Temporal and Spatial Characteristics of Intraseasonal Oscillations in the Meridional Wind Field over the Subtropical Northern Pacific

Based on the NCEP/NCAR reanalysis I daily data from 1958 to 2002,climatic characteristics of the 30-60-day intraseasonal oscillations（ISOs） of the zonal wind（u）,meridional wind（v）,and geopotential height（h） over global areas and especially the ISO of v over the subtropical northern Pacific are analyzed using the space-time spectrum analysis and wavelet transform methods.The results show that the ISO of v is very different from those of u and h,with the former representing the meridional low-frequency disturbances,which are the most active in the subtropics and mid-high latitudes,but very weak in the tropics.In the subtropical Northern Hemisphere,the energies of the ISOs of u and h are both concentrated on the waves with wave number of 1 and periods of 30-60 days,while the main energy of the ISO of v is concentrated on the waves with wave numbers of 4-6 and periods of 30-60 and 70-90 days.The westward propagating energies for the 30-60-day oscillations of u,v,and h are all stronger than the eastward propagating energies in the subtropics.In addition,the ISO of v is the strongest（weakest） in summer （winter） over the subtropics of East Asia and northwestern Pacific,while the situation is reversed over the subtropical northeastern Pacific,revealing a ＂seesaw＂ of the ISO intensity with seasons over the subtropics from the northwestern to northeastern Pacific.In the subtropical northwestern Pacific,the interannual and interdecadal changes of the ISO for v at 850 hPa indicate that its activities are significantly strong during 1958-1975,while obviously weak during 1976-1990,and are the strongest during 1991-2000,and its spectral energy is obviously abnormal but ruleless during the ENSO periods.However,in the 2-7-yr bandpass filtering series,the interannual changes of the v ISO over the subtropical northwestern Pacific contain distinct ENSO signals.And in the 9-yr low-pass filtering series,the v ISO changes over the subtropical northwestern Pacific are significantly out of phase with the changes of the Nino-3.4 SST,whereas the v ISO changes in the subtropical northeastern Pacific are significantly in phase with the changes of the Nino-3.4 SST.

INTERANNUAL LOW-FREQUENCY OSCILLATIONS OF MERIDIONAL WINDS OVER THE EQUATORIAL INDIAN-PACIFIC OCEANS

Based on analysis of the meridional winds over oceanic areas and SST for 1950--1979 extracted from the data sets of COADS, the long-term variability of the meridional winds over the equatorial Indian-Pacific oceans and its relationship to the onset and development of El Nino events have been studied. The major results are as follows: (1) There is a great similarity between ITCZ over the Pacific and SST in the seasonal trend, with ITCZ and high SST found in the Southern Hemisphere in winter and in the Northern Hemisphere in summer. During El Nino years, unusual meridional winds were often observed, with significant convergence of meridional winds occurring over near-equatorial regions. (2) For the near-equatorial meridional winds, there are three types of interannual LFO: QBO, SO, FYO. QBO plays an important role in the unusual behavior of meridional winds for El Nino years, while SO is very important for both El Nino and cold water years. These two oscil- lations may fit well to the observed variation in the meridional wind. FYO may enhance the variation of mer- idional winds. (3) Interannual LFO of meridional winds originates in the Indian Ocean-Maritime Continent and coastal area of East Pacific. Unusual activities of winter monsoon in both hemispheres and trade winds off the coastal area of East Pacific are believed to be their major cause. (4) Monsoon-trade interaction shows up in the significant amplification of the disturbances of meridional winds while they propagate eastward from monsoon area to trade wind area.

Application of the nonlinear time series prediction method of genetic algorithm for forecasting surface wind of point station in the South China Sea with scatterometer observations

The present work reports the development of nonlinear time series prediction method of genetic algorithm（GA） with singular spectrum analysis（SSA） for forecasting the surface wind of a point station in the South China Sea（SCS） with scatterometer observations.Before the nonlinear technique GA is used for forecasting the time series of surface wind,the SSA is applied to reduce the noise.The surface wind speed and surface wind components from scatterometer observations at three locations in the SCS have been used to develop and test the technique.The predictions have been compared with persistence forecasts in terms of root mean square error.The predicted surface wind with GA and SSA made up to four days（longer for some point station） in advance have been found to be significantly superior to those made by persistence model.This method can serve as a cost-effective alternate prediction technique for forecasting surface wind of a point station in the SCS basin.

