2. Strong intra-seasonal activity associated with the MJO
The March-May 2004 (MAM) season featured a continuation of ENSO-neutral conditions. Although slightly positive sea surface temperature (SST) anomalies were observed near the date line, the main area of above-average SSTs (+1C) remained west of the date line, and the 30C isotherm remained north of New Guinea (near 150E). This distribution of both SSTs and SST anomalies does not indicate the presence of developing El Niño conditions at this time.
The main evolution in SST anomalies during the season was observed over the eastern equatorial Pacific, where SSTs went from slightly below average in March and April to slightly above average in May. This evolution reflected a deepening of the oceanic thermocline in the east-central equatorial Pacific in response to continued strong intra-seasonal variability associated with the Madden-Julian Oscillation (MJO).
The low-level (850-hPa) and upper-level (200-hPa) equatorial zonal winds, 200-hPa streamfunction, 200-hPa velocity potential, and patterns of tropical convection [indicated by the Outgoing Longwave Radiation (OLR) anomalies], also exhibited considerable intra-seasonal variability in association with the MJO.
Above-average surface temperatures in the continental United States, Alaska, Europe, and in a broad area extending from the Middle East to eastern China
Below-average surface temperatures across the eastern half of Canada and central Siberia,
Above-average precipitation in the western U.S. and central Russia, and below-average precipitation in the south-central and mid-western U.S., Alaska, and northern Europe,
Strong intra-seasonal variability over the North Pacific associated with the MJO,
A pronounced negative phase (-3.5) of the Pacific/ North American (PNA) teleconnection pattern in May.
A continuation of exceptionally warm SSTs in the North Atlantic at both high latitudes and the subtropics.
In North America, the MAM season featured above-average surface temperatures in the United States and below-average temperatures in Canada. This anomaly pattern is related to persistent above-average 500-hPa heights across the eastern North Pacific and U.S., and to persistent below-average heights in the Hudson Bay region. In March nearly the entire continental U.S. experienced above-average temperatures. In April the anomalous warmth was concentrated along the northern tier of states, while in May it was concentrated across the South.
In Alaska, much of the anomalous warmth occurred during April and May in association with broad southwesterly flow upstream of a persistent 500-hPa ridge axis. Canada experienced most of its exceptionally cold weather during these months in response to anomalous northwesterly flow between the upper-level ridge and downstream trough axes. During May these conditions were partly related to a record negative phase of the PNA teleconnection pattern.
Intra-seasonal variability over the North Pacific
The circulation over the subtropical and extratropical North Pacific continued to reflect very strong month-to-month variability in response to the MJO. During March and May suppressed equatorial convection was observed over and east of the date line, which contributed cyclonic 200-hPa streamfunction anomalies in the subtropics of both hemispheres east of the date line. This anomaly pattern is associated with a retraction of the mean subtropical ridges toward the western Pacific in both hemispheres. In the Northern Hemisphere these anomalies were associated with a weakening of the East Asian jet in the vicinity of the date line, along with increased upper-level diffluence and an enhanced thermally-indirect transverse ageostrophic circulation in the jet exit region. North of the jet above-average 500-hPa heights covered the high latitudes of the North Pacific.
During April enhanced convection in the vicinity of the date line contributed to a strengthening and extension of the East Asian jet stream to well east of the date line. Consistent with these circulation anomalies, below-average 500-hPa heights covered the high latitudes of the North Pacific.
A particularly interesting aspect of the circulation occurred during May, when the strongest negative phase (-3.5 std. dev.) of the Pacific/ North American (PNA) teleconnection pattern was recorded for that month. Key aspects of this pattern included above-average 500-hPa heights over the Gulf of Alaska and southern U.S., and below-average heights over the central subtropical North Pacific and Canada. Associated with this anomaly pattern the mean upper-level ridge and trough axes were located over Alaska and western Canada, approximately 30 west of their climatological positions over the Rocky Mountains and eastern Canada, respectively.
