An active role of extratropical sea surface temperature anomalies in determining anomalous turbulent heat flux (original) (raw)
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Atmospheric Response to Sea Surface Temperature Anomalies Over the Equatorial Pacific Ocean
한국기상학회 학술대회 논문집, 1995
Atmospheric response to mid-latitude sea surface temperature (SST) anomalies is a long-standing and perplexing problem. There have been extensive studies on the issue of atmospheric response to mid-latitude SST anomalies from observational, theoretical, and modelling perspectives. This paper serves as a brief review focusing on large-scale SST anomalies. Here, convincing new observational evidence and modelling results are presented, and the process of noticing the importance of storm track and oceanic fronts is introduced. It has now been established that the atmospheric response to mid-latitude SST anomalies is largely controlled by the response of the storm track and that among the components of a mid-latitude SST anomaly, the disturbance to oceanic fronts plays a crucial role in simulating a significant storm track response. RÉSUMÉ [Traduit par la rédaction] La réaction de l'atmosphère aux anomalies de la température de surface de la mer (SST) des latitudes moyennes est depuis longtemps un problème préoccupant. Il existe des études approfondies portant sur la réaction de l'atmosphère aux anomalies de SST des latitudes moyennes. Elles se fondent sur des observations, la théorie et la modélisation. Nous présentons dans cet article une brève revue portant sur les anomalies de SST à grande échelle. Nous mentionnons de nouveaux résultats convaincants issus d'observations et de modélisation, et attirons aussi l'attention sur le processus qui consiste à remarquer l'importance de la trajectoire des systèmes et des fronts sur l'océan. Il est maintenant établi que la réaction de l'atmosphère aux anomalies de SST des latitudes moyennes est largement régie par la réaction de la trajectoire de la tempête et que, parmi les composantes d'une anomalie de SST des latitudes moyennes, la perturbation des fronts sur l'océan joue un rôle crucial dans la simulation d'une réaction évidente de la trajectoire du système.
Journal of Climate, 2014
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 ano...
Journal of Geophysical Research: Atmospheres, 2014
The East/Japan Sea (EJS) is a semi-enclosed marginal sea located in the upstream of the North Pacific storm track, where the leading modes of wintertime interannual variability in sea surface temperature (SST) are characterized by the basin-wide warming-cooling and the northeast-southwest dipole. Processes leading to local and remote atmospheric responses to these SST anomalies are investigated using the Weather Research and Forecast (WRF) model. The atmosphere in direct contact with anomalous diabatic forcing exhibits a linear and symmetric response with respect to the sign, pattern, and magnitude of SST anomalies, producing increased (decreased) wind speed and precipitation response over warm (cold) SSTs. This local response is due to modulation of both the vertical stability of the marine atmospheric boundary layer and the adjustment of sea level pressure, although the latter provides a better explanation of the quadrature relationship between SST and wind speed. The linearity in the local response suggests the importance of fine-scale EJS SSTs to predictability of the regional weather and climate variability. The remote circulation response, in contrast, is strongly nonlinear. An intraseasonal equivalent barotropic ridge emerges in the Gulf of Alaska as a common remote response independent of EJS SST anomalies. This downstream blocking response is reinforced by the enhanced storm track variability east of Japan via transient eddy vorticity flux convergence. Strong nonlinearity in remote response implies that detailed EJS SST patterns may not be critical to this downstream ridge response. Overall, results demonstrate a remarkably far-reaching impact of the EJS SSTs on the atmospheric circulation.
We examine processes that influence North Pacific sea surface temperature (SST) anomalies including surface heat fluxes, upper ocean mixing, thermocline variability, ocean currents, and tropical-extratropical interactions via the atmosphere and ocean. The ocean integrates rapidly varying atmospheric heat flux and wind forcing, and thus a stochastic model of the climate system, where white noise forcing produces a red spectrum, appears to provide a baseline for SST variability even on decadal time scales. However, additional processes influence Pacific climate variability including the "reemergence mechanism," where seasonal variability in mixed layer depth allows surface temperature anomalies to be stored at depth during summer and return to the surface in the following winter. Wind stress curl anomalies in the central/east Pacific drive thermocline variability that propagates to the west Pacific via baroclinic Rossby waves and influences SST by vertical mixing and the change in strength and position of the ocean gyres. Atmospheric changes associated with the El Niño-Southern Oscillation (ENSO) also influence North Pacific SST anomalies via the "atmospheric bridge." The dominant pattern of North Pacific SST anomalies, the Pacific Decadal Oscillation (PDO), exhibits variability on interannual as well as decadal time scales. Unlike ENSO, the PDO does not appear to be a mode of the climate system, but rather it results from several different mechanisms including (1) stochastic heat flux forcing associated with random fluctuations in the Aleutian Low, (2) the atmospheric bridge augmented by the reemergence mechanism, and (3) wind-driven changes in the North Pacific gyres.
