Empirical Orthogonal Function (EOF) was performed to investigate spatial variation in January precipitation over Pakistan using ground observed mean monthly precipitation data from 1950-2000 with a combination of gridded reanalysis data of sea level pressure (SLP) and 500 hPa geopotential height. The leading EOF mode captures 37.51% of the total variance and the spatial-temporal variability of January precipitation was consistent in the study area. The temporal changes explicate non-periodic interannual variability and some tacit interdecadal variation. The anomalous condition is more prominent along the western bordering mountains and northern high mountainous region than any other region of Pakistan. Based on results the study reveals spatial-temporal variation in January precipitation and possible links with global teleconnections located both in the proximity as well as in the remote areas from the study locus.
The Fourth Assessment Report of IPCC [
In this EOF analysis, two data sets have been utilized. The first set is comprised of ground observed mean monthly January precipitation over the period of 1950-2000, obtained from the Pakistan Meteorological Department (PMD). The 45 stations with less missing data were selected from all over the country (
The month of January was operated as continues time series from 1950-2000. Interpolation was done with grid resolution of 0.5˚ × 0.5˚. The interpolation with good spatial resolution can overcome the discrepancy exists in data due to either sparsely distributed observatories or missing data. The fine spatial resolution is instrumental to point out the environmental variability in the mountainous and other diversified areas where sharp climate gradient does exist [
The second set of data contains gridded reanalysis data of sea level pressure (SLP) and 500 hPa geopotentail height (GPF) field with spatial resolution of 2.5˚ × 2.5˚ was obtained from National Centre for Environmental Pre- diction/National Center for Atmospheric Research (NCEP/NCAR) at global scale over the period of 1950-2000.
The EOF provides us with statistical expressions and preferred spatial-temporal patterns of variability. Three EOFs have been performed; each set of analysis provides spatial pattern of ground variations, time series of precipitation anomalies, SLP and 500 hPa loading anomalous patterns acknowledged by correlation coefficient for January over the period of 1950-2000 that reflect statistical significant and less significant areas.
EOF1: The leading EOF mode captured 37.51% of the total variance. The observed spatial distribution of percentage departure of January precipitation from reference value in the time period of 1950-2000 has been shown in
The SLP and Φ500 hPa have been mapped with contour interval of 0.1, the blue color patches in the figures indicate the areas where correlation coefficient exceeds 95% significance level with respect to spatial-temporal variability of January precipitation. Reference to
The correlation pattern at 500 hPa level (
EOF2: The EOF second mode captured 15.79% of the total variance. The time series correlation coefficient (
The coefficient patterns that exhibit the relation with precipitation variability at SLP (
that extends from tropics to the Arctic region, simultaneously the high confidence area prevailed over the tropi- cal and subtropical western southern Atlantic basin. The considerable patches of teleconnections have been traced in the Antarctica region also. The Mediterranean, Middle East pattern centered over the Arabian Sea remained significant at 500 hPa level. Side by side, the atmospheric state over North America and adjacent oceans configures considerable relation with observed precipitation variability. The Antarctic region in the proximity of Antarctic Circle in the southern Atlantic shows area of significance, the rest of the region around Antarctica was found less significant in this case.
EOF3: The third EOF mode captured 10.32% of the total variance. The spatial variation over Pakistan shows (+), (−) and then (+) patterns from south to north and reflects changes in phase structure (
In the decade of 1950s, mostly Pakistan had precipitation above average, in 1960s, most areas were found with deficit in precipitation and the same was true for 1970s and 1980s, based on results the Peshawar Valley and surrounding got precipitation above normal in these decades. The 1990s have shown high internal variability and less decadal signature. Twenty years of the study period were found with above normal precipitation in most parts of the country excluding Peshawar Valley, Potwar region, Muree Area and Azad Kashmir where these months reflect precipitation below average. Twenty nine years were observed with rainfall below normal in most of Pakistan except Peshawar Valley, Potwar region, Muree Area and Azad Kashmir. The above said spatial-temporal variability in January precipitation has shown link with the atmospheric state at SLP (
The climate variability in Pakistan and surroundings has been linked with North Atlantic Oscillations (NAO) as documented by various researchers e.g. [
Based on results the rugged northern and western parts of the country show significant sptial-tempotral precipi-
tation variability in Pakistan. These areas include Kalat region, Quetta Valley, western Bordering Ranges, KPK and across Peshawar Valley to the northwestern lofty mountains up to the Karakoram Ranges in the extreme north of the country. The observed spatial-temporal variability in relation to anomalous atmospheric states at SLP and 500 hPa fields supports that precipitation variability in January can be connected to highly variable nature of westerlies. The interannual variability was salient feature in all modes and found consistently throughout time series of precipitation anomalies. The leading EOF mode 1 depicts correlation pattern at SLP and 500 hPa height field could be the acknowledgment of deep trough over the region, which encourages precipitation in winter. The interdecadal changes were not clear in the first two EOF modes but reasonable interdecadal changes were observed associated with EOF mode 3.
The NAO pressure system is known for its high variability, the WDs are mostly nurtured by this system, thus in this way have indispensable influence on the variability of precipitation in north and northwestern parts of the country. The results also lead us to the assumption that the precipitation variability under discussion had link with remote atmospheric condition round Antarctic region that needs further scientific investigation especially at 700 hPa field. We have put forward the January precipitation variability with regional detail in Pakistan; this geographical approach could be the potential base line study for further scientific investigation.
We are grateful to the PMD for provision of ground observed data. We are thankful to the anonymous reviewers
as their comments and suggestions improved the paper substantially. We are highly grateful to the NCEP/ ANCAR for making the data available.