scholarly journals Mechanisms for the Maintenance of the Wintertime Basin-Scale Atmospheric Response to Decadal SST Variability in the North Pacific Subarctic Frontal Zone

2017 ◽  
Vol 31 (1) ◽  
pp. 297-315 ◽  
Author(s):  
Satoru Okajima ◽  
Hisashi Nakamura ◽  
Kazuaki Nishii ◽  
Takafumi Miyasaka ◽  
Akira Kuwano-Yoshida ◽  
...  
Keyword(s):  
North Pacific ◽  
Frontal Zone ◽  
Atmospheric Response ◽  
The North ◽  
Model Response ◽  
Basin Scale ◽  
Sst Anomaly ◽  
Subarctic Frontal Zone ◽  
The North Pacific ◽  
Sst Anomalies

Abstract Mechanisms for the maintenance of a large-scale wintertime atmospheric response to warm sea surface temperature (SST) anomalies associated with decadal-scale poleward displacement of the North Pacific subarctic frontal zone (SAFZ) are investigated through the following two ensemble experiments with an atmospheric general circulation model (AGCM): one with climatological-mean SST and the other with positive SST anomalies along the SAFZ prescribed on top of the climatological-mean SST. As actually observed, the simulated January ensemble response over the North Pacific is anticyclonic throughout the depth of the troposphere, although its amplitude is smaller. This response is maintained through energy conversion from the ensemble climatological-mean circulation realized under the climatological SST as well as feedback from anomalous transient eddy activity, suggesting that the response may have characteristics as a preferred mode of variability (or “dynamical mode”). Conversions of both available potential energy and kinetic energy from the climatological-mean state are important for the observed anomaly, while the latter is less pronounced for the model response. Net transient feedback forcing is also important for both the observed anomaly and simulated response. These results imply that a moderate-resolution (~1°) AGCM may be able to simulate a basin-scale atmospheric response to the SAFZ SST anomaly through synoptic- and basin-scale dynamical processes. Weaker PNA-like internal variability in the model may lead to the weaker response, suggesting that misrepresentation of intrinsic atmospheric variability can affect the model response to the SST anomaly.

scholarly journals Seasonal Evolutions of Atmospheric Response to Decadal SST Anomalies in the North Pacific Subarctic Frontal Zone: Observations and a Coupled Model Simulation

2012 ◽  
Vol 25 (1) ◽  
pp. 111-139 ◽  
Author(s):  
Bunmei Taguchi ◽  
Hisashi Nakamura ◽  
Masami Nonaka ◽  
Nobumasa Komori ◽  
Akira Kuwano-Yoshida ◽  
...  
Keyword(s):  
North Pacific ◽  
Frontal Zone ◽  
Ocean Dynamics ◽  
The North ◽  
The Pacific ◽  
Decadal Scale ◽  
Anomaly Pattern ◽  
Subarctic Frontal Zone ◽  
The North Pacific ◽  
Sst Anomalies

Abstract Potential impacts of pronounced decadal-scale variations in the North Pacific sea surface temperature (SST) that tend to be confined to the subarctic frontal zone (SAFZ) upon seasonally varying atmospheric states are investigated, by using 48-yr observational data and a 120-yr simulation with an ocean–atmosphere coupled general circulation model (CGCM). SST fields based on in situ observations and the ocean component of the CGCM have horizontal resolutions of 2.0° and 0.5°, respectively, which can reasonably resolve frontal SST gradient across the SAFZ. Both the observations and CGCM simulation provide a consistent picture between SST anomalies in the SAFZ yielded by its decadal-scale meridional displacement and their association with atmospheric anomalies. Correlated with SST anomalies persistent in the SAFZ from fall to winter, a coherent decadal-scale signal in the wintertime atmospheric circulation over the North Pacific starts emerging in November and develops into an equivalent barotropic anomaly pattern similar to the Pacific–North American (PNA) pattern. The PNA-like signal with the weakened (enhanced) surface Aleutian low correlated with positive (negative) SST anomalies in the SAFZ becomes strongest and most robust in January, under the feedback forcing from synoptic-scale disturbances migrating along the Pacific storm track that shifts northward (southward) in accord with the oceanic SAFZ. This PNA-like signal, however, breaks down in February, which is suggestive of a particular sensitivity of that anomaly pattern to subtle differences in the background climatological-mean state. Despite its collapse in February, the PNA-like signal recurs the next January. This subseasonal evolution of the signal suggests that the PNA-like anomaly pattern may develop as a response to the persistent SST anomalies that are maintained mainly through ocean dynamics.