Over North America the negative PNA pattern was associated with an anomalous jet core across the northern U.S. and southeastern Canada, which separated near-record cold surface temperatures (2-5 below average) across most of Canada from anomalously warm temperatures (1.0-1.5C above average) across the southern and central United States. In Canada surface temperatures were generally in the lowest 10th percentile of occurrences. This exceptionally cold air mass is related to the northwesterly flow of arctic air from Alaska and the Beaufort Sea into western Canada in the area downstream of the mean upper-level ridge axis.
The anomalous circulation across North America also contributed to above-average rainfall across the northern United States. The largest surpluses were observed in the Midwest and Great Lakes regions where area-averaged totals reached the 90th percentile of occurrences. These regions were situated downstream of the mean upper-level trough axis within the region of enhanced surface temperature contrast.
Over the North Atlantic the mean 500-hpa circulation during MAM featured above-average heights in both middle and high latitudes. This circulation reflected a disappearance of the mean Icelandic Low similar to that observed during the previous season (December 2003-January 2004). This pattern was again associated with a pronounced split flow configuration over the eastern Atlantic and Europe. In addition to a continuation of anomalous warmth across Europe, this circulation contributed to a dipole pattern of precipitation anomalies over the continent, with northern Europe experiencing below-average precipitation downstream of the mean ridge axis, and southern Europe/northern Africa experiencing above-average precipitation within the enhanced southern branch of the jet stream and its associated storm track.
Also over the North Atlantic the MAM season featured a continuation of exceptionally warm SSTs at both high latitudes and the subtropics, in association with the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001). The anomalous warmth is also consistent with the disappearance of the mean Icelandic low and with a resulting decrease in rainfall across the high latitudes of the North Atlantic.
The Middle East to eastern China experienced exceptionally warm conditions during MAM 2004, with temperatures generally 1-2C above average throughout the region. Much of this anomalous warmth was observed during March and April when temperatures in many regions were 2-4C above average. Much of China also experienced well above-average surface temperatures during February.
During March the anomalous warmth was associated with above-average 500-hPa heights extending from northern Africa to central China. During April the upper-level circulation featured an anomalous split-flow configuration over eastern Asia, a below-average strength of the East Asian jet across China, and an anomalous eastward shift of that jet entrance region to Japan. These conditions contributed to exceptionally warm conditions over China, with temperatures in many areas exceeding the 90th percentile of occurrences.
Above-average precipitation to end South African rainy season.
Anomalously warm surface temperatures covered South America during March and April. This warmth was associated with a persistent pattern of positive 500-hPa height anomalies both northwest and northeast of South America, and a persistent trough located between these two features and situated just upstream of the continent. The resulting anomalous southwesterly flow downstream of the trough axis is a main contributor to the anomalous warmth during these months. In contrast, the southern half of South America experienced significantly below-average surface temperatures during May, with temperatures east of the Andes Mountains falling within the lowest 10th percentile of occurrences. These cool temperatures are related to the significant weakening of the westerly winds and associated downslope flow equatorward of a strong upper-level ridge centered over southern Argentina.
Surface temperatures during MAM averaged 1-2C above average in eastern Australia. The anomalous warmth was observed during March and April, with below-average temperatures covering southeastern Australia in May. During May the circulation reflected a strengthening and equatorward shift of the upper-level jet stream. Southeastern Australia experienced upper-level convergence and descending motion within the right entrance region of the jet, which resulted in below-average precipitation during the month. An anomalous inland penetration of cold air at 850-hPa from the Great Australian Bight was also evident during May, which resulted in anomalously cool surface temperatures across southeastern Australia.
In southern Africa the rainy season normally lasts from October to March. For the entire rainy season, area-averaged rainfall totals were above normal only during January and March. During March area-average totals approached the 90thpercentile, with the largest surpluses occurring across central and eastern South Africa, Mozambique, and Tanzania.