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  • 11
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    Importance of Resolving Kuroshio Front and Eddy Influence in Simulating the North Pacific Storm Track (2017)
    American Meteorological Society
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    Publication Date: 2017-02-23
    Description: 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.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 12
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    Influence of the Subtropical High and Storm Track on Low-Cloud Fraction and Its Seasonality over the South Indian Ocean (2018)
    American Meteorological Society
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    Publication Date: 2018-04-25
    Description: The south Indian Ocean is characterized by enhanced midlatitude storm-track activity around a prominent sea surface temperature (SST) front and unique seasonality of the surface subtropical Mascarene high. The present study investigates the climatological distribution of low-cloud fraction (LCF) and its seasonality by using satellite data, in order to elucidate the role of the storm-track activity and subtropical high. On the equatorward flank of the SST front, summertime LCF is locally maximized despite small estimated inversion strength (EIS) and high SST. This is attributable to locally augmented sensible heat flux (SHF) from the ocean under the enhanced storm-track activity, which gives rise to strong instantaneous wind speed while acting to relax the meridional gradient of surface air temperature. In the subtropics, summertime LCF is maximized off the west coast of Australia, while wintertime LCF is distributed more zonally across the basin unlike in other subtropical ocean basins. Although its zonally extended distribution is correspondent with that of LCF, EIS alone cannot explain the wintertime LCF enhancement, which precedes the EIS maximum under continuous lowering of SST and enhanced SHF in winter. Basinwide cold advection associated with the wintertime westward shift of the subtropical high contributes to the enhancement of SHF, especially around 15°–25°S, while seasonally enhanced storm-track activity augments SHF around 30°S. The analysis highlights the significance of large-scale controls, particularly through SHF, on the seasonality of the climatological LCF distribution over the south Indian Ocean, which reflect the seasonality of the Mascarene high and storm-track activity.
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    Topics: Geography , Geosciences , Physics
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  • 13
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    Vertical Structure and Energetics of the Western Pacific Teleconnection Pattern (2016)
    American Meteorological Society
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    Publication Date: 2016-08-26
    Description: The western Pacific (WP) pattern, characterized by north–south dipolar anomalies in pressure over the Far East and western North Pacific, is known as one of the dominant teleconnection patterns in the wintertime Northern Hemisphere. Composite analysis reveals that monthly height anomalies exhibit baroclinic structure with their phase lines tilting southwestward with height in the lower troposphere. The anomalies can thus yield not only a poleward heat flux across the climatological thermal gradient across the strong Pacific jet but also a westward heat flux across the climatological thermal gradient between the North Pacific and the cooler Asian continent. The resultant baroclinic conversion of available potential energy (APE) from the climatological-mean flow contributes most efficiently to the APE maintenance of the monthly WP pattern, acting against strong thermal damping effects by anomalous heat exchanges with the underlying ocean and anomalous precipitation in the subtropics and by the effect of anomalous eddy heat flux under modulated storm-track activity. Kinetic energy (KE) of the pattern is maintained through barotropic feedback forcing associated with modulated activity of transient eddies and the conversion from the climatological-mean westerlies, both of which act against frictional damping. The net feedback forcing by transient eddies is therefore not particularly efficient. The present study suggests that the WP pattern has a characteristic of a dynamical mode that can maintain itself through efficient energy conversion from the climatological-mean fields even without external forcing, including remote influence from the tropics.