Assessment of the Impacts of Tropical Cyclones Idai to the Western Coastal Area and Hinterlands of the South Western Indian Ocean

Tropical Cyclones (TCs) are among the atmospheric events which may trigger/enhance the occurrence of disasters to the society in most world basins including the Southwestern Indian Ocean (SWIO). This study analyzed the dynamics and the impacts of the Tropical Cyclone (TC) Idai (4th-21st March, 2019) which devastated most of the SWIO countries. The study used the Reanalysis 1 products of daily zonal (u) and meridional (v) winds, Sea Surface Temperatures (SSTs), amount of Precipitable Water (PRW), and relative humidity (Rh). The dynamics and movements of Idai were analyzed using the wind circulation at 850, 700, 500 and 200 mb, where the TC dynamic variables like vertical wind shear, vorticity, and the mean zonal wind were calculated using u and v components. Using the open Grid Analysis and Display System (GrADS) software the data was processed into three-time epochs of pre, during and post;and then analyzed to feature the state of the atmosphere before (pre), during and post TC Idai using all datasets. The amount of precipitable water was used to map the rainfall on pre, during, and post Idai as well as during its landfall. The results revealed that dynamics of TC Idai was intensifying the weather (over Mozambique) and clearing the weather equatorward or southward of 12°S, with low vertical wind shear over the landfall areas (-3 to 3 m/s) and higher shear values (10 - 40 m/s) northward and southward of the Mozambican channel. Higher moisture content (80 - 90%) and higher PRW (40 - 60 mm/day) mapped during Idai over the lowland areas of Mozambique propagating westward. Higher low-level vorticity values were also mapped over the landfall areas. More results revealed that countries laying equatorward of 12°S, e.g., the northern coastal areas of Kenya (Turkana and Baringo) and Tanzania, Idai disrupted the 2019 March to May (MAM) seasonal rainfall by inducing long dry spell which accelerated the famine over the northeastern Kenya (Turkana). Moreover, results revealed that the land falling of Idai triggered intensive flooding which affected a wide spectrum of socio-economic livelihoods including significant loss of lives, injuries, loss of material wealth, infrastructure;indeed, people were forced to leave their houses for quite a longtime;water-borne diseases like malaria, cholera among others were experienced. Furthermore, results and reports revealed that a large amount of funds were raised to combat the impacts of Idai. For instance, USAID/OFDA used about $14,146,651 for human aid and treatment of flood-prone diseases like Cholera in Mozambique ($13,296,651), Zimbabwe ($100,000), and Malawi ($280,000), respectively. Also a death toll of about 602 in Mozambique and 344 in Zimbabwe, and more than 2500 cases of injured people were reported. Conclusively the study has shown that TCs including Idai and other are among the deadliest natural phenomenon which great affects the human and his environments, thus extensive studies on TCs frequency, strength, tracks as well as their coast benefit analysis should be conducted to reduce the societal impacts of these TCs.

Atmospheric insight to climatic signals of δ^18O in a Laohugou ice core in the northeastern Tibetan Plateau during 1960–2006

Ice documentation and response to prominent warming, especially after the 1990s, is further investigated because it is concerned whether ice records have absence. A δ^18O series of a Laohugou （LHG） shallow ice core （20.12 m） in the northeastern Tibetan Plateau was reconstructed covering the period of 1960–2006. The ice core δ^18O record had sig-nificant positive correlations with the warm season （May–September） air temperatures at adjacent meteorological stations and the 500 hPa temperatures in boreal China, indicating that the δ^18O record could be considered a credible proxy of regional temperature. A clear, cold temperature event in 1967 and rapid warming after the 1990s were captured in the LHG δ^18O series, revealing that it could record extreme air-temperature events on both regional and global scales. The LHG δ^18O variations had evident positive correlations with both the summer surface outgoing longwave radiation （OLR） in the Mongolia region and the summer meridional wind at 500 hPa in the LHG region during 1960–2006, suggesting that the increased OLR in the Mongolia region might have intensified the Mongolia Low and expanded the pressure gradient to the LHG region （the Shulehe High）, which would have pushed the westerlies further north and suppressed southward incursions of cold air into the LHG region, and thus augmented the temperature rise. The regional atmospheric circulation difference （1985–2006 minus 1960–1984） suggested that the anticyclone in the Mongolia region might have developed the easterly wind, which transported warmer air from the east toward the LHG region and weakened the cold penetration of the westerlies, resulting in the temperature rise since the middle 1980s.