Modeling North Pacific SST anomalies as a response to anomalous atmospheric forcing
J. Mar. Syst., 1: 155-168, 1990
"Large-scale sea surface temperature anomalies (SSTA) in the North Pacific ocean are often persistent for several months during wintertime. There is observational evidence that these patterns are forced by anomalous atmospheric circulation. Since the latter is in part related to the tropical El Nino/Southern Oscillation (ENSO) phenomenon it is hypothesized that part of the North Pacific SSTA's may be interpreted as remote oceanic response to anomalous equatorial Pacific SSTA's. Two experiments with a multi-level primitive equation model of the North Pacific have been conducted to study the influence of such anomalous atmospheric circulation on the SST. In both experiments anomalous wind stress as derived from the 1950-1979 COADS subset is specified as anomalous forcing. In experiment 1 no anomalous heat flux is introduced whereas in experiment 2 anomalous heat fluxes are estimated from anomalous surface winds and a simple advective atmosphere. In both experiments the GCM SSTA response are able to reproduce the main features of the time series of observed SSTA, in particular in winter. In experiment 1, however, the magnitudes are systematically too low. The addition of anomalous heat fluxes in experiment 2 significantly improves the simulation. The ENSO signal is clearly present in both simulations."
Sea-Surface Temperature Anomaly Generation in Relation to Atmospheric Storms
Bulletin of the American Meteorological Society, 1978
An extended period of reduced surface heat and momentum fluxes due to the absence of atmospheric storms may result in upper-ocean temperature anomalies that persist for months. The predominance of either anomalously high or low temperatures is related to the ocean thermal structure that is established on the transition date between the winter and summer regimes.
Quarterly Journal of the Royal Meteorological Society, 2010
† Lamont Doherty Earth Observatory Contribution Number X. El Niño-Southern Oscillation (ENSO) related precipitation anomalies in North America are related to changes in the paths of storm systems across the Pacific Ocean, with a more southern route into southwestern North America during El Niños and a more northern route into the Pacific Northwest during La Niñas. Daily reanalysis data are analyzed to confirm these changes. Seasonal mean upper tropospheric eddy statistics show, for El Niños (La Niñas), a pattern that is shifted southward (northward) compared with climatology. Paths of coherent phase propagation of transient eddies and of the propagation of wave packets are analyzed. A coherent path of propagation across the Pacific towards North America is identified that is more zonal during El Niño winters and, during La Niñas, has a dominant path heading northeastward to the Pacific Northwest. A second path heading southeastward from the central Pacific to the tropical east Pacific is more accentuated during La Niñas than El Niños. These changes in wave propagation are reproduced in an ensemble of seasonal integrations of a general circulation model forced by a tropical Pacific sea-surface temperature pattern, confirming that the changes are forced by changes in the mean atmospheric state arising from changes in tropical sea-surface temperature. A simplified model with a specified basic state is used to model the storm tracks for El Niño and La Niña winters. The results suggest that the changes in transient eddy propagation and the eddy statistics can be understood in terms of the refraction of transient eddies within different basic states.