scholarly journals Assessing the Importance of Prominent Warm SST Anomalies over the Midlatitude North Pacific in Forcing Large-Scale Atmospheric Anomalies during 2011 Summer and Autumn

2014 ◽  
Vol 27 (11) ◽  
pp. 3889-3903 ◽  
Author(s):  
Satoru Okajima ◽  
Hisashi Nakamura ◽  
Kazuaki Nishii ◽  
Takafumi Miyasaka ◽  
Akira Kuwano-Yoshida
Keyword(s):  
North Pacific ◽  
General Circulation ◽  
Large Scale ◽  
Storm Track ◽  
Anticyclonic Anomaly ◽  
The North ◽  
Basin Scale ◽  
Sst Anomaly ◽  
The North Pacific ◽  
Sst Anomalies

Abstract Sets of atmospheric general circulation model (AGCM) experiments are conducted to assess the importance of prominent positive anomalies in sea surface temperature (SST) observed over the midlatitude North Pacific in forcing a persistent basin-scale anticyclonic circulation anomaly and its downstream influence in 2011 summer and autumn. The anticyclonic anomaly observed in October is well reproduced as a robust response of an AGCM forced only with the warm SST anomaly associated with the poleward-shifted oceanic frontal zone in the midlatitude Pacific. The equivalent barotropic anticyclonic anomaly over the North Pacific is maintained under strong transient eddy feedback forcing associated with the poleward-deflected storm track. As the downstream influence of the anomaly, abnormal warmth and dryness observed over the northern United States and southern Canada in October are also reproduced to some extent. The corresponding AGCM response over the North Pacific to the tropical SST anomalies is similar but substantially weaker and less robust, suggesting the primary importance of the prominent midlatitude SST anomaly in forcing the large-scale atmospheric anomalies observed in October 2011. In contrast, the model reproduction of the atmospheric anomalies observed in summer was unsuccessful. This appears to arise from the fact that, unlike in October, the midlatitude SST anomalies accompanied reduction of heat and moisture release from the ocean, indicative of the atmospheric thermodynamic forcing on the SST anomalies. Furthermore, the distinct seasonality in the AGCM responses to the warm SST anomalies may also be contributed to by the seasonality of background westerlies and storm track.

scholarly journals Modulation of Atmospheric Response to North Pacific SST Anomalies under Global Warming: A Statistical Assessment

2012 ◽  
Vol 25 (19) ◽  
pp. 6554-6566 ◽  
Author(s):  
Bolan Gan ◽  
Lixin Wu
Keyword(s):  
Global Warming ◽  
North Pacific ◽  
Climate Model ◽  
Storm Track ◽  
Early Winter ◽  
The North ◽  
Basin Scale ◽  
Climate Model Simulations ◽  
The North Pacific ◽  
Sst Anomalies

Abstract In this study the modulation of ocean-to-atmosphere feedback over the North Pacific in early winter from global warming is investigated based on both the observations and multiple climate model simulations from a statistical perspective. It is demonstrated that the basin-scale atmospheric circulation displays an equivalent barotropic ridge in response to warm SST anomalies in the Kuroshio–Oyashio Extension (KOE) region. This warm SST–ridge response in early winter can be enhanced significantly by global warming, indicating a strengthening of air–sea coupling over the North Pacific. This enhancement is likely associated with the intensification of storm tracks and, in turn, the amplification of atmospheric transient eddy feedback in a warm climate, although the secular trend of enhanced storm-track activity over the North Pacific is suggested to be biased in reanalysis product.

