North Pacific Variability Research Articles

Decadal North Pacific Bred Vectors in a Coupled GCM

Abstract A number of recent studies done with simple numerical models suggest that the decadal variability in the extratropical North Pacific Ocean is a result of the excitation of low-frequency ocean basin modes. To test this assumption, low-frequency North Pacific variability was examined using a state-of-the-art coupled general circulation model (CGCM). Earlier studies had shown that slowly varying dynamical modes in a CGCM can be effectively isolated using the breeding technique. In this study, the breeding method was applied to the Center for Ocean-Land-Atmosphere Studies (COLA) anomaly coupled GCM (ACGCM), and it was found that several types of slow modes can be isolated depending on the parameters of the breeding cycle. Tropical bred vector SST and upper-ocean heat content are dominated by the ENSO, which is consistent with the results obtained earlier using other CGCMs. Extratropical bred vector upper-ocean heat content is dominated by oceanic instability localized east of Japan, varying on seasonal-to-interannual time scales, and decadal modes with the large-scale pattern over the central and eastern extratropical North Pacific. Similar to ocean basin modes, the decadal modes have a signature of westward-propagating long baroclinic Rossby waves, but do not exhibit the global imprint typical for global basin modes. The relationship between the decadal bred vectors and the background anomalies is consistent with linear damped dynamics. Presumably, the growth of the decadal bred vectors is due to the atmospheric stochastic forcing, but the existence of extratropical instability on decadal time scales still needs to be verified.

Arctic salinity anomalies and their link to the North Atlantic during a positive phase of the Arctic Oscillation

Many of the changes observed during the last two decades in the Arctic Ocean and adjacent seas have been linked to the concomitant abrupt decrease of the sea level pressure in the central Arctic at the end of the 1980s. The decrease was associated with a shift of the Arctic Oscillation (AO) to a positive phase, which persisted throughout the mid 1990s. The Arctic salinity distribution is expected to respond to these dramatic changes via modifications in the ocean circulation and in the fresh water storage and transport by sea ice. The present study investigates these different contributions in the context of idealized ice-ocean experiments forced by atmospheric surface wind-stress or temperature anomalies representative of a positive AO index. Wind stress anomalies representative of a positive AO index generate a decrease of the fresh water content of the upper Arctic Ocean, which is mainly concentrated in the eastern Arctic with almost no compensation from the western Arctic. Sea ice contributes to about two-third of this salinification, another third being provided by an increased supply of salt by the Atlantic inflow and increased fresh water export through the Canadian Archipelago and Fram Strait. The signature of a saltier Atlantic Current in the Norwegian Sea is not found further north in both the Barents Sea and the Fram Strait branches of the Atlantic inflow where instead a widespread freshening is observed. The latter is the result of import of fresh anomalies from the subpolar North Atlantic through the Iceland–Scotland Passage and enhanced advection of low salinity waters via the East Icelandic Current. The volume of ice exported through Fram Strait increases by 20% primarily due to thicker ice advected into the strait from the northern Greenland sector, the increase of ice drift velocities having comparatively less influence. The export anomaly is comparable to those observed during events of Great Salinity Anomalies and induces substantial freshening in the Greenland Sea, which in turn contributes to increasing the fresh water export to the North Atlantic via Denmark Strait. With a fresh water export anomaly of 7 mSv, the latter is the main fresh water supplier to the subpolar North Atlantic, the Canadian Archipelago contributing to 4.4 mSv. The removal of fresh water by sea ice under a positive winter AO index mainly occurs through enhanced thin ice growth in the eastern Arctic. Winter SAT anomalies have little impact on the thermodynamic sea ice response, which is rather dictated by wind driven ice deformation changes. The global sea ice mass balance of the western Arctic indicates almost no net sea ice melt due to competing seasonal thermodynamic processes. The surface freshening and likely enhanced sea ice melt observed in the western Arctic during the 1990s should therefore be attributed to extra-winter atmospheric effects, such as the noticeable recent spring–summer warming in the Canada–Alaska sector, or to other modes of atmospheric circulations than the AO, especially in relation to the North Pacific variability.

Interpretation of North Pacific Variability as a Short- and Long-Memory Process*

A major difficulty in investigating the nature of interdecadal variability of climatic time series is their shortness. An approach to this problem is through comparison of models. In this paper a first-order autoregressive [AR(1)] model is contrasted with a fractionally differenced (FD) model as applied to the winter-averaged sea level pressure time series for the Aleutian low [the North Pacific (NP) index] and the Sitka winter air temperature record. Both models fit the same number of parameters. The AR(1) model is a “short-memory” model in that it has a rapidly decaying autocovariance sequence, whereas an FD model exhibits “long memory” because its autocovariance sequence decays more slowly. Statistical tests cannot distinguish the superiority of one model over the other when fit with 100 NP or 146 Sitka data points. The FD model does equally well for short-term prediction and has potentially important implications for long-term behavior. In particular, the zero crossings of the FD model tend to be farther apart, so they have more of a “regimelike” character; a quarter century interval between zero crossings is 4 times more likely with the FD than the AR(1) model. The long-memory parameter δ for the FD model can be used as a characterization of regimelike behavior. The estimated δs for the NP index (spanning 100 yr) and the Sitka time series (168 yr) are virtually identical, and their size implies moderate long-memory behavior. Although the NP index and the Sitka series have broadband low-frequency variability and modest long-memory behavior, temporal irregularities in their zero crossings are still prevalent. Comparison of the FD and AR(1) models indicates that regimelike behavior cannot be ruled out for North Pacific processes.

Decadal Variability of the Aleutian Low and Its Relation to High-Latitude Circulation*

The January‐February mean central pressure of the Aleutian low is investigated as an index of North Pacific variability on interannual to decadal timescales. Since the turn of the century, 37% of the winter interannual variance of the Aleutian low is on timescales greater than 5 yr. An objective algorithm detects zero crossings of Aleutian low central pressure anomalies in 1925, 1931, 1939, 1947, 1959, 1968, 1976, and 1989. No single midtropospheric teleconnection pattern is sufficient to capture the variance of the Aleutian low. The Aleutian low covaries primarily with the Pacific‐North American (PNA) pattern but also with the Arctic Oscillation (AO). The change to a prominent deep Aleutian low after 1977 is seen in indices of both the PNA and AO; the return to average conditions after 1989 was also associated with a change in the AO. The authors’ analysis suggests an increasing covariability of the high- and midlatitude atmosphere after 1970.

Oceanic Odyssey of a satellite-tracked drifter: North Pacific variability delineated by a single drifter trajectory

A near-surface satellite-tracked drifter launched off the east coast of the Kuril Islands on September 4,1993 began a 2.5-year Odyssey across the North Pacific Ocean. During its travels, the drifter encountered numerous energetic oceanographic regimes as it moved from the region of the Kuril-Kamchatka Trench to the continental margin of the Kuril Islands, through Friza Strait into the Sea of Okhotsk, seaward again through Bussol’ Strait, and then eastward across the North Pacific. Oceanic features detected along the basin-wide trajectory include a quasi-permanent anticyclonic eddy over the Kuril-Kamchatka Trench, open-ocean wind-driven inertial oscillations, coastal-trapped diurnal shelf waves, semidiurnal tidal currents, transient cyclonic and anticyclonic eddies, through-strait flows, and wave-like mesoscale meanders. The single drifter track delineates the dynamically-rich variability of upper ocean currents, emphasizes the marked difference in flow dynamics between boundary and open ocean regions, and provides a time-scale for the movement of surface waters across the entire North Pacific.