Journal of Geophysical Research, 1995
We describe a heat anomaly transport in the upper ocean mixed layer in the Kuroshio extension region and the subtropical gyre of the northwest Pacific. Emphasis is on behavior in the cool season (December-March) during the Asian Winter Monsoon. The heat anomaly transport is estimated by applying an inversion technique to the stochastic partial differential equation for the heat anomaly balance of advection, diffusion, stabilizing feedback, and atmospheric forcing. The inversion consists of (1) derivation of statistical parametric model from the heat anomaly balance equation; (2) fitting the derived statistical model to the sea surface temperature (SST) anomaly covariances; and (3) calculation of the heat anomaly net advection velocity, horizontal diffusion coefficient, feedback factor and atmospheric forcing correlation from the parameters of the evaluated statistical model. The inversion was applied to the Comprehensive Ocean-Atmosphere Data Set Compressed Marine Reports SST dam, averaged at 1 ø latitude x 2' longitude boxes on a 10-day mean basis from 1965 to 1990. The estimates of the net advection velocity are consistent in magnitude and direction with the general circulation in the surface layer of the Northwest Pacific in winter. SST anomalies are transported to the west at -0.15 m s -• in the northern part of the North Equatorial Current. Between 21 ø and 29øN in the recirculating region, SST anomalies propagate westward with the mean velocity less than 0.1 m s '•. South and east of Honshu the observed pattern of the SST anomaly transport agrees broadly with the circulations of the Kuroshio current and its extension and the Oyashio current. South of Honshu, the eastward transport is about 200-300 km wide; its absolute velocity is up to 0.2 m s -•. One branch of the transport separates from the coast near the large meander path of the Kuroshio current and follows the east-southeast direction. The second separation from the coast occurs south of Hokkaido. Over the analysis domain the estimates of the diffusion coefficient are in the range of 3x 103 to 6x 103 m 2 s -•. The higher values of the diffusion coefficient confirm the enhancement of the mesoscale eddy processes near the subtropical convergence zone. The analysis supports Hasselmann's (1976) theory in which generation of midlatitude SST anomalies lasting the dominant timescale of atmospheric processes is primarily attributed to the short period stochastic weather forcing. However, the analysis indicates that the inertia of SST anomalies to their "memory" of earlier winds can not be neglected in the vicinity of the western boundary and in the tropics. 1. Introduction Since the late 1950s, sea surface temperature (SST) anomalies have been regarded as one of the key elements of climate variations [Bjerknes, 1959; Namias, 1959]. During the past three decades many publications have described the generation and evolution of SST anomalies (c.f. review of Frankignoul [1985]). They considered a heat budget of the top layer of the ocean, atmospheric forcing of the sea, feedbacks, and multiple timescale interactions in the coupled oceanatmosphere system. The concept of the uniform mixed layer [Kraus and Turner, 1967, Niiler and Kraus, 1977] played a major role in the formulation of a model for the upper sea heat anomaly balance. Statistical studies of the global SST Copyright 1995 by the American Geophysical Union. Paper number 94JC03041. 0148-0227/95/94JC-03041 $05.00 anomaly variability in terms of empirical orthogonal functions were originated by Davis [1976]. Numerical simulations of SST anomalies with ocean general circulation models were initiated by Haney et al. [1978]. At the beginning of the 1980s it was generally accepted that the SST anomaly behavior differs between the tropics and midlatitudes. The tropical SST anomalies were suggested to be generated by the large-scale ocean-atmosphere feedback processes [see Philander, 1990]. White et al. [1985] and Pazan et al. [1986] examined heat content redistribution in the tropical western Pacific during E1 Nifio-Southern Oscillation events. The heat content redistribution was shown to be associated with wind-driven baroclinic Rossby and Kelvin wave activity. Recent numerical experiments indicate that there are at least two classes of ocean-atmosphere modes in tropics. In the first class of modes, SST and surface wind variations can be in phase, but other oceanic parameters, for example, thermocline depth variations, have a phase lag that represents the inertia of the ocean and its "memory" of earlier 4845 4846 OSTROVSKII AND PITERBARG: HEAT ANOMALY TRANSPORT IN NW PACIFIC winds [Philander et al., 1992]. In numerical models that capture this class of modes the ocean response to the wind is of the "delayed oscillator" type, and the simulated Southern Oscillation can be made irregular by introducing highfrequency modes such as atmospheric "weather" forcing. The second class of ocean-atmosphere modes in the tropics is characterized by phase differences between SST and surface wind fluctuations [Lau et al., 1992].
Pacific interdecadal variability driven by tropical–extratropical interactions
Climate Dynamics, 2013
Interactions between the tropical and subtropical northern Pacific at decadal time scales are examined using uncoupled oceanic and atmospheric simulations. An atmospheric model is forced with observed Pacific sea surface temperatures (SST) decadal anomalies, computed as the difference between the 2000-2009 and the 1990-1999 period. The resulting pattern has negative SST anomalies at the equator, with a global pattern reminiscent of the Pacific decadal oscillation. The tropical SST anomalies are responsible for driving a weakening of the Hadley cell and atmospheric meridional heat transport. The atmosphere is then shown to produce a significant response in the subtropics, with wind-stress-curl anomalies having the opposite sign from the climatological mean, consistent with a weakening of the oceanic subtropical gyre (STG). A global ocean model is then forced with the decadal anomalies from the atmospheric model. In the North Pacific, the shallow subtropical cell (STC) spins down and the meridional heat transport is reduced, resulting in positive tropical SST anomalies. The final tropical response is reached after the first 10 years of the experiment, consistent with the Rossby-wave adjustment time for both the STG and the STC. The STC provides the connection between subtropical wind stress anomalies and tropical SSTs. In fact, targeted simulations show the importance of off-equatorial wind stress anomalies in driving the oceanic