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K-M. Lau
,
C-H. Ho
, and
I-S. Kang

Abstract

Using reanalysis data from the Goddard Earth Observing System (GEOS) Data Assimilation System, the authors have documented the basic three-dimensional features of anomalous atmospheric hydrologic processes observed during the El Niño–Southern Oscillation (ENSO). The most dominant anomaly pattern features a pair of subtropical temperature maxima straddling the equator in the upper troposphere coupled to a corresponding pair of temperature minima in the lower stratosphere in the form of a dipole. Over the Tropics and subtropics, the water vapor content is increased in regions of large-scale ascent with maximum response in the middle troposphere, whereas substantial drying is found in the descending branches of the Walker and Hadley circulations. While the temperature and moisture patterns in the lower troposphere are thermodynamically linked to the sea surface temperature anomaly pattern, the distribution of temperature and water vapor in the upper troposphere is largely controlled by dynamics and much less by thermodynamics. The troposphere–stratosphere temperature dipole is fundamentally due to the rising of the tropopause associated with hydrostatic expansion and vertical ascent in regions of enhanced deep convection. The rising motion pushes colder upper-tropospheric air into the lower stratosphere where the climatological temperature gradient reverses. No such dipole anomaly exists in the moisture field.

Numerical experiments with the GEOS GCM show that while atmospheric dynamics are principally responsible for the generation of the basic structures of the temperature and moisture anomalies observed during ENSO, the interaction between the hydrologic cycle and radiation plays an important role in enhancing and modifying the response. The role of hydrologic cycle–radiation interaction is most important in rendering the atmosphere more unstable both columnwise and locally, through enhanced longwave heating in the middle and lower troposphere and cooling above. The enhanced instability leads to intensified Hadley and Walker circulations, which are accompanied by stronger latent heating and a more vigorous hydrologic cycle. The intensified hydrologic cycle promotes further warming and moistening of the middle and lower troposphere, and cooling and drying in the stratosphere. The radiation–dynamics feedback leads to a new equilibrium climate state in which the increased heat transport by convection into the upper troposphere and stratosphere is balanced by increased radiative cooling, which removes the local excessive heat buildup.

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K-M. Lau
,
P-J. Sheu
, and
I-S. Kang

Abstract

In this paper, fundamental multiscale circulation modes in the global atmosphere are identified with the objective of providing better understanding of atmospheric low-frequency variabilities over a wide range of spatial and temporal scales. With the use of a combination of rotated principal component technique, singular spectrum analysis, and phase space portraits, three categories of basic multiscale modes in the atmosphere are found. The first is the interannual mode (IAM), which is dominated by time scales longer than a year and can be attributed to heating and circulation anomalies associated with the coupled tropical ocean-atmosphere, in particular the E1 Niño–Southern Oscillation. The second is a set of tropical intraseasonal modes consisting of three separate multiscale patterns (ISO-1, -2, -3) related to tropical heating that can be identified with the different phases of the Madden–Julian Oscillation (MJO), including its teleconnection to the extratropics. The ISO spatial and temporal patterns suggest that the extratropical wave train in the North Pacific and North America is related to heating over the Maritime Continent and that the evolution of the MJO around the equator may require forcing from the extratropics spawning convection over the Indian Ocean. The third category represents extratropical intraseasonal oscillations arising from internal dynamics of the basic-state circulation. In the Northern Hemisphere, there are two distinct circulation modes with multiple frequencies in this category: the Pacific/North America (PNA) and the North Atlantic/Eurasia (NAE). In the Southern Hemisphere, two phase-locked modes (PSA-1 and PSA-2) are found depicting an eastward propagating wave train from eastern Australia, via the Pacific South America to the South Atlantic. The extratropical modes exhibit temporal characteristics such as phase locking and harmonic oscillations possibly associated with quadratically nonlinear dynamical systems.

Additionally, the observed monthly and seasonal anomalies arise from a complex interplay of the various multiscale low-frequency modes. The relative dominance of the different modes varies widely from month to month and from year to year. On the monthly time scale, while one or two mechanisms may dominate in one year, no single mechanism seems to dominate for all years. There are indications that when the IAM, that is, ENSO heating patterns are strong, the extratropical modes may be suppressed and vice versa. For the seasonal mean, the interannual mode tends to dominate and the contribution from the PNA remains quite significant.