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  • 14
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    Importance of Midlatitude Oceanic Frontal Zones for the Annular Mode Variability: Interbasin Differences in the Southern Annular Mode Signature (2016)
    American Meteorological Society
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    Publication Date: 2016-08-18
    Description: A midlatitude oceanic frontal zone is a confluent region of warm and cool ocean currents, characterized by a strong meridional gradient in both sea surface temperature (SST) and surface air temperature (SAT). While recent observational and modeling studies indicate potential impacts of midlatitude oceanic fronts on the extratropical climatological circulation, including storm tracks and eddy-driven westerlies, their impacts on the atmospheric-dominant low-frequency variability (i.e., the annular mode) still remain to be understood. This study explores possible impacts of midlatitude oceanic frontal zones on annular mode signatures in the wintertime Southern Hemisphere (SH). To mimic the SH, sets of idealized aquaplanet experiments are conducted with zonally symmetric distributions of SST prescribed globally at the lower boundary of an atmospheric general circulation model. By systematically changing the latitude of frontal gradient in the SST profile, the experiments reveal that the characteristics of the wintertime annular mode exhibit strong sensitivity to the position of the SST front if situated at midlatitude or subpolar latitude. The annular mode may be interpreted as a manifestation of wobble of the extratropical tropospheric circulation between two “dynamical regimes”—one under the strong influence of SST gradient and the other under the strong control of atmospheric internal dynamics unrelated to the lower-boundary condition. In fact, this interpretation offers insight into the observed interbasin differences in the wintertime signature of the southern annular mode (SAM) that are embedded in the zonally symmetric anomalies. The findings suggest a possible reinterpretation of the climatological-mean state observed in the wintertime SH as the superposition of those two dynamical regimes.
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  • 15
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    The Pacific Decadal Oscillation, Revisited (2016)
    American Meteorological Society
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    Publication Date: 2016-06-08
    Description: The Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere–ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of processes often more independent of the tropics than is observed. Finally, it is suggested that the assessment of PDO-related regional climate impacts, reconstruction of PDO-related variability into the past with proxy records, and diagnosis of Pacific variability within coupled GCMs should all account for the effects of these different processes, which only partly represent the direct forcing of the atmosphere by North Pacific Ocean SSTs.
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  • 16
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    Interannual Modulations of Oceanic Imprints on the Wintertime Atmospheric Boundary Layer under the Changing Dynamical Regimes of the Kuroshio Extension (2016)
    American Meteorological Society
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    Publication Date: 2016-04-21
    Description: The Kuroshio Extension (KE) fluctuates between its different dynamic regimes on (quasi) decadal time scales. In its stable (unstable) regime, the KE jet is strengthened (weakened) and less (more) meandering. The present study investigates wintertime mesoscale atmospheric structures modulated under the changing KE regimes, as revealed in high-resolution satellite data and data from a particular atmospheric reanalysis (ERA-Interim). In the unstable KE regime, a positive anomaly in sea surface temperature (SST) to the north of the climatological KE jet accompanies positive anomalies in upward heat fluxes from the ocean, surface wind convergence, and cloudiness. As revealed in the atmospheric reanalysis, these positive anomalies coincide with local lowering of sea level pressure, weaker vertical wind shear, warming and thickening of the marine atmospheric boundary layer (MABL), anomalous ascent, and convective precipitation. In the stable KE regime, by contrast, the corresponding imprints of sharp SST gradients across the KE and Oyashio fronts on the wintertime MABL are separated more distinctly, and so are the surface baroclinic zones along those two SST fronts. In the ERA-Interim data, such mesoscale imprints of the KE variability as above are not well represented in a period during which the resolution of SST data prescribed is relatively low. The present study thus elucidates the importance of high-resolution SST data prescribed for atmospheric reanalysis in representing modulations of the MABL structure and air–sea fluxes by the variability of oceanic fronts and/or jets, including the modulations occurring with the changing KE regimes through the hydrostatic pressure adjustment and vertical mixing mechanisms.
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  • 17
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    Evidence for SST-Forced Anomalous Winds Revealed from Simultaneous Radiosonde Launches from Three Ships across the Kuroshio Extension Front (2016)
    American Meteorological Society
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    Publication Date: 2016-10-01
    Description: Simultaneous launches of radiosondes were conducted from three research vessels aligned meridionally across a sea surface temperature (SST) front on the flank of the Kuroshio Extension. The soundings carried out every 2 h over 5 days in early July 2012 provided a unique opportunity in capturing unambiguous data on anomalous easterly winds derived from a pronounced meridional SST gradient. The data indicate that a meridional contrast in surface heat fluxes from the underlying ocean enhanced the air temperature anomaly across the SST front, which was observed from the surface up to 300-m altitude. Correspondingly, high and low pressure anomalies that reached 800-m altitude formed on the north and south sides of the SST front, respectively. These temperature and pressure anomalies were maintained even during the passage of synoptic-scale disturbances. Although the free-tropospheric winds are overall westerly, winds below the 1000-m level were easterly due to geostrophic anomalies driven by the northward pressure gradient near the surface. During periods of the northerlies at the surface, especially over the warmer side of the SST front, the wind direction changed in a clockwise direction from 1500 m to the surface, in the opposite sense to the Ekman spiral. The vertical wind shear is apparently in the thermal wind balance ascribed to the meridional contrast in air temperature derived from the SST anomaly.
