Atlantic circulation variability based on high and low heating years over

For the subsets of high and low years (Table B.1), zonal wind, SLP, and SST data from MERRA-2 is averaged for an elongated swath along the equator and extending directly across the Atlantic Ocean, from 40oW to 10oW, 2.5oN to 2.5oS, or the region of the tropical Atlantic where the return flow of the Walker cell is to be expected (Fig. A.3, blue box). Additionally, 500 mb omega is plotted to verify relative magnitudes of rising motion. For each of the variables of interest, data was averaged across latitudes within the spatial domain, such that longitudinal variations would be highlighted.

3.3.1 Omega

Figure A.18 shows the annual cycle of omega at 500 mb for the high (top) and low (middle) heating years, as well as the difference in omega between the high and low heat- ing years (bottom). During both the high and low years, there is significant rising motion (represented by negative values) in the western Atlantic and weaker rising motion in the eastern Atlantic during the wet season months of January through May, with maximum rising motion during the month of April (the time of maximum heating over the Amazon). Positive vertical motion is still seen in the east during this period due to the presence of warm waters and troughing along the equator during these months (represented by April plots in Figures A.16 and A.17), but it is much smaller in magnitude relative to the west. Sinking motion (positive values) occurs across the equatorial Atlantic during the Ama- zonian dry season (July through November) as warm waters and the equatorial trough move northward (represented by June plots in Figures A.16 and A.17). During MAM, Horel (1982) showed that the ITCZ is approximately over the equator, but by August and September, the ITCZ is further north, around 12o_{N; thus, rising motion is significantly}

diminished.

During high heating years (Fig. A.18, top panel), the easternmost area transitions to sinking motion in June while the westernmost area sees sinking motion in mid- to late July. This lag can be explained by the magnitudes of rising motion - since the westernmost areas experience stronger rising motion, the switch to positive values for omega takes longer. For the low heating years (Fig. A.18, middle panel), the pattern is largely the same, except the magnitudes are significantly lower during MAM, leading to an earlier switch to sinking motion as the dry season approaches (occurring in early July).

The difference plot (Fig. A.18, bottom panel) illuminates the largest changes in 500-mb omega that are associated with high and low heating years in the Amazon; in particular, cooler colors during MAM signify the high years having stronger upward motion, and warmer colors signify the low years having stronger upward motion. Upward motion is stronger in the west (between 40o_{W and 35}o_{W) during most months for years of high la-}

tent heat release (even though high and low years were classified based on heat release during the wet season). This stronger latent heat release in the west appears to correlate with stronger sinking in the eastern equatorial Atlantic throughout the year. This is con- sistent with years of high latent heat release experiencing an enhancement of the Walker circulation, which correlates with greater rising motion in the west and greater subsidence in the east. However, during low heating years, February shows stronger upward motion in the western Atlantic and stronger downward motion in the central Atlantic and April shows stronger downward motion across the central and eastern Atlantic. These vertical motion anomalies are likely due to the evolution of heating during the low heating years, which has an early maximum in heating during February and an early pick up in the main maximum in April (Fig. A.15).

3.3.2 SLP

For both high and low heating years, there is a significant lag between the increased latent heat release during MAMJ in the Richter and Xie area of interest and the increase in SLP detected for the eastern tropical Atlantic (Fig. A.19, top and middle panels). While SLP is fairly constant across the MAMJ wet season, with even slightly lower pressures in the east, the biggest change occurs toward the end of the wet season, between April and June, when SLP across the tropical Atlantic nearly uniformly experiences an increase, which also coincides with the seasonal departure of the ITCZ from the equatorial Atlantic. Associated rising motion and thus lower surface pressures have migrated northward at this point, and as a result, relatively higher pressures result at the surface. The largest longitudinal variation occurs from July to August, with the highest values in the east and lower values in the west.