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S. Kaspari
,
P. A. Mayewski
,
M. Handley
,
S. Kang
,
S. Hou
,
S. Sneed
,
K. Maasch
, and
D. Qin

Abstract

A Mount Everest ice core analyzed at high resolution for major and trace elements (Sr, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, Bi, U, Tl, Al, S, Ca, Ti, V, Cr, Mn, Fe, Co) and spanning the period a.d. 1650–2002 is used to investigate the sources of and variations in atmospheric dust through time. The chemical composition of dust varies seasonally, and peak dust concentrations occur during the winter–spring months. Significant correlations between the Everest dust record and dust observations at stations suggest that the Everest record is representative of regional variations in atmospheric dust loading. Back-trajectory analysis in addition to a significant correlation of Everest dust concentrations and the Total Ozone Mapping Spectrometer (TOMS) aerosol index indicates that the dominant winter sources of dust are the Arabian Peninsula, Thar Desert, and northern Sahara. Factors that contribute to dust generation at the surface include soil moisture and temperature, and the long-range transport of dust aerosols appears to be sensitive to the strength of 500-mb zonal winds. There are periods of high dust concentration throughout the 350-yr Mount Everest dust record; however, there is an increase in these periods since the early 1800s. The record was examined for recent increases in dust emissions associated with anthropogenic activities, but no recent dust variations can be conclusively attributed to anthropogenic inputs of dust.

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Ling-Feng Hsiao
,
Melinda S. Peng
,
Der-Song Chen
,
Kang-Ning Huang
, and
Tien-Chiang Yeh

Abstract

Tropical cyclone (TC) track predictions from the operational regional nonhydrostatic TC forecast system of the Taiwanese Central Weather Bureau (CWB) are examined for their sensitivities to initial and lateral boundary conditions. Five experiments are designed and discussed, each using a combination of different initial and lateral boundary conditions coming either from the CWB or the National Centers for Environmental Prediction (NCEP) global forecast system. Eight typhoons in the western Pacific Ocean with 51 cases in 2004 and 2005 are tested with the five designed experiments for the 3-day forecast. The average track forecasts are the best when both the initial and lateral boundary conditions are from the NCEP global forecast system. This reflects the generally superior performance of the NCEP global forecast system relative to that of the CWB. Using different lateral boundary conditions has a greater impact on the track than using different initial conditions. Diagnostics using piecewise inversion of potential vorticity perturbations are carried out to identify synoptic features surrounding the featured typhoon that impact the track the most in each experiment. For the two cases demonstrated with the largest track improvement using NCEP global fields, the diagnostics indicate that the prediction of the strength and extent of the subtropical high in the western Pacific plays the major role in affecting these storm tracks. Using the analysis and predictions of the CWB global forecast system as the initial and lateral boundary conditions produces an overintensified subtropical ridge in the regional TC forecast model. Because most of the typhoons studied are located in the southwestern peripheral of the western Pacific subtropical high, the stronger steering from the more intense and extended high system is the main cause of the poleward bias in the predicted typhoon tracks in the operational run, which uses the CWB global forecast fields. The study suggests that, when efforts are made to improve a regional TC forecast model, it is also critically important to improve the global forecast system that provides the lateral boundary and initial conditions to the regional system.

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S. K. Kang
,
M. G. G. Foreman
,
W. R. Crawford
, and
J. Y. Cherniawsky

Abstract

Internal M 2 tides near Hawaii are investigated with a two-dimensional, two-layer numerical model. It is seen that along the Hawaiian Ridge barotropic tidal energy is transformed into baroclinic internal tides that propagate in both northeast and southwest directions, as previously hypothesized. The internal tide for a certain beam is seen to propagate well over 1000 km. with an approximate decay scale of 1000 km. An asymmetric pattern in the baroclinic energy flux is observed to the north and south of the Hawaiian Ridge due to the spatially inhomogeneous baroclinic energy sources.

