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In-Sik Kang

Abstract

Long-term variations of global-mean air temperature at the surface are investigated using data from a GFDL coupled ocean-atmosphere model 139-year simulation and the observed datasets of surface air temperature over the globe for 139 years and tropical Pacific SST for 42 years. The global-mean surface air temperatures of both the model and observation are characterized by interdecadal oscillations with timescales of about 20 years and 10 years and interannual fluctuations with timescales of 3–6 years. The global-mean temperature anomaly (GMTA) is related to the first empirical orthogonal function mode of surface air temperature over the globe, which, in turn, is associated with the first eigenmode of SST over the tropical Pacific. The first eigenmode of tropical Pacific SST has a spatial pattern similar to that during a mature phase of El Niño, and its associated time series is characterized by the 3–6-yr interannual and interdecadal timescales. It is demonstrated that both interannual and interdecadal variations of the GMTA are associated with the corresponding variations of tropical Pacific SST. Comparisons of the model results with observed counterparts show that the coupled model has a capability of simulating many important features of surface air temperature variations over the globe and SST fluctuations over the tropical Pacific, except that the interannual variations of the model with timescales of 3–6 years are relatively weak.

The sampling error of the observed GMTA for the 139 years 1854–1992 due to inhomogenous spatial distribution of the data over the globe is examined by using the coupled model data. Although the sampling error of monthly mean GMTA is not small even for recent years, the long-term variations of GMTA with timescales longer than few years are not much affected by the sampling problem.

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In-Sik Kang

Abstract

The fluctuation of stationary waves caused by zonal mean flow changes is investigated using a barotropic model and GCM simulated upper-level data. The EOF analysis of monthly mean ū fluctuations during winter shows that a positive anomaly at the jet stream accompanies a negative anomaly of zonal mean flow in the high latitudes. The typical magnitudes of the anomalous zonal mean flow are 5 and 8 m s−1 in the jet stream latitudes and in the high latitudes, respectively.

With these fluctuations of the basic state, the wave propagation characteristics are significantly modified from month to month. The zonal mean flow changes subject to fixed large-scale mountains result in a large part of the stationary wave fluctuations. It is also demonstrated that the precise location of the turning latitude plays an important role in the pattern of stationary wave responses to a fixed forcing.

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Lei Zhou and In-Sik Kang

Abstract

Influences of convective momentum transport (CMT) on tropical waves are analytically studied with an idealized model, which captures the first-order baroclinic structure in the vertical. The CMT has significant influence on the mixed Rossby–gravity (MRG) waves, especially over the Indo-Pacific warm pool. The westward-propagating MRG wave with a small wavenumber becomes unstable because of the CMT. The convergence and geopotential are no longer in a quadrature phase relation, which is different from the classical MRG wave. As a result, there is a net source of mechanical energy within one wave period and there is an upscale momentum transfer that can have impacts on slow variabilities in the tropics, such as the Madden–Julian oscillation. The unstable MRG wave mimics the temporal and spatial features of the observed 2-day waves in tropics. Within this framework, the asymmetric structure of the MRG waves and the 2-day waves with respect to the equator are well captured by both the idealized model and observations. In addition, the CMT is found to be critical for determining the meridional scale of tropical waves. The meridional scale in the two-layer model is wider than the Rossby radius of deformation RL over the Indo-Pacific warm pool, but narrower than RL from the central to the eastern Pacific Ocean and over the Atlantic Ocean. Such variation is consistent with observations.

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In-Sik Kang and Ngar-Cheung Lau

Abstract

Principal modes of low-frequency atmospheric variability and the influence of sea surface temperature anomalies on such modes are investigated by examining the output from two general circulation model experiments. In the first experiment (the “control run”), all boundary forcings were constrained to evolve through 15 identical annual cycles. In the second experiment (the “SST runs”), the sea surface temperature conditions in the tropical Pacific Basin were prescribed to follow the actual month-to-month changes observed during the period 1962–76, which encompasses several El Niño events. The analysis tools employed here include empirical orthogonal functions, teleconnections and composite charts.