scholarly journals Decadal Variability of Upper-Ocean Heat Content Associated with Meridional Shifts of Western Boundary Current Extensions in the North Pacific

2017 ◽  
Vol 30 (16) ◽  
pp. 6247-6264 ◽  
Author(s):  
Bunmei Taguchi ◽  
Niklas Schneider ◽  
Masami Nonaka ◽  
Hideharu Sasaki
Keyword(s):  
North Pacific ◽  
Frontal Zone ◽  
Heat Content ◽  
Ocean Heat Content ◽  
Upper Ocean ◽  
Upper Ocean Heat Content ◽  
Temperature Anomalies ◽  
The North ◽  
Subarctic Frontal Zone ◽  
The North Pacific

Generation and propagation processes of upper-ocean heat content (OHC) in the North Pacific are investigated using oceanic subsurface observations and an eddy-resolving ocean general circulation model hindcast simulation. OHC anomalies are decomposed into physically distinct dynamical components (OHC ρ) due to temperature anomalies that are associated with density anomalies and spiciness components (OHC χ) due to temperature anomalies that are density compensating with salinity. Analysis of the observational and model data consistently shows that both dynamical and spiciness components contribute to interannual–decadal OHC variability, with the former (latter) component dominating in the subtropical (subpolar) North Pacific. OHC ρ variability represents heaving of thermocline, propagates westward, and intensifies along the Kuroshio Extension, consistent with jet-trapped Rossby waves, while OHC χ variability propagates eastward along the subarctic frontal zone, suggesting advection by mean eastward currents. OHC χ variability tightly corresponds in space to horizontal mean spiciness gradients. Meanwhile, area-averaged OHC χ anomalies in the western subarctic frontal zone closely correspond in time to meridional shifts of the subarctic frontal zone. Regression coefficient of the OHC χ time series on the frontal displacement anomalies quantitatively agree with the area-averaged mean spiciness gradient in the region, and suggest that OHC χ is generated via frontal variability in the subarctic frontal zone.

scholarly journals Summertime Evolution of Decadal Sea Surface Temperature Anomalies in the Midlatitude North Pacific

2008 ◽  
Vol 21 (7) ◽  
pp. 1569-1588 ◽  
Author(s):  
Takashi Mochizuki ◽  
Toshiyuki Awaji
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Pacific ◽  
Radiation Flux ◽  
Sea Surface ◽  
Shortwave Radiation ◽  
The North ◽  
Sst Anomaly ◽  
The North Pacific ◽  
Sst Anomalies

Abstract To clarify the summertime evolution of decadal sea surface temperature (SST) anomalies and related physical processes in the midlatitudes of the North Pacific, numerical solutions of a three-dimensional bulk mixed layer model are analyzed, focusing on the contribution of the net shortwave radiative forcing at the sea surface. A quantitative heat budget analysis for the ocean mixed layer relating to late-1980s decadal SST change reveals that the decadal SST anomalies decay from late spring to early summer over the entire midlatitudes of the North Pacific. This quasi-seasonal decay of the decadal SST anomalies is controlled by an anomalous local thermal damping (i.e., anomalous surface heat fluxes). From midsummer to early autumn the anomalous net shortwave radiation flux associated with a meridional shift of the storm track acts to induce strong seasonal damping of the decadal SST anomaly in the northern Kuroshio–Oyashio Extension region. In contrast, in the north of the subtropical frontal region, the net shortwave radiation flux anomaly, which results from changes in low-level stratiform cloud cover, plays a major role in seasonally enhancing the decadal SST anomaly. Consequently, the SST anomalies formed by these radiative forcings cause significant variations in the local thermal damping rate at the sea surface over the period from late summer to early autumn.

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