Regime Change Behavior During Asian Monsoon Onset

As the ITCZ moves off the equator the Hadley circulation transitions from an equinoctial regime with two near symmetric, significantly eddydriven cells, to a monsoon-like regime with a strong, thermally direct crossequatorial cell, intense low-latitude precipitation, and a weak summer hemisphere cell. Dynamical feedbacks appear to accelerate the transition. This study investigates the relevance of this behavior to monsoon onset by using a primitive-equation model in two aquaplanet configurations and a configuration with a realistic configuration of Earth’s continents and topography. A change in the relationship between ITCZ latitude and overturning strength is identified once the ITCZ moves poleward of about ∼ 10◦. In the monsoon regime this relationship is similar in all simulations, suggesting that similar dynamics are occurring. Monsoon onset is associated with off-equatorial ascent, in regions of non-negligible planetary vorticity, and this is found to generate a vortex stretching tendency that reduces upper level absolute vorticity. This causes a transition to a cross-equatorial thermally direct regime, intensifying the overturning circulation. Analysis of the zonal momentum budget suggests a stationary wave, driven by topography and land-sea contrast, can trigger the transition in the more realistic model configuration, with the wave extending the ascent region of the Southern Hemisphere Hadley cell northward, with a thermally direct overturning then developing to the south. These two elements of the circulation resemble the East and South Asian monsoons. 8


Introduction
Prescribed, seasonally varying ocean heat-fluxes (Q-fluxes) are additionally included in the full configuration. These were derived from the atmosphere model intercomparison project (AMIP) 145 SSTs following the methodology outlined in Russell et al. (1985). The resulting fluxes have a 146 comparable structure to the net surface heat flux seen in reanalysis (e.g. compare Fig. 1 with p. 147 12, Kållberg et al. 2005). The addition of Q-fluxes helps to produce a climatology resembling that 148 of Earth.  Comparing the full experiment with the aquaplanets allows zonally symmetric and asymmetric 160 processes to be distinguished. In ap2 the peak precipitation shifts quickly between hemispheres. 161 Precipitation is strongest directly after a hemispheric shift. The rainfall is more intense than in 162 the full simulation, and is associated with larger shifts of the ITCZ from the equator. The strong 163 precipitation, and sharp transition of the ITCZ from southern to Northern Hemisphere, show that 164 zonal asymmetries are not essential for monsoon-like behavior. In contrast, in ap20, peak pre-165 cipitation undergoes much smaller excursions from the equator. This simulation does not exhibit 166 other features of monsoon onset, with no reversal of the zonal wind direction (not shown). This experiment is presented as an example of a climate that remains in the equinoctial regime. 168 To allow behavior before and after monsoon onset to be compared, 20 day periods, prior to and 169 post onset, have been selected for the full and ap2 experiments. For each latitude and longitude 170 over the monsoon region, the first 5 day period (pentad) at which precipitation exceeds 8 mm/day 171 was identified. This was used as an indicator of the arrival of the monsoon. Maps of this onset date 172 (not shown) were then used to identify pentad ranges before and after the monsoon rains develop 173 over South and East Asia, or over the equivalent latitudes for ap2. For both full and ap2, the 174 monsoon is well established by mid July, and a 20 day period between the 16th of July and 5th of 175 August was selected (note the use of 30 day months in the experiments). As can be seen from Fig.   176 2, monsoon onset is earlier and more gradual in full than in ap2, suggesting that land-sea contrast 177 and topography advance onset. Pre-onset periods were selected as 1st-20th of June for ap2, and 178 1st-20th April for full. 179 Before monsoon onset, in both the ap2 and full simulations, two overturning cells are seen, 180 roughly centered around the equator (Fig. 3). Strong, westerly zonal jets are found in both to the north of this, with air descending on the poleward side of the Tibetan Plateau and warm-188 ing at lower levels. In both simulations, after onset, eddy momentum flux convergence (colors) is enhanced in the tropics at upper levels, indicating that the circulation is not axisymmetric. If 190 absolute vorticity is low, it may, however, still be near to thermally direct, cf. Eq. (1).

