Meiji Honda

Interannual seesaw between the Aleutian and Icelandic lows. Part I: seasonal dependence and life cycle

Abstract The seasonal dependence and life cycle of the well-known interannual seesawlike oscillation between the intensities of the surface Aleutian and Icelandic lows (AL and IL, respectively) are investigated, based on the National Meteorological Center operational analyses for the period from 1973 to 1994. It is found that the correlation between the AL and IL intensities is significantly negative only from February to mid-March. It is also found that the seesaw exhibits an equivalent barotropic structure within the troposphere. For this late-winter period an index is defined that measures the intensity difference between the two lows. A linear lag regression analysis between this index and circulation anomalies averaged in each of the nine 45-day periods from early winter to midspring reveals that the stationary AL and IL anomalies constituting the seesaw do not start developing simultaneously over the respective ocean basins in the course of a particular winter season. Rather, the seesaw formation is...

Dynamic and Thermodynamic Characteristics of Atmospheric Response to Anomalous Sea-Ice Extent in the Sea of Okhotsk

Abstract Influence of sea-ice extent anomalies within the Sea of Okhotsk on the large-scale atmospheric circulation is investigated through an analysis of the dynamic and thermodynamic characteristics of the response in an atmospheric general circulation model to specified anomalous sea-ice cover. Significant response appears not only around the Sea of Okhotsk, but also downstream over the Bering Sea, Alaska, and North America in the form of a stationary wave train in the troposphere. This remote response, associated with wave activity flux emanating from the Okhotsk area to the downstream, is regarded as a stationary Rossby wave generated thermally by the anomalous turbulent heat fluxes from the ocean surface as a result of the anomalous sea-ice cover. The Pacific storm track in the model that extends zonally at 35°N is located too far south of the Sea of Okhotsk to exert substantial feedback forcing on the local and remote response. Since a similar stationary wave train is identified in the composite di...

Interannual Seesaw between the Aleutian and Icelandic Lows. Part II: Its Significance in the Interannual Variability over the Wintertime Northern Hemisphere

Abstract Through analysis of observational data for the period of 1973–94, the late-winter formation of an interannual seesaw between the surface Aleutian and Icelandic lows (AL and IL, respectively) is shown to significantly impact the covariance structure of the leading mode of the interannual variability in the geopotential height field over the extratropical Northern Hemisphere. The tropospheric leading mode for early winter (November–January) is characterized by a polar–midlatitude dipole over the Euro–Atlantic sector with a high degree of the annularity, coupled with the anomalous lower-stratospheric polar vortex. Over the North Pacific, no significant anomalies are associated with this mode. After the formation of the AL–IL seesaw, however, the dipole no longer dominates in the upper-tropospheric variability. The dipole signature is masked in late winter (February–April) by the predominant combined signature of the so-called Pacific–North American pattern and a meridional dipole over the northweste...

A negative phase shift of the winter AO/NAO due to the recent Arctic sea-ice reduction in late autumn

This paper examines the possible linkage between the recent reduction in Arctic sea-ice extent and the wintertime Arctic Oscillation (AO)/North Atlantic Oscillation (NAO). Observational analyses using the ERA interim reanalysis and merged Hadley/Optimum Interpolation Sea Surface Temperature data reveal that a reduced (increased) sea-ice area in November leads to more negative (positive) phases of the AO and NAO in early and late winter, respectively. We simulate the atmospheric response to observed sea-ice anomalies using a high-top atmospheric general circulation model (AGCM for Earth Simulator, AFES version 4.1). The results from the simulation reveal that the recent Arctic sea-ice reduction results in cold winters in mid-latitude continental regions, which are linked to an anomalous circulation pattern similar to the negative phase of AO/NAO with an increased frequency of large negative AO events by a factor of over two. Associated with this negative AO/NAO phase, cold air advection from the Arctic to the mid-latitudes increases. We found that the stationary Rossby wave response to the sea-ice reduction in the Barents Sea region induces this anomalous circulation. We also found a positive feedback mechanism resulting from the anomalous meridional circulation that cools the mid-latitudes and warms the Arctic, which adds an extra heating to the Arctic air column equivalent to about 60% of the direct surface heat release from the sea-ice reduction. The results from this high-top model experiment also suggested a critical role of the stratosphere in deepening the tropospheric annular mode and modulation of the NAO in mid to late winter through stratosphere-troposphere coupling.

Influence of Okhotsk sea‐ice extent on atmospheric circulation

An atmospheric general circulation model was used to investigate the influence of Okhotsk sea ice on large-scale atmospheric circulation. Significant differences in the model responses between heavy and light ice cover are evident not only around the Sea of Okhotsk but also downstream towards North America in the form of a stationary wavetrain in the troposphere. Because the wave activity flux associated with the wavetrain emanates downstream from the Sea of Okhotsk, this remote response is regarded as a stationary Rossby wave excited thermally through an anomalous surface heat flux over the Sea of Okhotsk. The wavetrain is thermally reinforced downstream by the anomalous heat flux associated with wind anomalies over the ocean surface, which are induced by the wavetrain itself.

