Inertial instability flow in the troposphere over Suriname during the South American Monsoon

Abstract

[1] Weekly sonde observations in Suriname, supported by ECMWF analyses and a linear stability analysis, are used to analyze the recurrence of inertial instability as the ITCZ migrates over land during the South American Monsoon. A layer of cool moist air from the Atlantic Ocean is then advected southward over Suriname in the shape of a cold front, displacing the warmer air over the continent. The return flow northward, by the upper branch of the Hadley cell, is a region where inertial instability pervades due to cross-equatorial advection of anticyclonic vorticity and the proximity of the subtropical jet as it migrates closer to the Equator. This unstable region evidently leads to the episodical formation of a meridional sub-cell below the tropopause, where the damping is calculated to be strong enough to stabilize flow at smaller vertical scales, and yet weak enough to allow the observed cell recurrence - at approximately the inertial frequency of the underlying latitudes. This instability should also contribute to the Hadley cell formation through northward acceleration in the upper branch. The moist saturated conditions in the lower troposphere do allow inertial instability here, but the high damping values within the boundary layer suggests that the observed southward acceleration in the lower branch of the Hadley cell has a cause other than inertial instability.

1. Introduction

[2] Suriname, one of the Guyana countries at the northern coast of South America, lies approximately in the middle of the annual migration range of the inter-tropical convergence zone (ITCZ) in this region, and hence experiences two wet and two dry seasons. Weekly balloon sonde releases at Paramaribo station (5.8°N 55.2°W) started in September 1999, measuring morning (around 13h00 GMT) profiles of ozone, temperature, water vapor and wind. These sonde observations are used to study the structure of the ITCZ, and its associated large-scale circulation, as it migrates over land during the South American Monsoon. Of particular interest is the regular occurrence of inertial instability in the upper troposphere above Suriname during this period. Inertial instability in the northern hemisphere (NH) occurs when the potential vorticity (PV) of air becomes negative, such that a disturbance experiences a meridional poleward or equatorward acceleration in the unstable area. A study by Stevens [1983] demonstrates that the north-south symmetry observed in zonal flow in the tropical atmosphere and oceans is not accidental, as a horizontal wind shear near the Equator will induce a negative PV that results in inertial instability and a flow that counteracts the wind shear. This flow will be one of mass overturning through vertically stacked meridional circulation cells, if the conditions for zonal symmetric instability are met, as first indicated from theory by Dunkerton [1981], hence called D81. Apart from evidence of such flow occurring in the extratropical troposphere [Bennetts and Hoskins, 1979; Emanuel, 1983], it has also been identified near the Equator by Hitchman et al. [1987], as a so-called “pancake structure” of alternating temperature extrema in the lower mesosphere. The current study presents evidence of inertially unstable flow occurring in the tropical troposphere over Suriname during the monsoon period, when the subtropical jet migrates closer to the Equator in NH winter.

2. Structure of the ITCZ During the Monsoon Period

[3] The chronology of wet and dry seasons experienced in Suriname can be traced in the recorded water vapor sonde profiles versus time in Figure 1. The onset of the short wet season in December occurs almost simultaneously with the transition of the trade winds from southeasterly to northeasterly flow (Figure 2, indicated with an “A”). In contrast, the onset of the long wet season around April happens well in advance of the reverse transition in meridional wind around July. When the ITCZ comes from the north and first reaches the northern coast of South America around Nov–Dec, there is an initial rapid outflow of moist air over the Amazon basin to the south as the locus of precipitation suddenly shifts to the thermal low created over land - as is typical for a monsoon system. Convection over Paramaribo is suppressed by subsidence on the equatorward flank of the North Atlantic high during the short dry season from Feb–Mar [Hastenrath, 2000], when the ITCZ reaches its southern-most position around the Equator. From this position, the rainy band then slowly retreats back northward along with the ITCZ over the Atlantic Ocean, resulting in the long wet season from Apr–Jul. The northeasterly flow in the boundary layer during the monsoon period consists of cooler air, which, coming from the Atlantic Ocean, is rich in moisture and produces abundant rain over the warmer continent. One would expect the cooler air to have a wedged shape as it displaces the warmer air over the continent. Assuming a gradual northward migration after the ITCZ has reached its southernmost point, a pole-ward slope of this wedge (from Figure 2, marked “B”) can be inferred of ∼1:500, which agrees well with the slope inferred from the ECMWF case study presented later on (cf. Figure 5c). The induced deep convection often penetrates through the boundary layer, up to maximal heights of around 12 km. At this height, a distinct northward flow is apparent (Figure 2, within the “C” area) corresponding with the upper branch of the Hadley cell.

Figure 1

Time-height cross section of relative humidity from sonde observations over Paramaribo; the inferred tropopause height is in bold black. The markings (in yellow) are described in the text. The first day of each month is marked on the x-axis.

Figure 2

a) Observed zonal wind (top), and b) meridional wind (bottom) over Paramaribo, similar to Figure 1. The markings (in yellow) are described in the text.

