Penulis: rais

Outgoing longwave radiation (OLR) sering digunakan sebagai proxy (variabel pengganti) untuk indikator aktivitas awan konvektif. OLR di permukaan bumi sangat bervariasi secara spasial dan temporal yang dapat dipengaruhi oleh berbagai faktor. Penelitian terbaru dari Bapak Faiz R. Fajary dan Bapak Tri W. Hadi yang diterbitkan di Journal of Climate mengidentifikasi beberapa faktor yang mempengaruhi OLR tersebut. Mereka menemukan bahwa variabilitas OLR dipengaruhi paling dominan oleh fenomena Madden-Julian Oscillation dan El Nino Southern Oscillation.

Referensi:

Fajary, F. R., T. W. Hadi, S. Yoden, 2019, Contributing Factors to Spatio-Temporal Variations of Outgoing Longwave Radiation (OLR) in the Tropics. Journal of Climate. Early Online Release. doi:10.1175/JCLI-D-18-0350.1. https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0350.1

Abstract. Factors governing spatio-temporal variations of daily outgoing longwave radiation (OLR) dataset are studied using 35-year (1979–2013) data records by employing multiple linear regression, wavelet transforms, and band-pass filtering methods. From the regression coefficients of nine predictors and the explained variances, we found that the largest contributions to OLR data variability are associated with the Madden-Julian Oscillation and the El Niño–Southern Oscillation (ENSO). The ENSO signatures on OLR show dipole patterns over the Maritime Continent (MC) and Pacific regions. The significant factor of the Indian Ocean dipole is confined to the Indian Ocean and Africa. Furthermore, the solar cycle and stratospheric aerosols show mainly negative correlations, while positive linear trend is observed mainly in the northern hemisphere. Contributions from the stratospheric quasi-biennial oscillation (QBO) are the least significant contributor to OLR. Time-longitude variations of the annual cycle (AC) show pairs of contrasting phases that characterize monsoon systems, in which the MC-Pacific regions are found to be in the same phase group. Most consistent AC signals are found to correspond with North and South American monsoons that respectively exhibit weakening and strengthening trends. Wavelet spectra and filtered OLR signals in the QBO and ENSO frequency bands show an interdependent relationship with large longitudinal dependence.

Pemanasan global (global warming) biasanya ditunjukkan dengan parameter temperatur permukaan yang terus menaik. Indikator ini hanya valid untuk bagian atmosfer permukaan namun tidak bisa menggambarkan karakter vertikal atmosfer. Penelitian baru-baru ini yang diterbitkan di Environmental Research Letters mengusulkan indikator tambahan global warming yaitu massa udara yang dintegrasi sampai suatu level isentropik. Dengan menunjukkan pola tren massa udara dingin di Kutub Utara, indikator tersebut bisa menggambarkan dengan jelas pengurangan konsisten udara dingin yang drastis di wilayah Artik (Arctic Amplification).

Referensi:

Kanno, Y., J. E. Walsh, M. Rais Abdillah, J. Yamaguchi, T. Iwasaki, 2019: Indicators and trends of polar cold airmass. Environmental Research Letters. 14. 025006.  doi:10.1088/1748-9326/aaf42b. https://iopscience.iop.org/article/10.1088/1748-9326/aaf42b

Abstract. Trends and variations in the amount of cold airmass in the Arctic and the Northern Hemisphere are evaluated for the 60 year period, 1959–2018. The two indicators are (1) polar cold air mass (PCAM), which is the amount of air below a potential temperature threshold, and (2) negative heat content (NHC), which includes a weighting by coldness. Because the metrics of coldness are based on multiple layers in the atmosphere, they provide a more comprehensive framework for assessment of warming than is provided by surface air temperatures alone. The negative trends of PCAM and NHC are stronger (as a % per decade) when the threshold is 245 K rather than 280 K, indicating that the loss of extremely cold air is happening at a faster rate than the loss of moderately cold air. The loss of cold air has accelerated, as the most rapid loss of NHC has occurred in recent decades (1989–2018). The spatial patterns of the trends of PCAM and NHC provide another manifestation of Arctic amplification. Of the various teleconnection indices, the Atlantic Multidecadal Oscillation shows the strongest correlations with the spatially integrated metrics of moderate coldness. Several Pacific indices also correlate significantly with these indicators. However, the amount of extremely cold air mass does not correlate significantly with the indices of internal variability used here.