The stratospheric pathway for Arctic impacts on midlatitude climate

Recent evidence from both observations and model simulations suggests that an Arctic sea ice reduction tends to cause a negative Arctic Oscillation (AO) phase with severe winter weather in the Northern Hemisphere, which is often preceded by weakening of the stratospheric polar vortex. Although this evidence hints at a stratospheric involvement in the Arctic-midlatitude climate linkage, the exact role of the stratosphere remains elusive. Here we show that tropospheric AO response to the Arctic sea ice reduction largely disappears when suppressing the stratospheric wave mean flow interactions in numerical experiments. The results confirm a crucial role of the stratosphere in the sea ice impacts on the midlatitudes by coupling between the stratospheric polar vortex and planetary-scale waves. Those results and consistency with observation-based evidence suggest that a recent Arctic sea ice loss is linked to midlatitudes extreme weather events associated with the negative AO phase.

Influence of the Northern Hemisphere annular mode on ENSO by modulating westerly wind bursts

and high latitudes. Conversely, Thompson and Lorenz [2004] attempted to detect the effects of both hemispheric annular modes (AMs) on the tropics. They showed coupling between the AMs and tropical atmosphere and that a wintertime positive (negative) AM generates the upper tropospheric zonal mean westerly (easterly) anomalies in the tropics after a 2-week lag. These results suggest a link between the NAM and tropical variability. However, they did not note any direct influences of the NAM on the ENSO. [3] This study examined the influences of NAM on the ENSO. Based on the results of previous studies, we examined the lead and lag relationships between the NAM and ENSO in all seasons using a reanalysis data set. The observational evidence revealed a statistically significant relationship between the NAM signature and the ENSO. To strengthen this finding, we also used an atmospheric general circulation model (AGCM) to simulate climatological monthly mean sea surface temperature (SST) distributions.

Impacts of the Aleutian–Icelandic Low Seesaw on Surface Climate during the Twentieth Century

Abstract An interannual seesaw between the intensities of the Icelandic and Aleutian lows and its impact on surface climate observed during the twentieth century are investigated. In a recent period from the late 1960s to the early 1990s, their seesaw relationship was particularly apparent in late winter. The associated anomalies in surface air temperature were significant in many regions over the extratropical Northern Hemisphere except in central portions of the continents. The seesaw also modified the ocean–atmosphere exchange of heat and moisture extensively over the North Atlantic and North Pacific by changing evaporation and precipitation. Since the seesaw formation was triggered by eastward propagation of stationary Rossby wave trains from the North Pacific into the North Atlantic, anomalous circulation over the North Pacific in January was identified as a good precursor for February surface air temperatures in the Euro–Atlantic sector during that period. The seesaw relationship between the two low...

Northern Hemisphere Extratropical Tropospheric Planetary Waves and their Low‐Frequency Variability: Their Vertical Structure and Interaction with Transient Eddies and Surface Thermal Contrasts

Climate Dynamic Geophysical Mon Copyright 2010 b 10.1029/2008GM Structure and dynamics of the Northern Hemisphere planetary waves, which cause geographically fixed longitudinal dependence to the climate, are examined through dynamical diagnoses applied tomodern global data sets. Summertime planetarywave signature in the Western Hemisphere includes surface maritime subtropical anticyclones, for which pronounced land-sea thermal contrasts across the west coasts of subtropical continents are important as thermal forcing. Its Eastern Hemisphere counterpart is dominated by continental-scale cyclone and anticyclone in the lower and upper troposphere, respectively, associated with Asian monsoon. Wintertime planetary waves are forced orographically and thermally in middle/subpolar latitudes, with pronounced land-sea thermal contrasts, including a contribution from diabatic heating along oceanic “storm tracks.” Wave activity thus generated propagates southeastward, maintaining an upper-level vorticity dipole over the Atlantic with an eddy-driven polar-front jet (PFJ) separated from a subtropical jet (STJ). Its Pacific counterpart is in the opposite sense with a predominant single jet with PFJ-STJ hybrid characteristics. Stationary circulation anomaly patterns that cause regional climate variability are strong in winter over the midlatitude ocean basins, extracting kinetic energy effectively from diffluent westerly jets and with feedback forcing by transient eddies along storm tracks. In the summertime Asian STJ exit, a stationary baroclinic anomaly pattern is dominant, maintaining itself by extracting potential energy from the jet and negating it by anomalous cumulus activity. Each of these patterns thus bears characteristics of a dynamical mode. Generation of shallow, cold surface anticyclones is discussed from a viewpoint of interaction of stationary Rossby waves with surface baroclinic zones.

Formation, Mechanisms, and Predictability of the Aleutian–Icelandic Low Seesaw in Ensemble AGCM Simulations

Abstract The potential predictability associated with the remote influence of midlatitude tropospheric anomalies over the North Pacific or the North Atlantic, via a seesawlike interannual oscillation between the surface Aleutian and Icelandic lows (AL and IL, respectively) is investigated. Data from a 24-member ensemble of 50-yr atmospheric general circulation model simulation forced with observed sea surface temperature (SST) conditions are analyzed by separating the total simulated fluctuations into the external component forced by the prescribed SST and the internal component generated by atmospheric internal dynamics. The AL–IL seesaw can be identified in both the external and internal components of the variability. In the external variability, determined through the ensemble mean, the seesaw is gradually formed from December to March through the development of a Pacific–North American (PNA) pattern–like wave train, remotely forced by the El Nino–Southern Oscillation. The amplitudes of the externally ...