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

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.

Download Full-text

Importance of Resolving Kuroshio Front and Eddy Influence in Simulating the North Pacific Storm Track

Journal of Climate ◽

10.1175/jcli-d-16-0154.1 ◽

2017 ◽

Vol 30 (5) ◽

pp. 1861-1880 ◽

Cited By ~ 44

Author(s):

Xiaohui Ma ◽

Ping Chang ◽

R. Saravanan ◽

Raffaele Montuoro ◽

Hisashi Nakamura ◽

...

Keyword(s):

North Pacific ◽

Large Scale ◽

Climate Model ◽

Kuroshio Extension ◽

Storm Track ◽

Small Scale ◽

Low Resolution ◽

The North ◽

The North Pacific ◽

Sst Anomalies

Abstract Local and remote atmospheric responses to mesoscale SST anomalies associated with the oceanic front and eddies in the Kuroshio Extension region (KER) are studied using high- (27 km) and low-resolution (162 km) regional climate model simulations in the North Pacific. In the high-resolution simulations, removal of mesoscale SST anomalies in the KER leads to not only a local reduction in cyclogenesis but also a remote large-scale equivalent barotropic response with a southward shift of the downstream storm track and jet stream in the eastern North Pacific. In the low-resolution simulations, no such significant remote response is found when mesoscale SST anomalies are removed. The difference between the high- and low-resolution model simulated atmospheric responses is attributed to the effect of mesoscale SST variability on cyclogenesis through moist baroclinic instability. It is only when the model has sufficient resolution to resolve small-scale diabatic heating that the full effect of mesoscale SST forcing on the storm track can be correctly simulated.

Download Full-text

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

Journal of Climate ◽

10.1175/jcli-d-11-00493.1 ◽

2012 ◽

Vol 25 (19) ◽

pp. 6554-6566 ◽

Cited By ~ 11

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.

Download Full-text

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

Journal of Climate ◽

10.1175/jcli-d-17-0200.1 ◽

2017 ◽

Vol 31 (1) ◽

pp. 297-315 ◽

Cited By ~ 6

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.

Download Full-text

Atlantic multidecadal oscillation drives interdecadal Pacific variability via tropical atmospheric bridge

Journal of Climate ◽

10.1175/jcli-d-20-0983.1 ◽

2021 ◽

pp. 1-46

Author(s):

Xiaohe An ◽

Bo Wu ◽

Tianjun Zhou ◽

Bo Liu

Keyword(s):

North Pacific ◽

General Circulation ◽

Circulation Model ◽

Atlantic Multidecadal Oscillation ◽

Tropical Pacific ◽

Convective Heating ◽

The North ◽

The North Pacific ◽

Sst Anomalies ◽

The Tropical Pacific

AbstractInterdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Oscillation (AMO), two leading modes of decadal climate variability, are not independent. It was proposed that ENSO-like sea surface temperature (SST) variations play a central role in the Pacific responses to the AMO forcing. However, observational analyses indicate that the AMO-related SST anomalies in the tropical Pacific are far weaker than those in the extratropical North Pacific. Here, we show that SST in the North Pacific is tied to the AMO forcing by convective heating associated with precipitation over the tropical Pacific, instead of by SST there, based on an ensemble of pacemaker experiments with North Atlantic SST restored to the observation in a coupled general circulation model. The AMO modulates precipitation over the equatorial and tropical southwestern Pacific through exciting an anomalous zonal circulation and an interhemispheric asymmetry of net moist static energy input into the atmosphere. The convective heating associated with the precipitation anomalies drive SST variations in the North Pacific through a teleconnection, but remarkably weaken the ENSO-like SST anomalies through a thermocline damping effect. This study has implications that the IPO is a combined mode generated by both AMO forcing and local air-sea interactions, but the IPO-related global-warming acceleration/slowdown is independent of the AMO.