3. Inertial Instability During the Monsoon Period

[4] Since the start of the Paramaribo observation program, collocated ECMWF analysis profiles were calculated each day. In general, the ECMWF wind profiles compare very well with the Paramaribo observations. Of particular interest is the fact that the northward Hadley branch is most often flanked, directly above and below, by southward flow (Figure 2a, square area marked with a “C”), and that this persistently coincides with negative PV values (Figure 3). This raises the question whether the return flow could be a manifest of the vertically stacked cellular flow predicted by D81 for symmetrical inertial instability, as is schematically depicted in Figure 4. This issue is investigated further with ECMWF operational analyses over a 10-day period, when the conditions for secondary flow resulting from symmetric inertial stability seem to be clearly met: from February 23 to March 3, 2000. During this period, there is a clear anti-correlation between meridional and zonal wind in the upper troposphere (Figure 2, at the “C” mark), a feature also used as an indicator by Hayashi et al. [2002] to mark the occurrence of symmetric inertial flow around the stratopause. Similar to their analysis, we will study the averages over a 10-day period in order to filter out transient disturbances on shorter timescales.

Figure 3

Time-height cross section of ECMWF potential vorticity over Paramaribo. The markings (in yellow) are described in the text.

Figure 4

Cartoon of the ITCZ and the induced meridional circulation cells resulting from negative PV (blue shaded) in the upper troposphere during the South American Monsoon. The pink arrows indicate zonal wind (westerly pointed downward), the Equator is at y = 0 and Paramaribo at Y_o.

[5] A latitude-height cross-section of the meridional and zonal wind field at the Paramaribo longitude (55°W) is shown in Figures 5a and 5b. Figure 5b also shows the wind vectors composed of the meridional wind and an enhanced (10×) vertical wind velocity. It is evident that the strongest northward winds of the upper Hadley branch coincide with the region of maximal zonal wind shear caused by the subtropical jet at the same altitude to the north. Above the Hadley cell, a sub-cell can be distinguished directly underneath the tropopause, with a latitudinal span between the Equator and 12°N and a maximal southward velocity around 7°N. The location of the meridional wind maximum southward (at 150 hPa) and northward (250 hPa) of this sub-cell approximately coincides with the zero PV line. Viewing the days of the 10-day episode separately, it is found that on 5 of the 10 days the sub-cellular structure can be identified, and on 3 occasions the entire sub-cell lies submerged in the negative PV region, with local meridional wind maxima about halfway between Equator and the outer edge of the unstable layer. This is conform the analytical two-dimensional solution of D81 for symmetric inertial instability, predicting stacked cellular flow contained within the unstable area and maximal meridional velocities at the latitudinal middle, shifting towards the Equator as the vertical extent of the unstable domain increases [Stevens, 1983]. However, the linear stability analysis of D81 also predicts the highest growth rates for infinite small vertical scales. Stacked flow with a finite vertical extent only becomes possible when one assumes that viscosity or eddy diffusivity locally stabilizes flow at smaller vertical scales, as suggested by D81. Assuming a Prandtl number and diffusivity equal to unity, and then subsituting the ECMWF parameter values of this case study in equation 2.6 of D81, gives an almost uniform vertical wavelength at which maximal growth occurs: between 2.2 to 3 km for the entire unstable region (not shown). As a second indication, one can also calculate the maximum vertical wavelength at which growth of the unstable flow is still theoretically possible:

as can be inferred from the dispersion relation in D81 for zonal symmetric disturbances of inviscid flow on a β-plane. Calculating equation (1) from ECMWF values for our case study, Figure 5a (bold dashed contours within the negative PV region) yields a vertical wavelength range of 2–10 km within the unstable area. This shows that, in principle, symmetric instability can grow on the vertical scale observed for the upper tropospheric sub-cell, given that viscosity or eddy diffusivity stabilizes the flow on smaller vertical scales. For the months Jan–Mar, when the deepest convection occurs near the Equator, it is found that the sub-cell reappears about each 5 days or so and lasts for 2–3 days. This is consistent with the inertial frequency (≅f, the Coriolis parameter) at this latitude.

Figure 5

ECMWF pressure (hPa) - latitude cross sections at 55°W, averaged over the period 23/2–3/3/2003, for: (a) meridional wind (colored contours), the zero line for PV (bold solid) and PV_eq (bold dotted), maximum vertical wavelength (see text, bold dashed in km); (b) zonal wind (colored contours), vectors (green) composed of meridional and (10×) vertical wind, minimal horizontal wavelength (see text, bold dashed in °longitude); (c) temperature difference with the meridional mean; (d) percentage relative humidity. Paramaribo station is indicated with a light blue vertical line.