The SLP difference plot (Fig. A.19, bottom panel) shows significant zonal striation during the wet season, representing a switch between the high years experiencing stronger SLP readings and the low years experiencing stronger readings. The months when SLP is higher are when there are latent heating local maxima during the low years (i.e., February and June). During the dry season, there are no significant oscillations. It is interesting to note that from June to September, SLP is higher for the years when latent heat release from the Richter and Xie area of interest was lower. Thus, there may be implications for boreal spring Amazon heating on hurricane formation conditions during boreal summer. 3.3.3 Zonal wind

The annual cycle of zonal wind speed for both high and low heating years show similar variations in equatorial easterlies across the Atlantic (Fig. A.20, top and middle panels). During a period when pressure is zonally constant (e.g. from April to June) there is little longitudinal variation in the magnitude of zonal winds, which is consistent with the magni-

tude of the pressure gradient (Fig. A.19). Winds are weaker in April due to the dominance of the ITCZ and associated weak horizontal winds. As the ITCZ departs and pressure in- creases to its peak in June through September, zonal winds become stronger as the Walker cell assumes dominance. The stronger pressure gradient in the west corresponds to a con- vergence of air into an area of low pressure in northern South America, which leads to the enhanced vertical lift (Fig. A.17) and precipitation totals seen in the region. Conversely, a weak pressure gradient in the east results in weak winds. An additional period when winds are significantly weaker in the east appears during the month of September, likely due to the seasonal migration of peak solar radiation over the equatorial region.

When comparing the high and low years (Fig. A.20, bottom panel), warmer colors represent areas when the low years experience stronger easterlies; cooler colors represent areas when the high years experience stronger easterlies. Overall, the west is dominated by stronger easterlies during low years, and the east is dominated by stronger easterlies during high years. There is a slight oscillation from east to west in peak difference, particularly from January through August and strongest during MAMJ, which can likely be linked to variations in the timing of pressure changes and the subsequent changes in zonal winds. 3.3.4 SST

When analyzing SSTs (Fig. A.21), a structure similar to that of SLP is seen for both high years and low years (top and middle panels), with warm SSTs across the equatorial Atlantic January through May shifting to much cooler waters in the eastern Atlantic from June to October. When the pressure gradient and thus zonal winds are strongest, the SSTs trend lower. The stronger trades, seen from May through August, advect warmer surface water westward, decreasing the depth of the thermocline and allowing cooler subsurface water to upwell.

west Atlantic is generally warmer and the east Atlantic is generally cooler during high years, consistent with the zonal wind differences in the previous section. The main ex- ception occurs in June when the entire equatorial Atlantic is warmer during high heating years. This signal potentially represents a delay or weakening in the setup of the equatorial cold tongue in the eastern Atlantic in June, linked to changes in the western Atlantic in April and May (e.g. see eastward propagating feature between 40o_{W and 30}o_W).

4. CONCLUSIONS

When looking at Amazonia, there is generally good agreement between latent heating retrievals derived from the SHK, CSH, SLH, and variational analysis algorithms, showing a strong wet season during FMA, a dry season during JAS, and peak latent heat release between 6-8 km; however, variations exist due to differences in rain type identification and different sources of rain rates.

Latent heating from the SHK algorithm was analyzed across the Richter and Xie spa- tial area in the northeastern Amazon, where many GCMs experience severe precipitation biases, and years were classified as having relatively high or low latent heating across the MAMJ wet season. In correlation with higher magnitudes of latent heat release, 500- mb omega values nearest to this area of interest are more strongly negative during boreal spring than during years of lower latent heat release, indicating stronger positive vertical motion and verifying the strongest location of rising motion with a concurrent decrease in upward motion in the eastern Atlantic, as would be expected from a strengthened Walker circulation. A corresponding increase in SLP, an increase in return surface easterlies, and a decrease in SST across the equatorial Atlantic in also observed during boreal spring for high latent heating years.

During the low years, the evolution of heating through the wet season affects the timing and magnitudes of vertical motion, such that an early peak in latent heat release during February and an earlier pick up in the main peak during the month of April yield short but significant enhancements in the Walker circulation. As a result, SLP readings are lower in comparison to the high years during these periods.

When comparing low and high heating years and their impact on the Atlantic Walker circulation, there is significant temporal lag such that the strongest SLP and easterly

anomalies appear in boreal summer during high years. This lag is likely due to the domi- nance of the equatorial trough over the Atlantic during the Amazonian wet season, which moves north during boreal summer. In addition, high latent heating years show a delay in the onset of the cold tongue in June and July. This feature, in conjunction with the lower SLP, may have an impact on conditions in the equatorial Atlantic during the North Atlantic hurricane season during high heating years over the Amazon.

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