The Madden-Julian Oscillation:
A coupled atmopshere-ocean phenomenon?
1. Introduction
2. The oceanic response to convection on intraseasonal
timescales
3. The organisation of convection by intraseasonal SST
anomalies
4. Implications for the MJO
1. Introduction.
1.1 Background.
The Madden-Julian Oscillation (MJO), or intraseasonal oscillation is
the dominant mode of subseasonal variability in the tropics (Madden
and Julian 1971,1972,1994). It is manifested as large-scale eastward
propagating circulation anomalies and associated convective anomalies
with timesclaes of about 30-60 days. The convective anomalies
associated with the intraseasonal oscillation are strongest over the
Indian Ocean, Maritime Continent and warm pool region of the west
Pacific. Over the cooler waters of the eastern and central Pacific and
the Atlantic Ocean, there is very little convective signature. The
oscillation is strongest and most frequent in the boreal winter and
spring. Over the Eastern Hemisphere were the convective signal is
strong, the MJO has a phase speed of around 5 ms-1. Away
from the convective signal the phase speed is about 10ms-1
(Hendon and Salby 1994). There is a considerable amount of variability
in the activity of the MJO, in terms of both the strength of the
anomalies and the number of events in the season (Slingo et al. 1999)
1.2 Theories and Models of the MJO.
Theories and Models of the MJO need to be able to represent and
predict the observed nature of the MJO, in terms of its propagation
characteristics, seasonality and spatial extent of the convective
anomalies. Comparisons of the MJO in general circulation models as
part of AMIP (Slingo et al. 1996) indicated that most GCMs had weak
intraseasonal activity; they tended to simulate slightly shorter
periods than observations and failed to capture the seasonality of the
MJO.
There have been many attempts to develop a theoretical framework to
explain the propagation characteristics of the MJO. Much of this
workhas centred around the modification of equatorially trapped waves
by considering the effect of moist processes, either through the
wave-CISK mechanism (e.g. Lau and Peng, 1987) or by considering the
role of evaporation-wind feedbacks (Emmanuel 1987; Neelin et al
1987). Both of these theories have difficulty predicting the correct
phase speed and spatial scale for the oscillation, but most
importantly, the evaportation-wind feedback mechanism requires basic
state easterlies at the surface. In the region where the convective
signal is acitve the climatological winds are westerly at the
surface.
1.3 Does the ocean play a role in the Madden-Julian Oscillation?
Observations from the Tropical Ocean Global Atmosphere Coupled Ocean
Atmosphere Response Experiment (TOGA COARE) have shown that
SSTs in the warm pool are modulated by the passage of the MJO (e.g
Weller and Anderson 1996; Hendon and Glick 1997; Lau and Sui
1997). The modulation of the SST is related to the change in surface
fluxes associated with the passage of the MJO.
Such observations have led to speculation that the MJO may be a
coupled mode of the atmosphere-ocean system (e.g. Flatau et al 1997;
Sperber et al 1997). Model experiments by Flatau et al (1997) and
Waliser et al (1999) have shown that incorporating a coupling between
the convection and SST can enhance the models representation of the
MJO. Wang and Xie (1998) have developed a simple linear model of the
tropical atmosphere system which, for parameters consistent with the
observed values for the warm pool, exhibits unstable eastward
propagating modes with phase speeds under 10ms-1. The large
scale features of this mode compare well with the structure of the MJO
as observed during TOGA COARE.
The hypothesis that the MJO is a coupled ocean-atmosphere phenomenon
requires that;
- The atmospheric MJO impacts on the ocean.
- Intraseasonal variations in SST can organise convection.
These two issues will be addressed individually, the first through the
analysis of observational data and the second through aquaplanet GCM
integrations.
2. The oceanic response to convection on intraseasonal timescales.
(Woolnough et al. 2000a)
2.1 Data.
The response of the ocean to intraseasonal variations in convection
has been examined using 15 years of data from 1982-1997
inclusive. Outgoing longwave radiation from the NOAA polar orbiting
satellites is used as a proxy for convection. Surface fluxes of latent
heat and shortwave radiation and the surface wind stresses were
obtained from the ECMWF reanalysis and operational analyses. The
Reynold's weekly SST data were interpolated onto a daily timeseries
for consistency with the OLR and surface fluxes. To isolate the
intraseasonal variability a 20-100day bandpass filter was applied. In
view of the very different nature of the tropical circulation and the
behaviour of the MJO during the boreal summer only the period of
October to May is analysed.