    Print ISSN: 0027-0644
    Electronic ISSN: 1520-0493
    Topics: Geography , Geosciences , Physics
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  • 18
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    Error and Energy Budget Analysis of a Nonhydrostatic Stretched-Grid Global Atmospheric Model (2016)
    American Meteorological Society
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    Publication Date: 2016-03-29
    Description: A nonhydrostatic stretched-grid (SG) model is used to analyze the large-scale errors generated by stretching horizontal grids and their influence on a region of interest. Simulations by a fully compressible, nonhydrostatic global atmospheric model, the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), and its SG regional model, stretched-NICAM, were performed for the months of March, April, and May of 2011 using various resolutions and stretching factors. A comparison of week-long accumulative precipitation amounts between the Tropical Rainfall Measuring Mission (TRMM) satellite data and the quasi-uniform and SG simulations showed that a stretched run better represents storms and associated precipitation because the errors generated in the outer regions with coarser grid spacing do not significantly affect the inner domain centered at the focal point. For season-long simulations, in one particular set of stretched runs with the focal point located in the eastern United States, the artificial suppression of baroclinic development of midlatitude eddies in the Southern Hemisphere weakened the eddy-driven polar-front jet (PFJ), which yielded a cold bias at mid- to high latitudes. However, in the Northern Hemisphere, in contrast, the aforementioned changes are less apparent. Therefore, for the SG runs, the mean temperature was maintained at the region of interest, and an increased amount of moderate to heavy precipitation, which is also frequently found in the TRMM data, was observed; thus, the benefits of increased resolution were realized. However, careful attention must be given when applying the SG model because a regional climate response to the change in the large-scale circulations may not be fully accounted for.
    Print ISSN: 0027-0644
    Electronic ISSN: 1520-0493
    Topics: Geography , Geosciences , Physics
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  • 19
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    Mechanisms for the Maintenance of the Wintertime Basin-Scale Atmospheric Response to Decadal SST Variability in the North Pacific Subarctic Frontal Zone (2017)
    American Meteorological Society
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    Publication Date: 2017-12-15
    Description: 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.
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    Topics: Geography , Geosciences , Physics
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  • 20
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    On the Response of the Aleutian Low to Greenhouse Warming (2017)
    American Meteorological Society
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    Publication Date: 2017-05-02
    Description: Past and future changes in the Aleutian low are investigated by using observation-based sea level pressure (SLP) datasets and CMIP5 models. It is found that the Aleutian low intensity, measured by the North Pacific Index (NPI), has significantly strengthened during the twentieth century, with the observed centennial trend double the modeled counterpart for the multimodel average of historical simulations, suggesting compound signals of anthropogenic warming and natural variability. As climate warms under the strongest future warming scenario, the climatological-mean Aleutian low will continue to intensify and expand northward, as manifested in the significant decrease (−1.3 hPa) of the multimodel-averaged NPI, which is 1.6 times its unforced internal variability, and the increase in the central area of low pressure (SLP 〈 999.0 hPa), which expands about 7 times that in the twentieth century. A suite of idealized experiments further demonstrates that the deepening of the Aleutian low can be driven by an El Niño–like warming of the tropical Pacific sea surface temperature (SST), with a reduction in the climatological-mean zonal SST gradient, which overshadows the dampening effect of a weakened wintertime land–ocean thermal contrast on the Aleutian low change in a warmer climate. While the projected deepening of Aleutian low on multimodel average is robust, individual model portrayals vary primarily in magnitude. Intermodel difference in surface warming amplitude over the Asian continent, which is found to explain about 31% of the variance of the NPI changes across models, has a greater contribution than that in the spatial pattern of tropical Pacific SST warming (which explains about 23%) to model uncertainty in the projection of Aleutian low intensity.
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    Topics: Geography , Geosciences , Physics
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