Tropospheric Temperatures

 

Global estimates of mean tropospheric temperatures are obtained from Channel 2R of the Microwave Sounding Unit (MSU) (Spencer et al. 1990; Spencer and Christy 1992) on the National Oceanographic and Atmospheric Administration (NOAA) series of polar-orbiting satellites. The peak in the Channel 2R weighting function is near 750-hPa. According to the MSU estimates, the annual mean global tropospheric temperature during 1999 was 0.06°C below the 1979–98 base period mean (Fig. 6), which is in stark contrast to the record positive anomaly of +0.46°C observed in 1998. Since the beginning of MSU measurements in 1979, there has been little evidence of a trend in lower-tropospheric temperatures. This contrasts with a pronounced upward trend in surface air temperatures over the past 20 years (Fig. 1a), but according to the National Research Council (2000) "in no way invalidates the conclusion that surface temperature has been rising".

The record decrease in mean tropospheric temperatures of approximately 0.5°C between 1998 and 1999 was linked to below-average temperatures throughout the global Tropics and subtropics [25°S–25°N, Fig. 7], with the largest negative anomalies in this latitude band located over the central and eastern Pacific throughout the year (Figs. 8a, b). These anomalies reflected below-average SSTs and suppressed convection across the eastern half of the equatorial Pacific in association with La Niña conditions (see sections 3c, 3d; Figs. 16, 17), as well as the La Niña-related subtropical cyclonic circulation anomalies observed at upper levels which flanked the region of suppressed tropical convection.

In the Northern Hemisphere, a notable feature during 1999 was above-average temperatures in the middle latitudes centered near 40°N, which is also evident in the mean surface temperature anomalies (Fig. 3). These anomalies, together with the cooler-than-normal temperatures in the Tropics, reflected a decreased strength of the normal north-south temperature gradient near 30°N. An anomalously weak meridional temperature gradient was also evident in the Southern Hemisphere over the South Pacific near (30°S) throughout the year. In both hemispheres, this reduced temperature gradient coincided with a large-scale pattern of anomalous easterly winds in the upper troposphere (see section 3e(1), Fig. 17).

This overall anomaly pattern was especially prominent over the central North Pacific during December 1998–March 1999, when it was associated with a marked westward retraction of the East Asian jet stream to the area west of the date line (see section 3e(1), Fig. 21), and with reduced jet stream winds and reduced storminess across the lower mid-latitudes of the eastern North Pacific. Elsewhere, anomalous lower-latitude easterlies at upper levels during August–October contributed to reduced hurricane activity over the eastern Pacific, and to enhanced hurricane activity over the tropical North Atlantic [see section 4a(2i)]. In contrast, an increased meridional temperature gradient was evident in the Northern Hemisphere between 40°–55°N, which is consistent with increased westerlies in this latitude band during much of the year. During the winter season, these enhanced westerlies contributed to exceptionally warm temperatures over large portions of North America, and to well above-average precipitation in the Pacific Northwest region of the United States [see section 4a(1)].

Over the North Atlantic, lower-tropospheric temperatures were above average between 30°–60°N throughout the year. This pattern reflected an overall positive phase of the North Atlantic Oscillation (NAO), and was particularly prominent during DJF (see section 6, Fig. 78). During this 3-month period the standardized NAO index, determined from a rotated EOF analysis of 700-hPa heights (see Halpert and Bell 1997, their Fig. 31a), was 2.0 standard deviations above the 1950–99 base period mean (not shown).

In the Southern Hemisphere middle latitudes between 40°–65°S, a wave-3 pattern of temperature anomalies was evident during both halves of the year. This pattern featured above-average temperatures extending from southern Australia eastward to the central South Pacific, over extreme southern South America, and in the region just poleward of South Africa, and below-average temperatures over the eastern South Pacific, the central South Atlantic, and the Indian Ocean. Over the Pacific sector, this anomaly pattern was very similar to its Northern Hemisphere counterpart, indicating a considerable inter-hemispheric symmetry of the anomalies in this region. Over the high latitudes of the eastern South Pacific, this anomaly pattern was associated with reduced blocking activity during much of the year, and with increased westerlies and periods of increased storminess across southern South America (see section 4d, Fig. 55).