[6] The sub-cell is mirrored below by a much bigger cell, the Hadley cell, which is confined to approximately the same latitude band and lies in a vertically split domain where either the PV (upper troposphere) or the PV_eq (lower troposphere) is less than zero. Bennetts and Hoskins [1979], and later Emanuel [1983], showed that symmetric inertial instability is likely to occur in a moist baroclinic atmosphere under the same conditions as for a dry atmosphere, except that a PV_eq (instead of PV) less than zero is now the criterion for the occurrence of inertial instability. Figure 5 shows (PV_eq, bold dotted) that the lower troposphere now also becomes unstable, which raises the question to which extent inertial instability plays a role in the formation of the Hadley cell. The equator-ward position of the southward velocity maximum in the lower branch of the Hadley cell is in agreement with the larger vertical extent of the Hadley cell, as mentioned earlier [Stevens, 1983]. However, the study by Tomas et al. [1999] argues against an acceleration by inertial instability within the boundary layer due to strong damping here, indicating that the observed southward acceleration has another cause which clearly needs further investigation. Calculations for the upper troposphere do however suggest that the upper branch of the Hadley cell will feel the northward acceleration by inertial instability that also feeds the upper sub-cell.

[7] Symmetric unstable flow as discussed above is valid for the specific case where the disturbance has no longitudinal dependence and results in a uniform vertical mass overturning over the entire unstable zonal band. In practice, a zonally asymmetric disturbance will often happen, which for the same background conditions as discussed before leads to a non-linear solution first presented by Dunkerton [1983]. Well-behaved solutions were found to exist only for zonal wave numbers below a certain shortwave cutoff value corresponding with . For discrete wavenumbers with respect to the spherical earth, a maximum growth rate between the symmetric value (k = 0) and the cutoff value is then possible. This minimal horizontal wavelength is given by the bold dashed contours within the negative PV region of Figure 5b, showing a range of 20 to 40 degrees longitude within the unstable zone. In this and other cases studied so far, the zonal extent of the unstable region is narrower than these cutoff wavelength values, indicating that a symmetric inertial flow will be enforced within the unstable region.

[8] Figure 5c shows the temperature difference from the meridional mean (25°S–25°N) for each pressure layer. The latitudinal slope (1:500) of the cooler boundary layer inferred earlier for the monsoon period can be viewed as being approximately 3 km depth over a 1500 km latitude span. It is clear that the cooler air coming from the Atlantic Ocean is advected over the warmer continent to just south of the Equator, where deep convection occurs and the upward branch of the Hadley cell is formed. These features are reflected in the ECMWF humidity field of Figure 5d. Interestingly enough, a dryer layer can be distinguished at the location of the sub-cell, with a similar meridional extent and depth. If water vapor is regarded as a passive tracer in the upper troposphere, this dry cell could be a signature of the meridional sub-cell as moisture is first advected northward by the upper Hadley branch, and then upward and southward just below the tropopause, leaving the stagnant air in the middle of the cell to dry. Unfortunately, humidity measurements of radiosondes become unreliable at these high altitudes, so that this signal cannot be confirmed by sonde observations.

4. Conclusions

[9] Paramaribo observations, supported by ECMWF analyses, show the systematic recurrence of an inertially unstable domain in the upper troposphere over Suriname during the South American Monsoon, reaching from the Equator towards the subtropical jet. This unstable field is the result of the subtropical jet migrating towards the Equator during NH winter (raising local vorticity above local Coriolis values), or comes from cross-equatorial northward transport of anticyclonic vorticity (from the SH upper-air anticyclone related to deep convection in the Amazone basin below). A linear stability analysis points out that the recurrence of a meridional sub-cell positioned on top of the Hadley cell can be due to symmetrical inertial instability, as maximum growth rates can occur on the vertical scale observed for typical damping values of the upper troposphere. Inertial instability may also play an important role in the formation of the Hadley cell, which is approximately confined over the same latitude band during this period, through a northward acceleration of the upper Hadley branch at the height of the subtropical jet. The moist saturated conditions in the lower troposphere do allow inertial instability here, but the high damping values within the boundary layer suggests that the observed southward acceleration in the lower branch of the Hadley cell has a cause other than inertial instability. During the monsoon period, it is found that the cool moist air advected over the Amazon basin by the northeasterly trade winds has the wedge-like shape of a cold front sloping poleward (about 1:500), as it replaces the warmer continental air and induces deep convective events that pierce through the boundary layer up to about the height of the upper Hadley branch at 12 km. This study presents first evidence, not conclusive proof, that inertial instability plays an important role in the formation of meridional circulation cells in the tropical troposphere. More extensive analyses and modeling are clearly needed to substantiate this and infer its validity on broader temporal and geographical scales.

Acknowledgments

[10] We thank the Suriname operator team for their dedication and good work. Paramaribo station has benefited from its membership to the SHADOZ network. The first author wishes to acknowledge the Radio Science Center for Space and Atmosphere (RASC) at Kyoto University, Japan, where most of this work was done during his stay as visiting scientist. The referees are acknowledged for the valuable suggestions that were incorporated in the revision. This study is part of the project RADCHIS (Research on Atmospheric Dynamics and Chemistry in Suriname), financed by the Netherlands Organization for Scientific Research (NWO-ALW).