Download Full-text

Baroclinic Development in Observations and NASA GSFC General Circulation Models

Monthly Weather Review ◽

10.1175/mwr3110.1 ◽

2006 ◽

Vol 134 (4) ◽

pp. 1161-1173 ◽

Cited By ~ 3

Author(s):

Dennis P. Robinson ◽

Robert X. Black

Keyword(s):

North Atlantic ◽

North Pacific ◽

General Circulation ◽

Storm Track ◽

Model Simulations ◽

The North ◽

The Pacific ◽

The North Atlantic ◽

The North Pacific ◽

Midwinter Suppression

Abstract Comparative diagnostic analyses of developing synoptic-scale baroclinic disturbances in NCEP–NCAR reanalyses and the NASA–NCAR (NASCAR) and Aries [NASA’s Seasonal-to-Interannual Prediction Project (NSIPP)] general circulation model simulations are performed. In particular, lag composite analyses of wintertime cyclonic and anticyclonic events occurring in the North Pacific and North Atlantic storm tracks are constructed to pursue a synoptic and dynamic characterization of eddy development. The data are also seasonally stratified to study aspects of the North Pacific midwinter suppression phenomenon. Winter-averaged results indicate that the model-simulated events are generally too weak in amplitude, particularly in the upper troposphere. For the North Pacific storm track, model-simulated events are also anomalously distended in the meridional direction. The existing model biases in eddy structure and magnitude lead to anomalously weak baroclinic energy conversions for both cyclonic and anticyclonic events over the North Pacific. For the North Atlantic storm track the NASCAR model provides a very good representation of the structure of developing cyclonic events. However, growing North Atlantic cyclones in the NSIPP model are anomalously weak and horizontally too isotropic (meridionally retracted). These latter two characteristics are also observed in both models for developing anticyclonic flow anomalies over the North Atlantic. The relative weakness of NSIPP synoptic events over the North Atlantic region is largely responsible for the 50% deficiency in areal-averaged baroclinic energy conversions. Conversely, the NASCAR model climatology features anomalously strong temperature gradients over the western North Atlantic that provide local enhancements to the baroclinic energy conversion field. A seasonally stratified diagnostic analysis reveals that the simulated climatological storm tracks over the North Pacific undergo larger spatial migrations during the cool season compared to observations. It is further determined that the suppression of synoptic eddy activity observed in the Pacific storm track is associated with a relative midwinter weakness in the magnitude of the growing cyclonic anomalies. Specifically, during midwinter the cyclonic perturbations entering the Pacific storm track are deficient in magnitude compared to their early and late winter counterparts. It is also discovered that the midwinter suppression pattern over the North Pacific region has a clear organized extension upstream into Siberia, the region from which incipient upper-tropospheric short-wave features emanate. This behavior is found in both observations and the model simulations. The results herein support the idea that the North Pacific midwinter suppression phenomenon is linked to a midwinter weakness in the upstream formation of upper-level short waves, leading to anomalously weak “seeding” of baroclinic disturbances in the Pacific storm track.

Download Full-text

Seasonal and Long-Term Coupling between Wintertime Storm Tracks and Sea Surface Temperature in the North Pacific

Journal of Climate ◽

10.1175/jcli-d-12-00724.1 ◽

2013 ◽

Vol 26 (16) ◽

pp. 6123-6136 ◽

Cited By ~ 26

Author(s):

Bolan Gan ◽

Lixin Wu

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

North Pacific ◽

Storm Track ◽

Sea Surface ◽

Storm Tracks ◽

The North ◽

The North Pacific ◽

Sst Anomalies

Abstract In this study, a lagged maximum covariance analysis (MCA) of the wintertime storm-track and sea surface temperature (SST) anomalies derived from the reanalysis datasets shows significant seasonal and long-term relationships between storm tracks and SST variations in the North Pacific. At seasonal time scales, it is found that the midlatitude warm (cold) SST anomalies in the preceding fall, which are expected to change the tropospheric baroclinicity, can significantly reduce (enhance) the storm-track activities in early winter. The storm-track response pattern, however, is in sharp contrast to the forcing pattern, with warm (cold) SST anomalies in the western–central North Pacific corresponding to a poleward (equatorward) shift of storm tracks. At interannual-to-decadal time scales, it is found that the wintertime SST and storm-track anomalies are mutually reinforced up to 3 yr, which is characterized by PDO-like SST anomalies with warming in the western–central domain coupled with basin-scale positive storm-track anomalies extending along 50°N.

