.