This study examines the potential impact of subsurface potential temperature and current conditions on the sea surface temperature (SST) and rainfall over the tropical region using Climate Forecast System Version 2 (CFSv2) ensemble seasonal reforecasts for 1958–2014. The climatological difference of earlier period (1958–78; P58‐78) and later period (1994–2014; P94‐14) in January initialized reforecasts (JIR) depicts mild‐warm SST over the equatorial eastern Pacific in January, but its magnitude becomes larger over there from May to August. The difference between JIR P58‐78 and P94‐14 depicts warm potential temperature (PT) up to 3°C over the equatorial central and eastern Pacific at depth from 155 to 70 m in January therefore 20°C isotherm of JIR P58‐78 in January tends to be deeper in the equatorial eastern Pacific than JIR P94‐14. The magnitude of equatorial undercurrent (EUC) in JIR P58‐78 is larger in the eastern Pacific at depth from 125 to 35 m from January to February than JIR P94‐14. Therefore, water upwelled to the surface through EUC was usually warm in January of JIR P58‐78. As lead months increase, the center of warm PT gradually moves upward in the eastern Pacific than its location in January, resulting in development of warm SST in the eastern Pacific in March. The difference between April initialized reforecasts P58‐78 and P94‐14 depicts warm PT up to 1.5°C in the eastern Pacific at depth from 135 to 35 m in April but as lead months increase, magnitude of warm PT gradually decreases, resulting in negligible difference SST over the eastern Pacific.
This study demonstrates the potential impact of land and atmospheric initial conditions (ICs) on the mean state of land surface temperature (SKT) and mid-troposphere circulation over Eurasia and North America (NA) using the Climate Forecast System version-2 (CFSv2) ensemble seasonal reforecasts for 1958–2014. The land ICs and atmosphere ICs have assembled from several different data sources before and after 1979 in January initialized reforecasts (JIR). Therefore, two-time periods have defined, earlier period (1958–1978) and later period (1994–2014). The climatological difference between JIR P58-78 and P94-14 depicts enhanced cold SKT and sub-surface temperature at 0–10 cm over northern Eurasia and northeast region of NA from May to June. Model overestimates snow cover fraction from May to June in JIR P58-78 than JIR P94-14, which leads to excessive upwards-shortwave radiation in the northern latitudes. Due to cold temperature from the surface to mid-troposphere, model depicts lower geopotential heights in mid-troposphere, which leads to increase westerly winds at mid-troposphere over northern latitudes from May to June. The climatological differences between earlier and later periods of JIR from May to June are similar to differences between reforecasts initialized in winter and spring as reported by some recent studies. The climatological difference between JIR P58-78 and P94-14 depicts enhanced cooling from surface to 600 hPa over northern latitudes in January. As lead months increase, the magnitude of cold temperature decreases gradually from February to March. Most prominent centers of geopotential height from surface to mid-troposphere over northern latitude in climatological differences between earlier and later periods of JIR in January vanish in February. The different sources of ICs and the nature of land conditions before and after 1979 are main causes of large differences in JIR from May to June over northern latitudes.
more » « less- NSF-PAR ID:
- 10206878
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Environmental Research Letters
- Volume:
- 15
- Issue:
- 12
- ISSN:
- 1748-9326
- Page Range / eLocation ID:
- Article No. 124045
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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This study evaluates the possible influence of the winter surface conditions in Eurasia on the summer circulation over the Asian continent and Indo‐Pacific region. We have analysed multi‐seasonal ensemble reforecasts for 30 years (1979–2008) using the National Centers for Environmental Prediction Climate Forecast System version 2 initialized at the beginning of each month from January to May. It is found that the reforecasts initialized in winter (e.g., February) overestimate the snow cover fraction, depth and water equivalent, as well as surface albedo in the excessively snow‐covered portion of Eurasia from March to June. These biases are generated and perpetuated by a snow‐albedo feedback, leading to excessive upwards shortwave radiation reflected from the overly snow‐covered surface and an intense cold bias from the surface to mid‐troposphere. Originating over land, the cold bias is extended eastwards over the northwestern North Pacific by the advection of prevailing westerly winds. The cold air temperature in the broad mid‐latitude Asian‐Pacific region causes significantly lower geopotential heights at pressure levels in the middle and upper troposphere and thus increases the upper‐level westerly winds on its southern flank over the Asian continent and Indo‐Pacific. A slower than observed snow melting rate helps the winter cold bias persists well into the summer season in these runs. As a result, compared with the reforecasts initialized in spring (e.g., May), winter‐initialized reforecasts feature lower geopotential heights in the upper troposphere over Eurasia and a stronger subtropical jet over the Asian continent and the North Pacific from May to September, especially in early summer. The CFSv2 reforecasts in both sets of cases have too little total cloud fraction over Eurasia during June–August, leading to enhanced downwards shortwave radiation.
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