Figure 1: 20-100days Bandpass anomalies of OLR, SST, shortwave
flux, latent heat flux and zonal wind stress, for 01 Oct 97 - 01 JUN
88, averaged between 5°N and 5°S. Negative OLR anomalies correspond to enhanced convetion, surface
flux anomalies are postive into the surface.
In figure 1 several eastward propagating OLR anomalies can be seen,
associated with these OLR anomalies there are varitations in the SST,
shortwave flux (SWF), latent heat flux and zonal wind stress (UST).
2.2 Lag-correlations
To investigate whether the relationships between the convection and
surface fields are coherent lag-correlations between the equatorially
averaged OLR and SST, SWF, LHF, and UST were calculated. Figure 2
shows the lag-correlation maps for the 15 seasons treated as a single
time series.
Figure 2: Lag-correlations of OLR with SST, SWF, LHF and
UST. Contours every 0.1, red indicates correlations > 0.1, blue
indicates correlations < 0.1. Positive lags correspond to OLR
anomalies preceding anomalies in the surface fields. The sign
convection is such that postive correlations correspond to enhanced
convection (negative OLR anomaly) being correlated with negative SST
anomalies, reduced SWF, westerly wind dtress anomalies and enhanced
evaporation.
The lag-correlation maps indicate that enhanced convection
associated with reduction in the shortwave flux at the surface as
expected. Between 60°E and 180° positve SST anomalies lead enhanced
convection by about 10 days and negative SST anomalies follow enhanced
convection by about 10 days. Also extending out in the central Pacific
enhanced convection is preceded by easterly wind stress anomalies and
followed by westerly windstress anomalies. In the region where the
climatological winds are westerly at the surface these windstress
anomalies corresopond to reduced evaporation and surface warming prior
to the convection and enhanced evaporation and surface cooling
following the convection. To the east of the dateline where the
surface winds becomed easterly this relationship betwen the convection
and the latent heat flux breaks down.
To investigate the robustness of these relationships from year to year
the lag-correlations where calculated for each season individually and
for 3 El Niño years and 3 La Niña years. In El Niño years the region over which the relationship between the convection, the SST and the LHF holds extends further beyond the dateline, related to the extension of the warm pool and westerly winds into the central Pacific during those years. Similarly in La Niña years the region over which these relationships hold is more confined.
Year to year variations are in these relationships are more difficult
to summarize but can be used to provide a measure of the robustness of
the mean relationship between the convection and the surface
fields. Figure 3 shows a summary of the relationship between the
convection and the surface fields based on the lag at which the maxima
and mininma in the lag-correlations occur, the points are determined
from the single timeseries correlation coefficients and the error bars
from the spread of lags from the individual seasons (see Woolnough et
al. (2000a) for the details).
Figure 3: Summary of the temporal relationships between the convection and
surface fields. The dots indicate the lags at which extrema in
correlation coefficients occur when the 15 seasons are treated as one
time series. Within each region the ordering of the events (relative
to a convective maximum ) from left to right is SWF max, UST min, LHF
max, SST max, SWF min, UST max, LHF min, SST min, SWF max. Here SWF
and LHF positive corresponds to fluxes into the surface. (see Woolnough et al. (2000a) for an explaination of the error bars)
The timing of each event relative to the convective maximum is
consistent from one region to another; High SWF, easterly wind stress
anomalies (giving light total winds), and reduced evaporation lead the
positive SST anomalies, consistent with the surface fluxes generating
the SST anomalies. Following the warm SST anomalies there is enhanced
convection and reduced SWF, and a few days later, enhanced westerly
winds and strong evaporation leading to negative SST anomalies. This
relationship is most robust in the Indian Ocean and West Pacific, but
holds less well over the Maritime Continent region where the surface
processes may be disrupted by the presence of the land. There is some
interannual variability in the timing of these events, but the general
relationship is maintained.
2.3 Composites
Composites of the Equatorially averaged OLR were calculated based on
propagating OLR minima (enhanced convection) every %°: of longitude
accross the Indian Ocean and West Pacific (see Woolnough et al. (2000a)
for the details). Figure 4 shows the time longitude composites for
enhanced convection at 82.5°E (close to the centre of the Indian
Ocean).
