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Larry W. O’Neill, Tracy Haack, and Theodore Durland

Abstract

Two methods of computing the time-mean divergence and vorticity from satellite vector winds in rain-free (RF) and all-weather (AW) conditions are investigated. Consequences of removing rain-contaminated winds on the mean divergence and vorticity depend strongly on the order in which the time-average and spatial derivative operations are applied. Taking derivatives first and averages second (DFAS_RF) incorporates only those RF winds measured at the same time into the spatial derivatives. While preferable mathematically, this produces mean fields biased relative to their AW counterparts because of the exclusion of convergence and cyclonic vorticity often associated with rain. Conversely, taking averages first and derivatives second (AFDS_RF) incorporates all RF winds into the time-mean spatial derivatives, even those not measured coincidentally. While questionable, the AFDS_RF divergence and vorticity surprisingly appears qualitatively consistent with the AW means, despite using only RF winds. The analysis addresses whether the AFDS_RF method accurately estimates the AW mean divergence and vorticity.

Model simulations indicate that the critical distinction between these two methods is the inclusion of typically convergent and cyclonic winds bordering rain patches in the AFDS_RF method. While this additional information removes some of the sampling bias in the DFAS_RF method, it is shown that the AFDS_RF method nonetheless provides only marginal estimates of the mean AW divergence and vorticity given sufficient time averaging and spatial smoothing. Use of the AFDS_RF method is thus not recommended.

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Masayo Ogi, Bunmei Taguchi, Meiji Honda, David G. Barber, and Søren Rysgaard

Abstract

Contemporary climate science seeks to understand the rate and magnitude of a warming global climate and how it impacts regional variability and teleconnections. One of the key drivers of regional climate is the observed reduction in end of summer sea-ice extent over the Arctic. Here the authors show that interannual variations between the September Arctic sea-ice concentration, especially in the East Siberian Sea, and the maximum Okhotsk sea-ice extent in the following winter are positively correlated, which is not explained by the recent warming trend only. An increase of sea ice both in the East Siberian Sea and the Okhotsk Sea and corresponding atmospheric patterns, showing a seesaw between positive anomalies of sea level pressures over the Arctic Ocean and negative anomalies over the midlatitudes, are related to cold anomalies over the high-latitude Eurasian continent. The patterns of atmospheric circulation and air temperatures are similar to those of the annually integrated Arctic Oscillation (AO). The negative annual AO forms colder anomalies in autumn sea surface temperatures both over the East Siberian Sea and the Okhotsk Sea, which causes heavy sea-ice conditions in both seas through season-to-season persistence.

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Xiaohui Ma, Ping Chang, R. Saravanan, Dexing Wu, Xiaopei Lin, Lixin Wu, and Xiuquan Wan

Abstract

Boreal winter (November–March) extreme flux events in the Kuroshio Extension region (KER) of the northwestern Pacific and the Gulf Stream region (GSR) of the northwestern Atlantic are analyzed and compared, based on NCEP Climate Forecast System Reanalysis (CFSR), NCEP–NCAR reanalysis, and NOAA Twentieth Century Reanalysis data, as well as the observationally derived OAFlux dataset. These extreme flux events, most of which last less than 3 days, are characterized by cold air outbreaks (CAOs) with an anomalous northerly wind that brings cold and dry air from the Eurasian and North American continents to the KER and GSR, respectively. A close relationship between the extreme flux events over KER (GSR) and the Aleutian low pattern (ALP) [east Atlantic pattern (EAP)] is found with more frequent occurrence of the extreme flux events during a positive ALP (EAP) phase and vice versa. A further lag-composite analysis suggests that the ALP (EAP) is associated with accumulated effects of the synoptic winter storms accompanied by the extreme flux events and shows that the event-day storms tend to have a preferred southeastward propagation path over the North Pacific (Atlantic), potentially contributing to the southward shift of the storm track over the eastern North Pacific (Atlantic) basin during the ALP (EAP) positive phase. Finally, lag-regression analyses indicate a potential positive influence of sea surface temperature (SST) anomalies along the KER (GSR) on the development of the extreme flux events in the North Pacific (Atlantic).

