2.2 Southwest Monsoon

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2.2 Southwest Monsoon While many manuals place their discussion of the northeast monsoon first-since it can be associated with January, the first month of the year-the southwest monsoon is presented

1 2.2 Southwest Monsoon While many manuals place their discussion of the northeast monsoon first-since it can be associated with January, the first month of the year-the southwest monsoon is presented first in this handbook. Since the author began closely observing and monitoring the Philippine Islands (PI) nearly three years ago, it has been obvious that the southwest monsoon regime presents much more interesting and challenging weather for the Philippines. In particular, during a 10-day visit during January 1990, the author found most of Luzon devoid of any rainfall, with daytime temperatures in the 80's ( F) and comfortably-low humidity. The likelihood of a tropical cyclone (TC) striking the PI during January (and other months during the northeast monsoon) is small and will be discussed in Section 2.3. Figure 2.3 displays the average 200-mb and surface wind flow patterns during August' As the northern hemisphere summer months approach, the warming of the Asian continent (with the commensurate increase of thickness in the lower and middle troposphere), establishes a high pressure center over Nepal and Tibet at 200 mb, with its associated tropical easterly jet over southern India and Sri Lanka (not shown). Fig. 2.3(a) shows the dominant northeasterly flow aloft over the PI during the southwest monsoon regime, with the neutral point (near 22 N, 134 E) identifying the western extension of the tropical upper tropospheric trough (TUTT), often extending from western Canada southwestward to the western Pacific Ocean in the upper troposphere. While the subtropical ridge at 200 mb is poleward (off the chart in Fig. 2.3(a)), the sub-equatorial ridge, with its associated divergence, is present just south of the TUTT. Its existence is supported by the release of latent heat in the monsoon trough below. At the surface, land station wind speeds may approach zero during the night and early morning hours. Figure 2.3(b) displays the typical compensating southwesterly surface wind. As displayed in the monthly progression of Appendix A, high surface pressure over Asia commences to weaken as the land heats up, during April and May. Then as lower surface pressure is established over China (Fig. 2.4(a)) and the surface pressure ridge recedes northward and eastward over the Pacific Ocean, the southwesterlies are established during July and August. Additionally, the sea surface temperature (SST) (Fig. 2.4(b)) is everywhere far above 26.5 C, generally accepted as the SST required to support tropical cyclone genesis (Elsberry et al. 1987). While there is no exact date for the commencement of the surface southwest monsoon flow in the South China Sea and Philippine Islands, Appendix A displays the normal progression from northeasterly surface winds in April, becoming southeasterly during May, southerly in June and finally southwesterly in July, August and September. During 1991 and 1992, the southwest monsoon regime commenced early (in June). The commencement of the southwest monsoon regime in 1991 was coincident with the eruption of Mount Pinatubo and is discussed in Section 'Often typical of months June through early November--except at 200 mb over Luzon, where the northeasterly flow is replaced by the subtropical ridge during October and November. 2-5

2 (a) (b) Figure 2.3: (a) Mean 200 mb Flow, August (Sadler and Wann 1984) Streamlines (solid, with arrow indicating direction of flow) and isotachs (dashed) in knots (b) Surface Wind Flow, August (adapted from Sadler et al. 1987a and 1987b) Streamlines (solid, with arrow indicating direction of flow) and isotachs (dashed) in m/sec 2-6

3 (a) '7 4' t ' e , ' "R I0 20e t A2 2 PiT -'? 2 0S %; 't42e to 5 O 205 } z e , 290 (b) I 9 Q82 I Ž Ze " s q5 2zq ' I I Figure 2.4: (a) Mean Sea-level Pressure, August The isobars are labeled in millibars (or hectopascals (hpa)) with the leading 100 omitted. (b) Sea-surface Temperature (SST) in 0 C, August (adapted from Sadler et al. 1987a and 1987b) 2-7

2.2.1 June-November (Months of Heavier Precipitation) General June-August Many references classify June as a transition month or a southwest monsoon formation month; however, this handbook presents