Two prominent modes of variability have been identified in the wintertime Northern Hemisphere eddy streamfunction of the SST experiment. The first mode bears a strong spatial resemblance to the observed characteristic circulation pattern over the North Pacific–North American sector. It is demonstrated that this mode is highly correlated with the changing SST forcing imposed over the tropical Pacific, and with various meteorological phenomena accompanying El Niño-Southern Oscillation (ENSO). The second mode exhibits no significant correlation with the SST forcing, but is linked instead to a characteristic structure of the zonally averaged zonal wind. The circulation features related to this mode are reminiscent of observed anomalies accompanying the North Atlantic Oscillation, as well as variations of the “zonal index.” The fluctuations associated with the first and second modes in the SST experiment are of comparable amplitudes. It is further demonstrated that the essential characteristics of the first and second modes are highly reproducible in another 15-year simulation initiated from a completely independent set of atmospheric conditions.

A parallel diagnosis of the model behavior in the control run reveals no signal of the first mode (i.e., that related to FNSO) as previously described. However, circulation features accompanying the second mode are still discernible in the control experiment. It is noted that the predominant anomalous phenomena in the control run are manifestations of the zonal/eddy relationship associated with the second mode. The mode appearing in both the control and SST runs is likely related to internal dynamical processes in the model atmosphere subjected to a fixed boundary forcing.

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In-Sik Kang and K-M. Lau

Abstract

This paper provides a description of the variability of global atmospheric angular momentum (GAM) and its relationship with principal modes of three-dimensional atmospheric circulation anomalies. The data used are 5-day mean global wind fields from the European Centre for Medium-Range Weather Forecasts initialized dataset for 1980–1989. Significant seasonal variation of GAM is observed with maxima in April and November and a minimum during late July. The amplitude of the annual cycle is largest in the upper troposphere and decreases toward the surface. Although the lower tropospheric contribution to the total angular momentum is relatively small, its annual cycle is out of phase with those of the upper atmosphere and GAM. Also identified is a distinct semiannual component, with double peaks appearing in April and November. This signal is most noticeable in the upper troposphere above the 300-mb level.

The principal modes of zonal-mean angular momentum and meridional circulation anomalies and their coupled modes are obtained by using empirical orthogonal function analysis and singular value decomposition. It is shown that the leading modes of the angular momentum and meridional circulation are coupled with each other and are responsible for much of the variability in GAM. The coupled modes represent fluctuations of upper-level subtropical zonal flow, which are linked to the modulation of Hadley circulation intensity in both hemispheres. It is found that GAM is highly correlated with the first eigenvector of upper-level streamfunction anomalies, which consists of a superrotational flow in the tropics and subtropics, except over the central Pacific where a “blocked” flow with two subtropical anticyclonic circulation cells straddling the equator is found. Much of the blocked flow is due to the establishment of dipole anomalies in the velocity potential with centers over the central Pacific and the Maritime Continent on the interannual time scale. On the intraseasonal time scale, GAM fluctuation is dominated by superrotational flow in the tropics, with the blocked flow present to a much lesser extent. The associated velocity potential anomaly has a weak dipole structure with centers over the Indian Ocean and the eastern Pacific. The implications of the above results on the total angular momentum balance of the earth-atmosphere system are also discussed.

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In-Sik Kang and Isaac M. Held

Abstract

The upper tropospheric circulation during northern summer produced by a general circulation model (GCM) is studied using linear and nonlinear barotropic models and by analysing a streamfunction budget. The model experiments and the budget calculations both show a simple Sverdrup balance to he a useful first approximation for the largest scales during this season. In this Sverdrup balance, the advection of planetary vorticity by the divergent component of the flow is found to be significant, particularly in the Southern Hemisphere tropics.

Nonlinear barotropic models improve the simulation of regional structures. The correct position of the Tibetan high is explained by Sverdrup balance, but its amplitude and structure are reasonably well simulated only with the nonlinear models. With climatological forcing, the time-averaged solutions of the nonlinear model are insensitive to the strength of the damping included in the model. The difference between the GCM's climatology and the GCM's flow in a particular summer is more difficult to model because of the large contribution of anomalous transients to the maintenance of the flow. However, strongly damped models produce simulations that bear some resemblance to the anomalous flow, at least in the tropics.

To estimate the potential importance of vertical transport of momentum during moist convection, a damping proportional to the precipitation rate in the GCM is added in the nonlinear model. The estimated damping time scale for the eddy streamfunction is ∼5 days in the northern tropics, but the changes in the predicted stationary eddy streamfunction are modest.