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Moist static energy (MSE) is also plotted in Fig. 2, which is defined: where c p is the specific heat capacity of dry air at constant pressure, T is temperature, L is the 193 latent heat of vaporization of water, q is specific humidity, g is the gravitational constant, and 194 z is height. In all simulations the strongest precipitation lies close to and slightly equatorward  Figs. 2 and 3 suggest some relationship between the strength of the meridional overturning cir-208 culation, Ψ, (and its associated precipitation), and the displacement of the ITCZ from the equator.

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The strongest precipitation occurs when the ITCZ is furthest north. Fig. 4 shows the peak strength 210 of the meridional overturning circulation associated with the winter cell, Ψ max , versus the latitude, 211 φ 0 , at which Ψ drops below 120 × 10 9 kg/s, both at 500 hPa. The latter is taken as indicative of 212 the northward extent of the winter cell, and will be referred to as the ITCZ latitude. A non-zero 213 threshold was chosen to isolate the southern component of the double overturning circulation over 214 the Asian monsoon region in full, seen in Fig. 3. Each point on the plot corresponds to a multi-year 215 pentad mean, so that the points closest to the equator correspond to the equinoctial season, while 216 those furthest from the equator correspond to Northern Hemisphere summer. We note that the 217 peak strengths in this figure are stronger than those found in reanalysis (e.g. Kållberg et al. 2005 The dashed lines show least squares best fits between the natural logarithms of both quantities.

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For the purposes of fitting, the data is divided into ITCZ latitudes north and south of 10 • N (this   The sign of these changes corresponds to a reduction in magnitude in both hemispheres. However, 265 an increase in absolute vorticity magnitude is found to the south of the Tibetan Plateau, between 0 266 and 30 • N.

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The causes of these changes can be investigated using the vorticity budget: where ζ is relative vorticity, u is the horizontal wind vector, and ω is the pressure velocity. From term. Note that over these pentad means, relative vorticity is evolving and is not in steady state.

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Imbalances in the budget may therefore be interpreted as the driver of changes to the upper level 280 absolute vorticity.

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When the ITCZ crosses the equator, both tendencies change sign. In Northern Hemisphere 282 summer, the tendency from horizontal advection is positive, and strongest close to the equator.

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Looking more closely at the structure of the advective and stretching vorticity tendencies, it can 292 be seen that monsoon onset and withdrawal are marked by peaks in both terms. We propose that 293 these peaks, at ∼ 10 • N, connect to the regime change seen in Fig. 4 the overturning, as seen in Fig. 4. The region of ascent expands as the ITCZ moves poleward, 302 so that the divergence is weaker and occupies a larger area. However, the stretching tendency is 303 still sufficient to lower absolute vorticity over a broad area (cf. Fig. 3), further strengthening the 304 circulation.

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The peak in the stretching term provides a physical justification for the 10 • N threshold used for 306 the fitting in Fig. 4. Once ascent moves sufficiently far from the equator that absolute vorticity 307 is not negligible, the stretching tendency increases. This lowers absolute vorticity over a larger 308 region, triggering a change to a thermally driven circulation.

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The peaks of the horizontal advection tendency also relate to the narrow near-equator ITCZ.

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The leading order component of this is v∂ (ζ + f )/∂ y. When the ITCZ is close to the equator, (1), driven by the feedbacks described above. While the present 322 paper discusses the processes involved over monsoon onset in a time-varying case, the steady state 323 budget will be explored further in future work.

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The key features of the ap2 vorticity budget are shared with the full experiment (Fig. 8). The 325 reduction in absolute vorticity magnitude during monsoon onset is less marked in full than in ap2, 326 but a broadening of the low magnitude region is still observed in Northern Hemisphere summer.
Horizontal vorticity advection and vortex stretching are again the dominant terms in the budget, 328 and act to reduce the magnitude of the southern and Northern Hemisphere absolute vorticity re-329 spectively. Differences from ap2 can, however, be seen, which relate to the structure of the land Panels e and f), similar behavior to ap2 is found. Divergence is strong along the equator, but, as 339 absolute vorticity is small here, vortex stretching is weak. Once ascent is forced further to the 340 north, stretching strengthens, and acts to lower absolute vorticity.
where u, v and ω are the zonal, meridional and vertical winds in pressure coordinates, F (x) de-362 scribes frictional damping, and Φ represents geopotential. As in Eq. (1), square brackets indicate  390 3), suggesting it relates to instability associated with the easterly jet. Regions of stronger eddy 391 activity are associated with regions of higher magnitude absolute vorticity, e.g. Eq.