The surface manifestation of the M 2 internal tide in the model is compared with analysis results from TOPEX/Poseidon satellite altimetry. The baroclinic short-wave variation of a few centimeters amplitude, superposed on the barotropic surface amplitude, agrees well with the altimeter analyses. This, together with snapshots of the interfacial disturbance, allows the authors to sketch the propagation pattern of internal waves emanating northward and southward from the Hawaiian Ridge. Tidal current ellipses in the upper layer are dominated by the baroclinic internal tide with large spatial variability in their magnitude compared to the barotropic tidal ellipses.

The M 2 baroclinic energy flux is over 10 kW m−1 for the strongest energy beam propagating toward the northeast. Along the western Hawaiian Ridge about 3.8 GW of tidal power is converted from barotropic to baroclinic motion. The average northward or southward flux density for the first baroclinic mode is about 1.35 kW m−1 in the western Hawaiian Ridge. Also, if 2.7 kW m−1 (1.35 kW m−1 to each direction) is assumed for the whole 2000-km-long Hawaiian Ridge, a total of 5.4 GW is obtained. This value indicates that there is still a large uncertainty in the rate of barotropic to radiating baroclinic energy conversion along the Hawaiian Ridge.

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K-M. Lau
,
J-Y. Lee
,
K-M. Kim
, and
I-S. Kang

Abstract

The role of the North Pacific as a regulator of boreal summer climate over Eurasia and North America is investigated using observational data. Two summertime interannual climate modes associated with sea surface temperature (SST) variability in the North Pacific are identified. The first mode shows an elongated zone of warm (cold) SST anomalies in the central North Pacific along 40°N, with temporal variability significantly correlated with El Niño during the preceding spring, but its subsequent evolution is quite different from El Niño. The second mode exhibits a seesaw SST variation between the northern and southern North Pacific and is independent of El Niño. Both modes are linked to coherent SST anomalies over the North Atlantic, suggesting the presence of an “atmospheric bridge” linking the two extratropical oceans.

Using the principal component of the most dominant mode as the North Pacific index (NPI), composite analyses show that the positive (negative) phase of NPI features a warm (cold) North Pacific associated with the formation of contemporaneous low-level stationary anticyclones (cyclones) over the North Pacific and North Atlantic, respectively. The anticyclones (cyclones) are linked by quasi-zonally symmetric circulation anomalies in the middle to upper troposphere spanning Eurasia and North America, accompanied by a poleward (equatorward) shift of the subtropical jet and storm tracks. Associated with the positive (negative) phase of NPI, are hot/dry (cool/wet) summers over Japan, Korea, and eastern-central China, which are linked to hot/dry (cool/wet) conditions in the Pacific Northwest, western Canada, the U.S. northern Great Plains, and the Midwest. Cumulative probability computed from pentad temperature and rainfall data show that the odds of occurrence of extreme events are impacted consistently with the mean climate shift during opposite phases of the NPI. The possible roles of air–sea interaction and transient-mean flow interaction in exciting and sustaining the climate modes are discussed.

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Greg M. McFarquhar
,
Roland List
,
David R. Hudak
,
Robert P. Nissen
,
J. S. Dobbie
,
N. P. Tung
, and
T. S. Kang

Abstract

During the Joint Tropical Rain Experiment of the Malaysian Meteorological Service and the University of Toronto, pulsating raindrop ensembles, hereafter pulses, were observed in and around Penang Island. Using a Doppler radar on 25 October 1990, a periodic variation of precipitation aloft 30 km from the radar site, with an approximate 8-min period, was established and seemed to be caused by the evolution and motion of horizontal inhomogeneities existing within the same cell. On 30 October 1990, using a new volume scanning strategy with a repetition cycle of 3.5 min, pulsations of the same frequency were observed up to 3 km above the radar and at the ground by a disdrometer. High concentrations of large drops were followed by high concentrations of successively smaller drops at the ground. This provides observational evidence to support the recent argument for using a time-varying release of precipitation-sized particles to model observed pulsating rainfall.

Many cases of nonsteady rain from convective clouds displayed repetition periods of between 8 and 25 min.

