Long-term variations in the characteristics of the equatorial stratospheric zonal winds

Stratospheric equatorial zonal winds from 1951 onwards up to the present show considerable long-term variations, more so at higher levels, These are rarely monotonic and often show multi-periodic structures, including a sunspot cycle (II year variations), Stratospheric temperatures and geopotential heights also show multi-periodic variations, A periodicity near 20 years is encountered often.

Influence of geomagnetic disturbances on mean winds and tides in the mesosphere/lower thermosphere at midlatitudes

Observations of upper mesosphere/lower thermosphere (MLT) wind have been performed at Collm (51.3∘ N, 13.0∘ E) and Kazan (56∘ N, 49∘ E), using two SKiYMET all-sky meteor radars with similar configuration. Daily vertical profiles of mean winds and tidal amplitudes have been constructed from hourly horizontal winds. We analyse the response of mean winds and tidal amplitudes to geomagnetic disturbances. To this end, we compare winds and amplitudes for very quiet (Ap ≤ 5) and unsettled/disturbed (Ap ≥ 20) geomagnetic conditions. Zonal winds in both the mesosphere and lower thermosphere are weaker during disturbed conditions for both summer and winter. The summer equatorward meridional wind jet is weaker for disturbed geomagnetic conditions. Tendencies for geomagnetic effects on mean winds over Collm and Kazan qualitatively agree during most of the year. For the diurnal tide, amplitudes in summer are smaller in the mesosphere and greater in the lower thermosphere, but no clear tendency is seen for winter. Semidiurnal tidal amplitudes increase during geomagnetic active days in summer and winter. Terdiurnal amplitudes are slightly reduced in the mesosphere during disturbed days, but no clear effect is visible for the lower thermosphere. Overall, while there is a noticeable effect of geomagnetic variability on the mean wind, the effect on tidal amplitudes, except for the semidiurnal tide, is relatively small and partly different over Collm and Kazan.

Comparison of quasi-biennial oscillations of stratospheric winds and atmospheric temperatures at different altitudes

During 1959-89, the 12-month running means of 50 hPa zonal winds, the average atmospheric temperatures in the northern and southern hemisphere at four altitude slabs (950 hPa, 850- 300 hPa, 300-100 hPa and 100-50 hPa), Pacific and Atlantic sea surface temperature (SST) and-30hPa temperatures at North Pole and average for (10°-90° N), all showed quasi-biennial oscillations (QBO). However, whereas the wind QBO had an average spacing of 29 months, only temperatures at 300-100 hPa and Atlantic SST had similar average spacing. Other temperatures as also SO index (represented by Tahiti minus Darwin atmospheric pressure) had larger average spacing. Spectral analysis showed that whereas wind QBO had only one prominent peak at T=2.33 years, other parameters had weak QBOs near T=2.5-2.6 years except Pacific SST and 30 hPa North Pole temperature which had small peaks near T=2.3 years. All the temperatures had prominent peaks in the 3-6 year region which matched with similar peaks in the SO index. There is some indication that stratospheric wind QBO had some relation with parameters at all altitudes in tropics and with North Pole, while ENSO had considerable influence at other latitudes/altitudes.

North Atlantic footprint of summer Greenland ice sheet melting on interannual to interdecadal timescales: A Greenland blocking perspective

Recent rapid melting of summer Greenland ice sheet (GrIS) and its impact on the Earth’s climate has attracted much attention. In this paper, we establish a connection between the melting of GrIS and the variability of summer sea surface temperature (SST) anomalies over North Atlantic on interannual to interdecadal timescales through changes in sub-seasonal Greenland blocking (GB). It is found that the latitude and width of GB are important for the spatial patterns of the GrIS melting. The melting of GrIS on interdecadal timescales is most prominent for the positive Atlantic Multidecadal Oscillation phase (AMO+) because the high latitude GB and its large width, long lifetime and slow decay are favored. However, the North Atlantic mid-high latitude warm-cold-warm (cold-warm-cold) tripole or NAT+ (NAT−) pattern on interannual timescales tends to strengthen (weaken) the role of AMO+ in the GrIS melting especially on the northern or northeastern periphery of Greenland by promoting (inhibiting) high-latitude GB and increasing (decreasing) its width. It is further revealed that AMO+ (NAT+) favors the persistence and width of GB mainly through producing weak summer zonal winds and small summer meridional potential vorticity gradient (PVy) in the North Atlantic mid-high latitudes 55°-70°N (55°-65°N) compared to the role of AMO− (NAT−). The event frequency and zonal width of GB events and their poleward shift are favored by the combination of NAT+ with AMO+. In contrast, the combination of NAT− and AMO+ tends to suppress reduced summer zonal winds and PVy, thus inhibiting the event frequency of GB events and their poleward shift and zonal width.