Figure 4: Time-longitude composite based on 36 propagating OLR
minima at 82.5°E. a) OLR (contour interval 5Wm-2), b)
SW flux (contour interval 5Wm-2), c) zonal wind stress
(contour interval 0.005Nm-2), d) latent heat flux (contour
interval 5Wm-2), e) SW+LH flux (contour interval
5Wm-2) f) SST (contour interval 0.05°C). In each plot
the blue shading and dashed contours indicate negative values and the
zero contour is suppressed.
The composites confirm the relationship temporal relationship between
the convection, surface fluxes and SST, but also reveal the spatial
structure of the flux anomalies. Associated with the enhanced
convection in the Indian Ocean there is a reduction in the SWF, to the
west of the convection there are westerly wind stress anomalies and
associated with these there is enhanced evaportation and to the east
of the convection there are westerly wind stress anomalies and in the
west Pacific there is an reduction in the evaporation. Out into the
central Pacific where the climatological winds are easterly the
easterly wind stress anomalies are associated with enhanced
evaporation. The surface flux anomalies lead to warm SST anomalies in
the west Pacific, and following the convection cold SST anomalies inA
the Indian Ocean. The composites show eastward progression of the
convective signal and the flux anomalies and show a coherent link
between the Indian Ocean and the west Pacific.
For convection in the Indian Ocean the SW flux anomalies contribute
the largest component of the surface fluxes, but because of the phase
difference between the SW fluxes and LH fluxes both contribute
significantly to the surface fluxes. Similar componsites for enhanced
convection in the west Pacific indiacte that although the basic
structure of the anomalies is the same there are some differnces in
the nature of the anomalies. For convection in the west Pacific there
is little evidence of the easterly anomalies to the east of the
convection nad hence there is no signal there in the latent heat
fluxes, secondly the evaporative cooling has a comparable magnitude to
the SW flux although beacuse of the phase differences between these
two components there is little change in the magnitude of the surface
forcing.
The spatial scale of the SST anomalies is such that there are about
60° of longitude between the maximum and mininmum in SST
anomaly.Although the magnitude of the composite anomalies is small
individual events can have SST anomalies up to 0.5-1°C. The
magnitude of the composite flux and SST anomalies are consistent with
the simple mixed layer depth caluculations with depths between
approximately 10m and 50m, these depths are comparable with the range
observed in the warm pool region.
2.4 Summary
The analysis of the observations has shown that the convection
associated with the Madden-Julian Oscillation can generate SST
anomalies on intraseasonal timescales, through the modulation of the
shortwave and latent heat fluxes at the surface. Positve SST
anomalies are found before and to the east, of the convection and
negative SST anomalies following and to the west of the convection.
3. The organisation of convection by SST anomalies on
intraseasonal timescales
(Woolnough et al. 2000b)
3.1 Experimental Design
To investigate whether intraseasonal SST anomalies, such as those
observed in the Indian Ocean and west Pacific associated with the MJO,
are simply an oceanic response to the convection or are able to
organise the convection on intraseasonal timescales, and hence play a
part in a coupled ocean-atmosphere phenomeon, a series of sensitivity
experiments with an aquaplanet version of the UKMO Unified Model have
been performed.
The aquaplanet configuration of the model retains all the physical
process of the full GCM but the surface is ocean covered. A control
integration is performed with a zonally symmetric SST profile
prescribed, desgined to mimic the SST profile in the warm pool region.
Perturbation experiments are then performed with prescribed SST
anomalies with a magnitude of 1°C and a spatial structure (Figure
5) similar to that of the observed SST anomalies associated with the
MJO. Experiments are performed with a stationary SST anomaly, and with the SST anomalies propagating eastwards with phase speeds corresponding to periods of 30, 60 and 90 days.
Figure 5: Spatial Structure of the imposed SST anomaly
For analysis the fields are composited relative to the centre of the
SST anomaly, for the moving SST anomalies longitudinal diplacements
can be interpreted as time lags.
3.2 The precipitation response
Figure 6 shows the equatorial precipitation anomaly from the zonal
mean for the four experiments. For the stationary SST anomaly the
maximum in precipitation is colocated with the maximum in SST, with
suppressed precipitaton over the cold SST anomaly. For the moving SST
anomalies the precipitation is shifted to the west of the maximum in
SST anomaly. For the moving SST anomalies the faster the propagation
speed the weaker the precipitation response.