4 2.2.1 June-November (Months of Heavier Precipitation) General June-August Many references classify June as a transition month or a southwest monsoon formation month; however, this handbook presents June with the following two months. While Fig. 2.3(b) indicates that the flow is generally from the southwest during August 2, the wind direction may vary, e.g., westerly, southerly, or even southeasterly. During periods of weak southwest monsoon flow, onshore (sea breeze) and offshore (land breeze) winds may dominate. The air mass may be classified as maritime equatorial, often extending to 10 km. The southwest monsoon may appear as early as May, attaining its maximum intensity by July or August (Flores and Balagot 1969). The southwest monsoon is cloudy 3, hot, humid and wet. Surface temperature maxima vary from the mid 80's to the 90's ( 0 F), while the temperature minima range from 65' (probably during an early onset of the NE monsoon) to 0 (see the table of Manila statistics on page 2-10). Relative humidities are high, though varying diurnally. Their average of 60-80% during the afternoon increases to 85-95% or more during the early morning. Cool temperatures only occur due to prolonged lack of insolation; however, the presence of deep moisture (with its back radiation) keeps nighttime temperatures warm. The absence of pronounced temperature inversions and the presence of large water vapor magnitudes aloft combine with temperature lapse rates near saturated adiabatic to produce frequent convective activity. While the air stream is fairly constant, the presence of a large tropical cyclone to the north (e.g., near the Luzon Strait or approaching Taiwan) may enhance the magnitude of the southwesterly wind. Such a condition may persist for a week or longer with an associated increase in rainfall and is one cause of a surge in the southwest monsoon (see case studies in Sections and 3.5.4). Although the June- August rainfall is high (see the table on page 2-10), it varies widely, from 4-10 inches a month to inches a month. The greatest rainfall occurs at western locations in the northern Philippines (with rain occurring days during a month), while only days elsewhere. Thunderstorms occur on 5-20 days per month. Gale force winds, though rare, can last for 5-10 minutes during thunderstorms, or longer during passage of "a tropical cyclone-although gale force wind may be intermittent for several days during "a deep surge (USAFETAC 1985). This period is the cloudiest for the Philippines, and the mean cloud cover varies from 60 to 95%. This increase in cloudiness is partially attributed to the monsoon trough moving northward during the period. The location of the trough is displayed in Appendix A, extending from northeastern Luzon toward the southeast during July (Fig. A-8(b)), and then extending from the Luzon Strait toward the east-southeast during August (Figure A-9(b)). As discussed in Section 2.3, tropical cyclones are often spawned within the monsoon trough and then move westward to affect the Philippine Islands. 2 See Appendix A for monthly climatological wind directions 'Clear = 0/10 sky cover, partly cloudy = 1/10-5/10, cloudy = 6/10-9/10 and overcast = 10/

5 September-November As described for the previous three months, the surface wind direction may deviate from the monthly averages displayed in Appendix A. Recognizing the effects or superposition of sea breezes and land breezes, the monthly surface winds change from southwesterly in September, when the monsoon trough is over northern Luzon (Fig. A-10(b)) to northeasterly in November, when the monsoon trough has returned south to Mindanao (Fig. A-12(b)). While the weather during September is dominated by the southwest monsoon, that of October and November experiences less of its effects, as attested by the decrease of Manila rainfall (see table on page 2-10). The normal disappearance of the southwest monsoon during October, is followed, on the average, by the appearance of northeasterly flow in the northern Philippines (north of 13'N) during November. However, occasionally the southwest monsoon may persist even until December (Flores and Balagot 1969). As mentioned earlier, the monsoon trough passes over the PI moving southward during this period providing cloudy, hot, humid and wet weather. However, the mean cloud cover decreases from 60-95% in September to 50-85% in November. Mean daily maximum temperatures are much like the previous three months: 80 F to 90'F, with mean minima decreasing somewhat: 60-0 F. Relative humidity remains high: 85-95% during the early morning, decreasing to 65-80% in the afternoon. While precipitation varies widely from 2 to 25 inches a month within the PI, rainfall increases on the windward northeastern coasts. In November, rainfall occurring only 5-10 days a month in western Luzon is contrasted to days a month at northeastern locations. Thunderstorm activity decreases in frequency from 4-15 days a month in September to 1-9 days in November. As in the previous three months (if tropical cyclones are omitted), gale force winds are rare except during thunderstorms when they are common and can last for 5-10 minutes (USAFETAC 1985). Flying Weather (USAFETAC 1985) June-August * Good, but conditions are poor in the mountains. * Ceilings and visibilities 4 < 5000/6 occur up to 30% of the time. * < 1500/3: up to 20% of the time. * < 500/1: about 2% of the time. 4 ceiling/visibility, e.g., 5000/6 indicates "ceiling of 5000 feet and/or visibility of 6 miles." 2-9

6 September-November * Good. * Ceilings and visibilities < 5000/6 occur from less than 5 to 30% of the time. * < 1500/3: less than 20% of the time. * < 500/1: about 2% of the time. Terminal Weather, Manila (USAFETAC 1985) June-August Good. Ceiling/visibility < 300/1 occurs less than 1% of the time. Thunderstorms occur 5-10 days of the month. Prevailing surface wind directions vary greatly, easterly through westerly, at 5-10 kt or less. September-November Good. Ceiling/visibility < 300/1 occurs less than 1% of the time. Thunderstorms occur on 8 days in September and 2 days in November. Surface winds are light from the north or northeast. Monthly temperature, precipitation, thunderstorms & twilight (USAFETAC, 1985) (See Appendix C for Climatic Normals compiled by PAGASA (Philippine Atmospheric, Geophysical and Astronomical Services Administration) for 60 Philippine stations other than Manila.) MANILA TEMPERATURE ( F) Absolute maximum Mean maximum Mean minimum Absolute minimum MEAN PRECIPITATION (INCHES) MEAN NUMBER OF DAYS Precipitation Thunderstorms CIVIL TWILIGHT (15th of month) First light (local standard time) Last light (local standard time) JUN JUL AUG SEP OCT NOV

AT350 EXAM #2 November 18, 2003

AT350 EXAM #2 November 18, 2003 Name and ID: Enter your name and student ID number on the answer sheet and on this exam. Record your answers to the 50 questions by using a No. 2 pencil to completely fill