Global balanced wind derived from SABER temperature and pressure observations and its validations

Zonal winds in the stratosphere and mesosphere play important roles in atmospheric dynamics and aeronomy. However, the direct measurement of winds in this height range is difficult. We present a dataset of the monthly mean zonal wind in the height range of 18–100 km and at latitudes of 50∘ S–50∘ N from 2002 to 2019, derived by the gradient balance wind theory and the temperature and pressure observed by the SABER instrument. The tide alias above 80 km at the Equator is replaced by the monthly mean zonal wind measured by a meteor radar at 0.2∘ S. The dataset (named BU) is validated by comparing with the zonal wind from MERRA2 (MerU), UARP (UraU), the HWM14 empirical model (HwmU), meteor radar (MetU), and lidar (LidU) at seven stations from around 50∘ N to 29.7∘ S. At 18–70 km, BU and MerU have (i) nearly identical zero wind lines and (ii) year-to-year variations of the eastward and westward wind jets at middle and high latitudes, and (iii) the quasi-biennial oscillation (QBO) and semi-annual oscillation (SAO) especially the disrupted QBO in early 2016. The comparisons among BU, UraU, and HwmU show good agreement in general below 80 km. Above 80 km, the agreements among BU, UraU, HwmU, MetU, and LidU are good in general, except some discrepancies at limited heights and months. The BU data are archived as netCDF files and are available at https://doi.org/10.12176/01.99.00574 (Liu et al., 2021). The advantages of the global BU dataset are its large vertical extent (from the stratosphere to the lower thermosphere) and 18-year internally consistent time series (2002–2019). The BU data is useful to study the temporal variations with periods ranging from seasons to decades at 50∘ S–50∘ N. It can also be used as the background wind for atmospheric wave propagation.

Contrasting movement of wind based equatorial trough and equatorial cloud zone over Indian southern peninsula and adjoining Bay of Bengal during the onset phase of northeast monsoon

ABSTRACT. The northeast monsoon sets in over southern parts of peninsular India after the retreat of southwest monsoon and in association with the southward movement of equatorial trough. The INSAT satellite imageries scrutinised during the past several years revealed that the cloud bands at the time of northeast monsoon onset moved from south Bay into the southern peninsula, a feature that contrasts with the north to south movement of the equatorial trough. The paper investigates this aspect based on a dataset of lower level upper winds of the peninsula, rainfall data and INSAT OLR data for the 20 year period 1981–2000. The super epoch profiles of zonal winds, latitudinal position of equatorial trough with reference to northeast monsoon onset dates have been derived and studied. The region with OLR values less than 230 W/m2 was defined as the equatorial cloud zone and the movement of northern limit of ECZ was studied based on the normal pentad OLR data and also the superposed epoch profiles. From these analysis it has been established that at the time of northeast monsoon onset, the wind based equatorial trough moves south of Comorin whereas the cloud zone in the Bay of Bengal moves from south to north. Reasons for the occurrence of such a contrasting feature have been ascribed to features such as decreasing strength of lower level easterlies from north to south over coastal Tamil Nadu, reversal of temperature gradient between Chennai and Thiruvananthapuram at the time of onset and the dynamics of 40-day oscillation. The northeast monsoon activity over coastal Tamil Nadu has been found to be negatively correlated with the low level zonal winds over the coast, the degree of relation decreasing from north to south and also from October to December. Based on the results derived in the study and also the other known features of northeast monsoon a thematic model of northeast monsoon onset listing the events that precede and succeed the onset has been postulated.

Operational use of improved CMVs derived from INSAT IR data in NWP Model over Indian region

