A relationship between submonthly variability on the Asian subtropicaljet and convection over China during Meiyu season (June – July)
Hatsuki Fujinami
Hydroshperic Atmospheric Research Center, Nagoya University,
Chikusa-ku, Nagoya, 464-8601, Japan

1. Introduction
Submonthly (6 – 25 days) variability(Vincent et al. 1998) is a dominant
intraseasonal mode over the Asian subtropicaljet during boreal summer (e.g. Terao 1998,Fujinami and Yasunari 2004). During June –July, convective activity (e.g. Meiyu front)
becomes active over eastern China in theseasonal march. Some studies investigated a
relationship between submonthly convective
variability over the western North Pacific andMeiyu front (e.g. Chen et al. 2000, Fukutomi
and Yasunari 1999). However, an effect ofsubmonthly variability over the Asian jet on
convective activity over China has not been
investigated precisely. Therefore, the objectivesof this study are 1) to examine characteristics ofthe submonthly intraseasonal variability on thesubtropical jet over China and 2) to show arelationship between the submonthly variability
and convection over China.
2. DataDaily averaged interpolated outgoing
longwave radiation (OLR) data produced by theNational
Oceanic
andAtmospheric
Administration (NOAA) Climatic Diagnostics
Center (CDC) are also used as a proxy forconvection (Liebmann and Smith 1996). Daily
averaged National Center for Environmental
Prediction (NCEP)/National Center for
Atmospheric Research (NCAR) reanalysis data
represent the large-scale circulation (Kalnay etal. 1996) from 1979 through 2004. Both OLRand reanalysis data are defined on global 2.5° x
2.5° grids.

3. Results
In this study, 7–20-day filtered meridionalwind (v) at 200 hPa over the base region (30° –40°N, 110° – 120°E) was used as an index tostudy submonthly variation of the subtropicaljet over China. 7–20-day band was chosen as
the submonthly timescale. This band was also
used in Fujinami and Yasunari (2004) to study
submonthly variation of the Asian jet aroundthe Tibetan Plateau.
Figure1 shows the interannual variation of
the 7 – 20 day variance of meridional wind (v)at 200 hPa over the base region. There is alarge interannual variation in the variance atthis timescale. We use top 5 years (1986, 1989
1993, 2001 and 2004) to examine strong
submonthly signals on the jet.

Fig.1: The interannual variability of meridional
wind (v) variance in the 7 – 20-day band over thebase region (28°–34°N, 85°–94°E).

Fig.2: 5-year averaged (1986, 1989, 1993, 2001, 2004)
7–20-day filtered 200-hPa meridional wind variance forJune–July. Shading represents the percent of the total
variance explained by the 7–20-day band.

Figure 2 shows 5-year averaged 200-hPa
meridional wind variance with timescales of
7–20 days during June–July. Shading denotes
the percent of the total variance explained by
the 7–20-day band (without the seasonal cycle).
Variance peaks appear to the north of theTibetan Plateau and around 35°N, 120°E along
the westerly jet. The 7–20-day varianceaccounts for about half the total variance overthese regions.
Temporal variation in the circulation andconvection associated with 200hPa submonthly
meridional wind fluctuations over the baseregion was examined using a time composite
technique. Eighteen cycles from June – July
were selected for the composite. Eight phases
comprise each cycle. Phases 3 and 7 correspondto the southerly and northerly extremes of
meridional wind over the base region. The
mean interval between phases is about 1.8 days.
Composite fields were made for each phase.
Figure 3 shows the evolution of
streamfunction and wind vector anomalies at
200 hPa from Phase 6 to 1. Wind vectors areplotted where either the u or v component
exceeds the 99% local confidence level. From
Phase6 to Phase7, the wave anomalies growdownstream from 40°N, 70°E to 40°N, 140°E,
suggesting Rossby wave energy propagation tothe east. The wave packet also moves eastwardslowly. In Phase7, when northerly wind
anomaly reaches maximum over the baseregion, anticyclonic circulation develops to thewest of the base region. In Phase 8, a cyclonic
circulation deepens around 30°N, 40°E. An
anticyclonic
circulation
also
develops
downstream of the cyclonic anomaly. From
Phase8 to Phase1, wavelike structures andsuccessive downstream development of newcenters of action occur from 40N, 40E. The
Phase 1 circulation pattern has approximately
reversed Phase 6.
Figure 4 shows successive composite mapsof the 7–20-day OLR anomalies from Phases 6
to 1. Open circles denote locally statisticallysignificant grids at the 99% confidence level. In

Phase 6, significant active convection signals
appear over the Taklamakan Desert and thenorthern Tibetan Plateau. The negative
anomalies are associated with the upper-levelsoutherly anomalies ahead of a trough.
Suppressed convection is observed over theplains of eastern China (around 32.5°N,117.5°E). Areas of suppressed convection
corresponds that of upper-level northerlies.
From Phase 6 to Phase 8, active convection
anomalies move eastward along the northernFig. 3: Spatial distribution of composite
streamfunction and locally statistically significant
wind vector anomalies in the 7–20-day band at 200hPa from Phase 6 through Phase 1. Shading
denotes the Tibetan Plateau.

fringe of the Tibetan Plateau. By Phase 1,
active convection anomalies reach the plains ofeastern China. In Phase 8, suppressed
convection anomalies develop over the westernpart of the Tibetan Plateau. The suppressed
convection signals, then, move eastward asphase progresses. It should be note thatsignificant convection anomalies extend mainly
over the midlatitudes. This result indicatessignificant OLR anomalies over China in thisstudy are induced by the midlatitude systems.
There is little relationship with submonthly
convective variation over the subtropics.
I also examined the submonthly variability of
the lower atmosphere and a process of theenhancement (suppression) of submonthly
convection over China.
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Fig. 4: Spatial distribution of composite OLR
anomalies in the 7–20-day band from Phase 6
through Phase 1. The contour interval is 5 W m2.
The zero contour is not plotted. OLR anomalies
less than -5 W m2 are shaded. Open circles indicate
locally statistically significant grids.