Interannual variability of sea ice in the Bering Sea and its relationship to atmospheric variability is analyzed using a singular value decomposition (SVD) analysis of sea ice concentrations (SICs) ...and 1000 hPa wind speeds in winter and spring seasons. The statistically significant first and second SVD modes, explaining 76.3% and 17.6% in winter and 54.6% and 29.6% in spring of the squared covariance between the two fields, are identified for SICs both in the winter and spring seasons with 1 month leading wind speeds. The spatial structures show that the first (second) SVD mode explains the SIC variability in the northeastern (northwestern) Bering Sea, related to the local northwesterly (northerly) wind anomalies for the positive SIC anomalies both in the winter and spring seasons. A comparison of the first SVD modes between the winter and spring seasons suggests that the difference of dominant patterns of wind anomalies results in the difference of SIC anomaly distributions between two seasons. The relationship between sea ice and atmospheric circulation anomalies indicates that one mode of the leading two SVD modes in each season is related to large‐scale atmospheric circulation associated with the Aleutian low and the other mode is related to relatively local atmospheric fluctuations related with pressure anomalies over Alaska. Furthermore, a slight difference of 700 hPa geopotential height anomalies results in the substantially different sea ice anomalies. These results suggest that in order to know the interannual sea ice variability in the Bering Sea, a better understanding of the wind anomalies over the Bering Sea are important.
The influence of the closure of the Panamanian Gateway during the late Cenozoic on climate in and around the North Pacific is investigated by using a coupled ocean‐atmosphere general circulation ...model with an open and closed gateway. In the case of an open (closed) gateway, deep convection is present (absent) in the North Pacific. The deep convection is associated with surface saline water transported from the subtropical Atlantic through the open gateway to the North Pacific. On the other hand, with the closed gateway, the lack of saline water transport from the Atlantic induces halocline formation over the subarctic Pacific with cold climate. The deep convection in the North Pacific leads to a vigorous thermohaline circulation with larger meridional heat transport, and causes warmer climate in and around the North Pacific. These results are generally consistent with paleoceanographic and paleoclimatic estimates related to the closure of the Panamanian Gateway.
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.
A new gridded water‐temperature data set of upper 200 m depths (0, 50, 100, 200 m depths) for the Okhotsk Sea was produced using an optimal interpolation technique from 1950 to 1996 using ...oceanographic observations in the World Ocean Database 1998. Temperature variability at 50, 100 and 200 m depths in the southern Okhotsk Sea (south of 52°N) in the warm‐season (May–October) was investigated by an Empirical Orthogonal Function (EOF) analysis from 1958 to 1994, for which sufficient data exist for an EOF analysis. The first EOF mode has a monopole structure with the maximal amplitude in the Kuril Basin, and the corresponding Principal Component (PC) exhibits prominent quasi‐decadal variability. The first EOF mode is closely related with the wintertime (December–February) sea surface temperature anomalies over the subarctic front or Oyashio front in the North Pacific, and with wintertime Sea level Pressure (SLP) differences between northern Eurasia and the northern North Pacific. This suggests that the temperature changes in the Okhotsk Sea are caused by changes in the strength of the Asian winter monsoon, which are associated with the SLP difference. A quasi‐decadal oscillation, similar to that of the PC‐1, is observed in the SLP difference since the 1960s, and shared by the Polar/Eurasian (POL) pattern, Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). Some hints of the relating variability are observed in coastal sea level difference between Wakkanai and Abashiri, which was used as a proxy for transport in the Soya Warm Current. Also, some features of sea ice extents co‐vary with the PC‐1.
Relations in year-to-year variability between wintertime Sea-Ice Concentrations (SICs) in the Okhotsk Sea and atmospheric anomalies consisting of zonal and meridional 1000-hPa wind speeds and 850-hPa ...air temperatures are studied using a singular value decomposition analysis. It is revealed that the late autumn (October–November) atmospheric conditions strongly influence sea-ice variability from the same season (late autumn) through late winter (February—March), in which sea-ice extent is at its maximum. The autumn atmospheric conditions for the positive sea-ice anomalies exhibit cold air temperature anomalies over the Okhotsk Sea and wind anomalies blowing into the Okhotsk Sea from Siberia. These atmospheric conditions yield anomalous ocean-to-atmosphere heat fluxes and cold sea surface temperature anomalies in the Okhotsk Sea. Hence, these results suggest that the atmospheric conditions affect the sea-ice through heat anomalies stored in sea-ice and oceanic fields. The late autumn atmosphere conditions are related to large 700-hPa geopotential height anomalies over the Bering Sea and northern Eurasia, which are related to a stationary Rossby wave propagation over the North Pacific and that from the North Atlantic to Eurasia, respectively. In addition, the late autumn atmospheric preconditioning also plays an important role in the decreasing trend in the Okhotsk sea-ice extent observed from 1980 to the mid-1990s. Based on the lagged sea-ice response to the late autumn atmosphere, a simple seasonal prediction scheme is proposed for the February–March sea-ice extent using four-month leading atmospheric conditions. This scheme explains 45% of the variance of the Okhotsk sea-ice extent.
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.
Two major fluids of the earth are the ocean and the atmosphere, each of which influence the other. The influence of the atmosphere onto the ocean is strong and clear, while the opposing oceanic ...influence onto the atmosphere is not well understood. In this paper, several results of co-variability of the ocean and the atmosphere and interaction between them are introduced. Specific topics are seasonal variability over the tropical Pacific, decadal variability over and around the North Pacific, and mid-latitude air-sea interaction from a point of view of process-oriented studies. In order to identify oceanic impacts onto the atmosphere, especially using data analysis, it is useful to detect key features. Such key features are propagation signal commonly found in the atmosphere and the ocean with a speed of oceanic propagation for annual variability over the tropical Pacific, oscillation in the atmosphere for the decadal variability, and atmospheric signatures on oceanic spatial scales for the mid-latitude air-sea interactions. Some ideas behind the studies are also introduced.
Sea level variability around Japan from 1906 to 2010 is examined using a regional ocean model, along with observational data and the CMIP5 historical simulations. The regional model reproduces ...observed interdecadal sea level variability, for example, high sea level around 1950, low sea level in the 1970s, and sea level rise during the most recent three decades, along the Japanese coast. Sensitivity runs reveal that the high sea level around 1950 was induced by the wind stress curl changes over the North Pacific, characterized by a weakening of the Aleutian low. In contrast, the recent sea level rise is primarily caused by heat and freshwater flux forcings. That the wind-induced sea level rise along the Japanese coast around 1950 is as large as the recent sea level rise highlights the importance of natural variability in understanding regional sea level change on interdecadal time scales.
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°: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 /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.
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.
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