Lkkj & balSV ls izkIr lh- ,e- oh- dh xq.krk vk’oklu ¼D;w- ,-½ tk¡p izkjaHk esa ,u- lh- bZ- ih- okf’kaxVu vFkok bZ- lh- ,e- MCY;w- ,Q- ls izkIr 12 ?kaVs ds fuEu foHksnu iwokZuqeku ds vk¡dM+ksa dk mi;ksx djds dh tkrh gSA fgan egklkxj ds vi;kZIr vk¡dM+ksa okys {ks= ls balSV vFkok vU; mixzgksa ls izkIr lh- ,e- oh- dk mi;ksx bu iwokZuqekuksa esa izkjaHk esa ugha fd;k tkrk FkkA vU; dsUnzksa dks balSV ls izkIr lh- ,e- oh- dh xq.krk fo’oluh; Lrj dh ugha yxhA tqykbZ 98 ls igys] balSV ds vykok fdlh vU; mixzg ls fgan egklkxj ds vi;kZIr vk¡dM+ksa okys {ks= ds vk¡dM+s izkIr ugha gq,A tqykbZ 98 esa] ehfVvkslSV&5 dks fgan egklkxj ds Åij 63 fMxzh iwoZ dh vksj f[kldk;k x;kA balSV ekSle foKku vk¡dM+k lalk/ku ra= ¼vkbZ- ,e- Mh- ih- ,l-½ esa balSV ls lh- ,e- oh- izkIr djus dh izpkyukRed ;kstuk esa dqN egRoi.kZ lq/kkj fd, x, ftlls mi;ksxdrkZvksa dks csgrj xq.krk ds mixzg ls izkIr iou ds vk¡dM+s miyC/k djok, tk ldsaA la’kksf/kr ,yxksfjFe esa xq.krk vk’oklu tk¡p esa ,y- ,- ,e- iwokZuqeku dk mi;ksx fd;k x;k gSA LiSDVªe dh 'khr ifjf/k ij es?kksa ds dqN izfr’kr ds vkSlr rkieku ds vk/kkj ij nkc vkSj m¡pkbZ;k¡ fu/kkZfjr dh tkrh gSaA balSV vkSj ehfV;kslsV&5 ls izkIr lh- ,e- oh dk foLrkj ls rqyukRed v/;;u fd;k x;k gSA balSV vkSj ehfV;kslsV-5 ls izkIr ifj"Ñr lh- ,e- oh- ds chp xq.krk ds vk/kkj ij vPNh vuq:irk dk irk pyk gSA vfHkufr vkSj vkj- ,e- ,l esa Hkh lq/kkj gq, gSaA lh- ,e- oh- esa igys izsf{kr {ks=h; iouksa esa Hkh lq/kkj gqvk gSA ehfV;kslsV&5 dh rqyuk esa balSV ls izkIr lh- ,e- oh- dh de la[;k dk dkj.k balSV jsfM;ksehVj dk fuEu LFkkfud foHksnu gSA fp=ksa ds f=d ls rS;kj lh- ,e- oh- ds nks lSVksa dks lfEefyr fd;k x;k ftlesa nks lSVksa esa LFkkfir lh- ,e- oh- dks 'kkfey ugha djus dk /;ku j[kk x;k gSA balSV ls izkIr ifj"Ñr lh- ,e- oh- ls ekWMy }kjk fd, tkus okys iwokZuqeku ij ldkjkRed izHkko iMk gSA Earlier Quality Assurance (QA) tests of INSAT derived CMVs were done using low resolution 12 hour forecast from NCEP Washington or ECMWF. These forecasts earlier did not use satellite derived CMVs from INSAT or other satellites from data sparse Indian Ocean. Other centers did not find INSAT CMVs of reliable quality. Before July 1998, no other satellite except INSAT covered the data sparse Indian Ocean. In July 1998, METEOSAT-5 was shifted over Indian Ocean at 63 degree East. Certain vital changes have been incorporated in the operational scheme of INSAT CMVs derivation at INSAT Meteorological Data Processing System (IMDPS) for providing satellite derived winds of improved quality to the users. The modified algorithm uses LAM forecast in QA tests. Pressures and heights are assigned on the basis of mean temperature of certain percentage of cloud population at the cold end of the spectrum. Detailed comparison of CMVs from INSAT and METEOSAT-5 has been done. The improved CMVs from INSAT and METEOSAT-5 show quite good agreement qualitatively. The bias and RMS also show improvements. Zonal winds observed earlier in CMVs have shown improvement. The lesser number of INSAT CMVs is due to lower spatial resolution of INSAT radiometer compared to METEOSAT-5. The two sets of CMVs generated from triplet of images are being combined, taking care of excluding the collocated CMVs in the two sets. The improved INSAT derived CMVs have shown positive impact on the Model forecast.