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The equivalent budget for the full simulation is shown in Fig. 10. Eddy quantities are here 394 defined relative to a zonal mean state spanning 60-150 • E. As in ap2, the arrival of monsoonal Asian monsoon regions lie in the area of this wave where the sign of the geopotential gradient is 415 such as to extend the overturning circulation further north, as seen in Fig. 3. South of 20 • N, where 416 geopotential gradient is weak, and streamlines are roughly oriented east-west, the wave forces a 417 localized thermally direct circulation, with dynamics similar to that of the aquaplanets.

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The Asian monsoon is often considered to consist of two interacting systems, the South and strength per degree latitude can be described by a power law, with exponent 0.10±0.07 (Fig. 4).

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After a threshold latitude of ∼ 10 • N is passed, a larger increase in strength with latitude is ob-440 served, and the best-fit exponent increases to 0.33±0.11. The uncertainty ranges confirm that the 441 two power laws are significantly different, and that some change in behaviour occurs as the ITCZ can be expected to be sensitive to changes to midlatitude stationary waves, with changes to the 474 South Asian monsoon then occurring as a consequence of this.

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These results reconcile some of the apparent disagreement between the regime changes previ-476 ously discussed. The key process for monsoon onset is demonstrated to be a reduction of upper 477 level absolute vorticity, but this may be caused by a variety of mechanisms. Shallow aquaplanets 478 respond strongly to the seasonal cycle, shifting the ITCZ off of the equator and instigating a feed-479 back which lowers absolute vorticity (cf. Bordoni and Schneider 2008). In this case, we find that 480 the resulting circulation is more thermally direct, but not necessarily axisymmetric, with low lati-481 tude eddy activity not appearing to hamper the transition (e.g. Fig. 3). Stationary planetary waves 482 have previously been shown to be able to produce an absolute vorticity reversal, which results in a 483 thermally direct circulation over areas with a broad region of low absolute vorticity (Shaw 2014).

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This case is more relevant to our more realistic experiment, where the cyclone that forms over the 485 low pressure region associated with the Tibetan plateau seems responsible for the onset of a low 486 latitude, thermally direct circulation, localized over Asia.

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This study raises several avenues for future work. Although we have established that ascent 488 over the Asian continent is important in triggering monsoon onset, the individual roles of land-sea 489 contrast and topography, and the effect of continental geometry, remain to be investigated. While  Averages are over all longitudes for the aquaplanets, and between 60 and 150 • E for full.

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The 8  (    7. Schematic summarising the behaviour in the equinoctial regime and over monsoon onset. Shading indicates absolute vorticity, arrows indicate the direction and strength of the overturning circulation. a) Equinoctial regime: Ascent occurs close to the equator where absolute vorticity is near zero. Off the equator, absolute vorticity and its gradient are non-negligible, but the weak zonal mean tendencies are balanced by eddies. The total vorticity tendency is small, and the circulation is near to the 'weak overturning' steady-state solution of Eq. (1). b) At monsoon onset: Ascent occurs off the equator where absolute vorticity is non-negligible. This results in a negative upper level vorticity tendency due to vortex stretching in the Northern Hemisphere, reducing the magnitude of the absolute vorticity. The circulation begins to transition to the thermally direct steady-state solution of Eq. (1), and the overturning strengthens. The cross-equatorial meridional flow is associated with a positive vorticity tendency due to horizontal advection, which reduces absolute vorticity magnitude in the Southern Hemisphere. The vorticity tendencies due to vortex stretching and horizontal advection therefore both act to further reduce the magnitude of absolute vorticity. This positive feedback allows overturning strength, and the associated precipitation, to increase rapidly over onset.
∂ p , (f) ∂ u/∂t. All quantities are multi-year pentad and zonal averages. Units are ms −1 day −1 . The 8 mm/day precipitation contour is marked on all plots in gray as an indicator of monsoon onset. Absolute vorticity is overplotted in black on (b), with a contour interval of 2 day −1 .