Download Full-text

Downstream Suppression of Baroclinic Waves

Journal of Climate ◽

10.1175/jcli-d-20-0483.1 ◽

2021 ◽

Vol 34 (3) ◽

pp. 919-930

Author(s):

Lina Boljka ◽

David W. J. Thompson ◽

Ying Li

Keyword(s):

North Atlantic ◽

North Pacific ◽

General Circulation ◽

Wave Packets ◽

Storm Track ◽

Eddy Activity ◽

Baroclinic Waves ◽

The North ◽

The North Pacific ◽

Downstream Development

AbstractBaroclinic waves drive both regional variations in weather and large-scale variability in the extratropical general circulation. They generally do not exist in isolation, but rather often form into coherent wave packets that propagate to the east via a mechanism called downstream development. Downstream development has been widely documented and explored. Here we document a novel but also key aspect of baroclinic waves: the downstream suppression of baroclinic activity that occurs in the wake of eastward propagating disturbances. Downstream suppression is apparent not only in the Southern Hemisphere storm track as shown in previous work, but also in the North Pacific and North Atlantic storm tracks. It plays an essential role in driving subseasonal periodicity in extratropical eddy activity in both hemispheres, and gives rise to the observed quiescence of the North Atlantic storm track 1–2 weeks following pronounced eddy activity in the North Pacific sector. It is argued that downstream suppression results from the anomalously low baroclinicity that arises as eastward propagating wave packets convert potential to kinetic energy. In contrast to baroclinic wave packets, which propagate to the east at roughly the group velocity in the upper troposphere, the suppression of baroclinic activity propagates eastward at a slower rate that is comparable to that of the lower to midtropospheric flow. The results have implications for understanding subseasonal variability in the extratropical troposphere of both hemispheres.

Download Full-text

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

Journal of Climate ◽

10.1175/2007jcli1853.1 ◽

2008 ◽

Vol 21 (7) ◽

pp. 1569-1588 ◽

Cited By ~ 4

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.

Download Full-text

Stationary Waves and Upward Troposphere-Stratosphere Coupling in Operational Subseasonal Forecasting Models

10.5194/egusphere-egu21-5453 ◽

2021 ◽

Author(s):

Chen Schwartz ◽

Chaim Garfinkel

Keyword(s):

North Pacific ◽

General Circulation ◽

Large Scale ◽

General Circulation Models ◽

Stationary Waves ◽

Central Pacific ◽

The North ◽

Realistic Representation ◽

Longitudinal Position ◽

The North Pacific

Realistic representation of large-scale stationary waves (SWs) in general circulation models is crucial, as they modulate the trajectories of mid-latitude storms, and shape the distribution of surface temperatures along comparable latitude bands over densely populated areas in the Northern Hemisphere.In this work, we assess the fidelity of NH wintertime SWs in 5 operational subseasonal-to-seasonal models. In the troposphere, we found that biases in the North Pacific are more pronounced in NCEP, ECMWF and UKMO models compared to the North Atlantic, while in the CMA and BoM models, large biases in amplitude and phase are present in both sectors. These biases in tropospheric SWs directly affect the simulated SWs in the stratosphere.Finally, we attribute the biases in the North Pacific sector, in part, to the mean state biases in the tropics. Longitudinal shifts in the time-mean tropical convection over the Maritime Continent and central Pacific, affect the longitudinal position of the North Pacific trough in the models.

Download Full-text