Figure 6: Precipitation anomaly (mmday-1) from the
zonal mean, averaged between 2.5°N and 2.5°S composited
relative to the centre of the SST anomaly. Stationary SST anomaly
(black line), and propagating anomalies with phase speeds
corresponding to periods of 90 days (red line), 60 days (green line)
adn 30 days (blue line). The thick black line shows the SST
anomaly.
Examination of the Convective Available Potential Energy (CAPE) and
Convective Inhibition (CIN) shows that the precipitation anomalies are
not just a simple manifestation of the low-level instability. For the
propagating SST anomalies the maxima in CAPE and minima in CIN (the
most convectively unstable situations) are colocated with the SST
maxima, indicating the low-level buoyancy is responds very quickly to
the SST anomaly.
The westard shift of the precipitation and the dependence of the
strength of the anomaly on the precipitation speed can be explained by
considering the specific humidity (Fig. 7).
Figure 7: Specific humidity anomaly (g kg-1) from the
zonal mean for each of the SST anomaly experiments, composited
relative to the centre of the SST anomaly.
The boundary layer adjusts very quickly to the surface anomaly and
the enhanced low-level instability leads to enhanced
convection. However, entrainment of relatively dry air from the
environment reduces the parcel buoyancy and inhibits the
convection. As the convection moistens the environment, the effect of
the entrainment is reduced, and more of the moisture carried in the
convective plume is converted to precipitation. The time taken for the
convection to moisten the environment explains the westward shift of
the precipitation for the propagating anomalies. For the faster SST
anomalies the convection has less time to moisten the lower
troposhpere before the surface anomaly passes and the low-level
instability is removed, leading to the weaker precipitation
response.
3.3 The dynamical response
For the 30 day SST anomaly the dynamical response shows the
characteristic structure of the linear response to tropical heating,
with upper level anticylcones to the west of the convection and
cyclones to the east. The zonal wind anomalies (Fig. 8) show the low
level easterlies and upper level westerlies to the east of the
convection, associated with the Kelvin Wave response. For the slower
moving SST anomalies the structure is very different, the upper level
cyclone pair is weaker and the anticyclones are shifted to the
east. In the zonal winds, the Kelvin Wave signal to the east is much
weaker, despite the stronger heating and the surface westerlies to the
west of the convection are much deeper. At 30 days the forcing period
is close to the natural frequency of the unforced moist Kelvin wave in
the model and the forcing generates a large response in the Kelvin
Wave, at slower freqencies the response in the Kelvin Wave is much
weaker. The abscence of any resonance of the precipitation with the
eastward propagating natural modes of the system suggests that the
dynamical structure is a response to the heating associated with the
convection rather than being responsible for organising the
convection.
Figure 7: Zonal wind anomaly (ms-1) from the
zonal mean for the 30 day and 60 day SST anomaly experiments, composited
relative to the centre of the SST anomaly.
3.4 Summary
The model experiments have demonstrated that eastward propagating
intraseasonal SST anomalies can organise convection with structures
similar to that observed in the MJO. Furthermore the longer the period
of the SST anomalies the greater the magnitude of the precipitation
anomaly.
4. Implications for the coupled MJO
- The observations have shown that the convection can generate SST anomalies.
- The model integrations have shown that intraseasonal SST anomalies with a structure similar to the obsevered SST anomalies can organise convection on intraseasonal timescales
Both components of the system which are required for a coupled mechanism to work have been demonstared here. The timescale of such a coupled mechanism will be determined by the nature of the coupling.
- The strength of the precipitation increases for increasing period
of the SST anomaly
- The strength of the surface fluxes will also increase for
increasing period of the SST anomaly
- The timescale for the SST anomaly is determined by the strength
of the surface flux anomalies.
- Long period SST anomalies will be associated with large surface
flux anomalies and will tend to speed up
- Short period SST anomalies will be associated with weak surface
flux anomalies and will tend to slow down.
The timescale of the coupled MJO will then have a prefered timescale
which is determined by; the dependence of the strength of the
convection and surface fluxes on the timescale of the SST anomalies,
and the heat capacity of the mixed layer.
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