Precipitation Patterns in Cape Verde Islands:Santiago Island Case Study

The Cape Verde archipelago is located between 14。N - 18。N and 22。W - 28。W in the Atlantic Ocean. Previous studies associated Cape Verde’s rainfall regime with ITCZ and African squall lines. This hypothesis is revisited here using rainfall time series?of the 4 Santiago Island station network of Cape Verde National Institute of Meteorology and Geophysics. Rainfall monthly totals are standardized to produce indices in the 1981-2009 period. Time section plots of zonal and meridional wind components anomalies are made using reanalysis data from ESRL/PSD/NOAA website. Daily outgoing long wave radiation and sea level pressure time section plots are also made as proxies for weather systems propagation. Results show that Santiago presents a seasonal rainfall regime characterized by dry (November-June) and wet (July-October) seasons, with short transition period. In general, rainfall totals increase with altitude. Weather systems within a wet year rainy season were associated with positive anomalies of zonal and meridional wind components of relative short duration, while negative anomalies dominated in dry years. These results suggest that winds coming from southwestern quadrant over warm Atlantic Ocean, associated with frontal systems traveling eastward, are the rain-producing events in wet years, not ITCZ or African squall lines. Winds coming from the northeastern quadrant produce dry years. Apparently, decadal-long wet periods are related to PDO cold phase. In the current PDO cold phase, there is only one year (2002) slightly dry. Considering that each PDO phases lasts 25 to 30 years and the current PDO cold phase started in 1999, it is possible that wet years predominate in the next 10 to 15 years.

Intensity Evolution of Zonal Shear Line over the Tibetan Plateau in Summer:A Perspective of Divergent and Rotational Kinetic Energies

Based on the ERA5 reanalysis datasets during 1980-2019,a total of eleven zonal shear lines(ZSLs)that caused heavy precipitation and lasted more than 60 hours over the Tibetan Plateau in summer are selected for composite analysis.By decomposing the kinetic energy(K)near the ZSL into divergent and rotational kinetic energies(K_(D)and K_(R))and the kinetic energy of interaction between the divergent wind and the rotational wind(K_(R)D),the influence of the rotational and divergent winds on the evolution of the ZSL intensity is investigated from the perspective of K_(D)and K_(R).The main results are as follows.The ZSL is a comprehensive reflection of rotation and convergence.The intensity evolution of ZSL is essentially synchronized with those of K,K_(R),and K_(RD)but lags behind K_(D)by about three hours.The enhancement of K is mainly contributed by K_(R),which is governed by the conversion from K_(D)to K_(R).Furthermore,the increase in the conversion from K_(D)to K_(R)is controlled by the geostrophic effect term Af,which is determined by the joint enhancement of the zonal rotational and meridional divergent wind components(u_(R)and v_(D)).Therefore,the joint enhancement of u_(R)and v_(D)controls the increase of the ZSL intensity,leading to increased precipitation.

ON WESTERLY WIND BURSTS IN EQUATORIAL WESTERN PACIFIC BEFORE AND DURING THE ONSET AND INITIAL DEVELOPMENT PHASES OF ENSO

By means of NCEP/NCAR reanalysis dataset,the origins of westerly wind anomalies at low level over equatorial western Pacific Ocean before and during the onset and initial development phase of ENSO are explored.Evidences show that westerly anomalies in the equatorial western Pacific(140—180°E)are characterized by two remarkable enhancements in the spring and summer of the year when El Nine emerges.The enhancements are not only.to some extem.due to the eastward propagation of low-level westerlies in equatorial Indian Ocean.but also predominantly resulting from Ihe intense convergence of the meridional wind from both hemispheres.The latitudinal convergence leads to the local intensification of zonal pressure gradient so as to cause the reinforcement and bursts of westerly wind over warm pool.Besides,by virtue of the effect of earth rotation,the northeasterlies(southeasterlies)from the Northern(Southern)Hemisphere turn into northwesterlies(southwesterlies)progressively in the near-equatorial zone.which directly strengthens the westerly velocity.Comparing the contributions of the meridional wind from both hemispheres to westerly wind bursts,is seems that southeasterlies from the Southern Hemisphere are much stronger and more stable than northwesterlies of Northern Hemisphere.It is evident that the southeasterlies to the east of Australia originate from the southern mid-and high latitudes and are in close association with the Southern Oscillation.