The Variability Of The Ferrel Cell In The Northern Hemisphere During Boreal Winters

The analysis is based on NCEP/NCAR monthly reanalysis datasets, climate indices from CPC and NOAA extended reconstructed sea surface temperature (ERSST) datasets. By an EOF analysis and a filtering method, we examine the variability of the Ferrel cell in the Northern Hemisphere in boreal winters during 1948-2013, capturing its first two modes and their corresponding variations on both interannual and interdecadal timescales. By a composite analysis and a correlation analysis, we also bring about the possible climate backgrounds corresponding to the variations, trying to figure out the potential connections between the Ferrel cell and various climate signals.To investigate different modes of the cell’s variability, we use an EOF transformation, capturing the intensity-associated mode (51.34%) and the north-southward-shift mode of the circulation (22.08%). The interannual signals corresponding to the 1st mode exhibits a cycle of 5.5-8 years while the ones related to the 2rd mode exhibits significant cycles of 4 and 8.5 years with increasing amplitudes in recent years. In interdecadal respect, the intensity-related signal implies an overall strengthening trend during 1960-1993 and a slightly weakening trend afterwards. The position-related signal, on the other hand, exhibits a period of ～30 years. During 1995-1974 and 1989-2003, the circulation shows an equatorward-shifting trend while it is the reverse during 1974-1988 and 2003-2013. In addition, the position-related signal varies with its greatest amplitude during 1965-1995.The strength variation of the Ferrel cell is related to significant differences of climate backgrounds, including the zonal wind field, the air pressure field, the sea surface temperature and the precipitation distributions. On interannual timescale, a strengthened Ferrel cell is characterized by a poleward shift of the westerly jet streams at both upper and lower levels above the North Atlantic and the North Pacific. The strengthening of the Ferrel cell is in accompany with a strengthened subtropical high (including the Pacific Subtropical High and the Azores High) and an enhanced Iceland Low with larger spatial coverages. In addition to that, considerable warming signals occurred over the interior North Pacific (～20-30°N) when the Ferrel cell is intensified, while negative anomalies appears at the central tropical Pacific which may be related to La nina events. But note that there are no significant anomalies at the central tropical Pacific when the cell is weakened. Corresponding to SSTs anomalies, when the Ferrel cell is strengthened there exist evident enhanced rainfalls at the interior North Pacific (～20-30°N) and a reduction at central equatorial Pacific, while there are no significant anomalies at the central tropical Pacific when the cell is weakened. On the interdecadal timescale, a strengthened cell is related to an AO positive phase and the global warming event. The strengthening of the circulation is probably the atmospheric feedback under the force of global warming.The north-southward shift of the Ferrel cell is in concurrence with significant changes in climate backgrounds as well. On the interannual timescale, when the Ferrel cell shifts polewards, the westerly jet streams over the North Pacific and the North Atlantic tend to shift polewards and slightly weaken in their centers; the subtropical westerlies at upper level over the North Atlantic would on the contrary increase in strength. Accompanying that, the Azores High tends to expand polewards with strengthening low pressures all over the polar region. Additionally, the north-southward shift of the cell is significantly related to the SSTs anomalies at the interior North Pacific and along the west coastlines of the North America. When the cell shifts polewards, a conspicuous feature is the warm SSTs in the central northern Pacific (～50°N,～20°W) and cold ones off the west coast of the contiguous United States which closely resembles the pattern of PDO. What notable is, the SSTs anomalies at tropics exhibit a significant nonlinearity corresponding to positive and negative phases. Considerable warming signals occurred at the central tropical Pacific when the Ferrel cell shifts equatorwards, suggesting a linkage with El Nino events, while there are no significant anomalies when the Ferrel cell shifts polewards. Corresponding to SSTs anomalies, there exist reduced rainfalls at the interior North Pacific with reinforced ones at central-eastern equatorial Pacific when the cell shifts equatorwards, while there are no significant anomalies at the central tropical Pacific when the cell shifts polewards. On the interdecadal timescale, a poleward shifted cell is related to AO positive phases and NP positive events.