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Ran Zhang, Jiabei Fang, and Xiu-Qun Yang


The basin-scale subtropical oceanic front zone (STFZ) is a key region for midlatitude air–sea interaction in the North Pacific. However, previous studies considered midlatitude sea surface temperature (SST) variabilities as a response to atmospheric stochastic forcing. With reanalysis and observational data, this study investigates what kinds of atmospheric anomalies drive the wintertime North Pacific STFZ intensity variation. Lead correlations show that prior to the STFZ’s enhancement, there exist persistent atmospheric anomalies characterized by a negative-phase Arctic Oscillation (AO) and a positive-phase Pacific–North American (PNA) pattern, lasting for up to 80 and 50 days and peaking at 20- and 8-day leads, respectively. It is further found that the long-lasting negative-phase AO is conducive to stronger low-tropospheric baroclinicity at around 40°N over North Pacific where there is a climatological baroclinic region. The stronger baroclinicity leads to more synoptic transient eddy activities, promoting an equivalent barotropic low geopotential height anomaly north of STFZ via transient eddy vorticity forcing. The geopotential height anomaly propagates downstream, triggering a PNA-like pattern. With such an AO-promoted atmospheric internal wave–flow feedback, the regional PNA pattern is intensified and embedded in the annular AO mode, accompanied with an intensified Aleutian low and surface westerly wind that peak at an 8-day lead, preconditioning a persistent (nonstochastic) atmospheric forcing on the STFZ. The intensified surface westerly predominantly tends to drive a southward Ekman transport and increase upward surface turbulent heat fluxes into the atmosphere through increasing surface wind speed and sea–air temperature difference, amplifying the underlying negative SST anomaly and cross-frontal meridional SST gradient, ultimately intensifying the STFZ.

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Jia-Bei Fang and Xiu-Qun Yang


Following Goodman and Marshall (hereinafter GM), an improved intermediate midlatitude coupled ocean–atmosphere model linearized around a basic state is developed. The model assumes a two-layer quasigeostrophic atmosphere overlying a quasigeostrophic upper ocean that consists of a constant-depth mixed layer, a thin entrainment layer, and a thermocline layer. The SST evolution is determined within the mixed layer by advection, entrainment, and air–sea flux. The atmospheric heating is specified at midlevel, which is parameterized in terms of both the SST and atmospheric temperature anomalies. With this coupled model, the dynamical features of unstable ocean–atmosphere interactions in the midlatitudes are investigated. The coupled model exhibits two types of coupled modes: the coupled oceanic Rossby wave mode and the SST-only mode. The SST-only mode decays over the entire range of wavelengths, whereas the coupled oceanic Rossby wave mode destabilizes over a certain range of wavelengths (∼10 500 km) when the atmospheric response to the heating is equivalent barotropic. The relative roles of different physical processes in destabilizing the coupled oceanic Rossby wave are examined. The main processes emphasized are the influence of entrainment and advection for determining SST evolution, and the atmospheric heating profile. Although either entrainment or advection can lead to unstable growth of the coupled oceanic Rossby wave with similar wavelength dependence for each case, the advection process is found to play the more important role, which differs from GM’s results in which the entrainment process is dominant. The structure of the unstable coupled mode is sensitive to the atmospheric heating profile. The inclusion of surface heating largely reduces the growth rate and stabilizes the coupled oceanic Rossby wave. In comparison with observations, it is demonstrated that the structure of the growing coupled mode derived from the authors’ model is closer to reality than that from the previous model.

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Lingfeng Tao, Xiu-Qun Yang, Jiabei Fang, and Xuguang Sun


Observed wintertime atmospheric anomalies over the central North Pacific associated with the Pacific decadal oscillation (PDO) are characterized by a cold/trough (warm/ridge) structure, that is, an anomalous equivalent barotropic low (high) over a negative (positive) sea surface temperature (SST) anomaly. While the midlatitude atmosphere has its own strong internal variabilities, to what degree local SST anomalies can affect the midlatitude atmospheric variability remains unclear. To identify such an impact, three atmospheric general circulation model experiments each having a 63-yr-long simulation are conducted. The control run forced by observed global SST reproduces well the observed PDO-related cold/trough (warm/ridge) structure. However, the removal of the midlatitude North Pacific SST variabilities in the first sensitivity run reduces the atmospheric response by roughly one-third. In the second sensitivity run in which large-scale North Pacific SST variabilities are mostly kept, but their frontal-scale meridional gradients are sharply smoothed, simulated PDO-related cold/trough (warm/ridge) anomalies are also reduced by nearly one-third. Dynamical diagnoses exhibit that such a reduction is primarily due to the weakened transient eddy activities that are induced by weakened meridional SST gradient anomalies, in which the transient eddy vorticity forcing plays a crucial role. Therefore, it is suggested that midlatitude North Pacific SST anomalies make a considerable (approximately one-third) contribution to the observed PDO-related cold/trough (warm/ridge) anomalies in which the frontal-scale meridional SST gradient (oceanic front) is a key player, although most of those atmospheric anomalies are determined by the SST variabilities outside of the midlatitude North Pacific.

Open access
Tianyi Wang, Xiu-Qun Yang, Jiabei Fang, Xuguang Sun, and Xuejuan Ren


This study investigates the role of air–sea interaction in the 30–60-day boreal summer intraseasonal oscillation (BSISO) over the western North Pacific with daily outgoing longwave radiation (OLR), CFSR, and OAFlux datasets for 1985–2009. The BSISO events are identified with the first principal component of 30–60-day bandpass filtered OLR anomalies. Composite analysis of these events reveals that during the northward migration of BSISO, the convection can interact with underlying sea surface temperature (SST). A near-quadrature phase relationship exists between the convection and SST anomalies. An active (a suppressed) convection tends to induce a cold (warm) underlying SST anomaly by reducing (increasing) downward solar radiation but a warm SST anomaly in its northern (southern) portion by reducing near-surface wind and upward latent and sensible heat fluxes, resulting in a 10-day delayed maximized warm SST anomaly ahead of the active convection. In turn, this warm SST anomaly tends to increase upward surface sensible and latent heat fluxes via amplifying sea–air temperature and humidity differences. This oceanic feedback acts to heat, moisten, and destabilize the low-level atmosphere, favoring the trigger of shallow convection, which can further develop into deep convection. The maximum warm SST anomaly lies in the southern (northern) portion of the convectively suppressed (enhanced) area, which weakens the anomalous descending motion in the southern portion of convectively suppressed area and preconditions the boundary layer to promote convection development in the northern portion of convectively enhanced area. Such a spatial and temporal phase relationship between the convection and SST anomalies suggest that air–sea interaction can play a delayed negative feedback role in the BSISO cycle and provide an alternative mechanism responsible for its northward propagation.

Open access