Distribution of mean monthly surface/upper air parameters during July-August months of 1982-83 and 1987- 88

Lkkj & Hkkjrh; xzh"edkyhu ekulwu dks leqnz vkSj /kjkry dh feyh&tqyh ok;qeaMyh; ifj?kVuk ekurs gq, bldk v/;;u HkweaMyh; izÑfr ds ifjn`’; esa fd;k x;k gSA bl v/;;u esa nks fo"ke ifjfLFkfr;ksa esa ekulwu ds O;ogkj dks le>us ds fy, Øe’k% de vkSj vf/kd nksuksa rjg dh o"kkZ okys nks o"kksaZ ¼1982] 1987½] nks o"kksaZ ¼1983] 1988½ esa xzh"edkyhu ekulwu ds nks izeq[k eghuksa tqykbZ vkSj vxLr ds nkSjku 'kwU; va’k m- ls 40 va’k m-@40 va’k iw- ls 100 va’k iw- ds {ks= esa ek/; ekfld /kjkryh; izkpyksa ds forj.k dks fy;k x;k gSA blds vfrfjDr xzh"edkyhu ekulwu ij {kksHkeaMyh; if’peh gokvksa ds izHkko vkSj if’peh fo{kksHkksa dh xfrfof/k dk ewY;kadu djus ds fy, fuEu {kksHkeaMy dh dfVca/kh; iouksa ds forj.k vFkkZr~] tqykbZ] vxLr ds eghuksa esa 850 gSDVkikLdy vkSj 700 gSDVkikLdy ds Lrjksa tuojh] ebZ] tqykbZ vkSj vxLr ds eghuksa ds fy, 500 gSDVkikLdy ij HkwfLFkfrt Å¡pkb;ksa dk v/;;u fd;k x;k gSA bl v/;;u ls izkIr gq, ifj.kkeksa ij fopkj&foe’kZ fd;k x;k gSA Indian summer monsoon is considered as an ocean-land-atmosphere coupled phenomenon and also of global nature. In present study, the distribution of mean monthly surface parameters within 0° N – 40° N / 40° E – 100° E region during the two representative months of summer monsoon, July and August, in both deficient years (1982, 1987) and excess years (1983, 1988) was taken up to understand the behaviour of monsoon during two contrasting situations. Apart from this, the distribution of lower tropospheric zonal winds viz., 850 hPa and 700 hPa levels during July, August months, 500 hPa geopotential heights for the months of January, May, July and August months studied to assess the influence of tropospheric westerlies and activity of Western Disturbances on the summer monsoon. The results discussed.

Observed of Equatorial Currents in the Indian Ocean during Two Contrasting IOD Events: 2006 and 2010

The evolution of Indian Ocean Dipole (IOD) events in 2006 and 2010 is investigated using observational data products that are made to understand several processes in the positive (negative) phase of IOD events. Two Acoustic Doppler Current Profiler (ADCP) moorings mounted at 90°E and 80.5°E along the equator were used to evaluate the zonal current variation during two contrasting Indian Ocean Dipole (IO) events. Westward anomalies of the zonal current were observed at 0°, 80.5°E during the peak phase of the positive IOD event from October to December 2006. Meanwhile, the observed zonal currents at 0°, 90°E only showed the short-term westward anomalies during October 2006. On the other hand, during the negative IOD event in 2010, the observed zonal current at both mooring locations indicated strong intraseasonal variations of the eastward anomalies from August to December 2010. Strong easterly (westerly) anomalies of the surface zonal winds were observed during the peak phase of the positive (negative) IOD event in 2006 (2010). These easterly (westerly) anomalies forced upwelling (downwelling) equatorial Kelvin waves indicated by the negative (positive) sea surface height anomalies. Strengthening (weakening) of upwelling (downwelling) along the equatorial Indian Ocean would be a significant factor for further understanding of IOD evolution.

Analysis of climate indicator association with hotspots in Indonesia using heterogeneous correlation map

In several regions, land and forest fires of Indonesia occurred almost annually during the drought season. The severity of Indonesia's drought season is mainly influenced by the Australian Monsoon, local cloud formation controlled by Sea Surface Temperature (SST) around Indonesia. Moreover, it affects the severity of land and forest fires itself indirectly. This research aims to examine the association of the Australian Monsoon and local SST with land and forest fires in Indonesia. This research uses the Australian Monsoon Index (AUSMI) as an indicator for the Australian Monsoon and SST in the Karimata Strait and the Java Sea as indicators of local SST. An indicator of land and forest fires that will be used is the number of hotspots. A heterogeneous Correlation Map (HCM) is used to describe hotspots associated with AUSMI and local SST. The analysis shows that the east wind pattern of AUSMI associated with hotspots in Indonesia, especially in years when zonal winds enter an upward phase more slowly. Karimata Strait’s SST is associate with hotspots in the coastal part of Riau. Meanwhile, Java Sea’s SST is associate with hotspots in Lampung, South Sumatra, Jambi, and Kalimantan.