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Ryusuke Masunaga, Hisashi Nakamura, Takafumi Miyasaka, Kazuaki Nishii, and Youichi Tanimoto

Abstract

Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as climatological imprints of oceanic fronts within the Kuroshio–Oyashio Extension (KOE) region east of Japan are investigated by taking advantage of high horizontal resolution of the ERA-Interim global atmospheric reanalysis data, for which the resolution of sea surface temperature (SST) data has been improved. These imprints, including locally enhanced sensible and latent heat fluxes and local maxima in cloudiness and precipitation in association with locally strengthened surface-wind convergence in the vicinities of SST fronts along the warm Kuroshio Extension and cool Oyashio to its north, are also identified in high-resolution satellite data. In addition to these mesoscale MABL features, meridionally confined near-surface baroclinic zones and zonally oriented sea level pressure (SLP) minima associated with the dual SST fronts are represented in ERA-Interim only in the period of high-resolution SST, but those imprints of the Oyashio front are missing in the low-resolution SST period. In the presence of the prevailing monsoonal northerlies, latitudinal displacements of the SLP trough, baroclinic zone, and the peak meridional gradient of the turbulent heat fluxes from each of the corresponding SST fronts are also found to be sensitive to the frontal width that depends on the SST resolution. The analysis herein suggests that the converging surface northerlies into the SLP minima can contribute positively to the formation of a surface baroclinic zone along the Kuroshio Extension, while a stronger baroclinic zone along the Oyashio front is maintained primarily through the pronounced cross-frontal contrast in sensible heat release from the ocean.

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Dimitry Smirnov, Matthew Newman, Michael A. Alexander, Young-Oh Kwon, and Claude Frankignoul

Abstract

The local atmospheric response to a realistic shift of the Oyashio Extension SST front in the western North Pacific is analyzed using a high-resolution (HR; 0.25°) version of the global Community Atmosphere Model, version 5 (CAM5). A northward shift in the SST front causes an atmospheric response consisting of a weak surface wind anomaly but a strong vertical circulation extending throughout the troposphere. In the lower troposphere, most of the SST anomaly–induced diabatic heating is balanced by poleward transient eddy heat and moisture fluxes. Collectively, this response differs from the circulation suggested by linear dynamics, where extratropical SST forcing produces shallow anomalous heating balanced by strong equatorward cold air advection driven by an anomalous, stationary surface low to the east. This latter response, however, is obtained by repeating the same experiment except using a relatively low-resolution (LR; 1°) version of CAM5. Comparison to observations suggests that the HR response is closer to nature than the LR response. Strikingly, HR and LR experiments have almost identical vertical profiles of . However, diagnosis of the diabatic quasigeostrophic vertical pressure velocity (ω) budget reveals that HR has a substantially stronger response, which together with upper-level mean differential thermal advection balances stronger vertical motion. The results herein suggest that changes in transient eddy heat and moisture fluxes are critical to the overall local atmospheric response to Oyashio Front anomalies, which may consequently yield a stronger downstream response. These changes may require the high resolution to be fully reproduced, warranting further experiments of this type with other high-resolution atmosphere-only and fully coupled GCMs.

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Kohei Takatama, Shoshiro Minobe, Masaru Inatsu, and R. Justin Small

Abstract

The mechanisms acting on near-surface winds over the Gulf Stream are diagnosed using 5-yr outputs of a regional atmospheric model. The diagnostics for the surface-layer momentum vector, its curl, and its convergence are developed with a clear separation of pressure adjustment from downward momentum inputs from aloft in the surface-layer system. The results suggest that the downward momentum mixing mechanism plays a dominant role in contributing to the annual-mean climatological momentum curl, whereas the pressure adjustment mechanism plays a minor role. In contrast, the wind convergence is mainly due to the pressure adjustment mechanism. This can be explained by the orientation of background wind to the sea surface temperature front. The diagnostics also explain the relatively strong seasonal variation in surface-layer momentum convergence and the small seasonal variation in curl. Finally, the surface-layer response to other western boundary currents is examined using a reanalysis dataset.