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Soon-Il An and In-Sik Kang

Abstract

The recharge oscillator paradigm for ENSO is further investigated by using a simple coupled model, which externally includes the equatorial wave dynamics represented by the Kelvin and gravest symmetric Rossby waves. To investigate the role of eddies in the Pacific basin–wide adjustment to the wind forcing, particularly at the western and eastern boundaries, the zonal mean and eddy parts are treated separately in the current model.

It is clearly demonstrated that the basin-wide adjustment of the tropical ocean is accomplished by the net mass transport induced by the meridional transport over the tropical ocean interior and the zonal fluxes at the boundaries. With a reasonable choice of the reflection coefficient, particularly at the western boundary, the meridional transport plays a bigger role than the zonal boundary flux and determines the sign of zonal-mean thermocline depth tendency, in a way that the discharge of equatorial mass in the warm phase and recharge in the cold phase serve as a phase transition mechanism of the coupled system. The meridional mass transport is induced mainly by a geostrophic current associated with the east–west slope of thermocline depth, established quickly by the wind forcing. Also discussed in this paper is the difference between the recharge oscillator and the delayed oscillator in explaining the phase transition mechanism of ENSO.

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Fei Liu, Bin Wang, and In-Sik Kang

Abstract

Both observational data analysis and model simulations suggest that convective momentum transport (CMT) by cumulus convection may play a significant role in the intraseasonal oscillations (ISO) by redistributing atmospheric momentum vertically through fast convective mixing process. The authors present a simple theoretical model for the ISO by parameterizing the cumulus momentum transport process in which the CMT tends to produce barotropic wind anomalies that will affect the frictional planetary boundary layer (PBL). In the model with equatorial easterly vertical wind shear (VWS), it is found that the barotropic CMT tends to select most unstable planetary-scale waves because CMT suppresses the equatorial Ekman pumping of short waves, which reduces the shortwave instability from the PBL moisture convergence and accelerates the shortwave propagation. The model with subtropical easterly VWS has behavior that can be qualitatively different from the model with equatorial easterly VWS and has robust northward propagation. The basic mechanism of this northward propagation is that the CMT accelerates the barotropic cyclonic wind to the north of ISO, which will enhance the precipitation by PBL Ekman pumping and favor the northward propagation. The simulated northward propagation is sensitive to the strength and location of the seasonal-mean easterly VWS. These results suggest that accurate simulation of the climatological-mean state is critical for reproducing the realistic ISO in general circulation models.

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Jong-Seong Kug and In-Sik Kang

Abstract

A feedback process of the Indian Ocean SST on ENSO is investigated by using observed data and atmospheric GCM. It is suggested that warming in the Indian Ocean produces an easterly wind stress anomaly over Indonesia and the western edge of the Pacific during the mature phase of El Niño. The anomalous easterly wind in the western Pacific during El Niño helps a rapid termination of El Niño and a fast transition to La Niña by generating upwelling Kelvin waves. Thus, warming in the Indian Ocean, which is a part of the El Niño signal, operates as a negative feedback mechanism to ENSO. This Indian Ocean feedback appears to operate mostly for relatively strong El Niños and results in a La Niña one year after the mature phase of the El Niño. This 1-yr period of phase transition implies a possible role of Indian Ocean–ENSO coupling in the biennial tendency of the ENSO. Atmospheric GCM experiments show that Indian Ocean SST forcing is mostly responsible for the easterly wind anomalies in the western Pacific.

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Isaac M. Held and In-Sik Kang

Abstract

A series of linear and nonlinear barotropic models are used to interpret the extratropical response to El Niflo equatorial surface temperatures as simulated by an atmospheric general circulation model (GCM). The divergence, time-mean vorticity tendency due to transients, and the zonal mean tlow are specified from the GCM, and the deviation of the streamfunction from its zonal mean at an upper-tropospheric level is predicted. Nonlinearsteady-state models suggest that the extratropical wave train is primarily forced from the central rather than the western Pacific and that subtropical divergence anomalies are of more importance than tropical anomalies. These nonlinear solutions can be reproduced with little loss in accuracy by linearizing about the zonally asymmetric climatological flow. If one linearizes about the zonally symmetric flow, the part of the solution forced from thewestern Pacific deteriorates significantly. The solution in the tropics and subtropics also deteriorates if advection of vorticity by the divergent flow is omitted.

Forcing by transients plays a secondary role in generating the extratropical wave train in these barotropic models, but it is pointed out that the subtropical convergence that forces the bulk of this wave train could itself be closely related to anomalies in the transient forcing.

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