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Y. P. Kim
,
S-G. Shim
,
K. C. Moon
,
C-G. Hu
,
C. H. Kang
, and
K. Y. Park

Abstract

Gaseous species and total suspended particles were measured at Kosan, Cheju Island, Korea, between 11 March and 19 April 1994. The concentrations of nonsea salt (nss) ions were higher than those measured in clean marine areas of Japan and other background marine areas of the world. In particular, the nss sulfate concentration was comparable to that measured in Seoul. The average SO2 and NO x concentrations were approximately 0.97 and 3.5 ppb, respectively, which were lower than those at other urban areas in Korea but higher than those of other remote areas in the world. In contrast, the average O3 concentration was approximately 55 ppb, which is comparable to or higher than those at remote sites in Japan that were influenced by long-range transport of air pollutants. Half of the air parcels during the period were from northern China and about 30% of the air parcels from southern China. The main difference of air pollutant levels between the two areas was higher crustal species and lower nss sulfate concentrations for air parcels from northern China. The nss SO2− 4 concentrations had a strong correlation with nss K+, NH+ 4 , and O3 concentrations. In addition, the nss Ca2+ concentrations had a strong correlation with the nss K+ and nss Mg2+ concentrations. It was suggested that nss K+ had two sources:anthropogenic and crustal.

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Matthew T. DeLand
,
Richard P. Cebula
,
Liang-Kang Huang
,
Steven L. Taylor
,
Richard S. Stolarski
, and
Richard D. McPeters

Abstract

Satellite measurements using the backscattered ultraviolet technique provide a powerful method for the observation of stratospheric ozone. However, rapid input signal variations over three to four orders of magnitude in several minutes can lead to problems with instrument response. Inflight data have recently been used to characterize a “hysteresis” problem on the NOAA-9 SBUV/2 instrument, which affects measurements made shortly after emerging from darkness. Radiance values observed under these conditions can be up to 2%–3% lower than expected. A correction has been derived for NOAA-9 data that is solar zenith angle dependent and varies in amplitude and time. Typical changes to affected polar total ozone values are on the order of 1% but can reach 5% in some cases. Profile ozone changes are altitude dependent, with maximum values of 4%–5% at 1 hPa. The NOAA-11 and NOAA-14 SBUV/2 instruments have a much smaller hysteresis effect than that observed for NOAA-9 SBUV/2 due to a change in photomultiplier tubes. The Nimbus-7 SBUV instrument also shows a hysteresis effect, which has not been fully characterized at this time.

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Hyacinth C. Nnamchi
,
Jianping Li
,
Fred Kucharski
,
In-Sik Kang
,
Noel S. Keenlyside
,
Ping Chang
, and
Riccardo Farneti

Abstract

Equatorial Atlantic variability is dominated by the Atlantic Niño peaking during the boreal summer. Studies have shown robust links of the Atlantic Niño to fluctuations of the St. Helena subtropical anticyclone and Benguela Niño events. Furthermore, the occurrence of opposite sea surface temperature (SST) anomalies in the eastern equatorial and southwestern extratropical South Atlantic Ocean (SAO), also peaking in boreal summer, has recently been identified and termed the SAO dipole (SAOD). However, the extent to which and how the Atlantic Niño and SAOD are related remain unclear. Here, an analysis of historical observations reveals the Atlantic Niño as a possible intrinsic equatorial arm of the SAOD. Specifically, the observed sporadic equatorial warming characteristic of the Atlantic Niño (~0.4 K) is consistently linked to southwestern cooling (~−0.4 K) of the Atlantic Ocean during the boreal summer. Heat budget calculations show that the SAOD is largely driven by the surface net heat flux anomalies while ocean dynamics may be of secondary importance. Perturbations of the St. Helena anticyclone appear to be the dominant mechanism triggering the surface heat flux anomalies. A weakening of the anticyclone will tend to weaken the prevailing northeasterlies and enhance evaporative cooling over the southwestern Atlantic Ocean. In the equatorial region, the southeast trade winds weaken, thereby suppressing evaporation and leading to net surface warming. Thus, it is hypothesized that the wind–evaporation–SST feedback may be responsible for the growth of the SAOD events linking southern extratropics and equatorial Atlantic variability via surface net heat flux anomalies.

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