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Shusaku Sugimoto

Abstract

Variations in the turbulent heat flux (THF; the sum of the sensible and latent heat fluxes) in the eastern Kuroshio–Oyashio confluence region (EKOCR; 36°–40°N, 155°–160°E) were investigated over a period of 27 consecutive winters (December–February) from 1985/86 to 2011/12. The THF was calculated from a bulk formula using daily variables [surface wind speed, surface air specific humidity, surface air temperature, and sea surface temperature (SST)] of the objectively analyzed air–sea fluxes (OAFlux) dataset and bulk coefficients based on the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm 3.0. The winter THF over the EKOCR showed low-frequency variations, with larger THF values in the early 2000s and smaller values in the late 1990s and late 2000s. The heat release in the early 2000s was up to ~40% greater than that in the late 1990s and late 2000s. By performing experiments using combinations of daily raw data values and daily climatological data, the relative contributions of SST, surface air specific humidity, surface air temperature, and surface wind speed were quantitatively assessed in determining the THF over the EKOCR. Results showed that SST predominantly determines the THF: large amounts of heat are released during times of positive SST anomalies. By using Argo float (temperature–salinity) profiles of 2003–12 and a satellite altimetry dataset of 1992–2012, it was found that the warm–salty water transported by an occurrence of the Kuroshio bifurcation was responsible for the generation of positive SST anomalies in the EKOCR.

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Bunmei Taguchi and Niklas Schneider

Abstract

Upper ocean heat content (OHC) is at the heart of natural climate variability on interannual-to-decadal time scales, providing climate memory and the source of decadal prediction skill. In the midlatitude North Pacific Ocean, OHC signals are often found to propagate eastward as opposed to the frequently observed westward propagation of sea surface height, another variable that represents the ocean subsurface state. This dichotomy is investigated using a 150-yr coupled GCM integration. Simulated OHC signals are distinguished in terms of two processes that can support eastward propagation: higher baroclinic Rossby wave (RW) modes that are associated with density perturbation, and spiciness anomalies due to density-compensated temperature and salinity anomalies. The analysis herein suggests a unique role of the Kuroshio–Oyashio Extension (KOE) region as an origin of the spiciness and higher mode RW signals. Wind-forced, westward-propagating equivalent barotropic RWs cause meridional shifts of the subarctic front in the KOE region. The associated anomalous circulation crosses mean temperature and salinity gradients and thereby generates spiciness anomalies. These anomalies are advected eastward by the mean currents, while the associated surface temperature anomalies are damped by air–sea heat exchange. The accompanying surface buoyancy flux generates higher baroclinic, eastward-propagating RWs. The results suggest that the large OHC variability in the western boundary currents and their extensions is associated with the spiciness gradients and axial variability of oceanic fronts.

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Atsuhiko Isobe, Shin’ichiro Kako, and Shinsuke Iwasaki

Abstract

Atmospheric responses to biological heating caused by the spring phytoplankton bloom in the Sea of Japan are investigated. Sea surface temperature (SST) is first computed using a mixed-layer model with an ocean reanalysis product. Satellite-derived surface chlorophyll concentrations representing phytoplankton population are input to an equation for attenuation coefficients of solar radiation penetrating the mixed layer. Two sets of SST are obtained by this model, using the attenuation coefficients with and without phytoplankton. It is found that the phytoplankton bloom increases SST by up to 0.8°C by mid-May, especially in the northern Sea of Japan. Thereafter, two experiments using a regional atmospheric numerical model are conducted for April and May. One imposes SST synthesized by multiple satellite observations on the lower boundary of the model (the green case). The satellite-derived SST includes influences of biological heating by phytoplankton in the actual ocean. The other uses SST reduced by differences between SSTs computed by the mixed-layer model with and without phytoplankton (the blue case). Under modest wind conditions, extratropical cyclones east and south of the Japan Islands in the blue case develop more rapidly than in the green case. Cyclones are likely initiated by the cool and dry air mass that enhances lower-level baroclinicity above oceanic fronts. This cool and dry air mass is transported from the Sea of Japan, where SST decreases in the absence of phytoplankton. Therefore, incorporating ocean biology is potentially capable of improving regional atmospheric and ocean general circulation models.

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Satoru Okajima, Hisashi Nakamura, Kazuaki Nishii, Takafumi Miyasaka, and Akira Kuwano-Yoshida

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.

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