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Editor's Note: Due to computer problems, October products derived from the Climate Data Assimilation System (CDAS) were not available in time for inclusion in this issue of the Climate Diagnostics Bulletin. Instead we have used analyses available from the Global Data Assimilation System (GDAS). Anomalies were computed based on the CDAS (1979-95) base period means. The main differences between CDAS and GDAS analyses result from differences in model resolution and from the use of two analyses per day (GDAS) instead of four analyses per day (CDAS) to compute the daily means, which were then used to compute the monthly averages. Differences in model resolution have a strong impact on sea level pressure, both mean and anomalous, in the vicinity of elevated terrain. These differences may also affect other low-level fields such as the 850-hPa winds in the same regions. Also, the use of two analyses per day instead of four to compute daily averages impacts those variables that display a strong semi-diurnal tide, such as geopotential height and sea level pressure in the Tropics.

Additionally, the GDAS archive at CPC does not contain the same vertical structure as the CDAS archive. This has produced a gap in the data appearing in the longitude- and latitude-height diagrams (Figs. T29-T32). In these same figures you will also notice that GDAS contains relative humidity at all levels whereas the data from CDAS has no relative humidity data above 300 hPa.

We still have not been able to recover computer files that are used to compute the teleconnection indices (Table E1 and Figs. E6 and E7) and the CCA SST forecasts (Figs. F1-F2).


Tropical Highlights - October 1999

Cold episode conditions continued across the tropical Pacific during October, as sea surface temperatures (SSTs) averaged more than 1.0°C below normal across the central and eastern tropical Pacific (Fig. T18). The equatorial oceanic thermocline remained shallower than normal across the east-central and eastern Pacific during the month, and deeper than normal in the western Pacific (Fig. T15). Consistent with this pattern, temperatures at thermocline depth remained more than 4°C below normal in the eastern Pacific and 1-2°C above normal in the western Pacific (Fig. T17). This thermocline structure in consistent with the ongoing La Niņa conditions.

Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was suppressed over the western and central equatorial Pacific, and enhanced over Indonesia during October (Fig. T25). Convection has been suppressed across the central equatorial Pacific since the onset of cold episode conditions in late May 1998 (Fig. T8). Elsewhere, convection was enhanced over much of the northern Indian Ocean and over the Caribbean Sea, both in association with periods of enhanced tropical cyclone activity (Fig. E4).

Enhanced low level (850 hPa) easterly winds also persisted across the central and western tropical Pacific during October (Fig. T20). Anomalous easterlies have prevailed in this region since May 1998 (Fig. T7), in association with ongoing La Niņa conditions. Elsewhere, low-level westerly wind anomalies again covered the subtropical North Atlantic during the month, consistent with above-normal rainfall across the African Sahel. The upper-level atmospheric circulation (200 hPa) in the Tropics and subtropics also remained consistent with continuing cold episode conditions, with well-developed upper-level troughs observed over the low-latitudes of the western and central Pacific in both hemispheres and amplified subtropical and lower mid-latitude ridges observed across most of the remainder of both hemispheres (Figs. T21, T22).

The Southern Oscillation Index (SOI) was 0.9 during October (Table T1, Fig. T1), and the equatorial SOI was 1.0 (Fig. T2), indicating continued cold episode conditions. The pattern of sea level pressure (SLP) anomalies during the month was also consistent with cold episode conditions, with positive anomalies observed across the tropical Pacific and negative anomalies observed over Indonesia and the Indian Ocean (Fig. T19).

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